Somatic diversification of S107 from an antiphosphocholine to an anti-DNA autoantibody is due to a single base change in its heavy chain variable region

SLE classification criteria: Is "The causality principle" integrated and operative - and do the molecular and genetical network, on which criteria depend on, support the definition of SLE as "a one disease entity" - A theoretical discussion

Molecular and cellular aspects of the autoimmune pathophysiology in SLE is linked to the "The causality principle". SLE Classification Criteria identify per definition disease measures (here: synonymous with classification criteria), but not diagnostic criteria within a classical framework. These two mostly theoretical criteria collections represent a salient conflict between phenomenology and the causality principle - between disease measures and molecular interactions that promote such measures, in other words their cause(s). Essentially, each criterion evolves from immunogenic and inflammatory signals - some are interconnected, some are not. Disparate signals instigated by disparate causes. These may promote clinically heterogenous SLE cohorts with respect to organ affection, autoimmunity, and disease course. There is today no concise measures or arguments that settle whether SLE cohorts evolve from one decisive etiological factor (homogenous cohorts), or if disparate patho-biological factors promote SLE (heterogenous cohorts). Current SLE cohorts are not ideal substrates to serve as study objects if the research aims are to describe etiology, and molecular interactions that cause - and link - primary and secondary pathophysiological events together - events that account for early and progressive SLE. We have to develop SLE criteria allowing us to identify definable categories of SLE in order to describe etiology, pathophysiology and diagnostic criteria of delimitated SLE versions. In this regard, the causality principle is central to define dominant etiologies of individual SLE categories, and subsequent and consequent down-stream diagnostic disease measures. In this sense, we may whether we like it or not identify different SLE categories like "genuine SLE" and "SLE-like non-SLE" syndromes. Many aspects of this problem are thoroughly discussed in this study.

Characterization of gene use and efficacy of mouse monoclonal antibodies to Streptococcus pneumoniae serotype 8

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia in the United States and globally. Despite the availability of pneumococcal capsular polysaccharide (PPS) and protein conjugate-based vaccines, the prevalence of antibiotic-resistant pneumococcal strains, serotype (ST) replacement in nonconjugate vaccine strains, and uncertainty as to whether the PPS vaccine that is used in adults protects against pneumonia emphasize the need for continued efforts to understand the nature of protective PPS antibody responses. In this study, we generated mouse monoclonal antibodies (MAbs) to a conjugate consisting of the PPS of serotype 8 (PPS8) S. pneumoniae and tetanus toxoid. Thirteen MAbs, including four IgMs that bound to PPS8 and phosphorylcholine (PC) and five IgMs and four IgG1s that bound to PPS8 but not PC, were produced, and their nucleotide sequences, epitope and fine specificity, and efficacy against lethal challenge with ST8 S. pneumoniae were determined. MAbs that bound to PPS8 exhibited gene use that was distinct from that exhibited by MAbs that bound to PC. Only PPS8-binding MAbs that did not bind PC were protective in mice. All 13 MAbs used germ line variable-region heavy (V(H)) and light (V(L)) chain genes, with no evidence of somatic hypermutation. Our data reveal a relationship between PPS specificity and V(H) gene use and MAb efficacy in mice. These findings provide insight into the relationship between antibody molecular structure and function and hold promise for the development of novel surrogates for pneumococcal vaccine efficacy.

Covalent binding antibodies suppress advanced glycation: on the innate tier of adaptive immunity

Non-enzymatic protein glycation is a source of metabolic stress that contributes to cytotoxicity and tissue damage. Hyperglycemia has been linked to elevation of advanced glycation endproducts, which mediate much of the vascular pathology leading to diabetic complications. Enhanced glycation of immunoglobulins and their accelerated vascular clearance is proposed as a natural mechanism to intercept alternative advanced glycation endproducts, thereby mitigating microvascular disease. We reported that antibodies against the glycoprotein KLH have elevated reactivity for glycopeptides from diabetic serum. These reactions are mediated by covalent binding between antibody light chains and carbonyl groups of glycated peptides. Diabetic animals that were immunized to induce reactive antibodies had attenuated diabetic nephropathy, which correlated with reduced levels of circulating and kidney-bound glycation products. Molecular analysis of antibody glycation revealed the preferential modification of light chains bearing germline-encoded lambda V regions. We previously noted that antibody fragments carrying V regions in the germline configuration are selected from a human Fv library by covalent binding to a reactive organophosphorus ester. These Fv fragments were specifically modified at light chain V region residues, which map to the combining site at the interface between light and heavy chains. These findings suggest that covalent binding is an innate property of antibodies, which may be encoded in the genome for specific physiological purposes. This hypothesis is discussed in context with current knowledge of the natural antibodies that recognize altered self molecules and the catalytic autoantibodies found in autoimmune disease.

Molecular identification of pathogenetic IdLNF+1 autoantibody idiotypes derived from the NZBxSWR F1 model for systemic lupus erythematosus

The acceleration of nephritis in SNF(1) mice by CD4(+) T-cell clones reactive with a nephritogenic idiotype, Id(LN)F(1) [1], as well as the ability of anti-Id(LN)F(1) antisera to down-regulate the production of Id(LN)F(+)(1) immunoglobulin (Ig) in vivo and delay nephritis [2], suggests that dysregulation of this idiotype may contribute to the development of SNF(1) nephritis. Herein, we show that a monoclonal Id(LN)F(1)-expressing antibody, 540, significantly (P< or = 0.01) stimulated Id(LN)F(1)-reactive T-cell clones B6 and D2 to proliferate, while other Id(LN)F+1 antibodies did not. Further, injection of 540-producing hybridoma cells into nonautoimmune (SWRxBalb/c)F(1) mice resulted in the deposition of Id(LN)F(+)(1) Ig in the kidneys, in a pattern indicative of early nephritis. To identify the pathogenetic Id(LN)F(1) epitope(s) at the molecular level, we compared the deduced amino acid sequences of the heavy and light chain variable regions of pathogenetic and non-pathogenetic Id(LN)F(1)-expressing Igs 540, 317, and 533. Two overlapping peptides derived from the V(H) sequence of 540 (aa 54-66 and 62-73), which both contain the triple basic amino acid motif K(X)K(X)K, stimulated SNF(1) T cells and T-cell clones B6 and D2. These results further support the involvement of a subset of Id(LN)F(1)-expressing Ig in SNF(1) nephritis.

The structural basis of repertoire shift in an immune response to phosphocholine

The immune response to phosphocholine (PC)-protein is characterized by a shift in antibody repertoire as the response progresses. This change in expressed gene combinations is accompanied by a shift in fine specificity toward the carrier, resulting in high affinity to PC-protein. The somatically mutated memory hybridoma, M3C65, possesses high affinity for PC-protein and the phenyl-hapten analogue, p-nitrophenyl phosphocholine (NPPC). Affinity measurements using related PC-phenyl analogues, including peptides of varying lengths, demonstrate that carrier determinants contribute to binding affinity and that somatic mutations alter this recognition. The crystal structure of an M3C65-NPPC complex at 2.35-A resolution allows evaluation of the three light chain mutations that confer high-affinity binding to NPPC. Only one of the mutations involves a contact residue, whereas the other two have indirect effects on the shape of the combining site. Comparison of the M3C65 structure to that of T15, an antibody dominating the primary response, provides clear structural evidence for the role of carrier determinants in promoting repertoire shift. These two antibodies express unrelated variable region heavy and light chain genes and represent a classic example of the effect of repertoire shift on maturation of the immune response.

Apotosis of mature B cells

Antigen receptor (BCR) transduces either pro-apoptotic or anti-apoptotic signals of mature B cells depending on the nature of stimuli. Mature B cells also undergo apoptosis by signaling through CD95. Those apoptotic signals through BCR or CD95 are blocked by various transmembrane signaling such as those via CD40, BCR, CD21 and IL-4 receptor, presumably generated by interaction with T helper cells or the components of innate immunity such as complements. Induction of B cell apoptosis and its regulation are likely to play important roles in humoral immunity. Indeed, spontaneous models as well as patients of systemic autoimmune diseases show defects in apoptosis of mature B cells mediated by BCR or CD95.

Molecular Modeling of an Anti-DNA Autoantibody (V-88) and Mapping of Its V Region Epitopes Recognized by Heterologous and Autoimmune Antibodies

Anti-DNA autoantibodies are a characteristic feature of human systemic lupus erythematosus (SLE) and lupus diseases in the mouse. V-88 is an IgG1/κ ssDNA-binding Ab, derived from a lupus mouse, that bears a cross-species, cross-reactive Id (CRI) that has been implicated in the pathogenesis of both human and murine disease. A linear epitope map of V-88 has been determined with anti-idiotypic antisera obtained from rabbits, and candidate sequences for the idiotopes of the CRI have been proposed. We now report the modeling of the three-dimensional structure of the V regions of Ab V-88, to map the location of these idiotopes. The V region framework structure was derived from those of crystallographically determined Ab structures, and the complementarity determining region (CDR) structures were based upon the set of canonical structures adopted by these loop regions in Abs of known structure. One of the idiotopes is an extensive, highly accessible epitope consisting of framework regions spatially adjacent to CDR2 in the heavy chain. Epitopes recognized by an anti-idiotypic rabbit antiserum were compared with those recognized by autoimmune sera from SLE-prone mice, and common features were identified. By analogy with the crystal structure of an anti-DNA Ab BV04-01 complexed with a trinucleotide, the modeled structure also suggests a mode of binding of ssDNA to V-88. The location of the candidate CRI, although within the framework region of VH, is such that it could influence Ag specificity.

Rearrangement and Selection in the Developing Vκ Repertoire of the Mouse: An Analysis of the Usage of Two Vκ Gene Segments

Detailed analysis of the rearrangement and expression of two mouse Vκ genes has been used to examine B cell repertoire development. The Vκ1-A gene is used by a large proportion (9.6%) of splenic B cells in the adult primary repertoire, whereas the Vκ22 gene is used at a much lower frequency (0.16%). Consistent with these results, quantitative PCR (Q-PCR) assays revealed that the number of splenic B cells with rearranged Vκ1-A genes is much greater than the number with rearranged Vκ22 genes. Q-PCR was also performed on both normal bone marrow pre-B cells and transformed pre-B cells induced to rearrange their κ loci at high frequency. In contrast to splenic B cell rearrangements, the numbers of Vκ1-A and Vκ22 rearrangements in pre-B cells differ by only two- or threefold, suggesting that the intrinsic rearrangement frequencies of these two Vκ genes are not significantly different. Further evidence of disproportionate selection was obtained by comparing the percentages of productive rearrangements amplified from genomic splenic DNA. Sequence analysis showed 84% (37 of 44) of the Vκ1-A rearrangements but only 57% (29 of 51) of the Vκ22 rearrangements to be in-frame. Together these results suggest that B cells expressing Vκ1-A-encoded light chains are preferentially selected either in the periphery or in the transition from pre-B to B cell. Sequence data also reveal a surprisingly restricted diversity of VJ junctions, apparently due to biases introduced by the rearrangement mechanism.

Harmful somatic mutations: lessons from the dark side

The ability of somatic mutation to modify the course of an immune response is well documented. However, emphasis has been placed almost exclusively on the ability of somatic mutation to improve the functional characteristics of representative antibodies. The harmful effects of somatic mutation, its dark side, have been far less well characterized. Yet evidence suggests that the number of B cells directed to wastage pathways as a result of harmful somatic mutation probably far exceeds the number of cells whose antibodies have been improved. Here we review our recent findings in understanding the structural and functional consequences of V-region mutation.

Anti-idiotype monoclonal antibodies specific for the MOPC167 anti-phosphocholine transgene-encoded antibody

Four rat x mouse hybridomas secreting monoclonal anti-idiotypic (anti-Id) antibodies (MAb) specific for the transgene-encoded antibody of the 207-4 transgenic mouse line, which carries the VH1/V kappa 24 gene segments of the IgA, phosphocholine-(PC) specific MOPC167 myeloma, were developed from a fusion of Ag8-X63.653 mouse cells with spleen cells from a rat immunized with MOPC167 and HPCM27 anti-PC antibodies. The anti-Id MAb were shown by ELISA to be specific for PC-binding proteins of VH1/V kappa 24 H and L chains of various isotypes. They did not bind VH1/V kappa 22, VH1/V kappa 8, or VH1/V kappa 1 PC-binding proteins or other IgA or IgM myeloma proteins. Analysis by flow cytometry demonstrated that these MAb bind to the transgene-encoded membrane immunoglobulin (sIgM) as expressed on > 95% of the B220 positive 207-4 spleen cells. All four MAb were able to inhibit the binding of MOPC167 to PC conjugated to bovine serum albumin. Differences in fine specificity of binding were demonstrated by differential staining of spleen cells of the 216-7 mu kappa delta Mem MOPC167 transgenic mice. In these mice endogenous H chains associate with the transgene encoded L chain to form MOPC167 crossreactive idiotopes. Two of the MAb, 28-4-3 and 28-6-20, stained significant numbers of cells, while MAb 28-5-15 did not bind to 216-7 cells. Three of the MAb, 28-5-15, 28-6-20, and 28-4-3, when conjugated to Sepharose beads, were able to induce DNA synthesis in cultures of 207-4 transgenic spleen cells. None of the MAb were able to induce an antibody response in vivo. These MAb should prove useful in staining PC-transgenic B cells for flow cytometry studies and in defining early cellular events in the activation of idiotype positive B cells by anti-Id antibodies.

Expression studies of catalytic antibodies

We have examined the positive influence of human constant regions on the folding and bacterial expression of active soluble mouse immunoglobulin variable domains derived from a number of catalytic antibodies. Expression yields of eight hybridoma- and myeloma-derived chimeric Fab fragments are compared in both shake flasks and high density fermentations. In addition the usefulness of this system for the generation of in vivo expression libraries is examined by constructing and expressing combinations of heavy and light chain variable regions that were not selected as a pair during an immune response. A mutagenesis study of one of the recombinant catalytic Fab fragments reveals that single amino acid substitutions can have dramatic effects on the expression yield. This system should be generally applicable to the production of Fab fragments of catalytic and other hybridoma-derived antibodies for crystallographic and structure-function studies.

The role of the immunoglobulin heavy chain in human anti-DNA antibody binding specificity

OBJECTIVE

To investigate the structural basis for DNA binding of the natural human IgM lambda monoclonal antibody KIM4.6.

METHODS

An IgM lambda, non-DNA-reactive variant hybridoma was derived during in vitro subcloning of the anti-DNA antibody KIM4.6. The variable (V)-region heavy (H) and light (L) chain genes expressed by the variant hybridoma were amplified by polymerase chain reaction, cloned, sequenced, and compared with those of the KIM4.6 parent and other DNA-binding and non-DNA-binding antibodies.

RESULTS

The VL chain of the variant was identical to that of KIM4.6. In contrast, the VH chain was completely different from the VH chain of the parent but was similar or identical, except in the diversity (D) and joining regions, to the VH chain of the systemic lupus erythematosus (SLE) IgG anti-DNA antibody T14 and SLE IgM nephritogenic anti-DNA antibodies NE-1 and NE-13.

CONCLUSION

The expression of the KIM4.6 VL chain is not sufficient for DNA specificity. The VH chain and its D region play a key role in conferring DNA binding of the KIM4.6 anti-DNA antibody.

Human B-cell clones expressing lupus nephritis-associated anti-DNA idiotypes are preferentially expanded without somatic mutation

Human monoclonal anti-single/double-stranded (ss/ds) DNA antibodies (NE-1 and NE-13) expressed cross-reactive idiotypes (Id), NE-1 Id, which have been detected on the lupus glomeruli-deposited anti-DNA antibodies. The nucleotide sequences of the variable regions of NE-1 and NE-13 clones were analogous except for one nucleotide difference in the Vk region. The VH and Vk gene segments of NE-13 clone were identical with germline genes VH4.21 and Vb (or Vb'), respectively. CDR3s of NE-1 and NE-13 heavy chains were arginine rich and CDR1s contained an amino acid stretch, SGYY, the inverted sequence of YYGS, which was shared among CDR3s of several anti-DNA antibodies. Clonal frequency analysis using a limiting dilution method revealed that NE-1 Id-positive clones at precursor cell level increased in lupus patients. These findings suggest that some IgM anti-DNA clones which express NE-1 Id associated with lupus nephritis use germline genes without mutation and they may be preferentially expanded at the precursor cell levels as well as at the mature cell level.

Antigen-induced apoptotic death of Ly-1 B cells responsible for autoimmune disease in transgenic mice

Studies on transgenic mice expressing immunoglobulins against self-antigens have shown that self-tolerance is maintained by active elimination (clonal deletion), functional inactivation (clonal anergy) of self-reactive B cells, or a combination of both. We have established and characterized a transgenic mouse line expressing an anti-erythrocyte autoantibody. In contrast to other autoantibody transgenic lines, about 50% of the animals of this transgenic line suffer from autoimmune disease, indicating a loss of self-tolerance. Here we show that peritoneal Ly-1 B cells (also known as B-1 cells) are responsible for this autoimmune disease in our transgenic mice. A few self-reactive Ly-1 B cells that have somehow escaped the deletion mechanism expand in the peritoneum because of the absence of self-antigen. These Ly-1 B cells are eliminated in vivo by apoptosis once exposed to self-antigen. On the basis of these results we propose a novel autoantibody production mechanism whereby self-reactive B cells sequestered in compartments free of self-antigens may survive, proliferate and be activated for generation of pathogenic autoantibodies in autoimmune diseases.

Antibodies that are specific for a single amino acid interchange in a protein epitope use structurally distinct variable regions

We have analyzed how the immune system generates antibodies that are specific for analogues of an epitope on the influenza virus hemagglutinin (HA) that differ solely by the presence of Asp or Gly at amino acid 225. Most antibodies induced in response to HA(Asp225) use one of a few closely related variable (V) region structures that are encoded by characteristic VH/Vk gene segment combinations. Remarkably, none of these VH/Vk combinations was induced in response to HA(Gly225). Instead of modifying the HA(Asp225)-specific V regions by junctional variation or somatic mutation to recognize the altered epitope, new VH/Vk combinations were used. The expression of unique VH/Vk combinations appears to confer exquisite specificity to the selection of HA-specific B cells from the pre-immune repertoire.

Altered reactivity of immunoglobulin produced by human-human hybridoma cells transfected by pSV2-neo gene

The HB4C5 and HF10B4 cell lines are human-human hybridomas producing human IgM monoclonal antibodies (MAbs) reactive to porcine carboxypeptidase A (CPase), but not to double stranded DNA (ds DNA). We obtained G418-resistant HB4C5 and HF10B4 cells by an introduction of pSV2-neo DNA. Almost all of the G418-resistant clones produced MAbs reactive to not only the CPase but the ds DNA. The results of the inhibition ELISA suggested that the cross-reactivity of the antibodies from G418-resistant clones to CPase and ds DNA was responsible for the alteration on their antigen specificity. HB4C5 and HF10B4 cells and their G418-resistant clones produced antibodies having glycosylated lambda chain. The antibodies produced by tunicamycin-treated G418-resistant subclones of HB4C5 and HF10B4 lost the ability to bind to ds DNA, but retained the ability to bind to CPase. These results suggest that an introduction of pSV2-neo DNA into these hybridomas alters the specificities of their MAbs, and that the alteration to antigen binding specificities of their MAbs may be associated with glycosylation of the MAbs by these hybridomas.

Instability of immunoglobulin genes in S107 cell line

Somatic mutation occurs frequently in rearranged and expressed immunoglobulin variable region genes in vivo. In contrast, V region hypermutation seldom occurs in antibody-forming cells in culture. The S107 mouse myeloma cell line is one of the few cell lines that has been observed to generate V region mutations frequently and spontaneously in vitro. Detailed examination reveals that both the S107 tumor and the cell line derived from it contain and express a duplicated heavy-chain gene. In culture, only one of the two heavy-chain genes undergoes both V and C region mutation, and variants with complex phenotypes and genotypes arise as a result of mutation and segregation of these duplicated genes.

A potential role for microbial superantigens in the pathogenesis of systemic autoimmune disease

We have attempted herein to demonstrate how microbial superantigens could promote an abnormal form of "cognate" T helper-B cell interaction, analogous to that which may occur during GVH disease, leading to B cell activation and systemic autoimmunity. In vitro studies performed at our laboratory and others have demonstrated that resting human B cells bind microbial superantigens and present them to superantigen-reactive autologous T helper cells, resulting in T cell activation and polyclonal IgM and IgG production by the superantigen-bearing B cells. In vitro studies of microbial superantigen-mediated murine T helper-B cell interactions demonstrate preferential help for B cells that have encountered specific antigen. Both in humans and in mice, the cellular interactions involved and the B cell responses induced are highly analogous to those mediated by allospecific T helper-B cell interaction. Finally, the results of studies carried out on T cell-deficient (nude) mice suggest that microbial superantigens may trigger similar T helper cell-dependent polyclonal IgM and IgG responses in vivo. These mice will be studied over time and tested for the development of autoantibodies characteristic of SLE and of autoimmune organ system damage, the occurrence of which are predicted by our model.

Characterization of somatically mutated S107 VH11-encoded anti-DNA autoantibodies derived from autoimmune (NZB x NZW)F1 mice

We have studied 19 S107 heavy chain variable region gene (VH11)-encoded monoclonal antibodies from NZBWF1 mice. These studies show that a single VH gene can encode both antibodies to foreign antigens (anti-phosphorylcholine) and to self antigens (anti-double-stranded DNA) in the same animal. All of the anti-DNA antibodies contain many somatic mutations compared with the relevant germline genes. Since the anti-DNA antibodies were extensively somatically mutated and had undergone isotype switching, the response seems to be T cell dependent. While some of the antibodies appear to be the products of an antigen-driven and antigen-selected response, a number of characteristics of the antibodies suggest that forces other than antigen are contributing to the stimulation and selection of this response.

Structural characteristics of the variable regions of immunoglobulin genes encoding a pathogenic autoantibody in murine lupus

We have studied several monoclonal anti-double-stranded (ds) DNA antibodies for their ability to accelerate lupus nephritis in young NZB X NZW F1 female mice and to induce it in BALB/c mice. Two identified as pathogens in both strains have characteristics previously associated with nephritogenicity: expression of IgG2a isotype and IdGN2 idiotype. Both pathogenic antibodies used the combination of genes from the VHJ558 and VK9 subfamilies. Two weak pathogens failed to accelerate nephritis in young BW mice, but induced lupus nephritis in BALB/c mice. They both express IdGN2; one is cationic and an IgG3, the other is an IgG2a. Additional MAbs (some IgG2a, one IdGN2-positive) did not accelerate or induce nephritis. We have cloned and sequenced the variable regions of the immunoglobulin genes of one pathogenic autoantibody. No unique V, D, or J gene segments and no evidence of unusual mechanisms in generating diversity were used to construct this antibody. These data argue against use of unique abnormal Ig genes by systemic lupus erythematosus individuals to construct pathogenic autoantibody subsets. Instead, the major abnormality may be immunoregulatory.

The biologic significance of human natural autoimmune responses: relationship to the germline, early immune and malignant B cell variable gene repertoire

The potential for autoreactivity that has been well documented in normal individuals implies that natural autoimmune responses must serve some physiologic function. To investigate the genetic mechanisms involved in the emergence of such responses, we have determined the sequences of heavy (VH) and light (VL) chain variable region genes for several human monoclonal autoantibodies and compared these with corresponding sequences reported for other antibodies and autoantibodies. Our data reveal that natural autoantibodies can be encoded by nonmutated germline VH and VL genes which are essentially identical to V genes expressed in early B cell ontogeny as well as in some B-lineage tumors. Taken together with other structural data on human autoantibodies, these findings suggest that natural autoimmune responses originate early in ontogeny and that such antibodies may play a regulatory role in development of the normal immune repertoire and possibly in suppressing pathogenic autoimmune or malignant responses.

Significant structural and functional change of an antigen-binding site by a distant amino acid substitution: proposal of a structural mechanism

To study the molecular basis for antibody diversity and the structural basis for antigen binding, we have characterized the loss of phosphocholine (P-Cho) binding both experimentally and computationally in U10, a somatic mutant of the antibody S107. Nucleotide sequencing of U10 shows a single base change in JH1, substituting Asp-101 with Ala, over 9 A distant from the P-Cho-binding pocket. Probing with antiidiotypic antibodies suggests local, not global, conformational changes. Computational results support a specific structural mechanism for the loss of P-Cho binding. The U10 mutation eliminates the charged interaction between Asp-101 and Arg-94, which allows the Arg-94 side chain to disrupt P-Cho binding sterically and electrostatically by folding into the P-Cho-binding site. These results specifically show the importance of the Arg-94 to Asp-101 side chain salt bridge in the heavy-chain CDR3 conformation and suggest that residues distant from the binding site play an important role in antibody diversity and inducible complementarity.

The role of somatic hypermutation in the generation of antibody diversity

The immune system is capable of establishing an enormous repertoire of antibodies before its first contact with antigen. Most antibodies that express germ-line sequences are of relatively low affinity. Once antigen enters the system, it stimulates a somatic mutational mechanism that generates antibodies of higher affinity and selects for the expression of those antibodies to produce a more effective immune response. The details of the mechanism and regulation of somatic hypermutation remain to be elucidated.

The molecular origin of anti-DNA antibodies

The in vitro observation that a single point mutation in the protective anti-phosphorylcholine anti-bacterial antibody, S107, converts it into an autoantibody that reacts with dsDNA has focused our attention on the role of somatic mutation in generating autoantibodies. It has also led us to examine the significance of an individual's prior response to environmental antigens on the subsequent production of autoantibodies. The fact that genes of the S107 heavy chain variable region family could encode autoantibodies made it possible to clone and sequence the relevant germline genes of this small family from autoimmune (NZB x NZW)F1 mice and to compare these to the comparable genes in non-autoimmune mice. The germline genes from the normal and autoimmune mice are quite homologous and the small number of polymorphisms are not likely to predispose the autoimmune mice to the production of autoantibodies. (NZB x NZW)F1 mice respond to immunization with phosphorylcholine with a response that is largely encoded by the VH1 gene of the S107 family. However, when these same mice begin to make autoantibodies, their anti-DNA antibodies which are encoded by this family are in fact derived from the VH11 gene. The VH11 encoded anti-DNA antibodies which have been sequenced are all of the IgG2a subclass, react with dsDNA, and have undergone significant somatic diversification from the germline gene. Analysis of the ratio and location of the replacement and silent mutations suggests that the regulation of the autoantibody response differs from that of the normal response to foreign antigens. Our studies suggest that the utilization of a particular VH germline gene in the immune response to foreign antigens early in life does not lead to the preferential utilization of that same gene in the subsequent production of autoantibodies.

Anti-DNA antibodies, autoimmunity and the immunoglobulin repertoire

The advent of hybridoma and recombinant DNA technology about a decade ago has allowed a detailed analysis the structure, properties and molecular genetics of antibodies. These techniques, combined with studies of idiotypes and of Abelson-transformed and other cell lines, have resulted in major findings which are of particular importance to both the normal immune system and to autoimmunity. The rearrangement and expression of antibody genes in the normal immune system are discussed first, as a background for an appreciation of the significance of the molecular genetics of autoantibodies. We then turn to autoantibody genes, with an emphasis on anti-DNA antibodies and their role in the autoimmune disease, systemic lupus erythematosus. A model for the genetics of lupus which includes a possible role for Ig genes is considered.

Somatic diversification of the S107 (T15) VH11 germ-line gene that encodes the heavy-chain variable region of antibodies to double-stranded DNA in (NZB x NZW)F1 mice

A variety of studies suggest that members of the S107 (T15) heavy-chain variable-region gene family contribute to the autoimmune response of mice and humans to DNA. To identify the germ-line gene(s) involved and the degree of somatic diversification that occurs in such autoantibodies, we determined the mRNA sequence of the heavy and light chains of a group of monoclonal anti-DNA antibodies encoded by the S107 VH11 germ-line gene in (NZB x NZW)F1 mice. We also cloned and sequenced the VH11 germ-line gene of the NZB and NZW parental strains. The VH11 coding sequences of the two strains were identical. Comparison with this heavy-chain germ-line sequence showed that the variable regions of the monoclonal antibodies had undergone considerable somatic diversification.

The relationship between induced and spontaneous autoantibodies in MRL mice: the role of Ly-1 B cells?

The murine response to bromelain-treated mouse red blood cells (BrMRBC) is derived from Ly-1 B cells. It has been proposed that this B-cell subset produces a variety of other autoantibodies and is elevated in autoimmune mouse strains. We have studied the ability of MRL lpr/lpr and the non-autoimmune congenic MRL +/+ mice to make autoantibodies to BrMRBC and immunoglobulin (rheumatoid factors, RF). Following lipopolysaccharide (LPS) stimulation we found the numbers of autologous plaque-forming cells (PFC) to be low in both lpr and MRL +/+ mice, suggesting low Ly-1 B-cell numbers. This observation is consistent with the view that Ly-1 B cells in the mouse may not give rise to pathologically relevant RF.

Identification of mutant monoclonal antibodies with increased antigen binding

Sib selection and an ELISA have been used to isolate hybridoma subclones producing mutant antibodies that bind antigen better than the parental monoclonal antibody. Such mutants arise spontaneously in culture at frequencies of 2.5-5 X 10(-5). The sequences of the heavy and light chain variable regions of the mutant antibodies are identical to that of the parent and the Ka values of the mutants and the parent are the same. The increase in binding is associated with abnormalities of the constant region polypeptide and probably reflect changes in avidity of these antibodies.

Isolation and characterization of a light chain variable region gene for human rheumatoid factors

Previously, we isolated a Vk gene (Humkv325) from a human placenta that encodes RF light chains bearing the PSL2 and PSL3 CRI markers. Here we report the isolation and characterization of a second human Vk gene (Humkv328) that can be used for RF synthesis. This Vk gene probably encodes at least two 6B6.6 CRI+ RF light chains (Les and Pom) from unrelated subjects, and thus may be related to the light chain-associated 6B6.6 CRI.

Evolutionary and somatic selection of the antibody repertoire in the mouse

The repertoire of antibody variable (V) regions has been subject to evolutionary selection, affecting both the diversity of V region genes in the germline and their expression in the B lymphocyte population and its subsets. In ontogeny, contact with an antigen leads to the expansion of B cells expressing antibodies complementary to it. In a defined phase of B cell differentiation, new sets of V regions are generated from the existing repertoire through somatic hypermutation. Cells carrying advantageous antibody mutants are selected into the memory compartment and produce a stable secondary response upon reexposure to the antigen.