Interleukin-2-activated rat natural killer cells express inducible nitric oxide synthase that contributes to cytotoxic function and interferon-gamma production

Natural killer (NK) cells are large granular lymphocytes capable of destroy ing cells infected by virus or bacteria and susceptible tumor cells without prior sensitization and restriction by major histocompatability complex (M HC) antigens, Their cytotoxic activity could be strongly enhanced by interl eukin-2 (IL-2), Previous findings, even if obtained with indirect experimen tal approaches, have suggested a possible involvement of the inducible nitr ic oxide (iNOS) pathway in the NK-mediated target cell killing. The aim of the present study was first to directly examine the induction of iNOS in IL -2-activated rat NK cells isolated from peripheral blood (PB-NK) or spleen (S-NK), and second to investigate the involvement of the iNOS-derived NO in the cytotoxic function of these cells. Our findings clearly indicate the i nduction of iNOS expression in IL-2-activated PB-NK and S-NK cells, as eval uated either at mRNA and protein levels. Accordingly, significantly high le vels of iNOS activity were shown, as detected by the L-arginine to L-citrul line conversion in appropriate assay conditions. The consequent NO generati on appears to partially account for NK cell-mediated DNA fragmentation and lysis of sensitive tumor target cells, In fact, functional inhibition of iN OS through specific inhibitors, as well as the almost complete abrogation o f its expression through a specific iNOS mRNA oligodeoxynucleotide antisens e, significantly reduced the lytic activity of IL-2-activated NK cells. Mor eover, IL-2-induced interferon-gamma production appears also to be dependen t at least in part, on iNOS induction, (C) 1999 by The American Society of Hematology.