Lack of feedback inhibition of V kappa gene rearrangement by productively rearranged alleles

Contribution of secondary Igkappa rearrangement to primary immunoglobulin repertoire diversification

Abs reactive to DNA and DNA/histone complexes are a distinguished characteristic of primary immunoglobulin repertoires in autoimmune B6.MRL-Fas^lpr and MRL/MpJ-Fas^lpr mice. These mice are defective in Fas receptor, which is critical for the apoptosis of autoreactive B cells by an extrinsic pathway. In the present study, we explored the possibility that bone marrow small pre-B and immature B cells from adult B6.MRL-Fas^lpr mice and MRL/MpJ-Fas^lpr mice respectively, which contain autoreactive B-cell antigen receptors (BCR) and manifest autoimmune syndromes, exhibit enhanced receptor editing patterns. Indeed, FAS^lpr pre B and immature B cells were shown to possess more ongoing replacements of non-productive (nP) than productive (P) primary VκJκ rearrangements. Significantly, the P vs nP ratios of these replaced primary rearrangements were 1:2, thus indicating that κ light-chain production appears not to inhibit secondary rearrangements. In addition, we identified multiple atypical rearrangements, such as Vκ cRS (cryptic recombination signals) cleavages. These results suggest that the onset of light chain secondary rearrangements persists similarly as a non-selected mode and independent of BCR autoreactivity during certain developmental windows of bone marrow B cells in lupus-prone mice and control, and leads us to propose the function of secondary, de novo Igκ rearrangements to increase BCR diversity.

Skewed primary Igκ repertoire and V-J joining in C57BL/6 mice: implications for recombination accessibility and receptor editing

Previous estimates of the diversity of the mouse Ab repertoire have been based on fragmentary data as a result of many technical limitations, in particular, the many samples necessary to provide adequate coverage. In this study, we used 5'-coding end amplification of Igκ mRNAs from bone marrow, splenic, and lymph node B cells of C57BL/6 mice combined with amplicon pyrosequencing to assess the functional and nonfunctional Vκ repertoire. To evaluate the potential effects of receptor editing, we also compared V/J associations and usage in bone marrows of mouse mutants under constitutive negative selection or an altered ability to undergo secondary recombination. To focus on preimmune B cells, our cell sorting strategy excluded memory B cells and plasma cells. Analysis of ~90 Mbp, representing >250,000 individual transcripts from 59 mice, revealed that 101 distinct functional Vκ genes are used but at frequencies ranging from ~0.001 to ~10%. Usage of seven Vκ genes made up >40% of the repertoire. A small class of transcripts from apparently nonfunctional Vκ genes was found, as were occasional transcripts from several apparently functional genes that carry aberrant recombination signals. Of 404 potential V-J combinations (101 Vκs × 4 Jκs), 398 (98.5%) were found at least once in our sample. For most Vκ transcripts, all Jκs were used, but V-J association biases were common. Usage patterns were remarkably stable in different selective conditions. Overall, the primary κ repertoire is highly skewed by preferred rearrangements, limiting Ab diversity, but potentially facilitating receptor editing.

Liver-expressed Igkappa superantigen induces tolerance of polyclonal B cells by clonal deletion not kappa to lambda receptor editing

Analysis of tolerance in a polyclonal wild-type B cell population demonstrates apoptosis of cells reactive to antigen expressed on liver membrane.

Little is know about the nature of peripheral B cell tolerance or how it may vary in distinct lineages. Although autoantibody transgenic studies indicate that anergy and apoptosis are involved, some studies claim that receptor editing occurs. To model peripheral B cell tolerance in a normal, polyclonal immune system, we generated transgenic mice expressing an Igκ–light chain–reactive superantigen targeted to the plasma membrane of hepatocytes (pAlb mice). In contrast to mice expressing κ superantigen ubiquitously, in which κ cells edit efficiently to λ, in pAlb mice, κ B cells underwent clonal deletion. Their κ cells failed to populate lymph nodes, and the remaining splenic κ cells were anergic, arrested at a semi-mature stage without undergoing receptor editing. In the liver, κ cells recognized superantigen, down-regulated surface Ig, and expressed active caspase 3, suggesting ongoing apoptosis at the site of B cell receptor ligand expression. Some, apparently mature, κ B1 and follicular B cells persisted in the peritoneum. BAFF (B cell–activating factor belonging to the tumor necrosis factor family) overexpression rescued splenic κ B cell maturation and allowed κ cells to populate lymph nodes. Our model facilitates analysis of tissue-specific autoimmunity, tolerance, and apoptosis in a polyclonal B cell population. The results suggest that deletion, not editing, is the major irreversible pathway of tolerance induction among peripheral B cells.

Receptor editing in self-reactive bone marrow B cells. The Journal of Experimental Medicine. 1993. 177: 1009-1020

A central paradigm of immunology is clonal selection: lymphocytes displaying clonally distributed antigen receptors are generated and subsequently selected by antigen for growth or elimination. Here we show that in mice transgenic for anti-H-2Kk,b antibody genes, in which a homogeneous clone of developing B cells can be analyzed for the outcome of autoantigen encounter, surface immunoglobulin M+/idiotype+ immature B cells binding to self-antigens in the bone marrow are induced to alter the specificity of their antigen receptors. Transgenic bone marrow B cells encountering membrane-bound Kb or Kk proteins modify their receptors by expressing the V(D)J recombinase activator genes and assembling endogenously encoded immunoglobulin light chain variable genes. This (auto)antigen-directed change in the specificity of newly generated lymphocytes is termed receptor editing.

B cell receptor editing in tolerance and autoimmunity

Receptor editing is the process of ongoing antibody gene rearrangement in a lymphocyte that already has a functional antigen receptor. The expression of a functional antigen receptor will normally terminate further rearrangement (allelic exclusion). However, lymphocytes with autoreactive receptors have a chance at escaping negative regulation by "editing" the specificities of their receptors with additional antibody gene rearrangements. As such, editing complicates the Clonal Selection Hypothesis because edited cells are not simply endowed for life with a single, invariant antigen receptor. Furthermore, if the initial immunoglobulin gene is not inactivated during the editing process, allelic exclusion is violated and the B cell can exhibit two specificities. Here, we describe the discovery of editing, the pathways of receptor editing at the heavy (H) and light (L) chain loci, and current evidence regarding how and where editing happens and what effects it has on the antibody repertoire.

[B lymphocytes--a dogma revisited]

The one B cell-one antibody hypothesis proposed by M.F. Burnet (1957) was recently challenged by results showing that one B cell can simultaneously express an auto-reactive BCR, and a BCR directed against non-self antigens. The latter allows this auto-reactive B cell to escape negative selection. Burnet's theory was thus challenged from the beginning. Namely, Liacopoulos et al. demonstrated that normal B cells produce simultaneously or sequentially two antibody molecules against two unrelated haptens. It thus appears that 'double producers' provide new prospects in auto-immunity. Our previous work using heterologous antigens also suggest that 'double producers' provide an amplifying factor for immunoglobulin repertoire.

TCR delta gene rearrangements revealed by fine structure of the recombination junction in mice

The standard products of V(D)J recombination of immunoglobulin and T cell receptor genes are two kinds of DNA junction, a coding joint and a signal joint. TCR delta V-D and D-D signal joints in adult mouse thymocytes were sequenced following PCR amplification. We observed differential nucleotide insertions at the V delta-D delta signal joints, depending on the V delta and D delta gene usage in the developmental stage. Nucleotide insertions at the V delta-D delta 1 signal joints were less frequent for the V delta 4, 5 genes preferentially utilized in adult thymocytes than for the V delta 3, 6 genes, infrequently rearranged to D delta 1. In addition to standard signal joints, unexpectedly, novel nonstandard products, "replacement joints" of D delta 1 substituted downstream by the recombination signal sequence of V delta were also found. However, no D delta 2-associated replacement joints other than V delta 5 were found. The other replacement joints of D delta 1-D delta 2 recombination were also observed. The mutation in TCR beta gene affected the frequency of nucleotide insertions at the V delta-D delta signal joints and inhibited the formation of replacement joint. Recombination mechanism generating the replacement joint and the possible role of TCR beta in up-regulation of TCR delta gene rearrangements are discussed.

Antigen receptor selection by editing or downregulation of V(D)J recombination

Clonal selection is central to immune function, but it is complemented by "receptor selection", which regulates the immune repertoire not by cell death or proliferation but through the control of antigen receptor gene recombination. Inappropriate receptors, such as those that are autoreactive, underexpressed, or that fail to promote positive selection of thymocytes or B cells, stimulate secondary V-to-J recombinations that destroy and replace receptor genes. These processes play a central role in lymphocyte repertoire development. Recent work on the role of receptor selection in B and T cells has uncovered evidence for and against antigen-induced editing in thymocytes. Many studies suggest that editing plays a central role in B and T lymphocyte repertoire development. Important recent evidence has been uncovered addressing the role of tolerance-induced editing in thymocytes.

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.

Essential roles of the kappa light chain intronic enhancer and 3' enhancer in kappa rearrangement and demethylation

The kappa intronic (MiE(kappa)) and 3' (3'E(kappa)) enhancers are both quantitatively important to, but not essential for, immunoglobulin kappa rearrangement. To determine the functional redundancy between these two enhancers, B cells derived from mutant embryonic stem cells--in which both MiE(kappa) and 3'E(kappa) were deleted on both kappa alleles--were analyzed for kappa rearrangement. Our findings indicate that these double-mutant B cells have essentially no kappa rearrangement but do rearrange and express lambda. Therefore, these two kappa enhancers share essential roles in activating V(kappa)J(kappa) rearrangement. Our findings also indicate that the two kappa enhancers play overlapping and distinct roles in the demethylation of kappa in B cells.

Antibody regulation of B cell development

Antibodies on the surface of B lymphocytes trigger adaptive immune responses and control a series of antigen-independent checkpoints during B cell development. These physiologic processes are regulated by a complex of membrane immunoglobulin and two signal transducing proteins known as Ig alpha and Ig beta. Here we focus on the role of antibodies in governing the maturation of B cells from early antigen-independent through the final antigen-dependent stages.

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.

B-cell-receptor-dependent positive and negative selection in immature B cells

This review touches on only a small part of the complex biology of B cells, but serves to illustrate the point that the antigen receptor is the most important of many cell-surface receptors affecting cell-fate decisions. Receptor expression is necessary, but not sufficient, for cell survival. It is also essential that a B cell's antigen-receptor specificity be appropriate for its environment. The need to balance reactivity with self tolerance has resulted in an intricate feedback control (affected by both the recombinase and cell survival) that regulates independent selection events at the level of the receptor and the cell.

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.

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.

Four of five RAG-expressing JCkappa-/- small pre-BII cells have no L chain gene rearrangements: detection by high-efficiency single cell PCR

Single cell PCR assays have been further developed that detect over 80% of all VkappaJkappa, VkappaRS, and VlambdaJlambda rearrangements at efficiencies between 70% and 90%. These IgL chain gene rearrangement assays were used with small pre-BII cells that develop in comparably high numbers in the bone marrow of wild-type, Ckappa-deficient, and JCkappa-deficient homozygous and heterozygous mice. In all of these mice, only 15%-25% of all small pre-BII cells carry VlambdaJlambda rearrangements. These results confirm that lambdaL chain gene rearrangements occur independently of kappaL chain gene rearrangement and expression. They also show that a large part of the small pre-BII cells that express the rearrangement machinery can develop without IgL chain gene rearrangements.

A human immunoglobulin lambda locus is similarly well expressed in mice and humans

Transgenic mice carrying a 380-kb region of the human immunoglobulin (Ig) lambda light (L) chain locus in germline configuration were created. The introduced translocus on a yeast artificial chromosome (YAC) accommodates the most proximal Iglambda variable region (V) gene cluster, including 15 Vlambda genes that contribute to >60% of lambda L chains in humans, all Jlambda-Clambda segments, and the 3' enhancer. HuIglambdaYAC mice were bred with animals in which mouse Igkappa production was silenced by gene targeting. In the kappa-/- background, human Iglambda was expressed by approximately 84% of splenic B cells. A striking result was that human Iglambda was also produced at high levels in mice with normal kappa locus. Analysis of bone marrow cells showed that human Iglambda and mouse Igkappa were expressed at similar levels throughout B cell development, suggesting that the Iglambda translocus and the endogenous kappa locus rearrange independently and with equal efficiency at the same developmental stage. This is further supported by the finding that in hybridomas expressing human Iglambda the endogenous L chain loci were in germline configuration. The presence of somatic hypermutation in the human Vlambda genes indicated that the Iglambda-expressing cells function normally. The finding that human lambda genes can be utilized with similar efficiency in mice and humans implies that L chain expression is critically dependent on the configuration of the locus.

Light Chain Shifting: Identification of a Human Plasma Cell Line Actively Undergoing Light Chain Replacement

We identified an antibody-secreting human B-cell line (HTD8), which actively replaces the production of the original λ light chain with a new λ chain (light chain shifting) at a high rate. Loss of the original rearranged λ light chain occurs by significantly reducing the amount of transcript expressed. Expression of the new λ chain, which replaces the original λ chain, occurs by rearranging new VJ segments on a previously excluded allele. V λ gene usage of these new rearrangements are biased toward Vλ4, Vλ6, and Vλ10 families, which are known to be the least frequently used. In striking contrast to the plasma cell phenotype, recombination activating genes, RAG-1 and RAG-2, were expressed in the HTD8 cells and were shown to be necessary, but insufficient for inducing expression of the new λ chain. These results suggest that human plasma cells have the potential to actively undergo light chain replacement.

Light Chain Shifting: Identification of a Human Plasma Cell Line Actively Undergoing Light Chain Replacement

We identified an antibody-secreting human B-cell line (HTD8), which actively replaces the production of the original λ light chain with a new λ chain (light chain shifting) at a high rate. Loss of the original rearranged λ light chain occurs by significantly reducing the amount of transcript expressed. Expression of the new λ chain, which replaces the original λ chain, occurs by rearranging new VJ segments on a previously excluded allele. V λ gene usage of these new rearrangements are biased toward Vλ4, Vλ6, and Vλ10 families, which are known to be the least frequently used. In striking contrast to the plasma cell phenotype, recombination activating genes, RAG-1 and RAG-2, were expressed in the HTD8 cells and were shown to be necessary, but insufficient for inducing expression of the new λ chain. These results suggest that human plasma cells have the potential to actively undergo light chain replacement.

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.

B-cell development: a comparison between mouse and man

A common variable immunodeficiency (CVID) patient, who carries mutations on both alleles of the gene encoding the surrogate light chain component lambda 5/14.1, shows a similar phenotype of B-cell deficiency as the lambda 5-deficient mutant mouse. As discussed here by Paolo Ghia and colleagues, this points to a remarkably similar developmental pathway of B cells in humans and mice.

The 3' part of the immunoglobulin kappa locus of the mouse

A detailed restriction map of a 430-kb contig comprising the single Ckappa, the 5 Jkappa and the adjoining 22 Vkappa gene segments is presented. The first 12 Vkappa genes following the JkappaCkappa region belong to the Vkappa21 family, the subsequent ones to the closely related families Vkappa8 and Vkappal 9/ 28. Previous difficulties in cloning all Vkappa21 genes can now be explained by the presence of a duplicated region in this part of the locus. The structure was established by analysis of yeast artificial chromosome, bacterial artificial chromosome and cosmid clones and by the so-called long template PCR technique. The distance between Ckappa and the proximal Vkappa21 gene is 22 kb and the average distances between the Vkappa genes are about 20 kb. Of the 12 Vkappa21 genes 5 were sequenced for the first time and 8 of the 12 genes were found to be expressed. Of the 10 Vkappa8 and Vkappa19/28 germline genes 9 are new; expression products of 8 of the 10 genes were known. The known 5', 3' polarities allow to specify for the 22 Vkappa genes whether they are rearranged to the JkappaCkappa element by a deletion or an inversion mechanism. Also the formation of interesting rearrangement products in classical cell lines as MPC11, MOPC41 and PC 7043 can be explained now. The non-Vkappa sequence L10 whose rearrangement by inversion has been described earlier (Hoechtl and Zachau, Nature 1983. 302: 260-263) was now localized downstream of JkappaCkappa.

Receptor editing in a transgenic mouse model: site, efficiency, and role in B cell tolerance and antibody diversification

Mice carrying transgenic rearranged V region genes in their IgH and Igkappa loci to encode an autoreactive specificity direct the emerging autoreactive progenitors into a pre-B cell compartment, in which their receptors are edited by secondary Vkappa-Jkappa rearrangements and RS recombination. Editing is an efficient process, because the mutant mice generate normal numbers of B cells. In a similar nonautoreactive transgenic strain, neither a pre-B cell compartment nor receptor editing was seen. Thus, the pre-B cell compartment may have evolved to edit the receptors of autoreactive cells and later been generally exploited for efficient antibody diversification through the invention of the pre-B cell receptor, mimicking an autoreactive antibody to direct the bulk of the progenitors into that compartment.

Self-antigen does not accelerate immature B cell apoptosis, but stimulates receptor editing as a consequence of developmental arrest

In pre-B lymphocytes, productive rearrangement of Ig light chain genes allows assembly of the B cell receptor (BCR), which selectively promotes further developmental maturation through poorly defined transmembrane signaling events. Using a novel in vitro system to study immune tolerance during development, we find that BCR reactivity to auto-antigen blocks this positive selection, preventing down-regulation of light chain gene recombination and promoting secondary light chain gene rearrangements that often alter BCR specificity, a process called receptor editing. Under these experimental conditions, self-antigen induces secondary light chain gene rearrangements in at least two-thirds of autoreactive immature B cells, but fails to accelerate cell death at this stage. These data suggest that in these cells the mechanism of immune tolerance is receptor selection rather than clonal selection.

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.

Molecular mechanisms and selective influences that shape the kappa gene repertoire of IgM+ B cells

To analyze the human kappa chain repertoire and the influences that shape it, a single cell PCR technique was used that amplified Vkappa Jkappa rearrangements from genomic DNA of individual human B cells. More than 350 productive and 250 nonproductive Vkappa Jkappa rearrangements were sequenced. Nearly every functional Vkappa gene segment was used in rearrangements, although six Vkappa gene segments, A27, L2, L6, L12a, A17, and O12/O2 were used preferentially. Of these, A27, L2, L6, and L12a showed evidence of positive selection based on the variable region and not CDR3, whereas A17 was overrepresented because of a rearrangement bias based on molecular mechanisms. Utilization of Jkappa segments was also nonrandom, with Jkappa1 and Jkappa2 being overrepresented and Jkappa3 and Jkappa5 underrepresented in the nonproductive repertoire, implying a molecular basis for the bias. In B cells with two Vkappa Jkappa rearrangements, marked differences were noted in the Vkappa segments used for the initial and subsequent rearrangements, whereas Jkappa segments were used comparably. Junctional diversity was generated by n-nucleotide addition in 60% and by exonuclease trimming in 75% of the Vkappa Jkappa rearrangements analyzed. Despite this large degree of diversity, a strict CDR3 length was maintained in both productive and nonproductive rearrangements. More than 23% of the productive rearrangements, but only 7% of the nonproductive rearrangements contained somatic hypermutations. Mutations were significantly more frequent in Vkappa sequences derived from CD5- as compared with CD5+ B cells. These results document that the gene segment utilization within the Vkappa repertoire is biased by both intrinsic molecular processes as well as selection after light chain expression. Moreover, IgM+ memory cells with highly mutated kappa genes reside within the CD5- but not the CD5+ B cell compartment.

Molecular Ig gene analysis reveals that monocytoid B cell lymphoma is a malignancy of mature B cells carrying somatically mutated V region genes and suggests that rearrangement of the kappa-deleting element (resulting in deletion of the Ig kappa enhancers) abolishes somatic hypermutation in the human

Five cases of monocytoid B cell lymphoma (MBCL) were analyzed for somatic mutations in the rearranged V region genes. Somatic mutations were found in four of the five cases, whereas one unusual CD5+ lymphoma harbored unmutated V region genes. Since somatic mutations are introduced into V region genes of antigen-activated B cells in the course of T cell-dependent immune responses, these results suggest a derivation of the tumor B cells in MBCL from antigen-experienced mature B cells. An analysis of the kappa-deleting element in two of the cases in which mutated VH but unmutated and nonfunctional V kappa gene rearrangements were found suggests that somatic hypermutation does not take place in human rearranged V kappa region genes when the C kappa gene and the kappa enhancers have been deleted in cis by rearrangement of the kappa-deleting element.

Inducing the loss of immunoglobulin lambda light chain production and the rearrangement of a previously excluded allele in human plasma B cell lines with concanavalin A

We investigated the expression of differential lambda light chains in human B cell lines secreting immunoglobulin (Ig). When these cell lines were cultured with concanavalin A for a long period of time, a subpopulation of some but not all of these cell lines was induced to express new lambda light chains replacing the original lambda chain (light chain shifting). Production of the new lambda chain, which replaces the original lambda chain, results from a VJ rearrangement at a previously excluded allele and a dramatic reduction of the original lambda chain transcript, although no difference was found in the level of heavy chain transcript. Recombination activating genes RAG-1 and RAG-2, which are normally expressed during specific early stages of lymphocyte development, were expressed in not only the light chain shifting-inducible lines but also in the non-inducible cells. Treatment of these Ig secreting cell lines with dibutyryl cAMP, which is known to enhance expression of the RAG genes, could not induce the creation of new lambda light chain-producing cells from the inducible lines, suggesting that the expression of the two RAG genes is not sufficient for inducing new lambda light chain production. Concanavalin A induced a gradual but significant production lost of the original lambda chain in a subpopulation of the light chain shifting-inducible cells but not in the non-inducible cells. Association of new lambda light chain production with loss of original lambda chain raises the possibility that, when the RAG genes are expressed, concanavalin A may act on a novel intracellular mechanism controlling lambda light chain allelic exclusion in these plasma cell lines.

Clonal selection and learning in the antibody system

Each antibody-producing B cell makes antibodies of unique specificity, reflecting a series of ordered gene rearrangements which must be successfully performed if the cell is to survive. A second selection process occurs during immune responses in which a new antibody repertoire is generated through somatic hypermutation. Here only mutants binding antigen with high affinity survive to become memory cells. Cells expressing autoreactive receptors are counter-selected at both stages. This stringent positive and negative selection allows the generation and diversification of cells while rigorously controlling their specificity.

Down-regulation of RAG1 and RAG2 gene expression in preB cells after functional immunoglobulin heavy chain rearrangement

Two waves of immunoglobulin gene rearrangements, first of the heavy, then of the light chain chain gene loci form functional immunoglobulin genes during B cell development. In mouse bone marrow the differential surface expression of B220 (CD45R), c-kit, CD25, and surrogate light chain as well as the cell cycle status allows FACS separation of the cells in which these two waves of rearrangements occur. The gene products of two recombination activating genes, RAG1 and RAG2 are crucial for this rearrangement process. Here, we show that the expression of the RAG genes is twice up- and down-regulated, at the transcriptional level for RAG1 and RAG2, and at the postranscriptional level for RAG2 protein. Expression levels are high in D-->JH and VH-->DJH rearranging proB and preB-I cells, low in preB cells expressing the preB cell receptor on the cell surface, and high again in VL-->JL rearranging small preB-II cells. In immature B cells expressing on the cell surface RAG1 and RAG2 mRNA is down-regulated, whereas RAG2 protein levels are maintained. Down-regulation of RAG1 and RAG2 gene expression after productive rearrangement at one heavy chain allele might be part of the mechanisms that prevent further rearrangements at the other allele.

Immature B cells from human and mouse bone marrow can change their surface light chain expression

The capacity of bone marrow-derived surface immunoglobulin-positive (sIg+) human and mouse immature B cells, generated either in vitro or in vivo, to change their light (L) chain expression, has been assayed by the number of cells which change in vitro from one type of L chain to the other type, or to no sIg at all. Immature sIg+ B cells were generated in vitro from sIg- precursor cells from human or mouse bone marrow. The immature sIg+ cells expressed RAG-1. Human sIg+ cells expressed kappa and lambda L chains in ratios between 1:1 and 3:1, whereas in mouse cells, this ratio ranged from 10:1 to 20:1. Upon reculture of the human and mouse kappa+ sIg+ cells, about half of them remained kappa+, a quarter became lambda+, and another quarter became sIg-. Between 1 and 3% expressed both kappa and lambda chains. Of the human lambda+ cells, about two-thirds remained lambda+, only 1 to 2% became kappa+, while the other third became sIg-. Again, between 1 and 3% expressed both kappa and lambda L chains. These results indicate that expression of sIgM in the B cell membrane does not terminate L chain gene rearrangement, and that some order exists in kappa versus lambda gene rearrangements. Hence, human and mouse kappa+ immature B cells can become lambda+, but very few of the lambda+ cells can become kappa+, and both can become sIg-. Further, human CD10+/sIg+ kappa+ and lambda+ cells and mouse B220low/sIglow kappa+ cells enriched from bone marrow, i.e. immature B cells differentiated in vivo, changed their Ig phenotype upon in vitro culture, but in lower frequencies. By contrast, human and mouse mature B cells did not change their L chain or Ig phenotype. Hence, at least a part of the sIg+ immature B cells in bone marrow retain the capacity to change their L chain and Ig phenotype, and this capacity is lost when they become mature, peripheral B cells.

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.

Development of B cells in scid mice with immunoglobulin transgenes: implications for the control of V(D)J recombination

The inability of scid pro-B cells to progress to the pre-B and B cell stages is believed to be caused by a defective recombinase activity that fails to resolve chromosomal breaks resulting from attempted V(D)J recombination. In support of this model, we report that certain immunoglobulin transgenes, specifically those which strongly inhibit endogenous VH-to-DJH and V kappa-to-J kappa rearrangement in wild-type mice, allow scid pro-B cells to progress to the pre-B and B cell stages. This rescue of scid B cell differentiation is associated with a dramatic reduction in expression of the recombination activation genes, RAG1 and RAG2, and with reduced transcription of the kappa locus.

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

The genetic organization of the kappa and lambda light chain loci permits multiple, successive rearrangement attempts at each allele. Multiple rearrangements allow autoreactive B cells to escape clonal deletion by editing their surface receptors. Editing may also facilitate efficient B cell production by salvaging cells with nonproductive light chain (L chain) rearrangements. To study receptor editing of kappa L chains, we have characterized B cells from mice hemizygous for the targeted inactivation of kappa (JCkD/wt) which have an anti-DNA heavy chain transgene, 3H9. Hybridomas from JCkD/wt mice exhibited an increased frequency of rearrangements to downstream Jk segments (such as Jk5) compared with most surveys from normal mice, consistent with receptor editing by sequential kappa locus rearrangements in JCkD/wt. We observed an even higher frequency of rearrangements to Jk5 in 3H9 JCkD/wt animals compared with nontransgenic JCkD/wt, consistent with editing of autoreactive kappa in 3H9 JCkD/wt. We also recovered a large number of 3H9 JCkD/wt lines with Vk12/13-Jk5 rearrangements and could demonstrate by PCR and Southern analysis that up to three quarters of these lines underwent multiple kappa rearrangements. To investigate editing at the lambda locus, we used homozygous kappa-deficient animals (JCkD/JCkD and 3H9 JCkD/JCkD). The frequencies of V lambda 1 and V lambda 2 rearrangements among splenic hybridomas in 3H9 JCkD/JCkD were reduced by 75% whereas V lambda X was increased 5-10-fold, compared with nontransgenic JCkD/JCkD animals. This indicates that V lambda 1 and V lambda 2 are negatively regulated in 3H9 JCkD/JCkD, consistent with earlier studies that showed that the 3H9 heavy chain, in combination with lambda 1 binds DNA. As successive lambda rearrangements to V lambda X do not inactivate V lambda 1, the consequence of lambda editing in 3H9 JCkD/JCkD would be failed allelic exclusion at lambda. However, analysis of 18 3H9 JCkD/JCkD hybridomas with V lambda 1 and V lambda X DNA rearrangements revealed that most of these lines do not have productive lambda 1 rearrangements. In sum, both kappa and lambda loci undergo editing to recover from nonproductive rearrangement, but only kappa locus editing appears to play a substantial role in rescuing autoreactive B cells from deletion.

Promotion and prevention of autoimmunity by B lymphocytes

B lymphocytes are normally subject to heavy and ongoing selection through their antigen-specific Ig receptors. Self tolerance mediated through antigen-receptor crosslinking on B cells appears to function in a variety of different and perhaps complementary ways, leading to cell death or editing of the antigen-receptor genes. The consequences of defects in these processes are unclear, but may be sufficient to explain systemic autoimmune disease.

Immature surface Ig+ B cells can continue to rearrange kappa and lambda L chain gene loci

Pro and pre B cells possess the long-term capacity to proliferate in vitro on stromal cells and interleukin 7 (IL-7) and can differentiate to surface immunoglobulin (sIg+) cells upon removal of IL-7 from the cultures. A key event in this differentiation is the extensive cell loss due to apoptosis. Because the proto-oncogene bcl-2 can promote cell survival, we established pre-B cell lines from E mu-bcl-2 transgenic mice. These pre-B cells have the same properties as those derived from non-bcl-2 transgenic mice except that they do not die by apoptosis. This allowed us to study the fate of newly formed B cells in vitro for a longer period of time. Here we show that early during the differentiation of pre-B cells, upregulation of RAG-1 and RAG-2 expression go hand in hand with rearrangements of the Ig gene loci. Moreover, the newly formed sIg+ B cells continue to express RAG-1 and RAG-2 and continue to rearrange L chain gene loci, even in the absence of proliferation, in an orderly fashion, so that kappa L+ sIg+ cells can become lambda L+ sIg+ or sIg- cells, whereas lambda L+ sIg+ cells can become sIg-, but not kappa L+ sIg+ cells. Thus, deposition of a complete Ig molecule on the surface of a B cell does not automatically stop the Ig-rearrangement machinery.

Restricted kappa chain expression in early ontogeny: biased utilization of V kappa exons and preferential V kappa-J kappa recombinations

To determine the extent of kappa chain diversity in the preimmune repertoire early in development, kappa cDNA libraries were analyzed from 15-d old fetal omentum, 18-d-old fetal liver, and 3-wk old bone marrow. An anchored polymerase chain reaction approach was used to avoid bias for particular V kappa families. From the sequence analysis of 27 bone marrow clones, 10 different families and 20 unique V kappa genes were identified. In contrast, the V kappa expression in the fetus is highly restricted and clearly differs from the broader distribution see in 3-wk-old bone marrow. Although several V kappa families were represented in the fetal library including V kappa 9, V kappa 10, V kappa 4,5, V kappa 8, and V kappa 1, one or two members of individual families were observed repeatedly. The fetal liver and omentum libraries were found to be largely overlapping. Given the V kappa families/exons identified in the fetal sequences, the mechanism of kappa rearrangements in the early repertoire appears to occur predominantly by inversion. Importantly, the fetal repertoire was further restricted by dominant V kappa-J kappa combinations such as V kappa 4,5-J kappa 5, V kappa 9-J kappa 4, and V kappa 10-J kappa 1. Since in some cases independent rearrangements could be established, the results indicate a bias for particular V kappa-J kappa joins. The results also suggest that clonal expansion/selection in the fetal repertoire takes place after light chain rearrangement as opposed to at the pre-B cell level in the bone marrow. The restriction observed in kappa light chain expression together with known restrictions in gene usage and junctional diversity at the heavy chain level indicate a remarkably conserved fetal repertoire.

B lymphocytes may escape tolerance by revising their antigen receptors

To explore mechanisms that prevent autoreactivity in nonautoimmune mice, endogenous immunoglobulin (Ig) light (L) chains that associate with a transgenic anti-DNA heavy chain were analyzed. The antibodies from splenic B cell hybridomas of such mice did not bind double-stranded DNA (dsDNA) and their L chain sequences showed a biased use of V kappa and J kappa gene segments. The 44 L chains in this survey were coded for by just 18 germline genes. Six of the genes, each belonging to a different V kappa group, were used more than once and accounted for three fourths of all sequences. Based on the distribution of V kappa genes, the L chain repertoire in this line of transgenic mice was estimated at 37 V kappa genes. The most frequently observed gene, a member of the V kappa 12/13 group, was identified in 16 hybrids. In addition, the majority of V kappa genes used J kappa 5. We interpret the skewed representation of V kappa and J kappa gene segments to result from negative selection. Based on the data, we suggest that V kappa rearrangements giving rise to anti-dsDNA reactivity are removed from the repertoire by a corrective mechanism capable of editing self-reactive Ig.

Receptor editing in self-reactive bone marrow B cells

A central paradigm of immunology is clonal selection: lymphocytes displaying clonally distributed antigen receptors are generated and subsequently selected by antigen for growth or elimination. Here we show that in mice transgenic for anti-H-2Kk,b antibody genes, in which a homogeneous clone of developing B cells can be analyzed for the outcome of autoantigen encounter, surface immunoglobulin M+/idiotype+ immature B cells binding to self-antigens in the bone marrow are induced to alter the specificity of their antigen receptors. Transgenic bone marrow B cells encountering membrane-bound Kb or Kk proteins modify their receptors by expressing the V(D)J recombinase activator genes and assembling endogenously encoded immunoglobulin light chain variable genes. This (auto)antigen-directed change in the specificity of newly generated lymphocytes is termed receptor editing.

Receptor editing: an approach by autoreactive B cells to escape tolerance

To determine the fate of anti-DNA antibody-bearing B cells in normal mice, we generated transgenic mice bearing the heavy (H) and light (L) chain genes of a well-characterized anti-double-stranded DNA antibody. This antibody was originally isolated from a diseased MRL/lpr mouse and has characteristics common to spontaneously arising anti-DNA antibodies. Results show that the H/L transgene (tg) immunoglobulin receptor is not expressed by animals bearing both tgs, although single tg animals (H or L) express their transgenes. Young H/L tg animals express few B cells, whereas adult H/L tg animals maintain almost normal B cell numbers. Analysis of the immunoglobulin receptors used by adult B cells shows that all contain the tg H chain in association with endogenous L chains. These B cells transcribe the L tg as well as the rearranged endogenous L chain gene, and loss of endogenous L chain gene transcription results in resurrection of the 3H9 H/L tg product. Examination of the endogenous L chains used by these cells shows that they represent a highly restricted subset of V genes. Taken together, these data suggest that autoreactive transgenic B cells can rearrange endogenous L chain genes to alter surface receptors. Those L chains that compete successfully with the L tg for H chain binding, and that create a nonautoreactive receptor, allow the B cell to escape deletion. We suggest that this receptor editing is a mechanism used by immature autoreactive B cells to escape tolerance.