PAF and perforin-mediated NK target-cell lysis.

(A) The phospholipid PAF enhances perforin-mediated NK target-cell lysis. Resting NK cells were used in standard cytotoxic assays against the MHC class I and FasR-negative K562 and Daudi cells, both of which were shown to be capable of inducing NK-cell activation and degranulation. Perforin-induced cell lysis was measured by target-cell release of sodium-⁵¹Cr–labeled cytoplasmic proteins via membrane damage and perforin pores. Resting human NK cells were mixed with labeled target cells at the various E-to-T ratios of 50:1, 25:1, 12:1, and 6:1, and target-cell lysis was measured at 4 hours. Naive NK cells were very efficient in provoking ⁵¹Cr release from K562 targets. Following addition of PAF to targets (PAF C16:0 alkyl moieties, [1 μmol/L]), K562 cells displayed a higher susceptibility to the lytic activity of perforin, as reflected by an increase in the amount of ⁵¹Cr target-cell release. In contrast, increasing extracellular concentrations of C16:0 PAF turned out to be inefficient in overcoming the failure of naive NK cells to induce Daudi cell lysis. (B) Inhibition of the PAF receptor. PAF receptor activity on K562 target cells was inhibited either by blocking its IFN-γ–induced expression using a neutralizing anti–IFN-γ antibody or by adding specific PAF receptor antagonists, such as WEB 2086 and SR 27417. Resting NK cells were used as effectors. These processes led to a potent inhibitory activity on NK-cell lysis, as shown by a significant decrease in the amount of NK/target-cell⁵¹Cr release.