c-maf Expression Correlates with IL-10 Production in Th1, Th2, and Th17 Cells
To investigate further the downstream factors involved in regulating IL-10 production, we differentiated DO11.10 CD4+ T cells with increasing doses of OVA, in the presence or absence of IL-12, and quantified the expression of cytokines and transcription factors by real-time RT-PCR. Low-dose antigen resulted in transcription of Il4 and this was abrogated by both high antigen doses and IL-12 (Figure 6A). A low amount of transcription of Ifnγ was induced by IL-12 when cells were differentiated with low antigen dose, but this effect of IL-12 was markedly upregulated with increasing doses of antigen (Figure 6A). A low amount of Il10 transcription was observed at low doses of antigen accompanying Il4 expression (Th2 cell response), and this was abrogated by increased doses of antigen as was Il4 expression (Figure 6A). At low doses of antigen, IL-12 had little effect to increase IL-10 mRNA expression (Figure 6A) in keeping with the protein data (Figure 1A). However, IL-12 induced a high amount of Il10 transcription as well as Ifnγ expression with increased antigen doses (Figure 6A), again in keeping with the protein data (Figure 1A).
CD4+ T cells differentiated with increasing doses of antigen did not express high amounts of Tbx-21 (T-bet) mRNA, unless they were cocultured with IL-12 (Figure 6B). In contrast, high amounts of GATA-3 mRNA expression were only observed under Th2 cell differentiation conditions (low-dose antigen) (Figure 6B), and this expression was markedly downregulated by both increasing antigen dose and coculture in IL-12 (Figure 6B). Differentiation of T cells under low antigen dose led to expression of c-maf, in keeping with the Th2 cell profile (Ho et al., 1996), which was almost completely abrogated by increasing doses of antigen (Figure 6B). Interestingly, IL-12 sustained the high expression of c-Maf mRNA even at the highest antigen dose (Figure 6B). Moreover, IL-12 maintenance of c-maf expression required STAT4 activation (data not shown).
In Th17 cells that expressed IL-17a as well as IL-10 mRNA (Figure 6C), T-bet and GATA-3 mRNA were undetectable (data not shown), whereas that of ROR-γt was high (Figure 6C) (Ivanov et al., 2007). Th17 cells also expressed high amounts of c-maf (Figure 6C), confirming a recent report (Bauquet et al., 2009). c-Maf is therefore expressed in all IL-10-expressing T cell populations tested (Figures 6B and 6C) and may not be just a Th2 cell-specific transcription factor as originally thought (Ho et al., 1996). We showed also that like Il10 expression, c-maf expression was inhibited in Th1 and Th17 cells in the presence of the MEK1 and MEK2 inhibitor (PD184352), whereas T-bet and RORγt expression was hardly affected (Figure 6D).