Andrea J. Gerth

NFATc1 and NFATc2 Together Control Both T and B Cell Activation and Differentiation

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.

Sequential involvement of NFAT and Egr transcription factors in FasL regulation.

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.

Cutting Edge: Recipient MHC Class II Expression Is Required to Achieve Long-Term Survival of Murine Cardiac Allografts After Costimulatory Blockade

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.

Regulation of IFN-γ Production by B Effector 1 Cells: Essential Roles for T-bet and the IFN-γ Receptor

This manuscript systematically identifies the molecular mechanisms that regulate the ability of B cells to produce the critical type 1 cytokine, IFN-γ. B cells produce IFN-γ in response to IL-12 and IL-18 and when primed by Th1 cells. We show that development of IFN-γ-producing B cells by either Th1 cells or IL-12/IL-18 is absolutely dependent on expression of the IFN-γR 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-γR-mediated signals, STAT1-deficient B cells up-regulate T-bet and produce IFN-γ, indicating that additional transcriptional activators must be coupled to the IFN-γR in B cells. Finally, we show that although IL-12/IL-18 or IFN-γ-producing Th1 cells are required to initiate transcription of the IFN-γ gene in B cells, sustained expression of IFN-γ and T-bet by B cells is dependent on an IFN-γ/IFN-γR/T-bet autocrine feedback loop. These findings have significant implications, because they suggest that IFN-γ-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.

SH2D1A regulates T-dependent humoral autoimmunity

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.

Active Inhibition of Plasma Cell Development in Resting B Cells by Microphthalmia-associated Transcription Factor

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.

Further analysis of the T-cell subsets and pathways of murine cardiac allograft rejection.

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.