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This manuscript systematically identifies the molecular mechanisms that regulate the ability of B cells to produce the critical type 1 cytokine, IFN-gamma. B cells produce IFN-gamma in response to IL-12 and IL-18 and when primed by Th1 cells. We show that development of IFN-gamma-producing B cells by either Th1 cells or IL-12/IL-18 is absolutely dependent on expression of the IFN-gammaR and the T-box transcription factor, T-bet. Interestingly, although T-bet up-regulation in developing B effector 1 (Be1) cells is controlled by IFN-gammaR-mediated signals, STAT1-deficient B cells up-regulate T-bet and produce IFN-gamma, indicating that additional transcriptional activators must be coupled to the IFN-gammaR in B cells. Finally, we show that although IL-12/IL-18 or IFN-gamma-producing Th1 cells are required to initiate transcription of the IFN-gamma gene in B cells, sustained expression of IFN-gamma and T-bet by B cells is dependent on an IFN-gamma/IFN-gammaR/T-bet autocrine feedback loop. These findings have significant implications, because they suggest that IFN-gamma-producing B cells not only amplify Th1 responses, but also imprint a type 1 phenotype on B cells themselves. In the case of immune responses to bacterial or viral pathogens, this B cell-driven autocrine feedback loop is likely to be beneficial; however, in the case of B cell responses to autoantigens, it may result in amplification of the autoimmune loop and increased pathology.

The signaling lymphocytic activation molecule (SLAM)/CD150 family includes a family of chromosome 1-encoded cell surface molecules with costimulatory functions mediated in part by the adaptor protein SH2D1A (SLAM-associated protein, SAP). Deficiency in SH2D1A protects mice from an experimental model of lupus, including the development of hypergammaglobulinemia, autoantibodies including anti-double stranded DNA, and renal disease. This protection did not reflect grossly defective T or B cell function per se because SH2D1A-deficient mice were susceptible to experimental autoimmune encephalomyelitis, a T cell-dependent disease, and they were capable of mounting normal T-independent antigen-specific immunoglobulin responses. Instead, T-dependent antibody responses were impaired in SH2D1A-deficient mice, reflecting defective germinal center formation. These findings demonstrate a specific role for the SLAM-SH2D1A system in the regulation of T-dependent humoral immune responses, implicating members of the CD150-SH2D1A family as targets in the pathogenesis and therapy of antibody-mediated autoimmune and allergic diseases.

B cell terminal differentiation involves development into an antibody-secreting plasma cell, reflecting the concerted activation of proplasma cell transcriptional regulators, such as Blimp-1, IRF-4, and Xbp-1. Here, we show that the microphthalmia-associated transcription factor (Mitf) is highly expressed in naive B cells, where it antagonizes the process of terminal differentiation through the repression of IRF-4. Defective Mitf activity results in spontaneous B cell activation, antibody secretion, and autoantibody production. Conversely, ectopic Mitf expression suppresses the expression of IRF-4, the plasma cell marker CD138, and antibody secretion. Thus, Mitf regulates B cell homeostasis by suppressing the antibody-secreting fate.

Unmethylated CpG-containing DNA plays a critical role in immunity via the augmentation of Th1 but suppression of Th2 T cell responses. We describe here that CpG motifs also redirect isotype production by murine B cells to "Th1-like" Ig isotypes (IgG2a, IgG2b, and IgG3) while suppressing Th2 isotypes (IgG1 and IgE). Using genetically mutant B cells, we find that the IgG2a, IgG2b and IgG3 isotypes are transcriptionally regulated via the promotion of class-switching, in a manner critically dependent upon TLR9 and MyD88. Thus, CpG DNA redirects Ig isotype production by regulating the specificity of class-switch recombination.

Background & Aims: Nuclear factor of activated T cells transcription factors plays a central role in immunity by regulating the expression of multiple cytokines and other regulatory molecules, many of which have been heavily implicated in the pathogenesis of inflammatory bowel disease. However, few studies have directly investigated the nuclear factor of activated T cells proteins in inflammatory bowel disease. We describe here a specific role for nuclear factor of activated T cells c2 in the pathogenesis of murine inflammatory bowel disease., Methods: Mice deficient for nuclear factor of activated T cells c2, recombinase activating gene-2, or both and transgenic or nontransgenic for an anti-ovalbumin T-cell receptor or an anti-hen egg lysozyme B-cell receptor were studied. Adoptive transfers were performed of T or B cells or both from nuclear factor of activated T cells c2-deficient mice into nuclear factor of activated T cells c2-deficient recombinase activating gene-deficient animals, in the presence or absence of antibodies that neutralize interleukin-10 activity., Results: Nuclear factor of activated T cells c2-deficient, recombinase activating gene-deficient animals spontaneously developed a severe inflammatory bowel syndrome that resembled ulcerative colitis but was composed entirely of nonlymphocytes. The disease was suppressed by the adoptive transfer of polyclonal B-cell populations, even on neutralization of interleukin-10, but not by the presence of monoclonal T or B cells., Conclusions: Nuclear factor of activated T cells plays a critical role in the regulation of bowel inflammation by nonlymphoid immune cells, and B cells suppress bowel inflammation by innate immune cells. Such findings indicate a novel, interleukin-10-independent role for nuclear factor of activated T cells in the regulation of innate immunity and in intestinal immune tolerance.

In many models of organ-specific autoimmune diseases, mast cells provide a critical cellular link between autoantibodies and end-organ inflammation, both initiating and propagating disease. However, their role in systemic autoimmunity remains speculative. We therefore examined the role of mast cells in a murine model of systemic immune complex-related autoimmune disease, lupus nephritis, expecting to observe the development of humoral autoimmunity in the absence of end-organ disease. Surprisingly, not only did mast cell-deficient animals develop characteristic humoral features of lupus, including hypergammaglobulinemia and autoantibodies, they also developed immune complex glomerulonephritis, as evidenced by renal immune deposits, glomerular disease, and proteinuria. These findings implicate the presence of distinct effector pathways to end-organ damage in humoral autoimmune diseases: one involving the interaction between autoantibodies and mast cells to recruit inflammation in organ-specific autoimmunity, and another involving a more direct--mast cell-independent--interaction between autoantibodies and circulating inflammatory mediators in systemic autoimmunity.

Forkhead transcription factors play key roles in the regulation of immune responses. Here, we identify a role for one member of this family, Foxj1, in the regulation of T cell activation and autoreactivity. Foxj1 deficiency resulted in multiorgan systemic inflammation, exaggerated Th1 cytokine production, and T cell proliferation in autologous mixed lymphocyte reactions. Foxj1 suppressed NF-kappaB transcription activity in vitro, and Foxj1-deficient T cells possessed increased NF-kappaB activity in vivo, correlating with the ability of Foxj1 to regulate IkappaB proteins, particularly IkappaBbeta. Thus, Foxj1 likely modulates inflammatory reactions and prevents autoimmunity by antagonizing proinflammatory transcriptional activities. These results suggest a potentially general role for forkhead genes in the enforcement of lymphocyte quiescence.

The IgG2a Ig subclass plays a critical role in the pathogenesis of humoral autoimmunity and protection against pathogens. The T-box transcription factor T-bet has been implicated as a critical mediator of class-switch recombination (CSR) to IgG2a, but its relative importance to this process in various immune contexts remains incompletely defined. We report here that, surprisingly, T-bet is selectively required for IgG2a class switching in response to T-independent, but not T-dependent, stimuli. Specifically, T-dependent signaling through CD40, in contrast to T-independent signaling via lipopolysaccharide, can bypass a requirement for T-bet in IgG2a germline transcription and subsequent isotype switching. In contrast, T-bet-deficient B cells undergo class switching to other IgG isotypes at least as well as wild-type counterparts. Thus, T-bet is a class-specific regulator of IgG CSR and represents a unique regulator of B cell differentiation by participating in a T-independent, but not a T-dependent, activation pathway. T-bet-deficient B cells therefore represent a novel paradigm by which to investigate the regulation of humoral immune responses.

The present study examined the role of CD4+ and CD8+ T cells in cardiac allograft rejection when either the direct or indirect pathway was eliminated for the CD4+ portion of the response. To study the pathways in vivo, we used genetically altered mouse strains that lack class II antigens as either the donors or recipients for cardiac transplantation. In contrast to earlier published studies, which used different strain combinations, we found that either CD4- or CD8-depletion prolonged cardiac allograft survival moderately, but not indefinitely, in an MHC-mismatched, minor-matched combination. When the CD4+ indirect pathway was eliminated, rapid graft rejection occurred when both T-cell subsets were present and when either CD4+ or CD8+ T cells were depleted. When the CD4+ direct pathway was eliminated, rapid graft rejection occurred when both T-cell subsets were present, there was slow rejection when CD4+ T cells were eliminated, and no rejection was seen for more than 100 days when CD8+ T cells were eliminated. However, the long-surviving allografts on the recipients with only CD4+ cells and an indirect pathway did show evidence of chronic vasculopathy. Thus, either CD4+ or CD8+ T cells can mediate acute cardiac allograft rejection in these experiments when both pathways are available. In addition, CD4+ T cells can provide help for acute rejection through either the direct or indirect pathway. Finally, recipients who have only CD4+ cells and an indirect pathway do not demonstrate acute rejection, but do show evidence of chronic rejection.

To study the role of the direct and indirect pathways in achieving tolerance, we used genetically altered mouse strains in two ways: 1) MHC class II-deficient mice were used as donors of skin and cardiac grafts to eliminate the direct CD4(+) T cell response, and 2) B6 II(-)4(+) mice, which are MHC class II-deficient mice expressing an MHC class II transgene only on thymic epithelium, were used as recipients of normal grafts. These mice cannot mount an indirect response. Eliminating the indirect pathway actually made it more difficult to achieve prolonged allograft survival when we used costimulatory blockade than when both pathways were available. Costimulatory blockade was ineffective even when CD4(+) T cells from normal animals were transferred into recipients that lacked MHC class II molecules. These results suggest that an active CD4(+) response through the indirect pathway is necessary for costimulatory blockade to be effective in prolonging allograft survival.

NFAT transcription factors play critical roles in gene transcription during immune responses. To investigate further the two most prominent NFAT family members, NFATc1 and NFATc2, we generated mice bearing lymphoid systems devoid of both. Doubly deficient T cells displayed cell surface markers of activation yet were significantly deficient in the development of multiple effector functions, including Th cytokine production, surface effector molecule expression, and cytolytic activity. Nevertheless, doubly deficient B cells were hyperactivated, as evidenced by extremely elevated serum IgG1 and IgE, as well as plasma cell expansion and infiltration of end organs. Thus, in T cells, NFATc1 and NFATc2 are dispensable for inflammatory reactivity but are required for effector differentiation, while in B cells, NFATs regulate both normal homeostasis and differentiation.

The critical function of NFAT proteins in maintaining lymphoid homeostasis was revealed in mice lacking both NFATp and NFAT4 (DKO). DKO mice exhibit increased lymphoproliferation, decreased activation-induced cell death, and impaired induction of FasL. The transcription factors Egr2 and Egr3 are potent activators of FasL expression. Here we find that Egr2 and Egr3 are NFAT target genes. Activation of FasL occurs via the NFAT-dependent induction of Egr3, as demonstrated by the ability of exogenously provided NFATp to restore Egr-dependent FasL promoter activity in DKO lymph node cells. Further, Egr3 expression is enriched in Th1 cells, suggesting a molecular basis for the known preferential expression of FasL in the Th1 versus Th2 subset.