Shengli Xu

TACI and BAFF-R mediate isotype switching in B cells

The tumor necrosis factor family members BAFF and APRIL induce Ig isotype switching in human B cells. We analyzed the ability of BAFF and APRIL to induce isotype switching in murine B cells to IgG1, IgA, and IgE. APRIL and BAFF each engage two receptors, transmembrane activator and calcium-modulator and cytophilin ligand interactor (TACI) and B cell maturation antigen (BCMA), on B cells. In addition, BAFF engages a third receptor on B cells, BAFF-R. To determine the role of these receptors in isotype switching, we examined B cells from mice deficient in TACI, BCMA, and BAFF-R. The results obtained indicate that both TACI and BAFF-R are able to transduce signals that result in isotype switching.

Cutting Edge: The Dependence of Plasma Cells and Independence of Memory B Cells on BAFF and APRIL

Memory B (BMEM) cells and long-lived bone marrow plasma cells (BM-PCs) persist within local environmental survival niches that afford cellular longevity. However, the factors supporting BMEM cell survival within the secondary lymphoid organs and allowing BM-PC persistence in the bone marrow remain poorly characterized. We report herein that long-lived BMEM cell survival and function are completely independent of BAFF (B cell-activating factor of the TNF family) or APRIL (a proliferation-inducing ligand). Thus, BMEM cells represent the only mature B2 lineage subset whose survival is independent of these ligands. We have previously shown that the TNFR family member receptor BCMA (B cell maturation Ag) is a critical survival receptor for BM-PC survival in vivo. We identify in this study the ligands critical for BM-PC survival and show that either BAFF or APRIL supports the survival of BM-PCs in vivo. These data define the BAFF/APRIL-dependent and -independent components of long-lived humoral immunity.

Phospholipase Cγ2 Is Critical for Dectin-1-mediated Ca2+ Flux and Cytokine Production in Dendritic Cells

Dectin-1 is a C-type lectin that recognizes β-glucan in the cell walls of fungi and plays an important role in anti-fungal immunity. It signals via tyrosine kinase Syk and adaptor protein Card9 to activate NF-κB leading to proinflammatory cytokine production in dendritic cells (DCs). Other than this, not much else is known of the mechanism of Dectin-1 signaling. We demonstrate here that stimulation of DCs with zymosan triggers an intracellular Ca2+ flux that can be attenuated by a blocking anti-Dectin-1 antibody or by pre-treatment of cells with the phospholipase C (PLC) γ-inhibitor U73122, suggesting that Dectin-1 signals via a PLCγ pathway to induce Ca2+ flux in DCs. Interestingly, treatment of DCs with particulate curdlan, which specifically engages Dectin-1, results in the phosphorylation of both PLCγ1 and PLCγ2. However, we show that PLCγ2 is the critical enzyme for Dectin-1 signaling in DCs. PLCγ2-deficient DCs have drastic impairment of Ca2+ signaling and are defective in their secretion of interleukin 2 (IL-2), IL-6, IL-10, IL-12, IL-23, and tumor necrosis factor α. PLCγ2-deficient DCs also exhibit impaired activation of ERK and JNK MAPKs and AP-1 and NFAT transcription factors in response to Dectin-1 stimulation. In addition, PLCγ2-deficient DCs are also impaired in their activation of NF-κB upon Dectin-1 engagement due to defective assembly of the Card9-Bcl10-Malt1 complex and impaired IKKα/β activation and IκBα degradation. Thus, our data indicate that pattern recognition receptors such as Dectin-1 could elicit Ca2+ signaling and that PLCγ2 is a critical player in the Dectin-1 signal transduction pathway.

Bruton's tyrosine kinase phosphorylates Toll-like receptor 3 to initiate antiviral response

Toll-like receptor 3 (TLR3) mediates antiviral response by recognizing double-stranded RNA. Its cytoplasmic domain is tyrosine phosphorylated upon ligand binding and initiates downstream signaling via the adapter TIR-containing adaptor inducing interferon–β (TRIF). However, the kinase responsible for TLR3 phosphorylation remains unknown. We show here that Brutons tyrosine kinase (BTK)-deficient macrophages failed to secrete inflammatory cytokines and IFN-β upon TLR3 stimulation and were impaired in clearing intracellular dengue virus infection. Mutant mice were also less susceptible to d-galactosamine/p(I:C)-induced sepsis. In the absence of BTK, TLR3-induced phosphoinositide 3-kinase (PI3K), AKT and MAPK signaling and activation of NFκB, IRF3, and AP-1 transcription factors were all defective. We demonstrate that BTK directly phosphorylates TLR3 and in particular the critical Tyr759 residue. BTK point mutations that abrogate or led to constitutive kinase activity have opposite effects on TLR3 phosphorylation. Loss of BTK also compromises the formation of the downstream TRIF/receptor-interacting protein 1 (RIP1)/TBK1 complex. Thus, BTK plays a critical role in initiating TLR3 signaling.

The RNase III enzyme Dicer is essential for germinal center B-cell formation.

MicroRNAs (miRNAs) are short noncoding RNAs that regulate gene expression and are important for pre-B and follicular B lymphopoiesis as demonstrated, respectively, by mb-1-Cre- and cd19-Cre-mediated deletion of Dicer, the RNase III enzyme critical for generating mature miRNAs. To explore the role of miRNAs in B-cell terminal differentiation, we use Aicda-Cre to specifically delete Dicer in activated B cells where activation-induced cytidine deaminase is highly expressed. We demonstrate that mutant mice fail to produce high-affinity class-switched antibodies and generate memory B and long-lived plasma cells on immunization with a T cell-dependent antigen. More importantly, germinal center (GC) B-cell formation is drastically compromised in the absence of Dicer, as a result of defects in cell proliferation and survival. Dicer-deficient GC B cells express higher levels of cell cycle inhibitor genes and proapoptotic protein Bim. Ablation of Bim could partially rescue the defect in GC B-cell formation in Dicer-deficient mice. Taken together, our data suggest that Dicer and probably miRNAs are critical for GC B-cell formation during B-cell terminal differentiation.

Bruton's Tyrosine Kinase Separately Regulates NFκB p65RelA Activation and Cytokine Interleukin (IL)-10/IL-12 Production in TLR9-stimulated B Cells

B lymphocytes express both B cell receptor and Toll-like receptors (TLR). We show here that Brutons tyrosine kinase (Btk), a critical component in B cell receptor signaling, is also involved in TLR9 signaling in B cells. Stimulation of B cells with TLR9 ligand CpG oligodeoxynucleotide (ODN) leads to transient phosphorylation of Btk, and in the absence of Btk, TLR9-induced proliferation of B cells is impaired. Interestingly, Btk–/– B cells secrete significantly more interleukin (IL)-12 but much less IL-10 compared with wild type B cells upon TLR9 stimulation. Immunization of Btk–/– mice with CpG ODN also leads to elevated levels of IL-12 in vivo and consequently, a greater -fold increment in the production of Th1 type IgG2b and IgG3 antibodies in these mice compared with wild type controls. The addition of exogenous recombinant IL-10 could suppress IL-12 production by TLR9-activated Btk–/– B cells, suggesting that in B cells, Btk negatively regulates IL-12 through the induction of autocrine IL-10 production. TLR9 signaling also leads to the activation of NFκB, including the p65RelA subunit in wild type B cells. The lack of Btk signaling affects the activation of NFκB and impairs the translocation of the p65RelA subunit to the nucleus of B cells upon TLR9 stimulation. However, p65RelA–/– B cells could respond similarly to wild type B cells in terms of IL-10 and IL-12 secretion when stimulated with CpG ODN, suggesting that the defect in NFκB p65RelA activation is additional to the impairment in cytokine production in TLR9-activated Btk–/– B cells. Thus, Btk plays an important role in TLR9 signaling and acts separately to regulate NFκB RelA activation as well as IL-10 and IL-12 production in B cells.

Deficiency in TNFRSF13B (TACI) expands T-follicular helper and germinal center B cells via increased ICOS-ligand expression but impairs plasma cell survival.

Mutations in TNFRSF13B, better known as transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), contribute to common variable immunodeficiency and autoimmunity in humans. How TACI regulates these two opposing conditions is unclear, however. TACI binds the cytokines BAFF and APRIL, and previous studies using gene KO mice indicated that loss of TACI affected only T-cell–independent antibody responses. Here we demonstrate that Taci−/− mice have expanded populations of T follicular helper (Tfh) and germinal center (GC) B cells in their spleens when immunized with T-cell–dependent antigen. The increased numbers of Tfh and GC B cells in Taci−/− mice are largely a result of up-regulation of inducible costimulator (ICOS) ligand on TACI-deficient B cells, given that ablation of one copy of the Icosl allele restores normal levels of Tfh and GC B cells in Taci−/− mice. Interestingly, despite the presence of increased Tfh and antigen-specific B cells, immunized Taci−/− mice demonstrate defective antigen-specific antibody responses resulting from significantly reduced numbers of antibody-secreting cells (ASCs). This effect is attributed to the failure to down-regulate the proapoptotic molecule BIM in Taci−/− plasma cells. Ablation of BIM could rescue ASC formation in Taci−/− mice, suggesting that TACI is more important for the survival of plasma cells than for the differentiation of these cells. Thus, our data reveal dual roles for TACI in B-cell terminal differentiation. On one hand, TACI modulates ICOS ligand expression and thereby limits the size of Tfh and GC B-cell compartments and prevents autoimmunity. On the other hand, it regulates the survival of ASCs and plays an important role in humoral immunity.

Identification of HEXIM1 as a Positive Regulator of p53

Background: To investigate the involvement of HEXIM1 in the p53 signaling pathway, we examine the functional connection between HEXIM1 and p53. Results: HEXIM1 directly interacts with p53 and enhances the protein stability of p53. Conclusion: HEXIM1 is a novel regulator of p53. Significance: Our results reveal a new function of HEXIM1 in regulating the protein stability and induction of p53. Hexamethylene bisacetamide-inducible protein 1 (HEXIM1) is best known as the inhibitor of positive transcription elongation factor b (P-TEFb), which regulates the transcription elongation of RNA polymerase II and controls 60–70% of mRNA synthesis. Our previous studies show that HEXIM1 interacts with two key p53 regulators, nucleophosmin and human double minute-2 protein (HDM2), implying a possible connection between HEXIM1 and the p53 signaling pathway. Here we report the interaction between p53 and HEXIM1 in breast cancer, acute myeloid leukemia, and colorectal carcinoma cells. The C-terminal regions of p53 and HEXIM1 are required for the protein-protein interaction. Overexpression of HEXIM1 prevents the ubiquitination of p53 by HDM2 and enhances the protein stability of p53, resulting in up-regulation of p53 target genes, such as Puma and p21. Induction of p53 can be achieved by several means, such as UV radiation and treatment with anti-cancer agents (including doxorubicin, etoposide, roscovitine, flavopiridol, and nutlin-3). Under all the conditions examined, elevated protein levels of p53 are found to associate with the increased p53-HEXIM1 interaction. In addition, knockdown of HEXIM1 significantly inhibits the induction of p53 and releases the cell cycle arrest caused by p53. Finally, the transcription of the p53 target genes is regulated by HEXIM1 in a p53-dependent fashion. Our results not only identify HEXIM1 as a positive regulator of p53, but also propose a novel molecular mechanism of p53 activation caused by the anti-cancer drugs and compounds.

Fas apoptosis inhibitory molecule regulates T cell receptor-mediated apoptosis of thymocytes by modulating Akt activation and Nur77 expression.

Fas apoptosis inhibitory molecule (FAIM) has been demonstrated to confer resistance to Fas-induced apoptosis of lymphocytes and hepatocytes in vitro and in vivo. Here, we show that FAIM is up-regulated in thymocytes upon T cell receptor (TCR) engagement and that faim−/− thymocytes are highly susceptible to TCR-mediated apoptosis with increased activation of caspase-8 and -9. Furthermore, injection of anti-CD3 antibodies leads to augmented depletion of CD4+CD8+ T cells in the thymus of faim−/− mice compared with wild-type control, suggesting that FAIM plays a role in thymocyte apoptosis. Cross-linking of the TCR on faim−/− thymocytes leads to an elevated protein level of the orphan nuclear receptor Nur77, which plays a role in thymocyte apoptosis. Interestingly, in the absence of FAIM, there are reduced ubiquitination and degradation of the Nur77 protein. Faim−/− thymocytes also exhibit a defective TCR-induced activation of Akt whose activity we now show is required for Nur77 ubiquitination. Further analyses utilizing FAIM-deficient primary thymocytes and FAIM-overexpressing DO-11.10 T cells indicate that FAIM acts upstream of Akt during TCR signaling and influences the localization of Akt to lipid rafts, hence affecting its activation. Taken together, our study defined a TCR-induced FAIM/Akt/Nur77 signaling axis that is critical for modulating the apoptosis of developing thymocytes.

Mir-17-92 regulates bone marrow homing of plasma cells and production of immunoglobulin G2c.

The polycistronic mir-17-92 cluster, also known as oncomir-1, was previously shown to be essential for early B lymphopoiesis. However, its role in late-stage B-cell differentiation and function remains unexplored. Here we ablate mir-17-92 in mature B cells and demonstrate that mir-17-92 is dispensable for conventional B-cell development in the periphery. Interestingly, mir-17-92-deficiency in B cells leads to enhanced homing of plasma cells to the bone marrow during T-cell-dependent immune response and selectively impairs IgG2c production. Mechanistically, mir-17-92 directly represses the expression of Sphingosine 1-phosphate receptor 1 and transcription factor IKAROS, which are, respectively, important for plasma cell homing and IgG2c production. We further show that deletion of mir-17-92 could reduce IgG2c anti-DNA autoantibody production and hence mitigate immune complex glomerulonephritis in Shp1-deficient mice prone to autoimmunity. Our results identify important roles for mir-17-92 in the regulation of peripheral B-cell function.