Various chemokine receptors, cytokine receptors, and pattern recognition receptors are expressed by γδ T cells and these receptors have been shown to be involved in the activation of γδ T cells, especially for the induction of IL-17 (Fig. 1). IL-1, IL-6, IL-18, IL-23, and transforming growth factor beta
1 (TGF-β) have each been implicated in promoting IL-17 production by γδ T cells. Furthermore, activation via Toll-like receptor 2 (TLR2) and DC-associated C-type lectin 1 (dectin 1), as well as the internal receptor aryl hydrocarbon receptor (AhR), has also been associated with IL-17 production by see more γδ T cells [30]. However, highly purified γδ T cells do not appear to produce IL-17 following stimulation with TLR agonists in the absence of exogenous cytokines (Sutton, Mielke, and Mills, unpublished data). Furthermore, γδ T cells from IL-6−/−
mice produce IL-17 PLX-4720 mouse at comparable levels to wild-type mice [31], while ablation of TGF-β leads to a reduction but not a total loss of IL-17, suggesting that there may be a non-essential role for these cytokines in promoting IL-17 production by γδ T cells. In contrast, γδ T cells in a spleen cell preparation from IL-1 type I receptor-defective (IL-1RI−/−) mice fail to secrete IL-17 in response to IL-23 and/or TLR agonists (Sutton and Mills unpublished data). Furthermore, IL-1α or IL-1β in synergy with IL-23, has been shown to play a crucial role in the induction of IL-17 from γδ T cells in both mice and humans [6, 25, 32, 33]. Interestingly, γδ T cells express IL-1RI and have high levels of IL-18R on their cell surface, and it has recently been demonstrated that IL-18 can synergize with IL-23 to promote IL-17 production by γδ T cells [29]. It appears that the activation
of the inflammasome in DCs and macrophages, and the consequent processing of the cytokines IL-1β and IL-18, from an inactive precursor to an active form as a result RVX-208 of inflammasome-triggered pathways, is important for the generation of IL-17-secreting γδ T cells [29]. A defect in the response of IL-17+ γδ T cells, but not IFN-γ+ γδ T cells, to malaria infection has been reported in MyD88-deficient mice [34]. This provides further evidence that activation of TLR (and hence MyD88) signaling and the consequent production of inflammatory cytokines, such as IL-1 (that also signals via MyD88), IL-23, and IL-6, are important steps in driving IL-17 production from γδ T cells. CCR6, the chemokine receptor for CCL20, has been shown to be associated with IL-17+ RORγt+ CD4+ T cells and has also been shown to be present on IL-17+ γδ T cells [30]. IL-2, which has been shown to constrain Th17-cell differentiation [35], appears to have a role in inducing IL-17 production from γδ T cells.