The influence of cyclosporin A on the alternative pathways of human T cell activation in vitro

To gain further insight into the mechanism of action of the immunosuppressant cyclosporin A (CyA), we investigated the influence of CyA on proliferative responses of human T lymphocytes, induced via different membrane molecules. As was previously shown, activation of T cells via the T cell receptor (Ti)/CD3 complex with an anti‐CD3 monoclonal antibody was inhibited by CyA. Likewise, triggering of T lymphocytes via the alternative, CD2(T11)‐mediated pathway of activation was strongly inhibited. In contrast, responses induced by phorbol myristate 13‐acetate (PMA; 100 ng/ml) or the combination of an anti‐CD28 monoclonal antibody and a suboptimal concentration of PMA (1 ng/ml) were found to be insensitive to CyA. CyA‐induced inhibition of both anti‐CD3 and anti‐CD2‐mediated proliferation could not be reversed by addition of either PMA (1 ng/ml) or anti‐CD28. An increase in the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) is an early event observed after stimulation of T cells via CD3 or CD2, whereas stimulation with PMA and anti‐CD28 does not lead to a rise in [Ca²⁺]_i This suggests that the inhibitory action of CyA is related to Ca²⁺‐dependent signaling pathways. Since we observed that CyA does not interfere with anti‐CD3‐ or anti‐CD2‐induced increases of [Ca²⁺]_i, our data suggest that CyA‐mediated inhibition is related to a later event in these intracellular signaling pathways.