INDUCTION OF THE NITRIC OXIDE-SYNTHESIZING PATHWAY IN FRESH AND INTERLEUKIN 2-CULTURED RAT NATURAL-KILLER-CELLS

Several lines of evidence suggest that nitric oxide (NO), generated th rough nitric oxide synthase (NOS) by cleavage of terminal guanidino ni trogen from L-arginine, mediates tumor cell killing by mononuclear pha gocytes. Natural killer (NK) cells are cytotoxic effector cells that l yse a variety of tumor and virus-infected cells in a MHC-unrestricted manner. NE; cells cultured with interleukin 2 proliferate and acquire the ability to lyse a wide range of targets, including NK-resistant tu mor cells (LAK activity). The present study was designed to investigat e whether a NOS pathway exists in fresh or IL-2-activated NK cells and to assess the importance of NO synthesis in their activation and cyto toxic functions. NKR-P1 triggering, which is known to induce NK cell a ctivation and mediate reverse ADCC, was able to induce arginine metabo lism with consequent increase of nitrite and citrulline levels. Moreov er, stimulated NO synthesis leads to guanylate cyclase activity with c onsequent cGMP generation. We also report that cytotoxic activities of fresh or IL-2-activated NK cells appear to be dependent on arginine l evels in medium. Tumoricidal activity of both these effector cells, as sessed against YAC-1 and P815 target cells, respectively, was indeed s ignificantly reduced when cytotoxic assays were performed in arginine- free medium or in the presence of the L-arginine analog L-N-monomethyl -arginine, which inhibits nitroxide formation from L-arginine. Normal levels of cytotoxic activities could be restored by addition of exogen ous L-arginine. NO generation by NK and LAK cells, determined as nitri te, citrulline, and cGMP synthesis, correlated well with their cytotox ic activities. Moreover, NOS activity gradually increased during the L AK generation and correlated well with the increasing capability of IL -2-activated NK cells to lyse NK-resistant targets, such as P815. (C) 1994 Academic Press, Inc.