title: Binding of human C-reactive protein to mouse macrophages is mediated by distinct receptors.
abstract: Human C-reactive protein (CRP) is known to activate mouse macrophages (M phi) to a tumoricidal state and to serve as an opsonin for M phi. Therefore, cell surface receptors for CRP on mouse M phi were characterized and their relationship to the IgG FcR determined. The specific binding of 125I-CRP to resident or elicited mouse M phi was saturable, reversible, and involved both a high and a low affinity receptor population. Binding of CRP to the mouse M phi cell lines PU5 1.8 and J774 was nearly identical to that observed with peritoneal M phi. The high affinity receptor population had a calculated K of 10 nM and a receptor density of approximately 10(5) sites per cell. Mouse Ig of the IgG2a, IgG2b, or IgG1 isotypes inhibited binding of 125I-CRP to PU5 1.8 cells at concentrations five-fold greater than that of the homologous ligand. In the converse experiment, unlabeled CRP failed to inhibit specific binding of 125I-labeled IgG2a, IgG2b or IgG1. Isolation of CRP binding proteins from surface iodinated PU5 1.8 cells by ligand-affinity chromatography or chemical cross-linking yielded a major protein band of 57 to 60 kDa which appeared to be distinct from the IgG1/IgG2b FcR (FcR-II) membrane proteins. Removal of radiolabeled IgG2b/IgG1 binding membrane proteins by affinity chromatography did not remove CRP-binding proteins. The rat mAb 2.4G2 which inhibits binding of radiolabeled mouse IgG2b, did not inhibit the binding of CRP. A rat polyclonal antiserum to CRP-binding membrane proteins of PU5 1.8 cells inhibited 125I-CRP binding, but not 125IgG2b binding. The rat polyclonal antibody reacted with two 57 to 60 kDa membrane proteins from PU5 1.8 cells that appear to be of a similar size on Western blots. The 125I-CRP was internalized via endosomes and intact CRP subunits could be detected intracellularly. The findings suggest that binding of CRP occurs through a receptor that is distinct from the IgG FcRs, but that CRP-R activity may be influenced by an association with an IgG FcR.
authors.affiliation: Department of Microbiology, Ohio State University, Columbus 43210.
authors.forename: K
authors.initials: K
authors.lastname: Zahedi
authors.forename: J M
authors.initials: JM
authors.lastname: Tebo
authors.forename: J
authors.initials: J
authors.lastname: Siripont
authors.forename: G F
authors.initials: GF
authors.lastname: Klimo
authors.forename: R F
authors.initials: RF
authors.lastname: Mortensen
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal; D001667:Binding, Competitive; D002097:C-Reactive Protein / Q000187:drug effects / Q000378:metabolism*; D002460:Cell Line; D003600:Cytosol / Q000378:metabolism; D006801:Humans; D007106:Immune Sera; D007074:Immunoglobulin G / Q000502:physiology; D007700:Kinetics; D008262:Macrophage Activation; D008264:Macrophages / Q000187:drug effects / Q000276:immunology / Q000378:metabolism*; D008565:Membrane Proteins / Q000302:isolation & purification; D051379:Mice; D010529:Peritoneal Cavity; D011961:Receptors, Fc / Q000276:immunology; D011971:Receptors, Immunologic / Q000187:drug effects / Q000302:isolation & purification / Q000502:physiology*; D000685:Serum Amyloid A Protein / Q000494:pharmacology
chemical_list: D000911:Antibodies, Monoclonal; D007106:Immune Sera; D007074:Immunoglobulin G; D008565:Membrane Proteins; D011961:Receptors, Fc; D011971:Receptors, Immunologic; D000685:Serum Amyloid A Protein; D002097:C-Reactive Protein