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Although evidence of the protective immunity conferred by B-1b cells (CD19+B220+IgMhiMac1+CD5−) has been established, the mechanisms governing the maintenance and activation of B-1b cells following pathogen encounter remain unclear. B cell-activating factor (BAFF) and a proliferation-inducing ligand (APRIL) mediate their function in mature B cells through the BAFF receptor (BAFFR) and transmembrane activator and CAML interactor (TACI). BAFFR-deficient mice have lower numbers of B-1b cells, and this reduction is directly proportional to BAFFR levels. The generation of B-1b cells is also dependent on the strength of B cell receptor (BCR) signaling. Mice with impaired BCR signaling, such as X-linked immunodeficient ( xid) mice, have B-1b cell deficiency, indicating that both BCR- and BAFFR-mediated signaling are critical for B-1b cell homeostasis. Borrelia hermsii induces expansion and persistence of B-1b cells in xid mice, and these B-1b cells provide a heightened protective response. Toll-like receptor (TLR)-mediated stimulation of xid B cells results in a significant increase in TACI expression and restoration of TACI-mediated functions. The activation of TLR signaling by B. hermsii and BCR/TLR costimulation-mediated upregulation of BAFFR and TACI on B-1b cells suggests that B-1b cell maintenance and function following bacterial exposure may depend on BAFFR- and TACI-mediated signaling. In fact, the loss of both BAFFR and TACI results in a greater impairment in anti- B. hermsii responses compared to deficiency of BAFFR or TACI alone. [ABSTRACT FROM AUTHOR]

Background The B-cell receptor transmembrane activator and calcium modulator ligand interactor (TACI) is important for T-independent antibody responses. One in 200 blood donors are heterozygous for the TACI A181E mutation. Objective We sought to investigate the effect on B-cell function of TACI A181E heterozygosity in reportedly healthy subjects and of the corresponding TACI A144E mutation in mice. Methods Nuclear factor κB (NF-κB) activation was measured by using the luciferase assay in 293T cells cotransfected with wild-type and mutant TACI. TACI-driven proliferation, isotype switching, and antibody responses were measured in B cells from heterozygous TACI A144E knock-in mice. Mouse mortality was monitored after intranasal pneumococcal challenge. Results Levels of natural antibodies to the pneumococcal polysaccharide component phosphocholine were significantly lower in A181E-heterozygous than TACI-sufficient Swedish blood donors never immunized with pneumococcal antigens. Although overexpressed hTACI A181E and mTACI A144E acted as dominant-negative mutations in transfectants, homozygosity for A144E in mice resulted in absent TACI expression in B cells, indicating that the mutant protein is unstable when naturally expressed. A144E heterozygous mice, such as TACI +/− mice, expressed half the normal level of TACI on their B cells and exhibited similar defects in a proliferation-inducing ligand–driven B-cell activation, antibody responses to TNP-Ficoll, production of natural antibodies to phosphocholine, and survival after intranasal pneumococcal challenge. Conclusion These results suggest that TACI A181E heterozygosity results in TACI haploinsufficiency with increased susceptibility to pneumococcal infection. This has important implications for asymptomatic TACI A181E carriers. [ABSTRACT FROM AUTHOR]

Background: Transmembrane activator, calcium modulator, and cyclophilin ligand interactor (TACI) expression on B cells is upregulated by Toll-like receptor (TLR) 4. Objective: We sought to examine whether TACI synergizes with TLR4 in driving immunoglobulin production by B cells and to examine the mechanism of this synergy. Methods: Purified mouse naive B cells were stimulated with the TACI ligand a proliferation-inducing ligand (APRIL) and with suboptimal concentrations of the TLR4 ligand LPS in the presence or absence of IL-4. Immunoglobulin secretion was measured by means of ELISA. Surface IgG1–positive B cells and CD138+ plasmacytoid cells were enumerated by means of FACS. Expression of γ1 and ε germline transcripts, activation-induced cytidine deaminase, and γ1 and ε mature transcripts was measured by means of RT-PCR. Results: APRIL synergized with LPS in driving B-cell proliferation and IgM, IgG1, IgG3, IgE, and IgA production. This was mediated by TACI because it was preserved in B-cell maturation antigen−/−, but not TACI−/−, B cells. APRIL and LPS synergized to promote isotype switching, as evidenced by increased expression of activation-induced cytidine deaminase and γ1 and ε mature transcripts and generation of surface IgG1–positive cells. More importantly, APRIL and LPS strongly synergized to drive the plasma cell differentiation program, as evidenced by an increase in CD138+ cells and expression of B lymphocyte induced maturation protein-1 (Blimp-1), interferon regulatory factor-4 (IRF-4), and the spliced form of X-box binding protein-1 (XBP-1). TACI−/− mice had impaired IgM and IgG1 antibody responses to immunization, with a suboptimal dose of the type I T cell–independent antigen 2, 4, 6- Trinitrophenol (TNP)-LPS. Conclusions: These observations suggest that TACI cooperates with TLR4 to drive B-cell differentiation and immunoglobulin production in vitro and in vivo. [Copyright &y& Elsevier]

Deficiencies in transmembrane activator and CAML interactor (TACI) result in common variable immune deficiency, a syndrome marked by recurrent infections with encapsulated microorganisms, impaired production of antibodies, and lymphoproliferation. How TACI promotes antibody production and inhibits lymphoproliferation is not understood. To answer this question, we studied the generation of immunity to protein antigens in both TACI-deficient and TACI-proficient mice. We show that TACI promotes sustained Blimp-1 expression by B cells responding to antigen, which in turn limits B-cell clonal expansion and facilitates differentiation of long-lived antibody-secreting cells. Short-term IgG secretion occurs independently of TACI as DNA double-strand breaks associated with isotype class switching induce Blimp-1 transiently, independently of TACI. Our results showing that TACI induces and maintains Blimp-1 provide, for the first time, a unified molecular and cellular mechanism explaining the primary features of common variable immune deficiency, exquisite vulnerability to infection with encapsulated organisms, lymphoproliferation, and hypogammaglobulinemia. [ABSTRACT FROM AUTHOR]

Calcium-modulating cyclophilin ligand (CAML) is a ubiquitously expressed protein that is important during thymopoiesis. However, whether it serves a function in mature lymphocytes is unknown. In this article, we show that CAML is essential for survival of peripheral follicular (Fo) B cells. Conditional deletion of CAML in CD19-Cre transgenic mice caused a significant reduction in Fo cell numbers and increased rates of homeostatic proliferation. CAML-deficient Fo cells showed increased cellular turnover and normal proliferative ability. Although CAML-deficient Fo cells responded to AgR stimulation and to B cell activating factor, they displayed decreased survival and increased apoptosis following stimulation with LPS and IL-4 in vitro. Failure to survive was not due to aberrant B cell development in the absence of CAML, because induced deletion of the gene in mature cells resulted in a similar phenotype. These data establish an essential and ongoing role for CAML in the long-term survival of mature B cells. [ABSTRACT FROM AUTHOR]

Mechanistic understanding of RP105 has been con-founded by the fact that this TLR homolog has appeared to have opposing, cell type-specific effects on TLR4 sig-naling. Although RP105 inhibits TLR4-driven signaling in cell lines and myeloid cells, impaired LPS-driven pro-liferation by B cells from RP105 -/- mice has suggested that RP105 facilitates TLR4 signaling in B cells. In this article, we show that modulation of B cell proliferation by RP105 is not a function of B cell-intrinsic expression of RP105, and identify a mechanistic role for dysregu-lated BAFF expression in the proliferative abnormalities of B cells from RP105 -/- mice: serum BAFF levels are elevated in RP105 -/- mice, and partial BAFF neutral-ization rescues aberrant B cell proliferative responses in such mice. These data indicate that RP105 does not have dichotomous effects on TLR4 signaling and em-phasize the need for caution in interpreting the results of global genetic deletion. [ABSTRACT FROM AUTHOR]

Antiviral Abs, for example those produced in response to influenza virus infection, are critical for virus neutralization and defense against secondary infection. While the half-life of Abs is short, Ab titers can last a lifetime due to a subset of the Ab-secreting cells (ASCs) that is long lived. However, the mechanisms governing ASC longevity are poorly understood. Here, we have identified a critical role for extrinsic cytokine signals in the survival of respiratory tract ASCs in a mouse model of influenza infection. Irradiation of mice at various time points after influenza virus infection markedly diminished numbers of lung ASCs, suggesting that they are short-lived and require extrinsic factors in order to persist. Neutralization of the TNF superfamily cytokines B lymphocyte stimulator (BLyS; also known as BAFF) and a proliferation-inducing ligand (APRIL) reduced numbers of antiviral ASCs in the lungs and bone marrow, whereas ASCs in the spleen and lung-draining lymph node were surprisingly unaffected. Mice deficient in transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), a receptor for BLyS and APRIL, mounted an initial antiviral B cell response similar to that generated in WT mice but failed to sustain protective Ab titers in the airways and serum, leading to increased susceptibility to secondary viral challenge. These studies highlight the importance of TACI signaling for the maintenance of ASCs and protection against influenza virus infection. [ABSTRACT FROM AUTHOR]