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 t ransmembrane efflux pump. While the physiologic significance of P-gp e xpression in leukocytes is unknown, the relatively high levels of P-gp in CD56(+) and CD8(+) cells suggest that P-gp may function in cell-me diated cytolysis. To explore this possibility we examined the effect o f four inhibitors of P-gp efflux [(R)-verapamil (R-ver), (S)-verapamil (S-ver), cyclosporine A (CsA), and PSC833 (PSC)] on both the inhibiti on of natural killer cell (NK) function and on P-gp efflux. NK functio n was assayed by measuring the lysis of Cr-51-labeled K562 target cell s in the presence and absence of inhibitors. All four P-gp efflux inhi bitors inhibited NK-mediated cytolysis in a dose-dependent manner. The stereoisomers of verapamil were more potent inhibitors of cell-mediat ed 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 inhibite d P-gp efflux at 3 mu M, but only minimally inhibited cell-mediated cy tolysis. The verapamil compounds demonstrated closer correlation betwe en efflux inhibition and inhibition of NK-mediated cytolysis. The data support a role for P-gp in NK-mediated cytolysis; however, these stud ies also suggest that the NK cytolytic process is multifaceted and tha t inhibition of the P-gp-mediated efflux mechanism only partially abro gates this process.