Protein kinase activation and the immunosuppressant cyclosporine

Cyclosporine (CsA), a potent immunosuppressant for the prevention of transplant rejection, modulates T lymphocyte activation by blocking antigen stimulation and the production of interleukin-2. The mode of action by which CsA generates this immunosuppressive effect is unknown. We have studied two early intracellular enzymes associated with mitogen activation. They include calcium/phospholipid-dependent protein kinase (C-kinase) and cAMP-dependent protein kinase (cAMPd PK). Changes in protein kinase activation were correlated with the immunosuppression of polyamine and DNA synthesis measured by ornithine decarboxylase (ODC) induction and 3H-thymidine incorporation, respectively. These studies utilized murine T cell tumor lines sensitive to the effects of CsA. Similar to the mitogen activation of human peripheral blood lymphocytes, CsA was capable of inhibiting the induction of ODC and 3H-thymidine uptake of T cell tumor lines cultured with either fresh serum or mitogen. In contrast, C-kinase and cAMPd PK activation stimulated by the addition of fresh serum was not affected by CsA. Further, CsA did not inhibit the direct activation of C-kinase with phorbol esters or the activation of cAMPd PK with exogenous cAMP. We conclude that CsA does not affect the activation of either C-kinase or cAMPd PK in T cell tumor lines when activated by either fresh serum or when stimulated with chemical agents. The suppression of ODC induction and 3H-thymidine incorporation associated with CsA treatment cannot be accounted for with changes in C-kinase and cAMPd PK activation.