IL-17–producing CD4⁺T helper (Th17) cells have recently been defined as a unique subset of proinflammatory helper cells whose development depends on signaling initiated by IL-6 and TGF-β, autocrine activity of IL-21, activation of STAT3, and induction of the orphan nuclear receptor RORγt. The maintenance, expansion, and further differentiation of the committed Th17 cells depend on IL-1β and IL-23. IL-17 was originally found produced by circulating human CD45RO⁺memory T cells. A recent study found that human Th17 memory cells selectively express high levels of CCR6. In this study, we report that human peripheral blood and lymphoid tissue contain a significant number of CD4⁺FOXP3⁺T cells that express CCR6 and have the capacity to produce IL-17 upon activation. These cells coexpress FOXP3 and RORγt transcription factors. The CD4⁺FOXP3⁺CCR6⁺IL-17–producing cells strongly inhibit the proliferation of CD4⁺responder T cells. CD4⁺CD25^high-derived T-cell clones express FOXP3, RORγt, and IL-17 and maintain their suppressive function via a cell-cell contact mechanism. We further show that human CD4⁺FOXP3⁺CCR6⁻regulatory T (Treg) cells differentiate into IL-17 producer cells upon T-cell receptor stimulation in the presence of IL-1β, IL-2, IL-21, IL-23, and human serum. This, together with the finding that human thymus does not contain IL-17–producing Treg cells, suggests that the IL-17⁺FOXP3⁺Treg cells are generated in the periphery. IL-17–producing Treg cells may play critical roles in antimicrobial defense, while controlling autoimmunity and inflammation.

The nuclear receptors RORα and RORγt (retinoic acid receptor-related orphan receptors α and γt) are essential for the development of TH17 cells, the T-helper cells that produce interleukin-17. Two groups report the identification of RORγt inhibitors, compounds that could have potential in the treatment of autoimmune diseases. Huh et al. used a chemical screen in an insect-cell-based reporter system to identify the cardiac glycoside digoxin and various derivatives as inhibitors of the transcriptional activity of RORγt. Through this mechanism, these compounds block the differentiation of TH17 cells in mice, and inhibit interleukin-17 production in vitro in human T cells. Solt et al. describe a synthetic ligand, named SR1001, that functions as an inverse agonist for RORα and RORγt, and show that it blocks TH17 development in vitro and inhibits experimental encephalomyelitis in mice. CD4+ T helper lymphocytes that express interleukin-17 (TH17 cells) have critical roles in mouse models of autoimmunity, and there is mounting evidence that they also influence inflammatory processes in humans. Genome-wide association studies in humans have linked genes involved in TH17 cell differentiation and function with susceptibility to Crohn’s disease, rheumatoid arthritis and psoriasis1,2,3. Thus, the pathway towards differentiation of TH17 cells and, perhaps, of related innate lymphoid cells with similar effector functions4,5, is an attractive target for therapeutic applications. Mouse and human TH17 cells are distinguished by expression of the retinoic acid receptor-related orphan nuclear receptor RORγt, which is required for induction of IL-17 transcription and for the manifestation of TH17-dependent autoimmune disease in mice6. By performing a chemical screen with an insect cell-based reporter system, we identified the cardiac glycoside digoxin as a specific inhibitor of RORγt transcriptional activity. Digoxin inhibited murine TH17 cell differentiation without affecting differentiation of other T cell lineages and was effective in delaying the onset and reducing the severity of autoimmune disease in mice. At high concentrations, digoxin is toxic for human cells, but non-toxic synthetic derivatives 20,22-dihydrodigoxin-21,23-diol and digoxin-21-salicylidene specifically inhibited induction of IL-17 in human CD4+ T cells. Using these small-molecule compounds, we demonstrate that RORγt is important for the maintenance of IL-17 expression in mouse and human effector T cells. These data indicate that derivatives of digoxin can be used as chemical templates for the development of RORγt-targeted therapeutic agents that attenuate inflammatory lymphocyte function and autoimmune disease.

Dietary vitamin A during pregnancy is required for the formation of secondary lymphoid organs of the developing embryo and affects the offspring’s immune competence in adulthood. This comparison of pregnant mice on diets completely lacking vitamin A, and with low and high vitamin A content, reveals a role for maternal vitamin A metabolites — notably retinoic acid — in establishing innate immunity in the developing fetus. The vitamin is required for the formation of secondary lymphoid organs during embryonic development and affects the offspring's immune competence during adulthood. The impact of nutritional status during fetal life on the overall health of adults has been recognized1; however, dietary effects on the developing immune system are largely unknown. Development of secondary lymphoid organs occurs during embryogenesis and is considered to be developmentally programmed2,3. Secondary lymphoid organ formation depends on a subset of type 3 innate lymphoid cells (ILC3) named lymphoid tissue inducer (LTi) cells2,3,4,5. Here we show that mouse fetal ILC3s are controlled by cell-autonomous retinoic acid (RA) signalling in utero, which pre-sets the immune fitness in adulthood. We found that embryonic lymphoid organs contain ILC progenitors that differentiate locally into mature LTi cells. Local LTi cell differentiation was controlled by maternal retinoid intake and fetal RA signalling acting in a haematopoietic cell-autonomous manner. RA controlled LTi cell maturation upstream of the transcription factor RORγt. Accordingly, enforced expression of Rorgt restored maturation of LTi cells with impaired RA signalling, whereas RA receptors directly regulated the Rorgt locus. Finally, we established that maternal levels of dietary retinoids control the size of secondary lymphoid organs and the efficiency of immune responses in the adult offspring. Our results reveal a molecular link between maternal nutrients and the formation of immune structures required for resistance to infection in the offspring.

Dendritic cells (DC) play a crucial role in initiating immune responses to tumors. DC can efficiently present antigens from apoptotic tumor cells, but apoptotic cells are thought to lack the inflammatory signals required to induce DC maturation. Here, we show that apoptosis of 67NR mouse carcinoma cells via the Fas (CD95) pathway or induced by the anticancer drug bortezomib (PS-341) but not by ultraviolet irradiation is associated with the production of maturation signals for DC. These data have important implications for the effects of chemotherapy on antitumor immunity in solid and hematologic malignancies.