Spatial transcriptome of a germinal center plasmablastic burst hints at MYD88/CD79B mutants-enriched diffuse large B-cell lymphomas.

The GC reaction results in the selection of B cells acquiring effector Ig secreting ability by progressing toward plasmablastic differentiation. This transition is associated with exclusion from the GC microenvironment. The aberrant expansion of plasmablastic elements within the GC fringes configures an atypical condition, the biological characteristics of which have not been defined yet. We investigated the in situ immunophenotypical and transcriptional characteristics of a nonclonal germinotropic expansion of plasmablastic elements (GEx) occurring in the tonsil of a young patient. Compared to neighboring GC and perifollicular regions, the GEx showed a distinctive signature featuring key regulators of plasmacytic differentiation, cytokine signaling, and cell metabolism. The GEx signature was tested in the setting of diffuse large B-cell lymphoma (DLBCL) as a prototypical model of lymphomagenesis encompassing transformation at different stages of GC and post-GC functional differentiation. The signature outlined DLBCL clusters with different immune microenvironment composition and enrichment in genetic subtypes. This report represents the first insight into the transcriptional features of a germinotropic plasmablastic burst, shedding light into the molecular hallmarks of B cells undergoing plasmablastic differentiation and aberrant expansion within the noncanonical setting of the GC microenvironment.

TNF receptor-associated factor 3 restrains B-cell receptor signaling in normal and malignant B cells.

TRAF3 has diverse signaling functions, which vary by cell type. Uniquely in B lymphocytes, TRAF3 inhibits homeostatic survival. Highlighting the role of TRAF3 as a tumor suppressor, loss-of-function TRAF3 mutations are associated with human B-cell malignancies, while B-cell-specific deletion of TRAF3 in mice leads to autoimmunity and lymphoma development. The role of TRAF3 in inhibiting noncanonical NF-κB activation, CD40 and BAFF-R signaling to B cells is well documented. In contrast, TRAF3 enhances many T-cell effector functions, through associating with and enhancing signaling by the T-cell receptor (TCR)-CD28 complex. The present study was designed to determine the role of TRAF3 in signaling via the B-cell antigen receptor (BCR). The BCR is crucial for antigen recognition, survival, proliferation, and antibody production, and defects in BCR signaling can promote abnormal survival of malignant B cells. Here, we show that TRAF3 is associated with both CD79B and the BCR-activated kinases Syk and Btk following BCR stimulation. BCR-induced phosphorylation of Syk and additional downstream kinases was increased in TRAF3-/- B cells, with regulation observed in both follicular and marginal zone B-cell subsets. BCR stimulation of TRAF3-/- B cells resulted in increased surface expression of MHC-II, CD80, and CD86 molecules. Interestingly, increased survival of TRAF3-/- primary B cells was resistant to inhibition of Btk, while TRAF3-deficient malignant B-cell lines showed enhanced sensitivity. TRAF3 serves to restrain normal and malignant BCR signaling, with important implications for its role in normal B-cell biology and abnormal survival of malignant B cells.

Bruton's tyrosine kinase inhibition induces rewiring of proximal and distal B-cell receptor signaling in mice.

Bruton's tyrosine kinase (Btk) is a crucial signaling molecule in BCR signaling and a key regulator of B- cell differentiation and function. Btk inhibition has shown impressive clinical efficacy in various B-cell malignancies. However, it remains unknown whether inhibition additionally induces changes in BCR signaling due to feedback mechanisms, a phenomenon referred to as BCR rewiring. In this report, we studied the impact of Btk activity on major components of the BCR signaling pathway in mice. As expected, NF-κB and Akt/S6 signaling was decreased in Btk-deficient B cells. Unexpectedly, phosphorylation of several proximal signaling molecules, including CD79a, Syk, and PI3K, as well as the key Btk-effector PLCγ2 and the more downstream kinase Erk, were significantly increased. This pattern of BCR rewiring was essentially opposite in B cells from transgenic mice overexpressing Btk. Importantly, prolonged Btk inhibitor treatment of WT mice or mice engrafted with leukemic B cells also resulted in increased phosho-CD79a and phospho-PLCγ2 in B cells. Our findings show that Btk enzymatic function determines phosphorylation of proximal and distal BCR signaling molecules in B cells. We conclude that Btk inhibitor treatment results in rewiring of BCR signaling, which may affect both malignant and healthy B cells.

The BAFF receptor transduces survival signals by co-opting the B cell receptor signaling pathway.

Follicular B cell survival requires signaling from BAFFR, a receptor for BAFF and the B cell antigen receptor (BCR). This "tonic" BCR survival signal is distinct from that induced by antigen binding and may be ligand-independent. We show that inducible inactivation of the Syk tyrosine kinase, a key signal transducer from the BCR following antigen binding, resulted in the death of most follicular B cells because Syk-deficient cells were unable to survive in response to BAFF. Genetic rescue studies demonstrated that Syk transduces BAFFR survival signals via ERK and PI3 kinase. Surprisingly, BAFFR signaling directly induced phosphorylation of both Syk and the BCR-associated Igα signaling subunit, and this Syk phosphorylation required the BCR. We conclude that the BCR and Igα may be required for B cell survival because they function as adaptor proteins in a BAFFR signaling pathway leading to activation of Syk, demonstrating previously unrecognized crosstalk between the two receptors.

Phosphoflow Protocol for Signaling Studies in Human and Murine B Cell Subpopulations.

BCR signaling, involving phosphorylation of various downstream molecules, including kinases, lipases, and linkers, is crucial for B cell selection, survival, proliferation, and differentiation. Phosphoflow cytometry (phosphoflow) is a single-cell-based technique to measure phosphorylated intracellular proteins, providing a more quantitative read-out than Western blotting. Recent advances in phosphoflow basically allow simultaneous analysis of protein phosphorylation in B cell (sub)populations, without prior cell sorting. However, fixation and permeabilization procedures required for phosphoflow often affect cell surface epitopes or mAb conjugates, precluding the evaluation of the phosphorylation status of signaling proteins across different B cell subpopulations present in a single sample. In this study, we report a versatile phosphoflow protocol allowing extensive staining of B cell subpopulations in human peripheral blood or various anatomical compartments in the mouse, starting from freshly isolated or frozen cell suspensions. Both human and mouse B cell subpopulations showed different basal and BCR stimulation-induced phosphorylation levels of downstream signaling proteins. For example, peritoneal B-1 cells and splenic marginal zone B cells exhibited significantly increased basal (ex vivo) signaling and increased responsiveness to in vitro BCR stimulation compared with peritoneal B-2 cells and splenic follicular B cells, respectively. In addition, whereas stimulation with anti-IgM or anti-Igκ L chain Abs resulted in strong pCD79a and pPLCγ2 signals, IgD stimulation only induced CD79a but not pPLCγ2 phosphorylation. In summary, the protocol is user friendly and quantifies BCR-mediated phosphorylation with high sensitivity at the single-cell level, in combination with extensive staining to identify individual B cell development and differentiation stages.

Composite CD79A/CD40 co-stimulatory endodomain enhances CD19CAR-T cell proliferation and survival.

Adoptively transferred CD19 chimeric antigen receptor (CAR) T cells have led to impressive clinical outcomes in B cell malignancies. Beyond induction of remission, the persistence of CAR-T cells is required to prevent relapse and provide long-term disease control. To improve CAR-T cell function and persistence, we developed a composite co-stimulatory domain of a B cell signaling moiety, CD79A/CD40, to induce a nuclear translocating signal, NF-κB, to synergize with other T cell signals and improve CAR-T cell function. CD79A/CD40 incorporating CD19CAR-T cells (CD19.79a.40z) exhibited higher NF-κB and p38 activity upon CD19 antigen exposure compared with the CD28 or 4-1BB incorporating CD19CAR-T cells (CD19.28z and CD19.BBz). Notably, we found that CD19.79a.40z CAR-T cells continued to suppress CD19+ target cells throughout the co-culture assay, whereas a tendency for tumor growth was observed with CD19.28z CAR-T cells. Moreover, CD19.79a.40z CAR-T cells exhibited robust T cell proliferation after culturing with CD19+ target cells, regardless of exogenous interleukin-2. In terms of in vivo efficiency, CD19.79a.40z demonstrated superior anti-tumor activity and in vivo CAR-T cell proliferation compared with CD19.28z and CD19.BBz CD19CAR-T cells in Raji-inoculated mice. Our data demonstrate that the CD79A/CD40 co-stimulatory domain endows CAR-T cells with enhanced proliferative capacity and improved anti-tumor efficacy in a murine model.

Monophosphorylation of CD79a and CD79b ITAM motifs initiates a SHIP-1 phosphatase-mediated inhibitory signaling cascade required for B cell anergy.

Anergic B cells are characterized by impaired signaling and activation after aggregation of their antigen receptors (BCR). The molecular basis of this impairment is not understood. In studies reported here, Src homology-2 (SH2)-containing inositol 5-phosphatase SHIP-1 and its adaptor Dok-1 were found to be constitutively phosphorylated in anergic B cells, and activation of this inhibitory circuit was dependent on Src-family kinase activity and consequent to biased BCR immunoreceptor tyrosine-based activation motif (ITAM) monophosphorylation. B cell-targeted deletion of SHIP-1 caused severe lupus-like disease. Moreover, absence of SHIP-1 in B cells led to loss of anergy as indicated by restoration of BCR signaling, loss of anergic surface phenotype, and production of autoantibodies. Thus, chronic BCR signals maintain anergy in part via ITAM monophosphorylation-directed activation of an inhibitory signaling circuit involving SHIP-1 and Dok-1.

H3K36me3-mediated mismatch repair preferentially protects actively transcribed genes from mutation.

Histone H3 trimethylation at lysine 36 (H3K36me3) is an important histone mark involved in both transcription elongation and DNA mismatch repair (MMR). It is known that H3K36me3 recruits the mismatch-recognition protein MutSα to replicating chromatin via its physical interaction with MutSα's PWWP domain, but the exact role of H3K36me3 in transcription is undefined. Using ChIP combined with whole-genome DNA sequencing analysis, we demonstrate here that H3K36me3, together with MutSα, is involved in protecting against mutation, preferentially in actively transcribed genomic regions. We found that H3K36me3 and MutSα are much more co-enriched in exons and actively transcribed regions than in introns and nontranscribed regions. The H3K36me3-MutSα co-enrichment correlated with a much lower mutation frequency in exons and actively transcribed regions than in introns and nontranscribed regions. Correspondingly, depleting H3K36me3 or disrupting the H3K36me3-MutSα interaction elevated the spontaneous mutation frequency in actively transcribed genes, but it had little influence on the mutation frequency in nontranscribed or transcriptionally inactive regions. Similarly, H2O2-induced mutations, which mainly cause base oxidations, preferentially occurred in actively transcribed genes in MMR-deficient cells. The data presented here suggest that H3K36me3-mediated MMR preferentially safeguards actively transcribed genes not only during replication by efficiently correcting mispairs in early replicating chromatin but also during transcription by directly or indirectly removing DNA lesions associated with a persistently open chromatin structure.

Distinct patterns of B-cell receptor signaling in non-Hodgkin lymphomas identified by single-cell profiling.

Kinases downstream of B-cell antigen receptor (BCR) represent attractive targets for therapy in non-Hodgkin lymphoma (NHL). As clinical responses vary, improved knowledge regarding activation and regulation of BCR signaling in individual patients is needed. Here, using phosphospecific flow cytometry to obtain malignant B-cell signaling profiles from 95 patients representing 4 types of NHL revealed a striking contrast between chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL) tumors. Lymphoma cells from diffuse large B-cell lymphoma patients had high basal phosphorylation levels of most measured signaling nodes, whereas follicular lymphoma cells represented the opposite pattern with no or very low basal levels. MCL showed large interpatient variability in basal levels, and elevated levels for the phosphorylated forms of AKT, extracellular signal-regulated kinase, p38, STAT1, and STAT5 were associated with poor outcome. CLL tumors had elevated basal levels for the phosphorylated forms of BCR-signaling nodes (Src family tyrosine kinase, spleen tyrosine kinase [SYK], phospholipase Cγ), but had low α-BCR-induced signaling. This contrasted MCL tumors, where α-BCR-induced signaling was variable, but significantly potentiated as compared with the other types. Overexpression of CD79B, combined with a gating strategy whereby signaling output was directly quantified per cell as a function of CD79B levels, confirmed a direct relationship between surface CD79B, immunoglobulin M (IgM), and IgM-induced signaling levels. Furthermore, α-BCR-induced signaling strength was variable across patient samples and correlated with BCR subunit CD79B expression, but was inversely correlated with susceptibility to Bruton tyrosine kinase (BTK) and SYK inhibitors in MCL. These individual differences in BCR levels and signaling might relate to differences in therapy responses to BCR-pathway inhibitors.

Immobile BCRs: the safety on the signal trigger.

B cell receptor (BCR)-mediated tonic signals are important for B cell survival and development. In this issue of Immunity, Treanor et al. (2010) show that triggering of downstream responses is kept in check with BCR immobilization by actin.

The membrane skeleton controls diffusion dynamics and signaling through the B cell receptor.

Early events of B cell activation after B cell receptor (BCR) triggering have been well characterized. However, little is known about the steady state of the BCR on the cell surface. Here, we simultaneously visualize single BCR particles and components of the membrane skeleton. We show that an ezrin- and actin-defined network influenced steady-state BCR diffusion by creating boundaries that restrict BCR diffusion. We identified the intracellular domain of Igbeta as important in mediating this restriction in diffusion. Importantly, alteration of this network was sufficient to induce robust intracellular signaling and concomitant increase in BCR mobility. Moreover, by using B cells deficient in key signaling molecules, we show that this signaling was most probably initiated by the BCR. Thus, our results suggest the membrane skeleton plays a crucial function in controlling BCR dynamics and thereby signaling, in a way that could be important for understanding tonic signaling necessary for B cell development and survival.

Zebrafish B Cell Development without a Pre-B Cell Stage, Revealed by CD79 Fluorescence Reporter Transgenes.

CD79a and CD79b proteins associate with Ig receptors as integral signaling components of the B cell Ag receptor complex. To study B cell development in zebrafish, we isolated orthologs of these genes and performed in situ hybridization, finding that their expression colocalized with IgH-µ in the kidney, which is the site of B cell development. CD79 transgenic lines were made by linking the promoter and upstream regulatory segments of CD79a and CD79b to enhanced GFP to identify B cells, as demonstrated by PCR analysis of IgH-µ expression in sorted cells. We crossed these CD79-GFP lines to a recombination activating gene (Rag)2:mCherry transgenic line to identify B cell development stages in kidney marrow. Initiation of CD79:GFP expression in Rag2:mCherry+ cells and the timing of Ig H and L chain expression revealed simultaneous expression of both IgH-µ- and IgL-κ-chains, without progressing through the stage of IgH-µ-chain alone. Rag2:mCherry+ cells without CD79:GFP showed the highest Rag1 and Rag2 mRNAs compared with CD79a and CD79b:GFP+ B cells, which showed strongly reduced Rag mRNAs. Thus, B cell development in zebrafish does not go through a Raghi CD79+IgH-µ+ pre-B cell stage, different from mammals. After the generation of CD79:GFP+ B cells, decreased CD79 expression occurred upon differentiation to Ig secretion, as detected by alteration from membrane to secreted IgH-µ exon usage, similar to in mammals. This confirmed a conserved role for CD79 in B cell development and differentiation, without the requirement of a pre-B cell stage in zebrafish.

Identification of Novel STAT6-Regulated Proteins in Mouse B Cells by Comparative Transcriptome and Proteome Analysis.

The transcription factor STAT6 plays a key role in mediating signaling downstream of the receptors for IL-4 and IL-13. In B cells, STAT6 is required for class switch recombination to IgE and for germinal center formation during type 2 immune responses directed against allergens or helminths. In this study, we compared the transcriptomes and proteomes of primary mouse B cells from wild-type and STAT6-deficient mice cultured for 4 d in the presence or absence of IL-4. Microarray analysis revealed that 214 mRNAs were upregulated and 149 were downregulated >3-fold by IL-4 in a STAT6-dependent manner. Across all samples, ∼5000 proteins were identified by label-free quantitative liquid chromatography/mass spectrometry. A total of 149 proteins was found to be differentially expressed >3-fold between IL-4-stimulated wild-type and STAT6-/- B cells (75 upregulated and 74 downregulated). Comparative analysis of the proteome and transcriptome revealed that expression of these proteins was mainly regulated at the transcriptional level, which argues against a major role for posttranscriptional mechanisms that modulate the STAT6-dependent proteome. Nine proteins were selected for confirmation by flow cytometry or Western blot. We show that CD30, CD79b, SLP-76, DEC205, IL-5Rα, STAT5, and Thy1 are induced by IL-4 in a STAT6-dependent manner. In contrast, Syk and Fc receptor-like 1 were downregulated. This dataset provides a framework for further functional analysis of newly identified IL-4-regulated proteins in B cells that may contribute to germinal center formation and IgE switching in type 2 immunity.

Anaplastic variant of diffuse large B-cell lymphoma: Reappraisal as a nodal disease with sinusoidal involvement.

Anaplastic variant (av) of diffuse large B-cell lymphoma (DLBCL) is morphologically defined in the 2017 World Health Organization classification, but still an enigmatic disease in its clinicopathologic distinctiveness, posing the differential diagnostic problem from gray zone lymphoma (GZL) and classic Hodgkin lymphoma (cHL). Thirty-one cases previously diagnosed as avDLBCL were reassessed. Of these, 27 (87%) and 4 (13%) were node-based and extranodal diseases, respectively. They were further reclassified into nodal avDLBCL (n = 18), nodal CD30+ DLBCL with T-cell/histiocyte-rich large B-cell lymphoma-like features (CD30+ DLBCL-THRLBCL) (n = 6), GZL with features intermediate between DLBCL and cHL (n = 3) and CD30+ extranodal DLBCL, NOS (n = 4). The nodal avDLBCL cases had a sheet-like proliferation of large cells and/or Hodgkin/Reed-Sternberg (HRS)-like cells in 12 (67%) notably with a sinusoidal pattern in 16 (89%). They showed an expression of CD20 and/or CD79a in all and CD30 in 15 of 18. All of them were negative for PD-L1 on tumor cells, although HRS-like cells showed negativity or partial loss of other B-cell markers to varying degrees. The present study highlighted the distinctiveness of the nodal avDLBCL with sinusoidal pattern, but without neoplastic PD-L1 expression, which provide refined diagnostic criteria for a more precise pathologic and clinical characterization of this disease.

In vitro and in vivo evidence for uncoupling of B-cell receptor internalization and signaling in chronic lymphocytic leukemia.

B-cell receptor activation, occurring within lymph nodes, plays a key role in the pathogenesis of chronic lymphocytic leukemia and is linked to prognosis. As well as activation of downstream signaling, receptor ligation triggers internalization, transit to acidified endosomes and degradation of ligand-receptor complexes. Herein, we investigated the relationship between these two processes in normal and leukemic B cells. We found that leukemic B cells, particularly anergic cases lacking the capacity to initiate downstream signaling, internalize and accumulate ligand in acidified endosomes more efficiently than normal B cells. Furthermore, ligation of either surface CD79B, a B-cell receptor component required for downstream signaling, or surface Immunoglobulin M (IgM) by cognate agonistic antibody, showed that the two molecules internalize independently of each other in leukemic but not normal B cells. Since association with surface CD79B is required for surface retention of IgM, this suggests that uncoupling of B-cell receptor internalization from signaling may be due to the dissociation of these two molecules in leukemic cells. A comparison of lymph node with peripheral blood cells from chronic lymphocytic leukemia patients showed that, despite recent B-cell receptor activation, lymph node B cells expressed higher levels of surface IgM. This surprising finding suggests that the B-cell receptors of lymph node- and peripheral blood-derived leukemic cells might be functionally distinct. Finally, long-term therapy with the Bruton's tyrosine kinase inhibitors ibrutinib or acalabrutinib resulted in a switch to an anergic pattern of B-cell receptor function with reduced signaling capacity, surface IgM expression and more efficient internalization.

Canine Splenic Nodular Lymphoid Lesions: Immunophenotyping, Proliferative Activity, and Clonality Assessment.

Canine splenic lymphoid nodules are currently classified as indolent lymphomas (marginal zone lymphoma [MZL], mantle cell lymphoma [MCL]) or nodular hyperplasia (lymphoid [LNH] or complex [CNH] type). Their differentiation can be difficult on morphology, because of similar histologic appearance and poorly defined diagnostic criteria. Thirty-five surgical samples of splenic lymphoid nodules were reviewed in order to assess the diagnostic contribution of immunophenotyping, proliferative activity and clonality (PARR) in differentiating between hyperplastic and neoplastic lesions. Proliferative activity was evaluated by double immunolabeling for Ki-67 and CD79a, in order to separately assess the proliferative activity of B cells and non-B cells. Definitive diagnoses were MZL ( n = 11), MCL ( n = 4), LNH ( n = 10), and CNH ( n = 10). The overall concordance between histology and PARR was above 90%. Lymphomas had a significantly higher percentage of CD79a-positive areas (mean, 36.30%; P = .0004) and a higher B-cell proliferative activity (median Ki-67 index, 5.49%; P = .0012). The threshold value most accurately predicting a diagnosis of lymphoma was ≥28% of B-cell areas, with a Ki-67 index above 3%. Dogs were monitored for a median follow-up time of 870 days (IQR, 569-1225), and no relapses were documented. Overall median survival time was 1282 days. The combination of histology, immunohistochemistry and PARR can improve the diagnostic accuracy for canine splenic lymphoid nodules, although the long-term behavior of these lesions appears similar.

Survival of human lymphoma cells requires B-cell receptor engagement by self-antigens.

The activated B-cell-like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL) relies on chronic active B-cell receptor (BCR) signaling. BCR pathway inhibitors induce remissions in a subset of ABC DLBCL patients. BCR microclusters on the surface of ABC cells resemble those generated following antigen engagement of normal B cells. We speculated that binding of lymphoma BCRs to self-antigens initiates and maintains chronic active BCR signaling in ABC DLBCL. To assess whether antigenic engagement of the BCR is required for the ongoing survival of ABC cells, we developed isogenic ABC cells that differed solely with respect to the IgH V region of their BCRs. In competitive assays with wild-type cells, substitution of a heterologous V region impaired the survival of three ABC lines. The viability of one VH4-34(+) ABC line and the ability of its BCR to bind to its own cell surface depended on V region residues that mediate the intrinsic autoreactivity of VH4-34 to self-glycoproteins. The BCR of another ABC line reacted with self-antigens in apoptotic debris, and the survival of a third ABC line was sustained by reactivity of its BCR to an idiotypic epitope in its own V region. Hence, a diverse set of self-antigens is responsible for maintaining the malignant survival of ABC DLBCL cells. IgH V regions used by the BCRs of ABC DLBCL biopsy samples varied in their ability to sustain survival of these ABC lines, suggesting a screening procedure to identify patients who might benefit from BCR pathway inhibition.

Mechanisms of NF-κB deregulation in lymphoid malignancies.

Deregulations promoting constitutive activation of canonical and non-canonical NF-κB signaling are a common feature of many lymphoid malignancies. Due to their cellular origin and the pivotal role of NF-κB for the normal function of B lymphocytes, B-cell malignancies are particularly prone to genetic aberrations that affect the pathway. Key positive regulators of NF-κB signaling can act as oncogenes that are often prone to chromosomal translocation, amplifications or activating mutations. Negative regulators of NF-κB have tumor suppressor functions and are frequently inactivated either by genomic deletions or point mutations. Whereas some aberrations are found in a variety of different lymphoid malignancies, some oncogenic alterations are very restricted to distinct lymphoma subsets, reflecting the clonal and cellular origin of specific lymphoma entities. NF-κB activation in many lymphoma cells is also driven by the microenvironment or chronic signaling that does not rely on genetic alterations. A number of drugs that target the NF-κB pathway are in preclinical or clinical development, revealing that there will be new options for therapies in the future. Since each lymphoma entity utilizes distinct mechanisms to activate NF-κB, a major challenge is to elucidate the exact pathological processes in order to faithfully predict clinical responses to the different therapeutic approaches.

The IgG1 B-cell receptor provides survival and proliferative signals analogue to the Igα but not the Igß co-receptor.

The function of the IgM B-cell receptor (BCR) is dependent on intact signaling of the co-receptors Igα and Igß, both of which contain a cytoplasmic tail bearing an immunoreceptor tyrosine-based activation motif. We have previously demonstrated that the cytoplasmic tail of the IgG1 BCR can partially compensate for the loss of the signaling moiety of Igα. Here, we show that unlike Igα, Igß signaling is indispensable for the development and function of IgG1-expressing B cells. Deletion of the cytoplasmic signaling tail of Igß compromised the survival and proliferation not only of IgM(+) B cells but also of IgG1-expressing B cells. In the absence of the signaling tail of Igß, the transcription levels of the antiapoptotic gene bcl-xl and the cell-cycle gene ccnd2 were reduced, consistent with the observed defects in survival and proliferation. These results demonstrate functional differences between Igα and Igß in the transduction of IgG1 BCR signal.