V(D)J recombinase activity in a subset of germinal center B lymphocytes

Selection at multiple checkpoints focuses V(H)12 B cell differentiation toward a single B-1 cell specificity

Phosphatidyl choline (PtC)-specific B cells segregate to the B-1 subset, where they comprise up to 10% of the B-1 repertoire. About half express V(H)12 and Vkappa4/5H and are restricted in V(H)CDR3. We have previously reported that anti-PtC V(H)CDR3 is enriched among V(H)12-expressing cells by selective elimination of pre-B cells. We report here a bias for Vkappa4/5H expression among V(H)12-expressing B cells, even among those that do not bind PtC and are not B-1. This is due in part to an inability of V(H)12 to associate with many light (L) chains but must also be due to a selective advantage in survival or clonal expansion in the periphery for Vkappa4/5H-expressing cells. Thus, the bias for Vkappa4/5H expression is independent of PtC binding, and, as segregation to B-1 occurs after Ig gene expression, it precedes segregation to the B-1 subset. In 6-1 mice, splenic B-1 cells reside in follicles but segregate to follicles distinct from those that contain B-2 cells. These data indicate that selection at multiple developmental checkpoints ensures the co-expression of an anti-PtC V(H)CDR3 and L chain in a high frequency of V(H)12 B cells. This focus toward specificity for PtC facilitates the development of a large anti-PtC B-1 repertoire.

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.

Allelic exclusion in B and T lymphopoiesis

The plasticity of the immune system relies on stochastic, i.e. random, decisions as well as on controlled events. V(D)J rearrangement of antigen receptors on B and T cells are mediated through the action of compound elements containing enhancer sequences. These elements function in a developmentally stage-specific and a cell-type-specific manner to attract machineries that demethylate DNA, remodel chromatin structure, and induce V(D)J recombination on one allele preferentially.

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.

Continued RAG expression in late stages of B cell development and no apparent re-induction after immunization

Models of B-cell development in the immune system suggest that only those immature B cells in the bone marrow that undergo receptor editing express V(D)J-recombination-activating genes (RAGs). Here we investigate the regulation of RAG expression in transgenic mice carrying a bacterial artificial chromosome that encodes a green fluorescent protein reporter instead of RAG2. We find that the reporter is expressed in all immature B cells in the bone marrow and spleen. Endogenous RAG messenger RNA is expressed in immature B cells in bone marrow and spleen and decreases by two orders of magnitude as they acquire higher levels of surface immunoglobulin M (IgM). Once RAG expression is stopped it is not re-induced during immune responses. Our findings may help to reconcile a series of apparently contradictory observations, and suggest a new model for the mechanisms that regulate allelic exclusion, receptor editing and tolerance.

Relaxed negative selection in germinal centers and impaired affinity maturation in bcl-xL transgenic mice

The role of apoptosis in affinity maturation was investigated by determining the affinity of (4-hydroxy-3-nitrophenyl)acetyl (NP)-specific antibody-forming cells (AFCs) and serum antibody in transgenic mice that overexpress a suppressor of apoptosis, Bcl-xL, in the B cell compartment. Although transgenic animals briefly expressed higher numbers of splenic AFCs after immunization, the bcl-xL transgene did not increase the number or size of germinal centers (GCs), alter the levels of serum antibody, or change the frequency of NP-specific, long-lived AFCs. Nonetheless, the bcl-xL transgene product, in addition to endogenous Bcl-xL, reduced apoptosis in GC B cells and resulted in the expansion of B lymphocytes bearing VDJ rearrangements that are usually rare in primary anti-NP responses. Long-lived AFCs bearing these noncanonical rearrangements were frequent in the bone marrow and secreted immunoglobulin G(1) antibodies with low affinity for NP. The abundance of noncanonical cells lowered the average affinity of long-lived AFCs and serum antibody, demonstrating that Bcl-xL and apoptosis influence clonal selection/maintenance for affinity maturation.

Concanavalin A stimulation enhanced secondary VlambdaJlambda rearrangement in some human plasma B cells without up-regulation of recombination-activating gene expression and Vlambda germline transcription

Light chain shifting is a phenomenon that occurs in certain human antibody-producing plasma B cells which, when stimulated with concanavalin A (Con A), shift production of the original light chain to new light chains. Here we investigated the effect of Con A stimulation on these light chain shift-inducible cells. Analysis of transcripts and VJ-coding joints for new light chains revealed that a leaky amount of secondary VlambdaJlambda rearrangement occurs spontaneously, without Con A stimulation, and that Con A stimulation markedly increases VJ-coding joints and transcripts for new light chains. It was also shown that new light chain producers, which have carried out secondary rearrangement, do not further rearrange their light chain genes, even when stimulated with Con A. Recombination-activating gene (RAG) products and Vlambda germline transcription were constitutively expressed in these cell lines and their expression levels were not affected by Con A stimulation. These results suggest that Con A stimulation enhanced secondary VlambdaJlambda rearrangement, but this was not a result of the up-regulation of RAG expression and Vlambda germline transcription, which are believed to be sufficient for the process of VlambdaJlambda rearrangement.

RAG2:GFP knockin mice reveal novel aspects of RAG2 expression in primary and peripheral lymphoid tissues

We generated mice in which a functional RAG2:GFP fusion gene is knocked in to the endogenous RAG2 locus. In bone marrow and thymus, RAG2:GFP expression occurs in appropriate stages of developing B and T cells as well as in immature bone marrow IgM+ B cells. RAG2:GFP also is expressed in IgD+ B cells following cross-linking of IgM on immature IgM+ IgD+ B cells generated in vitro. RAG2:GFP expression is undetectable in most immature splenic B cells; however, in young RAG2:GFP mice, there are substantial numbers of splenic RAG2:GFP+ cells that mostly resemble pre-B cells. The latter population decreases in size with age but reappears following immunization of older RAG2:GFP mice. We discuss the implications of these findings for current models of receptor assembly and diversification.

Surface Molecule Loss and Bleb Formation by Human Germinal Center B Cells Undergoing Apoptosis: Role of Apoptotic Blebs in Monocyte Chemotaxis

Human tonsil germinal center (GC) B cells rapidly undergo apoptosis in culture. Annexin-V binding shows an early event in this process. In the present study, this method has been used to label apoptotic GC B cells and to analyze additional surface molecules. The expression of all of the molecules studied was reduced in apoptotic (annexin-V+) GC B cells, and the reduction was more marked for CD11a, CD21, CD22, CD49d, and CD54, molecules that participate in survival interaction for GC B cells. The analysis of CD54, one of the molecules that was more drastically reduced, showed that GC, but not mantle zone, B cells actively secrete CD54 to the culture supernatant (SN). The secreted CD54 was partly released from the GC B cells in a particulate form as demonstrated by centrifugation. Further experiments using filtration, fluorescence microscopy, electron microscopy, and flow cytometry analysis showed that GC B cells released to the culture SN a population of spherical membranous vesicles of about 0.18 μm in size, similar to the blebs described in other apoptosis systems. Bleb formation depended on active metabolism, Ca2+, and, in part, on microfilament integrity. GC B-cell–derived blebs were clearly associated with apoptosis, as antiapoptotic stimuli prevented their formation. In addition, GC B-cell–derived blebs contained the adhesion molecules previously studied. Consequently, bleb formation might contribute to the surface molecule loss occurring in apoptotic GC B cells. Finally, a chemotaxis assay showed that GC B-cell blebs were chemotactic for human monocytes, suggesting that this mechanism might operate in vivo.

Genetic pathway to recurrent chromosome translocations in murine lymphoma involves V(D)J recombinase

Chromosome translocations involving antigen receptor loci are a genetic hallmark of non-Hodgkin's lymphomas in humans. Most commonly, these translocations result in juxtaposition of the immunoglobulin heavy-chain (IgH) locus with one of several cellular proto-oncogenes, leading to deregulated oncogene expression. The V(D)J recombinase, which mediates physiologic rearrangements of antigen receptor genes, may play a mechanistic role in some lymphoma translocations, although evidence is indirect. A high incidence of B-lineage lymphomas has been observed in mice with severe combined immunodeficiency (SCID) and p53-null mutations. We show that these tumors are characteristic of the pro-B-cell stage of development and that they harbor recurrent translocations involving chromosomes 12 and 15. Fluorescence in situ hybridization (FISH) shows retention of IgH sequences on the derivative chromosome 12, implying that breakpoints involve the IgH locus. Pro-B-cell lymphomas were suppressed in SCID p53(-/-) mice by a Rag-2-null mutation, demonstrating that DNA breaks generated during V(D)J recombination are required for oncogenic transformation, and suggesting that t(12;15) arise during attempted IgH rearrangement in pro-B cells. These studies indicate that the oncogenic potential inherent in antigen receptor diversification is controlled in vivo by efficient rejoining of DNA ends generated during V(D)J recombination and an intact cellular response to DNA damage.

Secondary rearrangements and hypermutation generate sufficient B cell diversity to mount protective antiviral immunoglobulin responses

Variable (V) region gene replacement was recently implicated in B cell repertoire diversification, but the contribution of this mechanism to antibody responses is still unknown. To investigate the role of V gene replacements in the generation of antigen-specific antibodies, we analyzed antiviral immunoglobulin responses of "quasimonoclonal" (QM) mice. The B cells of QM mice are genetically committed to exclusively express the anti-(4-hydroxy-3-nitrophenyl) acetyl specificity. However, approximately 20% of the peripheral B cells of QM mice undergo secondary rearrangements and thereby potentially acquire new specificities. QM mice infected with vesicular stomatitis virus (VSV), lymphocytic choriomeningitis virus, or poliovirus mounted virus-specific neutralizing antibody responses. In general, kinetics of the antiviral immunoglobulin responses were delayed in QM mice; however, titers similar to control animals were eventually produced that were sufficient to protect against VSV-induced lethal disease. VSV neutralizing single-chain Fv fragments isolated from phage display libraries constructed from QM mice showed VH gene replacements and extensive hypermutation. Thus, our data demonstrate that secondary rearrangements and hypermutation can generate sufficient B cell diversity in QM mice to mount protective antiviral antibody responses, suggesting that these mechanisms might also contribute to the diversification of the B cell repertoire of normal mice.

Antigen-Induced Somatic Diversification of Rabbit IgH Genes: Gene Conversion and Point Mutation

During T cell-dependent immune responses in mouse and human, Ig genes diversify by somatic hypermutation within germinal centers. Rabbits, in addition to using somatic hypermutation to diversify their IgH genes, use a somatic gene conversion-like mechanism, which involves homologous recombination between upstream VH gene segments and the rearranged VDJ genes. Somatic gene conversion and somatic hypermutation occur in young rabbit gut-associated lymphoid tissue and are thought to diversify a primary Ab repertoire that is otherwise limited by preferential VH gene segment utilization. Because somatic gene conversion is rarely found within Ig genes during immune responses in mouse and human, we investigated whether gene conversion in rabbit also occurs during specific immune responses, in a location other than gut-associated lymphoid tissue. We analyzed clonally related VDJ genes from popliteal lymph node B cells responding to primary, secondary, and tertiary immunization with the hapten FITC coupled to a protein carrier. Clonally related VDJ gene sequences were derived from FITC-specific hybridomas, as well as from Ag-induced germinal centers of the popliteal lymph node. By analyzing the nature of mutations within these clonally related VDJ gene sequences, we found evidence not only of ongoing somatic hypermutation, but also of ongoing somatic gene conversion. Thus in rabbit, both somatic gene conversion and somatic hypermutation occur during the course of an immune response.

Abortive apoptosis as an initiator of chromosomal translocations

Apoptosis is a well-recognized regulator of a cell populations size and structure. Irreversible stages of apoptosis lead to activation of different enzymatic cascades, changes in cell morphology and DNA fragmentation. However, little is known about nuclear events which accompany the initial stages of apoptosis. These events are connected with introduction of limited amounts of double strand breaks into genomic DNA, some of which may be subsequently rejoined. We hypothesize here that the initial stages of apoptotic DNA fragmentation may be reversible and connected with the initiation of recombinational events and certain chromosomal translocations. The factors influencing apoptosis reversibility and cell survival after delivery of apoptotic stimuli may provide new insights into mechanisms of lymphocyte development and tumorigenesis.

Molecular Analysis of Single B Cells From T-Cell–Rich B-Cell Lymphoma Shows the Derivation of the Tumor Cells From Mutating Germinal Center B Cells and Exemplifies Means by Which Immunoglobulin Genes Are Modified in Germinal Center B Cells

T-cell–rich B-cell lymphoma (TCRBCL) belongs to the group of diffuse large cell lymphomas (DLL). It is characterized by a small number of tumor B cells among a major population of nonmalignant polyclonal T cells. To identify the developmental stage of the tumor progenitor cells, we micromanipulated the putative neoplastic large CD20+ cells from TCRBCLs and amplified and sequenced immunoglobulin (Ig) V gene rearrangements from individual cells. In six cases, clonal Ig heavy, as well as light chain, gene rearrangements were amplified from the isolated B cells. All six cases harbored somatically mutated V gene rearrangements with an average mutation frequency of 15.5% for heavy (VH) and 5.9% for light (VL) chains and intraclonal diversity based on somatic mutation. These findings identify germinal center (GC) B cells as the precursors of the transformed B cells in TCRBCL. The study also exemplifies various means how Ig gene rearrangements can be modified by GC B cells or their malignant counterparts in TCRBCL: In one case, the tumor precursor may have switched from κ to λ light chain expression after acquiring a crippling mutation within the initially functional κ light chain gene. In another case, the tumor cells harbor two in-frame VH gene rearrangements, one of which was rendered nonfunctional by somatic mutation. Either the tumor cell precursor entered the GC with two potentially functional in-frame rearrangements or the second VHDHJHrearrangement occurred in the GC after the initial in-frame rearrangement was inactivated by somatic mutation. Finally, in each of the six cases, at least one cell contained two (or more) copies of a clonal Ig gene rearrangement with sequence variations between these copies. The presence of sequence variants for V region genes within single B cells has so far not been observed in any other normal or transformed B lymphocyte. Fluorescence in situ hybridization (FISH) points to a generalized polyploidy of the tumor cells.

Production of High Affinity Autoantibodies in Autoimmune New Zealand Black/New Zealand White F1 Mice Targeted with an Anti-DNA Heavy Chain

Lupus-prone, anti-DNA, heavy (H) chain “knock-in” mice were obtained by backcrossing C57BL/6 mice, targeted with a rearranged H chain from a VH11(S107)-encoded anti-DNA hybridoma (D42), onto the autoimmune genetic background of New Zealand Black/New Zealand White (NZB/NZW) F1 mice. The targeted female mice developed typical lupus serologic manifestations, with the appearance of transgenic IgM anti-DNA autoantibodies at a young age (2–3 mo) and high affinity, somatically mutated IgM and IgG anti-DNA Abs at a later age (6–7 mo). However, they did not develop clinical, lupus-associated glomerulonephritis and survived to at least 18 mo of age. L chain analysis of transgenic anti-DNA Abs derived from diseased NZB/NZW mouse hybridomas showed a very restricted repertoire of Vκ utilization, different from that of nonautoimmune (C57BL/6 × BALB/c)F1 transgenic anti-DNA Abs. Strikingly, a single L chain was repetitively selected by most anti-DNA, transgenic NZB/NZW B cells to pair with the targeted H chain. This L chain had the same Vκ-Jκ rearrangement as that expressed by the original anti-DNA D42 hybridoma. These findings indicate that the kinetics of the autoimmune serologic manifestations are similar in wild-type and transgenic lupus-prone NZB/NZW F1 mice and suggest that the breakdown of immunologic tolerance in these mice is associated with the preferential expansion and activation of B cell clones expressing high affinity anti-DNA H/L receptor combinations.

Molecular Analysis of Single B Cells From T-Cell–Rich B-Cell Lymphoma Shows the Derivation of the Tumor Cells From Mutating Germinal Center B Cells and Exemplifies Means by Which Immunoglobulin Genes Are Modified in Germinal Center B Cells

T-cell–rich B-cell lymphoma (TCRBCL) belongs to the group of diffuse large cell lymphomas (DLL). It is characterized by a small number of tumor B cells among a major population of nonmalignant polyclonal T cells. To identify the developmental stage of the tumor progenitor cells, we micromanipulated the putative neoplastic large CD20+ cells from TCRBCLs and amplified and sequenced immunoglobulin (Ig) V gene rearrangements from individual cells. In six cases, clonal Ig heavy, as well as light chain, gene rearrangements were amplified from the isolated B cells. All six cases harbored somatically mutated V gene rearrangements with an average mutation frequency of 15.5% for heavy (VH) and 5.9% for light (VL) chains and intraclonal diversity based on somatic mutation. These findings identify germinal center (GC) B cells as the precursors of the transformed B cells in TCRBCL. The study also exemplifies various means how Ig gene rearrangements can be modified by GC B cells or their malignant counterparts in TCRBCL: In one case, the tumor precursor may have switched from κ to λ light chain expression after acquiring a crippling mutation within the initially functional κ light chain gene. In another case, the tumor cells harbor two in-frame VH gene rearrangements, one of which was rendered nonfunctional by somatic mutation. Either the tumor cell precursor entered the GC with two potentially functional in-frame rearrangements or the second VHDHJHrearrangement occurred in the GC after the initial in-frame rearrangement was inactivated by somatic mutation. Finally, in each of the six cases, at least one cell contained two (or more) copies of a clonal Ig gene rearrangement with sequence variations between these copies. The presence of sequence variants for V region genes within single B cells has so far not been observed in any other normal or transformed B lymphocyte. Fluorescence in situ hybridization (FISH) points to a generalized polyploidy of the tumor cells.

Characterization of anti-DNA B cells that escape negative selection

One of the challenges in the study of autoimmunity is to understand which autoreactive cells are subject to regulation and what mechanisms of regulation are operative. In mice transgenic for the R4A-gamma2b heavy chain of an anti-double stranded (ds) DNA antibody, the gamma2b heavy chain can pair with the full spectrum of endogenous light chains to produce a multitude of antibodies, including anti-dsDNA antibodies of different affinities and fine specificities. We have previously demonstrated the existence of two populations of anti-DNA B cells in non-autoimmune hosts: a high-affinity population which is rendered anergic in vivo, and a second high-affinity population which is deleted. We have now identified a third population of dsDNA-binding B cells. These cells produce germ-line-encoded antibodies with an apparent affinity for dsDNA that is 1 to 4 logs lower than the apparent affinities of antibodies made by anergic or deleted B cells, and represent a non-tolerized population which escapes regulation. Based on its characterization, we can define a molecular threshold for tolerance induction, and can speculate on the fate of these B cells when they are recruited to an immune response and undergo somatic mutation to become high-affinity anti-DNA B cells.

Hypermutation in antibody affinity maturation

By studying the role of mismatch repair in hypermutation at the immunoglobulin loci, the field of antibody hypermutation has been integrated into the larger area of DNA repair. Trans-acting factors - Ku70, Ku80 and possibly SWAP-70 - have been identified for the temporally related but not mechanistically related immunoglobulin heavy-chain class-switch.

Gene Conversion and Hypermutation During Diversification of VH Sequences in Developing Splenic Germinal Centers of Immunized Rabbits

The young rabbit appendix and the chicken bursa of Fabricius are primary lymphoid organs where the B cell Ab repertoire develops in germinal centers (GCs) mainly by a gene conversion-like process. In human and mouse, V-gene diversification by somatic hypermutation in GCs of secondary lymphoid organs leads to affinity maturation. We asked whether gene conversion, somatic hypermutation, or both occur in rabbit splenic GCs during responses to the hapten DNP. We determined DNA sequences of rearranged heavy and light chain V region gene segments in single cells from developing DNP-specific GCs after immunization with DNP-bovine γ-globulin and conclude that the changes at the DNA level that may lead to affinity maturation occur by both gene conversion and hypermutation. Selection was suggested by finding some recurrent amino acid replacements that may contribute increased affinity for antigen in the complementarity-determining region sequences of independently evolved clones, and a narrower range of complementarity-determining region 3 lengths at day 15. Some of the alterations of sequence may also lead to new members of the B cell repertoire in adult rabbits comparable with those produced in gut associated lymphoid tissues of young rabbits.

Distinct factors regulate the murine RAG-2 promoter in B- and T-cell lines

The recombination activating genes RAG-1 and RAG-2 are expressed in a lymphoid-cell-specific and developmentally regulated fashion. To understand the transcriptional basis for this regulation, we have cloned and characterized the murine RAG-2 promoter. The promoter was lymphoid cell specific, showing activity in various B- and T-cell lines but little activity in nonlymphoid cells. To our surprise, however, the promoter was regulated differently in B and T cells. Using nuclear extracts from B-cell lines, we found that the B-cell-specific transcription factor BSAP (Pax-5) could bind to a conserved sequence critical for promoter activity. BSAP activated the promoter in transfected cells, and the BSAP site was occupied in a tissue-specific manner in vivo. An overlapping DNA sequence binding to a distinct factor was necessary for promoter activity in T cells. Full promoter activity in T cells was also dependent on a more distal DNA sequence whose disruption had no effect on B-cell activity. The unexpected finding that a B-cell-specific factor regulates the RAG-2 promoter may explain some of the recently observed differences in the regulation of RAG transcription between B and T cells.

Induction of autoantibodies to mouse CCR5 with recombinant papillomavirus particles

The vertebrate immune system has evolved to respond vigorously to microbial infection but to ignore self-antigens. Evidence has emerged that B cell responses to viruses are initiated by immune recognition of ordered arrays of antigen on the viral surface. To test whether autoantibodies against a self-antigen can be induced by placing it in a context that mimics the ordered surface of a viral particle, a peptide representing an extracellular loop of the mouse chemokine receptor CCR5 was incorporated into an immunodominant site of the bovine papillomavirus virus L1 coat protein, which self-assembles into virus-like particles. Mice inoculated with chimeric L1-CCR5 particles generated autoantibodies that bound to native mouse CCR5, inhibited binding of its ligand RANTES, and blocked HIV-1 infection of an indicator cell line expressing a human-mouse CCR5 chimera. These results suggest a general method for inducing autoantibodies against self-antigens, with diverse potential basic research and clinical applications.

Sjögren's syndrome: lymphoma predisposition coupled with a reduced frequency of t(14;18) translocations in blood lymphocytes

Sjögren's syndrome (SS) is a systemic autoimmune disorder with a strong tumor predisposition (a 44-fold elevated incidence of non-Hodgkin's lymphoma has been reported). By polymerase chain reaction analysis of t(14;18), a key lymphomagenic event in peripheral blood lymphocytes, we found a lower frequency in a subset of 12 SS patients positive for SS-A/SS-B autoantibodies than in 21 healthy subjects and 20 SS patients lacking these SS marker autoantibodies (P < 0.001). All 14 mutants sequenced displayed signs typical of V(D)J recombinase activity. This perplexing result of a low rate of t(14;18) in a population strongly predisposed to t(14;18)-associated tumor development may be explained by a constitutive deficiency in V(D)J recombinase leading to autoimmunity and increased lymphoproliferation.

Negative selection of immature B cells by receptor editing or deletion is determined by site of antigen encounter

Immature B cells that encounter self-antigen are eliminated from the immune repertoire by negative selection. Negative selection has been proposed to take place by two distinct mechanisms: deletion by apoptosis or alteration of the antigen receptor specificity by receptor editing. While convincing evidence exists for each, the two models are inherently contradictory. In this paper, we propose a resolution to this contradiction by demonstrating that the site of first antigen encounter dictates which mechanism of negative selection is utilized. We demonstrate that the bone marrow microenvironment provides signals that block antigen-induced deletion and promote RAG reinduction. In the periphery, the absence of these signals allows the immature B cell to default to apoptosis as a result of BCR engagement.

Ig Heavy Chain Expression and Class Switching In Vitro from an Allele Lacking the 3′ Enhancers DNase I-Hypersensitive hs3A and hs1,2

The murine Ig heavy chain (IgH) 3′ regulatory region contains four enhancers: hs3A, hs1,2, hs3B, and hs4. Various studies have suggested a role for these enhancers in regulating IgH expression and class switching. Here we assess the role of hs3A and hs1,2 in these processes by exploiting a naturally occurring deletion of these enhancers from the expressed, C57BL/6 allele of the F1 pre-B cell line, 70Z/3. Equivalent μ expression in 70Z/3 and 18-81 (which has an intact 3′ region) indicated that hs3A and hs1,2 were not essential for μ expression at the pre-B cell stage. To further examine the role of hs3A and hs1,2 in IgH function at the plasma cell stage, we fused 70Z/3 with the plasmacytoma NSO. Electromobility shift assay analysis of the 70Z/3-NSO hybrids revealed a transcription factor complement conducive to the activation of the 3′ enhancers. Despite the lack of enhancers, hs3A and hs1,2, the level of μ RNA and protein in the 70Z/3-NSO fusion hybrids was substantially elevated relative to its pre-B parent and comparable with that observed in a number of μ-producing spleen cell hybridomas. Additionally, ELISAspot assays showed that the 70Z/3-NSO hybrid underwent spontaneous class switching in culture to IgG1 at a frequency comparable with that of most hybridomas. These results indicate that hs3A and hs1,2 are not essential for high levels of IgH expression or for spontaneous class switching in a plasma cell line.

Lymph node germinal centers form in the absence of follicular dendritic cell networks

Follicular dendritic cell networks are said to be pivotal to both the formation of germinal centers (GCs) and their functions in generating antigen-specific antibody affinity maturation and B cell memory. We report that lymphotoxin beta-deficient mice form GC cell clusters in the gross anatomical location expected of GCs, despite the complete absence of follicular dendritic cell networks. Furthermore, antigen-specific GC generation was at first relatively normal, but these GCs then rapidly regressed and GC-phase antibody affinity maturation was reduced. Lymphotoxin beta-deficient mice also showed substantial B cell memory in their mesenteric lymph nodes. This memory antibody response was of relatively low affinity for antigen at week 4 after challenge, but by week 10 after challenge was comparable to wild-type, indicating that affinity maturation had failed in the GC phase but developed later.

V(D)J hypermutation and receptor revision: coloring outside the lines

At least three mechanisms increase potential genetic diversity in peripheral B lymphocytes: hypermutation, gene conversion and secondary V(D)J rearrangements. These diversifying activities were once believed to be strictly confined to the immunoglobulin loci and B cells. Recent experiments demonstrate that this is not the case. Hypermutation has now been shown to diversify the BCL-6 genes of germinal-center B cells. The role, if any, of these mutations in the immune response remains unknown but the notion that the hypermutation mechanism is targeted solely to immunoglobulin genes is no longer tenable. Peripheral T cells may also diversify their antigen receptors by the reactivation of RAG (recombination-activating gene)1 and RAG2 and secondary V(D)J rearrangements. These new findings suggest a remarkable genetic plasticity in subsets of antigen-reactive lymphocytes and may frame new questions of clonal selection and self tolerance.

Developmental regulation of immune system gene rearrangement

Rearrangement of antigen receptors in the immune system is mediated through the action of complex enhancers which function in both a developmentally stage-specific and a cell-type-specific manner to demethylate DNA, open chromatin structure and tether the recombination machinery to one preferred allele at each locus.

Serum starvation induced secondary V lambda J lambda rearrangement in a human plasma B cell line

HB4C5 is a human antibody producing plasma B cell line that expresses the recombination activating gene-1 (RAG-1) and RAG-2 constitutively, but undergoes few secondary immunoglobulin gene rearrangements when cultured in fetal bovine serum-containing medium. Here, we found that depletion of serum from the culture media induces secondary VlambdaJlambda rearrangement in this cell line. To investigate the induction mechanism of secondary VlambdaJlambda rearrangement, we assessed the expression levels of RAG-1 and RAG-2 products, Vlambda germline transcription level and the amount of Vlambda signal broken ends (SBE) in HB4C5 cells cultured in serum-supplemented or serum-free medium. Western-blot analysis showed that the expression level for the RAG-1 and RAG-2 proteins was not affected by the serum depletion. Vlambda germline transcript was found to be constitutively expressed in HB4C5 cell line and this transcription level was not affected by the lack of serum. On the other hand, the amount of Vlambda SBE was shown to be increased in HB4C5 cells cultured in serum-free medium, suggesting that this increased formation of Vlambda SBE at least partly contributed to the enhanced occurrence of secondary VlambdaJlambda rearrangement in HB4C5 cells cultured in the serum-free condition. These results indicate that expression of RAG proteins and Vlambda germline transcription is not enough to undergo secondary VlambdaJlambda rearrangement in this cell line.

B lymphocyte migration to the bone marrow of humans is not random

This research is a result of analysing a total of 20 independently isolated immunoglobulin gene sequences from a defined population of mature B cells found in the bone marrow of five healthy adults. Each individual exhibited at least two identical gene sequences in our sample. Each variable immunoglobulin sequence is the product of the recombination of three gene segments, V, D and J genes. All sequences sampled contained a specific V gene, V5-51. Comparing the particular J and D genes expressed together with V5-51 as well as the junctional modifications between these genes established the relatedness of the sequences. The likelihood of finding at least two out of k identical gene sequences, if they occur randomly, is found and compared to the experimentally determined sequences. We conclude that it is unlikely that the B cells containing the related sequences found in the bone marrow is by chance. This suggests that this particular population of B cells migrate to the bone marrow in a co-ordinated fashion.

Secondary V(D)J recombination in B-1 cells

B-1 B cells are a self-renewing population of B cells that differ from conventional B cells (B-2 cells) in that they are particularly predisposed to auto-antibody production. Although much is known about the signalling pathways that control B-1-cell growth and development, less is known about why these cells are prone to produce autoreactive antibodies. Here we show that B-1 cells, like germinal-centre B cells, can express recombinase-activating genes 1 and 2 (RAG1 and RAG2) and undergo secondary V(D)J recombination of immunoglobulin genes. In addition, B cells from autoimmune-prone NZB mice show high levels of RAG messenger RNA and recombination. We propose that secondary immunoglobulin-gene rearrangements outside organized lymphoid organs may contribute to the development of autoreactive antibodies.

Analysis of heavy and light chain pairings indicates that receptor editing shapes the human antibody repertoire

In the bone marrow, diversity in the primary antibody repertoire is created by the combinatorial rearrangement of different gene segments and by the association of different heavy and light chains. During the secondary response in the germinal centres, antibodies are diversified by somatic mutation and possibly by further rearrangements, or "receptor editing". Here, we have analysed the pairings of heavy and light chain variable domains (VH and VL) in 365 human IgG+ B cells from peripheral blood, and established that these pairings are largely random. The repertoire is dominated by a limited number of pairings of segments and folds. Among these pairings we identified two identical mutated heavy chains in combination with two different mutated light chains (one kappa and one lambda). This shows that receptor editing occurs in the human periphery and that the same antibody lineage can be subjected to both receptor editing and somatic hypermutation. This suggests that receptor editing may be used together with somatic mutation for the affinity maturation of antibodies. We also propose that receptor editing has shaped variable gene segment use and the evolution of V gene families.

Activated Ras signals developmental progression of recombinase-activating gene (RAG)-deficient pro-B lymphocytes

To elucidate the intracellular pathways that mediate early B cell development, we directed expression of activated Ras to the B cell lineage in the context of the recombination-activating gene 1 (RAG1)-deficient background (referred to as Ras-RAG). Similar to the effects of an immunoglobulin (Ig) mu heavy chain (HC) transgene, activated Ras caused progression of RAG1-deficient progenitor (pro)-B cells to cells that shared many characteristics with precursor (pre)-B cells, including downregulation of surface CD43 expression plus expression of lambda5, RAG2, and germline kappa locus transcripts. However, these Ras-RAG pre-B cells also upregulated surface markers characteristic of more mature B cell stages and populated peripheral lymphoid tissues, with an overall phenotype reminiscent of B lineage cells generated in a RAG- deficient background as a result of expression of an Ig mu HC together with a Bcl-2 transgene. Taken together, these findings suggest that activated Ras signaling in pro-B cells induces developmental progression by activating both differentiation and survival signals.

Insertion of excised IgH switch sequences causes overexpression of cyclin D1 in a myeloma tumor cell

Oncogenes are often dysregulated in B cell tumors as a result of a reciprocal translocation involving an immunoglobulin locus. The translocations are caused by errors in two developmentally regulated DNA recombination processes: V(D)J and IgH switch recombination. Both processes share the property of joining discontinuous sequences from one chromosome and releasing intervening sequences as circles that are lost from progeny cells. Here we show that these intervening sequences may instead insert in the genome and that during productive IgH mu-epsilon switch recombination in U266 myeloma tumor cells, a portion of the excised IgH switch intervening sequences containing the 3' alpha-1 enhancer has inserted on chromosome 11q13, resulting in overexpression of the adjacent cyclin D1 oncogene.

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.

Immune receptor editing: revise and select

Secondary antigen receptor gene recombination occurs in developing lymphocytes, and in more mature T and B cells. In the developing lymphocyte, "editing" occurs in response to receptor ligation by autoantigens. As a result of receptor editing, anti-self-reactive cells are converted to non-self-reactive cells, and self-reactive clones are thereby salvaged before negative selection. This antigen-induced change in the receptor specificity was not foreseen in the clonal selection theory. However, editing could be incorporated into the clonal selection theory by limiting receptor selection to a specific stage in lymphocyte development that precedes cellular selection as suggested by Jerne (1971). We know much less about the origin, regulation, or function of V(D)J recombination in mature lymphocytes. Nevertheless, antigen-induced receptor selection is likely to play an important role in shaping immune responses.

Changes in thymic function with age and during the treatment of HIV infection

The thymus represents the major site of the production and generation of T cells expressing alphabeta-type T-cell antigen receptors. Age-related involution may affect the ability of the thymus to reconstitute T cells expressing CD4 cell-surface antigens that are lost during HIV infection; this effect has been seen after chemotherapy and bone-marrow transplantation. Adult HIV-infected patients treated with highly active antiretroviral therapy (HAART) show a progressive increase in their number of naive CD4-positive T cells. These cells could arise through expansion of existing naive T cells in the periphery or through thymic production of new naive T cells. Here we quantify thymic output by measuring the excisional DNA products of TCR-gene rearrangement. We find that, although thymic function declines with age, substantial output is maintained into late adulthood. HIV infection leads to a decrease in thymic function that can be measured in the peripheral blood and lymphoid tissues. In adults treated with HAART, there is a rapid and sustained increase in thymic output in most subjects. These results indicate that the adult thymus can contribute to immune reconstitution following HAART.

The complete nucleotide sequence of the human immunoglobulin heavy chain variable region locus

The complete nucleotide sequence of the 957-kb DNA of the human immunoglobulin heavy chain variable (VH) region locus was determined and 43 novel VH segments were identified. The region contains 123 VH segments classifiable into seven different families, of which 79 are pseudogenes. Of the 44 VH segments with an open reading frame, 39 are expressed as heavy chain proteins and 1 as mRNA, while the remaining 4 are not found in immunoglobulin cDNAs. Combinatorial diversity of VH region was calculated to be approximately 6,000. Conservation of the promoter and recombination signal sequences was observed to be higher in functional VH segments than in pseudogenes. Phylogenetic analysis of 114 VH segments clearly showed clustering of the VH segments of each family. However, an independent branch in the tree contained a single VH, V4-44.1P, sharing similar levels of homology to human VH families and to those of other vertebrates. Comparison between different copies of homologous units that appear repeatedly across the locus clearly demonstrates that dynamic DNA reorganization of the locus took place at least eight times between 133 and 10 million years ago. One nonimmunoglobulin gene of unknown function was identified in the intergenic region.

Analyses of Single B Cells by Polymerase Chain Reaction Reveal Rearranged VH with Germline Sequences in Spleens of Immunized Adult Rabbits: Implications for B Cell Repertoire Maintenance and Renewal

We used PCR to amplify rearranged VHDJHgenes in single cells collected by micromanipulation from splenic germinal centers of immunized adult rabbits. In the course of the study, the objective of which was to analyze diversification of rearranged VHDJH sequences, we were surprised to find cells 7 and 10 days after immunization with rearranged VH1a2 as well as a-negative (y33 and x32) sequences that were identical or close to germline (10 or fewer changes). About 58% (82/140) of the sequences had unique CDR3 regions and were unrelated. In seven different germinal centers, we found one to four different clones with two to seven members. Clonally related cells underwent diversification by hypermutation and gene conversion. We found that contrary to published reports, adult rabbits indeed have newly diversifying B cell receptors in splenic germinal centers. The attractive idea that the rabbit, like the chicken, develops its B cell repertoire early in life and depends upon self-renewing cells in the periphery to maintain its B lymphocyte pool throughout life, is challenged by the current finding. Although a major population of B lymphocytes may be generated early in life, diversified extensively, and maintained by self-renewal in the periphery, some sources of cells with sequences close to germline do exist in adult rabbits and appear in the developing germinal centers. Although considerable repertoire diversity is generated in young rabbits, mechanisms for continued generation of B cell receptor diversity are retained in adult life, where they may confer survival advantage.

Human immunoglobulin (Ig)M+IgD+ peripheral blood B cells expressing the CD27 cell surface antigen carry somatically mutated variable region genes: CD27 as a general marker for somatically mutated (memory) B cells

Immunoglobulin (Ig)M+IgD+ B cells are generally assumed to represent antigen-inexperienced, naive B cells expressing variable (V) region genes without somatic mutations. We report here that human IgM+IgD+ peripheral blood (PB) B cells expressing the CD27 cell surface antigen carry mutated V genes, in contrast to CD27-negative IgM+IgD+ B cells. IgM+IgD+CD27(+) B cells resemble class-switched and IgM-only memory cells in terms of cell phenotype, and comprise approximately 15% of PB B lymphocytes in healthy adults. Moreover, a very small population (<1% of PB B cells) of highly mutated IgD-only B cells was detected, which likely represent the PB counterpart of IgD-only tonsillar germinal center and plasma cells. Overall, the B cell pool in the PB of adults consists of approximately 40% mutated memory B cells and 60% unmutated, naive IgD+CD27(-) B cells (including CD5(+) B cells). In the somatically mutated B cells, VH region genes carry a two- to threefold higher load of somatic mutation than rearranged Vkappa genes. This might be due to an intrinsically lower mutation rate in kappa light chain genes compared with heavy chain genes and/or result from kappa light chain gene rearrangements in GC B cells. A common feature of the somatically mutated B cell subsets is the expression of the CD27 cell surface antigen which therefore may represent a general marker for memory B cells in humans.

RAG reexpression and DNA recombination at T cell receptor loci in peripheral CD4+ T cells

Under most circumstances, allelic exclusion at the T cell receptor (TCR)beta locus is tightly regulated. Here, we describe a system in which TCRbeta allelic exclusion is overcome as a result of V(D)J recombination in peripheral CD4+ T cells. In TCRbeta chain transgenic mice, tolerogen-mediated chronic peripheral selection against cells expressing the transgene leads to surface expression of endogenous TCRbeta chains. Peripheral CD4+ T cells reexpress the recombination activating genes, RAG1 and RAG2, and contain signal end intermediates indicative of ongoing V(D)J recombination. The rescue from deletion of mature T cells expressing newly generated TCRbeta chains suggests that receptor revision plays a role in the maintenance of peripheral T cell tolerance.

Re-expression of RAG-1 and RAG-2 genes and evidence for secondary rearrangements in human germinal center B lymphocytes

V(D)J recombination occurs in immature B cells within primary lymphoid organs. However, recent evidence demonstrated that the recombination activating genes RAG-1 and RAG-2 can also be expressed in murine germinal centers (GC) where they can mediate secondary rearrangements. This finding raises a number of interesting questions, the most important of which is what is the physiological role, if any, of secondary immunoglobulin (Ig) gene rearrangements. In the present report, we provide evidence that human GC B cells that have lost surface immunoglobulin re-express RAG-1 and RAG-2, suggesting that they may be able to undergo Ig rearrangement. Furthermore, we describe two mature B cell clones in which secondary rearrangements have possibly occurred, resulting in light chain replacement. The two clones carry both kappa and lambda light chains productively rearranged, but fail to express the x chain on the cell surface due to a stop codon acquired by somatic mutation. Interestingly, the analysis of the extent of somatic mutations accumulated by the two light chains might suggest that the lambda chain could have been acquired through a secondary rearrangement. Taken together, these data suggest that secondary Ig gene rearrangements leading to replacement may occur in human GC and may contribute to the peripheral B cell repertoire.

Localization of Splenic B Cells Activated for Switch Recombination by In Situ Hybridization with Iγ1 Switch Transcript and Rad51 Probes

B cells are activated for switch recombination by signals from Th cells, but the site at which this first occurs in vivo has yet to be identified. By in situ hybridization of splenic sections using riboprobes specific for the Iγ1 switch transcript and Rad51 mRNA, we have visualized B cells that are newly activated for switch recombination and characterized the spatial and temporal patterns of Iγ1 and Rad51 mRNA expression. Within 2 days after immunization with (4-hydroxy-3-nitrophenyl)acetyl-chicken gamma-globulin, expression of Iγ1 switch transcripts and Rad51 mRNA was evident and was localized to B220+ B cells clustered within the T cell-rich periarteriolar lymphoid sheath (PALS) and surrounding follicles. By Ab staining, we have shown previously that cells switching from IgM to IgG expression can be visualized at 3 to 5 days postimmunization and colocalize to clusters of Rad51+ cells. Hybridization of adjacent sections with probes for Cμ and Cγ1 mRNA now shows that switching from μ to γ expression occurs within Rad51+Iγ1+ regions of the PALS and peaks between days 3 and 5. Colocalized expression of Iγ1 and Rad51 transcripts was observed from days 2 through 12 of the immune response. Iγ1 and Rad51 transcripts were down-regulated but still detectable at 12 days postimmunization, when they were evident in peanut agglutinin-positive germinal center B cells. Taken together, these observations show that B cells are first activated for switch recombination in the T cell-rich PALS.

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.

V(H) gene replacement occurs in the spleen and bone marrow of non-autoimmune quasi-monoclonal mice

Genes encoding the heavy chain portion of immunoglobulin molecules arise from the combinatorial association of V, D and J gene segments, which occurs during discrete stages of B lineage development in the bone marrow. Recently, V(H) replacement, a form of receptor editing, has been described, in which the variable region of an existing VDJ(H) rearrangement is replaced by another V(H) gene segment in a recombination event believed to involve an embedded heptamer within the coding region of the V(H). Studies of transgenic mice with "knocked-in" VDJ(H) genes encoding anti-DNA specificity have demonstrated that receptor editing of the heavy chain is one mechanism by which autoreactive B cell receptors can be modified. Another mouse, the "quasi-monoclonal", which encodes a "knocked-in" VDJ(H) for the hapten NP also contains B lineage cells that undergo V(H) replacement. This suggests that V(H) replacement may play a role in the normal diversification of the antibody repertoire. Using a ligation-mediated PCR assay, we have identified V(QM) double-stranded DNA breaks indicative of V(H) replacement intermediates from bone marrow and splenic B lineage cells of quasi-monoclonal mice in the absence of immunization. V(QM) to J558 recombination deletion products consistent with V(H) replacement were also detected in both the bone marrow and spleen of non-immunized quasi-monoclonal mice. Moreover, RAG-1 transcripts were detected in the spleen. These data suggest that V(H) replacement can be part of the mechanism(s) used by B lineage cells to generate diversity throughout B lineage development, including later stages occurring in secondary lymphoid tissues.