Naïve T cells undergo robust proliferation in lymphopenic conditions, while they remain quiescent in steady-state conditions. However, a mechanism by which naïve T cells are kept from proliferating under steady-state conditions remains unclear. Here we report that memory CD4 T cells are able to limit naïve T cell proliferation within lymphopenic hosts by modulating stimulatory functions of DC. The inhibition was mediated by IL-27, which was primarily expressed in CD8+ DC subsets as the result of memory CD4 T cell-DC interaction. IL-27 appeared to be the major mediator of inhibition as naïve T cells deficient in IL-27R were resistant to memory CD4 T cell mediated inhibition. Finally, IL-27-mediated regulation of T cell proliferation was also observed in steady-state conditions as well as during Ag-mediated immune responses. We propose a new model for maintaining peripheral T cell homeostasis via memory CD4 T cells and CD8+ DC-derived IL-27 in vivo.

Within lymphopenic recipients, naïve T cells undergo proliferation that is induced by homeostatic mechanisms. Earlier studies have demonstrated that commensal antigens play a key role in inducing the proliferation. However, a relative contribution of endogenous self antigens in this process has not been formally investigated. In this study, we utilized a pharmacologic inhibitor that blocks T cell egress from the lymphoid tissues, antibiotics, and germ-free animals to examine the role of commensal and self antigens. The results suggest that T cell proliferation under lymphopenic conditions is a heterogeneous process triggered by both exogenous commensal and endogenous self antigens.

A proportional balance between αβ and γδ T cell subsets in the periphery is exceedingly well maintained via a homeostatic mechanism. However, a cellular mechanism underlying the regulation remains undefined. We recently reported that a subset of developing γδ T cells spontaneously acquire IL-17-producing capacity even within naïve animals via a TGFβ1-dependent mechanism, thus considered ‘einnate’ IL-17-producing cells. Here we report that γδ T cells generated within αβ T cell (or CD4 T cell)-deficient environments displayed altered cytokine profiles; particularly, ‘einnate’ IL-17 expression was significantly impaired compared to those in wild type mice. Impaired IL-17 production in γδ T cells was directly related to the CD4 T cell deficiency, because depletion of CD4 T cells in wild type mice diminished and adoptive CD4 T cell transfer into TCRβ−/− mice restored IL-17 expression in γδ T cells. CD4 T cell-mediated IL-17 expression required TGFβ1. Moreover, Th17 but not Th1 or Th2 effector CD4 T cells were highly efficient in enhancing γδ T cell IL-17 expression. Taken together, our results highlight a novel CD4 T cell-dependent mechanism that shapes the generation of IL-17+ γδ T cells in naïve settings.

Th17 cells have been implicated in the pathogenesis of colitis; however, a cellular mechanism by which colitogenic Th17 immunity arises in vivo remains unclear. In this study, we report that a subset of IL-17+γδ T cells plays a crucial role in enhancing in vivo Th17 differentiation and T cell-mediated colitis. TCRβ-/- mice were highly susceptible to T cell-mediated colitis, while TCRβδ-/- mice were resistant to the disease. Importantly, cotransfer of IL-17+ but not of IL-17-γδ T cells with CD4 T cells was sufficient to enhance Th17 differentiation and induce full-blown colitis in TCRβδ-/- recipients. Collectively, our results provide a novel function of IL-17+γδ T cell subsets in supporting in vivo Th17 differentiation and possibly in fostering the development of intestinal inflammation.

In naïve animals, γδ T cells are innate sources of IL-17, a potent proinflammatory cytokine mediating bacterial clearance as well as autoimmunity. However, mechanisms underlying the generation of these cells in vivo remain unclear. Here we show that TGFβ1 plays a key role in the generation of IL-17+ γδ T cells, and that it mainly occurs in the thymus particularly during the postnatal period. Interestingly, IL-17+ γδ TCR+ thymocytes were mainly CD44highCD25low cells, which seem to derive from DN4 γδ TCR+ cells that acquired CD44 and IL-17 expression. Our findings identify a novel developmental pathway during which IL-17-competent γδ T cells arise in the thymus by a TGFβ1-dependent mechanism.