Concomitant inhibition of janus kinase 3 and calcineurin-dependent signaling pathways synergistically prolongs the survival of rat heart allografts

The cytoplasmic localized Janus tyrosine kinase 3 (Jak3) is activated by mu ltiple cytokines, including IL-2, IL-4, and IL-7, through engagement of the IL-2R common gamma -chain, Genetic inactivation of Jak3 is manifested as S CID in humans and mire. These findings have suggested that Jak3 represents a pharmacological target to control certain lymphoid-derived diseases. Usin g the rat T cell line Nb2-11c, we document that tyrphostin AG-490 blocked i n vitro IL-2-induced cell proliferation (IC50 similar to 20 muM), Jak3 auto phosphorylation, and activation of its key substrates, Stat5a and Stat5b, a s measured by tyrosine/serine phosphorylation analysis and DNA-binding expe riments. To test the notion that inhibition of Jak3 provides immunosuppress ive potential, a 7-day course of i.v. therapy with 5-20 mg/kg AG-490 was us ed to inhibit rejection of heterotopically transplanted Lewis (RTI1) heart allografts in ACI (RT1(a)) recipients. In this study, we report that AG-490 significantly prolonged allograft survival, but also acted synergistically when used in combination with the signal 1 inhibitor cyclosporin A, but no t the signal 3 inhibitor, rapamycin, Finally, AG-490 treatment reduced graf t infiltration of mononuclear cells and Stat5a/b DNA binding of es vivo IL- 2-stimulated graft infiltrating of mononuclear cells, but failed to affect IL2R alpha expression, as judged by RNase protection assays. Thus, inhibiti on of Jak3 prolongs allograft survival and also potentiates the immunosuppr essive effects of cyclosporin A, but not rapamycin.