Moesin functions as a lipopolysaccharide receptor on human monocytes

Bacterial endotoxin (lipopolysaccharide [LPS]), a glycolipid found in the o uter membranes of gram-negative bacteria, induces the secretion of proinfla mmatory cytokines such as tumor necrosis factor alpha (TNF-alpha), interleu kin-1 (IL-1), and IL-6 by monocytes/macrophages. The secretion of these bio logically active compounds leads to multiple pathological conditions, such as septic shock There is substantial evidence that chronic exposure to LPS mediates, at least in part, the tissue destruction associated with gram-neg ative infection. CD14, a 55-kDa protein, has been identified as an LPS rece ptor. In conjunction with a serum protein, LPS binding protein (LBP), LPS-C D14 interactions mediate many LPS functions in the inflammatory response. H owever, CD14 lacks a cytoplasmic domain, or any known signal transduction s equence motif, suggesting the existence of another cell surface domain capa ble of transducing signals. In this paper, we report a second, CD14-indepen dent LPS binding site, which, based on biological activity, appears to be a functional LPS receptor. Cross-linking experiments were performed to ident ify LPS binding sites. Two molecules were identified: a 55-kDa protein (CD1 4) and a second, 78-kDa band. Sequencing of the 78-kDa protein by mass spec troscopic analysis revealed 100% homology with moesin (membrane-organizing extension spike protein). Antibody to CD14 induced partial blocking of the LPS response. However, antimoesin monoclonal antibody completely blocked th e LPS-induced TNF-alpha response in human monocytes, without blocking CD14 binding of LPS. Irrelevant isotype controls had no effect. Additional exper iments were performed to evaluate the specificity of the antimoesin blockin g. Separate experiments evaluated antimoesin effects on monocyte chemotaxis , IL-1 production in response to IL-1 stimulation, and TNF-cu secretion in response to Staphylococcus aureus stimulation. Antimoesin blocked only LPS- mediated events. The data suggest that moesin functions as an independent L PS receptor on human monocytes. The role of moesin in transduction of CD14- mediated signals is discussed.