Inhibition of cytotoxic T lymphocyte and natural killer cell-mediated lysis by O,S,S-trimethyl phosphorodithioate is at an early postrecognition step

O,S,S-Trimethyl phosphorodithioate (OSS-TMP), an organophosphate esterase inhibitor, has been shown to block the effector phase of the cytolytic reaction mediated by murine and human cytotoxic T lymphocytes (CTL) and human natural killer cells. The murine interleukin 2-dependent CTLL-1 (anti-Ia(d)) clone was used to determine the phase of the cytolytic pathway inhibited by OSS-TMP. Pretreatment of the CTL or target cell with OSS-TMP was not effective at blocking lysis; however, inhibition of lysis was achieved if the reaction was carried out in the continuous presence of OSS-TMP (IC₅₀ = 55 μM) or when CTL-target conjugates were performed and incubated with OSS-TMP (IC₅₀ = 640 μM). Two structural analogues of OSS-TMP were unable to inhibit CTL-mediated lysis. In contrast to OSS-TMP, N-α-p-tosyl-L-lysine chloromethylketone required only a 5-min preincubation with the CTL to inhibit lysis. OSS-TMP did not block recognition-adhesion step(s) of the reaction since the ability to form conjugates was not impaired; however, the lytic efficiency of individual CTL-target pairs were blocked. OSS-TMP did not appear to be an inhibitor of the major granule-associated protease that cleaves the substrate, N-α-benzyloxycarbonyl-L-lysine thiobenzylester. Ca²⁺ pulse and kinetic experiments indicated that the OSS-TMP-sensitive site was at a pre-Ca²⁺-dependent phase but after recognition-adhesion. Human CTL and natural killer cell activity was also inhibited by OSS-TMP, suggesting the presence of a common site of action among these cytolytic systems. The results indicate that OSS-TMP may be a useful reagent in characterizing the early post-recognition events in the cytolytic pathway of CTL and natural killer effector cells.