Objective Tofacitinib, which is a Janus kinase (JAK) inhibitor, has shown clinical effects in the treatment of rheumatoid arthritis. JAKs are important kinases in lymphocyte differentiation; however, their function in dendritic cells (DCs) is unknown. In this study, the function of JAKs in DCs was investigated with tofacitinib. Methods The effects of tofacitinib on the maturation of human monocyte-derived DCs induced by lipopolysaccharide (LPS) stimulation were investigated. In addition, its effects on T cell stimulatory capability was investigated by coculturing with naïve CD45RA-positive T cells. Results Tofacitinib decreased expression of CD80/CD86 in a concentration-dependent manner in LPS-stimulated DCs; however, it did not affect HLA-DR expression. Tofacitinib suppressed tumour necrosis factor, interleukin (IL)-6 and IL-1β production without affecting transforming growth factor (TGF)-β and IL-10 production. Meanwhile, CD80/CD86 expression in DCs was enhanced by type I interferon (IFN) stimulation, and the LPS-induced CD80/CD86 expression was inhibited by an antibody to type I IFN receptor. Furthermore, tofacitinib suppressed production of type I IFN and activation of interferon regulatory factor (IRF)-7, which is a transcription factor involved in CD80/CD86 and type I IFN expression. Tofacitinib also decreased the T cell stimulatory capability of DCs and increased expression of indoleamine 2,3-dioxygenase (IDO)-1 and IDO-2. Conclusions Tofacitinib, a JAK1/JAK3 inhibitor, affected the activities of human DCs. It decreased CD80/CD86 expression and T cell stimulatory capability through suppression of type I IFN signalling. These results suggest a novel mode of action for tofacitinib and a pivotal role for JAKs in the differentiation of DCs.
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