Novel role for the T-helper 17 signature cytokine IL-17: CD4 resistance to immune suppression

Important immune regulatory mechanisms mediated by CD4+ and CD8+ T-cells keep untoward CD4+ T-cell responses in check. CD4+ T-helper 17 (Th17) cells, characterized by IL-17 production, play critical roles in the body’s response to infections and cancer and in the pathogenesis of autoimmune diseases such as multiple sclerosis, psoriasis, arthritis, IBD, among others. Here we demonstrate that human CD4+ T-cells cells exposed to a Th17-differentiating milieu are highly resistant to immune suppression by CD8+ T-cells, compared to control Th0 cells. This resistance is mediated, in part, through the action of IL-17A, IL-17F and IL-17AF heterodimer through their receptors (IL-17RA and IL-17RC) on CD4+ T-cells themselves, but not through their action on CD8+ T-cells or APC. We further show that IL-17 can directly act on non-Th17 effector CD4+ T-cells to induce suppressive resistance and this resistance can be reversed by blockade of IL-1, IL-6 or STAT3. These studies reveal a novel function for IL-17 cytokines in a CD4-intrinsic mechanism of immune resistance. The pathways induced in this process may serve as a critical target for intensive investigation and therapeutic intervention. Ex vivo-purified bulk CD4+CD25- T-cells isolated from 3 healthy donors and sequenced after in vitro for 48 hours in the presence of media alone (control), or 10 ng/ml of IL-17A, IL-17F or IL-17AF