Moesin: a potential LPS receptor on human monocytes

Abstract

Bacterial endotoxin (lipopolysaccharide, LPS), a glycolipid found in the outer membrane of Gram-negative bacteria, induces the secretion of pro-inflammatory cytokines such as tumor necrosis factor alpha (TNF-α), interleukin (IL)-1, and IL-6 by monocytes/macrophages. The secretion of these biologically active compounds leads to multiple pathological conditions, such as septic shock. There is substantial evidence that chronic exposure to LPS in periodontal diseases mediates, at least in part, the tissue destruction associated with the Gram-negative infection. LPS receptor has been shown to be CD14, a 55 kDa protein. LPS—CD14 interactions mediate many monocyte/macrophage functions in the inflammatory response. However, CD14 lacks a cytoplasmic domain, or any known signal transduction sequence motif, suggesting the existence of another cell surface domain capable of transducing signals. More recently, significant work has implicated Toll proteins in LPS-mediated signaling. The purpose of the present work was to investigate, identify, and characterize secondary LPS binding cell surface domain(s) on monocytes/macrophages. Initial experiments with anti-CD14 blocking antibody revealed only partial blocking of the LPS induced TNF-α response. The kinetics of these experiments suggested a second, low-affinity receptor. Cross-linking experiments were performed to identify LPS binding sites. Two domains were identified: a 55 kDa protein which was inhibited by anti-CD14 (presumably the CD14 receptor) and a second 78 kDa domain. Partial protein sequencing of the 78 kDa domain using mass spectroscopic analysis ascribed this domain to Moesin (membrane organizing extension spike protein). Preliminary experiments using anti-Moesin monoclonal antibody revealed a dose-dependent blocking of LPS induced TNF-α response with a total blocking at 50 µg/ml. Irrelevant isotype controls had no effect. Additional experiments were performed to evaluate the specificity of the anti-Moesin blocking. Separate experiments evaluated anti-Moesin effects on monocyte chemotaxis, IL-1 production in response to IL-1 stimulation, and TNF-α secretion in response to Staphylococcus aureus stimulation. Anti-Moesin antibody only blocked LPS-mediated events. Histological analysis of tissue sections harvested from LPS-induced skin lesions exhibited a 3-fold reduction of the polymorphonuclear neutrophil infiltrate in Moesin-deficient mice compared to wild type mice. The data suggest that Moesin functions as an independent LPS receptor on human monocytes.