Interleukin-17A (IL-17A) has been proven to participate in the process of various autoimmune diseases. The elevation of plasma homocysteine (Hcy), known as hyperhomocysteinemia (HHcy), is related to various chronic inflammatory diseases. Though HHcy-induced upregulation of IL-17A expression in T lymphocytes has been examined, the way in which IL-17A is regulated remains unclear. In this study, western blotting assays showed that Hcy (100 mu M) upregulated NOP2/Sun domain family, member 2 (NSun2) expression in rat T lymphocytes. HHcy-induced upregulation of IL-17A observed in plasma of wild-type rats was markedly decreased in NSun2(-/-) rats in vivo. Mechanistically, by using in vitro methylation assays and high-performance liquid chromatography-mass spectrum (HPLC-MS) analysis, we showed that the tRNA methyltransferase NSun2 methylated the IL-17A mRNA in an m5C pattern. The results from bisulfite sequencing indicated that NSun2 methylated IL-17A mRNA at cytosine C466 in vitro and in vivo. Furthermore, we analyzed the activity of pGL3-derived reporters bearing IL-17A mRNA fragments and found that methylation by NSun2 promoted the translation of IL-17A. In conclusion, NSun2 mediates HHcy-induced upregulation of IL-17A expression by methylating IL-17A mRNA and promoting its translation in T lymphocytes. (c) 2017 Elsevier Inc. All rights reserved.