Cyclosporin a permits the distinction between specific T and NK activity generated in a human MLC.

Human lymphocytes, activated in mixed lymphocyte culture (MLC), express cytolytic activity against a range of target cell types. Cyclosporin A (CyA) efficiently prevents the development of cytotoxic T cells directed against target cells expressing the phenotype of the stimulator cells whilst not preventing an increase in natural killing (NK) activity measured against tumor targets. The drug was further shown not to influence the short-term induction of increased NK activity by interferon. Already established T killer cells are not sensitive to CyA. The drug thus seems to be selectively involved in the induction of immune responses requiring T-cell proliferation.