Citation

Pro-inflammatory interleukin (IL)-17-producing (17) T cells are thought to develop exclusively in the thymus during fetal/perinatal life, as adult bone marrow precursors fail to generate 17 T cells under homeostatic conditions. Here, we employ a model of experimental autoimmune encephalomyelitis (EAE) in which hematopoiesis is reset by bone marrow transplantation and demonstrate unequivocally that V4(+) 17 T cells can develop de novo in draining lymph nodes in response to innate stimuli. In vitro, T cells from IL-17 fate-mapping reporter mice that had never activated the Il17 locus acquire IL-17 expression upon stimulation with IL-1 and IL-23. Furthermore, IL-23R (but not IL-1R1) deficiency severely compromises the induction of 17 T cells in EAE, demonstrating the key role of IL-23 in the process. Finally, we show, in a composite model involving transfers of both adult bone marrow and neonatal thymocytes, that induced 17 T cells make up a substantial fraction of the total IL-17-producing V4(+) T-cell pool upon inflammation, which attests the relevance of this novel pathway of peripheral 17 T-cell differentiation.