text: IL-10 Production by Th1 Cells Is Dependent on STAT4 but Not on STAT6, IFN-gamma, or IL-4 Signaling
  To further elucidate the mechanisms required for the development of Th1 cells producing IL-10, we investigated the role of STAT4, one of the signaling pathways activated by IL-12 (Murphy etal., 2000). Naive CD4+ D011.10 Tcells deficient in STAT4 (Ouyang etal., 1998) were cultured in the presence of IL-12 and OVA. Again, IL-10-producing Th1 cells were differentiated at the high antigen dose in the presence of IL-12 in DO11.10 Tcells (Figure2A). In contrast, in the absence of STAT4, the percentage of cells expressing IFN-gamma was dramatically diminished as expected and resulted in an increase in the percentage of cells expressing IL-4, but not IL-10 (Figure2A), suggesting that STAT4 contributes to IL-10 expression by Th1 cells. 
  Because IL-10 expression is associated with an IL-4-induced Th2 cell phenotype, we investigated whether the differentiation of the IL-10-producing Th1 cells depended on signaling through the IL-4 receptor via STAT6 activation (Glimcher and Murphy, 2000; Murphy etal., 2000). The absence of STAT6 did not impair the differentiation of IL-10-producing Th1 cells in the presence of IL-12 and OVA (Figure2A). In fact, a higher percentage of STAT6-deficient cells compared with WT cells produced both IL-10 and IFN-gamma (Figure2A), which may be the result of the loss of Th2 cell control over a Th1 cell response. As expected, lack of STAT6 abrogated both IL-4 and IL-10 production by Tcells developed with IL-4 or with low antigen dose (Figures 2B and 2C). However, in the absence of STAT4 signaling, IL-10 and IL-4 production by Th2 cells was if anything increased (Figures 2B and 2C). Thus, in contrast to what was observed under Th1 conditions, IL-10 expression by Th2 cells depended on STAT6, but not on STAT4, signaling (Figures 2B and 2C). 
  To investigate whether the inability of STAT4-deficient Tcells to produce IL-10 might be due to the absence of IFN-gamma, as suggested before (Shaw etal., 2006), we differentiated DO11.10 or DO11.10 IFN-gamma-deficient naive CD4+ Tcells in the presence of IL-12 and increasing doses of OVA. The secretion of IL-10 as induced by high antigen dose, and IL-12 was not affected by an absence of IFN-gamma (Figure2D), showing that the expression ofIL-10 by Th1 cells is independent of IFN-gamma. In the absence of IFN-gamma, we observed an increase in the secreted IL-4 as expected (data not shown). 
  We also tested for any potential role of IL-4 in the development of Th1 cells producing IL-10 by culturing DO11.10 or DO11.10 IL-4-deficient naive CD4+ Tcells with IL-12 and increasing doses of antigen. As observed in the absence of STAT6 (Figure2A), IL-4 deficiency had no effect on the development of Th1 cells producing IL-10 (Figure2E), but compromised the development of Th2 cells producing IL-10 (Figure2F). Thus, our data suggested that IL-10 production by Th1 or Th2 cells was dependent on the specific signaling pathways required for their differentiation, given that STAT4 is required for the induction of IL-10 production by Th1 cells and STAT6 for Th2 cells. 
  