The γ-PGA-induced FoxP3+ cells expressed CD25, GITR and cytotoxic T lymphocyte antigen 4 (CTLA-4) Selleck MK-2206 at levels equivalent to those in nTreg cells and higher than those in TGF-β-induced aTreg cells (Fig. 2d). Taken together, these results demonstrate that the presence of γ-PGA during priming converts naive non-Treg cells to FoxP3+ aTreg cells with phenotypes equivalent to those of nTreg cells. Because TGF-β is a well-known and potent inducer of FoxP3 [7,8], it seemed possible that γ-PGA induced FoxP3 expression by first stimulating CD4+ T cells to produce

TGF-β. Because the culture medium supplemented with 10% fetal bovine serum (FBS) contained a substantial amount of TGF-β and the cells did not survive under

the serum-free condition, we failed to quantitate the level of TGF-β secreted in the culture supernatant. Instead, we found that CD4+ T cells stimulated in the presence of γ-PGA produced approximately 3·5-fold more TGF-β transcripts than in the absence of γ-PGA (Fig. 3a). This TGF-β seemed to contribute to the induction of FoxP3 because neutralizing antibody to TGF-β reduced significantly the number of γ-PGA-induced FoxP3+ cells (Fig. 3b). Therefore, we conclude that the mechanism by which γ-PGA induces FoxP3 expression is at least partially dependent on the TGF-β produced in response to γ-PGA. Previously selleck chemicals we found that γ-PGA activated dendritic cells via TLR-4 [24]. Therefore, we needed to confirm that the ability of γ-PGA to suppress the development of Th17 cells (Fig. 1) was due solely to its action on naive CD4+ T cells rather than on dendritic cells. To this end, we completely eliminated CD4+CD11c+ dendritic cells from a CD4+ population. γ-PGA still rendered these cells refractory to Th17-polarizing conditions, indicating that γ-PGA acts directly on naive CD4+ T cells (Fig. 4a). Furthermore, in addition to the master Forskolin regulator RORγt, γ-PGA significantly inhibited the induction of other Th17-related factors, such as STAT-3, IRF-4 and Ahr, while increasing the

expression of FoxP3 and SOCS3 (Fig. 4b). Under Th17-polarizing conditions, γ-PGA inhibited TGF-β expression – the opposite outcome to that obtained in neutral conditions. However, the reduced expression of TGF-β may not interfere with the action of γ-PGA, because we found that reducing the concentration of exogenous TGF-β from 5 ng/ml to 2 ng/ml in the Th17-polarizing conditions did not affect the development of Th17 cells (data not shown). Because of the importance of RORγt as a Th17 lineage-determining factor, we tested whether γ-PGA affects the expression of RORγt at the transcriptional level. Mouse thymoma EL4 cells were transfected with a luciferase reporter spanning 2 kb upstream of exon 1 of the Rorc gene encoding RORγt and cultured under Th17 conditions in the presence and absence of γ-PGA. The presence of γ-PGA significantly reduced luciferase activity (Fig. 4c).