Elimination of N-linked glycosylation sites from the human IgA1 constant region: effects on structure and function

IgA1 Abs possess conserved N-linked glycosylation sites in the second C region and secreted tailpiece domains. To understand the role of these carbohydrates in the structure and function of human IgA1, site-directed mutants that produce human IgA1 lacking either one or both of the N-linked carbohydrate sites have been produced. When the mutant heavy chains are expressed in myeloma lines producing the relevant kappa-light chain, efficient secretion of the monomer and dimer forms of IgA1 is seen. In addition, higher polymer forms of the IgA molecules lacking the third domain carbohydrate, either singly or in the double mutant, are present. Functional analysis of the IgA1 proteins has shown significant differences between the various mutants and wild-type IgA. The carbohydrate mutants show a reduced affinity for their target Ag, dansyl. All of the IgA1 molecules retained the ability to bind to the polymeric Ig receptor. C3 binding was observed for all of the IgA molecules, with the IgA mutants lacking the third domain carbohydrate showing a reduced ability to bind C3; however, IgA did not effectively activate the alternative pathway, as determined by factor B cleavage and terminal complex binding. These studies demonstrate that N-linked glycosylation in the constant domain of human IgA1 plays an important role in the biologic properties of IgA1.

The hinge as a spacer contributes to covalent assembly and is required for function of IgG

Chimeric IgG3 lacking a genetic hinge but with Cys residues in CH2 between position 231 and 232 (IgG3 deltaH+Cys) and position 279 (IgG3 deltaH+2Cys) produced by site-directed mutagenesis have been found to be deficient in their intermolecular assembly. Although some H2L2 molecules are formed, significant quantities of assembly intermediates, in particular HL, are found in the secretions. Both IgG3 deltaH+Cys and IgG3 deltaH+2Cys were greatly reduced in their ability to bind Fc gammaRI. They also failed to bind C1q and activate the complement cascade. Addition of the tailpiece from IgM (microtp) to IgG3 deltaH+Cys (yielding IgG3 deltaH+Cys microtp) failed to lead to efficient oligomerization. IgG3 deltaH+Cys microtp failed to bind Fc gammaRII. However, at high concentrations it was able to bind C1q and effect complement-dependent cytolysis. It consumed complement in the absence of added Ag, indicating that removing the flexible hinge was not sufficient to yield polymeric IgG that resembled IgM in its dependence on added Ag for complement activation.

Effect of glycosylation on antibody function: implications for genetic engineering

Antibodies are able to both bind antigens and trigger the responses that eliminate them from circulation. All antibodies are glycosylated at conserved positions in their constant regions, and the presence of carbohydrate can be critical for antigen clearance functions such as complement activation. The structure of the attached carbohydrate can also affect antibody activity. Antibody glycosylation can be influenced by the cell in which it is produced, the conformation of the antibody and cell culture conditions. These variables should be considered in the design and production of antibodies with selected specificity and function.

Residues critical for H-L disulfide bond formation in human IgA1 and IgA2

There are two subclasses of human IgA, IgA1 and IgA2. IgA2 exists as two known allotypes, IgA2 m(1) and IgA2 m(2) with a recently reported novel IgA2 (IgA2(n)) possibly representing a third allotype. The variants of human IgA differ in their H and L chain disulfide-bonding pattern; in IgA1, IgA2(n), and IgA2 m(2), a disulfide bond connects a cysteine residue in CH1 of the H chain with the L chains while human IgA2 m(1) has been reported to lack a covalent bond between the H and L chains. Here we have used site-directed mutagenesis to demonstrate that Cys133 is essential for the formation of the H-L disulfide bond in IgA1. However, IgA2 m(2) and the IgA2(n) but not IgA2 m(1) form an H-L disulfide in the absence of Cys133. IgA2 m(1) differs from IgA2 m(2) and the IgA2(n) at two positions in CH1; IgA2 m(1) has Pro212 and Pro221 whereas IgA2 m(2) and the IgA2(n) have Ser212 and Arg221. Our studies demonstrate that it is the presence of Pro221 in IgA2 m(1) that interferes with the H-L disulfide in the absence of Cys133. Contrary to what has been previously reported, protein purified from culture supernatants of IgA2 m(1) show some HL, H2L2, and H4L4J, suggesting that IgA2 m(1) can exist either as a form lacking H-L disulfide bonds or as a form with H-L disulfides.

Residues critical for H-L disulfide bond formation in human IgA1 and IgA2

There are two subclasses of human IgA, IgA1 and IgA2. IgA2 exists as two known allotypes, IgA2 m(1) and IgA2 m(2) with a recently reported novel IgA2 (IgA2(n)) possibly representing a third allotype. The variants of human IgA differ in their H and L chain disulfide-bonding pattern; in IgA1, IgA2(n), and IgA2 m(2), a disulfide bond connects a cysteine residue in CH1 of the H chain with the L chains while human IgA2 m(1) has been reported to lack a covalent bond between the H and L chains. Here we have used site-directed mutagenesis to demonstrate that Cys133 is essential for the formation of the H-L disulfide bond in IgA1. However, IgA2 m(2) and the IgA2(n) but not IgA2 m(1) form an H-L disulfide in the absence of Cys133. IgA2 m(1) differs from IgA2 m(2) and the IgA2(n) at two positions in CH1; IgA2 m(1) has Pro212 and Pro221 whereas IgA2 m(2) and the IgA2(n) have Ser212 and Arg221. Our studies demonstrate that it is the presence of Pro221 in IgA2 m(1) that interferes with the H-L disulfide in the absence of Cys133. Contrary to what has been previously reported, protein purified from culture supernatants of IgA2 m(1) show some HL, H2L2, and H4L4J, suggesting that IgA2 m(1) can exist either as a form lacking H-L disulfide bonds or as a form with H-L disulfides.

In vivo properties of an IgG3-IL-2 fusion protein. A general strategy for immune potentiation

In this report, we describe a strategy for enhancing the immunogenicity of a wide variety of Ags by linking them to IL-2 via an IgG3-IL-2 fusion protein with high affinity for a convenient hapten Ag, dansyl (DNS; N,N-dimethyl-1-aminonaphthalene-5-sulfonyl chloride). This fusion protein, anti-DNS-IgG3-IL-2, combines the functional characteristics of its constituents and has pharmacokinetic properties that are greatly improved over those of IL-2 and a previously described IgG1-IL-2 fusion. The molecule is intact and recoverable from the blood of mice hours after i.p. injection and reaches distant organs throughout the animal. The 7-h in vivo half-life of this molecule is much longer than that of IL-2, addressing a major obstacle in the application of IL-2 to human diseases, including cancer and AIDS. Additionally, the Ab's specificity for the hapten dansyl and the convenient chemistry of dansyl provide a means to link IL-2 to virtually any molecule of interest without the complexities and uncertainties of making IL-2 fusions with each molecule individually. Using hapten-conjugated-BSA (DNS-BSA) as a model Ag we show that the Ab response elicited by anti-DNS-IgG3-IL-2-bound DNS-BSA-Sepharose injected into mice is increased over that of DNS-BSA-Sepharose or anti-DNS-IgG3-bound DNS-BSA-Sepharose. Anti-DNS-IgG3-IL-2 also increased the Ab response to soluble DNS-BSA after a booster injection. This system should be useful in testing the ability of IL-2 to potentiate the immune response to Ag and in screening a large number of potential Ags for use in vaccines. The dramatically improved pharmacokinetics should also overcome one of the major difficulties in applying IL-2 to the treatment of human disease, its short half-life.

In vivo properties of an IgG3-IL-2 fusion protein. A general strategy for immune potentiation

In this report, we describe a strategy for enhancing the immunogenicity of a wide variety of Ags by linking them to IL-2 via an IgG3-IL-2 fusion protein with high affinity for a convenient hapten Ag, dansyl (DNS; N,N-dimethyl-1-aminonaphthalene-5-sulfonyl chloride). This fusion protein, anti-DNS-IgG3-IL-2, combines the functional characteristics of its constituents and has pharmacokinetic properties that are greatly improved over those of IL-2 and a previously described IgG1-IL-2 fusion. The molecule is intact and recoverable from the blood of mice hours after i.p. injection and reaches distant organs throughout the animal. The 7-h in vivo half-life of this molecule is much longer than that of IL-2, addressing a major obstacle in the application of IL-2 to human diseases, including cancer and AIDS. Additionally, the Ab's specificity for the hapten dansyl and the convenient chemistry of dansyl provide a means to link IL-2 to virtually any molecule of interest without the complexities and uncertainties of making IL-2 fusions with each molecule individually. Using hapten-conjugated-BSA (DNS-BSA) as a model Ag we show that the Ab response elicited by anti-DNS-IgG3-IL-2-bound DNS-BSA-Sepharose injected into mice is increased over that of DNS-BSA-Sepharose or anti-DNS-IgG3-bound DNS-BSA-Sepharose. Anti-DNS-IgG3-IL-2 also increased the Ab response to soluble DNS-BSA after a booster injection. This system should be useful in testing the ability of IL-2 to potentiate the immune response to Ag and in screening a large number of potential Ags for use in vaccines. The dramatically improved pharmacokinetics should also overcome one of the major difficulties in applying IL-2 to the treatment of human disease, its short half-life.

Identification and functional characterization of a highly conserved sequence in the intron of the kappa light chain gene

A highly conserved 225 bp sequence was identified within the J-C intron of the murine kappa light-chain immunoglobulin gene and its nuclear protein-binding and regulatory function were examined. The binding of nuclear proteins to this fragment was found to reflect the differentiation state of the cell used to prepare the nuclear extracts and three different complexes are seen with this fragment: CI, CII and CIII. CIII is present in all cell types. CI is present in fibroblasts, T cells and early B cells, but not mature B cells. Moreover, nuclear extracts prepared from the early pre-B cell line, 70Z/3, that was treated with agents which activate kappa gene transcription have a reduced ability to form CI. Therefore, the presence of CI correlates with the absence of kappa gene transcription. CII is present in all stages of B cell development, however its composition changes with B cell maturation. Contained within the 225 bp element is the ets family-binding motif GGAA and the B-cell-and-macrophage-specific family member, PU.1 binds this sequence and participates in CII formation. The 225 bp fragment showed modest augmentation of expression in CAT reporter constructs containing the heavy chain enhancer (HCE) and a light chain promoter in the plasmacytoma, S194, and uninduced 70Z/3 cells and mediated a small but reproducible response to IFN-gamma in 70Z/3 cells. Thus, the 225 bp sequence contained within the J-C intron may function as a regulatory element for kappa light chain gene expression.

An IgG3-IL-2 fusion protein has higher affinity than hrIL-2 for the IL-2R alpha subunit: real time measurement of ligand binding

The alpha subunit of the interleukin-2 (IL-2) receptor (IL-2R alpha)3 has the highest individual affinity for IL-2 and is the only subunit not known to bind other cytokines. The interactions between IL-2 and IL-2R alpha studied in cell binding assays have revealed a number of factors which may vary significantly in different cell lines used for these assays in different laboratories. In order to avoid the problems associated with cellular assays we used an optical biosensor to examine the interaction between IL-2R alpha and hrIL-2. Real-time measurement of association and dissociation resulted in a calculated KD of 1.9 x 10(-7) M for this interaction. We then examined the IL-2R alpha binding of a potentially bivalent IgG3-IL2 fusion protein previously shown to have a higher affinity than hrIL-2 for the high affinity IL-2R but not the intermediate affinity IL-2R. Biosensor measurements of association and dissociation of IgG3-IL2 to IL-2R alpha yielded a similar association rate but a decreased dissociation rate compared to hrIL-2, resulting in a KD of 5.3 x 10(-8) M. This system is applicable to the numerous IL-2 mutants with different affinities and activities and is generalizable to other cytokine/receptor interactions.

Expression of novel secreted isoforms of human immunoglobulin E proteins

Four human IgE isoforms produced by alternative splicing of the epsilon primary transcript were expressed as chimeric mouse/human anti 5-dimethylamino-1-naphthalenesulfonyl antibodies in the murine myeloma cell line Sp2/0. The four isoforms include the classic secreted form and three novel isoforms with altered carboxyl termini. All of these isoforms lack the transmembrane region encoded by the M1/M1' exon and are therefore predicted to be secreted proteins. When expressed in Sp2/0 cells, three of the IgE isoforms are assembled into complete molecules of two Ig heavy chains and two Ig light chains, whereas the fourth isoform is predominately assembled into half-molecules of one Ig heavy chain and one Ig light chain. All four isoforms are secreted with similar kinetics. In contrast, when the isoform containing the C epsilon4 domain joined directly to the M2 exon (IgE grandé) is expressed in the J558L cell line, it is degraded intracellularly, suggesting a cell line-dependent regulation of secretion. These data show that these novel isoforms of human IgE, predicted to occur from in vivo and in vitro mRNA analysis, can be produced and secreted by mammalian cells. The different forms of IgE may have physiologically relevant but distinct roles in human IgE-mediated immune inflammation. The availability of purified recombinant human IgE isoforms makes it possible to analyze the functional differences among them.

Analysis of the pH dependence of the neonatal Fc receptor/immunoglobulin G interaction using antibody and receptor variants

The neonatal Fc receptor (FcRn) binds maternal immunoglobulin G (IgG) from ingested milk in the gut (pH 6.0-6.5) and delivers it to the bloodstream of the newborn (pH 7.0-7.5). A soluble version of FcRn reproduces the physiological pH-dependent interaction with IgG, showing high-affinity binding at pH 6.0-6.5 but weak or no binding at pH 7.0-7.5. We have studied the pH dependence of the FcRn/IgG interaction using a surface plasmon resonance assay to measure kinetic and equilibrium constants. We show that the affinity of FcRn for IgG is reduced about 2 orders of magnitude as the pH is raised from 6.0 to 7.0. A hill put analysis suggests that several titrating residues participate in the pH-dependent affinity transition. Histidine side chains are likely candidate for residues that titrate between pH 6.0 and 7.0, and previous biochemical and structural work identified several histidines on the Fc portion of IgG that are located at the FcRn binding site. Using mutant IgG molecules and IgG subtype variants that differ in the number of histidines at the IgG/FcRn interface, we demonstrate that IgG histidines located at the junction between the CH2 and CH3 domains (residues 310 and 433) contribute to the pH-dependent affinity transition. Experiments with a mutant FcRn molecule show that two histidines on the FcRn heavy chain (residues 250 and 251) also contribute to the pH dependence of the FcRn/IgG interaction. There results are interpreted using the crystal structures of FcRn and an FcRn/Fc complex.

Glycosylation of the variable region of immunoglobulin G--site specific maturation of the sugar chains

The structure of the N-linked sugar chains attached to three IgG antibodies, identical in amino acid sequence except for the changes required to introduce the carbohydrate addition sites, has been determined. All three antibodies are specific for dextran but differ in their ability to bind antigen. The heavy chains with a murine variable region (V region) attached to the human gamma 4 constant region were expressed in a murine hybridoma synthesizing the specific light chain. In addition to the glycosylation site in the Fc portion, each antibody has a different glycosylation site in the second complementarity determining region (CDR2) of the heavy chain (Asn54, Asn58, or Asn60). The sugar chains were released from purified Fab and Fc fragments by hydrazinolysis and converted to radioactive oligosaccharides by reduction with sodium borotritide. The structures of these radioactive oligosaccharides were determined by a combination of sequential exoglycosidase digestion and Bio-Gel P-4 and lectin column chromatography. For all three antibodies, the carbohydrate attached to the Fc portion was a mixture of complex-type biantennary sugar chains. The variable region carbohydrate structures attached at Asn54 and Asn58 were also complex-type but more highly sialylated than were the Fc-associated sugars. Moreover, unlike the Fc-associated sugars, a significant population of Fab-associated sugars contained a Gal alpha 1-->3 residue as a non-reducing terminus. In contrast, the carbohydrate attached at Asn60 was a high mannose structure. These results demonstrate that slight changes in the position of carbohydrate attachment within CDR2 of the variable region of the heavy chain can substantially alter carbohydrate processing and that complex-type carbohydrates contained within the same polypeptide chain can have different structures.

An IgG3-IL2 fusion protein activates complement, binds Fc gamma RI, generates LAK activity and shows enhanced binding to the high affinity IL-2R

The therapeutic value of Interleukin 2 (IL-2) is limited by its short half life and systemic toxicity. One approach to overcoming these problems is to fuse this protein to an antibody, a protein with a long half life and the ability to target a unique antigen within the body. To examine the biochemical properties of such a molecule a fusion protein was constructed linking the N-terminus of human IL-2 to the C-terminus of IgG3. A similar fusion between IgG1 and IL-2 has previously been shown to bind antigen, generate antibody-dependent cellular cytotoxicity (ADCC) and stimulate T cell proliferation and cytotoxicity. We now extend these studies and show that the fusion protein, termed IgG3-IL2, is appropriately N-glycosylated within the IgG3 CH2 domain, binds the human high affinity Fc receptor (Fc gamma RI) with an affinity slightly lower than that of IgG3, and is able to activate complement via the classical pathway to lyse antigen coated sheep red blood cells (SRBC). When used to stimulate the proliferation of the IL-2 dependent cell line CTLL-2, IgG3-IL2 has a specific activity slightly lower than that of human recombinant IL-2 (hrIL-2). In marked contrast, when comparable unit concentrations, as defined by the standard CTLL-2 proliferation assay, are used to stimulate human peripheral blood lymphocytes (PBL), IgG3-IL2 generates significantly greater lymphokine activated killer (LAK) cell cytotoxicity than does hrIL-2. Competition studies show that IgG3-IL2 binds the intermediate affinity form of the IL-2 receptor (IL-2R), consisting of the beta and gamma subunits, with an affinity slightly less than that of hrIL-2. In contrast, IgG3-IL2 shows a greater affinity than hrIL-2 for the high affinity IL-2R, consisting of alpha, beta and gamma subunits. Our studies show that the IgG3-IL2 fusion protein possesses a combination of the biological properties of IgG3 and IL-2 including antigen binding, complement activation, Fc gamma RI binding, IL-2R binding and stimulation of both proliferation and LAK activity. This combination of activities may allow IgG3-IL2 to target humoral and cell-mediated immune activation to the site of an antigen of interest or target an antigen to IL-2R bearing cells or organs.

Genetically engineered antibodies and their application to brain delivery

Techniques of genetic engineering and expression have been applied to the production of antibodies in a variety of expression systems. Combinatorial libraries produced in bacteriophage may present an alternative to animal immunization as a source of antigen-binding specificities. Transfectomas which express genetically engineered antibody genes provide one approach to overcoming some of the limitations inherent in classical monoclonal antibodies. Novel antibodies have been produced with a variety of modifications: as chimeric antibodies, as 'humanized' antibodies, with catalytic groups, as bifunctional or fusion proteins and as functional fragments such as Fabs or Fvs. The domain structure of the antibody is favorable to such manipulation; the novel proteins often retain their antibody-derived activity and acquire new properties as well. Chimeric and CDR-grafted antibodies have been effective in immunotherapy, but problems of immunogenicity remain. Careful analysis and comparison of effector functions among immunoglobulin isotypes may be applied to the design of effective therapeutic antibodies. In addition, antibody combining specificities can be joined with non-immunoglobulin sequences thereby providing properties not usually found in antibodies. In particular, antibodies fused with the growth factors insulin-like growth factor(IGF)-1, IGF-2 and transferrin have shown increased uptake into the brain parenchyma. These fusion proteins provide a family of reagents with many potential applications.

Structure and function of several anti-dansyl chimeric antibodies formed by domain interchanges between human IgM and mouse IgG2b

Two pairs of chimeric, domain-switched immunoglobulins with identical murine, anti-dansyl (5-dimethylaminonaphthalene-1-sulfonyl) variable domains have been generated, employing as parent antibodies a human IgM and a mouse IgG2b. The first pair of chimeric antibodies mu mu gamma mu and gamma gamma mu gamma was generated by switching the C mu 3 and C gamma 2 domains between IgM and IgG2b. The second pair of chimeras mu mu gamma gamma and gamma gamma mu mu were formed by switching both C mu 3 and C mu 4 with C gamma 2 and C gamma 3. SDS-polyacrylamide gel electrophoresis and analytical ultracentrifugation showed that over half (57 and 71%) of the two chimeric antibodies possessing the C mu 4 domain and tail piece formed disulfide-linked IgM-like polymers. In contrast, the two chimeric antibodies lacking the C mu 4 domain were almost entirely monomeric. Both monomeric chimeras had reduced ability to activate complement. The chimera gamma gamma mu gamma had no activity under any of the assay conditions, whereas mu mu gamma gamma caused only a small amount of cell lysis but was fully active in consuming complement at 4 degrees C. The polymeric chimera gamma gamma mu mu was much less active than IgM, bound C1 weakly and caused some cell lysis but consumed little complement with soluble antigen. The polymeric chimera mu mu gamma mu bound C1 strongly and was the most active antibody in all assays, even more active than the parental IgG2b and IgM antibodies; it was the only antibody that exhibited antigen-independent activity. The results suggest that C mu 3 alone does not constitute the complement binding site in IgM but requires both C mu 1-2 and C mu 4 for full activity.

Transferrin-antibody fusion proteins are effective in brain targeting

In the present study, the receptor binding potential of transferrin (Tf) was linked to an antibody binding specificity. Human Tf was fused to mouse-human chimeric IgG3 at three positions: at the end of heavy chain constant region 1 (CH1), after the hinge, and after CH3. The resulting Tf-antibody fusion proteins were able to bind antigen and the Tf receptor. The CH3-Tf fusion protein showed no complement-mediated cytolysis but possessed IgG receptor I (Fc gamma RI) binding activity. Most importantly, all of the fusion proteins demonstrated significant uptake into brain parenchyma, with 0.3% of the injected dose of the hinge-Tf fusion protein rapidly targeted to the brain. Recovery of iodinated CH3-Tf fusion protein from the brain parenchyma demonstrated that the fusion proteins can cross the blood-brain barrier intact. The binding specificity of these fusion proteins can be used for brain delivery of noncovalently bound ligands, such as drugs and peptides, or for targeting antigens present within the brain.

Addition of a mu-tailpiece to IgG results in polymeric antibodies with enhanced effector functions including complement-mediated cytolysis by IgG4

The 18-amino acid carboxyl-terminal tailpiece from IgM (mutp) has now been added to the carboxyl-termini of IgG1, IgG2, IgG3, and IgG4 constant regions to produce recombinant IgM-like IgGs. Polymeric IgGs obtained by this approach possess up to six Fcs and 12 antigen-combining sites, greatly increasing the avidity of their interactions with other molecules. Not surprisingly, the C activity of normally active IgG1 and IgG3 and somewhat less active IgG2 Abs is shown to be dramatically enhanced upon polymerization. The multiple Fcs present in a single molecule apparently allow for more efficient interactions with the multiple C1q heads present in C1, the first component of the classical C cascade. An unexpected result however, is that IgG4, normally devoid of C activity, when polymerized in the same fashion directs C-mediated lysis of target cells almost as effectively as the other polymers. Interestingly though, IgG4mutp does not deplete C activity in a standard consumption assay using soluble Ag. The other gamma mutp isotypes are capable of depleting 100% of the serum lytic ability even in the absence of Ag, whereas IgG4mutp shows no evidence of activity in this assay under any of the conditions tested. Additionally, we show that, in contrast to monomeric IgG, polymeric IgGs bind with very high affinity to Fc gamma receptor II (Fc gamma RII), a low affinity receptor for wild-type antibodies; however, binding to Fc gamma Rl, the high affinity receptor, appears to be unaltered. Finally, the in vivo t1/2 of the gamma mutp proteins is decreased relative to wild-type IgG, apparently because of rapid clearance of the polymeric fraction.

Addition of a mu-tailpiece to IgG results in polymeric antibodies with enhanced effector functions including complement-mediated cytolysis by IgG4

The 18-amino acid carboxyl-terminal tailpiece from IgM (mutp) has now been added to the carboxyl-termini of IgG1, IgG2, IgG3, and IgG4 constant regions to produce recombinant IgM-like IgGs. Polymeric IgGs obtained by this approach possess up to six Fcs and 12 antigen-combining sites, greatly increasing the avidity of their interactions with other molecules. Not surprisingly, the C activity of normally active IgG1 and IgG3 and somewhat less active IgG2 Abs is shown to be dramatically enhanced upon polymerization. The multiple Fcs present in a single molecule apparently allow for more efficient interactions with the multiple C1q heads present in C1, the first component of the classical C cascade. An unexpected result however, is that IgG4, normally devoid of C activity, when polymerized in the same fashion directs C-mediated lysis of target cells almost as effectively as the other polymers. Interestingly though, IgG4mutp does not deplete C activity in a standard consumption assay using soluble Ag. The other gamma mutp isotypes are capable of depleting 100% of the serum lytic ability even in the absence of Ag, whereas IgG4mutp shows no evidence of activity in this assay under any of the conditions tested. Additionally, we show that, in contrast to monomeric IgG, polymeric IgGs bind with very high affinity to Fc gamma receptor II (Fc gamma RII), a low affinity receptor for wild-type antibodies; however, binding to Fc gamma Rl, the high affinity receptor, appears to be unaltered. Finally, the in vivo t1/2 of the gamma mutp proteins is decreased relative to wild-type IgG, apparently because of rapid clearance of the polymeric fraction.

Human/mouse chimeric monoclonal antibodies with human IgG1, IgG2, IgG3 and IgG4 constant domains: electron microscopic and hydrodynamic characterization

The unique structure of the human IgG3 constant region with its greatly extended hinge can clearly be seen in electron micrographs, which compare a series of recombinant proteins with the same murine anti-dansyl variable domain but constant domains from human IgG1, IgG2, IgG3 and IgG4. The hinge region of IgG3 was found to be very long, with some measurements extending to 100 A. It exhibited considerable flexibility allowing the Fc to be displaced far toward either side. Upon addition of bivalent hapten, all of the monoclonal antibodies formed complexes. IgG1, IgG3 and IgG4 formed circular dimers, composed of two antibodies forming a ring-shaped complex, presumably through the binding of two bivalent haptens. IgG2, on the other hand, showed a distribution of complexes which was noticeably different from the other subclasses. Some circular dimers, some linear dimers and a large amount of monomer were seen. This was interpreted in terms of an energy barrier to ring closure arising from the orientation of the Fab arms of IgG2 probably leading to linear dimers as the predominate complex seen with the analytical ultracentrifuge. A substantial number of these dimers probably dissociated upon dilution for examination in the electron microscope. The distribution of the angles between the Fab arms of the monoclonal antibodies forming the circular dimers has been measured for the different subclasses. Most were open at wide angles (> 100 degrees) but some formed very shallow angles, with the Fab arms being nearly parallel to each other. The free energy for this transition was calculated from the ratio of open/closed angles, and it was found to be proportional to the length of the upper hinge of the monoclonal antibody, in agreement with previous nanosecond depolarization results (Dangl et al., Eur. molec. Biol. Org. J. 7, 1989-1994, 1988).

Effect of altered CH2-associated carbohydrate structure on the functional properties and in vivo fate of chimeric mouse-human immunoglobulin G1

Immunoglobulin G (IgG) molecules are glycosylated in CH2 at Asn297; the N-linked carbohydrates attached there have been shown to contribute to antibody (Ab) stability and various effector functions. The carbohydrate attached to the IgG constant region is a complex biantennary structure. Alterations in the structure of oligosaccharide have been associated with human diseases such as rheumatoid arthritis and osteoarthritis. To study the effects of altered carbohydrate structure on Ab effector function, we have used gene transfection techniques to produce mouse-human chimeric IgG1 Abs in the Chinese hamster ovary (CHO) cell line Lec 1, which is incapable of processing the high-mannose intermediate through the terminal glycosylation steps. We also produced IgG1 Abs in Pro-5, the wild-type CHO cell line that is the parent of Lec 1. The Pro-5-produced Ab (IgG1-Pro-5) was similar to IgG1-My 1, a myeloma-produced IgG1 Ab of the same specificity, in its biologic properties such as serum half-life, ability to effect complement-mediated cytolysis, and affinity for Fc gamma RI. Although the Lec 1-produced Ab, IgG1-Lec 1, was properly assembled and retained antigen specificity, it was incapable of complement-mediated hemolysis and was substantially deficient in complement consumption, C1q binding, and C1 activation. IgG1-Lec 1 also showed reduced but significant affinity for Fc gamma R1 receptors. The in vivo half-life of IgG1-Lec 1 was shorter than that of either the myeloma- or Pro-5-produced counterpart, with more being cleared during the alpha-phase and with more rapid clearance during the beta-phase. Clearance of IgG1-Lec 1 could be inhibited by the administration of yeast-derived mannan. Thus the uptake of IgG1-Lec 1 appears to be accelerated by the presence of terminally mannosylated oligosaccharide. Therefore, certain Ab functions as well as the in vivo fate of the protein are dramatically affected by altered carbohydrate structure. Expression of Igs in cell lines with defined glycosylation mutations is shown to be a useful technique for investigating the contribution of carbohydrate structure to Ab function.

Structure-function studies of anti-3-fucosyllactosamine (Le(x)) and galactosylgloboside antibodies

We are studying murine mAbs against two carbohydrate epitopes, 3-fucosyllactosamine (Le(x), CD15) and galactosylgloboside. The VH domains of both panels of Ab are encoded by VH441, a member of the X24 family of Ig genes. To evaluate the contribution of the heavy chain CDR3 to the affinity of the anti-3-fucosyllactosamine Ab, CDR3-H of PMN6, a low affinity Ab, was replaced by the CDR3 of PM81, a higher affinity Ab. The affinity of the chimeric 6/81 Ab was increased when the heavy chain was paired with the PM81 light chain, but not when paired with another light chain (M5), which differs from PM81 light chain by three amino acids. To evaluate the contribution of somatic mutations to the binding of GalGb4, the 3A9 VH sequence, which contains three amino acid substitutions, was replaced by a germ-line sequence encoded by either VH441 or VHX24. The chimeric Ab, 441/3A9 and X24/3A9, bound Ag as well as the wild-type 3A9 Ab. Computer models of the Fv fragments of PM81 and 3A9 were compared with the crystal structure of the Fv fragment of J539, a galactan-binding myeloma protein that is encoded by the same VH and VK genes as 3A9. The surfaces of 3A9 and J539 have shallow pockets that are potential Ag-binding sites. Replacement of CDR3-H Tyr99, which is a prominent component of the pocket, by Ala abolished the binding of Ag. In contrast, the Fv surface of PM81 contains a large cleft rather than a pocket. These models indicate how the same VH gene segment can be used to encode Abs that exhibit different specificities.

Structure-function studies of anti-3-fucosyllactosamine (Le(x)) and galactosylgloboside antibodies

We are studying murine mAbs against two carbohydrate epitopes, 3-fucosyllactosamine (Le(x), CD15) and galactosylgloboside. The VH domains of both panels of Ab are encoded by VH441, a member of the X24 family of Ig genes. To evaluate the contribution of the heavy chain CDR3 to the affinity of the anti-3-fucosyllactosamine Ab, CDR3-H of PMN6, a low affinity Ab, was replaced by the CDR3 of PM81, a higher affinity Ab. The affinity of the chimeric 6/81 Ab was increased when the heavy chain was paired with the PM81 light chain, but not when paired with another light chain (M5), which differs from PM81 light chain by three amino acids. To evaluate the contribution of somatic mutations to the binding of GalGb4, the 3A9 VH sequence, which contains three amino acid substitutions, was replaced by a germ-line sequence encoded by either VH441 or VHX24. The chimeric Ab, 441/3A9 and X24/3A9, bound Ag as well as the wild-type 3A9 Ab. Computer models of the Fv fragments of PM81 and 3A9 were compared with the crystal structure of the Fv fragment of J539, a galactan-binding myeloma protein that is encoded by the same VH and VK genes as 3A9. The surfaces of 3A9 and J539 have shallow pockets that are potential Ag-binding sites. Replacement of CDR3-H Tyr99, which is a prominent component of the pocket, by Ala abolished the binding of Ag. In contrast, the Fv surface of PM81 contains a large cleft rather than a pocket. These models indicate how the same VH gene segment can be used to encode Abs that exhibit different specificities.

Generation and characterization of a mouse/human chimeric antibody directed against extracellular matrix protein tenascin

The murine anti-tenascin monoclonal antibody 81C6, following iodination, has been shown to be an efficient localizing and therapeutic agent in both subcutaneous and intracranial human glioma xenograft models in athymic mice and rats. Similarly, effective monoclonal antibody 81C6 localization has been demonstrated in glioma patients, and Phase I trials with the intact murine IgG2b kappa molecule are currently in progress. In order to maximize the potential for repeated administration by minimizing murine Fc-mediated immunogenicity and reducing Fc-mediated immune effects, we created murine 81C6 variable region/human IgG2 chimeric monoclonal antibodies by the molecular cloning of the variable region genes of mouse 81C6 and their genetic linkage to human constant region exons. The resulting chimeric constructs were introduced into SP2/0 cells, and stable transfectomas were selected by G418 and mycophenolic acid resistance. The resistant clones were screened for anti-tenascin activity on tenascin-coated plates by enzyme-linked immunosorbent assay. The N-terminal amino acid sequence of both heavy and light chains of the purified chimeric 81C6 antibody matched exactly with that of the native mouse 81C6 as well as with that deduced from the nucleotide sequence. The production level of chimeric 81C6 (13.9 mg/ml) from ascites in the highest expressing transfectoma was much higher than that of native mouse 81C6 (2.5 mg/ml). The chimeric antibody showed the same specificity and equivalent affinity for human intact tenascin or tenascin-expressing cells as the native mouse 81C6 antibody. Direct comparison of radioiodinated chimeric and radioiodinated mouse 81C6 biodistribution in subcutaneous and intracranial xenograft-bearing mice showed higher tumor-to-normal tissue ratios for chimeric 81C6 as compared with native mouse 81C6. The improved localizing and clearance characteristics of chimeric 81C6 in xenograft model systems suggests that chimeric 81C6 would be an improved reagent for intracompartmental therapy of tenascin-expressing tumors in the human central nervous system.