Radiodetoxified endotoxin-induced tolerance. Effect on endotoxin lethality and macrophage arachidonic acid metabolism

Abstract

Endotoxin (LPS) tolerance produces changes in macrophage mediator production which is thought to be responsible for the acquired LPS resistance. Detoxification of LPS by gamma irradiation has been reported to diminish certain noxious properties while retaining its tolerance inducing actions. We compared the efficacy of LPS and radiodetoxified (RD)-LPS from Escherichia coli O101 on stimulating rat peritoneal macrophage arachidonic acid (AA) metabolism, measured by thromboxane (TXB2). Changes in macrophage production of these mediators were also assessed after tolerance induction. LPS tolerance was induced by i.p. injection of LPS, RD-LPS or vehicle on day 1 (100 micrograms/kg, i.p.) and day 2 (500 micrograms/kg, i.p.). On day 5 or 4 weeks after pretreatment, peritoneal macrophages were harvested for in vitro studies, or rats were tested for lethality resistance. Macrophages were incubated +/- LPS (0.1 ng to 50 micrograms/ml), lipid A (1 or 10 micrograms/ml) or Ca+2 ionophore A23187 (10 microM) for determination of TXB2 production. Minimum effective concentrations of LPS and RD-LPS for stimulation (P < 0.05) of TXB2 were 100 ng/ml and 1 microgram/ml, respectively. Maximal stimulation of TXB2 occurred at 10 micrograms/ml of LPS or RD-LPS. Macrophages from LPS or RD-LPS tolerized rats were refractory to stimulated TXB2 with LPS or RD-LPS (0.1 ng to 50 micrograms/ml). The suppressed in vitro macrophage TXB2 production was apparent 4 weeks after rats were tolerized with LPS or RD-LPS. In subsequent mortality studies, LPS challenge of control or tolerance rats at day 5 in vivo with Salmonella enteritidis LPS (15 mg/kg, i.v.) produced a 90% mortality in control rats (N = 22), versus 13% mortality in the LPS pretreated group (N = 23) and a 20% in the RD-LPS pretreated group (N = 10) (P < 0.05 vs control). However, this lethality resistance was not apparent at 4 weeks after LPS or RD-LPS pretreatment. Both LPS and RD-LPS appear to be equipotent in inducing macrophage alterations, and in lethality resistance during LPS tolerance induction. However, these observations suggest that during LPS tolerance suppression of LPS-stimulated AA in peritoneal macrophage metabolism persists longer than acquired lethality resistance.