V kappa gene usage, idiotype expression, and antigen binding among clones expressing the VHX24 gene family derived from naive and anti-idiotype immune BALB/c mice

The BALB/c myeloma protein ABPC48 binds beta(2-6)-linked fructosans and expresses genes derived from the VHX24 and V kappa 10 gene families. We have selected 30 hybridomas expressing the VHX24 gene family derived from mitogen-stimulated spleen cells of naive BALB/c mice and mice injected at birth with the syngeneic monoclonal anti-ABPC48Id, IDA10. The majority of mAb with kappa L chains uses V kappa 1. Antibodies reacting with IDA10 use both V kappa 10 and V kappa 1. Most of these VHX24+ mAb reacted with one or more members of a limited panel of predominantly polysaccharide Ag that have been previously observed to interact with antibodies expressing the VHX24 gene family. Nucleotide sequencing of selected VH and V kappa genes shows a very low frequency of somatic mutation. The effect of neonatal anti-Id injection on VHX24-V kappa pairing and Id expression is discussed.     

Sequences of the VH and VL regions of murine monoclonal antibodies against 3-fucosyllactosamine

Many mAb that bind the carbohydrate antigenic determinant 3-fucosyl-lactosamine (3-FL), Gal beta 1-4[Fuc alpha-3]GlcNAc-R have been raised in BALB/c mice, and we are studying the structure and regulation of these antibodies. In this report, we present the first information about their amino acid sequences and the Ig gene segments used to encode them. V regions of the H and L chains of three anti-3-FL antibodies, PMN6, PMN29, and PM81, were sequenced by a combination of mRNA and amino acid sequencing. The L chain sequences of PMN6 and PM81 antibodies indicate that their VK and JK regions are encoded by VK24B and JK1 germ-line genes, respectively. The nucleotide and amino acid sequences of the H chains suggest that the three anti-3-FL antibodies are encoded by the VH441 gene segment of the X24 VH family, and this conclusion was supported by Southern filter hybridization with VH441 and JH3-JH4 probes. PMN29 has at least 11 amino acid substitutions, which is an unusually large amount of somatic mutation for an IgM antibody. Previous analyses of BALB/c genomic libraries with VHX24 and VH441 probes make it unlikely that this VH family contains additional germ-line genes, but this possibility cannot be excluded. All three antibodies use the DQ52 and JH4 gene segments. The single VH and VL gene segments used to encode the anti-3-FL antibodies is in contrast to the multiple VH and VL segments used by antibodies against other carbohydrate Ag such as alpha 1-6 dextran and group A streptococcal carbohydrate. VH441 also encodes the VH regions of antibodies against galactan and levan (beta 2-6 fructosan). The similarities among VH segments of antibodies against 3-FL, levan, and galactan, and the striking differences in their CDR3 sequences, suggest that CDR3 plays an important role in the formation of the Ag binding site. The use of a single VH segment from the smallest VH gene family by antibodies against at least three different carbohydrate determinants is noteworthy. It raises the possibility that the amino acid sequence encoded by VH441 has some general structural features that make it particularly well adapted for binding to carbohydrate sequences.     

Mutational analysis of avidity and fine specificity of anti-levan antibodies

Using the polyfructose, bacterial levan, as a model polysaccharide, we analyzed how V regions affect binding in anti-polysaccharide mAbs. Previously, panels of mAb were constructed from bacterial levan-immunized BALB/c and CBA/Ca mice. The BALB/c mAb were mostly germline VHJ606:Vkappa11, and a subset contained presumed somatic mutations in the complementarity-determining regions (CDRs) that correlated with increases in avidity for the beta(2-->1) inulin linkage of levan. The CBA/Ca mAb were more heterogeneous in V gene usage, but a subset of inulin-nonreactive mAb were VHJ606:Vlambda and had VH sequence differences in the CDRs from the VHJ606 regions of the BALB/c mAb. In this report, VHJ606 Abs containing various combinations of specifically mutated H and L chains were produced by engineered transfectants and tested for inulin avidity and levan binding. Two presumed somatic mutations seen in CDRs of the BALB/c hybridomas were shown to directly cause marked increases in avidity for inulin (VH N53H, 9-fold; VL N53I, 20-fold; together, 46-fold) but not for beta(2-->6) levan. Exchange of either positions 50 or 53 in VH or the H3 loop between the BALB/c and CBA/Ca mAb resulted in either fine specificity shift or total loss of bacterial levan binding. Three-dimensional models of the V regions suggested that residues that affect binding to inulin alone are near the edge of the CDR surface, while residues involved with binding both forms of levan and affecting fine specificity are in the VH:VL junctional area.     

Structural basis of a conserved idiotope expressed by an autoreactive human B cell lymphoma. Evidence that a VH CDR3 mutation alters idiotypy and specificity

Our laboratory has previously investigated the relationship of autoimmune disease and B cell neoplasia in a patient with a diffuse, well differentiated splenic B cell lymphoma and associated autoimmune hemolysis due to an anti-Pr2 antibody. EBV-immortalized B cell clones, established from this lymphoma, were shown to secrete the same pathologic anti-Pr2 antibody. The antiidiotypic mAb, RI.1, defined a private Id (IdRI.1) of the anti-Pr2 antibody that was related to the Ag-binding site and was expressed by both the lymphoma and derived cell lines. This unique Id was expressed by the majority of splenic tumor B cells and also was conserved over a period of 4 yr. In this report, the structural basis of IdRI.1 expression was investigated by analysis of Id- variants isolated by flow microfluorimetry using RI.1. Six Id- cell lines that secrete IgM kappa but lack Pr2 specificity were generated from an Id+ cell line, LS2. These lines were shown to be related to LS2 and the lymphoma by karyotype and by restriction fragment analysis of Ig gene rearrangements. Shared and unshared nucleotide substitutions in the VH and VL regions of the six independent clones were used to construct a genealogic tree relating the Id- clonal members to a common Id+ precursor. The tree illustrates that the base changes occurred sequentially, suggesting that they were introduced by somatic point mutation. Only one VH CDR3 bp difference from the LS2 nucleic acid sequence is common to all Id- sequences, resulting in an amino acid substitution of cysteine 108 to tyrosine. Taken together, these findings suggest that both the expression of IdRI.1 and Ag binding are affected by a single mutation localized to the D region of the anti-Pr2 antibody.     

Strain-dependent restricted VH and VL usage by anti-bacterial levan monoclonal antibodies.

The immune response to polysaccharides is highly regulated and has several distinguishing features, including restricted clonotype and isotype expression. The basis for this highly restricted response is not fully understood. To address these questions in a systematic manner, we have generated a panel of 102 mAb from CBA/CaHN (CBA/Ca) and BALB/cAnN (BALB/c) mice after one and two injections of bacterial levan (BL), a beta(2----6)-linked polyfructosan with beta(2----1)-linked fructose branch points (inulin determinant, In). This panel of mAb was examined for isotype, fine specificity, VH and VL region gene family usage and relationships between these parameters. After one or two injections of BL in both strains, mAb were IgM and IgG3. Fine specificity and VH/VL gene family usage differed markedly, however, between the two strains. Only 4% (2/51) of CBA/Ca mAb recognized the In determinant, whereas 77% (40/51) of BALB/c mAb recognized this epitope. In both strains, VH usage was restricted and certain families were overrepresented. In CBA/Ca mice, the overall response to BL was dominated by VHJ558 (45%, 23/51), the largest VH family, but VH36-60 (27%, 14/51) and VHJ606 (25%, 13/51) were also highly utilized and overrepresented. In BALB/c mice, the overall response to BL was dominated by VHJ606 (79%, 39/49 designated), a relatively small VH family. More importantly, after a single immunization with BL one particular VH/VL pair (VHJ606/ kappa 11) was used by 88% (36/41) of BALB/c mAb and was associated with In reactivity. In summary, BALB/c and CBA/Ca responses to BL differ in fine specificity and VH/VL usage.     

Junctional diversity of H and L chains allows the coexpression of two mutually exclusive idiotopes (IdI104 and IdI558).

The molecular basis for the unexpected coexpression of the individual Id (IdI)558 and IdI104 Id by anti-alpha(1-3) DEX antibody (Ab) (126.33 and 414.2) derived from the MPW wild mouse strain has been investigated by the comparison of the structures of their VH and V lambda 1 chain regions with those of two other MPW-derived Ab (262.9 and 16.3) expressing either IdI558 or IdI104 Id. Our data show that 262.9 and 16.3 Ab display identical V lambda 1 and very similar VH regions when compared with BALB/c anti-alpha (1-3) dextran Ab expressing IdI104 or IdI558, respectively. The two Ab (414.2 and 126.33) that express both IdI104 and IdI558 Id display two main features. First, their VH CDR3 are different from those found in IdI104 or IdI558 expressing anti-alpha(1-3) dextran Ab. Second, their V lambda 1 are identical to those from BALB/c origin except for the presence of an additional residue, a phenylalanine at position 95A of CDR3. This additional residue is encoded by the V lambda 1 gene segment and results from a hitherto undescribed V lambda 1-J lambda 1 junction. The alteration of the length of the V lambda 1 CDR3 loop, in conjunction with particular residues within VH CDR3, allows the coexpression of two Id that were found to be mutually exclusive in laboratory mice.     

Structure, map position, and evolution of two newly diverged mouse Ig VH gene families.

We have characterized two novel mouse VH gene families, VH3609N and VHSM7. These VH families have recently diverged from previously defined VH families. The VH3609N family, which may contain only one member in most inbred strains of mice, shares sequence similarity with the VHJ606 family and is located to the 3' side of VHJ606. VHSM7, with at least three members, is related to the VHJ558 family but maps 3' of VHJ558. These findings suggest that physical displacement of VH sequences may facilitate their subsequent divergence. During the early stages of VH gene family evolution that are exemplified by these new families, amino acid replacements have been selected against in frame-work regions and selected for in complementarity-determining regions. This pattern of nucleotide substitution appears to reflect evolutionary pressures to maintain germ-line VH diversity and, possibly, to select for new antibody specificities, as well as to select against mutations resulting in aberrant Ig. The classification of VH sequences with borderline similarity to previously defined VH families is discussed.     

Neonatal and adult primary B cells use the same germ-line VH and V kappa genes in their (T,G)-A-L-specific repertoire

Although there is a nonrandom usage of VH gene families by primary B cells early in ontogeny, at issue is whether the preferential rearrangement of 3' germ-line VH genes, e.g., VH7183 and VHQ52 family genes, influences the neonatal B cell repertoire that can be expressed in response to Ag. In order to address this issue, and to determine whether neonatal B cells can use the same germ-line VH and V kappa genes as adult B cells in their primary response, we have analyzed at the molecular level the neonatal antibody response to (T,G)-A-L and compared it with the adult primary response. Among the TGB5 Id+, GT+ antibodies, which dominate the neonatal response to (T,G)-A-L, two VH gene families were used: J558 (high frequency) and 36-60 (low frequency). The majority of Id+ neonatal hybridomas used the same germ-line VH gene (H10, from the VHJ558 family), but with enormous diversity in the D region, and one of two germ-line V kappa 1 genes (V kappa 1A, V kappa 1C). These are the same germ-line V-genes used by most primary adult Id+ hybridomas, and the frequency of expression of this germ-line V-gene combination appears equivalent in the neonatal and adult primary repertoires. Therefore, it is clear from this study that as early as day 5, neonatal B cells can use the same germ-line V-genes as adult primary B cells in their Ag-specific repertoire.     

A set of closely related antibodies dominates the primary antibody response to the antigenic site CB of the A/PR/8/34 influenza virus hemagglutinin

Approximately 50% of the primary antibody response of BALB/c mice to the A/PR/8/34 influenza virus hemagglutinin is directed to the Cb site, one of the four major antigenic regions of the molecule. To determine the structural basis of the anti-Cb site response, we have examined the paratypic and genetic diversity exhibited by a panel of 24 primary and 4 secondary response mAb specific for this antigenic region. Reactivity pattern analysis demonstrated 20 distinct fine specificities among these antibodies, and V region gene sequence analysis showed that they are encoded by 17 different VH gene segments from 6 VH gene families and 14 different VK gene segments from 6 VK gene groups. Despite this overall diversity, many of the antibodies can be placed in a limited number of sets based on the shared expression of VH and/or VK genes. One set contains antibodies encoded by a single gene of the VK4/5 group in combination with one of two closely related genes from the J558 VH family. This set accounts for half of the Cb site-specific primary response hybridomas, indicating that the representation of the various anti-Cb site B cell specificities during the primary response to A/PR/8/34 influenza virus is not uniform. The preferential participation of B cells expressing this VH/VK combination is largely responsible for the dominance of anti-Cb site antibodies in the primary anti-hemagglutinin response.     

Somatic evolution of diversity among anti-phosphocholine antibodies induced with Proteus morganii

The variable region sequences of light and heavy chains (VL and VH) were determined for 11 hybridoma antibodies produced in response to the PC moiety on Proteus morganii. These hybridomas were derived from two separate fusions, one obtained from mice early in a secondary response and the other from late in a secondary response. All of these antibodies possessed a cross-reactive idiotype found on anti-PC antibodies in the M603 family, and exhibited preferential specificity for PC in the context of P. morganii. We found that all of the antibodies were derived from a single VH/VL pair. VH was encoded by V1, DFL16.1 and JH1, and VL was encoded by a consensus VK8 gene and JK5. Antibodies differed from each other by somatic point mutations that occurred at a high rate. The mutations in VL were approximately one-third as abundant as those in VH and were randomly distributed throughout the molecule. Mutations in VH were concentrated in CDR 2 and 3 and had a replacement to silent ratio that was three to six times greater than predicted from random accumulation. Based on the sequence data, a single genealogic tree with multiple branches could accommodate all the hybrids from a fusion. We concluded that in both examples the anti-PC response arose by somatic mutation and stepwise selection from a single precursor. Antigen binding studies with these 11 hybridomas and a 12th that had no mutations revealed that the acquisition of preferential specificity for antigen was dependent on somatic mutation of germline genes. Additional binding studies demonstrated that continued selection during clonal expansion was probably antigen driven. An unexpected finding was five independently selected antibodies from one fusion that had identically mutated VH and VL sequences. We suggest that the hypermutation mechanism is not a continuously active process during clonal expansion and that it is regulated, probably during the mid to late phase of the primary response.     

Nucleotide and translated amino acid sequences of cDNA coding for the variable regions of the light and heavy chains of mouse hybridoma antibodies to blood group A and B substances

This is the first report of nucleotide and translated amino acid sequences of the variable region light (VL) and heavy (VH) chains of mouse monoclonal hybridoma anti-blood group A and B substances, the combining sites of which have been mapped. Monoclonal hybridoma anti-A and anti-B produced in BALB/c mice by immunization with A or B blood group substances, with A1 erythrocytes, and water-soluble blood group A substance or with synthetic B determinants coupled to bovine serum albumin or to O erythrocytes have been characterized immunochemically. To relate the immunochemical properties of the monoclonals to their primary structures, we have cloned and sequenced cDNAs of variable regions of light and heavy chains of two anti-A and two anti-B. The anti-A hybridomas have very similar combining site specificities and have almost identical VH sequences belonging to the J558 germ-line family, but their VL are from different germ-line VK gene families. The two anti-B hybridomas have different combining site specificities and use the same VL which differs completely from the anti-A VL; their VH are derived from different VH germ-line genes belonging to the J606 family. The results suggest that the heavy chains play a major role in determining the specificities of the antibody combining sites, with only minor contribution of VL. Additional sequence data on monoclonal antibodies of defined specificity for blood group substances are needed for further insights into the genetic and structural basis for their specificities.     

Amino acid substitutions in VH CDR2 change the idiotype but not the antigen-binding of monoclonal antibodies to alpha(1----6)dextrans

An idiotype defined by mAb and polyclonal antibodies to 10.16.1, an anti-alpha(1----6) dextran was previously reported to be expressed on most BALB/c anti-alpha(1----6)dextrans with groove-type sites and to involved CDR3 and probably CDR2. By comparing amino acid sequences of VH and VL derived from cDNA of idiotype+ and idiotype- anti-alpha(1----6)dextran hybridoma proteins, an idiotope was assigned to VH CDR2. Substitution of phenylalanine for leucine at residue 52 in CDR2 coupled with amino acid changes at either residue 58 or residues 57 and 60 abolished expression of this idiotype without affecting Ag binding.     

Two families of monoclonal antibodies to alpha(1----6)dextran, VH19.1.2 and VH9.14.7, show distinct patterns of J kappa and JH minigene usage and amino acid substitutions in CDR3.

Nine groove-type mAb to alpha(1----6)dextran were cloned and sequenced. Together with previous reports from this laboratory, the VH and VL of 34 mAb have been sequenced, in which 10 VH19.1.2 and 11 VH9.14.7 combined with the V kappa-Ox1 gene to form two major families of anti-alpha(1----6)dextrans. The same D minigene (DFL16) was used by all VH19.1.2 and VH9.14.7 mAb; however, the patterns of JH and J kappa usage are quite different. VH19.1.2 mAb used only JH3 and J kappa 2, whereas VH9.14.7 mAb used three JH (JH1, JH2, and JH3) and all four active J kappa (J kappa 1, J kappa 2, J kappa 4, and J kappa 5). Relative uniformity in the lengths of VH CDR3 and the junctional sequences is seen in both families. Some mAb from different mouse strains share common structural features. The differences in idiotypic specificities and in the amino acid sequences suggest that VH19.1.2 and VH9.14.7 may differ in the conformation of CDR1 and CDR2. Combining with V kappa-Ox1 gene to generate groove-type combining sites to the single site-filling epitope of alpha(1----6)dextran, the two VH chains may require certain conformations of CDR3. Whether such conformational requirements influence the choice of J minigenes, the selection of the length of VH CDR3 and the sequences at junctions, are discussed.     

Recognition of DNA by VH and Fv domains of an IgG anti-poly(dC) antibody with a singly mutated VH domain

Secondary antigen stimulation usually produces IgG antibodies with hypermutated V segments. Studying a strong secondary response to the polynucleotide antigen poly(dC), however, we found a highly selective IgG antibody (mAb dC7) with only one mutation (a conservative Leu to Ileu substitution) throughout the whole VH domain. To investigate the roles of VH and VL domains in selective binding by this mAb, we prepared its VH, VL and single-chain Fv (scFv) fragments. A bacterial expression system produced soluble monomeric V region proteins. CD spectra confirmed that they had the beta-secondary structure expected for Ig domains. Both the scFv and VH fragments bound to single-stranded non-protonated poly(dC) and to ssDNA but not to protonated, more structured poly(dC) or dsDNA. The VL domain alone did not bind to nucleic acids, but VL association modified the VH binding, giving the scFv a 10-fold higher affinity than the VH for poly(dC) and greatly increasing the cytosine-dependent selectivity. Non-ionic interactions were prominent in the Fv reaction with a (dC)( n) sequence. Ionic interactions were revealed in Fv cross-reactions with ssDNA, and were more prominent in binding of either poly(dC) or ssDNA by VH alone, consistent with the lesser base selectivity of the VH. Thus, the Fv and VH alone bind to a single antigen, poly(dC), but mechanistic differences result from additional subsites in the Fv. Generation of a selective IgG with very few CDR mutations in either VH or VL, which was accompanied by IgM antibodies with unmutated V regions, also suggests that nucleic acid binding activity is a property of the B cell repertoire even before immunization.     

Complete heavy and light chain variable region sequence of anti-arsonate monoclonal antibodies from BALB/c and A/J mice sharing the 36-60 idiotype are highly homologous

Structural and serologic studies on murine A/J monoclonal anti-arsonate antibodies resulted in the identification of a second idiotype family (Id36-60) in addition to the predominant idiotype family (IdCR). Id36-60, unlike IdCR, is a dominant idiotype in the BALB/c strain but is a "minor" idiotype in the A/J strain. The complete heavy and light chain variable region (VH and VL) amino acid sequences of a representative Id36-60 hybridoma protein from both the A/J and BALB/c strains have been determined. There are only four amino acid sequence differences between the VH of antibody 36-60 (A/J) and antibody 1210.7 (BALB/c). Two of these differences arise from single nucleotide changes in which the A/J and BALB/c Id36-60 VH germline gene sequences differ. The two other differences are the result of somatic mutation in hybridoma protein 36-60. In addition, Id36-60 heavy chains employ the same D and JH3 segments in both strains. The entire Vk2 VL of 36-60 and 1210.7 differ by only two amino acids, suggesting that like the heavy chains, they are derived from highly homologous VL genes. The same Jk segment is used in both antibodies. A comparison of the amino acid sequence data from Id36-60-bearing hybridomas suggests that a heavy chain amino acid difference accounts for the diminished arsonate binding by the 1210.7 hybridoma protein. Because the 1210.7 heavy chain is the unmutated product of the BALB/c VH gene, somatic mutation in VH may be required to enhance Ars affinity in this system.     

Genetic mechanisms for dominant VH gene expression. The VHB512 gene.

A total of 37 mAb with reactivity for dextran B512 have been studied; 30 of them were products of independent rearrangements and 21 made use of the same VH gene, the VHB512 gene. These results unambiguously established that the immune response to dextran in the high responder mouse strain C57BL/6 was restricted. Idiotypic determinants are located all over the Ig V region. Many but not all Id described so far can be ascribed to protein structures encoded by VH or VL gene segments. The expression of the major Id, 17-9 Id, in C57BL/6 was not absolutely correlated with the expression of the dominant VHB512 gene in the same mouse strain. Inspection of amino acid sequences of the CDR3 of idiotypic positive and negative clones suggested that idiotypic structures may be associated with the expression of Tyr at position 95 and Phe or Leu at position 96 in the H and L chains, respectively. Therefore the indiscriminate use of idiotypic markers to characterize VH genes and the relevance of idiotypic regulation in VH gene expression are questioned. Id-positive and Id-negative clones displayed similar affinity values for dextran, indicating that idiotypic and binding structures were probably separated. The exchange of Asp65 for Gly65 in one of the clones reduced affinity for dextran, suggesting the involvement of CDR2 in dextran binding. The dominant expression of VH genes can be explained by somatic and/or genetic mechanisms. Because somatic mechanisms such as idiotypic regulation or selection based on affinity for dextran did not seem to influence the expression of the VHB512 gene we favor a genetic alternative. We discuss a model based on the distance between VH genes and D and JH elements. This model is compatible with somatic and genetic regulation in other systems and provides a new theoretical approach to the understanding of immune VH dominance and low responsiveness.     

Predominance of VH-D-JH junctions occurring at sites of short sequence homology results in limited junctional diversity in neonatal antibodies.

Sequence diversity at the junctions of Ig genes differs between newborn and adult mice in two respects: 1) fetal/newborn Ig lack N regions; and 2) these N- junctional sequences very often contain 1 to 6 nucleotides that could have been encoded by either of the two joined gene segments. We address the hypothesis that such short homologies preferentially direct recombination to that site, and we analyze the effect of such homology-directed recombination upon the neonatal Ig repertoire. We examined 546 CDR3 sequences that were generated from polymerase chain reaction-amplified DNA from fetal and newborn liver using primers from three different VH families: S107, 7183, and J558. All junctional sequences using 14 frequently occurring IgH V-D and D-J gene combinations were analyzed. In 12 of the 14 combinations analyzed, there were 1 to 3 short sequence homologies, and the junctional sequences that would be created by those homologies were observed with high frequency. The D-J junctions often had two to three predominant junctional sequences, whereas the V-D junctions had one dominant junctional sequence. The only exceptions were the VHJ558-D junctions, where homology-directed recombination using the sequence homology between VHJ558 genes and most D genes would result in an out-of-frame join, and most of our sequences were productive. This latter result further suggests that homology-directed recombination may play a role in the nonrandom VH gene usage observed in fetal and newborn mice. Thus, most neonatal IgH junctions show limited diversity, not only due to the lack of N regions, but also because of nonrandom junctional sequences. Inasmuch as the few adult N- junctions also show a high frequency of homology-directed junctional sequences, V-D-J recombination throughout life may involve pairing via short homologies, with addition of N regions obscuring its role in the formation of adult IgH junctions.     

V region sequences of anti-DNA and anti-RNA autoantibodies from NZB/NZW F1 mice

The V region sequences of two anti-DNA (A52, D42) and two anti-RNA (D44, D444) autoantibodies, derived from lupus prone NZB/NZW F1 female mice, were determined by mRNA sequencing. The sequences had the following features: 1) there was no clear sequence relationship between anti-DNA and anti-RNA antibodies; 2) there were no major similarities between any of the L chain sequences and each VL gene segment belonged to a different mouse VK subgroup; 3) the H chains of the two anti-RNA antibodies showed closely related sequences of VH gene segments and very similar third complementarity determining regions (CDR3); 4) the H chains of the two anti-DNA antibodies had VH segments belonging to different VH gene families but had a unique and similar combination of D segments and junctional sequences, suggesting a common recognition element for Ag and/or for idiotypic regulation in the H chain CDR3; and 5) the VH gene segment of one anti-DNA antibody (D42) was found to be very similar to the VH gene segment of a CBA mouse hybridoma antibody (6G6) which binds to the environmental Ag phosphocholine. The three-dimensional structure of the Fv-region of the anti-DNA antibody (D42) was modeled by computer and a stretch of poly(dT), ssDNA was docked to a cleft in the antibody combining site, formed by the three H chain CDR and by CDR1 and CDR3 of the L chain. The cleft is characterized by a preponderance of arginine and tyrosine residues, lining both the walls and base of the cleft.     

VH-related idiotopes detected by site-directed mutagenesis. A study induced by the failure to find CD4 anti-idiotypic antibodies mimicking the cellular receptor of HIV.

The function of the CD4 cell surface protein as coreceptor on T helper lymphocytes and as receptor for HIV makes this glycoprotein a prime target for an immune intervention with mAb. A detailed understanding of the structural determinants on the therapeutic CD4 mAb that are involved in Ag binding or are recognized by anti-idiotypic mAb (anti-Id) may be important for designing antibodies with optimal therapeutic efficacy. Seven anti-Id raised against the CD4 mAb M-T310 were selected from a large panel with the intention to obtain CD4 mimicking structures with specificity for HIV gp120. The selected anti-Id did not react with other CD4-specific mAb cross-blocking M-T310. Among these, mAb M-T404, although having the same L chain as M-T310 and a VH region sequence differing only at 14 amino acid positions, was not recognized by the anti-Id. M-T310 H chain complexed with the J558L L chain reacted with all anti-Id, thus demonstrating that the recognized idiotopes are located within the VH region. To identify the idiotopes of M-T310 seen by the anti-Id, variants of M-T404 containing one or more of the M-T310-derived substitutions were generated by oligonucleotide-directed mutagenesis. The reactivity pattern of the mutant proteins with the anti-Id demonstrated that the idiotopes reside within the complementarity determining region (CDR) 2 and CDR3 loops of the VH region. A major idiotope was defined by a single amino acid in CDR2 that was recognized by three anti-Id, whereas the four other anti-Id reacted with determinants of CDR3. Although the performed amino acid substitutions did influence the Id recognition, Ag binding was not significantly affected, suggesting that none of the anti-Id can be considered as a mimicry of the CD4 Ag.     

The critical role of arginine residues in the binding of human monoclonal antibodies to cardiolipin

Previously we reported that the variable heavy chain region (VH) of a human beta2 glycoprotein I-dependent monoclonal antiphospholipid antibody (IS4) was dominant in conferring the ability to bind cardiolipin (CL). In contrast, the identity of the paired variable light chain region (VL) determined the strength of CL binding. In the present study, we examine the importance of specific arginine residues in IS4VH and paired VL in CL binding. The distribution of arginine residues in complementarity determining regions (CDRs) of VH and VL sequences was altered by site-directed mutagenesis or by CDR exchange. Ten different 2a2 germline gene-derived VL sequences were expressed with IS4VH and the VH of an anti-dsDNA antibody, B3. Six variants of IS4VH, containing different patterns of arginine residues in CDR3, were paired with B3VL and IS4VL. The ability of the 32 expressed heavy chain/light chain combinations to bind CL was determined by ELISA. Of four arginine residues in IS4VH CDR3 substituted to serines, two residues at positions 100 and 100 g had a major influence on the strength of CL binding while the two residues at positions 96 and 97 had no effect. In CDR exchange studies, VL containing B3VL CDR1 were associated with elevated CL binding, which was reduced significantly by substitution of a CDR1 arginine residue at position 27a with serine. In contrast, arginine residues in VL CDR2 or VL CDR3 did not enhance CL binding, and in one case may have contributed to inhibition of this binding. Subsets of arginine residues at specific locations in the CDRs of heavy chains and light chains of pathogenic antiphospholipid antibodies are important in determining their ability to bind CL.