Mechanisms of human cell-mediated cytotoxicity. III. Dependence of natural killing on microtubule and microfilament integrity.

To date, the mechanisms of human natural killer (NK) cell activity have been poorly defined. Since microtubules and microfilaments are important in many non-NK leukocytic processes, the potential roles of these cellular elements in natural killing were assessed in simultaneously performed 51-chromium-release microcytotoxicity and single cell cytotoxicity assays. The microtubule inhibitors colchicine, vincristine, and vinblastine suppressed chromium-release in a concentration-dependent fashion. This suppression could be prevented by the promoters of microtubule assembly, deuterium oxide and cyclic GMP. Microtubule inhibitors did not affect the binding of natural killer cells to target cells but did suppress the lysis of bound targets. The "recycling" of natural killer cells to repeat the lytic sequence was not altered by microtubule-disrupting agents. Cytochalasin B, a microfilament inhibitor, induced dose-dependent suppression of lytic activity in the chromium-release assay. Concentrations of cytochalasin B greater than 2 micrograms/ml prevented the binding of effector cells to target cells. Concentrations of cytochalasin B less than 2 micrograms/ml depressed killing in the chromium-release assay but did not alter target cell binding or the lysis of bound targets indicative of an inhibition of effector cell recycling. Concentrations of cytochalasin B greater than 2 micrograms/ml did not affect the lysis of bound targets once binding had occurred. The roles of microtubules and microfilaments in the lytic sequence can therefore be subdivided based on these data: 1) the binding of natural killer cells to target cells in dependent on microfilaments; 2) the lysis of bound targets requires microtubule assembly; and 3) the subsequent post-lytic recycling of natural killer cells necessitates a intact microfilament system.