DIFFERENTIAL RECOVERY OF MACROPHAGES FROM ENDOTOXIN-TOLERANT STATES ELICITED BY LIPOPOLYSACCHARIDE AND ENZYMATIC TREATMENTS

Exposure of macrophages to endotoxin (lipopolysaccharide, LPS) leads t o a suppression of their capacity to bind LPS and to produce cytokines after reexposure to LPS. This phenomenon is termed endotoxin toleranc e, or LPS-induced desensitization. LPS also stimulates the secretion o f serine proteases in macrophages, and activates membrane phospholipas es. We have investigated the role of trypsin (a serine protease) and o f a phosphatidylinositol-specific phospholipase C (PI-PLC, which cleav es GPI-anchored molecules such as CD14), on LPS-induced desensitizatio n. The results obtained by treatment with PI-PLC or in the presence of protease inhibitors, suggested that activation of phospholipases and proteases are not involved in LPS-induced desensitization. However, tr ypsin treatment of macrophages abolished both LPS binding and cytokine responses. The recovery of macrophages from this trypsin-induced tole rance (restoration of TNF-alpha synthesis without reexpression of LPS- binding sites) was very different from that following LPS-induced tole rance (reexpression of LPS-binding sites without restoration of TNF-al pha synthesis). The results are consistent with the hypothesis that si gnaling LPS-receptors might be synthesized de novo after trypsin degra dation, whereas non-signaling LPS-receptors might be internalized and recycled after preexposure to LPS.