Light chain editing in kappa-deficient animals: a potential mechanism of B cell tolerance

Haplotype exclusion and receptor editing: irreconcilable differences?

Features of antibody genes and their regulation hinder two properties thought to be critical for clonal selection: haplotype exclusion and receptor diversity. These properties include: (1) the retention of multiple independent L-chain isotypes, which compounds the problem of allelic exclusion with one of isotype exclusion; (2) the process of receptor editing, in which recombination continues in cells already expressing antigen receptors; and (3) non-random associations and quasi-ordered rearrangements of the elements that generate light chain genes, which promote editing at the expense of allelic exclusion and receptor diversification. In contrast, heavy chain gene structure seems to promote haplotype exclusion and receptor diversity. It appears that requirements of receptor selection, such as the need for receptor editing as an immune tolerance mechanism and positive selection as a quality control checkpoint for receptor functionality, impose independent selections that shape the organization and regulation of the antibody genes. Despite these features, B cell development still achieves a significant level of phenotypic haplotype exclusion, suggesting that there is indeed significant selection for antibody monospecificity that is accommodated along with receptor editing. Thus, the immune system achieves both receptor selection and clonal selection, despite their partly antagonistic mechanisms.

Analysis of B cell receptor production and rearrangement. Part I. Light chain rearrangement

A probabilistic model of allelic exclusion fails to explain the status of receptor genes and the receptor phenotype of most B cells. A large proportion of B cells have incompletely rearranged H and/or L chain genes (e.g. kappa0/kappa+) and most B cells express only one receptor. These properties seem to require deterministic features of B cell development such as special mechanisms that stop rearrangement. However, receptor editing has revealed that rearrangement-stop is not stable and that multi-receptor lymphocytes make up a significant fraction of certain B and T cell populations. Consequently we have revived the purely probabilistic approach in a model that now includes receptor editing and allows for some multi-receptor B cells. We find that this model can explain the observed properties of B cells when the frequency of self-reactive B cells is high. Indeed, as we illustrate for anti-DNA, this is the case. Hence the probabilistic model has life and assiduous use of the model suggests unexpected but not unrealistic features of lymphocyte development.

Receptor revision and systemic lupus

Studies over the past 10 years have shown that B cells can undergo secondary heavy- or light-chain immunoglobulin (Ig) rearrangements at various stages of their normal development, a process termed receptor editing. In the bone marrow, this mechanism is important to maintain tolerance because it can extinguish a self-reactive specificity without having to physically eliminate a potentially autoreactive B cell. In the periphery, secondary rearrangements may also play a role in the diversification and maturation of an immune response, although conclusive evidence for this process is still required. Individuals with systemic autoimmune diseases, such as lupus, show evidence of intricate abnormalities in receptor editing. On the one hand, decreased editing may not eliminate the self-reactive specificities that emerge during B-cell development in the bone marrow. Conversely, excessive secondary rearrangements, especially in the periphery where tolerance mechanisms are less effective, can result in the production of autoantibodies by edited B cells. It will be important to assess whether the complex editing defects observed during lupus are a primary susceptibility factor to this disease or if they are secondary to other abnormalities of lymphocyte development in these autoimmune patients.

Chronic graft-versus-host in Ig knockin transgenic mice abrogates B cell tolerance in anti-double-stranded DNA B cells

Anti-dsDNA Abs are specific diagnostic markers of systemic lupus erythematosus, and are also implicated in kidney pathology. Anti-dsDNA B cells have been shown to be tolerized in nonautoimmune mice. The immunodysregulation that causes these cells to break tolerance is presumably part of the fundamental defects in systemic lupus erythematosus. To explore these mechanisms, we used the chronic graft-versus-host model mediated by MHC class II differences. Induction of chronic graft-vs-host in anti-DNA H chain knockin (3H9.KI) transgenic mice on a nonautoimmune background resulted in specific activation of anti-dsDNA B cells, as evidenced by high titers of soluble Ab in sera and a high frequency (70%) of anti-dsDNA B cell clones recovered as hybridomas. In addition, the lambda(+)-anti-dsDNA B cells developed increased expression of cell surface activation markers, and concentrated in the T cell area of the follicle with an Ab-forming cell-compatible phenotype. Genetic analysis of the hybridoma clones showed strong evidence of secondary rearrangements of the L chain associated with anti-dsDNA reactivity. Thus, our study indicates that alloreactive T cell help can break tolerance in a complex manner, involving several events.

Editors and editing of anti-DNA receptors

Receptor editing is a means by which immature bone marrow B cells can become self-tolerant. Rearrangements of heavy (H) and/or light (L) chain genes are induced by encounter with autoantigens to change the specificity from self to nonself. We have developed site-directed transgenic mice (sd-tg) whose transgenes code for the H chain of antibodies that bind DNA. B cells that express the transgenic H chain associate mainly with four of the 93 functional Vkappa genes of the mouse. Numerous aspartate residues that might inhibit DNA binding by the V(H) domain distinguish these L chain Vkappa sequences, but engaging these Vkappa editors often requires multiple rearrangements. Among the edited B cells is a subset of multispecific cells that express multiple receptors. One consequence of multispecificity is partial autoreactivity; these multispecific B cells may contribute to autoimmunity.

Role of B cell antigen receptor in regulation of V(D)J recombination and cell survival

B lymphocytes learn through the interaction of the B cell receptor with antigens in the context of B cell developmental stage and environmental cues. B cells can respond by proliferation and antibody secretion, programmed cell death, or modification of the antibody genes themselves through secondary immunoglobulin gene rearrangements or somatic point mutation. A critical learning process is that of self/nonself-discrimination. We have shown that one potent mechanism for immune self-tolerance in B cells is ongoing antibody light chain gene rearrangements, which can result in "receptor editing" that changes antigen receptor specificity. This process appears to be developmentally regulated, because it is confined to cells at an immature stage of development. Cells at later stages of development can be tolerized by apoptosis, but probably not by receptor editing.

B cell receptor expression level determines the fate of developing B lymphocytes: receptor editing versus selection

During B lymphocyte development, antibody genes are assembled by DNA recombination. Successful cell surface expression of IgM promotes developmental progression. However, when antigen receptors bind autoantigen, development is blocked and ongoing antibody gene recombination occurs, which often alters antibody specificity in a process called receptor editing. We demonstrate here a significant role of developmental block and receptor editing in B cell receptor quality control. During development a functional, non-self-reactive receptor undergoes receptor editing if its expression is below a certain threshold. Doubling the receptor gene dose promotes development in the absence of autoantigen, but allows editing when autoantigen is present. Thus, both underexpressed and harmful B cell receptors can undergo correction by receptor editing.

A VH11V kappa 9 B cell antigen receptor drives generation of CD5+ B cells both in vivo and in vitro

B lymphocytes can be divided into different subpopulations, some with distinctive activation requirements and probably mediating specialized functions, based on surface phenotype and/or anatomical location, but the origins of most of these populations remain poorly understood. B cells constrained by transgenesis to produce an Ag receptor derived from a conventional (B-2) type cell develop a B-2 phenotype, whereas cells from mice carrying a B-1-derived receptor acquire the B-1 phenotype. In this study transgenic enforced expression of a B cell receptor (mu/kappa) originally isolated from a CD5+ (B-1a) B cell generates B-1 phenotype cells in bone marrow cultures that show a distinctive B-1 function, survival in culture. Despite their autoreactivity, we find no evidence for receptor editing or that the paucity of B-2 cells is the result of tolerance-induced selection. Finally, Ca2+ mobilization studies reveal a difference between transgenic B-1 cells in spleen and peritoneal cavity, with cells in spleen much more responsive to anti-B cell receptor cross-linking. We discuss these results in terms of specificity vs lineage models for generation of distinctive B cell subpopulations.

Affiliation to mature B cell repertoire and positive selection can be separated in two distinct processes

Using an 'oligoclonal' model, we have previously shown that mice transgenic for a mu chain (H3) and deficient for kappa chain expression display a mature B cell repertoire largely dominated by the H3/lambda1 pair, while the four H3/lambda available combinations can be observed in the immature B cell compartment. This led us to propose the existence of a positive selection process. To test this hypothesis, we have introduced the SJL lambda locus coding for a defective lambda1 chain (lambda1(s)) that creates a dysfunctional Ig receptor complex during B cell differentiation. Our results show that the lambda1(s) defect impairs the development of mature B cells when the H3-mu transgene insert is present in the hemizygous state. This suggests that the Gly --> Val substitution present in the C(lambda)1(s) chain at position 155 is sufficient to abrogate the selection of the H3/lambda1 pair. Unexpectedly, when the H3-mu transgene array is present in a homozygous state in lambda1(s) mice but not in 'wild-type' lambda1 mice (lambda1(+)), a significant number of mature B cells expressing all H3/lambda combinations can be developed. These results indicate that the overriding H3/lambda1 dominance observed in lambda1(+) mice is due to a positive selection process and not to a negative selection of other H3/lambda combinations. They also show that the export of B cells to the periphery can be controlled by the expression of the mu chain.

Receptor selection in B and T lymphocytes

The process of clonal selection is a central feature of the immune system, but immune specificity is also regulated by receptor selection, in which the fate of a lymphocyte's antigen receptor is uncoupled from that of the cell itself. Whereas clonal selection controls cell death or survival in response to antigen receptor signaling, receptor selection regulates the process of V(D)J recombination, which can alter or fix antigen receptor specificity. Receptor selection is carried out in both T and B cells and can occur at different stages of lymphocyte differentiation, in which it plays a key role in allelic exclusion, positive selection, receptor editing, and the diversification of the antigen receptor repertoire. Thus, the immune system takes advantage of its control of V(D)J recombination to modify antigen receptors in such a way that self/non-self discrimination is enhanced. New information about receptor editing in T cells and B-1 B cells is also discussed.

Restricted immunoglobulin variable region (Ig V) gene expression accompanies secondary rearrangements of light chain Ig V genes in mouse plasmacytomas

The many binding studies of monoclonal immunoglobulin (Ig) produced by plasmacytomas have found no universally common binding properties, but instead, groups of plasmacytomas with specific antigen-binding activities to haptens such as phosphorylcholine, dextrans, fructofuranans, or dinitrophenyl. Subsequently, it was found that plasmacytomas with similar binding chain specificities not only expressed the same idiotype, but rearranged the same light (V(L)) and heavy (V(H)) variable region genes to express a characteristic monoclonal antibody. In this study, we have examined by enzyme-linked immunosorbent assay five antibodies secreted by silicone-induced mouse plasmacytomas using a broader panel of antigens including actin, myosin, tubulin, single-stranded DNA, and double-stranded DNA. We have determined the Ig heavy and light chain V gene usage in these same plasmacytomas at the DNA and RNA level. Our studies reveal: (a) antibodies secreted by plasmacytomas bind to different antigens in a manner similar to that observed for natural autoantibodies; (b) the expressed Ig heavy genes are restricted in V gene usage to the V(H)-J558 family; and (c) secondary rearrangements occur at the light chain level with at least three plasmacytomas expressing both kappa and lambda light chain genes. These results suggest that plasmacytomas use a restricted population of B cells that may still be undergoing rearrangement, thereby bypassing the allelic exclusion normally associated with expression of antibody genes.

Receptor editing: genetic reprogramming of autoreactive lymphocytes

The clonal selection theory postulates that immune tolerance mediated selection occurs at the level of the cell. The receptor editing model, instead, suggests that selection occurs at the level of the B-cell receptor, so that self-reactive receptors that encounter autoantigen in the bone marrow are altered through secondary rearrangement. Recent studies in transgenic model systems and normal B cells, both in vivo and in vitro, have demonstrated that receptor editing is a major mechanism for inducing B-cell tolerance.

The roles of preB and B cell receptors in the stepwise allelic exclusion of mouse IgH and L chain gene loci

Membrane-bound preBCR of wild-type mice, and probably also preBCR-like V(preB) muH chain complexes in lambda5-deficient mice, signal allelic exclusion so that < 0.1% of all preB-II cells and all subsequent B lineage cells express two muH chains on their surface. On the other hand a large number of muH chains which are originally generated at the transition of preB-I to preB-II cells cannot pair with surrogate L chains, cannot form a preBCR on the surface and, hence, allow two H chain alleles to be productively rearranged in one B-lineage cell. By contrast membrane-bound BCR on immature B cells does not signal allelic or isotypic exclusion Of Ig kappaL and lambdaL chain gene loci. This allows the rearrangement machinery to remain active, and secondary L chain rearrangements on one kappaL chain allele are frequently observed. Rapid selection of fitting H/L chain pairs, forming BCR on the surface, allows B-lineage cells to enter the mature B cell pool where the rearrangement machinery is shut off, securing allelic exclusion of L chain loci in most B cells.

Somatic mutation and light chain rearrangement generate autoimmunity in anti-single-stranded DNA transgenic MRL/lpr mice

Antibodies to single-stranded (ss)DNA are expressed in patients with systemic lupus erythematosus and in lupus-prone mouse models such as the MRL/Mp-lpr/lpr (MRL/lpr) strain. In nonautoimmune mice, B cells bearing immunoglobulin site-directed transgenes (sd-tgs) that code for anti-ssDNA are functionally silenced. In MRL/lpr autoimmune mice, the same sd-tgs are expressed in peripheral B cells and these autoantibodies gain the ability to bind other autoantigens such as double-stranded DNA and cell nuclei. These new specificities arise by somatic mutation of the anti-ssDNA sd-tgs and by secondary light chain rearrangement. Thus, B cells that in normal mice are anergic can be activated in MRL/lpr mice, which can lead to the generation of pathologic autoantibodies. In this paper, we provide the first direct evidence for peripheral rearrangement in vivo.

Frequencies of multiple IgL chain gene rearrangements in single normal or kappaL chain-deficient B lineage cells

PCR analyses of the kappaL chain locus in single B-lineage cells of wild-type, Ckappa-, or JCkappa-deficient homozygous or heterozygous mice often detect multiple in- and out-of-frame rearrangements at the kappaL and lambdaL loci. They are most frequent in small pre-BII cells and equally so in wild-type and kappaL chain-deficient cells. Hence, kappaL chain production appears not to inhibit secondary rearrangements. Around 20% of all small preBII cells express IgL chains in their cytoplasm. Cells with a first productive rearrangement on one allele are favored to enter the immature B cell compartment. Thus, allelic exclusion might be secured by control of accessibility of IgL chain loci for rearrangement and by rapid selection of cells with a fitting over those with a nonfitting IgL chain.

Models for Antigen Receptor Gene Rearrangement. I. Biased Receptor Editing in B Cells: Implications for Allelic Exclusion

Recent evidence suggests that lymphocyte Ag receptor gene rearrangement does not always stop after the expression of the first productively rearranged receptor. Light chain gene rearrangement in B cells, and α-chain rearrangement in T cells can continue, which raises the question: how is allelic exclusion maintained, if at all, in the face of continued rearrangement? In this and the accompanying paper, we present comprehensive models of Ag receptor gene rearrangement and the interaction of this process with clonal selection. Our B cell model enables us to reconcile observations on the κ:λ ratio and on κ allele usage, showing that B cell receptor gene rearrangement must be a highly ordered, rather than a random, process. We show that order is exhibited on three levels: a preference for rearranging κ rather than λ light chain genes; a preference to make secondary rearrangements on the allele that has already been rearranged, rather than choosing the location of the next rearrangement at random; and a sequentiality of J segment choice within each κ allele. This order, combined with the stringency of negative selection, is shown to lead to effective allelic exclusion.

Ig λ and Heavy Chain Gene Usage in Early Untreated Systemic Lupus Erythematosus Suggests Intensive B Cell Stimulation

To determine the distribution of Vλ and Jλ as well as VH and JH gene usage in a patient with systemic lupus erythematosus (SLE), productive and nonproductive VJ and V(D)J rearrangements were amplified from individual peripheral CD19+ B cells and were analyzed. No differences in the Vλ and Jλ or the VH and JH gene usage in the nonproductive gene repertoire of this SLE patient were found compared with the distribution of genes found in normal adults, whereas marked skewing of both Vλ and VH was noted among the productive rearrangements. The distribution of productive Vλ rearrangements was skewed, with significantly greater representation of the Jλ distal cluster C Vλ genes and the Vλ distal Jλ7 element, consistent with the possibility that there was receptor editing of the Vλ locus in this patient. Significant bias in VH gene usage was also noted with VH3 family members dominating the peripheral B cell repertoire of the SLE patient (83%) compared with that found in normal subjects (55%; p &lt; 0.001). Notably, a clone of B cells employing the VH3-11 gene for the heavy chain and the Vλ1G segment for the light chain was detected. These data are most consistent with the conclusion that extreme B cell overactivity drives the initial stages of SLE leading to remarkable changes in the peripheral V gene usage that may underlie on fail to prevent the emergence of autoimmunity.

B Cell Deletion, Anergy, and Receptor Editing in “Knock In” Mice Targeted with a Germline-Encoded or Somatically Mutated Anti-DNA Heavy Chain

To study the relative contributions of clonal deletion, clonal anergy, and receptor editing to tolerance induction in autoreactive B cells and their dependence on B cell receptor affinity, we have constructed “knock in” mice in which germline encoded or somatically mutated, rearranged anti-DNA heavy (H) chains were targeted to the H chain locus of the mouse. The targeted H chains were expressed on the vast majority of bone marrow (BM) and splenic B cells and were capable of Ig class switching and the acquisition of somatic mutations. A quantitative analysis of B cell populations in the BM as well as of Jκ utilization and DNA binding of hybridoma Abs suggested that immature B cell deletion and light (L) chain editing were the major mechanisms affecting tolerance. Unexpectedly, these mechanisms were less effective in targeted mice expressing the somatically mutated, anti-DNA H chain than in mice expressing the germline-encoded H chain, possibly due to the greater abundance of high affinity, anti-DNA immature B cells in the BM. Consequently, autoreactive B cells that showed features of clonal anergy could be recovered in the periphery of these mice. Our results suggest that clonal deletion and receptor editing are interrelated mechanisms that act in concert to eliminate autoreactive B cells from the immune system. Clonal anergy may serve as a back-up mechanism for central tolerance, or it may represent an intermediate step in clonal deletion.

Regulation of anti-DNA B cells in recombination-activating gene-deficient mice

Anti-DNA antibodies are regulated in normal individuals but are found in high concentration in the serum of systemic lupus erythematosus (SLE) patients and the MRL lpr/lpr mouse model of SLE. We previously studied the regulation of anti-double-stranded (ds)DNA and anti-single-stranded (ss)DNA B cells in a nonautoimmune background by generating mice carrying immunoglobulin transgenes coding for anti-DNAs derived from MRL lpr/lpr. Anti-dsDNA B cells undergo receptor editing, but anti-ssDNA B cells seem to be functionally silenced. Here we have investigated how anti-DNA B cells are regulated in recombination- activating gene (RAG)-2-/- mice. In this setting, anti-dsDNA B cells are eliminated by apoptosis in the bone marrow and anti-ssDNA B cells are partially activated.

Receptor editing occurs frequently during normal B cell development

Allelic exclusion is established in development through a feedback mechanism in which the assembled immunoglobulin (Ig) suppresses further V(D)J rearrangement. But Ig expression sometimes fails to prevent further rearrangement. In autoantibody transgenic mice, reactivity of immature B cells with autoantigen can induce receptor editing, in which allelic exclusion is transiently prevented or reversed through nested light chain gene rearrangement, often resulting in altered B cell receptor specificity. To determine the extent of receptor editing in a normal, non-Ig transgenic immune system, we took advantage of the fact that lambda light chain genes usually rearrange after kappa genes. This allowed us to analyze kappa loci in IgMlambda+ cells to determine how frequently in-frame kappa genes fail to suppress lambda gene rearrangements. To do this, we analyzed recombined VkappaJkappa genes inactivated by subsequent recombining sequence (RS) rearrangement. RS rearrangements delete portions of the kappa locus by a V(D)J recombinase-dependent mechanism, suggesting that they play a role in receptor editing. We show that RS recombination is frequently induced by, and inactivates, functionally rearranged kappa loci, as nearly half (47%) of the RS-inactivated VkappaJkappa joins were in-frame. These findings suggest that receptor editing occurs at a surprisingly high frequency in normal B cells.

Immunoglobulin kappa chain receptor editing in systemic lupus erythematosus

To determine whether receptor editing of Vkappa genes was involved in the pathogenesis of systemic lupus erythematosus (SLE), the usage of Vkappa and Jkappa gene elements from individual peripheral CD19(+) B cells obtained from a patient with untreated SLE was examined. No differences in the Vkappa and Jkappa gene usage in the nonproductive gene repertoire of this SLE patient were noted compared with the distribution of genes found in normal adults. However, an increased usage of Jkappa5 segments, and a significant overrepresentation of the Vkappa1 and Vkappa4 families, especially the L15, O14/O4, and B3 genes characterized the productive Vkappa gene repertoire of the SLE patient. Furthermore, Jkappa5-containing Vkappa gene rearrangements in the productive but not the nonproductive repertoire manifested significantly fewer mutations compared with Vkappa genes recombined with Jkappa1-4. These data are consistent with the conclusion that receptor editing of Vkappa is much more apparent in this SLE patient than in normals and suggest that a deficiency in this means to counteract the emergence of autoimmunity is not an essential feature of SLE.

Generation of the Germline Peripheral B Cell Repertoire: VH81X-λ B Cells Are Unable to Complete All Developmental Programs

The generation of VH81X heavy chain λ-light chain-expressing B cells (VH81X-λ+ B cells) was studied in VH81X heavy chain transgenic mice as well as in VH81X JH −/− and VH81X JH −/− Ck −/− mice, in which competition resulting from expression of heavy and light chains from the endogenous heavy and κ light chain loci was prevented. We show that although λ light chain gene rearrangements occur normally and give rise to light chains that associate with the transgenic heavy chain to form surface and soluble IgM molecules, further B cell development is almost totally blocked. The few VH81X-λ+ B cells that are generated progress into a mature compartment (expressing surface CD21, CD22, CD23, and low CD24 and having a relatively long life span) but they also have reduced levels of surface Ig receptor and express higher amounts of Fas Ag than VH81X-κ+ B cells. These VH81X-λ+ B cells reach the peripheral lymphoid organs and accumulate in the periarteriolar lymphoid sheath but are unable to generate primary B cell follicles. In other heavy chain transgenic mice (MD2, M167, and M54), λ+ B cells are generated. However, they seem to be preferentially selected in the peripheral repertoire of some transgenic heavy chain mice (M54) but not in others (MD2, M167). These studies show that a crucial selection step is necessary for B cell survival and maintenance in which B cells, similar to T cells, receive signals depending on their clonal receptors.

V(H) replacement is unlikely to contribute significantly to receptor editing due to an ineffectual embedded recombination signal sequence

Receptor editing is a process consisting of replacement of pre-existing H or L chain rearrangements by secondary rearrangements. This process could serve to remove autoreactive specificities, or to rescue loci with non-functional rearrangements. At the H chain locus, functional replacement of a V(H)DJ(H) rearrangement by an upstream V(H) requires the presence of an embedded RSS located in reverse orientation near the 3' end of the V(H) segment. Although most V(H) genes contain a fairly consensus embedded heptamer, the nonamer sequence bears little resemblance to the consensus RSS nonamer. Therefore, the physiologic rate of H chain editing by V(H) replacement is yet unknown. In this study, we used both conventional and sensitive competition recombination substrate assays to determine the recombination frequency of the V(H)1X embedded RSS relative to consensus and non-consensus RSS's. Results show no detectable recombination of the 81X embedded RSS in a recombination substrate, and the competition substrate allows us to estimate that the 81X embedded RSS recombines at least 1300 fold less often than a consensus RSS. This suggests that V(H) gene replacement is not responsible for the decrease in representation of the 81X gene during differentiation. Furthermore, since the sequence of the embedded RSS is very similar for many V(H) genes, our results suggest that receptor editing of the H chain will be an infrequent event, leaving L chain editing as the main mode of avoiding autoreactive specificities in vivo.

The B-cell biology of aging

Although both the number and responsiveness of peripheral B cells in aged mice remain relatively intact, there are dramatic changes in B-cell generation. Alterations in B-cell development include both a skewing of V-gene utilization, especially in cells responsive to phosphorylcholine, and a decrease in the generation of various developmental B-cell subsets. The altered representation of these subsets appears to be the consequence of two developmental blocks. The first developmental block occurs during the maturation of pro-B cells and is evidenced by a decrease in the number of pre-B cells. The second developmental block occurs at the earliest stage of sIg(+)-cell maturation (sIgMvery lo). Because of this block in B-cell maturation, in spite of a decrease in incoming pre-B cells, the number of sIgMvery lo cells appears to increase in aged mice. Additionally, the time of residence of cells within this maturational stage increases dramatically, while the proportion of cells in more mature (sIgMhi) stages of bone marrow development are decreased. In addition to the decreased number of maturing bone marrow B cells, the population of splenic B cells that represent recent bone marrow émigrés (HSAvery hi) is markedly decreased. In the face of this decrease in newly emerging cells from the bone marrow, the population of mature splenic B cells is maintained by their increased longevity.

Regulation of anti-double-stranded DNA B cells in nonautoimmune mice: localization to the T-B interface of the splenic follicle

Systemic lupus erythematosus (SLE) and the MRL-lpr/lpr murine model for SLE are characterized by the presence of serum anti-double-stranded (ds)DNA antibodies (Abs), whereas nonautoimmune individuals have negligible levels of these Abs. To increase the frequency of anti-DNA B cells and identify the mechanisms involved in their regulation in nonautoimmune mice, we have used Ig transgenes (tgs). In the present study, we used the VH3H9 heavy (H) chain tg which expresses an H chain that was repeatedly isolated from anti-dsDNA Abs from MRL-lpr/lpr mice. Because the VH3H9 H chain can pair with endogenous L chains to generate anti-single-stranded DNA, anti-dsDNA, and non-DNA B cells, this allowed us to study the regulation of anti-dsDNA B cells in the context of a diverse B cell repertoire. We have identified anti-dsDNA B cells that are located at the T-B interface in the splenic follicle where they have an increased in vivo turnover rate. These anti-dsDNA B cells exhibit a unique surface phenotype suggesting developmental arrest due to antigen exposure.

Analysis of the molecular basis of synovial rheumatoid factors in rheumatoid arthritis

The objective of this study was to better understand the molecular basis of IgM rheumatoid factor in rheumatoid arthritis (RA). We recently generated 10 different monoclonal IgM RF (mRF) molecules isolated from the synovium of a single patient with RA. The heavy (H) and light chain (L) variable region (V) genes of these 10 mRFs were cloned and sequenced. Six mRFs used kappa light chains and 4 mRFs used lambda light chains. Of particular interest, 8 of 10 heavy chains used the JH4 joining region gene, and all five VH4 heavy chains used the DK4 diversity region gene with the JH4. Four of the VH4 clones used the same germline gene, likely representing a novel but closely related germline gene to VH4.18, and may be clonally related because of the extensive homology in their heavy chain sequence. Two VH4 clones shared the same light chain gene, VkappaIIIb kv325 (99% homology) and the same JK4 joining region gene, while three VH4 clones used two different light chain genes, an uncommon Vkappa4 and a Vlambda4 gene, respectively. In this RA patient, there was recurrent utilization of VH4-DK4-21/10-JH4 genes and a recurring association with gene elements Vkappa3 and Vlambda4. Recurring usage of Vkappa3 (kv325) and Vlambda4 (lv418) gene elements may result from a light chain editing process whereby immature autoreactive B cells encountering self-antigen attempt, and often succeed, in altering their specificities through secondary Ig light chain gene rearrangement. Moreover, the oligoclonality of these RFs suggest clonal relatedness secondary to an antigen-driven response.

Does positive selection determine the B cell repertoire?

To know whether each newly formed B cell has an equal chance of survival in the organism, we analyzed the composition of the B cell repertoire of extremely limited diversity by generating mu-transgenic kappa-knockout mice. Surprisingly, in both types of mice studied, the B cell repertoire is mainly composed of cells expressing the mu-transgene-encoded chain associated with only one out four available lambda types depending on the mu transgene. Moreover, B cell differentiation cultures in vitro show that newly formed B cells can express the various lambda types regardless of the presence or absence of the mu transgenes. These results show a drastic impact of the heavy chain on the lambda light chain repertoire expressed in the periphery. The overexpression of a unique heavy/light chain pairing therefore results from selective processes. The immature B cells may be positively selected to provide the immunocompetent B cells in the periphery.

BCR ligation induces receptor editing in IgM+IgD- bone marrow B cells in vitro

The ability of BCR cross-linking to stimulate receptor editing was analyzed in vitro using bone marrow B cells from immunoglobulin (Ig) transgenic (Tg) and non-Tg mice. In cultured Ig-Tg cells, BCR ligation induced receptor editing as measured by up-regulation of RAG gene expression, light chain gene DNA rearrangements, and expression of lambda-light chain protein in cells that previously expressed kappa. In the culture conditions used, BCR ligation induced light chain rearrangements in most immature IgM+IgD- bone marrow B cells in the absence of significant cell death or cell growth. Receptor editing in non-Tg B cells was also documented in cultures treated with anti-immunoglobulin. These results provide direct evidence for the ability of BCR ligation to stimulate immunoglobulin light chain gene rearrangements in immature B cells.

Editing disease-associated autoantibodies

We have generated site-directed transgenic mice whose transgenes code for anti-DNA antibodies. These antibodies are representative of the lupus-associated anti-DNAs seen in mouse models of autoimmunity and human SLE, and have the usual characteristics of pathogenic autoantibodies. As conventional transgenics in nonautoimmune mice, anti-DNA B cells have been shown to be deleted or inactivated. Autoreactive B cells can also escape negative regulation by a process called receptor editing. Here we describe two combined immunoglobulin H and L chain site-directed transgenic mouse models and characterize their editing phenotypes. One model, 3H9R/Vkappa4R, has a deletion-prone phenotype and undergoes editing, primarily by inactivation of the light chain by leap-frogging events. In the other model, 3H9R/Vkappa8R, B cells are susceptible to anergy and maintain most of their HR and LR chains. These studies clarify the relationship between editing and other mechanisms of tolerance.

B cells are exquisitely sensitive to central tolerance and receptor editing induced by ultralow affinity, membrane-bound antigen

To assess the sensitivity of B cell tolerance with respect to receptor/autoantigen affinity, we identified low affinity ligands to the 3-83 (anti-major histocompatibility complex class I) antibody and tested the ability of these ligands to induce central and peripheral tolerance in 3-83 transgenic mice. Several class I protein alloforms, including Kbm3 and Dk, showed remarkably low, but detectable, affinity to 3-83. The 3-83 antibody bound Kb with K lambda approximately 2 x 10(5) M-1 and bound 10-fold more weakly to the Kbm3 (K lambda approximately 2 x 10(4) M-1) and Dk antigens. Breeding 3-83 immunoglobulin transgenic mice with mice expressing these ultralow affinity Kbm3 and Dk ligands resulted in virtually complete deletion of the autoreactive B cells from the peripheral lymphoid tissues. These low affinity antigens also induced receptor editing, as measured by elevated RAG mRNA levels in the bone marrow and excess levels of id- variant B cells bearing lambda light chains in the spleen. Reactive class I antigens were also able to mediate deletion of mature B cells when injected into the peritoneal cavity of 3-83 transgenic mice. Although the highest affinity ligand, Kk, was consistently able to induce elimination of the 3-83 peritoneal B cells, the lower affinity ligands were only partially effective. These results demonstrate the remarkable sensitivity of the deletion and receptor-editing mechanisms in immature B cells, and may suggest a higher affinity threshold for deletion of peripheral, mature B cells.

The lambda B cell repertoire of kappa-deficient mice

Analysis of the B cell repertoire is complicated by the huge diversity inherent in the germ line determined combinatory. Making use of knockout technology, kappa-deficient mice have been obtained. They constitute a shrewd model to follow the expression of an Ig minilocus, such as the lambda one, in the normal condition compared with classical transgenic models. Indeed, in contrast to wild type mice, in which only 5% of lambda B cells are produced, these mutant mice exclusively produce lambda positive B cells. Although, the lambda locus is well characterized and has a relatively simple organization, the mechanistic and selective pressures that govern its utilization are still poorly understood. The analysis of the lambda B cell repertoire in kappa-deficient mice, should therefore bring more conclusive informations. Here we present the lambda subtype distribution in the various cellular compartments of the kappa-deficient mice, and discuss the rules that can be responsible for this distribution. Our recent data indicate that the lambda subtype proportions in the bone marrow and the spleen result, for the major part, from mechanistic processes (i.e., recombinase accessibility, production of V-J functional joint and H/L pairings) while the lambda proportions found in the peritoneal cavity ensue from selective processes. Finally, the capacity to respond to various antigens is discussed from such a generated lambda B cell repertoire.

Immunoglobulin heavy chain gene replacement: a mechanism of receptor editing

We have generated a site-directed transgenic (sd-tg) mouse model in which the JH locus has been replaced with a rearranged VDJ coding for the heavy chain of an anti-DNA antibody. In these mice, B cells expressing the anti-dsDNA specificity are negatively regulated. We observe a novel mechanism for B cell tolerance, receptor editing at the heavy chain locus. In most sd-tg B cells, the inserted anti-DNA VH gene has been replaced by the upstream endogenous VH, or DH, or both genes through recombination with the heptamer embedded at the 3' end of most VH genes. Three types of recombination events have been identified. VH-to-VDJ, DH-to-VDJ, and VH-to-DH-VDJ. Analysis of the junctional sequences revealed features of classical V(D)J rearrangement, namely N sequence addition and nucleotide deletion. A conserved nonamer was found 12 bp upstream of the embedded heptamer. This nonamer may represent a novel recombination signal sequence used for VH editing. The sd-tg model thus provides direct evidence for secondary rearrangement at VH-D-JH. This process may play a role in tolerance by editing autoreactive receptors and may also serve to diversify the VH repertoire.

Light chain replacement: a new model for antibody gene rearrangement

A functional B cell antigen receptor is thought to regulate antibody gene rearrangement either by stopping further rearrangement (exclusion) or by promoting additional rearrangement (editing). We have developed a new model to study the regulation of antibody gene rearrangement. In this model, we used gene targeting to replace the J kappa region with a functional V kappa-J kappa light chain gene. Two different strains of mice were created; one, V kappa 4R, has a V kappa 4-J kappa 4 rearrangement followed by a downstream J kappa 5 segment, while the other, V kappa 8R, has a V kappa 8-J kappa 5 light chain. Here, we analyze the influence of these functional light chains on light chain rearrangement. We show that some V kappa 4R and V kappa 8R B cells only have the V kappa R light chain rearrangement, whereas others undergo additional rearrangements. Additional rearrangement can occur not only at the other kappa allele or isotype (lambda), but also at the targeted locus in both V kappa 4R and V kappa 8R. Rearrangement to the downstream J kappa 5 segment is observed in V kappa 4R, as is deletion of the targeted locus in both V kappa 4R and V kappa 8R. The V kappa R models illustrate that a productively rearranged light chain can either terminate further rearrangement or allow further rearrangement. We attribute the latter to editing of autoantibodies and to corrections of dysfunctional receptors.