Human Th17 Cells Comprise Heterogeneous Subsets Including IFN-γ–Producing Cells with Distinct Properties from the Th1 Lineage

Th17 cells have been named after their signature cytokine IL-17 and accumulating evidence indicates their involvement in the induction and progression of inflammatory diseases. In addition to IL-17 single-producing T cells, IL-17/IFN-γ double-positive T cells are found in significantly elevated numbers in inflamed tissues or blood from patients with chronic inflammatory disorders. Because IFN-γ is the classical Th1-associated cytokine, the origin and roles of these subsets remain elusive. In this paper, we show that not only IL-17+/IFN-γ+ but also IFN-γ+ (IL-17−) cells arise under Th17-inducing condition and have distinct properties from the Th1 lineage. In fact, these populations displayed characteristics reminiscent to IL-17 single-producing cells, including production of IL-22, CCL20, and induction of antimicrobial gene expression from epithelial cells. Live sorted IL-17+ and Th17–IFN-γ+ cells retained expression of IL-17 or IFN-γ after culture, respectively, whereas the IL-17+/IFN-γ+ population was less stable and could also become IL-17 or IFN-γ single-producing cells. Interestingly, these Th17 subsets became “Th1-like” cells in the presence of IL-12. These results provide novel insights into the relationship and functionality of the Th17 and Th1 subsets and have direct implications for the analysis and relevance of IL-17 and/or IFN-γ–producing T cells present in patients’ peripheral blood and inflamed tissues.