Moesin: a potential LPS receptor on human monocytes

Bacterial endotoxin (lipopolysaccharide, LPS), a glycolipid found in the ou ter membrane of Gramnegative bacteria. induces the secretion of pro-inflamm atory cytokines such as tumor necrosis factor alpha (TNF-alpha), interleuki n (IL)-1. and IL-6 by monocytes/macrophages. The secretion of these biologi cally active compounds leads to multiple pathological conditions. such as s eptic shock. There is substantial evidence that chronic exposure to LPS in periodontal diseases mediates, at least in part. the tissue destruction ass ociated with the Gram-negative infection. LPS receptor has been shown to be CD14, a 55 kDa protein. LPS-CD 14 interactions mediate many monocyte/macro phage functions in the inflammatory response. However, CD 14 lacks a cytopl asmic domain, or any known signal transduction sequence motif, suggesting t he existence of another cell surface domain capable of transducing signals. More recently, significant work has implicated Toll proteins in LPS-mediat ed 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-alpha response. The kinetics of these experiments suggested a second, low-affinity receptor. Cross-linki ng experiments were performed to identify LPS binding sites. Two domains we re identified: a 55 kDa protein which was inhibited by anti-CD14 (presumabl y the CD14 receptor) and a second 78 kDa domain. Partial protein sequencing of the 78 kDa domain using mass spectroscopic analysis ascribed this domai n to Moesin (membrane organizing extension spike protein). Preliminary expe riments using anti-Moesin monoclonal antibody revealed a dose-dependent blo cking of LPS induced TNF-alpha response with a total blocking at 50 mug/ml. Irrelevant isotype controls had no effect. Additional experiments were per formed to evaluate the specificity of the anti-Moesin blocking. Separate ex periments evaluated anti-Moesin effects on monocyte chemotaxis, IL-1 produc tion in response to IL-1 stimulation, and TNF-alpha secretion in response t o Staphylococcus aureus stimulation. Anti-Moesin antibody only blocked LPS- mediated events. Histological analysis of tissue sections harvested from LP S-induced skin lesions exhibited a 3-fold reduction of the polymorphonuclea r 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.