Lipopolysaccharide (LPS) Recognition in Macrophages. Participation of LPS-binding Protein and CD14 in LPS-induced Adaptation in Rabbit Peritoneal Exudate Macrophages

Exposure of rabbit peritoneal exudate macrophages (PEM) or whole blood to picomolar concentrations of LPS induces adaptation or hyporesponsiveness to LPS. Because of the importance of plasma LPS-binding protein (LBP) and the macrophage cell membrane protein CD14 in recognition of LPS, we examined the effect of LBP on LPS-induced adaptation in PEM. PEM exposed to LPS in the presence of LBP for 8 h were markedly less responsive to subsequent stimulation by LPS than monocytes/macrophages (M phi) adapted in the absence of LBP. LPS-induced expression of TNF was sharply reduced in LBP-LPS-adapted PEM, but in contrast these cells remained fully responsive to Staphylococcus aureus peptidoglycan. We considered that specific hyporesponsiveness in LPS-adapted M phi or in blood monocytes could be due to decreased expression of CD14 or diminished binding of LBP-LPS complexes to CD14. However, flow cytometry analysis revealed only minimal reduction of CD14 expression or CD14-dependent binding of a fluorescent LPS derivative when normo- and hyporesponsive cells were compared. These results show that complexes of LPS and LBP are more effective than LPS alone in inducing adaptation to LPS, and LPS-induced hyporesponsiveness probably results from changes in cellular elements distinct from CD14 that are involved in either LPS recognition or LPS-specific signal transduction.