A similar trend was observed under IL-23 polarizing conditions (Fig. 1a and data not shown). In addition, G-1-mediated IL-10 expression was blocked by the recently described GPER antagonist G15.40 The induction of a population of IL-10+ IL-17A+ cells suggests that G-1 can elicit IL-10 expression within cells that have differentiated to the Th17 lineage. Taken together, these data show that G-1 can elicit IL-10 production within the Th17 compartment, a response that is blocked by the GPER-selective antagonist G15. Interleukin-10 production within Th populations has been shown to be dependent on signalling through extracellular

signal-regulated kinases ERK1/2,12,13 one of three MAP kinase cascades, the others comprising JNK1/2 and p38. GPER has been shown to activate the FDA approved Drug Library cell assay ERK pathway, although predominantly in cancer cells.42 To test whether G-1-mediated induction of IL-10 was dependent on MAP kinase signalling, naive T cells were treated with either PD98059, an inhibitor of the ERK pathway, SB203580, an inhibitor of the p38 pathway, or the JNK II inhibitor, and stimulated under Th17-polarizing conditions as before. Consistent with other published reports,13 we found that inhibition of p38 had no effect on IL-10 expression in Th17-polarized cells. Similarly, MG-132 ic50 JNK signalling appeared not

to be required for G-1-mediated induction of IL-10 (Fig. 4a). In contrast, there was no difference in the percentage of IL-10+ cells observed between control and G-1-treated cultures when cells were cultured with the ERK inhibitor PD98059 (Fig. 4a,b), consistent with a role for ERK signalling specifically in G-1-mediated IL-10 induction. These data suggest that G-1 mediates IL-10 expression by activating ERK signalling in CD4+ T cells. The ERK pathway is known to be a potent activator of cell proliferation. To determine if G-1-mediated increases in

IL-10 were the result of increased proliferation of cells expressing IL-10 rather than induction of IL-10 de novo, naive T cells were stained with the proliferation dye eFluor670 before stimulation in culture. We were unable to detect any significant difference in the proportion of dividing cells following G-1 Interleukin-2 receptor treatment. The observation that G-1-treated cultures demonstrate attenuated dilution of the eFluor dye compared with the DMSO-treated cultures (Fig. 5) indicates that the increase in IL-10+ cells following G-1 treatment is not the result of an increase in cell proliferation, and in fact shows that proliferating cells are going through fewer divisions when treated with G-1, perhaps because of the action of IL-10. In addition, the dramatic increase in the number of non-dividing cells expressing IL-10 in G-1-treated cultures (as indicated in the upper right quadrant in Fig. 5b) suggests that G-1 can specifically drive expression of IL-10 independent of cell division.