EXPRESSION AND FUNCTION OF MULTIDRUG-RESISTANCE P-GLYCOPROTEIN IN A CULTURED NATURAL-KILLER CELL-RICH POPULATION REVEALED BY MRK16 MONOCLONAL-ANTIBODY AND AHC-52

Natural killer (NK) cells have been reported recently to be the highes t in expressing multidrug resistance (MDR) P-glycoprotein among normal mature lymphoid cells. Using a cultured NK cell-rich population, we h ave examined the expression and function of P-glycoprotein, in particu lar its role in NK cell-mediated cytotoxicity, by employing two MDR-re versing agents (nicardipine and AHC-52, a nicardipine analog almost de void of calcium channel blocking activity) and monoclonal antibody aga inst P-glycoprotein (MRK-16). The expression of P-glycoprotein was det ected by flaw cytometry and polymerase chain reaction of reverse trans cribed mRNA. P-glycoprotein was functional in terms of rhodamine dye e xcretion and its susceptibility to the MDR-reversing agents. Since the concentration of nicardipine required for 50% inhibition (IC50) of rh odamine dye excretion (2 mu M) was close to that of AHC-52 (5 mu M), i t was suggested that their inhibitory effects were not due to calcium channel blocking activity, and that AHC-52 is a selective inhibitor fo r P-glycoprotein. The IC50 Of nicardipine for NK cell-mediated cytotox icity (33 mu M) was also close to that of AHC-52 (26 mu M), indicating that P-glycoprotein is involved in NK cell-mediated cytotoxicity. In support of this, MRK16 inhibited NK cell-mediated cytotoxicity in a co ncentration-dependent manner. Both binding of target cells to NK cells and post-binding events were affected by AHC-52, suggesting that P-gl ycoprotein is involved in several steps in NK cell-mediated cytotoxici ty.