TNFR-1 (p55) and Fas share a death domain which is critical for apoptosis signaling whereas TNFR-p55 and TNFR-2 (p75) can activate NF-kappaB leading to anti-apoptotic proteins expression such as A1. The purpose of this study was to elucidate the role(s) of TNFR-p55 and TNFR-p75 in Fas-mediated neutrophil apoptosis and A1 expression in a mouse model of endotoxemia. Gene knockout (KO) (p55-/-, p75-/-, p55(-/-)/p75(-/-)) or wild type (WT) mice were injected i.p. with saline or LPS (4 microg/g) followed by collecting peripheral blood after 24 h. Neutrophil apoptosis was assessed by propidium iodide staining using two-color flow cytometry with granulocyte-specific Gr1-FITC after 6-h whole blood culture with or without Fas agonist Jo2 (300 ng/ml) in the presence or absence of cycloheximide (CHX, 30 microg/ml). Membrane-associated receptors (Fas, TNFR-p55 and TNFR-p75) and cytoplasmic A1 expression of freshly isolated neutrophils were assessed by one-color flow cytometry and western blotting respectively. Compared with the group-WT/Sal, Jo2 induced apoptosis only in the presence of CHX (J+C). J+C-induced apoptosis was significantly lower in the group-p55(-/-)/Sal and p55(-/-)/p75(-/-)/Sal but not in the group-p75(-/-)/Sal. J+C-induced apoptosis was inhibited similarly in all the LPS-injected WT and KO mice. Strong A1 expression was also induced similarly in all the LPS-injected WT and KO mice. Fas and TNFR-p55 expression was normal and TNFR-p75 was significantly increased in all the LPS-injected WT and KO mice although absence of the appropriate surface receptors was confirmed in the KO mice. We conclude that p55 normally plays a proapoptotic role, but p75 appears to play a minimal role in Fas-mediated neutrophil apoptosis. During endotoxin-induced systemic inflammation, both TNFR-p55 and TNFR-p75 appear to be of minimal importance for modulation of Fas-mediated apoptosis and associated A1 protein expression despite normal Fas/TNFR-p55 and increased TNFR-p75 expression in neutrophils.