Protective Effects of Endotoxin in a Rat Model of Chronic Inflammation are Accompanied By Suppressed Secretion of Pro‐Inflammatory Cytokines and Biphasic Alteration in Hypothalamo‐Pituitary‐Adrenal Axis Activity

We have previously demonstrated that Gram-negative bacterial endotoxin can exert long-term protective effects against the chronic inflammatory disease adjuvant arthritis in rats. The present study was designed to investigate the mechanisms and time-course of hypothalamo-pituitary-adrenocortical (HPA) axis activity and cytokine secretion underlying this phenomenon. Rats were injected with endotoxin (lipopolysaccharide) and blood was collected either 7 or 21 days later. Priming with endotoxin induced a biphasic alteration in secretion of adrenocorticotrophic hormone and corticosterone in response to a second injection of endotoxin, with decreased secretion observed after 7 days whereas robust secretion was observed at 21 days. Seven days following priming with endotoxin, plasma concentrations of pro-inflammatory cytokines interleukin (IL)-6 and interferon (IFN)-y were reduced by 90%, and tumour necrosis factor (TNF)-a by 70%, compared to saline-treated rats, whereas robust secretion of the anti-inflammatory cytokine IL-10 was maintained in both groups. A similar net change favouring an anti-inflammatory cytokine secretory milieu was also observed 21 days following priming with endotoxin. This study provides evidence that the long-term protective effects of endotoxin on inflammation are associated with a sustained reduction in secretion of pro-inflammatory cytokines. HPA axis hypoactivity at 7 days suggests that corticosterone is not involved in suppressing IL-6, IFN-y and TNF-a at this time point. Conversely, hypersecretion of corticosterone at 21 days may underlie synchronous suppression of IL-6 and IFN-γ. These data provide novel insight into interactions between HPA axis activity and cytokine secretion following endotoxin priming prior to induction of inflammatory disease.