A new class of inflammatory CD4؉ T cells that produce interleukin-17 (IL-17) (termed Th17) has been identified, which plays a critical role in numerous inflammatory conditions and autoimmune diseases. Interleukin-17A (IL-17A)-producing T cells are a subset of CD4 ϩ T cell lineage, termed Th17, distinct from Th1, Th2, and T regulatory (T reg ) subsets (52). IL-17 is involved in the pathogenesis of autoimmune inflammation and has been implicated in numerous autoimmune diseases, including systemic lupus erythematosus, rheumatoid arthritis, and multiple sclerosis (MS) (10,21,26,41). IL-17 mRNA and protein levels in patients with MS have been shown to be increased in mononuclear cells isolated from blood, in cerebrospinal fluid, and in brain lesions (39,41). IL-17 is also increased in lymphocytes derived from mice with experimental autoimmune encephalomyelitis (EAE; mouse model for multiple sclerosis) (33). In IL-17A knockout (KO) mice, EAE is markedly suppressed, indicating that IL-17 contributes to the development of EAE (33). Although it has been reported that the transcription factors nuclear factor for activated T cells (NFAT), retinoid orphan nuclear receptor ␥t (ROR␥t), and Runt-related transcription factor 1 (Runx1) are important for the T cell receptor (TCR)-mediated transcriptional regulation of IL-17A (24, 29, 38, 74), knowledge of the factors involved in the cellular and molecular regulation of IL-17A remains limited.The principle function of the active form of vitamin D,, is the maintenance of calcium and phosphate homeostasis (13). However, vitamin D has numerous other functions, including downregulation of autoimmunity (7,8,25,55). 1,25(OH) 2 D 3 has been reported to at least partially protect against a number of experimental autoimmune diseases, including EAE (7,8,11,35,40,53). In addition, numerous epidemiological studies have indicated a negative correlation between increased sun exposure, which would result in a higher vitamin D synthetic rate, and diets rich in vitamin D and MS prevalence (34,55,70
