Cyclosporin-A and the OKT3 monoclonal antibody exert their respective inhibitory and mitogenic activity on T cells by interacting with the same receptors.

Cyclosporin-A strongly suppressed proliferation of T cells induced by the OKT3 monoclonal antibody when added at the beginning of the cultures but not when added 72 hr later. The inhibitory activity of Cyclosporin-A became apparent during the first 48 hr and was maintained throughout the culture period. Cyclosporin-A significantly inhibited binding of OKT3, but not OKT4 or OKT8, antibodies to T cells as determined by indirect immunofluorescence microscopy. In addition, Cyclosporin-A suppressed the killing of 51Cr labelled T cells mediated by OKT3 antibody plus complement, whereas Cyclosporin-A did not alter the lysis of T cells by OKT4 antibody plus complement treatment. These results strongly suggest that Cyclosporin-A and OKT3 antibody exert their respective suppressive and mitogenic activity on T cells by interacting with the same receptor.