Allelic exclusion of immunoglobulin gene rearrangement and expression: why and how?

Decoding B Cells in Autoimmune Diseases Through ScRNA + BCR-Seq: Current Knowledge and Future Directions

The combined application of single-cell RNA sequencing (scRNA-seq) and single-cell B-cell receptor sequencing (scBCR-seq) offers a multidimensional perspective for dissecting the immunopathological mechanisms of B cells in autoimmune diseases. This review systematically summarizes the principles of these techniques, the analytical framework, and their key applications in diseases such as systemic lupus erythematosus et. al. It reveals the dynamic correlations between the transcriptome of B-cell subsets and B-cell receptor (BCR) clones. Furthermore, we focus on the potential roles of dual BCR B cells and B/T biphenotypic cells in autoimmunity, emphasizing their exacerbation of disease progression through abnormal clonal expansion and autoantibody secretion. By sorting through cutting-edge advancements and bottleneck issues, this article aims to propel the innovation of multi-omics research and precision treatment paradigms for autoimmune diseases.

scRNA+ TCR-seq revealed dual TCR T cells antitumor response in the TME of NSCLC

The intricate origins, subsets, and characteristics of TCR (T Cell Receptor) s, along with the mechanisms underpinning the antitumor response of tumor-infiltrating T lymphocytes within the tumor microenvironment (TME) remain enigmatic. Recently, the advent of single-cell RNA+TCR-sequencing (scRNA+TCR seq) has revolutionized TME analysis, providing unprecedented insight into the origins, cell subsets, TCR CDR3 compositions, and the expression patterns of response/depletion factors within individual tumor-infiltrating T lymphocytes. Our analysis of the shared scRNA+TCR seq dataset revealed a substantial presence of dual TCR T cells, characterized by clonal hyperplasia and remarkable migratory prowess across various tissues, including blood, normal, peritumoral, and tumor tissues in non-small cell lung cancer patients. Notably, dual TCR CD8+T cells predominantly fell within the CXCL13+subset, displaying potent antitumor activity and a strong preference for tumor tissue residency. Conversely, dual TCR CD4+T cells were predominantly classified as CD5+ or LMNA+subsets, exhibiting a more even distribution across diverse tissue types. By harnessing scRNA+TCR seq and other cutting-edge technologies, we can delve deeper into the effects and mechanisms that regulate the antitumor response or tolerance of dual TCR T cells. This innovative approach holds immense promise in offering fresh perspectives and avenues for advancing research on TIL (Tumor infiltrating lymphocyte)s within the TME.

scRNA-seq revealed the special TCR β & α V(D)J allelic inclusion rearrangement and the high proportion dual (or more) TCR-expressing cells

Allelic exclusion, one lymphocyte expresses one antigen receptor, is a fundamental mechanism of immunological self-tolerance and highly specific immune responses to pathogens. However, the phenomenon of V(D)J allelic inclusion (incomplete allelic exclusion or allelic escape) rearrangement and dual TCR T cells have been discovered by multiple laboratories. Despite continuous new discoveries, the proportion and underlying mechanism of dual TCR has been puzzling immunologists. In this study, we observed the presence of single T cells expressing multiple TCR chains in all samples, with the proportion of 15%, 10%, and 20% in the human thymus, human peripheral blood, and mouse lymphoid organs, respectively. The proportion of T cells possessing multiple T-cell receptors (TCR) varied significantly in different physiological states and developmental stages. By analyzing RSS category, RSS direction, and V(D)J gene position at TR locus of T cells which contain multiple TCR chains, we creatively found that one of TCR β (or TCR α) should originate from the transcription of V(D)J combination in T-cell receptor excision circle (TREC) formed after the twice successful rearrangement in the same chromosome. Moreover, human V30 (or mouse V31) gene may participate in reverse recombination and transcription to prevent allelic exclusion. In general, high proportion of T cells with multiple TCR at the transcriptome level was first made public, and we proposed a novel mechanism of secondary (or more) TCR rearrangement on a single chromosome. Our findings also indicated that the single-cell sequencing data should be classified according to the single, multiple, and abnormal TCR when analyzing the T-cell repertoire.

The role of B cells in multiple sclerosis: Current and future therapies

While it was long held that T cells were the primary mediators of multiple sclerosis (MS) pathogenesis, the beneficial effects observed in response to treatment with Rituximab (RTX), a monoclonal antibody (mAb) targeting CD20, shed light on a key contributor to MS that had been previously underappreciated: B cells. This has been reaffirmed by results from clinical trials testing the efficacy of subsequently developed B cell-depleting mAbs targeting CD20 as well as studies revisiting the effects of previous disease-modifying therapies (DMTs) on B cell subsets thought to modulate disease severity. In this review, we summarize current knowledge regarding the complex roles of B cells in MS pathogenesis and current and potential future B cell-directed therapies.

Mechanisms of central tolerance for B cells

Immune tolerance hinders the potentially destructive responses of lymphocytes to host tissues. Tolerance is regulated at the stage of immature B cell development (central tolerance) by clonal deletion, involving apoptosis, and by receptor editing, which reprogrammes the specificity of B cells through secondary recombination of antibody genes. Recent mechanistic studies have begun to elucidate how these divergent mechanisms are controlled. Single-cell antibody cloning has revealed defects of B cell central tolerance in human autoimmune diseases and in several human immunodeficiency diseases caused by single gene mutations, which indicates the relevance of B cell tolerance to disease and suggests possible genetic pathways that regulate tolerance.

Long-Range Control of V(D)J Recombination & Allelic Exclusion: Modeling Views

Allelic exclusion of immunoglobulin (Ig) and T-cell receptor (TCR) genes ensures the development of B and T lymphocytes operating under the mode of clonal selection. This phenomenon associates asynchronous V(D)J recombination events at Ig or TCR alleles and inhibitory feedback control. Despite years of intense research, however, the mechanisms that sustain asymmetric choice in random Ig/TCR dual allele usage and the production of Ig/TCR monoallelic expressing B and T lymphocytes remain unclear and open for debate. In this chapter, we first recapitulate the biological evidence that almost from the start appeared to link V(D)J recombination and allelic exclusion. We review the theoretical models previously proposed to explain this connection. Finally, we introduce our own mathematical modeling views based on how the developmental dynamics of individual lymphoid cells combine to sustain allelic exclusion.

Immunoglobulin sequence analysis and prognostication in CLL: guidelines from the ERIC review board for reliable interpretation of problematic cases

Immunoglobulin gene sequence analysis is widely utilized for prognostication in chronic lymphocytic leukemia (CLL) and the definition of standardized procedures has allowed reliable and reproducible results. Occasionally, a straightforward interpretation of the sequences is not possible because of the so-called 'problematic sequences' that do not fit the 'classic' interpretation and pose scientific questions at the cross-road between hematology and immunology. Thanks to a dedicated effort within the European Research Initiative on CLL (ERIC), we have now the possibility to present such cases, offer a scientific explanation and propose recommendations in terms of prognostication.

Initiation of allelic exclusion by stochastic interaction of Tcrb alleles with repressive nuclear compartments

Studies of antigen-receptor loci have linked directed monoallelic association with pericentromeric heterochromatin to the initiation or maintenance of allelic exclusion. Here we provide evidence for a fundamentally different basis for T cell antigen receptor-beta (Tcrb) allelic exclusion. Using three-dimensional immunofluorescence in situ hybridization, we found that germline Tcrb alleles associated stochastically and at high frequency with the nuclear lamina or with pericentromeric heterochromatin in developing thymocytes and that such interactions inhibited variable-to-diversity-joining (V(beta)-to-D(beta)J(beta)) recombination before beta-selection. The introduction of an ectopic enhancer into Tcrb resulted in fewer such interactions and impaired allelic exclusion. We propose that initial V(beta)-to-D(beta)J(beta) recombination events are generally monoallelic in developing thymocytes because of frequent stochastic, rather than directed, interactions of Tcrb alleles with repressive nuclear compartments. Such interactions may be essential for Tcrb allelic exclusion.

Immunoglobulin gene rearrangements and the pathogenesis of multiple myeloma

The ability to rearrange the germ-line DNA to generate antibody diversity is an essential prerequisite for the production of a functional repertoire. While this is essential to prevent infections, it also represents the “Achilles heal” of the B-cell lineage, occasionally leading to malignant transformation of these cells by translocation of protooncogenes into the immunoglobulin (Ig) loci. However, in evolutionary terms this is a small price to pay for a functional immune system. The study of the configuration and rearrangements of the Ig gene loci has contributed extensively to our understanding of the natural history of development of myeloma. In addition to this, the analysis of Ig gene rearrangements in B-cell neoplasms provides information about the clonal origin of the disease, prognosis, as well as providing a clinical useful tool for clonality detection and minimal residual disease monitoring. Herein, we review the data currently available on both Ig gene rearrangements and protein patterns seen in myeloma with the aim of illustrating how this knowledge has contributed to our understanding of the pathobiology of myeloma.

Ig allotypic inclusion does not prevent B cell development or response

B cells expressing two different Ig kappa L chains (allotype included) have been occasionally observed. To determine frequency and function of these cells, we have analyzed gene-targeted mice that carry a human and a mouse Igk C region genes. Using different methodologies, we found that cells expressing two distinct kappa-chains were 1.4-3% of all B cells and that they were present in the follicular, marginal zone, and B1 mature B cell subsets. When stimulated in vitro with anti-IgM, dual kappa surface-positive cells underwent activation that manifested with cell proliferation and/or up-regulation of activation markers and similar to single kappa-expressing B cells. Yet, when activated by divalent reagents that bound only one of the two kappa-chains, dual kappa B cells responded suboptimally in vitro, most likely because of reduced Ag receptor cross-linking. Nonetheless, dual kappa B cells participated in a SRBC-specific immune response in vivo. Finally, we found that Ig allotype-included B cells that coexpress autoreactive and nonautoreactive Ag receptors were also capable of in vitro responses following BCR aggregation. In summary, our studies demonstrate that Ig kappa allotype-included B cells are present in the mouse mature B cell population and are responsive to BCR stimulation both in vitro and in vivo. Moreover, because in vitro activation in response to anti-IgM was also observed in cells coexpressing autoreactive and nonautoreactive Abs, our studies suggest a potential role of allotype-included B cells in both physiological and pathological immune responses.

Attenuation of IL-7 Receptor Signaling Is Not Required for Allelic Exclusion

Allelic exclusion prevents pre-B cells from generating more than one functional H chain, thereby ensuring the formation of a unique pre-BCR. The signaling processes underlying allelic exclusion are not clearly understood. IL-7R-dependent signals have been clearly shown to regulate the accessibility of the Ig H chain locus. More recent work has suggested that pre-BCR-dependent attenuation of IL-7R signaling returns the H chain loci to an inaccessible state; this process has been proposed to underlie allelic exclusion. Importantly, this model predicts that preventing pre-BCR-dependent down-regulation of IL-7R signaling should interfere with allelic exclusion. To test this hypothesis, we made use of transgenic mice that express a constitutively active form of STAT5b (STAT5b-CA). STAT5b-CA expression restores V(D)J recombination in IL-7R(-/-) B cells, demonstrating that IL-7 regulates H chain locus accessibility and V(D)J recombination via STAT5 activation. To examine the effects of constitutively active STAT5b on allelic exclusion, we crossed STAT5b-CA mice (which express the IgM(b) allotype) to IgM(a) allotype congenic mice. We found no difference in the percentage of IgM(a)/IgM(b)-coexpressing B cells in STAT5b-CA vs littermate control mice; identical results were observed when crossing STAT5b-CA mice with hen egg lysozyme (HEL) H chain transgenic mice. The HEL transgene enforces allelic exclusion, preventing rearrangement of endogenous H chain genes; importantly, rearrangement of endogenous H chain genes was suppressed to a similar degree in STAT5b-CA vs HEL mice. Thus, attenuation of IL-7R/STAT5 signaling is not required for allelic exclusion.

The κ-like pre-B receptor: Surplus biology or a missing link?

After the demonstration that surrogate JCkappa polypeptides could covalently bind mu heavy chain and upon the characterization of the Vkappa-like component of the kappa-like pre-B cell receptor, it became evident that germline transcription is not sterile. The present review discusses the concept of the alternative usage of kappa-like pre-B cell receptors and classical pre-B cell receptors utilizing the lambda-like surrogate light chain composed of lambda5 and VpreB. We propose that both kappa-like and lambda-like pre-B cell receptors work in concert in a fail-safe mechanism to promote light chain rearrangement, heavy chain allelic exclusion and B-lymphocyte maturation.

Receptor editing can lead to allelic inclusion and development of B cells that retain antibodies reacting with high avidity autoantigens

Receptor editing is a major B cell tolerance mechanism that operates by secondary Ig gene rearrangements to change the specificity of autoreactive developing B cells. In the 3-83Igi mouse model, receptor editing operates in every autoreactive anti-H-2K(b) B cell, providing a novel receptor without additional cell loss. Despite the efficiency of receptor editing in generating nonautoreactive Ag receptors, we show in this study that this process does not inactivate the autoantibody-encoding gene(s) in every autoreactive B cell. In fact, receptor editing can generate allelically and isotypically included B cells that simultaneously express the original autoreactive and a novel nonautoreactive Ag receptors. Such dual Ab-expressing B cells differentiate into transitional and mature B cells retaining the expression of the autoantibody despite the high avidity interaction between the autoantibody and the self-Ag in this system. Moreover, we find that these high avidity autoreactive B cells retain the autoreactive Ag receptor within the cell as a consequence of autoantigen engagement and through a Src family kinase-dependent process. Finally, anti-H-2K(b) IgM autoantibodies are found in the sera of older 3-83Igi mice, indicating that dual Ab-expressing autoreactive B cells are potentially functional and capable of differentiating into IgM autoantibody-secreting plasma cells under certain circumstances. These results demonstrate that autoreactive B cells reacting with ubiquitous membrane bound autoantigens can bypass mechanisms of central tolerance by coexpressing nonautoreactive Abs. These dual Ab-expressing autoreactive B cells conceal their autoantibodies within the cell manifesting a superficially tolerant phenotype that can be partially overcome to secrete IgM autoantibodies.

The pre-B cell receptor in B cell development: recent advances, persistent questions and conserved mechanisms

B cell development is a process tightly regulated by the orchestrated signaling of cytokine receptors, the pre-B cell receptor (BCR) and the B cell receptor (BCR). It commences with common lymphoid progenitors (CLP) up-regulating the expression of B cell-related genes and committing to the B cell lineage. Cytokine signaling (IL-7, stem cell factor, FLT3-L) is essential at this stage of development as it suppresses cell death, sustains proliferation and facilitates heavy chain rearrangements. As a result of heavy chain recombination, the pre-BCR is expressed, which then becomes the primary determiner of survival, cell cycle entry and allelic exclusion. In this review, we discuss the mechanisms of B cell lineage commitment and describe the signaling pathways that are initiated by the pre-BCR. Finally, we compare pre-BCR and pre-TCR structure, signal transduction and function, drawing parallels between early pre-B and pre-T cell development.

Mechanisms controlling termination of V-J recombination at the TCRgamma locus: implications for allelic and isotypic exclusion of TCRgamma chains

Analyses of Vgamma-Jgamma rearrangements producing the most commonly expressed TCRgamma chains in over 200 gammadelta TCR(+) thymocytes showed that assembly of TCRgamma V-region genes display properties of allelic exclusion. Moreover, introduction of functionally rearranged TCRgamma and delta transgenes results in a profound inhibition of endogenous TCRgamma rearrangements in progenitor cells. The extent of TCRgamma rearrangements in these cells is best explained by a model in which initiation of TCRgamma rearrangements at both alleles is asymmetric, occurs at different frequencies depending on the V or J segments involved, and is terminated upon production of a functional gammadelta TCR. Approximately 10% of the cells studied contained two functional TCRgamma chains involving different V and Jgamma gene segments, thus defining a certain degree of isotypic inclusion. However, these cells are isotypically excluded at the level of cell surface expression possibly due to pairing restrictions between different TCRgamma and delta chains.

Immunoglobulin locus silencing and allelic exclusion

Lymphocytes are characterised by monoclonal expression of antigen receptors. This is achieved by silencing of one of two homologous antigen receptor alleles, a process known as allelic exclusion. This process is regulated both before and after V(D)J recombination, by a variety of mechanisms. These include nuclear localisation, changes in chromatin structure and histone modifications, non-coding sense and antisense RNA transcription, epigenetic alterations at the DNA level, feedback signalling from expressed alleles, locus contraction and decontraction, recruitment to heterochromatin. This review will focus on recent advances in the immunoglobulin heavy and kappa light chain loci. The current picture is of a complex, temporally ordered sequence of events, in which these loci share many contributory mechanisms, but clear and intriguing differences are emerging.

Basal immunoglobulin signaling actively maintains developmental stage in immature B cells

In developing B lymphocytes, a successful V(D)J heavy chain (HC) immunoglobulin (Ig) rearrangement establishes HC allelic exclusion and signals pro-B cells to advance in development to the pre-B stage. A subsequent functional light chain (LC) rearrangement then results in the surface expression of IgM at the immature B cell stage. Here we show that interruption of basal IgM signaling in immature B cells, either by the inducible deletion of surface Ig via Cre-mediated excision or by incubating cells with the tyrosine kinase inhibitor herbimycin A or the phosphatidylinositol 3-kinase inhibitor wortmannin, led to a striking “back-differentiation” of cells to an earlier stage in B cell development, characterized by the expression of pro-B cell genes. Cells undergoing this reversal in development also showed evidence of new LC gene rearrangements, suggesting an important role for basal Ig signaling in the maintenance of LC allelic exclusion. These studies identify a previously unappreciated level of plasticity in the B cell developmental program, and have important implications for our understanding of central tolerance mechanisms.

Gene rearrangement is a hallmark of B cell maturation. By interrupting basal cell signaling through the rearranged IgM receptor, immature B cells "back-differentiate" to an earlier stage in their development

Tsix transcription- versus RNA-based mechanisms in Xist repression and epigenetic choice

Recent inquiries have revealed a surprisingly large number (>2500) of naturally occurring antisense transcripts, but their function remains largely undiscovered. A better understanding of antisense mechanisms is clearly needed because of their potentially diverse roles in gene regulation and disease. A well-documented case occurs in X inactivation, the mechanism by which X-linked gene expression is equalized between XX females and XY males. The antisense gene Tsix determines X chromosome choice and represses the noncoding silencer, Xist. In principle, Tsix action may involve RNA, the act of transcription, or local chromatin. Here, we create novel Tsix alleles to distinguish transcription- versus RNA-based mechanisms. When Tsix transcription is terminated before Xist (TsixTRAP), Tsix cannot block Xist upregulation, suggesting the importance of overlapping antisense transcription. To separate the act of transcription from RNA, we knocked in Tsix cDNA in the reverse orientation (Tsix(cDNA)) to restore RNA levels in cis without concurrent transcription across Xist. However, Tsix(cDNA) cannot complement TsixTRAP. Surprisingly, both mutations disrupt choice, indicating that this epigenetic step requires transcription. We conclude that the processed antisense RNA does not act alone and that Tsix function specifically requires antiparallel transcription through Xist. A mechanism of transcription-based feedback regulation is proposed.

Regulation of activation and recombination of the murine Igkappa locus

The murine immunoglobulin (Ig) kappa locus has been intensively studied in an attempt to understand its developmentally regulated activation for both transcription and V(D)J recombination. A variety of signaling proteins, cis-acting DNA elements, and trans-acting DNA-binding proteins have been discovered and shown to be involved in the regulated changes in chromatin structure, which are associated with recombinase accessibility. In addition, key roles have been suggested for DNA methylation and replication in kappa-locus expression and rearrangement. This review summarizes data in this area and considers what studies of the murine kappa locus have revealed about the lineage specificity, order, and allelic exclusion of lymphoid V(D)J recombination.

The molecular basis and biological significance of VH replacement

First observed in mouse pre-B-cell lines and then in knock-in mice carrying self-reactive IgH transgenes, VH replacement has now been shown to contribute to the primary B-cell repertoire in humans. Through recombination-activating gene (RAG)-mediated recombination between a cryptic recombination signal sequence (RSS) present in almost all VH genes and the flanking 23 base pair RSS of an upstream VH gene, VH replacement renews the entire VH-coding region, while leaving behind a short stretch of nucleotides as a VH replacement footprint. In addition to extending the CDR3 region, the VH replacement footprints preferentially contribute charged amino acids. VH replacement rearrangement in immature B cells may either eliminate a self-reactive B-cell receptor or contribute to the generation of self-reactive antibodies. VH replacement may also rescue non-productive or dysfunctional VHDJH rearrangement in pro-B and pre-B cells. Conversely, VH replacement of a productive immunoglobulin H gene may generate non-productive VH replacement to disrupt or temporarily reverse the B-cell differentiation process. VH replacement can thus play a complex role in the generation of the primary B-cell repertoire.

Editing anti-DNA B cells by Vlambdax

Receptor editing is performed by replacement of Vkappa genes that contribute to autoreactivity. In addition, the Ckappa locus can be deleted by Vkappa rearrangement to intronic or 3' of Ckappa RS sequences (also referred to as kappa deletion elements). B cells that delete the Ckappa can then express lambda light chains. However, the lambda locus, either of man or mouse, does not allow V gene replacement. Nor does it appear to be deleted. Therefore, editing of autoreactive lambda B cells may require alternative pathways. We have found that in anti-DNA heavy chain transgenic mice (tgs) VH3H9/56R, B cells that express anti-DNA receptors comprised of lambda1 in association with an anti-DNA heavy chain often coexpress a kappa chain that prevents DNA binding. We speculate that such isotypically included cells may have low anti-DNA receptor densities, a feature that may lead to self-tolerance. Here we describe a mechanism of preventing DNA binding by expression of a rarely used member of the Vlambda family, Vlambdax. The lambdax B cells of the tgs also express CD25 and may represent B cells that have exhausted light chain editing possibilities.

Histone 3 lysine 4 methylation during the pre-B to immature B-cell transition

The relationship between chromatin modification and lymphocyte development is still poorly understood. Here we show a correlation between methylation of lysine 4 on histone 3 (H3-K4) and activation of several loci required for the pre-B cell to immature B-cell developmental transition. A critical step in this transition is the induction of V(D)J recombination at the Igkappa locus. Upon activation of Igkappa recombination, a >10-fold enrichment of both di- and trimethylated H3-K4 is observed at Jkappa targeting signals, but not at an analogous targeting signal in the T-cell receptor alpha locus or, surprisingly, at several Vkappa signals. However, H3-K4 methylation is restricted to the actively recombining fraction of Jkappa recombination targeting signals, consistent with a direct relationship between H3-K4 methylation and signal activity. Correlations between increased H3-K4 methylation and induction of transcription are also observed at some, but not all, loci where transcription is induced. H3-K4 methylation may therefore be a widely used but not universal means for controlling chromatin activity in this developmental transition.

Biallelic IgH rearrangements in patients with indolent lymphoproliferative disorders: Molecular and practical implications

We report a group of patients (pts) with indolent lymphoproliferative disorder who had both alleles for the immunoglobulin heavy chain genes rearranged (biIgH). This group of 17 pts consisted of 9 small lymphocytic lymphomas (SLL) and 8 chronic lymphocytic leukemia (CLL). The polymerase chain reaction (PCR) amplification of clonal immunoglobulin heavy (IgH) rearrangement using the complementarity determining region III (CDRIII) constantly retrieved two distinct bands in all PCR informative samples of those pts. To rule out biclonality, we evaluated samples by fluorescein activated cell sorting (FACS) analysis and sequenced the PCR products. We were able to obtain both IgH sequences from 12 patients. FACS suggested biclonality in one case, which also correlated with sequencing results as both IgH rearrangements were in-frame. Recently, we reported a patient who sustained transformation into an aggressive disease after biIgH was detected in the setting of monoclonal disease (Cerny et al., 2003b, Haematologica 88(05):ECR15 B.). We decided to compare clinical characteristics and prognosis of 17 pts with biIgH and 37 pts with monoIgH rearrangements. Although we found some minor differences in disease characteristics between both groups, these did not translate into a significantly different overall survival. Our findings suggest that true biclonal cases of CLL are rare.

Immunoglobulin mu heavy chains do not mediate tyrosine phosphorylation of Ig alpha from the ER-cis-Golgi

Signals delivered by Ig receptors guide the development of functional B lymphocytes. For example, clonal expansion of early mu heavy chain ( mu HC)-positive pre-B cells requires the assembly of a signal-competent pre-B cell receptor complex (pre-BCR) consisting of a mu HC, a surrogate L chain, and the signal dimer Ig alpha beta. However, only a small fraction of the pre-BCR is transported to the cell surface, suggesting that pre-BCR signaling initiates already from an intracellular compartment, e.g., the endoplasmic reticulum (ER). The finding that differentiation of pre-B cells and allelic exclusion at the IgH locus take place in surrogate L chain-deficient mice further supports the presence of a mu HC-mediated intracellular signal pathway. To determine whether a signal-competent Ig complex can already be assembled in the ER, we analyzed the consequence of pervanadate on tyrosine phosphorylation of Ig alpha in J558L plasmacytoma and 38B9 pre-B cells transfected with either a transport-competent IgL chain-pairing or an ER-retained nonpairing micro HC. Flow cytometry, combined Western blot-immunoprecipitation-kinase assays, and confocal microscopy revealed that both the nonpairing and pairing mu HC assembled with the Ig alpha beta dimer; however, in contrast to a pairing mu HC, the nonpairing mu HC was retained in the ER-cis-Golgi compartment, and neither colocalized with the src kinase lyn nor induced tyrosine phosphorylation of Ig alpha after pervanadate treatment of cells. On the basis of these findings, we propose that a signal-competent Ig complex consisting of mu HC, Ig alpha beta, and associated kinases is assembled in a post-ER compartment, thereby supporting the idea that a pre-BCR must be transported to the cell surface to initiate pre-BCR signaling.

The pathogenetic role of oncogenes deregulated by chromosomal translocation in B-cell malignancies

Chromosomal translocations involving the immunoglobulin (IG) loci play a pivotal role in the pathogenesis of many subtypes of mature B-cell malignancy. Although all the common IG translocations have been cloned, cloning of rare but nonetheless recurrent translocations continues to allow identification of genes of importance to the development of both normal and malignant B-cells. Clustering of breakpoints within the IG gene segments has allowed development of polymerase chain reaction methods that facilitate cloning. IG translocations result in overexpression of a wide variety of genes ranging from cell surface receptors to transcriptional repressors. Genes recently shown to be involved in such translocations include BCL11A and MALT1. As with the acute leukemias, different translocations in B-cell lymphomas may target different proteins that interact directly. A common endpoint for several translocations is activation of the nuclear factor kappaB pathway. Analysis of the mechanisms of transformation may define new therapeutic strategies.

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

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