Requirements for triggering of lysis by cytolytic T lymphocyte clones.

Cloned murine cytolytic T lymphocytes (CTL) having defined specificity were triggered by the phorbol ester together with a calcium ionophore (either A23187 or Ionomycin) to lyse syngeneic or third party target cells efficiently. Neither phorbol 12-myristate 13-acetate (PMA) nor calcium ionophore alone induced efficient lysis. The characteristics of the lytic process induced by these signals are similar to those of antigen-specific or lectin-facilitated lysis by CTL. Lysis is calcium and temperature dependent and shows kinetics which are not grossly different from lysis mediated via the antigen receptor. Two helper T lymphocyte clones were not induced to lyse efficiently EL-4 target cells by concanavalin A or PMA + ionophore. Triggering of lysis induced with PMA plus ionophore by the CTL clone L3 differed from antigen-mediated lysis in specificity and in the susceptibility to inhibition by cytochalasin B. Properties of the target cell determine which cell surface associative recognition structures are important in the efficient lysis of these cells. Anti-LFA-1 monoclonal antibodies inhibited efficiently both antigen-mediated and PMA + ionophore-induced lysis of P-815 or EL-4 target cells which are of hematopoietic origin. However, anti-LFA-1 antibodies do not inhibit antigen-mediated, lectin-facilitated, or PMA + Ionomycin-induced CTL cytolysis of target cells derived from the L cell fibroblast line. We conclude that two intracellular signals, which can be provided by the combination of PMA + ionophore, are required for efficient lysis by antigen-specific murine CTL clones. When the T cell receptor for antigen is bypassed using PMA + ionophore to trigger lysis, we show that Lyt-2 and LFA-1 molecules may be required for efficient lysis. These associative recognition structures appear to play an important role in postactivation steps leading to efficient delivery of the lethal hit to the target cell.