This study investigated the effects of the novel Janus kinase (JAK) inhibitor CP-690,550 (now known as tofacitinib) on adaptive and innate immune responses, in order to establish the mode of action of JAK inhibitors in the setting of inflammatory diseases. The inhibition of specific JAK/STAT-dependent pathways by CP-690,550 was determined through analysis of cytokine stimulation of mouse and human T cells in vitro.The effects of CP-690,550 on Th-cell differentiation of naive murine CD4+...