Molecular mode of action of cyclosporin and FK506 in human thymocytes

The molecular mode of action of cyclosporin, three of its non-immunosuppressive analogs, N-methyl-l-alanyl cyclosporin, acetyl cyclosporin and cyclosporin S3, and of FK506 was studied in primary cultures of human thymocytes. Nuclear factors derived from thymocytes activated with phorbol myristate acetate and concanavalin A were tested for their ability to bind to a synthetic radiolabelled probe corresponding to the NF-AT region (-285 to -255) of the IL-2 gene. Binding was observed, and it was inhibited by CsA (100 ng/ml), while the analogs at ten-fold higher concentrations (1000 ng/ml) were only partially inhibitory. CsA in combination with FK506 inhibited binding of nuclear factors at the NF-AT site, and acted in concert.