JAK-3 has been shown to play a key role in cytokine signaling via gammac, e.g. IL-2, 4, 7, 9, 15, 21. The current study describes the immunosuppressive effects of CP-690550, a novel, small molecule inhibitor of JAK-3, in various murine models. In vitro, CP-690550 effectively inhibited a murine mixed lymphocyte reaction (MLR) (IC50= 91 nm). Mice chronically dosed with CP-690550 (1.5-15 mg/kg/day) demonstrated dose- and time-dependent alterations in lymphocyte subsets when examined by flow cytometry. The most dramatic change observed was a 96% reduction in splenic NK1.1 + TCRbeta- cell numbers following 21 days of treatment. Delayed-type hypersensitivity (DTH) responses in sensitized mice were reduced in a dose-dependent manner following treatment with the JAK-3 inhibitor (1.87-30 mg/kg, s.c.). Extended survival of neonatal Balb/c hearts implanted into the ear pinna of MHC mismatched C3H/HEN mice was observed with CP-690550 monotherapy (10-30 mg/kg/day), but improved upon combination with cyclosporin (10 mg/kg/day). These data support the participation of JAK-3 in various lymphocyte homeostatic functions in mature mice. Furthermore, the ability of CP-690550 to extend cardiac allograft survival in murine models suggests it may afford a new treatment for prevention of transplant rejection.