Antigen Receptor Cross-linking Research Articles

Effects of IL-9 on TRAF3-deficient B cell activation

Abstract The pleiotropic cytokine interleukin-9 (IL-9) signals to target cells by binding to a heterodimeric receptor consisting of the IL-9 receptor (IL-9R) and the common subunit γ chain. In the present study, we found that the expression of IL-9R was strikingly up-regulated in mouse naïve follicular B cells genetically deficient in TRAF3, a critical regulator of B cell survival and function. The highly up-regulated IL-9R on Traf3 −/−follicular B cells conferred responsiveness to IL-9, including IgM production and STAT3 phosphorylation. Interestingly, IL-9 significantly enhanced class switch recombination to IgG1 induced by B cell antigen receptor crosslinking plus IL-4 in Traf3 −/−B cells but not in wild type B cells. Blocking the JAK-STAT3 pathway abrogated the effects of IL-9 on Traf3 −/−B cells. Our study thus revealed a novel pathway that TRAF3 suppresses B cell activation and Ig isotype switching by inhibiting IL-9R-JAK-STAT3 signaling. Our findings provide new insights into the TRAF3-IL-9R axis in B cell function and have significant implications for the understanding and treatment of a variety of human diseases involving aberrant B cell activation. This study was supported by the National Institutes of Health grants R01 CA158402 (P. Xie) and R21 AI128264 (P. Xie), the Department of Defense grant W81XWH-13-1-0242 (P. Xie), and a Busch Biomedical Grant (P. Xie). FACS analyses were supported by the Flow Cytometry Core Facility with funding from NCI-CCSG P30CA072720.

Immune dysregulation caused by homozygous mutations in CBLB.

CBL-B is an E3 ubiquitin ligase that ubiquitinates proteins downstream of immune receptors to downregulate positive signaling cascades. Distinct homozygous mutations in CBLB were identified in 3 unrelated children with early-onset autoimmunity, one of whom also had chronic urticaria. Patient T cells exhibited hyperproliferation in response to anti-CD3 cross-linking. One of the mutations, p.R496X, abolished CBL-B expression, and a second mutation, p.C464W, resulted in preserved CBL-B expression. The third mutation, p.H285L in the SH2 domain of CBL-B, was expressed at half the normal level in the patient’s cells. Mice homozygous for the CBL-B p.H257L mutation, which corresponds to the patient’s p.H285L mutation, had T and B cell hyperproliferation in response to antigen receptor cross-linking. CblbH257L mice had increased percentages of T regulatory cells (Tregs) that had normal in vitro suppressive function. However, T effector cells from the patient with the p.H285L mutation and CblbH257L mice were resistant to suppression by WT Tregs. Bone marrow–derived mast cells from CblbH257L mice were hyperactivated after FcεRI cross-linking, and CblbH257L mice demonstrated exaggerated IgE-mediated passive anaphylaxis. This study establishes CBL-B deficiency as a cause of immune dysregulation.

Beginnings of coinhibition.

Costimulatory and coinhibitory mechanisms appear to be involved throughout immune responses to control their specificity and level. Many mechanisms operate; therefore, various theoretical models should be considered complementary rather than competing. One such coinhibitory model, pictured in 1971, involves the crosslinking of antigen receptors with inhibitory Fc receptors by antigen/antibody complexes. This model was prompted by observations that the Fc portion of antibody was required for potent suppression of immune responses by antibody. The signal via the antigen receptor wakes up T or B cells, providing specificity, while costimulators and coinhibitors stimulate or inhibit these awoken cells. The recent observations that administration of monoclonal anti-SARS-CoV-2 spike antibodies in early COVID-19 patients inhibits the induction of clinically damaging autoimmune antibodies suggest they may provide negative Fc signals that are blocked in COVID-19 patients. Furthermore, the reduced ability of SARS-CoV-2 antigen to localize to germinal centres in COVID-19 patients also suggests a block in binding of the Fc of antibody bound to antigen on FcγRIIb of follicular dendritic cells. The distinction between self and foreign is made not only at the beginning of immune responses but also throughout, and involves multiple mechanisms and models. There are past beginnings (history of models) and current and future beginnings for solving serious clinical problems (such as COVID-19) and different types of models used for understanding the complexities of fundamental immunology.

Gαi Signaling Promotes Marginal Zone B Cell Development by Enabling Transitional B Cell ADAM10 Expression.

The follicular (FO) versus marginal zone (MZ) B cell fate decision in the spleen depends upon BCR, BAFF, and Notch2 signaling. Whether or how Gi signaling affects this fate decision is unknown. Here, we show that direct contact with Notch ligand expressing stromal cells (OP9-Delta-like 1) cannot promote normal MZ B cell development when progenitor B cells lack Gαi proteins, or if Gi signaling is disabled. Consistent with faulty ADAM10-dependent Notch2 processing, Gαi-deficient transitional B cells had low ADAM10 membrane expression levels and reduced Notch2 target gene expression. Immunoblotting Gαi-deficient B cell lysates revealed a reduction in mature, processed ADAM10. Suggesting that Gαi signaling promotes ADAM10 membrane expression, stimulating normal transitional B cells with CXCL12 raised it, while inhibiting Gαi nucleotide exchange blocked its upregulation. Surprisingly, inhibiting Gαi nucleotide exchange in transitional B cells also impaired the upregulation of ADAM10 that occurs following antigen receptor crosslinking. These results indicate that Gαi signaling supports ADAM10 maturation and activity in transitional B cells, and ultimately Notch2 signaling to promote MZ B cell development.

Syk-dependent tyrosine phosphorylation of 3BP2 is required for optimal FcR\u03b3-mediated phagocytosis and chemokine expression in U937 cells

The adaptor protein c-Abl SH3 domain binding protein-2 (3BP2) is tyrosine phosphorylated by Syk in response to cross-linking of antigen receptors, which in turn activates various immune responses. Recently, a study using the mouse model of cherubism, a dominant inherited disorder caused by mutations in the gene encoding 3BP2, showed that 3BP2 is involved in the regulation of phagocytosis mediated by Fc receptor for IgG (FcγR) in macrophages. However, the molecular mechanisms underlying 3BP2-mediated regulation of phagocytosis and the physiological relevance of 3BP2 tyrosine phosphorylation remains elusive. In this study, we established various gene knockout U937 cell lines using the CRISPR/Cas9 system and found that 3BP2 is rapidly tyrosine phosphorylated by Syk in response to cross-linking of FcγRI. Depletion of 3BP2 caused significant reduction in the Fc receptor γ chain (FcRγ)-mediated phagocytosis in addition to the FcγRI-mediated induction of chemokine mRNA for IL-8, CCL3L3 and CCL4L2. Syk-dependent tyrosine phosphorylation of 3BP2 was required for overcoming these defects. Finally, we found that the PH and SH2 domains play important roles on FcγRI-mediated tyrosine phosphorylation of 3BP2 in HL-60 cells. Taken together, these results indicate that Syk-dependent tyrosine phosphorylation of 3BP2 is required for optimal FcRγ-mediated phagocytosis and chemokine expression.

Prolactin Rescues Immature B-Cells from Apoptosis Induced by B-Cell Receptor Cross-Linking.

Prolactin has an immunomodulatory effect and has been associated with B-cell-triggered autoimmune diseases, such as systemic lupus erythematosus (SLE). In mice that develop SLE, the PRL receptor is expressed in early bone marrow B-cells, and increased levels of PRL hasten disease manifestations, which are correlated with a reduction in the absolute number of immature B-cells. The aim of this work was to determine the effect of PRL in an in vitro system of B-cell tolerance using WEHI-231 cells and immature B-cells from lupus prone MRL/lpr mice. WEHI-231 cells express the long isoform of the PRL receptor, and PRL rescued the cells from cell death by decreasing the apoptosis induced by the cross-linking of the B-cell antigen receptor (BCR) as measured by Annexin V and active caspase-3. This decrease in apoptosis may have been due to the PRL and receptor interaction, which increased the relative expression of antiapoptotic Bcl-xL and decreased the relative expression of proapoptotic Bad. In immature B-cells from MRL/lpr mice, PRL increased the viability and decreased the apoptosis induced by the cross-linking of BCR, which may favor the maturation of self-reactive B-cells and contribute to the onset of disease.

Contributors to HMGB1 Release by Urothelial Carcinoma Cells in Response to Bacillus Calmette-Guérin

Our group previously noted that HMGB1 release by urothelial carcinoma cells occurs as a consequence of bacillus Calmette-Guérin induced nonapoptotic cell death. Additional studies demonstrated that HMGB1 release in response to bacillus Calmette-Guérin is required for the invivo tumor response to bacillus Calmette-Guérin. We evaluated the steps required for HMGB1 release by human urothelial carcinoma cells in response to bacillus Calmette-Guérin exposure. We used the T24 and 253J human urothelial carcinoma cell lines. HMGB1 concentrations in cell culture supernatant with and without bacillus Calmette-Guérin treatment served as the principal end point to assess the role of potentially involved variables. Specific techniques were used to determine the role of α5β1 antigen receptor cross-linking, TLR signaling, inducible nitric oxide synthase expression/nitric oxide production, p21 expression, and bacillus Calmette-Guérin adherence, internalization and viability. Cross-linking of α5β1 integrin or signaling through TLR2/4 did not contribute to HMGB1 release. Optimal HMGB1 release required bacillus Calmette-Guérin adherence and internalization. Bacillus Calmette-Guérin viability correlated with the magnitude of HMGB1 release. Inhibition of oxidative stress and p21 expression in response to bacillus Calmette-Guérin decreased the magnitude of HMGB1 release. Bacillus Calmette-Guérin induced nonapoptotic cell death and HMGB1 release occur as a consequence of a complex multistep process. Understanding the steps and mechanisms involved in the induced HMGB1 release would provide an opportunity for targeted strategies to improve bacillus Calmette-Guérin treatment efficacy.

586 CONTRIBUTORS TO HMGB1 RELEASE BY UROTHELIAL CARCINOMA CELLS IN RESPONSE TO BCG

You have accessJournal of UrologyBladder Cancer: Basic Research (I)1 Apr 2013586 CONTRIBUTORS TO HMGB1 RELEASE BY UROTHELIAL CARCINOMA CELLS IN RESPONSE TO BCG Guangjian Zhang, Fanghong Chen, Yanli Cao, Gopit Shah, and William See Guangjian ZhangGuangjian Zhang Milwaukee, WI More articles by this author , Fanghong ChenFanghong Chen Milwaukee, WI More articles by this author , Yanli CaoYanli Cao Milwaukee, WI More articles by this author , Gopit ShahGopit Shah Milwaukee, WI More articles by this author , and William SeeWilliam See Milwaukee, WI More articles by this author View All Author Informationhttps://doi.org/10.1016/j.juro.2013.02.1982AboutPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookTwitterLinked InEmail INTRODUCTION AND OBJECTIVES Recent work by our group has demonstrated that the powerful chemokine and marker of non-apoptotic cell death, high molecular group box 1 (HMGB1), is released by a portion of urothelial carcinoma (UC) tumor cells following BCG exposure. Elevated levels of HMGB1 have been observed in the urine of patients following intravesical BCG therapy. Most importantly, in vivo in models have demonstrated that tumor cells engineered to have reduced HMGB1 release fail to respond to intravesical BCG administration. Given the apparent importance of HMGB1 release for in vivo anti-tumor activity, this study evaluated the role of specific elements of BCG/tumor cell interaction in contributing to maximal HMGB1 release. METHODS Two human UC cell lines, T24 and 253J, were employed. HMGB1 concentrations in cell culture supernatant, with and without BCG treatment, served as the principal end point to assess the role of potentially involved variables. Specific techniques were utilized to determine the role of α5β1 antigen receptor crosslinking, BCG adherence, BCG internalization, BCG viability, iNOS expression/NO production, and p21 expression in mediating HMGB1 release by UC cells in response to BCG. RESULTS Crosslinking of α5β1 integrin was insufficient to induce HMGB1 release. HMGB1 release by crosslinked cells was not significantly different than untreated controls (p > 0.5). Treatment of cells with the peptide GRDGS to block α5β1 integrin and inhibit BCG adherence significantly inhibited HMGB1 release (p<0.001). Inhibition of BCG internalization by pretreatment of cells with Cytochalasin B significantly decreased HMGB1 release relative to BCG alone (p < 0.0001). Both viable and heat killed BCG significantly increased HMGB1 release by UC cells relative to controls (p < 0.01) The use of heat killed BCG significantly reduced HMGB1 release to 50% of values observed in response to viable BCG (p<0.0001). HMGB1 release by iNOS inhibited cells was significantly reduced (p < 0.01). Inducible knockdown of p21 expression in response to BCG significantly reduced HMGB1 release following BCG treatment (p < 0.01). CONCLUSIONS BCG induced non-apoptotic cell death and HMGB1 release occurs as a consequence of a complex multi-step process. Optimal HMGB1 release requires the adherence and internalization of viable BCG, iNOS activity, and p21 expression. An understanding of the steps and mechanisms involved in BCG induced HMGB1 release affords an opportunity for targeted strategies to improve BCG treatment efficacy. © 2013 by American Urological Association Education and Research, Inc.FiguresReferencesRelatedDetails Volume 189Issue 4SApril 2013Page: e239-e240 Advertisement Copyright & Permissions© 2013 by American Urological Association Education and Research, Inc.MetricsAuthor Information Guangjian Zhang Milwaukee, WI More articles by this author Fanghong Chen Milwaukee, WI More articles by this author Yanli Cao Milwaukee, WI More articles by this author Gopit Shah Milwaukee, WI More articles by this author William See Milwaukee, WI More articles by this author Expand All Advertisement Advertisement PDF downloadLoading ...

Interaction of TAPP adapter proteins with phosphatidylinositol (3,4)‐bisphosphate regulates B‐cell activation and autoantibody production

TAPP1 and TAPP2 (where TAPP is tandem PH domain containing protein) are dual PH domain adaptors that selectively bind PI(3,4)P2 (phosphatidylinositol (3,4)-bisphosphate). PI(3,4)P2 is a lipid messenger generated by phosphoinositide 3-kinase (PI3K) and SHIP, both of which are critical regulators of B-cell activation. To determine the functional role of TAPP-PI(3,4)P2 interactions, we utilized a double knock-in (KI) mouse bearing mutations within the PI-binding pocket of both TAPP1 and TAPP2. TAPP KI mice show evidence of altered B-cell development, but generate phenotypically normal mature B-cell populations. Total serum immunoglobulin IgM and IgG levels were found to be markedly elevated in TAPP KI mice. B cells purified from TAPP KI mice were hyper-responsive to antigen receptor cross-linking, showing increased proliferation, CD86 expression, and Akt phosphorylation on Ser473 and Thr308. Female TAPP KI mice developed elevated levels of anti-DNA and antinuclear antibodies with age, associated with IgG deposition in kidneys and significant glomerulonephritis pathology. Together our results indicate that interaction of TAPPs with PI(3,4)P2 mediates feedback inhibition impacting on BCR signaling, with functional significance for control of autoreactive B cells.

Control of T cell activation by Drak2 depends on protein kinase D and the interplay between calcium and reactive oxygen species (50.6)

Abstract Drak2 plays a negative regulatory role in acute T cell activation with direct consequences on proliferative capacity. The absence of Drak2 in T cells provokes responsiveness to sub-optimal stimuli, heightens calcium mobilization across a broad range of antigen receptor cross-linking, and increases production of Il-2, yet key survival factors are diminished within the context of Drak2 deficiency, ultimately limiting effector function. Increased co-stimulation, or transgenic expression of Bcl-xL, can rescue this proliferative defect. Thus, Drak2 provides a link between negative regulation of T cell activation and the requirement for adequate co-stimulation. The pathway through which this serine/threonine kinase functions has remained elusive. This work shows that Drak2 activity is dependent on protein kinase D (PKD), in a calcium-dependent manner, and that a PKD/Drak2 signaling axis serves to limit the influx of extracellular calcium following T cell activation. Furthermore, reactive oxygen species are required for Drak2 activity and are shown to play a key role in the initial phase of this negative feedback loop. Taken together, PKD and Drak2 are shown to limit the activation state of T cells through tuning those signals which initially converge to ultimately drive effector function.

CCL3 and CCL4 Plasma Levels Correlate with Established Prognostic Markers in Chronic Lymphocytic Leukemia: Towards a Simple, ELISA-Based Assay for Risk Assessment.

Abstract 358▪▪This icon denotes an abstract that is clinically relevant.During the induction of normal immune responses, activated B cells secrete the chemokines CCL3 and CCL4 for recruitment of regulatory T cells (Nat Immunol. 2:1126-32, 2001). This may represent a mechanism enabling cognate interactions between rare antigen-specific T and B cells. We previously reported that CCL3 and CCL4 RNA and protein are induced in CLL cells by co-culture with nurselike cells and after B cell antigen receptor (BCR) cross-linking (Blood 113:3050-8, 2009). We found higher levels of CCL3 and CCL4 in ZAP-70 positive cases, and CCL3 and CCL4 secretion was abrogated by the Syk inhibitor R406. These findings suggest that CCL3/CCL4 secretion by CLL cells correlates with the signaling capacity of the BCR. Also, we previously noticed higher plasma levels of CCL3 and CCL4 in CLL patients, when compared to healthy controls.In this study, we measured CCL3 and CCL4 plasma levels by ELISA in 351 CLL patients, and correlated CCL3 and CCL4 levels with various prognostic markers, including RAI stage, immunoglobulin heavy chain variable region gene (IgVH) mutational status, ZAP-70, β2 microglobulin, CD38, white blood count (WBC), and cytogenetic subgroups. We found that the concentrations of CCL3 and CCL4 were significantly higher in plasma samples from CLL patients who had CLL cells that used unmutated IgVH genes or that expressed ZAP-70 or CD38, or who had an advanced stage of their disease (see Table 1). For example, the mean CCL3 plasma level was 56.4 ± 8.8 pg/ml in patients with CLL cells that used unmutated IgVH (mean ± SEM, n=123) versus 14.5 ± 2.4 in patients with CLL cells that used mutated IgVH (mean ± SEM, n=139, p<0.001). The mean CCL4 level was 171.4 ± 26.3 in patients that had CLL cells that used unmutated IgVH and 92.5 ± 17.5 in patients that had CLL that expressed mutated IgVH. On the other hand, the relative absolute white cell count or presence of chromosomal abnormalities did not correlate with high plasma levels of CCL3 or CCL4. Immunohistochemistry revealed that CCL4 was predominantly expressed in proliferation centers in approximately 50% of the cases. Currently, we are evaluating whether high plasma levels of CCL3 and/or CCL4 are associated with a relatively short time from diagnosis to requiring initial treatment by the IWCLL-working group criteria. Also, we are exploring whether high plasma levels of CCL3 or CCL4 are associated with high proportions of infiltrating T cells in proliferation centers. [Display omitted] These studies suggest that patients can be stratified by their relative plasma levels of CCL3 and/or CCL4 and that high plasma-levels of theses chemokines define a characteristic that may be associated with aggressive disease. Disclosures:No relevant conflicts of interest to declare.

Foxo3-/- mice demonstrate reduced numbers of pre-B and recirculating B cells but normal splenic B cell sub-population distribution

B cell antigen receptor (BCR) cross-linking promotes proliferation and survival of mature B cells. Phosphoinositide-3-kinase-mediated down-regulation of pro-apoptotic and anti-mitogenic genes such as the Foxo family of transcription factors is an important component of this process. Previously, we demonstrated that BCR signaling decreases expression of transcripts for Foxo1, Foxo3 and Foxo4. We now show that BCR-induced down-regulation of Foxo3 and Foxo4 mRNA expression occurs via distinct mechanisms from those established for Foxo1. While Foxo1, Foxo3 and Foxo4 bind the same DNA sequence, the differential control of their expression upon B cell activation suggests that they may have unique functions in the B lineage. To begin to address this issue, we evaluated B cell development and function in Foxo3-/- mice. No effect of Foxo3 deficiency was observed with respect to the following parameters in the splenic B cell compartment: sub-population distribution, proliferation, in vitro differentiation and expression of the Foxo target genes cyclin G2 and B cell translocation gene 1. However, Foxo3-/- mice demonstrated increased basal levels of IgG2a, IgG3 and IgA. A significant reduction in pre-B cell numbers was also observed in Foxo3-/- bone marrow. Finally, recirculating B cells in the bone marrow and peripheral blood were decreased in Foxo3-/- mice, perhaps due to lower than normal expression of receptor for sphingosine-1 phosphate, which mediates egress from lymphoid organs. Thus, Foxo3 makes a unique contribution to B cell development, B cell localization and control of Ig levels.

Non-apoptotic caspase-8 activation balances T lymphocyte autophagy (82.14)

Abstract Apoptosis and autophagy are critical mediators of a functional immune response. FADD and caspase-8 (casp8) were originally discovered to transduce apoptotic signals delivered through TNF family members. Paradoxically, lymphocytes lacking FADD or casp8 function have impaired clonal expansion following antigen receptor crosslinking and succumb to a caspase-independent cell death. We show that T cells lacking FADD or casp8 are subject to hyperactive autophagy. T cell autophagy induces casp8 activation through its interaction with FADD:Atg5:Atg12 complexes. Blockade of autophagic signaling with shRNA knockdown of the autophagy-dependent protein Atg7 leads to a rescue of T cells lacking FADD and casp8. Similarly, inhibition of RIPK1 kinase activity with necrostatin-1 completely restored T cell cycle and survival. We hypothesize that the non-apoptotic casp8 activity serves to dampen the autophagic response of proliferating T cells through direct cleavage of RIPK1 or the casp8 homolog, c-FLIP, which may secondarily inhibit RIPK1-dependent authophagic signaling. While rapidly dividing T cells require autophagy, this negative feedback loop can prevent hyperactive autophagy. Linking apoptotic proteins to this signaling paradigm may serve to prevent an inflammatory response driven by autophagic cell death in lymphocytes during an immune response. (Funded by NIH T32CA09054 to B.D.B. and R01AI63419 to C.M.W.)

FADD and caspase-8 control the outcome of autophagic signaling in proliferating T cells

Fas-associated death domain protein (FADD) and caspase-8 (casp8) are vital intermediaries in apoptotic signaling induced by tumor necrosis factor family ligands. Paradoxically, lymphocytes lacking FADD or casp8 fail to undergo normal clonal expansion following antigen receptor cross-linking and succumb to caspase-independent cell death upon activation. Here we show that T cells lacking FADD or casp8 activity are subject to hyperactive autophagic signaling and subvert a cellular survival mechanism into a potent death process. T cell autophagy, enhanced by mitogenic signaling, recruits casp8 through interaction with FADD:Atg5-Atg12 complexes. Inhibition of autophagic signaling with 3-methyladenine, dominant-negative Vps34, or Atg7 shRNA rescued T cells expressing a dominant-negative FADD protein. The necroptosis inhibitor Nec-1, which blocks receptor interacting protein kinase 1 (RIP kinase 1), also completely rescued T cells lacking FADD or casp8 activity. Thus, while autophagy is necessary for rapid T cell proliferation, our findings suggest that FADD and casp8 form a feedback loop to limit autophagy and prevent this salvage pathway from inducing RIPK1-dependent necroptotic cell death. Thus, linkage of FADD and casp8 to autophagic signaling intermediates is essential for rapid T cell clonal expansion and may normally serve to promote caspase-dependent apoptosis under hyperautophagic conditions, thereby averting necrosis and inflammation in vivo.

PI3K is a negative regulator of IgE production

The production of IgE, a main player in allergic disorders such as asthma and atopic dermatitis, is strictly regulated and the serum concentrations of IgE are normally kept at a much lower level than other isotypes. We found that mice deficient for the p85alpha regulatory subunit of class IA phosphoinositide 3-kinase (PI3K) produced increasing amounts of serum IgE. Purified p85alpha-/- B cells produced more IgE than wild-type B cells in vitro in response to anti-CD40 mAb and IL-4. PI3K inhibitors wortmannin and IC87114 enhanced IgE production by wild-type B cells stimulated with anti-CD40 mAb and IL-4. Under the same condition, antigen receptor cross-linking induced the expression of inhibitor of differentiation-2 and suppressed the expression of activation-induced cytidine deaminase and class switch recombination (CSR) in a PI3K-dependent manner. IgE production was also suppressed in a concentrated cell culture condition, which was completely reversed by PI3K inhibition. The selective suppression of IgE production by PI3K was also observed at a protein level after CSR. Our results indicate that PI3K negatively regulates IgE production at both CSR and protein levels.

The 3BP2 Adapter Protein Is Required for Optimal B-Cell Activation and Thymus-Independent Type 2 Humoral Response

3BP2 is a pleckstrin homology domain- and Src homology 2 (SH2) domain-containing adapter protein that is mutated in the rare human bone disorder cherubism and which has also been implicated in immunoreceptor signaling. However, a function for this protein has yet to be established. Here we show that mice lacking 3BP2 exhibited a perturbation in the peritoneal B1 and splenic marginal-zone B-cell compartments and diminished thymus-independent type 2 antigen response. 3BP2(-/-) B cells demonstrated a proliferation defect in response to antigen receptor cross-linking and a heightened sensitivity to B-cell receptor-induced death via a caspase-3-dependent apoptotic pathway. We show that 3BP2 binds via its SH2 domain to the CD19 signaling complex and is required for optimum Syk phosphorylation and calcium flux.

Antigen receptor–mediated changes in glucose metabolism in B lymphocytes: role of phosphatidylinositol 3-kinase signaling in the glycolytic control of growth

AbstractThe bioenergetic response of B lymphocytes is subject to rapid changes following antigen encounter in order to provide ATP and anabolic precursors necessary to support growth. However, the pathways involved in glucose acquisition and metabolism are unknown. We find that B lymphocytes rapidly increase glucose uptake and glycolysis following B-cell antigen receptor (BCR) crosslinking. Inhibition of glycolysis blocks BCR-mediated growth. Prior to S-phase entry, glucose metabolism shifts from primarily glycolytic to include the pentose phosphate pathway. BCR-induced glucose utilization is dependent upon phosphatidylinositol 3-kinase (PI-3K) activity as evidenced by inhibition of glucose uptake and glycolysis with LY294002 treatment of normal B cells and impaired glucose utilization in B cells deficient in the PI-3K regulatory subunit p85α. Activation of Akt is sufficient to increase glucose utilization in B cells. We find that glucose utilization is inhibited by coengagement of the BCR and FcγRIIB, suggesting that limiting glucose metabolism may represent an important mechanism underlying FcγRIIB-mediated growth arrest. Taken together, these findings demonstrate that both growth-promoting BCR signaling and growth-inhibitory FcγRIIB signaling modulate glucose energy metabolism. Manipulation of these pathways may prove to be useful in the treatment of lymphoproliferative disorders, wherein clonal expansion of B lymphocytes plays a role.

Molecular basis of common variable immunodeficiency

Common variable immunodeficiency (CVID) is the most prevalent human primary immunodeficiency requiring medical attention. Until recently, the only known genetic defect specific to CVID was the inducible costimulatory receptor (ICOS) deficiency, which accounts for less than 1% of the patients. Recently, mutations in the TNF receptor family member transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), which mediates isotype switching in B cells, were found to be present in 10% to 20% of patients with CVID. Mutations in TACI were also found in relatives of patients with CVID who had IgA deficiency (IgAD), as well as in a patient with isolated IgAD. In the majority of patients described to date, only one TACI allele is mutated, showing an autosomal dominant transmission of the disease. B cells from individuals with TACI mutations did not produce IgG and IgA in response to the TACI ligand a proliferation-inducing ligand (APRIL), probably reflecting impaired isotype switching. These results suggest that TACI mutations can lead to CVID and IgAD.

Selective role of PKCβ enzymatic function in regulating cell survival mediated by B cell antigen receptor cross-linking

Cross-linking of the B cell antigen receptor (BCR) results in the activation of several protein tyrosine kinases leading to phospholipase C-γ2-dependent phospholipid hydrolysis and Ca 2+ mobilization, followed by activation of the protein kinase C (PKC) family members. Sustained Ca 2+ release in B lymphocytes is dependent on the membrane localization and activation of the protein tyrosine kinase BTK. Ca 2+ release is a tightly regulated process involving BTK membrane localization through its phosphorylation by PKCβ. A selective role of PKCβ in B cell signaling was first revealed by the characterization of PKCβ knockout mice, which displayed decreased B cell proliferation in response to various mitogenic stimuli. However, it is not clear whether the B cell defects displayed by the PKCβ knockout mice are due a B cell developmental defect or the scaffolding function of PKCβ, resulting in a defect in the recruitment or formation of signal transducing complex molecules. Thus, in this report we investigated the effects of pharmacologic inhibition of the catalytic function of PKCβ on B cell survival and growth. Treatment of Daudi B lymphoma cell line with a selective PKCβ inhibitor, LY333531, inhibited anti-IgM-induced phosphorylation of BTK on Ser180 in a concentration-dependent manner, which was concomitant with an increase in BTK activation, and Ca 2+ mobilization. In primary splenic B cells, LY333531 inhibited BCR-induced B cell proliferation, but did not affect basal or LPS-induced proliferation. Finally, LY333531 treatment resulted in the induction of apoptosis of anti-IgM-activated B cells, which corroborated with their inability to up-regulate pro-survival factors, Bcl-X L and Bcl-2. These results support the important and selective role of the PKCβ enzymatic function in controlling Ca 2+ release during BCR signaling leading to B lymphocyte survival and growth.

The BTB-kelch Protein KLHL6 Is Involved in B-Lymphocyte Antigen Receptor Signaling and Germinal Center Formation

BTB-kelch proteins can elicit diverse biological functions but very little is known about the physiological role of these proteins in vivo. Kelch-like protein 6 (KLHL6) is a BTB-kelch protein with a lymphoid tissue-restricted expression pattern. In the B-lymphocyte lineage, KLHL6 is expressed throughout ontogeny, and KLHL6 expression is strongly upregulated in germinal center (GC) B cells. To analyze the role of KLHL6 in vivo, we have generated mouse mutants of KLHL6. Development of pro- and pre-B cells was normal but numbers of subsequent stages, transitional 1 and 2, and mature B cells were reduced in KLHL6-deficient mice. The antigen-dependent GC reaction was blunted (smaller GCs, reduced B-cell expansion, and reduced memory antibody response) in the absence of KLHL6. Comparison of mutants with global loss of KLHL6 to mutants lacking KLHL6 specifically in B cells demonstrated a B-cell-intrinsic requirement for KLHL6 expression. Finally, B-cell antigen receptor (BCR) cross-linking was less sensitive in KLHL6-deficient B cells compared to wild-type B cells as measured by proliferation, Ca2+ response, and activation of phospholipase Cgamma2. Our results strongly point to a role for KLHL6 in BCR signal transduction and formation of the full germinal center response.