Endotoxin Tolerance: Independent Regulation of Interleukin-1 and Tumor Necrosis Factor Expression

The injection of lethal or sublethal doses of bacterial lipopolysaccharide (LPS) into mice results in transient increases in both serum tumor necrosis factor (TNF) and interleukin-1 (IL-1). The peak in serum TNF was detected prior to maximal elevation in endogenous corticosterone and was no longer apparent 3 to 4 h post-LPS injection, a point at which corticosterone and IL-1 levels had significantly increased. The initial increase in serum IL-1 may, in part, be modulated by the preceding TNF peak, as pretreating animals with a monoclonal antibody against murine TNF resulted in a significant decrease in IL-1 levels 3 h post-LPS injection. A second injection of LPS at 20 h failed to result in a secondary TNF peak, suggesting an endotoxin-tolerant state. However, in contrast to TNF, significant increases in serum IL-1 were detected in the endotoxin-tolerant animals following a repeated LPS stimulus. This secondary increase in IL-1 occurred despite the elevation in serum corticosterone. While peritoneal macrophages from endotoxin-tolerant mice demonstrated only a modest 10 to 15% increase in TNF and IL-1 mRNA relative to the levels after the primary 1-h LPS stimulus, a secondary increase in IL-1 but not TNF mRNA in the spleen was apparent following a second LPS injection. The spleen, however, was not essential for the increase in serum IL-1, as endotoxin-tolerant splenectomized mice had comparable increases in IL-1 following a repeated LPS stimulus. These results demonstrate the differential regulation of IL-1 and TNF in vivo during endotoxin tolerance.