ACTIVATION OF NUCLEAR FACTOR OF ACTIVATED T-CELLS IN A CYCLOSPORINE A-RESISTANT PATHWAY

The mechanism of action of the immunosuppressive drug cyclosporin A (C sA) is the inactivation of the Ca2+/calmodulin-dependent serine-threon ine phosphatase calcineurin by the drug-immunophilin complex, Inactive calcineurin is unable to activate the nuclear factor of activated T c ells (NFAT), a transcription factor required for expression of the int erleukin 2 (IL-2) gene. IL-2 production by CsA-treated cells is theref ore dramatically reduced, We demonstrate here, however, that NFAT can be activated, and significant levels of IL-2 can be produced by the Cs A-resistant CD28-signaling pathway. In transient transfection assays, both multicopy NFAT- and IL-2 promoter-beta-galactosidase reporter gen e constructs could be activated by phorbol 12-myristate 13-acetate (PM A)/alpha CD28 stimulation, and this activation was resistant to CsA. E lectrophoretic mobility shift assay showed the induction of a CsA-resi stant NFAT complex in the nuclear extracts of peripheral blood T cells stimulated with PMA plus alpha CD28. Peripheral blood T cells stimula ted with PMA/alpha CD28 produced IL-2 in the presence of CsA. Collecti vely, these data suggest that NFAT can be activated and IL-2 can be pr oduced in a calcineurin independent manner.