Nitric oxide synthase pathway may mediate human natural killer cell cytotoxicity

The present study provides evidence that the human natural killer (NK) cell effector mechanism causing target cytolysis has a requirement for L-arginine. In a deficient medium (DM) containing only salts, buffer system and glucose, NK cell-mediated cytotoxicity was found to decrease by 70% as compared to that obtained in a complete medium (CM). However, adding L-arginine to such DM could restore the activity of NK cells to the normal level. Many other components of CM, such as serum, glutamine and vitamins did not improve NK cell-mediated killing in DM. When all amino acids except L-arginine were added to DM only a partial recovery of NK cell functional cytolysis was seen. L-arginine enhanced the NK cell activity in a dose-dependent manner. Additionally, the inhibitor of both inducible and constitutive nitric oxide synthase, N-monomethyl-L-arginine (L-NMMA) inhibited NK cytolytic activity in DM supplemented with L-arginine indicating participation of nitric oxide (NO). The results also show that the stimulatory effect of L-arginine on human NK cell-mediated cytotoxicity was accompanied by an increase in NO formation as determined by accumulation of nitrite and citrulline. L-NMMA gave a dose-dependent reduction in NO generation as well. The nitrite and citrulline production dose-dependently correlated with not only the concentration of L-arginine in the cultivation medium, but also the enhanced NK cell-mediated cytolysis. Taken together, these findings could define a L-arginine/NO-linked effector mechanism in human NK cells. Nitrite and citrulline were not formed when NK cell-mediated target cell killing took place in a L-arginine-free DM supplemented with additives. Thus, it appears as if human NK cells may cause target cell killing via both NO-dependent and -independent processes.