ENDOTOXIN-INDUCED DESENSITIZATION OF MOUSE MACROPHAGES TS MEDIATED INPART BY NITRIC-OXIDE PRODUCTION

Refractoriness (tolerance) to endotoxin effects, such as induction of tumor necrosis factor alpha (TNF-alpha) secretion, can be elicited in vitro in macrophages by preexposure of cells to endotoxin (lipopolysac charide [LPS]) itself. The aim of this study was to determine whether this phenomenon is due to negative feedback mediated by the free radic al nitric oxide (NO) produced by cells when they are activated by LPS. Among several efficient inhibitors of NO production, N-G-monomethyl-L arginine did not induce concomitant inhibition of TNF-alpha secretion . Mouse macrophages that were exposed to LPS in the presence of N-G-mo nomethyl-L-arginine partially maintained the ability to secrete TNF-al pha in response to a second LPS stimulation, compared with cells preex posed to LPS alone, thus suggesting that NO is involved in part in LPS -induced desensitization of cells. Furthermore, direct exposure of cel ls to the NO-generating compounds sodium nitroprusside and S-nitroso-N -acetylpenicillamine mimicked LPS induced desensitization. However, lo w concentrations of a synthetic lipid (lipid M4) that is structurally analogous to the reducing end of the lipid A moiety of LPS induced des ensitization of mouse macrophages without concomitant production of NO . Taken together, these data suggest that although NO actually takes p art in LPS-induced desensitization of mouse macrophages, additional an d yet unknown mechanisms must also exist.