Digoxin and its derivatives suppress TH17 cell differentiation by antagonizing RORγt activity

Abstract

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.