Effects of JAK3 inhibition with CP-690,550 on immune cell populations and their functions in nonhuman primate recipients of kidney allografts

Background. Janus Kinase (JAK) 3 is a tyrosine kinase essential for proper signal transduction downstream of selected cytokine receptors and for robust T-cell and natural killer cells activation and function. JAK3 inhibition with CP-690,550 prevents acute allograft rejection. To provide further insight into the mechanisms of efficacy, we investigated the immunomodulatory effects of CP-690,550 in vitro and in vivo in nonhuman primates. Methods. Pharmacodynamic assessments of lymphocyte activation, function, proliferation and phenotype were performed in three settings: in vitro in whole blood isolated from untransplanted cynomolgus monkeys (cynos), in vivo in blood from untransplanted cynos dosed with CP-690,550 for 8 days, and in vivo in blood from transplanted cynos immunosuppressed with CP-690,550. Cell surface activation markers expression, IL-2- enhanced IFN-γ production, lymphocyte proliferation and immune cell phenotype analyzes were performed with multiparametric flow cytometry. Results. In vitro exposure to CP-690,550 resulted in significant reduction of IL-2-enhanced IFN-γ production by T-cells (maximum inhibition of 55-63%), T-cell surface expression of CD25 (50% inhibitory concentration (IC₅₀); 0.18 μM) and CD71 (IC₅₀; 1.6 μM), and T-cell proliferative capacities measured by proliferating cell nuclear antigen expression (IC₅₀; 0.87 μM). Similar results were observed in animals dosed with CP-690,550. In addition, transplanted animals displayed significant reduction of NK cell (90% from baseline) and T-cell numbers whereas CD8⁺ effector memory T-cell populations were unaffected. Conclusions. Potent in vitro and in vivo immunomodulatory effects of the JAK3 inhibitor CP-690,550 likely contribute to its efficacy in the prevention of organ allograft rejection.