Receptor Editing Research Articles

B Cell Receptor Basal Signaling Regulates Antigen-Induced Ig Light Chain Rearrangements

BCR editing in the bone marrow contributes to B cell tolerance by orchestrating secondary Ig rearrangements in self-reactive B cells. We have recently shown that loss of the BCR or a pharmacologic blockade of BCR proximal signaling pathways results in a global "back-differentiation" response in which immature B cells down-regulate genes important for the mature B cell program and up-regulate genes characteristic of earlier stages of B cell development. These observations led us to test the hypothesis that self-Ag-induced down-regulation of the BCR, and not self-Ag-induced positive signals, lead to Rag induction and hence receptor editing. Supporting this hypothesis, we found that immature B cells from xid (x-linked immunodeficiency) mice induce re-expression of a Rag2-GFP bacterial artificial chromosome reporter as well as wild-type immature B cells following Ag incubation. Incubation of immature B cells with self-Ag leads to a striking reversal in differentiation to the pro-/pre-B stage of development, consistent with the idea that back-differentiation results in the reinduction of genes required for L chain rearrangement and receptor editing. Importantly, Rag induction, the back-differentiation response to Ag, and editing in immature and pre-B cells are inhibited by a combination of phorbol ester and calcium ionophore, agents that bypass proximal signaling pathways and mimic BCR signaling. Thus, mimicking positive BCR signals actually inhibits receptor editing. These findings support a model whereby Ag-induced receptor editing is inhibited by BCR basal signaling on developing B cells; BCR down-regulation removes this basal signal, thereby initiating receptor editing.

Ig Knock-In Mice Producing Anti-Carbohydrate Antibodies: Breakthrough of B Cells Producing Low Affinity Anti-Self Antibodies

Natural Abs specific for the carbohydrate Ag Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal) play an important role in providing protective host immunity to various pathogens; yet little is known about how production of these or other anti-carbohydrate natural Abs is regulated. In this study, we describe the generation of Ig knock-in mice carrying functionally rearranged H chain and L chain variable region genes isolated from a B cell hybridoma producing alphaGal-specific IgM Ab that make it possible to examine the development of B cells producing anti-carbohydrate natural Abs in the presence or absence of alphaGal as a self-Ag. Knock-in mice on a alphaGal-deficient background spontaneously developed alphaGal-specific IgM Abs of a sufficiently high titer to mediate rejection of alphaGal expressing cardiac transplants. In the spleen of these mice, B cells expressing alphaGal-specific IgM are located in the marginal zone. In knock-in mice that express alphaGal, B cells expressing the knocked in BCR undergo negative selection via receptor editing. Interestingly, production of low affinity alphaGal-specific Ab was observed in mice that express alphaGal that carry two copies of the knocked in H chain. We suggest that in these mice, receptor editing functioned to lower the affinity for self-Ag below a threshold that would result in overt pathology, while allowing development of low affinity anti-self Abs.

Allelic and Isotypic Light Chain Inclusion in Peripheral B Cells from Anti-DNA Antibody Transgenic C57BL/6 and BALB/c Mice

Most mature B lymphocytes express one BCR L chain, kappa or lambda, but recent work has shown that there are exceptions in that some B lymphocytes express both kappa and lambda and some even bear two different kappa L chains. Using the anti-DNA H chain-transgenic mouse, 56R, we find that B cells with pre-existing autoreactivity are especially subject to L chain inclusion. Specifically, we show that isotypic and allelic inclusion enables autoreactive B cells to bypass central tolerance giving rise to B cells that retain dangerous features. One receptor in dual receptor B cells is an editor L chain, i.e., neutralizes or alters self-reactivity of the 56R H chain transgene. We compare the 56R mouse when on the C57/BL/6 background, a strain prone to autoimmunity, with that of 56R when on the BALB/c background, a strain that resists autoimmunity. In the B6.56R mouse, polyreactive B cells with dual L chain move to the follicular B cell compartment. Their localization in the follicular compartment may explain the ease with which B cells in the B6.56R differentiate into autoantibody-producing plasma cells. Likewise, in the BALB/c.56R mouse, dual L chain B cells are found in the follicular B cell compartment. Yet, the lack of autoantibody-producing plasma cells in the BALB/c.56R suggests that postfollicular tolerance checkpoints are intact. The Jkappa usage in dual kappa L chain B cells suggests increased receptor editing activity and is consistent with the expected distribution of Jkappa genes in our computational model for random selection of Jkappa.

MRL/lpr mice with a knock‐in BCR show much less efficient receptor‐editing to membrane‐bound antigen

Receptor editing is the main mechanism by which self‐reactive BCR are removed in immature B cells. Using 3‐83 BCR transgenic mice on the MRL/lpr and B10 background, we have previously shown that receptor editing is less efficient in MRL/lpr mice using indirect measures of receptor editing such as RAG upregulation. Now we have bred the 3‐83 gene‐targeted heavy and light chain genes onto the MRL/lpr background, on both the antigenic H‐2k and non‐antigenic H‐2d background. These mice can therefore undergo normal receptor editing since the targeted light chain is in the kappa locus. Unlike the B10 3‐83 knock‐in mice, where the B cell number is similar whether self antigen (H‐2k) is present or not, we find that the spleens of MRL/lpr knock‐in mice have greatly reduced B cell numbers if self‐antigen is present. Furthermore, the level of CD19 is greatly reduced in the presence of antigen. Finally, B10 mice increase their marginal zone compartment in the presence of antigen, while MRL/lpr mice lose the MZ compartment. Thus, in the MRL/lpr mice, clonal deletion appears to be the main mechanism of eliminating B cells reactive to membrane bound antigen, while in the B10 non‐autoimmune mice, receptor editing efficiently makes a non‐self reactive normal B cell repertoire.

An Effective PSO and AIS-Based Hybrid Intelligent Algorithm for Job-Shop Scheduling

The optimization of job-shop scheduling is very important because of its theoretical and practical significance. In this paper, a computationally effective algorithm of combining PSO with AIS for solving the minimum makespan problem of job-shop scheduling is proposed. In the particle swarm system, a novel concept for the distance and velocity of a particle is presented to pave the way for the job-shop scheduling problem. In the artificial immune system, the models of vaccination and receptor editing are designed to improve the immune performance. The proposed algorithm effectively exploits the capabilities of distributed and parallel computing of swarm intelligence approaches. The algorithm is examined by using a set of benchmark instances with various sizes and levels of hardness and is compared with other approaches reported in some existing literature works. The computational results validate the effectiveness of the proposed approach.

Functional Silencing of Immature B cells by Insulin

The majority of nascent B cells possess autoreactive specificities, thus constant maintenance of B cell tolerance is critical. While receptor editing and deletion effectively remove many autoimmune specificities from the repertoire, clonal anergy functions to censor autoreactive B cells without sacrificing repertoire diversity. Mature spleen B cells that harbor an anti‐insulin BCR transgene (125Tg) are rendered anergic in the periphery, as evidenced by impaired calcium mobilization and proliferation to various stimuli. To understand the ontogeny of this anergic state, an IL‐7‐driven bone marrow culture system was employed to generate antigen‐naïve or antigen‐experienced 125Tg immature B cells in vitro. When immature B cells encounter antigen during their genesis, they acquire many key hallmarks of anergy, including surface IgM downregulation and impaired intracellular calcium mobilization. Mimicking chronic antigen exposure in vitro also leads to impaired proliferative responses; thus immature B cell function is compromised by exposure to a moderate affinity, non‐crosslinking antigen. These data suggest that the small protein hormone, insulin, induces anergy in developing B cells in the bone marrow, and identify a therapeutic window for preventing insulin autoimmunity.

Receptor editing and receptor revision in rheumatic autoimmune diseases

Receptor editing is a key mechanism of B cell tolerance that modifies the B cell receptor (BcR) specificity of self-reactive lymphocytes. It acts through initiation of secondary immunoglobulin rearrangements, through generation of newly rearranged endogenous lambda chains that displace kappa chains, or through isotypic and allelic inclusion of dual BcRs (kappa(+)/lambda(+) or kappa(+)/kappa(+) B cells). Mounting evidence indicates that receptor editing is either impaired or accelerated in patients suffering from rheumatic autoimmune diseases. Remarkably, both alterations can promote the pathogenesis of autoimmune disorders by favoring the uncontrolled emergence and/or persistence of autoreactivity. Whereas impaired secondary rearrangements might result in ineffective silencing of B cells, exacerbation of receptor editing can give rise to autoreactive receptors from clones that were initially devoid of autoreactivity.

Rearrangement of Mouse Immunoglobulin Kappa Deleting Element Recombining Sequence Promotes Immune Tolerance and Lambda B Cell Production

The recombining sequence (RS) of mouse and its human equivalent, the immunoglobulin (Ig) kappa deleting element (IGKDE), are sequences found at the 3' end of the Ig kappa locus (Igk) that rearrange to inactivate Igk in developing B cells. RS recombination correlates with Ig lambda (Iglambda) light (L) chain expression and likely plays a role in receptor editing by eliminating Igk genes encoding autoantibodies. A mouse strain was generated in which the recombination signal of RS was removed, blocking RS-mediated Igk inactivation. In RS mutant mice, receptor editing and self-tolerance were impaired, in some cases leading to autoantibody formation. Surprisingly, mutant mice also made fewer B cells expressing lambda chain, whereas lambda versus kappa isotype exclusion was only modestly affected. These results provide insight into the mechanism of L chain isotype exclusion and indicate that RS has a physiological role in promoting the formation of lambda L chain-expressing B cells.

Multiple, conserved cryptic recombination signals in VH gene segments: detection of cleavage products only in pro–B cells

Receptor editing is believed to play the major role in purging newly formed B cell compartments of autoreactivity by the induction of secondary V(D)J rearrangements. In the process of immunoglobulin heavy (H) chain editing, these secondary rearrangements are mediated by direct VH-to-JH joining or cryptic recombination signals (cRSs) within VH gene segments. Using a statistical model of RS, we have identified potential cRSs within VH gene segments at conserved sites flanking complementarity-determining regions 1 and 2. These cRSs are active in extrachromosomal recombination assays and cleaved during normal B cell development. Cleavage of multiple VH cRSs was observed in the bone marrow of C57BL/6 and RAG2:GFP and μMT congenic animals, and we determined that cRS cleavage efficiencies are 30–50-fold lower than a physiological RS. cRS signal ends are abundant in pro–B cells, including those recovered from μMT mice, but undetectable in pre– or immature B cells. Thus, VH cRS cleavage regularly occurs before the generation of functional preBCR and BCR. Conservation of cRSs distal from the 3′ end of VH gene segments suggests a function for these cryptic signals other than VH gene replacement.

Tolerance induction to self-MHC antigens in fetal and neonatal mouse B cells

We have studied the mechanisms of tolerance induction to self-MHC antigens in mouse B cells during fetal development and the post-natal period. To monitor the fate of autoreactive B cell clones, we used the 3-83 micro delta B cell receptor (BCR)-transgenic (Tg) and -knock-in (KI) mouse models. These BCR-Tg and -KI B cells recognize the MHC class I molecules H-2K(k) and H-2K(b), with a high or moderate affinity, respectively. We compared the fate of BCR-Tg and -KI B cells in H-2K(b)-bearing animals and H-2K(b)-negative controls at various stages of their fetal development and post-natal life. Our data show that, in contrast to what occurs in adult B cells, anergy is the main component of tolerance induction in 3-83 micro delta BCR-Tg K(b+) autoreactive fetuses, while 3-83 BCR-KI fetuses primarily use receptor editing. Interestingly, autoreactive B cell deletion is absent or merely marginal before birth. Our results indicate that tolerance induction is effective as early as embryonic day 16.5 and that in the fetus and neonate, like in the adult, the main mechanism of B cell tolerance functioning in the 3-83 KI system is receptor editing. In contrast, in the 3-83 micro delta mice where receptor editing is hindered, adult and fetal B cells differ in their preferential use of mechanisms leading to self-tolerance (i.e. deletion versus anergy).

Transitional B Cells Lose Their Ability to Receptor Edit but Retain Their Potential for Positive and Negative Selection

Ligation of B cell receptors on immature bone marrow B cells, either by an endogenous Ag or by an anti-B cell receptor Ab induces secondary V(D)J gene rearrangements, termed receptor editing. Whether the same signal induces receptor editing in transitional B cells is not clear. In this study, we examined the responses of immature and transitional B cells from V(H)12Vkappa1A Ig transgenic mice to stimulation with an anti-Igbeta Ab. Our results demonstrated that immature B cells stimulated with a low concentration of anti-Igbeta Ab, mimicking Ag stimulation, underwent receptor editing both in vivo and in vitro, as evidenced by the detection of dsDNA breaks at Jkappa recombination signal sequences, whereas transitional B cells did not. The lack of dsDNA breaks in transitional B cells contrasts with their increased expression of RAG1 and RAG2, suggesting a novel mechanism that may prevent rearrangements. Furthermore, treatment of transitional B cells with high concentrations of anti-Igbeta Abs induced apoptosis, whereas low concentrations induced differentiation. Our results support the idea that transitional B cells lose the capacity to edit, but are sensitive to positive and negative selection.

KDE Analysis in Chronic Lymphocytic Leukemia Patients with B Cells Expressing IGHV3-21 and Discordant Mutational Status between Heavy and Light Chain.

Patients with chronic lymphocytic leukemia (CLL) cells that use IGHV3-21 typically have aggressive disease similar to that of patients with CLL cells that use unmutated IGHV. However, the IGHV3-21 genes expressed in CLL often have less than 98% sequence homology with the known germline IGHV3-21 and hence are considered mutated. However, many “mutated” IGHV3-21 in CLL have higher homology with the germline IGHV3-21 (e.g. >97%) than do most other IGHV in CLL that also are considered mutated (mean percent homology = 93 ± 4% (SD), N= 1,260). As such, some argue that 97% be used as the threshold for defining IGHV mutation status. To test whether this definition is valid for IGHV3-21 we examined the mutation status of the rearranged and/or expressed Ig light chain V genes. We examined 63 (2.6%) IgVH3-21 cases from a large cohort (N=2,457) of CLL patients evaluated by the CRC. Forty of 63 (63.5%) IGHV3-21 cases used ? and 23 (36.5%) used ? light chains. While the ?-expressing cases had perfect correlation in mutational status between IgH and IgL, 12 of 40 (30%) ?-expressing cases were discordant in that they had IGHV3-21 with 94–97.6% germline sequence homology and IGLV that were all unmutated. Nine of these 12 (75%) also displayed a strikingly homologous and short IgH third complementary determining region (CDR) motif (DANGMDV) and light chains encoded by IGLV3-21. To study the differentiation history of such cases, we examined for productively-rearranged, and formerly-expressed, IGKV alleles and non-functional IGKV alleles that had undergone rearrangement with the kappa deleting element (KDE). Analysis for IGKV-KDE rearrangements was performed on 7 of the 9 IGHV3-21/IGLV3-21 cases. Two cases had two non-functional IGKV alleles and five (71%) had one non-functional IGKV allele and one productively-rearranged IGKV allele that apparently was aborted via receptor editing. All 5 productively rearranged IGKV alleles had somatic mutations with germline-homology ranging from only 93.6% to 97.6% (median 96.3%). Non-conservative mutations were clustered in the CDRs, arguing that they were selected in an apparent antigen-driven response. All 9 non-functional IGKV alleles had 100% homology with known germline IGKV genes. For comparison, we examined for IGKV-KDE rearrangements in 4 cases of ?-expressing CLL cells that used the unmutated IGHV1-69 gene and IGLV3-09. All such cases each had two non-functional IGKV alleles that had 100% homology with known IGKV rearranged with KDE, indicating that none of these cases experienced IGKV receptor editing, a significantly lower incidence than that noted for cases using IGHV3-21/IGLV3-21 (P<0.01). These results indicate that despite sharing association with more aggressive disease, most CLL cases that use IGHV3-21/IGLV3-21 are distinctive from cases that use unmutated IGHV1-69 in their B cell differentiation history in that they are derived from B cells that formerly expressed ? light chains that had undergone somatic mutation and selection in an antigen-driven immune response prior to undergoing IGKV receptor editing. Despite expressing IGHV with >97% homology to the known germline IGHV3-21 and IGLV with >98% homology to known germline IGLV, these cases have experienced somatic mutation and apparent germinal center maturation prior to leukemogenesis.

Inhibition of TACE Activity Enhances the Susceptibility of Myeloma Cells to TRAIL.

TNF-related apoptosis-inducing ligand/Apo2 ligand (TRAIL/Apo2L) selectively induces apoptosis in various cancer cells including myeloma (MM) cells. Although TRAIL-mediated immunotherapy is as attractive maneuver against MM because of its tumor-specific cytotoxic activity, the susceptibility of MM cells to TRAIL has been demonstrated to vary among MM cells, which limits clinical applications of TRAIL-mediated immunotherapy. TRAIL is known to bind to 2 different proapoptotic receptors, death receptor 4 (DR4) and DR5. The membrane-bound TRAIL receptors have been reported to be subjected to proteolytic cleavage in melanoma cells in a manner inhibitable by tissue inhibitor of metalloproteinases-3 (TIMP-3), an endogenous inhibitor for TNF-alpha converting enzyme (TACE) (Ahonen M, Oncogene. 2003). Although TACE is regarded as a sheddase for TNF-like ligands/receptors, the role for TACE in TRAIL-mediated cytotoxicity in MM cells is largely unknown. In the present study, we therefore investigated the role for TACE in TRAIL and its receptor editing as well as the effect of TACE inhibition on TRAIL-triggered cytotoxicity in MM cells. MM cells constitutively expressed DR4 on their surface as determined in flow cytometry, while the expression of DR5 as well as TRAIL was negligible. TACE mRNA was constitutively overexpressed in all MM cell lines including RPMI8226, MM.1S, KMS12 and U266, whereas its endogenous inhibitor, TIMP-3, was not detected, suggesting enhanced TACE activity along with down-regulation of TIMP-3 in MM cells. TAPI-0, a TACE inhibitor, up-regulated the cell-surface expression of DR4 about 2-fold in RPMI8226 and U266 cells, while DR5 and TRAIL expression remained undetectable. Immunoblot analyses for DR4 revealed that conditioned media from RPMI8226 and U266 cells contained DR4 protein with a molecular size smaller than that observed in their cell lysates. Interestingly, TAPI-0 treatment down-regulated DR4 levels in culture supernatants of these MM cells, while increasing DR4 levels in their cell lysates as well as on their surface, suggesting ectodomain shedding of DR4 by TACE in MM cells. Importantly, TAPI-0 treatment potentiated TRAIL-triggered cytotoxicity against MM cells including RPMI8226 and MM.1S, which was almost completely antagonized by addition of osteoprotegerin, a soluble inhibitor for TRAIL, indicating a TRAIL-dependent mechanism. Taken together, the present observations demonstrate that cell-surface TRAIL receptor expression is post-transcriptionally down-regulated by endogenous TACE activity enhanced in MM cells, which may protect MM cells from TRAIL-mediated apoptosis by surrounding immune cells expressing TRAIL, and that up-regulation of TRAIL receptor expression by inhibiting TACE activity is able to restore the susceptibility of MM cells to TRAIL. Therefore, these preclinical data provide that TACE inhibitors in combination with TRAIL-based immunotherapy may be a novel therapeutic approach against MM which can not be cured by the present chemotherapeutic modalities.

Crosslineage T-Cell Receptor Genes Rearrangements and Active Receptor Editing in B-Cell Acute Lymphoblastic Leukemia.

B-cell acute lymphoblastic leukemia (B-ALL) results from clonal expansion of B-lymphocytes derived at different stage of differentiation. Immunoglobulin (Ig) heavy chain genes (IGH), light chain kappa (IGK) and lambda (IGL) genes rearrange during early B-lymphocyte differentiation. T-cell receptor (TCR) genes are considered to rearrange exclusively in normal T lymphocytes, but malignant B lymphoblasts often contain crosslineage rearranged TCR genes. The clonal leukemic cell population, carrying identical copies of rearranged Ig and/or TCR genes, can be identified above 95% of B-ALL patients. In our study Ig/TCR genes rearrangements were detected by multiplex PCR with heteroduplex analysis according to BIOMED-2 protocol. DNA was isolated by column method from mononuclear cells isolated from the peripheral blood/bone marrow samples obtained at initial diagnosis from 36 B-ALL patients. Monoclonal rearrangements of Ig genes were detected in 100% (36/36) of patients. The most frequent rearrangements were observed in IGH genes (94%), including complete IGHV-IGHJ in 83% (30/36) and incomplete IGHD-IGHJ in 22% (8/36) of patients. Among complete IGH rearrangements 2 biallelic rearrangements in IGHV1-7 and IGHJ genes (FR3) were found. Ig light chain genes rearrangements were identified in 26 patients (72%) (including 64% of IGKV-IGKJ, 47% IGKV/intron-Kde, and 22% IGLV-IGLJ) what indicates active receptor editing occurring during B lymphoblasts leukemogenesis. Crosslineage TCR genes rearrangements were found in 97% (35/36) of patients. TCR beta genes rearrangements were detected in 47% (17/36) of patients (complete TRBV-TRBJ in 25% (9/36), TRBD-TRBJ in 6/36 patients - 17%). TRGV-TRGV in 58% (21/36), TRDV-TRDJ in 58% (21/36); 17 monoallelic and 4 biallelic were found. The inactivation of potentially functional IGKV-IGKJ by secondary rearrangements indicates active receptor editing. Our data describe IGK and IGL genes rearrangements incidence, present allelic exclusion and active receptor editing in B-ALL patients. B-ALL lymphoblast undergoes rearrangement on the same IGK allele before IGL genes rearrangement occur. The data may suggest the possible of antigens in B-ALL immunopathogenesis. The results indicate also rearranged IGK, IGL and TCR genes as stable molecular marker for monitoring MRD in B-ALL.

Reduced receptor editing in lupus-prone MRL/lpr mice

The initial B cell repertoire contains a considerable proportion of autoreactive specificities. The first major B cell tolerance checkpoint is at the stage of the immature B cell, where receptor editing is the primary mode of eliminating self-reactivity. The cells that emigrate from the bone marrow have a second tolerance checkpoint in the transitional compartment in the spleen. Although it is known that the second checkpoint is defective in lupus, it is not clear whether there is any breakdown in central B cell tolerance in the bone marrow. We demonstrate that receptor editing is less efficient in the lupus-prone strain MRL/lpr. In an in vitro system, when receptor-editing signals are given to bone marrow immature B cells by antiidiotype antibody or after in vivo exposure to membrane-bound self-antigen, MRL/lpr 3-83 transgenic immature B cells undergo less endogenous rearrangement and up-regulate recombination activating gene messenger RNA to a lesser extent than B10 transgenic cells. CD19, along with immunoglobulin M, is down-regulated in the bone marrow upon receptor editing, but the extent of down-regulation is fivefold less in MRL/lpr mice. Less efficient receptor editing could allow some autoreactive cells to escape from the bone marrow in lupus-prone mice, thus predisposing to autoimmunity.

A Novel Clonal Selection Algorithm and Its Application to Traveling Salesman Problem

The Clonal Selection Algorithm (CSA) is employed by the natural immune system to define the basic features of an immune response to an antigenic stimulus. In the immune response, according to Burnet's clonal selection principle, the antigen imposes a selective pressure on the antibody population by allowing only those cells which specifically recognize the antigen to be selected for proliferation and differentiation. However ongoing investigations indicate that receptor editing, which refers to the process whereby antigen receptor engagement leads to a secondary somatic gene rearrangement event and alteration of the receptor specificity, is occasionally found in affinity maturation process. In this paper, we extend the traditional CSA approach by incorporating the receptor editing method, named RECSA, and applying it to the Traveling Salesman Problem. Thus, both somatic hypermutation (HM) of clonal selection theory and receptor editing (RE) are utilized to improve antibody affinity. Simulation results and comparisons with other general algorithms show that the RECSA algorithm can effectively enhance the searching efficiency and greatly improve the searching quality within reasonable number of generations.

Regulation of autoreactive antibodies

Significant progress has been made over recent years in uncovering the B-cell tolerance mechanisms that control development of autoreactive antibodies. This review examines current knowledge on the regulation and selection of autoreactive B cells in mouse models, and in healthy humans and patients with autoimmune disorders. Autoreactive B cells undergo stringent selection either in the bone marrow or peripheral circulation by deletion, induction of anergy, or receptor editing. There is growing evidence that receptor editing represents the primary physiologic B-cell tolerance mechanism. Several checkpoints against autoreactive B cells have been established in bone marrow and peripheral blood of healthy humans. Recent studies demonstrate that some autoimmune disorders are associated with several alterations in B-cell tolerance checkpoints and often lead to a greater number of autoreactive B cells in the circulation. Discovering the precise nature of B-cell tolerance alterations in patients with autoimmune diseases will lead to the identification of new targets for therapeutic interventions in patients with these disorders.

Phospholipase Cγ2 Contributes to Light-Chain Gene Activation and Receptor Editing

Phospholipase Cgamma2 (PLCgamma2) is critical for pre-B-cell receptor (pre-BCR) and BCR signaling. Current studies discovered that PLCgamma2-deficient mice had reduced immunoglobulin lambda (Iglambda) light-chain usage throughout B-cell maturation stages, including transitional type 1 (T1), transitional type 2 (T2), and mature follicular B cells. The reduction of Iglambda rearrangement by PLCgamma2 deficiency was not due to specifically increased apoptosis or decreased proliferation of mutant Iglambda+ B cells, as lack of PLCgamma2 exerted a similar effect on apoptosis and proliferation of both Iglambda+ and Igkappa+ B cells. Moreover, PLCgamma2-deficient IgHEL transgenic B cells exhibited an impairment of antigen-induced receptor editing among both the endogenous lambda and kappa loci in vitro and in vivo. Importantly, PLCgamma2 deficiency impaired BCR-induced expression of IRF-4 and IRF-8, the two transcription factors critical for lambda and kappa light-chain rearrangements. Taken together, these data demonstrate that the PLCgamma2 signaling pathway plays a role in activation of light-chain loci and contributes to receptor editing.

Receptor editing: how B cells stay tolerant.

In 1993, David Nemazee and Martin Weigert independently showed that autoreactive B cells could proofread, alter, and reexpress modified receptors to become nonautoreactive. This process, called “receptor editing,” has since gained prominence as the main mechanism of B cell tolerance.

B-cell tolerance checkpoint violations in systemic lupus erythematosus

B cells play a crucial role in the pathogenesis of the prototypic autoimmune disease systemic lupus erythematosus. The presence of autoantibodies in lupus is indicative of a breach in B-cell tolerance to self. Studies conducted over the past 15 years indicate that tolerance checkpoints exist at various stages of B-cell development to ensure that self-reactive B cells are censored and do not produce autoantibodies. These include an early checkpoint operative at the immature B-cell stage in the bone marrow, with receptor editing and deletion as the two main mechanisms. Additional checkpoints exist in the periphery and include receptor revision and anergy at the mature naive stage, with further checkpoints being operative at the germinal center and memory B-cell stages. In a recent study, it has been shown that mice harboring the lupus susceptibility gene Sle1bz/Ly108z display defective tolerance both at the immature as well as the mature B-cell stage in the periphery. This, along with other studies conducte...