U.S. patent application number 10/633699 was filed with the patent office on 2005-04-07 for glycosylation engineering of antibodies for improving antibody-dependent cellular cytotoxicity.
Invention is credited to Bailey, James E., Jean-Mairet, Joel, Umana, Pablo.
Application Number | 20050074843 10/633699 |
Document ID | / |
Family ID | 22172079 |
Filed Date | 2005-04-07 |
United States Patent
Application |
20050074843 |
Kind Code |
A1 |
Umana, Pablo ; et
al. |
April 7, 2005 |
Glycosylation engineering of antibodies for improving
antibody-dependent cellular cytotoxicity
Abstract
The present invention relates to the field glycosylation
engineering of proteins. More particular, the present invention is
directed to the glycosylation engineering of proteins to provide
proteins with improved therapeutic properties, e.g., antibodies,
antibody fragments, or a fusion protein that includes a region
equivalent to the Fc region of an immunoglobulin, with enhanced
Fc-mediated cellular cytotoxicity.
Inventors: |
Umana, Pablo; (Zurich,
CH) ; Jean-Mairet, Joel; (Zurich, CH) ;
Bailey, James E.; (Zurich, CH) |
Correspondence
Address: |
STERNE, KESSLER, GOLDSTEIN & FOX PLLC
1100 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Family ID: |
22172079 |
Appl. No.: |
10/633699 |
Filed: |
August 5, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10633699 |
Aug 5, 2003 |
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09294584 |
Apr 20, 1999 |
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6602684 |
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60082581 |
Apr 20, 1998 |
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Current U.S.
Class: |
435/69.1 ;
435/193; 435/320.1; 435/326; 435/358; 536/23.2 |
Current CPC
Class: |
A61P 43/00 20180101;
C07K 16/2896 20130101; C07K 2317/41 20130101; C12P 21/005 20130101;
C07K 16/00 20130101; A61P 11/00 20180101; A61P 17/00 20180101; A61P
15/00 20180101; A61P 13/12 20180101; C07K 2317/14 20130101; Y10S
977/802 20130101; A61P 25/00 20180101; A61P 35/00 20180101; C07K
16/2887 20130101; C07K 16/3053 20130101; C07K 2317/732 20130101;
C07K 2317/24 20130101; A61P 1/04 20180101; A61P 35/02 20180101;
C07K 2319/30 20130101 |
Class at
Publication: |
435/069.1 ;
435/320.1; 435/193; 435/326; 435/358; 536/023.2 |
International
Class: |
C12N 009/10; C12N
005/06 |
Claims
1-85. (Cancelled).
86. A glycoengineered, recombinant antibody comprising an Fc region
containing N-linked oligosaccharides, wherein said antibody has
been engineered to have an altered pattern of glycosylation in the
Fc region and wherein said antibody has increased Fc-mediated
cellular cytotoxicity as a result of said altered
glycosylation.
87. A glycoengineered, recombinant antibody comprising an Fc region
containing N-linked oligosaccharides, wherein said antibody has
been engineered to have an altered pattern of glycosylation in the
Fc region and wherein said antibody has increased Fc receptor
binding affinity as a result of said altered glycosylation.
88. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein said antibody is a chimeric antibody.
89. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein said antibody is a humanized antibody.
90. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein said antibody is an antibody fragment that
contains the Fc region.
91. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein said antibody is a fusion protein that
includes a Fc region of an immunoglobulin.
92. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein said altered pattern of glycosylation is an
increase in the proportion of GlcNAc residues compared to the
nonglycoengineered recombinant antibody.
93. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein said altered pattern of glycosylation is a
decrease in the proportion of fucose residues compared to the
nonglycoengineered recombinant antibody.
94. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein said altered pattern of glycosylation is an
increase in the proportion of GlcNAc residues relative to the
proportion of fucose residues compared to the nonglycoengineered
recombinant antibody.
95. A glycoengineered recombinant antibody according to claim 92,
wherein said oligosaccharides are bisected.
96. A glycoengineered, recombinant antibody according to claim 92,
wherein said oligosaccharides are bisected, complex.
97. A glycoengineered, recombinant antibody according to claim 92,
wherein said oligosaccharides are bisected, hybrid.
98. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein said antibody is isolated from an engineered
host cell selected from the group consisting of an engineered CHO
cell, an engineered BHK cell, an engineered NS0 cell, an engineered
SP2/0 cell, an engineered yeast cell, and an engineered plant
cell.
99. A glycoengineered, recombinant antibody according to claim 98,
wherein said antibody is isolated from an engineered CHO cell.
100. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein said antibody is a therapeutic antibody.
101. A glycoengineered, recombinant antibody according to claim
100, wherein said antibody selectively binds to a cancer
antigen.
102. A glycoengineered, recombinant antibody according to claim
100, wherein said antibody is a monoclonal antibody.
103. A glycoengineered recombinant antibody according to claim 100,
wherein said antibody is a selected from the group consisting of:
an anti-CD20 antibody, an anti-human neuroblastoma antibody, an
anti-human renal cell carcinoma antibody, an anti-HER2 antibody, an
anti-human colon, lung, and breast carcinoma antibody, an
anti-human 17-1A antigen antibody, a humanized anti-human
colorectal tumor antibody, an anti-human melanoma antibody, and an
anti-human squamous-cell carcinoma antibody.
104. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein said antibody is IgG.
105. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein the majority of the N-linked oligosaccharides
are bisected.
106. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein the majority of the N-linked oligosaccharides
are nonfucosylated.
107. A glycoengineered, recombinant antibody according to claim 86
or claim 87, wherein the majority of the N-linked oligosaccharides
are bisected, nonfucosylated.
108. A glycoengineered, recombinant antibody produced by a process
comprising: (a) providing a glycoengineered host cell comprising at
least one nucleic acid encoding a recombinant antibody and at least
one nucleic acid encoding a glycosyltransferase; (b) culturing said
host cell under conditions which permit expression of said
recombinant antibody having an altered pattern of glycosylation in
the Fc region of said antibody; and (c) isolating said recombinant
antibody; wherein said recombinant antibody has increased
Fc-mediated cellular cytotoxicity as a result of said altered
glycosylation.
Description
I. RELATION TO OTHER APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application Ser. No. 60/082,581, filed Apr. 20, 1998, incorporated
herein by reference in its entirety.
II. FIELD OF THE INVENTION
[0002] The present invention relates to the field of glycosylation
engineering of proteins. More particularly, the present invention
relates to glycosylation engineering to generate proteins with
improved therapeutic properties, including antibodies with enhanced
antibody-dependent cellular cytotoxicity.
III. BACKGROUND OF THE INVENTION
[0003] Glycoproteins mediate many essential functions in human
beings, other eukaryotic organisms, and some prokaryotes, including
catalysis, signalling, cell-cell communication, and molecular
recognition and association. They make up the majority of
non-cytosolic proteins in eukaryotic organisms. Lis and Sharon,
1993, Eur. J. Biochem. 218:1-27. Many glycoproteins have been
exploited for therapeutic purposes, and during the last two
decades, recombinant versions of naturally-occurring, secreted
glycoproteins have been a major product of the biotechnology
industry. Examples include erythropoietin (EPO), therapeutic
monoclonal antibodies (therapeutic mAbs), tissue plasminogen
activator (tPA), interferon-.beta., (IFN-.beta.),
granulocyte-macrophage colony stimulating factor (GM-CSF), and
human chorionic gonadotrophin (hCH). Cumming et al., 1991,
Glycobiology 1:115-130.
[0004] The oligosaccharide component can significantly affect
properties relevant to the efficacy of a therapeutic glycoprotein,
including physical stability, resistance to protease attack,
interactions with the immune system, pharmacokinetics, and specific
biological activity. Such properties may depend not only on the
presence or absence, but also on the specific structures, of
oligosaccharides. Some generalizations between oligosaccharide
structure and glycoprotein function can be made. For example,
certain oligosaccharide structures mediate rapid clearance of the
glycoprotein from the bloodstream through interactions with
specific carbohydrate binding proteins, while others can be bound
by antibodies and trigger undesired immune reactions. Jenkins et
al., 1996, Nature Biotechn. 14:975-981.
[0005] Mammalian cells are the preferred hosts for production of
therapeutic glycoproteins, due to their capability to glycosylate
proteins in the most compatible form for human application.
Cumming, 1991, supra; Jenkins et al., 1996, supra. Bacteria very
rarely glycosylate proteins, and like other types of common hosts,
such as yeasts, filamentous fungi, insect and plant cells, yield
glycosylation patterns associated with rapid clearance from the
blood stream, undesirable immune interactions, and in some specific
cases, reduced biological activity. Among mammalian cells, Chinese
hamster ovary (CHO) cells have been most commonly used during the
last two decades. In addition to giving suitable glycosylation
patterns, these cells allow consistent generation of genetically
stable, highly productive clonal cell lines. They can be cultured
to high densities in simple bioreactors using serum-free media, and
permit the development of safe and reproducible bioprocesses. Other
commonly used animal cells include baby hamster kidney (BHK) cells,
NS0- and SP2/0-mouse myeloma cells. More recently, production from
transgenic animals has also been tested. Jenkins et al., 1996,
supra.
[0006] The glycosylation of recombinant therapeutic proteins
produced in animal cells can be engineered by overexpression of
glycosyl transferase genes in host cells. Bailey, 1991, Science
252:1668-1675. However, previous work in this field has only used
constitutive expression of the glycoprotein-modifying glycosyl
transferase genes, and little attention has been paid to the
expression level.
IV. SUMMARY OF THE INVENTION
[0007] The present invention is directed, generally, to host cells
and methods for the generation of proteins having an altered
glycosylation pattern resulting in improved therapeutic values. In
one specific embodiment, the invention is directed to host cells
that have been engineered such that they are capable of expressing
a preferred range of a glycoprotein-modifying glycosyl transferase
activity which increases complex N-linked oligosaccharides carrying
bisecting GlcNAc. In other embodiments, the present invention is
directed to methods for the generation of modified glycoforms of
glycoproteins, for example antibodies, including whole antibody
molecules, antibody fragments, or fusion proteins that include a
region equivalent to the Fc region of an immunoglobulin, having an
enhanced Fc-mediated cellular cytotoxicity, and glycoproteins so
generated. The invention is based, in part, on the inventors'
discovery that there is an optimal range of glycoprotein-modifying
glycosyl transferase expression for the maximization of complex
N-linked oligosaccharides carrying bisecting GlcNAc.
[0008] More specifically, the present invention is directed to a
method for producing altered glycoforms of proteins having improved
therapeutic values, e.g., an antibody which has an enhanced
antibody dependent cellular cytotoxicity (ADCC), in a host cell.
The invention provides host cells which harbor a nucleic acid
encoding the protein of interest, e.g., an antibody, and at least
one nucleic acid encoding a glycoprotein-modifying glycosyl
transferase. Further, the present invention provides methods and
protocols of culturing such host cells under conditions which
permit the expression of said protein of interest, e.g., the
antibody having enhanced antibody dependent cellular cytotoxicity.
Further, methods for isolating the so generated protein having an
altered glycosylation pattern, e.g., the antibody with enhanced
antibody dependent cellular cytotoxicity, are described.
[0009] Furthermore, the present invention provides alternative
glycoforms of proteins having improved therapeutic properties. The
proteins of the invention include antibodies with an enhanced
antibody-dependent cellular cytotoxicity (ADCC), which have been
generated using the disclosed methods and host cells.
V. BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 depicts the representation of typical oligosaccharide
structures.
[0011] FIG. 2 depicts a Western blot analysis of
tetracycline-regulated expression of GnT III in two different
tTA-producing CHO clones. CHOt2 (lanes A and B) and CHOt17 (lanes C
and D) cells were transfected with the pUDH10-3GnTIIIm expression
vector and cultured for 36 h in the absence (lanes A and C) or
presence of tetracycline, at a concentration of 400 ng/ml (lanes B
and D). Cell lysates were then prepared for western blot analysis
probing with an antibody (9E10), which recognizes specifically the
c-myc tag added to GnT III at its carboxy-terminus.
[0012] FIG. 3 depicts determination of the range of tetracycline
concentrations where myc-tagged GnT III expression can be
controlled. CHOt17 cells were transfected with the pUDH10-3-GnTIIIm
expression vector and then cultured for 48 h in the presence of the
indicated concentrations of tetracycline. GnT III levels in cell
lysates from these cultures were compared using western blot
analysis. GnT III was detected via the c-myc tag using 9E10
antibody.
[0013] FIGS. 4A through 4B depict screening of CHO clones for
stable, tetracycline-regulated expression of GnT V (FIG. 4A) or
myc-tagged GnT III (FIG. 4B) glycosyltransferases by western blot
analysis. CHOt17 cells were co-transfected with a vector for
expression of puromycin resistance (pPUR) and either pUHD10-3GnTV
(FIG. 4A) or pUDH10-3GnTIIIm (FIG. 4B) and stable CHO clones were
selected for resistance to puromycin (7.5 .mu./ml), in the presence
of tetracycline (2 .mu.g/ml). Eight clones (1-8) for each
glycosyltransferase were cultured for 48 h in the absence or
presence (+) of tetracycline (2 .mu.g/ml) and analysed by western
blot using either an anti-GnT V antibody (FIG. 4A) or an anti-myc
(9E10) antibody (FIG. 4B).
[0014] FIGS. 5A and 5B depict verification of activity of
heterologous GnT V (FIG. 5A) and Gn T III (FIG. 5B)
glycosyltransferaseas in vivo by lectin blot analysis. Cellular
glycoproteins from various stable clones (numbered as in FIG. 4),
cultured in the absence or presence (+) of tetracycline (2
.mu.g/ml), were resolved by SDS-PAGE, blotted to a membrane, and
probed with either L-PHA (FIG. 5A) or E-PHA (FIG. 5B) lectins.
These lectins bind with higher affinity to the oligosaccharide
products of reactions catalyzed by GnT V and GnT III, respectively,
than to the oligosaccharide substrates of these reactions. A
molecular weight marker (MWM) was run in parallel. A comparison of
lectin blots in FIGS. 5A and 5B indicates a broader range of
substrates, among the endogenous CHO cell glycoproteins, for GnT
III (FIG. 5B) than for GnT V (FIG. 5A).
[0015] FIGS. 6A through 6D depict inhibition of cell growth upon
glycosyltransferase overexpression. CHO-tet-GnTIIIm cells were
seeded to 5-10% confluency and cultured in the absence (FIGS. 6A
and 6B) or presence (FIGS. 6C and 6D) of tetracycline. Cultures
were photographed 45 (FIGS. 6A and 6C) and 85 (FIGS. 16B and 6D)
hours after seeding.
[0016] FIG. 7 depicts sequences of oligonucleotide primers used in
PCRs for the construction of the chCE7 heavy chain gene. Forward
and reverse primers are identified by the suffixes ".fwd" and
".rev", respectively. Overlaps between different primers, necessary
to carry out secondary PCR steps using the product of a primary PCR
step as a template, are indicated. Restriction sites introduced,
sequences annealing to the CE7 chimeric genomic DNA, and the
synthetic leader sequence introduced, are also indicated.
[0017] FIG. 8 depicts sequences of oligonucleotide primers used in
PCRs for the construction of the chCE7 light chain gene. Forward
and reverse primers are identified by the suffixes ".fwd" and
".rev" respectively. Overlaps between different primers, necessary
to carry out secondary PCR steps using as a template the product of
a primary PCR step, are indicated. Restriction sites introduced,
sequences annealing to the CE7 chimeric genomic DNA, and the leader
sequence introduced, are also indicated.
[0018] FIG. 9 depicts MALDI/TOF-MS spectra of neutral
oligosaccharide mixtures from chCE7 samples produced either by
SP2/0 mouse myeloma cells (FIG. 9A, oligosaccharides from 50 .mu.g
of CE7-SP2/0), or by CHO-tetGnTIII-chCE7 cell cultures differing in
the concentration of tetracycline added to the media, and therefore
expressing the GnT III gene at different levels. In decreasing
order of tetracycline concentration, i.e., increasing levels of GnT
III gene. expression, the latter samples are: CE7-2000t (FIG. 9B,
oligosaccharides from 37.5 .mu.g of antibody), CE7-60t (FIG. 9C,
oligosaccharides from 37.5 .mu.g of antibody, CE7-30t (FIG. 9D,
oligosaccharides from 25 .mu.g of antibody) and CE7-15 t (FIG. 9E,
oligosaccharides from 10 .mu.g of antibody).
[0019] FIG. 10 depicts N-linked oligosaccharide biosynthetic
pathways leading to bisected complex oligosaccharides via a GnT
III-catalyzed reaction. M stands for Mannose; Gn,
N-acetylglucosamine (G1cNAc); G, galactose; Gn.sup.b, bisecting
G1cNAc; f, fucose. The oligosaccharide nomenclature consists of
enumerating the M, Gn, and G residues attached to the core
oligosaccharide and indicating the presence of a bisecting G1cNAc
by including a Gn.sup.b. The oligosaccharide core is itself
composed of 2 Gn residues and may or may not include a fucose. The
major classes of oligosaccharides are shown inside dotted frames.
Man I stands for Golgi mannosidase; GnT, G1cNAc transferase; and
GalT, for galactosyltransferase. The mass associated with the
major, sodium-associated oligosaccharide ion that is observed
MALDI/TOF-MS analysis is shown beside each oligosaccharide. For
oligosaccharides which can potentially be core-fucosylated, the
masses associated with both fucosylated (+f) and non-fucosylated
(-f) forms are shown.
[0020] FIG. 11 depicts N-linked oligosaccharide biosynthetic
pathway leading to bisected complex and bisected hybrid
oligosaccharides via GnT III-catalyzed reactions. M stands for
mannose; Gn N-acetylglucosamine (G1cNAc); G, galactose; Gn.sup.b,
bisecting G1cNAc; f, fucose. The oligosaccharide nomenclature
consists of enumerating the M, Gn, and G residues attached to the
common oligosaccharide and indicating the presence of bisecting
G1cNAc by including a Gn.sup.b. The oligosaccharide core is itself
composed of 2 Gn residues and may or not include a fucose. The
major classes of oligosaccharides are shown inside dotted frames.
Man I stands for Golgi mannosidase; TnT, G1cNAc transferase; and
GalT, for galactosyltransferase. The mass associated with major,
sodium-associated oligosaccharide ion that is observed in
MALDI/TOF-MS analysis is shown beside each oligosaccharide. For
oligosaccharides which can potentially be core-fucosylated, the
masses associated with both fucosylated (+f) and non-fucosylated
(-f) forms are shown.
[0021] FIG. 12 depicts ADCC activity of different chCE7 samples.
Lysis of IMR-32 neuroblastoma cells by human lymphocytes
(target:effector ratio of 1:19, 16 h incubation at 37.degree. C.),
mediated by different concentrations of chCE7 samples, was measured
via retention of a fluorescent dye. The percentage of cytotoxicity
is calculated relative to a total lysis control (by means of a
detergent), after subtraction of the signal in the absence of
antibody.
[0022] FIG. 13 depicts the GnT III expression of different cultures
of CHO-tet-GnTIII grown at different tetracycline concentrations
used to produce distinct C2B8 antibody samples. Cell lysates from
each culture grown at 2000 ng/ml (Lane C) and 25 ng/ml (Lane D)
tetracycline concentrations were resolved by SDS-PAGE, blotted onto
a membrane, and probed with 9E10 (see supra) and anti-mouse
horseradish peroxidase as primary and secondary antibodies,
respectively. Lane A depicts a negative control.
[0023] FIGS. 14A and 14B depict the specificity of antigen binding
of the C2B8 anti-CD20 monoclonal antibody using an indirect
immunofluorescence assay with cells in suspension. CD20 positive
cells (SB cells; ATCC deposit no.ATCC CCL120) and CD20 negative
cells (HSB cells; ATCC deposit no. ATCC CCL120.1), FIG. 14A and 14B
respectively, were utilized. Cells of each type were incubated with
C2B8 antibody produced at 25 ng/ml tetracycline as a primary
antibody. Negative controls included HBSSB instead of primary
antibody. An anti-human IgG Fc specific, polyclonal, FITC
conjugated antibody was used for all samples as a secondary
antibody.
[0024] FIG. 15 depicts the ADCC activity of different C2B8 antibody
samples at different antibody concentrations (0.04-5 .mu.g/ml).
Sample C2B8-nt represents the ADCC activity of the C2B8 antibody
produced in a cell line without GnT III expression. Samples
C2B8-2000t, C2B8-50t and C2B8-25t show the ADCC activity of three
antibody samples produced at decreasing tetracycline concentrations
(i.e., increasing GnT III expression).
VI. DEFINITIONS
[0025] Terms are used herein as generally used in the art, unless
otherwise defined in the following:
[0026] As used herein, the term antibody is intended to include
whole antibody molecules, antibody fragments, or fusion proteins
that include a region equivalent to the Fc region of an
immunoglobulin.
[0027] As used herein, the term glycoprotein-modifying glycosyl
transferase refers to an enzyme that effects modification of the
glycosylation pattern of a glycoprotein. Examples of
glycoprotein-modifying glycosyl transferases include, but are not
limited to glycosyl transferases such as GnT III, GnT V, GalT, and
Man II.
[0028] As used herein, the term glycosylation engineering is
considered to include any sort of change to the glycosylation
pattern of a naturally occurring polypeptide or fragment thereof.
Glycosylation engineering includes metabolic engineering of the
glycosylation machinery of a cell, including genetic manipulations
of the oligosaccharide synthesis pathways to achieve altered
glycosylation of glycoproteins expressed in cells. Furthermore,
glycosylation engineering includes the effects of mutations and
cell environment on glycosylation.
[0029] As used herein, the term host cell covers any kind of
cellular system which can be engineered to generate modified
glycoforms of proteins, protein fragments, or peptides of interest,
including antibodies and antibody fragments. Typically, the host
cells have been manipulated to express optimized levels of at least
one glycoprotein-modifying glycosyl transferase, including, but not
limited to GnT III, GnT V, GalT, and Man II, and/or at least one
glycosidase. Host cells include cultured cells, e.g., mammalian
cultured cells, such as CHO cells, BHK cells, NS0 cells, SP2/0
cells, or hybridoma cells, yeast cells, and insect cells, to name
only few, but also cells comprised within a transgenic animal or
cultured tissue.
[0030] As used herein, the term Fc-mediated cellular cytotoxicity
is intended to include antibody dependent cellular cytotoxicity
(ADCC), and cellular cytotoxicity directed to those cells that have
been engineered to express on their cell surface an Fc-region or
equivalent region of an immunoglobin G, and cellular cytotoxicity
mediated by a soluble fusion protein consisting of a target protein
domain fused to the N-terminus of an Fc-region or equivalent region
of an immunoglobulin G.
VII. DETAILED DESCRIPTION OF THE INVENTION
[0031] A. General Overview
[0032] The objective of the present invention is to provide
glycoforms of proteins, in particular antibodies, including whole
antibody molecules, antibody fragments, or fusion proteins that
include a region equivalent to the Fc region of an immunoglobulin,
to produce new variants of a therapeutic protein. The invention is
based, in part, on the inventors' discovery that the glycosylation
reaction network of a cell can be manipulated to maximize the
proportion of certain glycoforms within the population, and that
certain glycoforms have improved therapeutic characteristics. The
invention is further based, in part, on the discovery of ways to
identify glycoforms of proteins which have an improved therapeutic
value, and how to generate them reproducibly. The invention is
further based, in part, on the discovery that there is a preferred
range of glycoprotein-modifying glycosyl transferase expression in
the antibody-generating cell, for increasing complex N-linked
oligosaccharides carrying bisecting GlcNAc.
[0033] As such, the present invention is directed, generally, to
methods for the glycosylation engineering of proteins to alter and
improve their therapeutic properties. More specifically, the
present invention describes methods for producing in a host cell an
antibody which has an altered glycosylation pattern resulting in an
enhanced antibody dependent cellular cytotoxicity (ADCC). For the
practice of the methods, the present invention provides host cells
which harbor a nucleic acid encoding an antibody and at least one
nucleic acid encoding a glycoprotein-modifying glycosyl
transferase. Further, the present invention provides methods and
protocols of culturing such host cells under conditions which
permit the expression of the desired antibody having an altered
glycosylation pattern resulting in an enhanced antibody dependent
cellular cytotoxicity. Further, methods for isolating the so
generated antibody with enhanced antibody dependent cellular
cytotoxicity are described.
[0034] In more specific embodiments of the invention, two
monoclonal antibodies, namely the anti-neuroblastoma antibody
chCE7, and the anti-CD20 antibody C2B8, have been used as model
therapeutic glycoproteins, and the target glycoforms have been
those carrying a special class of carbohydrate, namely bi-antennary
complex N-linked oligosaccharides modified with bisecting
N-acetylglucosamine (GlcNAc). In the model system provided by the
invention, CHO cells are used as host cells, although many other
cell systems may be contemplated as host cell system. The glycosyl
transferase that adds a bisecting GlcNAc to various types of
N-linked oligosaccharides, GlcNAc-transferase III (GnT III), is not
normally produced by CHO cells. Stanley and Campell, 1984, J Biol.
Chem. 261:13370-13378.
[0035] To investigate the effects of GnT III overexpression
experimentally, a CHO cell line with tetracycline-regulated
overexpression of a rat GnT III cDNA was established. Using this
experimental system, the inventors discovered that overexpression
of GnT III to high levels led to growth inhibition and was toxic to
the cells. Another CHO cell line with tetracycline-regulated
overexpression of GnT V, which is a distinct glycosyl transferase,
showed the same inhibitory effect, indicating that this may be a
general feature of glycoprotein-modifying glycosyl transferase
overexpression. The effect of the enzyme expression on the cell
growth sets an upper limit to the level of glycoprotein-modifying
glycosyl transferase overexpression and may therefore also limit
the extent to which poorly accessible glycosylation sites can be
modified by engineering of glycosylation pathways and patterns
using unregulated expression vectors.
[0036] The production of a set of chCE7 mAb and C2B8 samples
differing in their glycoform distributions by controlling GnT III
expression in a range between basal and toxic levels are disclosed.
Measurement of the ADCC activity of the chCE7 mAb samples showed an
optimal range of GnT III expression for maximal chCE7 in vitro
biological activity. The activity correlated with the level of
Fc-associated bisected, complex oligosaccharides. Expression of GnT
III within the practical range, i.e., where no significant growth
inhibition and toxicity are observed, led to an increase of the
target bisected, complex structures for this set of chCE7 samples.
The pattern of oligosaccharide peaks in MALDI/TOF-mass
spectrometric analysis of chCE7 samples produced at high levels of
GnT III indicates that a significant proportion of potential GnT
III substrates is diverted to bisected hybrid oligosaccharide
by-products. Minimization of these by-products by firrther
engineering of the pathway could therefore be valuable.
[0037] B. Identification and Generation of Nucleic Acids Encoding a
Protein for Which Modification of the Glycosylation Pattern is
Desired
[0038] The present invention provides host cell systems suitable
for the generation of altered glycoforms of any protein, protein
fragment or peptide of interest, for which such an alteration in
the glycosylation pattern is desired. The nucleic acids encoding
such protein, protein fragment or peptide of interest may be
obtained by methods generally known in the art. For example, the
nucleic acid may be isolated from a cDNA library or genomic
library. For a review of cloning strategies which may be used, see,
e.g., Maniatis, 1989, Molecular Cloning, A Laboratory Manual, Cold
Springs Harbor Press, N.Y.; and Ausubel et al., 1989, Current
Protocols in Molecular Biology, (Green Publishing Associates and
Wiley Interscience, N.Y.).
[0039] In an alternate embodiment of the invention, the coding
sequence of the protein, protein fragment or peptide of interest
may be synthesized in whole or in part, using chemical methods well
known in the art. See, for example, Caruthers et al., 1980, Nuc.
Acids Res. Symp. Ser. 7:215-233; Crea and Horn, 1980, Nuc. Acids
Res. USA 9:2331; Matteucci and Caruthers, 1980, Tetrahedron Letters
21:719; Chow and Kempe, 1981, Nuc. Acids Res. 9:2807-2817.
Alternatively, the protein itself could be produced using chemical
methods to synthesize its amino acid sequence in whole or in part.
For example, peptides can be synthesized by solid phase techniques,
cleaved from the resin, and purified by preparative high
performance liquid chromatography. E.g., see Creighton, 1983,
Protein Structures And Molecular Principles, W.H. Freeman and Co.,
N.Y. pp. 50-60. The composition of the synthetic peptides may be
confirmed by amino acid analysis or sequencing (e.g., the Edman
degradation procedure; see Creighton, 1983, Proteins, Structures
and Molecular Principles, W.H. Freeman and Co., N.Y., pp.
34-49).
[0040] In preferred embodiments, the invention provides methods for
the generation and use of host cell systems for the production of
glycoforms of antibodies or antibody fragments or fusion proteins
which include antibody fragments with enhanced antibody-dependent
cellular cytotoxicity. Identification of target epitopes and
generation of antibodies having potential therapeutic value, for
which modification of the glycosylation pattern is desired, and
isolation of their respective coding nucleic acid sequence is
within the scope of the invention.
[0041] Various procedures known in the art may be used for the
production of antibodies to target epitopes of interest. Such
antibodies include but are not limited to polyclonal, monoclonal,
chimeric, single chain, Fab fragments and fragments produced by an
Fab expression library. Such antibodies may be useful, e.g., as
diagnostic or therapeutic agents. As therapeutic agents,
neutralizing antibodies, iLe., those which compete for binding with
a ligand, substrate or adapter molecule, are of especially
preferred interest.
[0042] For the production of antibodies, various host animals are
immunized by injection with the target protein of interest
including, but not limited to, rabbits, mice, rats, etc. Various
adjuvants may be used to increase the immunological response,
depending on the host species, including but not limited to
Freund's (complete and incomplete), mineral gels such as aluminum
hydroxide, surface active substances such as lysolecithin, pluronic
polyols, polyanions, peptides, oil emulsions, keyhole limpet
hemocyanin, dinitrophenol, and potentially useful human adjuvants
such as BCG (bacille Calmette-Guerin) and Corynebacterium
parvum.
[0043] Monoclonal antibodies to the target of interest may be
prepared using any technique which provides for the production of
antibody molecules by continuous cell lines in culture. These
include, but are not limited to, the hybridoma technique originally
described by Kohler and Milstein, 1975, Nature 256:495-497, the
human B-cell hybridoma technique (Kosbor et al., 1983, Immunology
Today 4:72; Cote et al., 1983, Proc. Natl. Acad. Sci. U.S.A.
80:2026-2030) and the EBV-hybridoma technique (Cole et al., 1985,
Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp.
77-96). In addition, techniques developed for the production of
"chimeric antibodies" (Morrison et al., 1984, Proc. Natl. Acad.
Sci. U.S.A. 81:6851-6855; Neuberger et al., 1984, Nature
312:604-608; Takeda et al., 1985, Nature 314:452-454) by splicing
the genes from a mouse antibody molecule of appropriate antigen
specificity together with genes from a human antibody molecule of
appropriate biological activity can be used. Alternatively,
techniques described for the production of single chain antibodies
(U.S. Pat. No. 4,946,778) can be adapted to produce single chain
antibodies having a desired specificity.
[0044] Antibody fragments which contain specific binding sites of
the target protein of interest may be generated by known
techniques. For example, such fragments include, but are not
limited to, F(ab').sub.2 fragments which can be produced by pepsin
digestion of the antibody molecule and the Fab fragments which can
be generated by reducing the disulfide bridges of the F(ab').sub.2
fragments. Alternatively, Fab expression libraries may be
constructed (Huse et al., 1989, Science 246:1275-1281) to allow
rapid and easy identification of monoclonal Fab fragments with the
desired specificity to the target protein of interest.
[0045] Once an antibody or antibody fragment has been identified
for which modification in the glycosylation pattern are desired,
the coding nucleic acid sequence is identified and isolated using
techniques well known in the art. See, supra.
[0046] C. Generation of Cell Lines for the Production of Proteins
with Altered Glycosylation Pattern
[0047] The present invention provides host cell expression systems
for the generation of proteins having modified glycosylation
patterns. In particular, the present invention provides host cell
systems for the generation of glycoforms of proteins having an
improved therapeutic value. Therefore, the invention provides host
cell expression systems selected or engineered to increase the
expression of a glycoprotein-modifying glycosyltransferase.
Specifically, such host cell expression systems may be engineered
to comprise a recombinant nucleic acid molecule encoding a
glycoprotein-modifying glycosyltransferase, operatively linked to a
constitutive or regulated promoter system. Alternatively, host cell
expression systems may be employed that naturally produce, are
induced to produce, and/or are selected to produce a
glycoprotein-modifying glycosyltransferase.
[0048] In one specific embodiment, the present invention provides a
host cell that has been engineered to express at least one nucleic
acid encoding a glycoprotein-modifying glycosyl transferase. In one
aspect, the host cell is transformed or transfected with a nucleic
acid molecule comprising at least one gene encoding a
glycoprotein-modifying glycosyl transferase. In an alternate
aspect, the host cell has been engineered and/or selected in such
way that an endogenous glycoprotein-modifying glycosyl transferase
is activated. For example, the host cell may be selected to carry a
mutation triggering expression of an endogenous
glycoprotein-modifying glycosyl transferase. This aspect is
exemplified in one specific embodiment, where the host cell is a
CHO lec10 mutant. Alternatively, the host cell may be engineered
such that an endogenous glycoprotein-modifying glycosyl transferase
is activated. In again another alternative, the host cell is
engineered such that an endogenous glycoprotein-modifying glycosyl
transferase has been activated by insertion of a regulated promoter
element into the host cell chromosome. In a further alternative,
the host cell has been engineered such that an endogenous
glycoprotein-modifying glycosyl transferase.has been activated by
insertion of a constitutive promoter element, a transposon, or a
retroviral element into the host cell chromosome.
[0049] Generally, any type of cultured cell line can be used as a
background to engineer the host cell lines of the present
invention. In a preferred embodiment, CHO cells, BHK cells, NS0
cells, SP2/0 cells, or a hybridoma cell line is used as the
background cell line to generate the engineered host cells of the
invention.
[0050] The invention is contemplated to encompass engineered host
cells expressing any type of glycoprotein-modifying glycosyl
transferase as defined herein. However, in preferred embodiments,
at least one glycoprotein-modifying glycosyl transferase expressed
by the host cells of the invention is GnT III, or, alternatively,
.beta.(1,4)-N-acetylgluco- saminyltransferase V (GnT V). However,
also other types of glycoprotein-modifying glycosyl transferase may
be expressed in the host system, typically in addition to GnT III
or GnT V, including .beta.(1,4)-galactosyl transferase (GalT), and
mannosidase II (Man II). In one embodiment of the invention, GnT
III is coexpressed with GalT. In another embodiment of the
invention, GnT III is coexpressed with Man II. In a further
embodiment of the invention, GnT III is coexpressed with GalT and
Man II. However, any other permutation of glycoprotein-modifying
glycosyl transferases is within the scope of the invention.
Further, expression of a glycosidase in the host cell system may be
desired.
[0051] One or several nucleic acids encoding a
glycoprotein-modifying glycosyl transferase may be expressed under
the control of a constitutive promoter or, alternately, a regulated
expression system. Suitable regulated expression systems include,
but are not limited to, a tetracycline-regulated expression system,
an ecdysone-inducible expression system, a lac-switch expression
system, a glucocorticoid-inducible expression system, a
temperature-inducible promoter system, and a metallothionein
metal-inducible expression system. If several different nucleic
acids encoding glycoprotein-modifying glycosyl transferases are
comprised within the host cell system, some of them may be
expressed under the control of a constitutive promoter, while
others are expressed under the control of a regulated promoter. The
optimal expression levels will be different for each protein of
interest, and will be determined using routine experimentation.
Expression levels are determined by methods generally known in the
art, including Western blot analysis using a glycosyl transferase
specific antibody, Northern blot analysis using a glycosyl
transferase specific nucleic acid probe, or measurement of
enzymatic activity. Alternatively, a lectin may be employed which
binds to biosynthetic products of the glycosyl transferase, for
example, E.sub.4-PHA lectin. In a further alternative, the nucleic
acid may be operatively linked to a reporter gene; the expression
levels of the glycoprotein-modifying glycosyl transferase are
determined by measuring a signal correlated with the expression
level of the reporter gene. The reporter gene may transcribed
together with the nucleic acid(s) encoding said
glycoprotein-modifying glycosyl transferase as a single MRNA
molecule; their respective coding sequences may be linked either by
an internal ribosome entry site (IRES) or by a cap-independent
translation enhancer (CITE). The reporter gene may be translated
together with at least one nucleic acid encoding said
glycoprotein-modifying glycosyl transferase such that a single
polypeptide chain is formed. The nucleic acid encoding the
glycoprotein-modifying glycosyl transferase may be operatively
linked to the reporter gene under the control of a single promoter,
such that the nucleic acid encoding the glycoprotein-modifying
glycosyl transferase and the reporter gene are transcribed into an
RNA molecule which is alternatively spliced into two separate
messenger RNA (mRNA) molecules; one of the resulting mRNAs is
translated into said reporter protein, and the other is translated
into said glycoprotein-modifying glycosyl transferase.
[0052] If several different nucleic acids encoding a
glycoprotein-modifying glycosyl transferase are expressed, they may
be arranged in such way that they are transcribed as one or as
several mRNA molecules. If they are transcribed as a single mRNA
molecule, their respective coding sequences may be linked either by
an internal ribosome entry site (IRES) or by a cap-independent
translation enhancer (CITE). They may be transcribed from a single
promoter into an RNA molecule which is alternatively spliced into
several separate messenger RNA (mRNA) molecules, which then are
each translated into their respective encoded
glycoprotein-modifying glycosyl transferase.
[0053] In other embodiments, the present invention provides host
cell expression systems for the generation of therapeutic proteins,
for example antibodies, having an enhanced antibody-dependent
cellular cytotoxicity, and cells which display the IgG Fc region on
the surface to promote Fc-mediated cytotoxicity. Generally, the
host cell expression systems have been engineered and/or selected
to express nucleic acids encoding the protein for which the
production of altered glycoforms is desired, along with at least
one nucleic acid encoding a glycoprotein-modifying glycosyl
transferase. In one embodiment, the host cell system is transfected
with at least one gene encoding a glycoprotein-modifying glycosyl
transferase. Typically, the transfected cells are selected to
identify and isolate clones that stably express the
glycoprotein-modifying glycosyl transferase. In another embodiment,
the host cell has been selected for expression of endogenous
glycosyl transferase. For example, cells may be selected carrying
mutations which trigger expression of otherwise silent
glycoprotein-modifying glycosyl transferases. For example, CHO
cells are known to carry a silent GnT III gene that is active in
certain mutants, e.g., in the mutant Lec10. Furthermore, methods
known in the art may be used to activate silent
glycoprotein-modifying glycosyl transferase genes, including the
insertion of a regulated or constitutive promoter, the use of
transposons, retroviral elements, etc. Also the use of gene
knockout technologies or the use of ribozyme methods may be used to
tailor the host cell's glycosyl transferase and/or glycosidase
expression levels, and is therefore within the scope of the
invention.
[0054] Any type of cultured cell line can be used as background to
engineer the host cell lines of the present invention. In a
preferred embodiment, CHO cells, BHK cells, NS0 cells, SP2/0 cells.
Typically, such cell lines are engineered to further comprise at
least one transfected nucleic acid encoding a whole antibody
molecule, an antibody fragment, or a fusion protein that includes a
region equivalent to the Fc region of an immunoglobulin. In an
alternative embodiment, a hybridoma cell line expressing a
particular antibody of interest is used as background cell line to
generate the engineered host cells of the invention.
[0055] Typically, at least one nucleic acid in the host cell system
encodes GnT III, or, alternatively, GnT V. However, also other
types of glycoprotein-modifying glycosyl transferase may be
expressed in the host system, typically in addition to GnT III or
GnT V, including GalT, and Man II. In one embodimentof the
invention, GnT III is coexpressed with GalT. In another embodiment
of the invention, GnT III is coexpressed with Man II. In a further
embodiment of the invention, GnT III is coexpressed with GalT and
Man II. However, any other permutation of glycoprotein-modifying
glycosyl transferases is within the scope of the invention.
Further, expression of a glycosidase in the host cell system may be
desired.
[0056] One or several nucleic acids encoding a
glycoprotein-modifying glycosyl transferase may be expressed under
the control of a constitutive promoter, or alternately, a regulated
expression system. Suitable regulated expression systems include,
but are not limited to, a tetracycline-regulated expression system,
an ecdysone-inducible expression system, a lac-switch expression
system, a glucocorticoid-inducible expression system, a
temperature-inducible promoter system, and a metallothionein
metal-inducible expression system. If several different nucleic
acids encoding glycoprotein-modifying glycosyl transferases are
comprised within the host cell system, some of them may be
expressed under the control of a constitutive promoter, while
others are expressed under the control of a regulated promoter. The
optimal expression levels will be different for each protein of
interest, and will be determined using routine experimentation.
Expression levels are determined by methods generally known in the
art, including Western blot analysis using a glycosyl transferase
specific antibody, Northern blot analysis using a glycosyl
transferase specific nucleic acid probe, or measurement of
enzymatic activity. Alternatively, a lectin may be employed which
binds to biosynthetic products of glycosyl transferase, for
example, E.sub.4-PHA lectin. In a further alternative, the nucleic
acid may be operatively linked to a reporter gene; the expression
levels of the glycoprotein-modifying glycosyl transferase are
determined by measuring a signal correlated with the expression
level of the reporter gene. The reporter gene may transcribed
together with the nucleic acid(s) encoding said
glycoprotein-modifying glycosyl transferase as a single mRNA
molecule; their respective coding sequences may be linked either by
an internal ribosome entry site (IRES) or by a cap-independent
translation enhancer (CITE). The reporter gene may be translated
together with at least one nucleic acid encoding said
glycoprotein-modifying glycosyl transferase such that a single
polypeptide chain is formed. The nucleic acid encoding the
glycoprotein-modifying glycosyl transferase may be operatively
linked to the reporter gene under the control of a single promoter,
such that the nucleic acid encoding the glycoprotein-modifying
glycosyl transferase and the reporter gene are transcribed into an
RNA molecule which is alternatively spliced into two separate
messenger RNA (mRNA) molecules; one of the resulting mRNAs is
translated into said reporter protein, and the other is translated
into said glycoprotein-modifying glycosyl transferase.
[0057] If several different nucleic acids encoding a
glycoprotein-modifying glycosyl transferase are expressed, they may
be arranged in such way that they are transcribed as one or as
several mRNA molecules. If they are transcribed as single mRNA
molecule, their respective coding sequences may be linked either by
an internal ribosome entry site (IRES) or by a cap-independent
translation enhancer (CITE). They may be transcribed from a single
promoter into an RNA molecule which is alternatively spliced into
several separate messenger RNA (mRNA) molecules, which then are
each translated into their respective encoded
glycoprotein-modifying glycosyl transferase.
[0058] 1. Expression Systems
[0059] Methods which are well known to those skilled in the art can
be used to construct expression vectors containing the coding
sequence of the protein of interest and the coding sequence of the
glycoprotein-modifying glycosyl transferase and appropriate
transcriptional/translational control signals. These methods
include in vitro recombinant DNA techniques, synthetic techniques
and in vivo recombination/genetic recombination. See, for example,
the techniques described in Maniatis et al., 1989, Molecular
Cloning A Laboratory Manual, Cold Spring Harbor Laboratory, N.Y.
and Ausubel et al., 1989, Current Protocols in Molecular Biology,
Greene Publishing Associates and Wiley Interscience, N.Y.
[0060] A variety of host-expression vector systems may be utilized
to express the coding sequence of the protein of interest and the
coding sequence of the glycoprotein-modifying glycosyl transferase.
Preferably, mammalian cells are used as host cell systems
transfected. with recombinant plasmid DNA or cosmid DNA expression
vectors containing the coding sequence of the protein of interest
and the coding sequence of the glycoprotein-modifying glycosyl
transferase. Most preferably, CHO cells, BHK cells, NS0 cells, or
SP2/0 cells, or alternatively, hybridoma cells are used as host
cell systems. In alternate embodiments, other eukaryotic host cell
systems may be contemplated, including, yeast cells transformed
with recombinant yeast expression vectors containing the coding
sequence of the protein of interest and the coding sequence of the
glycoprotein-modifying glycosyl transferase; insect cell systems
infected with recombinant virus expression vectors (e.g.,
baculovirus) containing the coding sequence of the protein of
interest and the coding sequence of the glycoprotein-modifying
glycosyl transferase; plant cell systems infected with recombinant
virus expression vectors (e.g., cauliflower mosaic virus, CaMV;
tobacco mosaic virus, TMV) or transformed with recombinant plasmid
expression vectors (e.g., Ti plasmid) containing the coding
sequence of the protein of interest and the coding sequence of the
glycoprotein-modifying glycosyl transferase; or animal cell systems
infected with recombinant virus expression vectors (e.g.,
adenovirus, vaccinia virus) including cell lines engineered to
contain multiple copies of the DNA encoding the protein of interest
and the coding sequence of the glycoprotein-modifying glycosyl
transferase either stably amplified (CHO/dhfr) or unstably
amplified in double-minute chromosomes (e.g., murine cell
lines).
[0061] For the methods of this invention, stable expression is
generally preferred to transient expression because it typically
achieves more reproducible results and also is more amenable to
large scale production. Rather than using expression vectors which
contain viral origins of replication, host cells can be transformed
with the respective coding nucleic acids controlled by appropriate
expression control elements (e.g., promoter, enhancer, sequences,
transcription termninators, polyadenylation sites, etc.), and a
selectable marker. Following the introduction of foreign DNA,
engineered cells may be allowed to grow for 1-2 days in an enriched
media, and then are switched to a selective media. The selectable
marker in the recombinant plasmid confers resistance to the
selection and allows selection of cells which have stably
integrated the plasmid into their chromosomes and grow to form foci
which in turn can be cloned and expanded into cell lines.
[0062] A number of selection systems may be used, including, but
not limited to, the herpes simplex virus thymidine kinase (Wigler
et al., 1977, Cell 11:223), hypoxanthine-guanine
phosphoribosyltransferase (Szybalska & Szybalski, 1962, Proc.
Natl. Acad Sci. USA 48:2026), and adenine phosphoribosyltransferase
(Lowy et al., 1980, Cell 22:817) genes, which can be employed in
tk.sup.-, hgprt.sup.- or aprt.sup.- cells, respectively. Also,
antimetabolite resistance can be used as the basis of selection for
dhfr, which confers resistance to methotrexate (Wigler et al.,
1980, Natl. Acad. Sci. USA 77:3567; O'Hare et al., 1981, Proc.
Natl. Acad Sci. USA 78:1527); gpt, which confers resistance to
mycophenolic acid (Mulligan & Berg, 1981, Proc. Natl. Acad.
Sci. USA 78:2072); neo, which confers resistance to the
aminoglycoside G-418 (Colberre-Garapin et al., 1981, J. Mol. Biol.
150:1); and hygro, which confers resistance to hygromycin (Santerre
et al., 1984, Gene 30:147) genes. Recently, additional selectable
genes have been described, namely trpB, which allows cells to
utilize indole in place of tryptophan; hisD, which allows cells to
utilize histinol in place of histidine (Hartman & Mulligan,
1988, Proc. Natl. Acad Sci. USA 85:8047); the glutamine synthase
system; and ODC (omithine decarboxylase) which confers resistance
to the ornithine decarboxylase inhibitor,
2-(difluoromethyl)-DL-omithine, DFMO (McConlogue, 1987, in: Current
Communications in Molecular Biology, Cold Spring Harbor Laboratory
ed.).
[0063] 2. Identification of Transfectants or Transformants That
Express the Protein Having a Modified Glycosylation Pattern
[0064] The host cells which contain the coding sequence and which
express the biologically active gene products may be identified by
at least four general approaches; (a) DNA-DNA or DNA-RNA
hybridization; (b) the presence or absence of "marker" gene
functions; (c) assessing the level of transcription as measured by
the expression of the respective mRNA transcripts in the host cell;
and (d) detection of the gene product as measured by immunoassay or
by its biological activity.
[0065] In the first approach, the presence of the coding sequence
of the protein of interest and the coding sequence of the
glycoprotein-modifying glycosyl transferase(s) inserted in the
expression vector can be detected by DNA-DNA or DNA-RNA
hybridization using probes comprising nucleotide sequences that are
homologous to the respective coding sequences, respectively, or
portions or derivatives thereof.
[0066] In the second approach, the recombinant expression
vector/host system can be identified and selected based upon the
presence or absence of certain "marker" gene functions (e.g.,
thymidine kinase activity, resistance to antibiotics, resistance to
methotrexate, transformation phenotype, occlusion body formation in
baculovirus, etc.). For example, if the coding sequence of the
protein of interest and the coding sequence of the
glycoprotein-modifying glycosyl transferase are inserted within a
marker gene sequence of the vector, recombinants containing the
respective coding sequences can be identified by the absence of the
marker gene function. Alternatively, a marker gene can be placed in
tandem with the coding sequences under the control of the same or
different promoter used to control the expression of the coding
sequences. Expression of the marker in response to induction or
selection indicates expression of the coding sequence of the
protein of interest and the coding sequence of the
glycoprotein-modifying glycosyl transferase.
[0067] In the third approach, transcriptional activity for the
coding region of the protein of interest and the coding sequence of
the glycoprotein-modifying glycosyl transferase can be assessed by
hybridization assays. For example, RNA can be isolated and analyzed
by Northern blot using a probe homologous to the coding sequences
of the protein of interest and the coding sequence of the
glycoprotein-modifying glycosyl transferase or particular portions
thereof. Alternatively, total nucleic acids of the host cell may be
extracted and assayed for hybridization to such probes.
[0068] In the fourth approach, the expression of the protein
products of the protein of interest and the coding sequence of the
glycoprotein-modifying glycosyl transferase can be assessed
immunologically, for example by Western blots, immunoassays such as
radioimmuno-precipitation, enzyme-linked immunoassays and the like.
The ultimate test of the success of the expression system, however,
involves the detection of the biologically active gene
products.
[0069] D. Generation and Use of Proteins and Protein Fragments
Having Altered Glycosylation Patterns
[0070] 1. Generation and Use of Antibodies Having Enhanced
Antibody-Dependent Cellular Cytotoxicity
[0071] In preferred embodiments, the present invention provides
glycoformns of antibodies and antibody fragments having an enhanced
antibody-dependent cellular cytotoxicity.
[0072] Clinical trials of unconjugated monoclonal antibodies (mAbs)
for the treatment of some types of cancer have recently yielded
encouraging results. Dillman, 1997, Cancer Biother. &
Radiopharm. 12:223-225; Deo et al., 1997, Immunology Today 18:127.
A chimeric, unconjugated IgG1 has been approved for low-grade or
follicular B-cell non-Hodgkin's lymphoma (Dillman, 1997, supra),
while another unconjugated mAb, a humanized IgG1 targeting solid
breast tumors, has also been showing promising results in phase III
clinical trials. Deo et al., 1997, supra. The antigens of these two
mAbs are highly expressed in their respective tumor cells and the
antibodies mediate potent tumor destruction by effector cells in
vitro and in vivo. In contrast, many other unconjugated mAbs with
fine tumor specificities cannot trigger effector functions of
sufficient potency to be clinically useful. Frost et al., 1997,
Cancer 80:317-333; Surfus et al., 1996, J. Immunother. 19:184-191.
For some of these weaker mAbs, adjunct cytokine therapy is
currently being tested. Addition of cytokines can stimulate
antibody-dependent cellular cytotoxicity (ADCC) by increasing the
activity and number of circulating lymphocytes. Frost et al., 1997,
supra; Surfus et al., 1996, supra. ADCC, a lytic attack on
antibody-targeted cells, is triggered upon binding of lymphocyte
receptors to the constant region (Fc) of antibodies. Deo et al.,
1997, supra.
[0073] A different, but complementary, approach to increase ADCC
activity of unconjugated IgG1s would be to engineer the Fc region
of the antibody to increase its affinity for the lymphocyte
receptors (Fc.gamma.Rs). Protein engineering studies have shown
that Fc.gamma.Rs interact with the lower hinge region of the IgG
CH2 domain. Lund et al., 1996, J. Immunol. 157:4963-4969. However,
Fc.gamma.R binding also requires the presence of oligosaccharides
covalently attached at the conserved Asn 297 in the CH2 region.
Lund et al., 1996, supra; Wright and Morrison, 1997, Tibtech
15:26-31, suggesting that either oligosaccharide and polypeptide
both directly contribute to the interaction site or that the
oligosaccharide is required to maintain an active CH2 polypeptide
conformation. Modification of the oligosaccharide structure can
therefore be explored as a means to increase the affinity of the
interaction.
[0074] An IgG molecule carries two N-linked oligosaccharides in its
Fc region, one on each heavy chain. As any glycoprotein, an
antibody is produced as a population of glycoforms which share the
same polypeptide backbone but have different oligosaccharides
attached to the glycosylation sites. The oligosaccharides normally
found in the Fc region of serum IgG are of complex bi-antennary
type (Wormald et al., 1997, Biochemistry 36:130-1380), with low
level of terminal sialic acid and bisecting N-acetylglucosamine
(GlcNAc), and a variable degree of terminal galactosylation and
core fucosylation (FIG. 1). Some studies suggest that the minimal
carbohydrate structure required for Fc.gamma.R binding lies within
the oligosaccharide core. Lund et al., 1996, supra. The removal of
terminal galactoses results in approximately a two-fold reduction
in ADCC activity, indicating a role for these residues in
Fc.gamma.R receptor binding. Lund et al., 1996, supra.
[0075] The mouse- or hamster-derived cell lines used in industry
and academia for production of unconjugated therapeutic mAbs
normally attach the required oligosaccharide determinants to Fc
sites. IgGs expressed in these cell lines lack, however, the
bisecting GlcNAc found in low amounts in serum IgGs. Lifely et al.,
1995, Glycobiology 318:813-822. In contrast, it was recently
observed that a rat myeloma-produced, humanized IgGl (CAMPATH-1H)
carried a bisecting GlcNAc in some of its glycoforms. Lifely et
al., 1995, supra. The rat cell-derived antibody reached a similar
in vitro ADCC activity as CAMPATH- 1H antibodies produced in
standard cell lines, but at significantly lower antibody
concentrations.
[0076] The CAMPATH antigen is normally present at high levels on
lymphoma cells, and this chimeric mAb has high ADCC activity in the
absence of a bisecting GlcNAc. Lifely et al, 1995, supra. Even
though in the study of Lifely et al., 1995, supra. the maximal in
vitro ADCC activity was not increased by altering the glycosylation
pattern, the fact that this level of activity was obtained at
relatively low antibody concentrations for the antibody carrying
bisected oligosaccharides suggests an important role for bisected
oligosaccharides. An approach was developed to increase the ADCC
activity of IgG1s with low basal activity levels by producing
glycoforms of these antibodies carrying bisected oligosaccharides
in the Fc region.
[0077] In the N-linked glycosylation pathway, a bisecting GlcNAc is
added by the enzyme .beta.(1,4)-N-acetylglucosaminyltransferase III
(GnT III). Schachter, 1986, Biochem. Cell Biol. 64:163-181. Lifely
et al., 1995, supra, obtained different glycosylation patterns of
the same antibody by producing the antibody in different cell lines
with different but non-engineered glycosylation machineries,
including a rat myeloma cell line that expressed GnT III at an
endogenous, constant level. In contrast, we used a single
antibody-producing CHO cell line, that was previously engineered to
express, in an externally-regulated fashion, different levels of a
cloned GnT III gene. This approach allowed us to establish for the
first time a rigorous correlation between expression of GnT III and
the ADCC activity of the modified antibody.
[0078] As demonstrated herein, see, Example 4, infra, C2B8 antibody
modified according to the disclosed method had an about
sixteen-fold higher ADCC activity than the standard, unmodified
C2B8 antibody produced under identical cell culture and
purification conditions. Briefly, a C2B8 antibody sample expressed
in CHO-tTA-C2B8 cells that do not have GnT III expression showed a
cytotoxic activity of about 31% (at 1 .mu.g/ml antibody
concentration), measured as in vitro lysis of SB cells (CD20+) by
human lymphocytes. In contrast, C2B8 antibody derived from a CHO
cell culture expressing GnT III at a basal, largely repressed level
showed at 1 .mu.g/ml antibody concentration a 33% increase in ADCC
activity against the control at the same antibody concentration.
Moreover, increasing the expression of GnT III produced a large
increase of almost 80% in the maximal ADCC activity (at 1 .mu.g/ml
antibody concentration) compared to the control at the same
antibody concentration. See, Example 4, infra.
[0079] Further antibodies of the invention having an enhanced
antibody-dependent cellular cytotoxicity include, but are not
limited to, anti-human neuroblastoma monoclonal antibody (chCE7)
produced by the methods of the invention, a chimeric anti-human
renal cell carcinoma monoclonal antibody (ch-G250) produced by the
methods of the invention, a humanized anti-HER2 monoclonal antibody
produced by the methods of the invention, a chimeric anti-human
colon, lung, and breast carcinoma monoclonal antibody (ING-1)
produced by the methods of the invention, a humanized anti-human
17-1A antigen monoclonal antibody (3622W94) produced by the methods
of the invention, a humanized anti-human colorectal tumor antibody
(A33) produced by the methods of the invention, an anti-human
melanoma antibody (R24) directed against GD3 ganglioside produced
by the methods of the invention, and a chimeric anti-human
squamous-cell carcinoma monoclonal antibody (SF-25) produced by the
methods of the invention. In addition, the invention is directed to
antibody fragment and fusion proteins comprising a region that is
equivalent to the Fc region of immunoglobulins. See, infra.
[0080] 2. Generation and Use of Fusion Proteins Comprising a Region
Equivalent to an Fc Region of an Immunoglobulin that Promote
Fc-Mediated Cytotoxicity
[0081] As discussed above, the present invention relates to a
method for enhancing the ADCC activity of therapeutic antibodies.
This is achieved by engineering the glycosylation pattern of the Fc
region of such antibodies, in particular by maximizing the
proportion of antibody molecules carrying bisected complex
oligosaccharides N-linked to the conserved glycosylation sites in
their Fc regions. This strategy can be applied to enhance
Fc-mediated cellular cytotoxicity against undesirable cells
mediated by any molecule carrying a region that is an equivalent to
the Fc region of an immunoglobulin, not only by therapeutic
antibodies, since the changes introduced by the engineering of
glycosylation affect only the Fc region and therefore its
interactions with the Fc receptors on the surface of effector cells
involved in the ADCC mechanism. Fc-containing molecules to which
the presently disclosed methods can be applied include, but are not
limited to, (a) soluble fusion proteins made of a targeting protein
domain fused to the N-terminus of an Fc-region (Chamov and
Ashkenazi, 1996, TIBTECH 14: 52) and (b) plasma membrane-anchored
fusion proteins made of a type II transmembrane domain that
localizes to the plasma membrane fused to the N-terminus of an Fc
region (Stabila, P. F., 1998, Nature Biotech. 16: 1357).
[0082] In the case of soluble fusion proteins (a) the targeting
domain directs binding of the fusion protein to undesirable cells
such as cancer cells, i.e., in an analogous fashion to therapeutic
antibodies. The application of presently disclosed method to
enhance the Fc-mediated cellular cytotoxic activity mediated by
these molecules would therefore be identical to the method applied
to therapeutic antibodies. See, Example 2 of U.S. Provisional
Application Ser. No. 60/082,581, incorporated herein by
reference.
[0083] In the case of membrane-anchored fusion proteins (b) the
undesirable cells in the body have to express the gene encoding the
fusion protein. This can be achieved either by gene therapy
approaches, i.e., by transfecting the cells in vivo with a plasmid
or viral vector that directs expression of the fusion
protein-encoding gene to undesirable cells, or by implantation in
the body of cells genetically engineered to express the fusion
protein on their surface. The later cells would normally be
implanted in the body inside a polymer capsule (encapsulated cell
therapy) where they cannot be destroyed by an Fc-mediated cellular
cytotoxicity mechanism. However should the capsule device fail and
the escaping cells become undesirable, then they can be eliminated
by Fc-mediated cellular cytotoxicity. Stabila et al., 1998, Nature
Biotech. 16:1357. In this case, the presently disclosed method
would be applied either by incorporating into the gene therapy
vector an additional gene expression cassette directing adequate or
optimal expression levels of GnT III or by engineering the cells to
be implanted to express adequate or optimal levels of GnT III. In
both cases, the aim of the disclosed method is to increase or
maximize the proportion of surface-displayed Fc regions carrying
bisected complex oligosaccharides.
[0084] The examples below explain the invention in more detail. The
following preparations and examples are given to enable those
skilled in the art to more clearly understand and to practice the
present invention. The present invention, however, is not limited
in scope by the exemplified embodiments, which are intended as
illustrations of single aspects of the invention only, and methods
which are functionally equivalent are within the scope of the
invention. Indeed, various modifications of the invention in
addition to those described herein will become apparent to those
skilled in the art from the foregoing description and accompanying
drawings. Such modifications are intended to fall within the scope
of the appended claims.
VII. EXAMPLES
A. Example 1
Tetracycline-Regulated Overexpression of Glycosyl Transferases in
Chinese Hamster Ovary Cells
[0085] To establish a cell line in which the expression of GnT III
could be extemally-controlled, a tetracycline-regulated expression
system was used. Gossen, M. and Bujard, H., 1992, Proc. Nat. Acad.
Sci. USA, 89: 5547-5551. The amount of GnT III in these cells could
be controlled simply by manipulating the concentration of
tetracycline in the culture medium. Using this system, it was found
that overexpression of GnT III to high levels led to growth
inhibition and was toxic to the cells. Another CHO cell line with
tetracycline-regulated overexpression of GnT V, a distinct
glycoprotein-modifying glycosyl transferase, showed the same
inhibitory effect, indicating that this may be a general feature of
glycoprotein-modifying glycosyl transferase overexpression. This
phenomenon has not been reported previously, probably due to the
fact that inventigators generally have used constitutive promoters
for related experiments. The growth effect sets an upper limit to
the level of glycoprotein-modifying glycosyl transferase
overexpression, and may thereby also limit the maximum extent of
modification of poorly accessible glycosylation sites.
[0086] 1. Materials And Methods
[0087] Establishment of CHO Cells with Tetracycline-Regulated
Expression of Glycosyltransferases. In a first step, an
intermediate CHO cell line (CHO-tTA) was first generated that
constitutively expresses a tetracycline-controlled transactivator
(tTA) at a level for the adequate for the regulation system. Using
Lipofectamine reagent (Gibco, Eggenfelden, Germany), CHO (DUKX)
cells were co-transfected, with pUHD15-1, a vector for constitutive
expression of the tTA gene (Gossen and Bujard, 1992, Proc. Nat.
Acad Sci. USA, 89: 5547-5551), and pSV2Neo, a vector for
constitutive expression of a neomycin resistance gene (Clontech,
Palo Alto, Calif.). Stable, drug-resistant clones were selected and
screened for adequate levels of tTA expression via transient
transfections with a tetracycline-regulated .beta.-galactosidase
expression vector, pUHG16-3. C-myc epitope-encoding DNA was added
to the 3' end of the rat GnT III cDNA (Nishikawa et al., 1992, J.
Biol. Chem. 267:18199-18204) by PCR amplification. Nilsson et al,
1993, J. Cell Biol. 120:5-13. The product was sequenced and
subcloned into pUHD10-3, a vector for tetracycline-regulated
expression (Gossen and Bujard, supra) to generate the vector
pUHD10-3-GnTIIIm. The human GnT V cDNA (Saito et al., 1995, Eur. J.
Biochem. 233:18-26), was directly subcloned into pUHD10-3 to
generate plasmid vector pUHD10-3-GnT V. CHO-tTA cells were
co-transfected using a calcium phosphate transfection method
(Jordan and Wurm, 1996, Nucleic Acids Res. 24:596-601), with pPur,
a vector for constitutive expression of puromycin resistance
(Clontech, Palo Alto, Calif.), and either the vector pUHD10-3-GnT
IIIm or the vector pUHD10-3-GnT V. Puromycin resistant clones were
selected in the presence of tetracycline, isolated and then
analyzed for tetracycline-regulated expression of GnT III or GnT V
via western blots analysis. See, infra.
[0088] Western and Lectin Blotting. For Western blot analysis of
GnT III or GnT V, cell lysates were separated by SDS-PAGE and
electroblotted to PVDF membranes (Millipore, Bedford, Mass.). GnT
III was detected using the anti-c-myc monoclonal antibody 9E10
(Nilsson et al., 1993, J. Cell Biol. 120:5-13) and GnT V using with
an anti-GnT V rabbit polyclonal antibody (Chen et al., 1995,
Glycoconjugate J. 12:813-823). Anti-mouse or anti-rabbit IgG-horse
radish peroxidase (Amersham, Arlington, Ill.) was used as secondary
antibody. Bound secondary antibody was detected using an enhanced
chemiluminescence kit (ECL kit, Amersham, Arlington, Ill.)
[0089] For lectin blot analysis of glycoproteins modified either by
GnT III- or GnT V-catalyzed reactions, biotinylated E-PHA (Oxford
Glycosciences, Oxford, United Kingdom) or L-PHA-digoxigenin
(Boehringer Mannheim, Mannheim, Germany), respectively, were used.
Merkle and Cummings, 1987, Methods Enzymol. 138:232-259.
[0090] 2. Results and Discussion
[0091] Establishment of CHO Cell Lines with Tetracycline-Regulated
Overexpression of Glycosyl Transferases. The strategy used for
establishment of glycosyl transferase overexpressing cell lines
consisted of first generating an intermediate CHO cell line
constitutively expressing the tetracycline-controlled
transactivator (tTA) at an adequate level for the system to work.
Yin et al., 1996, Anal. Biochem. 235:195-201. This level had to be
high enough. to activate high levels of transcription, in the
absence of tetracycline, from the minimal promoter upstream of the
glycosyl transferase genes. CHO cells were co-transfected with a
vector for constitutive expression for tTA, driven by the human
cytomegalovirus (hCMV) promoter/enhancer, and a vector for
expression of a neomycin-resistance (Neo.sup.R) gene. An excess of
the tTA-expression vector was used and neomycin-resistant clones
were isolated.
[0092] In mammalian cells, co-transfected DNA integrates adjacently
at random locations within the chromosomes, and expression depends
to a large extent on the site of integration and also on the number
of copies of intact expression cassettes. A mixed population of
clones with different expression levels of the transfected genes is
generated. Yin et al., 1996, supra. Selection for neomycin
resistance merely selects for integration of an intact Neo.sup.R
expression cassette, while the use of an excess of the
tTA-expression vector increases the probability of finding clones
with good expression of tTA. The mixed population of clones has to
be screened using a functional assay for tTA expression. Gossen and
Bujard, 1992, supra; Yin et al., 1996, supra. This was done by
transfection of each clone with a second vector harboring a
reporter gene, lacZ, under the control of the tet-promoter and
screening for tetracycline-regulated (tet-regulated), transient
expression (i.e., one to three days after transfection) of
.beta.-galactosidase activity. CHOt17, which showed the highest
level of tet-regulated .beta.-galactosidase activity among twenty
screened clones, was selected for further work.
[0093] CHOt17 cells were tested for tet-regulated expression of GnT
III by transfecting the cells with vector pUHDlO-3-GnT IIIm and
comparing the relative levels of GnT III after incubation of the
cells in the presence and absence of tetracycline for 36 h. GnT III
levels were compared by western blot analysis, using a monoclonal
antibody (9E 10) which recognizes the c-myc peptide epitope tag at
the carboxy-terminus of GnT III. The tag had been introduced
through a modification of the glycosyl transferase gene using PCR
amplification. Various reports have demonstrated addition of
peptide epitope tags to the carboxy-termini of glycosyl
transferases, a group of enzymes sharing the same topology, without
disruption of localization or activity. Nilsson et al., 1993,
supra; Rabouille et al., 1995, J. Cell Science 108:1617-1627. FIG.
2 shows that in clone CHOt17 GnT III accumulation is significantly
higher in the absence than in the presence of tetracycline. An
additional clone, CHOt2, which gave weaker activation of
transcription in the b-galactosidase activity assay, was tested in
parallel (FIG. 2). GnT III and .beta.-galactosidase expression
levels follow the same pattern of tetracycline-regulation for both
of these clones. The range of tetracycline concentrations where GnT
III expression can be quantitatively controlled was found to be
from 0 to 100 ng/ml (FIG. 3). This result agrees with previous
research using different cell lines and genes (Yin et al., 1996,
supra).
[0094] To generate a stable cell line with tet-regulated expression
of GnT III, CHOt17 cells were co-transfected with vector
pUHDlO-3-GnT IIIm and vector, pPUR, for expression of a puromycin
resistance gene. In parallel, CHOt17 cells were co-transfected with
pUHDlO-3-GnT V and pPUR vectors to generate an analogous cell line
for this other glycosyl transferase. A highly efficient calcium
phosphate transfection method was used and the DNA was linearized
at unique restriction sites outside the eucaryotic expression
cassettes, to decrease the probability of disrupting these upon
integration. By using a host in which the levels of tTA expressed
had first been proven to be adequate, the probability of finding
clones with high expression of the glycosyl transferases in the
absence of tetracycline is increased.
[0095] Stable integrants were selected by puromycin resistance,
keeping tetracycline in the medium throughout clone selection to
maintain glycosyl transferase expression at basal levels. For each
glycosyl transferase, sixteen puromycin resistant clones were grown
in the presence and absence of tetracycline, and eight of each were
analysed by western blot analysis (FIG. 4). The majority of the
clones showed good regulation of glycosyl transferase expression.
One of the GnT III-expressing clones showed a relatively high basal
level in the presence of tetracycline (FIG. 4B, clone 3), which
suggests integration of the expression cassete close to an
endogenous CHO-cell enhancer; while two puromycin-resistant clones
showed no expression of GnT III in the absence of tetracycline
(FIG. 4B, clones 6 and 8). Among the clones showing good regulation
of expression, different maximal levels of glycosyl transferase
were observed. This may be due to variations in the site of
integration or number of copies integrated. Activity of the
glycosyl transferases was verified by E-PHA and L-PHA lectin
binding to endogenous cellular glycoproteins derived from various
clones grown in the presence and absence of tetracycline (FIG. 5).
Lectins are proteins which bind to specific oligosaccharide
structures. E-PHA lectin binds to bisected oligosaccharides, the
products of GnT III-catalyzed reactions, and L-PHA binds to tri-
and tetra-antennary oligosaccharides produced by GnT V-catalyzed
reactions (Merkle and Cummings, 1987, Methods Enzymol.
138:232-259). For each glycosyl transferase, a clone with high
expression in the absence, but with undetectable expression in the
presence, of tetracycline (clone 6, FIG. 4A, CHO-tet-GnT V, and
clone 4, FIG. 4B, CHO-tet-GnT IlIm) was selected for further
work.
B. Example 2
Inhibition of Cell Growth Effected by Glycosyl Transferase
Overexpresseion
[0096] During screening of GnT III- and GnT V-expressing clones in
the absence of tetracycline, see, Example 1, supra, approximately
half of each set of clones showed a strong inhibition of growth.
The extent of growth-inhibition varied among clones, and comparison
with expression levels estimated from western blot analysis (FIG.
4) suggested a correlation between the degree of growth-inhibition
and glycosyl transferase overexpression. This correlation was
firmly established by growing the final clones, CHO-tet-GnT IIIm
and CHO-tet-GnT V, in different concentrations of tetracycline. A
strong inhibition of growth was evident after two days of culture
at low levels of tetracycline (FIG. 6). Growth-inhibited cells
displayed a small, rounded morphology instead of the typical
extended shape of adherent CHO cells. After a few days, significant
cell death was apparent from the morphology of the growth-inhibited
cells.
[0097] Growth-inhibition due to glycosyl transferase overexpression
has not hitherto been reported in the literature, probably due to
the widespread use of constitutive promoters. Those clones giving
constitutive expression of a glycosyl transferase at
growth-inhibiting levels, would be lost during the selection
procedure. This was avoided here by keeping tetracycline in the
medium, i.e., basal expression levels, throughout selection. Prior
to selection, the frequency of clones capable of expressing
glycosyl transferases to growth-inhibiting levels using traditional
mammalian vectors based on the constitutive hCMV promoter/enhancer
would be expected to be lower. This is due to the fact that, for
any given gene, the pUHDlO-3 vector in CHO cell lines selected for
high constitutive levels of tTA, gives significantly higher
expression levels than constitutive hCMV promoter/enhancer-based
vectors, as observed by others. Yin et al., 1996, supra.
[0098] Inhibition of cell growth could be due to a direct effect of
overexpression of membrane-anchored, Golgi-resident glycosyl
transferases independent of their in vivo catalytic activity, e.g.,
via misfolding in the endoplasmic reticulum (ER) causing saturation
of elements which assist protein folding in the ER. This could
possibly affect the folding and secretion of other essential
cellular proteins. Alternatively, inhibition of growth could be
related to increased in vivo activity of the glycosyl transferase
leading to a change of the glycosylation pattern, in a
function-disrupting fashion, of a set of endogenous glycoproteins
necessary for growth under standard in vitro culture
conditions.
[0099] Independent of the underlying mechanism, the
growth-inhibition effect has two consequences for engineering the
glycosylation of animal cells. First, it implies that
cotransfection of constitutive glycosyl transferase expression
vectors together with vectors for the target glycoprotein product
is a poor strategy. Other ways of linking expression of these two
classes of proteins, e.g., through the use of multiple constitutive
promoters of similar strength or use of multicistronic,
constitutive expression vectors, should also be avoided. In these
cases, clones with very high, constitutive expression of the target
glycoprotein, a pre-requisite for an economical bioprocess, would
also have high expression of the glycosyl transferase and would be
eliminated during the selection process. Linked, inducible
expression could also be problematic for industrial bioprocesses,
since the viability of the growth-arrested cells would be
compromised by the overexpression of the glycosyl transferase.
[0100] The second consequence is that it imposes an upper limit on
glycosyl transferase overexpression for glycosylation engineering
approaches. Clearly, the conversions of many glycosyl
transferase-catalyzed reactions in the cell, at the endogenous
levels of glycosyl transferases, are very high for several
glycosylation sites. However, glycosylation sites where the
oligosaccharides are somewhat inaccesible or are stabilized in
unfavorable conformations for specific glycosyl tranferases also
exist. For example, it has been observed that addition of bisecting
GlcNAc is more restricted to the oligosaccharides attached to the
Fc region than to those located on the variable regions of human
IgG antibodies. Savvidou et al., 1984, Biochemistry 23:3736-3740.
Glycosylation engineering of these restricted sites could be
affected by such a limit on glycosyl transferase expression.
Although this would imply aiming for an "unnatural" distribution of
glycoforms, these could be of benefit for special therapeutic
applications of glycoproteins.
C. Example 3
Engineering the Glycosylation of an Anti-Human Neuroblastoma
Antibody in Chinese Hamster Ovary Cells
[0101] In order to validatelthe concept of engineering a
therapeutic antibody by modifying its glycosylation pattern, a
chimeric anti-human neuroblastoma IgG1 (chCE7) was chosen which has
insignificant ADCC activity when produced by SP2/0 recombinant
mouse myeloma cells. ChCE7 recognizes a tumor-associated 190-kDa
membrane glycoprotein and reacts strongly with all neuroblastoma
tumors tested to date. It has a high affinity for its antigen
(K.sub.d of 10.sup.10M.sup.-1) and, because of its high
tumor-specificity, it is routinely used as a diagnostic tool in
clinical pathology. Amstutz et al., 1993, Int. J. Cancer
53:147-152. In recent studies, radiolabelled chCE7 has shown good
tumor localization in human patients. Durr, 1993, Eur. J. Nucl.
Med. 20:858. The glycosylation pattern of chCE7, an
anti-neuroblastoma therapeutic monoclonal antibody (mAb) was
engineered in CHO cells with tetracycline-regulated expression of
GnT III. A set of mAb samples differing in their glycoform
distribution was produced by controlling GnT III expression in a
range between basal and toxic levels, and their glycosylation
profiles were analyzed by MALDI/TOF-MS of neutral oligosaccharides.
Measurement of the ADCC activity of these samples showed an optimal
range of GnT III expression for maximal chCE7 in vitro biological
activity, and this activity correlated with the level of
Fc-associated bisected, complex oligosaccharides.
[0102] 1. Materials And Methods
[0103] Construction of chCE7 Expression Vectors. Plasmid vectors
10CE7VH and 98CE7VL, for expression of heavy (IgG1) and light
(kappa) chains, respectively, of anti-human neuroblastoma chimeric
antibody chCE7, which contain chimeric genomic DNA including the
mouse immunoglobulin promoter/enhancer, mouse antibody variable
regions, and human antibody constant regions (Amstutz et al., 1993,
Int. J. Cancer 53:147-152) were used as starting materials for the
construction of the final expression vectors, pchCE7H and pchCE7L.
Chimeric heavy and light chain chCE7 genes were reassambled and
subcloned into the pcDNA3.1(+) vector. During reassembly, all
introns were removed, the leader sequences were replaced with
synthetic ones, Reffet al., 1994, Blood 83:435-445, and unique
restriction sites joining the variable and constant region
sequences were introduced. Introns from the heavy constant region
were removed by splicing with overlap-extension-PCR. Clackson et
al., 1991, General Applications of PCR to Gene Cloning and
Manipulation, p. 187-214, in: McPherson et al. (ed.), PCR a
Practical Approach, Oxford University Press, Oxford.
[0104] Production of chCE7 in CHO Cells Expressing Different Levels
of GnT III. CHO-tet-GnT IIIm (see, supra) cells were co-transfected
with vectors pchCE7H, pchCE7L, and pZeoSV2 (for Zeocin resistance,
Invitrogen, Groningen, The Netherlands) using a calcium phosphate
transfection method. Zeocin resistant clones were transferred to a
96-well cell culture plate and assayed for chimeric antibody
expression using an ELISA assay specific for human IgG constant
region. Lifely et al, 1995, supra. Four chCE7 antibody samples were
derived from parallel cultures of a selected clone (CHO-tet-GnT
IIIm-chCE7), grown in FMX-8 cell culture medium supplemented with
10% FCS; each culture containing a different level of tetracycline
and therefore expressing GnT III at different levels. CHO-tet-GnT
IIIm-chCE7 cells were expanded and preadapted to a different
concentration of tetracycline during 7 days. The levels of
tetracycline were 2000, 60, 30, and 15 ng/ml.
[0105] Purification of chCE7Antibody Samples. Antibody was purified
from culture medium by Protein A affinity chromatography on a 1 ml
HiTrap Protein A column (Pharmacia Biotech, Uppsala, Sweden), using
linear pH gradient elution from 20 mM sodium phosphate, 20 mM
sodium citrate, 500 mM sodium chloride, 0.01% Tween 20, 1M urea, pH
7.5 (buffer A) to buffer B (buffer A without sodium phosphate, pH
2.5). Affinity purified chCE7 samples were buffer exchanged to PBS
on a 1 ml ResourceS cation exchange column (Pharmacia Biotech,
Uppsala, Sweden). Final purity was judged to be higher than 95%
from SDS-PAGE and Coomasie-Blue staining. The concentration of each
sample was estimated from the absorbance at 280 nm.
[0106] Binding of Antibodies to Neuroblastoma Cells. Binding
affinity to human neuroblastoma cells was estimated from
displacement of .sup.125I-labeled chCE7 by the CHO-produced
samples. Amstutz et al, 1993, supra.
[0107] Oligosaccharide Analysis by MALDI/TOF-MS. CE7-2000t, -60t,
-30t, and -15t samples were treated with A. urefaciens sialidase
(Oxford Glycosciences, Oxford, United Kingdom), following the
manufacturer's instructions, to remove any sialic acid
monosaccharide residues. The sialidase digests were then treated
with peptide N-glycosidase F (PNGaseF, Oxford Glycosciences,
Oxford, United Kingdom), following the manufacturer's instructions,
to release the N-linked oligosaccharides. Protein, detergents, and
salts were removed by passing the digests through microcolumns
containing, from top to bottom, 20 ml of SepPak C18 reverse phase
matrix (Waters, Milford, Mass.), 20 ml of Dowex AG 50W X8 cation
exchange matrix (BioRad, Hercules, Calif.), and 20 ml of AG 4X4
anion exchange matrix (BioRad, Hercules, Calif.). The microcolumns
were made by packing the matrices in a Gel Loader tip (Eppendorf,
Basel, Switzerland) filled with ethanol, followed by an
equilibration with water. Kuster et al., 1997, Anal. Biochem.
250:82-101. Flow through liquid and a 300 ml-water wash were
pooled, filtered, evaporated to dryness at room temperature, and
resuspended in 2 ml of deionized water. One microliter was applied
to a MALDI-MS sample plate (Perseptive Biosystems, Farmingham,
Mass.) and mixed with 1 ml of a 10 mg/ml dehydrobenzoic acid (DHB,
Aldrich, Milwakee, Wis.) solution in acetonitrile. The samples were
air dried and the resulting crystals were dissolved in 0.2 ml of
ethanol and allowed to recrystallize by air drying. Harvey, 1993,
Rapid Mass. Spectrom. 7:614-619. The oligosaccharide samples were
then analyzed by matrix-assisted laser desorption
ionization/time-of-flight-mass spectrometry (MALDI/TOF-MS) using an
Elite Voyager 400 spectrometer (Perseptive Biosystems, Farmingham,
Mass.), equipped with a delayed ion extraction MALDI-ion source, in
positive ion and reflector modes, with an accelaration voltage of
20 kV. One hundred and twenty eight scans were averaged. Bisected
biantennary complex oligosaccharide structures were assigned to
five-HexNAc-associated peaks. Non-bisected tri-antennary N-linked
oligosaccharides, the alternative five HexNAc-containing isomers,
have never been found in the Fc region of IgGs and their syntheses
are catalyzed by glycosyltransferases discrete from GnT III.
[0108] ADCC Activity Assay. Lysis of IMR-32 human neuroblastoma
cells (target) by human lymphocytes (effector), at a
target:effector ratio of 1:19, during a 16 h incubation at
37.degree. C. in the presence of different concentrations of chCE7
samples, was measured via retention of a fluorescent dye. Kolber et
al, 1988, J. Immunol. Methods 108: 255-264. IMR-32 cells were
labeled with the fluorescent dye Calcein AM for 20 min (final
concentration 3.3 .mu.M). The labeled cells (80,000 cells/well)
were incubated for 1 h with different concentrations of CE7
antibody. Then, monocyte depleted mononuclear cells were added
(1,500,000 cells/well) and the cell mixture was incubated for 16 h
at 37.degree. C. in a 5% CO.sub.2 atmosphere. The supernatant was
discarded and the cells were washed once with HBSS and lysed in
Triton X-100 (0.1%). Retention of the fluorescent dye in IMR-32
cells was measured with a fluorometer (Perkin Elmer, Luminscence
Spectrometer LS 50B, (Foster City, Calif.) and specific lysis was
calculated relative to a total lysis control, resulting from
exposure of the target to a detergent instead of exposure to
antibody. The signal in the absence of antibody was set to 0%
cytotoxicity. Each antibody concentration was analyzed by
triplicate, and the assay was repeated three separate times.
[0109] 2. Results and Discussion
[0110] Production of chCE7in CHO Cells Expressing Different Levels
of GnTIII. ChCE7 heavy and light chain expression vectors were
constructed incorporating the human cytomegalovirus (hCMV)
promoter, the bovine growth hormone termination and polyadenylation
sequences, and eliminating all heavy and light chain introns. This
vector design was based on reports of reproducible high-level
expression of recombinant IgG genes in CHO cells. Reff et al.,
1994, supra; Trill et al., 1995, Current Opinion Biotechnol.
6:553-560. In addition, a unique restriction sites was introduced
in each chain, at the junction between the variable and constant
regions. These sites conserve the reading frame and do not change
the amino acid sequence. They should enable simple exchange of the
mouse variable regions, for the production of other mouse-human
chimeric antibodies. Reff et al., 1994, supra. DNA sequencing
confirmed that the desired genes were appropriately assembled, and
production of the chimeric antibody in transfected CHO cells was
verified with a human Fc-ELISA assay.
[0111] CHO-tet-GnT IIIm-chCE7 cells, with stable,
tetracycline-regulated expression of GnT III and stable,
constitutive expression of chCE7, were established and scaled-up
for production of a set of chCE7 samples. During scale-up, four
parallel cultures derived from the same CHO clone were grown, each
at a different level of tetracycline and therefore only differing
in the level of expression of the GnT III gene. This procedure
eliminates any clonal effects from other variables affecting
N-linked glycoform biosynthesis, permitting a rigorous correlation
to be established between GnT III gene expression and biological
activity of the glycosylated antibody. The tetracycline
concentration ranged from 2000 ng/ml, i.e., the basal level of GnT
III expression, to 15 ng/ml, at which significant growth inhibition
and toxicity due to glycosyl transferase overexpression was
observed (see, supra). Indeed, only a small amount of antibody
could be recovered from the latter culture. The second highest
level of GnT III expression, using tetracycline at a concentration
of 30 ng/ml, produced only a mild inhibition of growth. The
purified antibody yield from this culture was approximately 70%
that from the remaining two lower levels of GnT III gene
overexpression.
[0112] The four antibody samples, CE7-2000t, -60t, -30t, and -15t,
numbers denoting the associated concentration of tetracycline, were
purified by affinity chromatography on Protein A and buffer
exchanged to PBS using a cation exchange column. Purity was higher
than 95% as judged from SDS-PAGE with Coomassie Blue staining.
Binding assays to human neuroblastoma cells revealed high affinity
to the cells and no significant differences in antigen binding
among the different samples (estimated equilibriurn dissociation
constants varied between 2.0 and 2.7.times.10.sup.-10 M). This was
as expected, since there are no potential N-linked glycosylation
sites in the CE7 variable regions.
[0113] Oligosaccharide Distributions and Levels of Bisected Complex
Oligosaccharides of Different chCE7Samples. Oligosaccharide
profiles were obtained by matrix-assisted laser
desorption/ionization mass spectrometry on a time-of-flight
instrument (MALDI/TOF-MS). Mixtures of neutral N-linked
oligosaccharides derived from each of the four CHO-produced
antibody samples and from a SP2/0 mouse myeloma-derived chCE7
(CE7-SP2/0) sample were analyzed using 2,5-dehydrobenzoic acid
(2,5-DHB) as the matrix (FIG. 9). Under these conditions, neutral
oligosaccharides appear essentially as single [M+Na.sup.+] ions,
which are sometimes accompanied by smaller [M+K.sup.+] ions,
depending on the potassium content of the matrix. Bergweff et al.,
1995, Glycoconjugate J. 12:318-330.
[0114] This type of analysis yields both the relative proportions
of neutral oligosaccharides of different mass, reflected by
relative peak height, and the isobaric monosaccharide composition
of each peak. Kuster et al., 1997, supra; Naven and Harvey, 1996,
Rapid Commun. Mass Spectrom. 10:1361 - 1366. Tentative structures
are assigned to peaks based on the monosaccharide composition,
knowledge of the biosynthetic pathway, and on previous structural
data for oligosaccharides derived from the same glycoprotein
produced by the same host, since the protein backbone and the cell
type can have a strong influence on the oligosaccharide
distribution. Field et al., 1996, Anal. Biochem. 239:92-98. In the
case of Fc-associated oligosaccharides, only bi-antennary complex
oligosaccharides have been detected in IgGs present in human serum
or produced by mammalian cell cultures under normal conditions.
Wormald et al., 1997, Biochemistry 36:1370-1380; Wright and
Morrison, 1997, Tibtech 15:26-31. The pathway leading to these
compounds is illustrated in FIG. 10, including the mass of the
[M+Na.sup.+] ion corresponding to each oligosaccharide. High
mannose oligosaccharides have also been detected on antibodies
produced in the stationary and death phases of batch cell cultures.
Yu Ip et al., 1994, Arch. Biochem. Biophys. 308:387-399.
[0115] The two major peaks in the CE7-SP2/0 sample (FIG. 9A)
correspond to masses of fucosylated oligosaccharides with four
N-acetylhexosamines (HexNAcs) containing either three (m/z 1486) or
four (m/z 1648) hexoses. See, FIG. 10, but note that the summarized
notation for oligosaccharides in this figure does not count the two
GlcNAcs of the core. This composition is consistent with core
fucosylated, bi-antannry complex oligosaccharide structures
carrying zero or one galactose residues, respectively, typical of
Fc-associated oligosaccharides, and as previously observed in NMR
analysis of Fc oligosaccharides derived from a chimeric IgGI
expressed in SP2/0 cells. Bergweff et al., 1995, supra.
[0116] GnT III-catalyzed transfer of a bisecting GlcNAc to these
bi-antennary compounds, which are the preferred GnT III acceptors,
would lead to oligosaccharides with five HexNAcs (m/z 1689 and
1851, non- and mono-galactosylated, respectively, FIG. 10), which
are clearly absent in the CE7-SP2/0 sample. The latter peaks appear
when chCE7 is expressed in CHO-tet-GnTIIIm cells. In the
CHO-expressed antibodies the four HexNAc-containing peaks are also
mainly fucosylated, although a small amount of non-fucosylated
structures is evident from the peak at m/z 1339 (see, FIG. 10). The
level of galactosylation is also not very different between the
CHO- and SP2/0-derived material. At the basal level of GnT III
expression (CE7-2000t sample, FIG. 9B), the molecules with five
HexNAcs are present in a lower proportion than those with four
HexNAcs. A higher level of GnT III expression (CE7-60t sample, FIG.
9C) led to a reversal of the proportions in favor of
oligosaccharides with five HexNAcs. Based on this trend, bisected,
bi-antennary complex oligosaccharide structures can be assigned to
compounds with five HexNAcs in these samples. Tri-antennary
N-linked oligosaccharides, the alternative five HexNAc-containing
isomers, have never been found in the Fc region of IgGs and their
syntheses are catalyzed by GlcNAc-transferases discrete from GnT
III.
[0117] A further increase in GnT III expression (CE7-30t sample,
FIG. 9D) did not lead to any significant change in the levels of
bisected complex oligosaccharides. Another peak (m/z 1543)
containing five HexNAcs appears at low, but relatively constant
levels in the CHO-GnTIII samples and corresponds in mass to a
non-fucosylated, bisected-complex oligosaccharide mass (FIG. 10).
The smaller peaks at m/z 1705 and 1867, also correspond to five
HexNAc-containing bi-antennary complex oligosaccharides. They can
be assigned either to potassium adducts of the peaks at m/z 1689
and 1851 (mass difference of 16 Da with respect to sodium adducts)
(Kuster et al., 1997, supra) or to mono- and bi-galactosylated,
bisected complex oligosaccharides without fticose (FIG. 10).
Together, the bisected complex oligosaccharides amount to
approximately 25% of the total in sample CE7-2000t and reach
approximately 45 to 50% in samples CE7-60t and CE7-30t.
[0118] Additional Information From the Oligosaccharide Profiles of
chCE7 Samples. Although the levels of bisected complex
oligosaccharides were not higher in sample CE730t, increased
overexpression of GnT III did continue to reduce, albeit to a small
extent, the proportions of substrate bi-antennary complex
oligosaccharide substrates. This was accompanied by moderate
increases in two different, four HexNAc-containing peaks (m/z 1664
and 1810). The latter two peaks can correspond either to
galactosylated bi-antennary complex oligosaccharides or to bisected
hybrid compounds (FIG. 11). A combination of both classes of
structures is also possible. The relative increase in these peaks
is consistent with the accumulation of bisected hybrid by-products
of GnT III overexpression. Indeed, the sample produced at the
highest level of GnT III overexpression, CE7-15t, showed a large
increase in the peak at m/z 1664, a reduction in the peak at m/z
1810 and a concomitant reduction of complex bisected
oligosaccharides to a level of approximately 25%. See, peaks with
m/z 1689 and 1851 in FIG. 9E and the corresponding structures in
FIG. 11. Higher accumulation of non-fucosylated (m/z 1664) bisected
hybrid by-products, instead of fucosylated ones (m/z 1810), would
agree with the fact that oligosaccharides which are first modified
by GnT III can no longer be biosynthetic substrates for core
.alpha.1,6-fucosyltransferase. Schachter, 1986, Biochem. Cell Biol.
64:163-181.
[0119] The peak at m/z 1257 is present at a level of 10 -15% of the
total in the CHO-derived samples and at a lower level in CE7-SP2/0
(FIG. 9). It corresponds to five hexoses plus two HexNAcs. The only
known N-linked oligosaccharide structure with this composition is a
five mannose-containing compound of the high-mannose type. Another
high mannose oligosaccharide, a six mannose one (m/z 1420), is also
present at much lower levels. As mentioned above, such
oligosaccharides have been detected in the Fc of IgGs expressed in
the late phase of batch cell cultures. Yu Ip et al., 1994,
supra.
[0120] Antibody Dependent Cellular Cytotoxicity of chCE7Samples.
ChCE7 shows some ADCC activity, measured as in vitro lysis of
neuroblastoma cells by human lymphocytes, when expressed in
CHO-tet-GnTIIIm cells with the minimum level of GnT III
overexpression (FIG. 12, sample CE7-2000t). Raising the level of
GnT III produced a large increase in ADCC activity (FIG. 12, sample
CE7-60t). Further overexpression of GnT III was not accompanied by
an additional increase in activity (FIG. 12, sample CE7-30t), and
the highest level of expression actually led to reduced ADCC (FIG.
12, sample CE7-15t). Besides exhibiting the highest ADCC
activities, both CE7-60t and CE7-30t samples show significant
levels of cytotoxicity at very low antibody concentrations. These
results show that there is an optimal range of GnT III
overexpression in CHO cells for ADCC activity, and comparison with
oligosaccharide profiles shows that activity correlates with the
level of Fc-associated, bisected complex oligosaccharides.
[0121] Given the importance of bisected complex oligosaccharides
for ADCC activity, it would be useful to engineer the pathway to
further increase the proportion of these compounds. Overexpression
of GnT III to levels approaching that used for sample CE7-30t is
within the biotechnologically practical range where no significant
toxicity and growth inhibiton are observed. At this level of
expression, the non-galactosylated, non-bisected, bi-antennary
complex oligosaccharides, ie., the preferred, potential GnT III
substrates, are reduced to less than 10% of the total. See, In/z
1486 peak, FIG. 9D. However, only 50% are converted to the desired
bisected biantennary complex structures. The rest are either
diverted to bisected, hybrid oligosaccharide byproducts or consumed
by the competing enzyme .beta.1,4-galactosyltransferase, GalT (FIG.
11).
[0122] Resolution of the bisected hybrid and the non-bisected,
galactosylated complex oligosaccharide peaks by complementary
structural analyses would determine how much each potential,
undesired route is consuming. The growth of the m/z 1664 and 1810
peaks at high GnT III overexpression levels suggests that at least
a fraction of these peaks corresponds to bisected hybrid
oligosaccharides (FIG. 11). In theory, a flux going to bisected
hybrid compounds can be reduced by co-overexpression of enzymes
earlier in the pathway such as mannosidase II together with GnT
III. On the other hand, competition between GnT III and GalT for
bisected complex oligosaccharide substrates could potentially be
biased towards GnT III-catalyzed reactions, by increasing the
intra-Golgi concentration of UDP-GlcNAc while overexpressing GnT
III. GnT III transfers a GlcNAc from the co-substrate UDP-GlcNAc to
the different oligosaccharides. Should the intra-Golgi
concentration of UDP-GlcNAc co-substrate be sub-saturating for GnT
III, then increasing it, either by manipulation of the culture
medium composition or by genetic manipulation of sugar-nucleotide
transport into the Golgi, could favor GnT III in a competition for
oligosaccharides with GalT.
[0123] It remains to be determined whether the increase in ADCC
activity results from the increase in both the galactosylated and
non-galactosylated, bisected complex oligosaccharides, or only from
one of these forms. See, peaks at m/z 1689 and 1851 in FIG. 9. If
it is found that galactosylated, bisected complex bi-antennary
oligosaccharides are the optimal structures for increased ADCC
activity, then maximizing the fraction of these compounds on the Fc
region would require overexpression of both GnT III and GalT. Given
the competitive scenario discussed previously, the expression
levels of both genes would have to be carefully regulated. In
addition, it would be valuable to try to re-distribute
overexpressed GalT as much as possible towards the TGN instead of
the trans-Golgi cistema. The latter strategy may be realized by
exchanging the transmembrane region-encoding sequences of GalT with
those of .alpha.2,6-sialyltransferase (Chege and Pfeffer, 1990, J.
Cell Biol. 11 1:893-899).
D. Example 4
Engineering the Glycosylation of the Anti-CD20 Monoclonal Antibody
C2B8
[0124] C2B8 is an anti-human CD20 chimeric antibody, Reff, M.E. et
al, 1994, supra. It recieved FDA approval in 1997 and is currently
being used, under the commercial name of Rituxan.TM., for the
treatment of Non-Hodgkin's lymphoma in the United States. It is
derived from CHO cell culture and therefore should not carry
bisected oligosaccharides. See, supra. In order to produce an
improved version of this antibody, the method demonstrated
previously for the chCE7 anti-neuroblastoma antibody was applied.
See, supra. C2B8 antibody modified according to the disclosed
method had a higher ADCC activity than the standard, unmodified
C2B8 antibody produced under identical cell culture and
purification conditions.
[0125] 1. Material And Methods
[0126] Synthesis of the Variable Light and Variable Heavy Chain
Regions of Chimeric Anti-CD20 Monoclonal Antibody (C2B8). The VH
and VL genes of the C2B8 antibody were assembled synthetically
using a set of overlapping single-stranded oligonucleotides
(primers) in a one-step process using PCR, Kobayashi et al, 1997,
Biotechniques 23: 500-503. The sequence data coding for mouse
immunoglobulin light and heavy chain variable regions (VL and VH
respectively) of the anti-CD20 antibody were obtained from a
published international patent application (International
Publication Number: WO 94/11026). The assembled DNA fragments were
subcloned into pBluescriptIIKS(+) and sequenced by DNA cycle
sequencing to verify that no mutations had been introduced.
[0127] Contruction of Vectors for Expression of Chimeric Anti-CD20
Monoclonal Antibody (C2B8). VH and VL coding regions of the C2B8
monoclonal antibody were subcloned in pchCE7H and pchCE7L
respectively. In the subcloning, the sequences coding for the
variable heavy and light chains of the anti-neuroblastoma CE7 (see,
supra) were exchanged with the synthetically assembled variable
heavy and variable light chain regions of C2B8.
[0128] Generation of CHO-tet-GnTIIIm Cells Expressing C2B8Antibody.
The method for the generation of a CHO-tet-GntIIIm cell line
expressing C2B8 antibody was exactly the same as for
CHO-tet-GnTIIIm-CE7. See, supra. The clone chosen for further work
was named CHO-tet-GnTIIIm-C2B8.
[0129] Generation of CHO-tTA Expressing C2B8 Antibody. CHO-tTA is
the parental cell line of CHO-tet-GnTIlIm. See, supra. The method
for the generation of a CHO-tTA cell line expressing C2B8 antibody
without GnT III expression was exactly the same as for
CHO-tet-GnTIIIm-C2B8 and CHO-tet-GnTIIIm-chCE7. See, supra. The
clone chosen for further work was named CHO-tTA-C2B8.
[0130] Production of C2B8 Antibody Samples. Two C2B8 antibody
samples were derived from parallel CHO-tet-GnTIIIm-C2B8 cultures;
each culture containing different levels of tetracycline and
therefore expected to express GnTIII at different levels. The
levels of tetracycline were 2000, 50, and 25ng/ml. The C2B8
antibody samples derived from these cultures were designated as
C2B8-2000t, C2B8-50t, and C2B8-25t, respectively. In parallel, one
antibody sample (C2B8-nt) was made from a CHO-tTA-C2B8 culture,
this cell line does not express GnT III. CHO-tTA-C2B8 cells were
cultured without tetracycline.
[0131] Analysis of GnTIII Expression. For Western blot analysis of
GnT III, cell lysates of each of the production cultures were
resolved by SDS-PAGE and electroblotted to polyvinylidene
difluoride membranes. Anti-c-myc monoclonal antibody 9E10 and
anti-mouse IgG-horseradish peroxidase (Amersham, Arlington, Ill.)
were used as primary and secondary antibodies respectively. Bound
antibody was detected using an enhanced chemiluminiscence kit
(Amersham, Arlington, Ill.).
[0132] Purification of C2B8 Antibody Samples. Antibody samples were
purified using the same procedure as for the chCE7 antibody
samples. See, supra. The concentration was measured using a
fluorescence based kit from Molecular Probes (Leiden, The
Netherlands).
[0133] Verification of Specific C2B8 Antigen Binding. The
specificity of antigen binding of the C2B8 anti-CD20 monoclonal
antibody was verified using an indirect immunofluorescence assay
with cells in suspension. For this study, CD20 positive cells (SB
cells; ATCC deposit no.ATCC CCL120) and CD20 negative cells (HSB
cells; ATCC deposit no. ATCC CCL 120.1) were utilized. Cells of
each type were incubated with C2B8 antibody produced at 25 ng/ml
tetracycline, as a primary antibody. Negative controls included
HBSSB instead of primary antibody. An anti-human IgG Fc specific,
polyclonal, FITC conjugated antibody was used for all samples as a
secondary antibody (SIGMA, St. Louis, Mo.). Cells were examined
using a Leica (Bensheim, Germany) fluorescence microscope.
[0134] ADCC Activity Assay. Lysis of SB cells (CD20+ target cells;
ATCC deposit no. ATCC CCL 120) by human monocyte depleted
peripheral blood mononuclear cells (effector cells) in the presence
of different concentrations of C2B8 samples was performed basically
following the same procedure described in Brunner et al., 1968,
Immunology 14:181-189. The ratio of effector cells to target cells
was 100:1.
[0135] 2. Results and Discussion
[0136] Gn TIII is Expressed at Different Levels in Different Cell
Lines And Cultures. The cells of the parallel CHO-tet-GnTIIIm-C2B8
cultures, each culture containing different levels of tetracycline
(2000, 50, and 25 ng/ml) and therefore expected to express GnTlIl
at different levels, were lysed and the cell lysates were resolved
by SDS-PAGE and detected by Western blotting. The lysates of the
culture grown at 25 ng/ml tetracycline showed an intense band at
the corresponding molecular weight of GnT III whereas cultures
grown at 50 and at 2000 ng/ml had much less expression of GnT III
as shown in FIG. 13.
[0137] Verification of Specific C2B8 Antigen Binding. C2B8 samples
produced from parallel cultures of cells expressing different
levels of GnT III were purified from the culture supernatants by
affinity chromatography and buffer exchanged to PBS on a cation
exchange column. Purity was estimated to be higher than 95% from
Coomassie Blue staining of an SDS-PAGE under reducing conditions.
These antibody samples were derived from expression of antibody
genes whose variable regions were synthesized by a PCR assembly
method. Sequencing of the synthetic cDNA fragments revealed no
differences to the original C2B8 variable region sequences
previously published in an international patent application
(International Publication Number WO 94/11026). Specific binding of
the samples to human CD20, the target antigen of C2B8, was
demonstrated by indirect immunofluorescence using a human
lymphoblastoid cell line SB expressing CD20 on its surface and an
HSB lymphoblastoid cell line lacking this antigen. Antibody sample
C2B8-25t gave positive staining of SB cells (FIG. 14A), but not of
HSB cells under identical experimental conditions (see FIG. 14B).
An additional negative control consisted of SB cells incubated with
PBS buffer instead of C2B8-25t antibody. It showed no staining at
all.
[0138] In Vitro ADCC Activity of C2B8 Samples. The antibody sample
C2B8-nt expressed in CHO-tTA-C2B8 cells that do not have Gnt III
expression (see, supra) showed 31 % cytotoxic activity (at 1
.mu.g/ml antibody concentration), measured as in vitro lysis of SB
cells (CD20+) by human lymphocytes (FIG. 15, sample C2B8-nt).
C2B8-2000t antibody derived from a CHO-tet-GnTIII culture grown at
2000 ng/ml of tetracycline (i e., at the basal level of cloned GnT
III expression) showed at 1 .mu.g/ml antibody concentration a 33%
increase in ADCC activity with respect to the C2B8-nt sample at the
same antibody concentration. Reducing the concentration of
tetracycline to 25 ng/ml (sample C2B8-25t), which significantly
increased GnTIII expression, produced a large increase of almost
80% in the maximal ADCC activity (at 1 .mu.g/ml antibody
concentration) with respect to the C2B8-nt antibody sample at the
same antibody concentration (FIG. 15, sample C2B8-25t).
[0139] Besides exhibiting the highest ADCC activity, C2B8-25t
showed significant levels of cytotoxicity at very low antibody
concentrations. The C2B8-25t sample at 0.06[tg/ml showed an ADCC
activity similar to the maximal ADCC activity of C2B8-nt at 1
.mu.g/ml. This result showed that sample C2B8-25t, at a 16- fold
lower antibody concentration, reached the same ADCC activity as
C2B8-nt. This result indicates that the chimeric anti-CD20 antibody
C2B8 produced in a cell line actively expressing GnT III was
significantly more active than the same antibody produced in a cell
line that did not express GnT III.
[0140] One advantage of this antibody using the methods of the
invention is that (1) lower doses of antibody have to be injected
to reach the same therapeutic effect, having a benefical impact in
the economics of antibody production, or (2) that using the same
dose of antibody a better therapeutic effect is obtained.
E. Example 5
Establishment of CHO Cell Lines with Constitutive Expression of
Glycosyltransferase Genes at Optimal Levels Leading to Maximal ADCC
Activity
[0141] In some applications of the method for enhancing the ADCC it
may be desirable to use constitutive rather than regulated
expression of GnT III on its own or together with other cloned
glycosyltransferases and/or glycosidases. However, the inventors
have demonstrated that ADCC activity of the modified antibody
depends on the expression level of GnT III. See, supra. Therefore,
it is important to select a clone with constitutive expression of
GnT III alone or together with other glycosyltransferase and/or
glycosidase genes at optimal or near optimal levels. The optimal
levels of expression of GnT III, either alone or together with
other glycosyl transferases such as .beta.(1,4)-galactosyl
transferase (GalT), are first determined using cell lines with
regulated expression of the glycosyl transferases. Stable clones
with constitutive expression of GnT III and any other cloned
glycosyltransferase are then screened for expression levels near
the optimum.
[0142] 1. Determination of Near-optimal Expression Levels
Construction of a Vector for Regulated GnT III
[0143] Expression Linked to GFP Expression. Each glycosyl
transferase gene is linked, via an IRES sequence, to a reporter
gene encoding a protein retained in the cell, e.g., green
fluorescent protein (GFP) or a plasma membrane protein tagged with
a peptide that can be recognized by available antibodies. If more
than one glycosyl transferase is being tested, a different marker
is associated with each glycosyl transferase, e.g., GnT III may be
associated to GFP and GalT may be associated to blue fluorescent
protein (BFP). An eucaryotic expression cassette consisting of the
GnT III cDNA upstream of an IRES element upstream of the GFP cDNA
is first assembled by standard subcloning and/or PCR steps. This
cassette is then subcloned in the tetracycline regulated expression
vector pUHD10-3 (see, supra), downstream of the tet-promoter and
upstream of the termination and polyadenylation sequences resulting
in vector pUHD10-3-GnTIII-GFP.
[0144] Establishment of CHO Cells with Regulated GnTIII Expression
Linked to GFP Expression and Constitutive chCE7Antibody Expression.
CHO-tTA cells (see, supra) expressing the tetracycline-responsive
transactivator, are co-transfected with vector pUHD10-3-GnTIII-GFP
and vector pPur for expression of a puromycin-resistance gene. See,
supra. Puromycin resistant clones are selected in the presence of
tetracycline. Individual clones are cultured by duplicate in the
presence (2 .mu.g/ml) or absence of tetracycline. Six clones that
show inhibition of growth in the absence of tetracycline, due to
glycosyltransferase overexpression (see, supra), are selected and
analyzed by fluorescence-activated cell sorting (FACS) for
detection of the GFP-associated signal. A clone giving the highest
induction ratio, defined as the ratio of fluorescence in the
absence of tetracycline to fluorescence in the presence of
tetracycline is chosen for further work and designated as
CHO-tet-GnTIII-GFP. CHO-tet-GnTIII-GFP are transfected with
expression vectors for antibody chCE7 and a clone with high
constitutive expression of this antibody is selected
CHO-tet-GnTIII-GFP-chCE7. See, supra.
[0145] Production of chCE7Samples, Measurement of ADCC Activity and
Determination of Optimal GnTIll Expression Levels. Parallel
cultures of CHO-tet-GnTIII-GFP-chCE7 are grown at different levels
of tetracycline, and therefore expressing GnTIII together with GFP
at different levels. chCE7 antibody samples are purified from the
culture supernatants by affinity chromatography. In parallel, the
cells from each culture are analyzed by FACS to determine the mean
level of GFP-associated fluorescence, which is correlated to the
expression level of GnT III, of each culture. The in vitro ADCC
activity of each chCE7 antibody sample is determined (see, supra)
and the maximal in vitro ADCC activity of each sample is plotted
against the mean fluorescence of the cells used to produce it.
[0146] 2. Establishment of a CHO Cell Line with Constitutive GnTIII
Expression at Near-Optimal Levels
[0147] The GnTIII-IRES-GFP cassette (see, supra) is subcloned in a
constitutive expression vector. CHO cells are stably co-transfected
with this vector and a vector for puromycin resistance. Puromycin
resistant cells are selected. This population of stably transfected
cells is then sorted via FACS, and clones are selected which
express the levels of reporter GFP gene near the within the range
where optimal or near-optimal ADCC activity is achieved. See,
supra. This final transfection step may be done either on CHO cells
already stably expressing a therapeutic antibody or on empty CHO
cells, e.g., DUKX or DG44 dhfr- CHO cells. In the latter case, the
clones obtained from the procedure described above will be
transfected with therapeutic antibody-expression vectors in order
to generate the final antibody-producing cell lines.
F. Example 6
Cell Surface Expression of a Human IgG Fc Chimera with Optimized
Glycosylation
[0148] Encapsulated cell therapy is currently being tested for a
number of diseases. An encapsulated cell implant is designed to be
surgically placed into the body to deliver a desired therapeutic
substance directly where it is needed. However, if once implanted
the encapsulated device has a mechanical failure, cells can escape
and become undesirable. One way to destroy escaped, undesirable
cells in the body is via an Fc-mediated cellular cytoxicity
mechanism. For this purpose, the cells to be encapsulated can be
previously engineered to express a plasma membrane-anchored fusion
protein made of a type II transmembrane domain that localizes to
the plasma membrane fused to the N-terminus of an Fc region.
Stabila, P. F., 1998, supra. Cells inside the capsule are protected
against Fc-mediated cellular cytoxicity by the capsule, while
escaped cells are accesible for destruction by lymphocytes which
recognize the surface-displayed Fc regions, i e., via an
Fc-mediated cellular cytoxicity mechanism. This example illustrates
how this Fc-mediated cellular cytoxicity activity is enhanced by
glycosylation engineering of the displayed Fc regions.
[0149] 1. Establishment of Cells Expressing the Fc Chimera on Their
Surface and Expressing GnTIII
[0150] Cells to be implanted for a particular therapy, for example
baby hamster kidney (BHK) cells, which already produce the
surface-displayed Fc chimera and a secreted, therapeutic protein,
are first stably transfected with a vector for constitutive
expression of GnTIII linked via an IRES element to expression of
GFP. See, supra. Stable transfectants are selected by means of a
marker incorporated in the vector, e.g., by means of a drug
resistance marker and selected for survival in the presence of the
drug.
[0151] 2. Screening of Cells Expressing Diffent Levels of GnTIII
and Measurement
[0152] Stable transfectants are analyzed by fluorescence-activated
cell sorting (FACS) and a series of clones with different mean
fluorescence levels are selected for further studies. Each selected
clone is grown and reanalyzed by FACS to ensure stability of GFP,
and therefore associated GnT III, expression.
[0153] 3. Verification of Different Levels of Bisected Complex
Oligosaccharides on the Displayed Fc Regions
[0154] Fc regions from three clones with different levels of
GFP-associated fluorescence and from the original BHK cells not
transfected with the GnTIII-IRES-GFP vector are solubilized from
the membrane by means of a detergent and then purified by affinity
chromatography. The oligosaccharides are then removed, purified and
analyzed by MALDI-TOF/MS. See, supra. The resulting MALDI-TOF/MS
profiles show that the Fc-regions of the modified, fluorescent
clones carry different proportions of bisected complex
oligosaccharides. The MALDI profile from the unmodified cells does
not show any peak associated to bisected oligosaccharides. The
clone with carrying the highest levels of bisected complex
oligosaccharides on the displayed Fc regions is chosen for further
work.
[0155] 4. In vitro Fc-mediated Cellular Cytoxicity Activity
Assay
[0156] Two Fc-mediated cellular cytoxicity activity assays are then
conducted in parallel. In one assay the target cells are derived
from the clone selected above. In the parallel assay the target
cells are the original cells to be encapsulated and which have not
been modified to express GnTIII. The assay is conducted using the
procedure described previously (see, supra) but in the absence of
any additional antibody, since the target cells already display Fc
regions. This experiment demonstrates that the Fc-mediated cellular
cytoxicity activity against the cells expressing GnT III is higher
than that against cells not expressing this
glycosyltransferase.
[0157] All references cited within the body of the instant
specification are hereby incorporated by reference in their
entirety.
Sequence CWU 1
1
14 1 50 DNA Artificial PCR Oligonucleotide Primer CE7VHPCR1.fwd 1
ttccttgtcg ctgttgctac gcgtgtcctg tcccaggtcc aactgcagca 50 2 63 DNA
Artificial PCR Oligonucleotide Primer CE7VHPCR2.fwd 2 gtgtgttaag
cttccaccat gggttggagc ctcatcttgc tcttccttgt cgctgttgct 60 acg 63 3
38 DNA Artificial PCR Oligonucleotide Primer CE7VHPCR(1+2).rev 3
gtgtgtgaat tcgctagctg aggagactgt gagagtgg 38 4 40 DNA Artificial
PCR Oligonucleotide Primer hGamma1CH1.fwd 4 gtttgtaagc ttgctagcac
caagggccca tcggtcttcc 40 5 59 DNA Artificial PCR Oligonucleotide
Primer hGamma1CH1.rev 5 ggcatgtgtg agttttgtca caagatttgg gctcaacttt
cttgtccacc ttggtgttg 59 6 57 DNA Artificial PCR Oligonucleotide
Primer hGamma1CH2.fwd 6 tcttgtgaca aaactcacac atgcccaccg tgcccagacc
tgaactcctg gggggac 57 7 49 DNA Artificial hGamma1CH2.rev 7
cctgtggttc tcggggctgc cctttggctt tggagatggt tttctcgat 49 8 22 DNA
Artificial PCR Oligonucleotide Primer hGamma1CH3.fwd 8 gggcagcccc
gagaaccaca gg 22 9 36 DNA Artificial PCR Oligonucleotide Primer
hGamma1CH2.rev 9 gtgtgtggat cctcatttac ccggagacag ggagag 36 10 56
DNA Artificial PCR Oligonucleotide Primer CE7VLPCR1.fwd 10
tgggtactgc tgctctgggt tccaggttcc actggtgaca tccagatgac acaatc 56 11
63 DNA Artificial PCR Oligonucleotide Primer CE7VLPCR2.fwd 11
gtgtgtaagc ttccaccatg gagacagaca cactcctgct atgggtactg ctgctctggg
60 ttc 63 12 37 DNA Artificial PCR Oligonucleotide Primer
CE7VLPCR(1+2).rev 12 gtgtgtgaat tccgtacgtt ttatttccaa ctctgtc 37 13
32 DNA Artificial PCR Oligonucleotide Primer hKappa.fwd 13
gtgtgtaagc ttcgtacggt ggctgcacca tc 32 14 33 DNA Artificial PCR
Oligonucleotide Primer hKappa.rev 14 gtgtgtggat ccctaacact
ctcccctgtt gaa 33
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