Increased processing of lymphocyte function-associated antigen-1 in human natural killer cells stimulated with IL-2

Previously we reported that surface expression of lymphocyte function-associated antigen-1 (LFA-1), the primary leukocyte integrin on human natural killer (NK) and lymphokine-activated killer (LAK) cells, does not differ between NK and LAK cells. In contrast to surface expression, we now report that much higher levels of both precursor and mature forms of LFA-1 molecules were found relative to MHC class I, another membrane glycoprotein, with metabolic labeling of IL-2-stimulated LAK cells compared with native NK cells. An 85-90 kDa glycoprotein, found in much higher quantities in LAK compared with NK cells, appeared to be a precursor of the 95 kDa beta chain of the beta 2 integrin family in human LAK cells because: (i) pulse-chase experiments using LAK cells demonstrated decreased 35S-labeling of the 85-90 kDa molecule with a concomitant increase in the radioactivity of the mature 95 kDa LFA-1 beta chain, (ii) results of protease treatment revealed that the two molecules share virtually identical peptide maps, and (iii) endoglycosaminidase F treatment of LAK cell lysates immunoprecipitated with antibody against LFA-1 beta resulted in the disappearance of both the 85-90 and 96 kDa LFA-1 beta signals, and appearance of a signal at approximately 76 kDa. Digestion of the same immunoprecipitates with neuraminidase resulted in the disappearance of the 95 kDa signal and revealed a single molecular weight signal corresponding to 85-90 kDa. These data suggest that a core protein of approximately 76 kDa becomes N-glycosylated, perhaps terminally with sialic acid residues, to mature into the 95 kDa form. Moreover, the rate of maturation of LFA-1 was more rapid in LAK than NK cells, with half times of 0.8 versus 1.5 h for the alpha chain and 3.7 versus 4.9 h for the beta chain for LAK versus NK cells respectively. IL-2 treatment of NK cells therefore alters the processing of LFA-1 molecules during the transition to LAK cells, providing a larger intracellular reservoir with which to replenish the surface molecule. Together with our previous observation that LFA-1 is phosphorylated and transduces signal more effectively in LAK than NK cells, the findings support the notion that adhesion molecules contribute to the increased function of LAK cells.