C1q binding to chimeric monoclonal IgG3 antibodies consisting of mouse variable regions and human constant regions with shortened hinge containing 15 to 47 amino acids

Population-specific diversity of the immunoglobulin constant heavy G chain (IGHG) genes

Human immunoglobulin G (IgG) molecules, IgG1, IgG2 and IgG3, exhibit substantial inter-individual variation in their constant heavy chain regions, as discovered by serological methods. This polymorphism is encoded by the IGHG1, IGHG2, and IGHG3 genes and may influence antibody function. We sequenced the coding fragments of these genes in 95 European Americans, 94 African Americans, and 94 Black South Africans. Striking differences were observed between the population groups, including extremely low amino acid sequence variation in IGHG1 among South Africans, and higher IGHG2 and IGHG3 diversity in individuals of African descent compared to individuals of European descent. Molecular definition of the loci illustrates a greater level of allelic polymorphism than previously described, including the presence of common IGHG2 and IGHG3 variants that were indistinguishable serologically. Comparison of our data with the 1000 Genome Project sequences indicates overall agreement between the datasets, although some inaccuracies in the 1000 Genomes Project are likely. These data represent the most comprehensive analysis of IGHG polymorphisms across major populations, which can now be applied to deciphering their functional impact.

Coming together at the hinges: Therapeutic prospects of IgG3

The human IgG3 subclass is conspicuously absent among the formats for approved monoclonal antibody therapies and Fc fusion protein biologics. Concern about the potential for rapid degradation, reduced plasma half-life, and increased immunogenicity due to marked variation in allotypes has apparently outweighed the potential advantages of IgG3, which include high affinity for activating Fcγ receptors, effective complement fixation, and a long hinge that appears better suited for low abundance targets. This review aims to highlight distinguishing features of IgG3 and to explore its functional role in the immune response. We present studies of natural immunity and recombinant antibody therapies that elucidate key contributions of IgG3 and discuss historical roadblocks that no longer remain clearly relevant. Collectively, this body of evidence motivates thoughtful reconsideration of the clinical advancement of this distinctive antibody subclass for treatment of human diseases. Abbreviations: ADCC - Antibody-Dependent Cell-mediated CytotoxicityADE - Antibody-dependent enhancementAID - Activation-Induced Cytidine DeaminaseCH - Constant HeavyCHF - Complement factor HCSR - Class Switch RecombinationEM - Electron MicroscopyFab - Fragment, antigen bindingFc - Fragment, crystallizableFcRn - Neonatal Fc ReceptorFcγR - Fc gamma ReceptorHIV - Human Immunodeficiency VirusIg - ImmunoglobulinIgH - Immunoglobulin Heavy chain geneNHP - Non-Human Primate.

Structural difference in the complement activation site of human IgG1 and IgG3

The C1q binding epicentre on IgG molecules involves residues Asp(270), Lys(322), Pro(329) and Pro(331) in the C(H)2 domain. IgG1 and IgG3 are usually the most efficient of the four human IgG subclasses in activating complement and they both share all these residues. To reveal possible differences in the structural requirement for complement activation, we created a number of NIP (5-iodo-4-hydroxy-3-nitro-phenacetyl) specific IgG1 and IgG3 antibodies with parallel mutations in or near the putative C1q binding site. The mutants were tested simultaneously for antibody induced, antibody-dependent complement-mediated lysis (ADCML) at high and low antigen concentration on the target cells using sera of human, rabbit and guinea pig as complement source. In addition, we tested the antibodies against target cells decorated with the NP hapten, which has 10-fold lower affinity for the antibodies compared to the NIP hapten. We also used ELISA methods to measure complement activation. We observed a clear difference between IgG1 and IgG3 localized to residues Asp(270), Leu(334), Leu(335). For all these residues, and especially for Asp(270), IgG1 was heavily reduced in complement activation, while IgG3 was only moderated reduced, by alanine substitution. This difference was independent of the long hinge region of IgG3, demonstrated by hinge region truncation of this isotype such that it resembles that of IgG1. This report indicates the presence of structural differences between human IgG1 and IgG3 in the C1q binding site, and points to a specialization of the two isotypes with respect to complement activation.

Modulation of the effector functions of a human IgG1 through engineering of its hinge region

We report here the engineering of a humanized anti-human EphA2 mAb (mAb 12G3H11) in an effort to explore the relationship between the hinge of a human IgG1 and its effector functions. mAb 12G3H11, used here as a model, is directed against the human receptor tyrosine kinase EphA2, which is an actively investigated target for cancer therapy due to its up-regulation in many cancer cells. Various rational modifications were introduced into the hinge region of mAb 12G3H11. These mutations were predicted to modulate the hinge's length, flexibility, and/or biochemical properties. We show that the upper and middle hinge both play important, although functionally distinct roles. In particular, middle hinge modifications predicted to decrease its rigidity or length as well as eliminating either one of its two cysteine residues had a strong negative impact on C1q binding and complement-dependent cytotoxicity. Disruption of covalent bonds between both H chains may account in part for these effects. We also describe middle hinge mutants with a significantly decreased ability to bind FcgammaRIIIA and trigger Ab-dependent cell-mediated cytotoxicity. Conversely, we also generated upper hinge mutants exhibiting an increase in C1q binding and complement-dependent cytotoxicity activity. Therefore, this approach represents a novel strategy to fine-tune the biological activity of a given human IgG1. We also define, for the first time in such a systematic fashion, the relationship between various characteristics of the middle and upper hinge and the corresponding effector functions.

A mutant human IgG molecule with only one C1q binding site can activate complement and induce lysis of target cells

There are potentially two binding sites for C1q on IgG, one on each C(H)2 domain of the gamma heavy chains, close to the lower hinge region. It is not clear whether the presence and involvement of both the C1q binding sites is necessary to induce the activation signal of human IgG. In order to clarify this issue, we made a hybrid mutant IgG1/IgG3 molecule where the IgG1 half of the molecule was made unable to activate complement through the introduction of a P329A mutation. The IgG3 half of the molecule was mutated to harbor a hinge region identical to that of IgG1, and for detection a peptide tag derived from p21ras was introduced into the FG loop of the C(H)1 domain. The hybrid IgG1P329A/IgG3h1-ras molecules were isolated by Protein A affinity chromatography and shown to activate complement and induce complement-mediated lysis at the same levels as wild-type IgG1 and IgG3h1-ras molecules. Thus, one C1q binding site per IgG is sufficient to induce activation. Wild-type human IgG molecules might also normally expose only one C1q binding site as already shown for interaction with FcgammaR, were IgG expose one binding site per molecule.

A point mutation in the Ch3 domain of human IgG3 inhibits antibody secretion without affecting antigen specificity

Immunoglobulins (Ig) require correct folding and assembly of both heavy (H) and light (L) chains to form a functional H2L2 dimer that is secreted from plasma cells. This process is dependent upon the endoplasmic reticulum (ER) chaperone BiP, which targets improperly, folded or assembled Ig molecules for degradation. While investigating the mechanism of low IgG3 secretion, we identified a missense mutation L368P in the Ch3 region of the human gamma3 H-chain that was associated with impaired secretion of intact and functional Ig. The non-secreted H-chains displayed slower electrophoretic migration than secreted H-chains, consistent with them being glycosylated in the ER but not fully processed in the golgi apparatus and secretory pathway. Reversion of the mutated codon to wild type restored secretion of the IgG3, which displayed the same fine specificity for antigen as non-secreted IgG3. However, the non-secreted IgG3 was not opsonic in an in vitro phagocytosis assay. The results indicate that correct IgG3 Ch3 domain folding is essential for secretion and effective function but does not affect specificity for antigen.

Differential segmental flexibility and reach dictate the antigen binding mode of chimeric IgD and IgM: implications for the function of the B cell receptor

Mature, naive B cells coexpress IgD and IgM with identical binding sites. In this study, the binding properties of such IgM and IgD are compared to determine how size and shape may influence their ability to bind Ag and thus function as receptors. To dissect their intrinsic binding properties, recombinant IgM and IgD were produced in soluble form as monomers of the basic H(2)L(2) Ab architecture, each with two Ag binding sites. Since these sites are connected with a hinge region in IgD and structural Ig domains in IgM, the two molecules differ significantly in this region. The results show that IgD exhibited the larger angle and longer distance between its binding sites, as well as having the greater flexibility. Relative functional affinity was assessed on two antigenic surfaces with high or low epitope density, respectively. At high epitope density, IgM had a higher functional affinity for the Ag compared with IgD. The order was reversed at low epitope density due to a decrease in the functional affinity of IgM. Studies of binding kinetics showed similar association rates for both molecules. The dissociation rate, however, was slower for IgM at high epitope density and for IgD at low epitope density. Taken together, the results show that IgM and IgD with identical Ag binding regions have different Ag binding properties.

Human and murine immunoglobulin expression vector cassettes

We describe the construction of new immunoglobulin (Ig) expression vectors and their use in the production of recombinant chimeric Ig molecules in transfected mammalian cells. The vectors contain the cDNA encoding the constant regions of human (mu, alpha1, gammal, gamma2, gamma3, gamma4, kappa) and murine (mu, gamma2a, kappa) Ig heavy and light chains. Unique restriction sites flanking the Ig variable region allow for replacement of variable regions generated by PCR. The CMV promoter allows for the transfection and expression of Ig in non-lymphoid cells. Distinct drug selection markers for heavy chain and light chain expression vectors allows for sequential or co-transfection of the vectors. We show that secretion of recombinant Ig can reach 1.2 microg/ml per million cells per day for transfected B cells. Replacement of the variable region results in the production of functional Ig retaining antigen specificity.

Structural requirements for incorporation of J chain into human IgM and IgA

J chain is associated with pentameric IgM and dimeric IgA via disulfide bonds involving the penultimate cysteine residue in the secretory tailpiece of the mu or the alpha heavy chain. We have investigated the structural basis for incorporation of J chain by analyzing several IgM mutants, IgA mutants and IgG/IgM hybrid molecules. IgM mutants with the mu secretory tailpiece replaced by the alpha secretory tailpiece and/or Cys414 replaced by serine incorporated J chain, although in reduced amounts correlating with reduced pentamer/polymer formation. In addition to pentamers, tetramers of IgMC414S contained J chain, while no J chain was associated with smaller polymers or hexamers of IgM. An IgA/IgM hybrid tailpiece abolished J chain incorporation to pentameric IgM. Analysis of IgG molecules that have added a secretory tailpiece and/or have IgM domain replacements showed that J chain incorporation depends on regions of the C(mu)4 domain in addition to the tailpiece. Features of the C(mu)3 domain other than Cys414 also play a role in efficient formation of pentamers and J chain incorporation, while the C(mu)2 domain is not specifically required. By analysis of two IgA mutants that formed larger polymers than IgAwt, we found J chain equally incorporated into dimers, trimers, tetramers and pentamers. Thus, the results show that J chain incorporation into IgA does not depend on the polymeric structure, while J chain incorporation into IgM is restricted to certain polymeric conformations.

Comparisons of the Ability of Human IgG3 Hinge Mutants, IgM, IgE, and IgA2, to Form Small Immune Complexes: A Role for Flexibility and Geometry

Various native and hinge-modified forms of Ig with identical Ids were reacted with an anti-Id mAb, and the resultant immune complexes were analyzed by negative stain immunoelectron microscopy. Complexes were scored for their geometry (linear versus ring complexes) and size (dimer, trimer, etc.). Ring dimers are the thermodynamically most favorable configuration, unless inhibited by steric and/or flexibility constraints. We found ring dimerization to correlate with the length of the upper, but not middle or lower, hinge. In contrast, the geometry and size of complexes of those molecules lacking formal hinges were unpredictable. A hingeless IgG mutant and native IgE readily formed ring dimers. Remarkably, monomeric IgM formed more ring dimers than any of the other Igs tested, including IgG3. We also tagged the Fab arms and measured the mean Fab-Fab angles and the degree of angular variation for each type of Ig. Surprisingly, IgM proved the most flexible by this assay. In hinged Igs, there was a correlation between length of the upper hinge and Fab-Fab flexibility. In contrast, we found no correlation between the mean Fab-Fab angle in uncomplexed Igs and their ability to dimerize with anti-Id mAb. These data suggest that the physicochemical methods typically used to evaluate molecular flexibility are often of low predictive value when tested in a functional assay.

IgM secretory tailpiece drives multimerisation of bivalent scFv fragments in eukaryotic cells

BACKGROUND

The monoclonal antibody (mAb) TP-3 binds selectively to human and canine osteosarcoma (OS) cells and is therefore a potential candidate for use as a targeting agent in radioimmunoimaging and therapy of OS metastases. However, intact murine mAbs have several drawbacks such as large size, delayed blood clearance and high immunogenicity, all of which can be overcome by genetic engineering.

OBJECTIVES

To construct and express bivalent and multivalent TP-3 scFv fragments from the mammalian expression vector, pLNO. This vector has unique restriction sites for simple cassette cloning of any individual variable (V) and constant (C) genes and has previously been used for expression of intact chimeric TP-3 mAbs and Fab fragments. Furthermore, it is also suitable for expression of any modified V region, such as a scFv fragment, fused to any modified C region or to non-immunoglobulin protein sequences.

STUDY DESIGN

Six different constructs were made; three scFv-CH3 fragments that differed in the design of linker between the scFv fragment and the IgG CH3 domain. These constructs were also made with the IgM secretory tailpiece (microtp) attached to the C terminus.

RESULTS

All constructs were secreted as bivalent antibody fragments with a molecular weight of about 100 kDa. A band corresponding to a dimer appeared in all the supernatants from TP-3 scFv-CH3 producing cells, whether microtp was present or not, whereas higher orders of multimers were not seen. However, pulse chase analyses of the cells revealed that a small fraction of higher order polymers was formed from genes including the fragment encoding microtp and that microtp conferred retention both to monomers and intermediate polymers. The recombinant TP-3 antibody fragments were shown to bind human OS cells.

CONCLUSION

Recombinant mAb fragments can be designed and cloned into the mammalian expression vector, pLNO. This vector is flexible in the sense that the genes encoding such fragments can be expressed from either cDNA or from genomic DNA. A microtp attached to the CH3 domain in these fragments was sufficient to drive polymerization, however inefficiently and intracellular retention of both monomers and intermediate polymers was observed.

Versatile vectors for transient and stable expression of recombinant antibody molecules in mammalian cells

We have developed new cassette expression vectors for the cloning of any intact V-region gene followed by any C-region gene. Both the heavy-and light chain vectors harbor a strong hCMV promoter, restriction site cassettes for cloning of both V- and C-region genes, transcription termination signals, fl-ori for single stranded DNA (ssDNA) synthesis, selection marker for Neomycin and SV40 ori for transient expression. The vectors accept VH and VL chain genes obtained by RT-PCR. Reamplification of the V genes is then performed with a new set of primers which are designed specifically for each individual V gene. Cloning into the vectors is aided by restriction sites located just outside the V-gene coding region, thus keeping the V-genes intact. The vectors also contain cloning sites for the exchange of genomic C-genes so that the resulting Ig genes may code for complete antibodies, antibody fragments or fusion proteins. A simple subcloning step permits the expression of both heavy and light chain genes from one single vector, thus avoiding co-transfection of the two vectors. The usefulness of the vectors was confirmed by construction of mouse-human chimeric antibodies. The V-genes were derived from a hybridoma cell line, TP-3, and was combined with human C kappa, C gamma 3 and C gamma 1 genes as well as with CH1 gamma 3. High yields of recombinant antibody products in NSO cells were obtained. Transient expression was also demonstrated.

The structural requirements for complement activation by IgG: does it hinge on the hinge?

The flexibility of antibody molecules principally derives from the structure of the hinge region. It has generally been accepted that the flexibility of the IgG hinge is necessary for complement activation to occur; however, recent studies dispute this premise. As described here by Ole Henrik Brekke, Terje Michaelsen and Inger Sandlie, it now appears that the only requirement of the hinge region for complement activation is the presence of inter-heavy-chain disulfide bond(s). Furthermore, the structural basis for the differences between IgG subclasses with respect to effector functions appear to be located within the CH2 domain of the immunoglobulin molecule.

Human IgG isotype-specific amino acid residues affecting complement-mediated cell lysis and phagocytosis

In this report we describe the construction of anti-5-iodo-4-hydroxy-3-nitrophenacetyl (NIP) mouse/human immunoglobulin (Ig) G4 chimeric molecules with altered amino acid residues in the CH2 domain. Three mutants are described. Gln-268 is substituted by His in gamma 4 Q268H, Ser-331 is substituted by Pro in gamma 4 S331P, and in gamma 4 Q268H/S331P both residues are substituted. The ability of the mutant molecules to induce complement-mediated cell lysis (CML) and phagocytosis by Fc gamma RII- and Fc gamma RIII-bearing polymorphonuclear leukocytes (PMN) were measured. In CML, gamma 4 Q268H was inactive, but both gamma 4 S331P and gamma 4 Q268H/S331P were active provided that the antigenic density on the target cells was high. In phagocytosis mediated by PMN, the mutants gamma 4 S331P and gamma 4 Q268H/S331P were both active only when complement was introduced. gamma 4 Q268H was not active in phagocytosis under any conditions. We conclude that His-268 in human IgG molecules does not modulate CML activity or phagocytosis mediated by Fc gamma RII and/or Fc gamma RIII. Pro-331 rescues CML activity in IgG4 molecules when the epitope density on the target cells is high, but does not affect Fc gamma RII/Fc gamma RIII-mediated phagocytosis. In this manner the mutants gamma 4 S331P and gamma 4 Q268H/S331P mimic human IgG2. This could indicate a structural similarity between IgG2 and these mutant molecules that distinguish them from both IgG1 and IgG3.

One disulfide bond in front of the second heavy chain constant region is necessary and sufficient for effector functions of human IgG3 without a genetic hinge

We have created four IgG3 mutants without a normal hinge region: (i) m0 without a genetic hinge; (ii) m0/C131S, where Cys-131 in m0 was mutated to Ser; (iii) m0/231C232 (formerly HM-1), where a Cys residue was inserted in m0 between Ala-231 and Pro-232; (iv) m0/C131S/231C232, which is a hybrid of m0/231C232 and m0/C131S. The wild-type IgG3 and all mutants bind 5-iodo-4-hydroxy-3-nitrophenacetyl groups. The wild type and mutants, m15 (with 15 aa in the hinge), m0/231C232, and m0/C131S/231C232, were all positive for complement-mediated lysis, antibody-dependent cellular cytotoxicity mediated by peripheral blood leukocytes, and phagocytosis by U937. m0/C131S/231C232 was only weakly positive and sometimes negative for respiratory burst activity mediated by peripheral blood neutrophils (polymorphonuclear leukocytes), whereas m15, m0/231C232, and wild-type IgG3 were strongly positive. The m0 and m0/C131S mutants were mainly negative for complement-mediated lysis, antibody-dependent cell-mediated cytotoxicity, and phagocytosis by U937 and polymorphonuclear leukocytes. The results indicate that a hinge spacer region is not necessary, but the correct alignment of the two second heavy chain constant regions in the IgG3 molecule by a minimum of one disulfide bond is necessary and sufficient for effector functions.

Monitoring the formation of soluble immune complexes composed of idiotype and anti-idiotype antibodies by electron microscopy

We have previously used immunoelectron microscopy (IEM) to generate a three-dimensional map of idiotypic (Id) and isotypic epitopes on the Fab arms of HGAC 39 (Roux et al., 1987, Proc. natn. Acad. Sci. U.S.A. 84, 4984-4988), a mouse IgG3 monoclonal antibody (Mab). In this report, we analyse the geometry of the various types of immune complexes formed by the interaction of HGAC 39 with Mab directed against four mapped epitopes. Moreover, by sampling of reaction mixtures over time, we show that the kinetics of each of the subpopulations of immune complexes, as defined by geometric configuration, can be determined. The data show that for each antibody (Ab)-HGAC 39 combination the rate of immune complex formation was greatest during the first 1.5-3.5 min but that additional complexes formed through the remainder of the half hour assay period. As anticipated, complexes composed of even number units predominated (primarily dimers and tetramers) and most of these were in the form of closed rings. The data also suggest that the location and orientation of the epitopes on HGAC 39 to which the monoclonal antibodies were bound has an influence on the types of immune complexes generated. Specifically we observed that those anti-idiotype Abs that bind to the distal tip of Fab arms (i.e. in the CDR) are less likely to produce bivalently associated ringed dimers than antibodies that bind to epitopes that are proximal to the CDR and that project laterally from the surface of the Fab arms. These data are interpreted in terms of restrictions on hinge mediated flexibility and steric inhibition between adjacent Fab arms on HGAC 39.

Human IgG3 can adopt the disulfide bond pattern characteristic for IgG1 without resembling it in complement mediated cell lysis

In this paper we describe the construction of mouse-human IgG3 mutant antibodies resembling IgG1 in their disulfide bond pattern between the heavy and light chain (H-L) and between the two heavy chains (H-H). The effector functions of these mutant antibodies were compared to normal IgG3 and IgG1. Changing only the disulfide bond pattern between the heavy and light chains did not alter the ability to induce complement mediated cell lysis (CML), regardless of the amount of corresponding antigen that had been introduced to the surface of the target cells. However, alteration of the disulfide bond pattern between the two heavy chains had a large effect on CML due to shortening of the hinge from 62 to 15 amino acids. No difference between the mutants and normal antibodies in antibody-dependent cell-mediated cytotoxicity (ADCC) was observed. This suggests that IgG3 can adopt the H-L disulfide bond pattern of IgG1 without obtaining the CML activity characteristic for IgG1.

The extended hinge region of IgG3 is not required for high phagocytic capacity mediated by Fc gamma receptors, but the heavy chains must be disulfide bonded

Fc gamma receptor (Fc gamma R) phagocytosis and respiratory burst were induced by chimeric mouse-human anti-(4-hydroxy-5-iodo-3-nitrophenyl) acetyl IgG3 antibodies with mutations in hinge and/or in CH1 region. IgG3 mutants with different hinge length ranging from 47 to 0 amino acids, an IgG3 molecule with an artificial hinge of just one cysteine residue (HM-1), and two hybrid IgG3 molecules with IgG4 hinge or IgG4 CH1-hinge were tested. Using the monocytic cell line U937 as effector cells, the mutated IgG3 molecules were very similar, revealing high activity, while the IgG3/IgG4 hybrids revealed a slightly reduced activity. However, the hingeless (0-h) mutant was negative, except after interferon-gamma stimulation when it became slightly positive. Interestingly, HM-1 was as active as the IgG3 mutants. With polymorphonuclear leucocytes (PMN) as effector cells we obtained some day-to-day variations, but all the IgG3 mutants were highly active, with the two shortest hinge mutants somewhat less active. The IgG3/IgG4 hybrid molecules revealed an intermediate activity, while IgG4 wild-type and the 0-h mutant were negative. However, the HM-1 molecule revealed an activity similar to that of the IgG3 mutants. The phagocytic activity of U937 was inhibited by monomeric IgG, indicating the importance of Fc gamma RI. In contrast, with PMN both blockage of Fc gamma RII and cleavage of Fc gamma RIII were required to significantly reduce the phagocytosis and respiratory burst, thus showing that both receptors contribute to the effect. These results demonstrate that the extended IgG3 hinge region is not necessary for a high phagocytic activity and that the major structural importance of the hinge is to connect the two heavy chains in this region.

Activation of complement by an IgG molecule without a genetic hinge

The hinge region links the two Fab arms to the Fc portion of the IgG molecule. It mediates flexibility to the molecule and serves as a connecting structure between the two heavy chains. In addition it provides space between the Fab and Fc parts. All three properties have been proposed to be important for the ability of IgG to initiate complement activation leading to complement-mediated cell lysis (CML). Here we report the construction of a hinge-deleted mouse-human chimaeric IgG3 molecule with specificity for the hapten NIP (3-iodo-4-hydroxy-5-nitrophenacetyl), HM-1. HM-1 lacks the genetic hinge, but has an introduced cysteine between Ala 231 (EU numbering) and Pro 232 in the lower hinge encoded by the CH2 exon. The introduced cysteine forms a disulphide bond between the two heavy chains of the molecule. In CML, HM-1 shows a greater activity than IgG3 wild type. This is the first time an IgG molecule without a genetic hinge has been found to be active in CML. We conclude that the hinge functioning as a spacer is not a prerequisite for complement activation. Rather, its major role seems to be to connect the heavy chains to each other in the amino-terminal part of CH2. Because HM-1 is expected to have low Fab-Fc flexibility, this molecular feature is probably of no importance for complement activation.

Peripheral selection of antigen receptor junctional features in a major human gamma delta subset

Recent studies demonstrating the existence of murine gamma delta T cell subsets with structurally identical T cell receptors (TcR) suggest that unlike alpha beta T cells, some gamma delta T cells are specialized in the recognition of a limited number of monomorphic antigens. However, this question still remains open in humans, since the TcR structural diversity of their peripheral gamma delta T cells was shown to be extensive. Here we have analyzed in detail the TcR chain genes expressed by human V gamma 9+V delta 2+ peripheral blood lymphocytes (PBL), a major peripheral gamma delta T cell subset in adults and present evidence for an antigen-driven peripheral selection of both TcR gamma and delta junctional motifs among these cells. First, it is shown that the proportion of V gamma 9+V delta 2+ cells expressing the V9JPC1 gamma chain is much higher among PBL than among thymus-derived clones, indicating that preferential use of this J gamma segment is not due to pairing or combinatorial constraints. Second, analysis of V9JPC1 gamma transcripts derived from V gamma 9+V delta 2+ PBL clones revealed a high prevalence of a unique V9JP gamma sequence with limited "N" nucleotide additions and VJ trimming, which could not be accounted for by enzymatic or antigen-independent structural limitations. Third, the TcR delta chain expressed by most V gamma 9+V delta 2+ PBL clones, though diverse in amino acid composition and length, carried a highly distinctive junctional motif, found at a much lower frequency among V2DJ delta sequences derived from V gamma 9-V delta 2+ PBL or V gamma 9+V delta 2+ thymocytes. Together, these results which demonstrate shared gamma and delta junctional features by cells using unique V gamma and V delta genes, suggest that in vivo selection of V gamma 9+V delta 2+ lymphocytes is mediated by a highly restricted number of nominal ligands.

Epitope mapping of human immunoglobulin-specific murine monoclonal antibodies with domain-switched, deleted and point-mutated chimeric antibodies

27 engineered chimeric antibodies possessing human gamma, epsilon, mu or alpha constant regions and V region specificity for nitrophenyl or dansyl were used to study the isotype specificity of 29 murine monoclonal antibodies (MAbs) specific for human immunoglobulins (IgG1-4, IgE, IgM, IgA or secretory piece). The isotype-restricted immunoreactivity observed with wild-type chimeric antibodies paralleled the pattern of each MAb's reactivity with purified human myeloma proteins. 16 mutant IgG anti-dansyl chimeric antibodies with genetically engineered domain switches, deletions or point-mutations were used as antigens to further characterize the epitopes recognized by the human IgG subclass-specific MAbs. The binding of three human IgG1-specific MAbs (HP6069, HP6070 and HP6091) was mapped to similar epitopes on the CH2 domain of human IgG1. Of the two anti-human IgG2 MAbs tested, HP6002 reacted with the CH2 of IgG2 while HP6014 bound to the CH1 domain. Both anti-human IgG3 MAbs (HP6047, HP6050) reacted with different regions of the IgG3 hinge. The anti-human IgG4 MAbs (HP6023, HP6025) bound to a similar epitope on the carboxyl terminus of CH2 or the CH3 of human IgG4. The three exclusion antibodies (HP6019, HP6030 and HP6058) bound to different epitopes in the CH2 domain of three of four IgG subclasses. The domain mapping was confirmed by competitive inhibition experiments. These results were used to select a group of IgG-reactive MAbs for construction of a poly-monoclonal anti-IgG capture and detection reagent that uniformly bound all four subclasses of human IgG. This study provides support for the use of engineered chimeric human chimeric antibodies as replacements for increasingly rare, purified human paraproteins in the specificity analysis of immunochemical reagents used in clinical and research laboratories for the detection and quantitation of human antibodies. Moreover, these studies demonstrate how the MAbs can serve as effective probes for examining conformational differences among the four human IgG subclasses.

Hybrid antibodies

One of the major advantages of genetic engineering is the ability to produce novel, hybrid antibodies. Hybrid antibodies can be assembled using fragments from different antibodies with the objective of assembling novel combinations of antibody-related effector functions. To efficiently achieve this goal it is necessary to have a precise understanding of the structure-function relationships within the antibody molecule. Secondly, it is possible to produce hybrids of antibodies with non-immunoglobulin proteins thereby achieving unique combination of functional properties. In this case it is necessary to consider both the desired functional properties and the means of assembling the protein components so as to maintain these properties. In all cases it is necessary to have the cloned gene segments, appropriate vectors and expression systems.

Antibody dependent cell-mediated cytotoxicity induced by chimeric mouse-human IgG subclasses and IgG3 antibodies with altered hinge region

A matched set of chimeric mouse-human NP-antibodies were studied for the capacity to induce cell mediated cytotoxicity (ADCC) by normal peripheral blood NK/K cells. The target cells were sheep red blood cells (SRBC) sensitized with the haptens NP or NIP. All four IgG subclasses and several IgG3 variants with altered hinge were tested for ADCC activity. The hierarchy of the ADCC capacity among the subclasses was found to be IgG3 greater than IgG1 greater than IgG4 greater than IgG2. The superiority of IgG3 was only revealed at low effector cell:target cell ratio. The ADCC activity was for the most part unaltered by shortening the hinge region of IgG3 from 62 to 15 amino acids. Also, when the hinge region of IgG3 was mutated to become identical to that of IgG4, the ADCC activity was mainly unchanged. However, an IgG3 variant with deletion of all four hinge exons showed a depressed ADCC activity compared to the wild type. The IgG subclass pattern of complement-mediated lysis (CML) and ADCC is different and the capacity to induce CML and ADCC is changed differently by hinge region modification. Thus CML and ADCC have different structural requirements in the Fc region of IgG.

Chimeric mouse human IgG3 antibodies with an IgG4-like hinge region induce complement-mediated lysis more efficiently than IgG3 with normal hinge

We have altered the amino acid sequence of the hinge and the first constant domain (CH1) of mouse/human chimeric IgG3 antibodies by site-directed mutagenesis, so as to make the sequences identical to those of IgG4. All the mutant antibodies with altered hinge region were more active in complement activation and complement-mediated lysis than native IgG3. The mutations in CH1, however, did not alter the activity. This demonstrates the importance of the hinge region in modulating this effector function. The results show that the primary structure of neither CH1 nor the hinge of IgG4 is responsible for the lack of complement activation shown by this subclass.

Human IgG subclass pattern of inducing complement-mediated cytolysis depends on antigen concentration and to a lesser extent on epitope patchiness, antibody affinity and complement concentration

The relative complement-mediated lytic capability of the IgG subclass isotypes was studied using a matched set of mouse-human chimeric anti-(4-hydroxy-3-nitrophenyl)acetyl (NP) antibodies. The subclass pattern was shown to be highly dependent on variations in antigen concentration and to lesser extent on variation in epitope patchiness, antibody binding affinity and complement concentration. In general, the IgG3 subclass was most effective in inducing cytolysis at the different conditions used and only at high antigen concentration did the IgG1 subclass mediated more efficient cytolysis than IgG3. The IgG2 isotype required a relative high antigen concentration to be cytolytic while the IgG4 isotype was not cytolytic at any of the conditions tested. These individual characters of each of the IgG subclasses makes it conceivable that a subtle system of immunoregulation exists among the subclasses.