Inhibition of cell-mediated cytolysis and P-glycoprotein function in natural killer cells by verapamil isomers and cyclosporine a analogs

We have previously shown that among normal leukocytes, CD56⁺ and CD8⁺ cells express relatively high levels of P-glycoprotein (P-gp), a transmembrane efflux pump. While the physiologic significance of P-gp expression in leukocytes is unknown, the relatively high levels of P-gp in CD56⁺ and CD8⁺ cells suggest that P-gp may function in cell-mediated cytolysis. To explore this possibility we examined the effect of four inhibitors of P-gp efflux [(R)-verapamil (R-ver), (S) -verapamil (S-ver), cyclosporine A (CsA), and PSC833 (PSC)] on both the inhibition of natural killer cell (NK) function and on P-gp efflux. NK function was assayed by measuring the lysis of⁵¹Cr-labeled K562 target cells in the presence and absence of inhibitors. All four P-gp efflux inhibitors inhibited NK-mediated cytolysis in a dose-dependent manner. The stereoisomers of verapamil were more potent inhibitors of cell-mediated cytolysis than the cyclosporines CsA and PSC. In contrast, CsA and PSC were more potent as inhibitors of P-gp-mediated rhodamine 123 dye efflux than the verapamil isomers. Both CsA and PSC maximally inhibited P-gp efflux at 3 μM, but only minimally inhibited cell-mediated cytolysis. The verapamil compounds demonstrated closer correlation between efflux inhibition and inhibition of NK-mediated cytolysis. The data support a role for P-gp in NK-mediated cytolysis; however, these studies also suggest that the NK cytolytic process is multifaceted and that inhibition of the P-gp-mediated efflux mechanism only partially abrogates this process.