IL-17-secreting CD4 T cells play a role in several physiologic and pathologic processes. In mice TGF-beta and IL-6 trigger Th17 differentiation of CD4 T cells, but their role in human Th17 commitment has not been evaluated. To investigate the signals driving human Th17 differentiation, sorted naïve CD4 T cells from peripheral-blood mononuclear cells were stimulated with anti-CD3/anti-CD28 coated beads in the presence of IL-4 and IFNgamma neutralizing antibodies and different cytokines. We found that TGF-beta alone or together with IL-6 failed to induce differentiation of Th17 cells. IL-1beta alone induced the differentiation of IL-17-secreting cells and synergizes with IL-6 to promote Th17 development. Accordingly, IL-1beta upregulated the mRNA for RORgammat (master regulator of Th17 commitment), and together with IL-6 ensured its sustained expression. Surprisingly, the addition of TGF-beta to the IL-1beta/IL-6 combination reduced the percentage of Th17 cells. We next evaluated the ability of different antigen presenting cells (APC) to prime Th17 cells. For this, human naïve CD4 T cells were primed with different APC activated with Toll-like receptor (TLR) agonists. Monocyte-derived dendritic cells (DC), that secreted low levels of IL-1beta and high levels IL12p70 upon activation, were poor inducers of Th17 cells and promoted Th1 differentiation. Circulating CD1c+ DC activated with TLR2 agonists secreted IL-1beta and induced Th17 differentiation. Importantly, circulating CD14+ monocytes, that produced high IL-1beta and IL-6 levels and low or no IL12p70 after stimulation, were very effective in inducing Th17 development mainly when activated with TLR2 agonists. Th17 induction was abrogated by the blockade of IL-1beta and IL-6, but enhanced by the inhibition of TGF-beta and IL-12p70. Our data demonstrate that the ability of an APC to prime Th17 is dependent on the cytokines secreted and that the cytokines that drive Th17 differentiation in humans are primarily IL-1beta and IL-6 but not TGF-beta.
