CALCINEURIN MUTANTS RENDER T-LYMPHOCYTES RESISTANT TO CYCLOSPORINE-A

The immunosuppressants cyclosporin A (CsA) and FK506 have been widely used to prevent and treat graft rejection after human organ and tissue transplantations. CsA and FK506 associate with intracellular binding proteins (i.e., CsA with cyclophilin A and FK506 with FKBP12) to form protein/drug complexes that suppress the immune system by preventing a ctivation of T cells in response to antigen presentation. The common t arget of CsA and FK506 is calcineurin, a Ca2+/calmodulin-regulated, se rine/threonine-specific protein phosphatase that regulates the nuclear import of a transcription factor, NF-AT, required for expression of T cell activation genes. In previous studies, we identified calcineurin mutations that block binding by the cyclophilin A/CsA or FKBP12/FK506 complexes and thereby render yeast cells resistant to the antifungal effects of CsA or FK506. in this report, we demonstrate that the corre sponding mutations in murine calcineurin render the T cell receptor si gnal transduction cascade CsA resistant in human Jurkat T cells. Our f indings support the recently determined calcineurin X-ray crystal stru cture, provide evidence that calcineurin is the only CsA-sensitive com ponent limiting signaling from the T cell receptor to the nucleus, and suggest a means to render cells and tissues resistant to the toxic si de effects of CsA and FK506.