Two key events occur during the differentiation of IFN-γ-secreting Th1 cells: up-regulation of IL-12Rβ2 and IL-12-driven up-regulation of IL-18Rα. We previously demonstrated that IL-12-driven up-regulation of IL-18Rα expression is severely impaired in IFN-γ−/− mice. However, it was unclear from these studies how IFN-γ influenced IL-18Rα since IFN-γ alone had no direct effect on IL-18Rα expression. In the absence of IL-4, IL-12-dependent up-regulation of IL-18Rα/IL-12Rβ2 was independent of IFN-γ. However, in the presence of IL-4, IFN-γ functions to limit the negative effects of IL-4 on both IL-18Rα and IL-12Rβ2. Neutralization of IL-4 restored IL-12-driven up-regulation of IL-18Rα/IL-12Rβ2 in an IFN-γ-independent fashion. In the absence of both IL-12 and IL-4, IFN-γ up-regulates IL-12β2 expression and primes IFN-γ-producing Th1 cells. When T cells were primed in the presence of IL-4, no correlation was found between the levels of expression of the IL-18Rα or the IL-12Rβ2 and the capacity of these cells to produce IFN-γ, suggesting that IL-4 may also negatively affect IL-12-mediated signal transduction and thus Th1 differentiation. These data clarify the role of IFN-γ in regulation of IL-18Rα/IL-12Rβ2 during both IL-12-dependent and IL-12-independent Th1 differentiation.
Interleukin (IL)-18 has been well characterized as a costimulatory factor for the induction of IL-12–mediated interferon (IFN)-γ production by T helper (Th)1 cells, but also can induce IL-4 production and thus facilitate the differentiation of Th2 cells. To determine the mechanisms by which IL-18 might regulate these diametrically distinct immune responses, we have analyzed the role of cytokines in the regulation of IL-18 receptor α chain (IL-18Rα) expression. The majority of peripheral CD4+ T cells constitutively expressed the IL-18Rα. Upon antigen stimulation in the presence of IL-12, marked enhancement of IL-18Rα expression was observed. IL-12–mediated upregulation of IL-18Rα required IFN-γ. Activated CD4+ T cells that expressed low levels of IL-18Rα could produce IFN-γ when stimulated with the combination of IL-12 and IL-18, while CD4+ cells which expressed high levels of IL-18Rα could respond to IL-18 alone. In contrast, T cell stimulation in the presence of IL-4 resulted in a downregulation of IL-18Rα expression. Both IL-4−/− and signal transducer and activator of transcription (Stat)6−/− T cells expressed higher levels of IL-18Rα after TCR stimulation. Furthermore, activated T cells from Stat6−/− mice produced more IFN-γ in response to IL-18 than wild-type controls. Thus, positive/negative regulation of the IL-18Rα by the major inductive cytokines (IL-12 and IL-4) determines the capacity of IL-18 to polarize an immune response.
CD4+ Th17 cells have emerged as a new T cell subset in the Th1/Th2 paradigm, and efforts have shifted toward understanding the factors that regulate their development in vivo. To analyze the role of the transcription factor T-bet in regulation of Th17 cells, we used a murine model of Trypanosoma cruzi infection, a protozoan parasite that causes Chagas disease in humans. Infection of Tbx21−/− mice led to normal, unimpaired development of Ag-specific CD4+ T cells producing IFN-γ. However, a robust Th17 response developed concomitant with Th1 responses. Despite significant IFN-γ production, the physiological effects of Th17 responses prevailed as there was a sharp increase in Gr-1+Ly6G+ neutrophils. Adoptive transfer of T cells from infected Tbx21−/− mice into Rag-2−/− mice (Tbx21+/+) revealed that CD4+ T cells maintained their IL-17-producing phenotype, including those cells capable of producing both IFN-γ and IL-17. Furthermore, and in contrast to the effects of IL-2 on Th17 development, IL-2 had no effect on IL-17 production by primed T cells. Importantly, adoptive transfer of T cells from naive Tbx21−/− mice into infected Rag-2−/− mice recapitulated the differentiation of T. cruzi-specific Th17 cells observed in infected Tbx21−/− mice. Conversely, transfer of wild-type T cells into infected Tbx21−/− mice did not reveal an increase in Th17 development. These results demonstrate that T-bet regulates the differentiation of T. cruzi-specific Th17 cells in vivo in a T cell-intrinsic manner. These data provide important insight into the role of T-bet in regulation of parasite-specific Th17 responses.
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