title: Identification of haptoglobin as an alternative ligand for CD11b/CD18.
abstract: Haptoglobin is an acute phase protein with presumed anti-inflammatory activities. We report that purified fluorescein-labeled haptoglobin 1-1 binds to THP1 and U937 promonocytic cell lines, to monocytes, to granulocytes, and to a subset of CD8+ T cells and to NK cells. Studies with radioiodinated haptoglobin on THP1 cells were consistent with specific binding to one class of receptors with a density of 1.7 x 10(5) binding sites per cell and a low affinity of 6.5 x 10(-6) Kd. Binding was increased by Ca2+ and by Ca2+ and ADP. Binding to THP1 and U937 cells could be inhibited by preincubation with nonfluoresceinated haptoglobin and by fibrinogen, but not by albumin, transferrin, or alpha1-acid glycoprotein. Fibrinogen binds to the CD11b/CD18 integrin. We therefore examined whether haptoglobin has the same receptor. The anti-CD11b mAb44 indeed inhibited the binding of fluoresceinated haptoglobin to THP1 and U937 cell lines, and haptoglobin inhibited the binding of the anti-CD11b mAb anti-Leu15 and mAb44 to both cell lines. An anti-CD18 mAb partly inhibited the binding of fluoresceinated haptoglobin to THP1 and U937, indicating that the beta-chain of MAC-1 is also involved in haptoglobin binding. There was no interference between the binding of anti-CD4, anti-CD11a, or anti-CD11c mAb and haptoglobin binding to THP1 cells. Binding of haptoglobin to purified CD11b/CD18 indicates that it binds directly to the receptor. Haptoglobin is an alternative low affinity ligand for the CD11b/CD18 integrin, suggesting that this acute phase protein might regulate MAC-1-dependent cell function in vivo.
authors.affiliation: Division of Clinical Immunology, Department of Medicine, University of Leuven, Belguim.
authors.forename: S M
authors.initials: SM
authors.lastname: El Ghmati
authors.forename: E M
authors.initials: EM
authors.lastname: Van Hoeyveld
authors.forename: J G
authors.initials: JG
authors.lastname: Van Strijp
authors.forename: J L
authors.initials: JL
authors.lastname: Ceuppens
authors.forename: E A
authors.initials: EA
authors.lastname: Stevens
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000911:Antibodies, Monoclonal; D018821:CD18 Antigens / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D002460:Cell Line; D005340:Fibrinogen / Q000378:metabolism; D016650:Fluorescein-5-isothiocyanate; D005456:Fluorescent Dyes; D006098:Granulocytes / Q000276:immunology / Q000378:metabolism; D006242:Haptoglobins / Q000037:antagonists & inhibitors / Q000276:immunology / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D007694:Killer Cells, Natural / Q000276:immunology / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D008214:Lymphocytes / Q000276:immunology / Q000378:metabolism; D016177:Macrophage-1 Antigen / Q000378:metabolism*; D009000:Monocytes / Q000276:immunology / Q000378:metabolism; D011485:Protein Binding / Q000187:drug effects; D016176:T-Lymphocyte Subsets / Q000276:immunology / Q000378:metabolism; D013696:Temperature
chemical_list: D000911:Antibodies, Monoclonal; D018821:CD18 Antigens; D005456:Fluorescent Dyes; D006242:Haptoglobins; D008024:Ligands; D016177:Macrophage-1 Antigen; D000244:Adenosine Diphosphate; D005340:Fibrinogen; D016650:Fluorescein-5-isothiocyanate; D002118:Calcium