Human NK cells express endothelial nitric oxide synthase, and nitric oxideprotects them from activation-induced cell death by regulating expression of TNF-alpha

Although NO appears important in rodent immune responses, its involvement i n the human immune system is unclear, We report that human NK cells express constitutive endothelial NO synthase mRNA and protein, but not detectable levels of inducible NO synthase. They produce NO following activation by co culture with target cells or cross-linking with anti-CD16 mAb, and producti on is increased in the presence of IL-2, N-monomethyl-L-arginine (L-NMA), a NOS inhibitor, partially inhibited NK cell lysis of four different target cells (<40% inhibition at 500 mu M L-NMA), but not granule release followin g coculture with target cells, or Fas ligand induction following cross-link ing with anti-CDl6 mAb, However, L-NMA augmented apoptosis of NK cells indu ced by activation through CD16 ligation or coculture with K562. An NO donor , S-nitroso-N-acetylpenicillamine (SNAP), suppressed apoptosis of NK cells induced by CD16 cross-linking or coculture with target cells, suggesting th at endogenous NO production is involved in protection of NK cells from acti vation-induced apoptosis, thereby maintaining NK activity. SNAP also suppre ssed, and L-NMA enhanced, expression of TNF-alpha, reported to be involved in activation-induced NK cell death, in response to CD16 cross-linking. Sup pression of anti-CD16-induced apoptosis by SNAP was reversed by the additio n of rTNF-alpha, DNA-binding activity of the transcription factor, NF-AT, w hich is involved in TNF-alpha induction upon ligation of CD16, was inhibite d by SNAP and enhanced by L-MMA, Our results suggest that down-regulation o f TNF-alpha expression, possibly due to suppression of NF-AT activation, is a mechanism by which endogenous NO protects NK cells from activation-induc ed apoptosis, and maintains lytic capacity.