Low dose CP-690,550 (tofacitinib), a pan-JAK inhibitor, accelerates the onset of experimental autoimmune encephalomyelitis by potentiating Th17 differentiation.

Th17 cells, which have been implicated in autoimmune diseases, require STAT3 signaling activated by IL-6 or IL-23 for their development. Other Th1 and Th2 cytokines such as IL-2, IFN-γ and IL-4 strongly suppress Th17 development. Recently, CP-690,550 (tofacitinib), originally developed as a JAK3 inhibitor, has been shown to be effective in phase III clinical trials of rheumatoid arthritis and collagen-induced arthritis (CIA) models, but the precise mechanism of the effect, especially with respect to Th17 cells, is poorly understood. To our surprise, a low dose CP-690,550 was found to accelerate the onset of experimental autoimmune encephalomyelitis (EAE) at a concentration that suppressed CIA. At an early stage after immunization, more IL-17 production was observed in 15mg/kg body weight CP-690,550-treated mice than in untreated mice. In vitro, CP-690,550 inhibited both Th1 and Th2 development, while promoting Th17 differentiation at 10-50nM concentrations. Enhancement of Th17 by CP-690,550 is probably due to suppression of IL-2 signaling, because anti-IL-2 antibodies cancel the Th17-promoting effect of CP-690,550. CP-690,550 selectively inhibited IFN--induced STAT1, IL-4-induced STAT6 and IL-2-induced STAT5 at 3-30nM, while suppression of IL-6-induced STAT3 phosphorylation required a concentration greater than 100nM. In HEK293T cells, CP-690,550 less effectively suppressed JAK1-mediated STAT3 phosphorylation compared with JAK3. These results suggest that CP-690,550 has a different effects among JAKs and STATs, thereby affecting helper T cell differentiation, and murine autoimmune disease models.