title: Legionella pneumophila lipopolysaccharide activates the classical complement pathway.
abstract: Legionella pneumophila is a gram-negative bacterium capable of entering and growing in alveolar macrophages and monocytes. Complement and complement receptors are important in the uptake of L. pneumophila by human mononuclear phagocytes. The surface molecules of L. pneumophila that activate the complement system are unknown. To identify these factors, we investigated the effects of L. pneumophila lipopolysaccharide (LPS) on the classical and alternative complement pathways of normal human serum by functional hemolytic assays. Although incubation of LPS in normal human serum at 37 degrees C resulted in the activation of both pathways, complement activation proceeded primarily through the classical pathway. Activation of the classical pathway by LPS was dependent on natural antibodies of the immunoglobulin M class that were present in various quantities in sera from different normal individuals but were absent in an immunoglobulin-deficient serum obtained from an agammaglobulinemic patient. Additional studies using sheep erythrocytes coated with LPS suggested that the antibodies recognized antigenic sites in the carbohydrate portion of LPS. The ability of LPS to interact with the complement system suggests a role for LPS in the uptake of L. pneumophila by mononuclear phagocytes.
authors.affiliation: Department of Microbiology and Immunology, University of Miami School of Medicine, Florida 33101.
authors.forename: C S
authors.initials: CS
authors.lastname: Mintz
authors.forename: D R
authors.initials: DR
authors.lastname: Schultz
authors.forename: P I
authors.initials: PI
authors.lastname: Arnold
authors.forename: W
authors.initials: W
authors.lastname: Johnson
mesh_terms: D003172:Complement C1 / Q000378:metabolism; D003175:Complement C2 / Q000378:metabolism; D015933:Complement C3d / Q000378:metabolism; D003181:Complement C4 / Q000378:metabolism; D003170:Complement Pathway, Alternative / Q000187:drug effects / Q000276:immunology; D003171:Complement Pathway, Classical / Q000187:drug effects* / Q000276:immunology*; D004306:Dose-Response Relationship, Immunologic; D004912:Erythrocytes; D006801:Humans; D007075:Immunoglobulin M / Q000502:physiology; D016952:Legionella pneumophila / Q000276:immunology*; D007877:Legionnaires' Disease / Q000276:immunology; D008070:Lipopolysaccharides / Q000276:immunology* / Q000302:isolation & purification
chemical_list: D003172:Complement C1; D003175:Complement C2; D003181:Complement C4; D007075:Immunoglobulin M; D008070:Lipopolysaccharides; D015933:Complement C3d