Janus kinase 3 (JAK-3) is a tyrosine kinase that has been shown to participate in the signaling of several cytokines that are believed to play a role in allergic airway disease, e.g. IL-2, 4 and 9. The current study describes the immunosuppressive effects of CP-690550, a novel, small molecule inhibitor of JAK-3, in a murine model of allergic pulmonary inflammation. In vitro, CP-690550 potently inhibited IL-4 induced upregulation of CD23 (IC(50)=57 nM) and class II major histocompatibility complex (MHCII) expression (IC(50)=71 nM) on murine B cells. Repeat aerosol exposure to ovalbumin in wild-type mice sensitized to the antigen resulted in preferential recruitment of Th2-like cells (IL-4+ and IL-5+) into bronchoalveolar lavage fluid (BAL). The importance of IL-4 in the development of pulmonary eosinophilia was supported by a marked (90%) reduction in the influx of these cells in IL-4KO mice similarly sensitized and ovalbumin exposed. Animals dosed with CP-690550 (15 mg/kg/d) during the period of antigen sensitization and boost demonstrated marked reductions in BAL eosinophils and levels of IL-13 and eotaxin following ovalbumin aerosol exposure. The JAK-3 inhibitor (1.5-15 mg/kg/d) also effectively reduced the same parameters when administered during the period of antigen challenge. In contrast, the calcineurin inhibitor tacrolimus (10 mg/kg) was effective only when administered during the period of ovalbumin aerosol exposure. These data support the participation of JAK-3 in processes that contribute to pulmonary eosinophilia in the allergic mouse model. CP-690550 represents an intriguing novel therapy for treatment of allergic conditions associated with airway eosinophilia including asthma and rhinitis.