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the treated cell preparations.

/ Ammonium-chloride-containing solutions (AC) are routinely used to lyse red blood cells during preparation of peripheral blood monnuclear cells (PBMC). Although exposure to AC has been described to affect the ultrastructural appearance of large granular lymphocytes and to temporarily inhibit cytolytic activity of PBMC preparations, the cellular basis of this phenomenon has not been studied. Here, the inhibitory effect of AC on human cytotoxic T lymphocyte cell (CTL) and natural killer (NK)-mediated cytotoxicity has been analyzed in 4-hr 51Cr-release assays. The results show that NK killing of K562 leukemia cells and xenogeneic endothelial cells is inhibited by AC exposure. The effect is dose-dependent and reversible, because recovery of cytotoxicity is observed within 15 hr of re-culturing. AC does not reduce the viability of NK cells and the inhibitory effect is not mediated by the exhaustive release of granzymes upon AC treatment. In contrast, antigen-specific CTL killing of epstein barr virus (EBV)-transformed B-lymphoblastoid cell lines and xenogeneic phytohemagglutinin (PHA) lymphoblasts was less sensitive to AC and data are presented suggesting that FasL-induced apoptosis is not inhibited by AC. In conclusion, perforin-mediated NK killing is AC-sensitive whereas CTL killing and FasL-mediated killing appear to be AC-resistant. Therefore, AC represents a powerful tool to study different mechanisms of cell-mediated cytotoxicity and may be helpful in assessing antigen-specific CTL cytotoxicity without the influence of NK cell-mediated background killing.