title: Inhibition of FcγR-mediated phagocytosis by IVIg is independent of IgG-Fc sialylation and FcγRIIb in human macrophages.
abstract: In immune thrombocytopenia and warm autoimmune hemolytic anemia, circulating immunoglobulin G (IgG)-opsonized blood cells are cleared from the circulation by macrophages. Administration of intravenous immunoglobulin (IVIg) can prevent uptake, but the exact working mechanism is not known. The prevailing theory from murine studies, which states that Fc-sialylated IgG alters the balance between activating and inhibitory Fc-gamma receptors (FcγRs) by inducing upregulation of the inhibitory FcγRIIb on effector macrophages, is currently debated. We studied phagocytosis of IgG-opsonized blood cells in a human system, assessing the effect of IVIg and blocking anti-FcγR F(ab')2 fragments on uptake by monocyte-derived macrophages (both M1 and M2 macrophages). Phagocytosis was remarkably sensitive to administration of IVIg, but unexpectedly, recombinant Fc-sialylated IgG or sialic acid-enriched IVIg were equally active as unsialylated IgG fractions in mediating this inhibition, independent of FcγRIIb expression. Instead, IVIg inhibited phagocytosis by direct blockade of FcγRs. IgG fractions enriched for IgG dimers with enhanced avidity for FcγRs showed increased inhibition compared with monomeric IgG fractions. Together, our data demonstrate that inhibition of IgG-mediated phagocytosis in human macrophages by IVIg is dependent on the capacity to directly bind FcγRs but is independent of FcγRIIb or sialylation of the Fc fragment in the human setting.
authors.affiliation: Department of Blood Cell Research.
authors.forename: Sietse Q
authors.initials: SQ
authors.lastname: Nagelkerke
authors.affiliation: Department of Experimental Immunohematology, and.
authors.forename: Gillian
authors.initials: G
authors.lastname: Dekkers
authors.affiliation: Department of Immunopathology, Sanquin Research, and Landsteiner Laboratory, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands;
authors.forename: Iwan
authors.initials: I
authors.lastname: Kustiawan
authors.affiliation: Department of Immunopathology, Sanquin Research, and Landsteiner Laboratory, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands;
authors.forename: Fleur S
authors.initials: FS
authors.lastname: van de Bovenkamp
authors.affiliation: Department of Blood Cell Research.
authors.forename: Judy
authors.initials: J
authors.lastname: Geissler
authors.affiliation: Center for Proteomics and Metabolomics, Leiden University Medical Center, Leiden, The Netherlands; and.
authors.forename: Rosina
authors.initials: R
authors.lastname: Plomp
authors.affiliation: Center for Proteomics and Metabolomics, Leiden University Medical Center, Leiden, The Netherlands; and.
authors.forename: Manfred
authors.initials: M
authors.lastname: Wuhrer
authors.affiliation: Department of Experimental Immunohematology, and.
authors.forename: Gestur
authors.initials: G
authors.lastname: Vidarsson
authors.affiliation: Department of Immunopathology, Sanquin Research, and Landsteiner Laboratory, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands;
authors.forename: Theo
authors.initials: T
authors.lastname: Rispens
authors.affiliation: Department of Blood Cell Research.
authors.forename: Timo K
authors.initials: TK
authors.lastname: van den Berg
authors.affiliation: Department of Blood Cell Research, Emma Children's Hospital, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Taco W
authors.initials: TW
authors.lastname: Kuijpers
mesh_terms: D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004912:Erythrocytes / Q000276:immunology; D005434:Flow Cytometry; D016178:Granulocyte-Macrophage Colony-Stimulating Factor / Q000276:immunology / Q000494:pharmacology; D006801:Humans; D007141:Immunoglobulin Fc Fragments / Q000276:immunology* / Q000378:metabolism; D007074:Immunoglobulin G / Q000276:immunology; D016756:Immunoglobulins, Intravenous / Q000276:immunology* / Q000494:pharmacology; D016173:Macrophage Colony-Stimulating Factor / Q000276:immunology / Q000494:pharmacology; D008264:Macrophages / Q000187:drug effects / Q000276:immunology* / Q000378:metabolism; D019158:N-Acetylneuraminic Acid / Q000276:immunology / Q000378:metabolism; D010587:Phagocytosis / Q000187:drug effects / Q000276:immunology*; D017452:Receptors, IgG / Q000276:immunology* / Q000378:metabolism
chemical_list: C499173:FCGR2B protein, human; D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D016756:Immunoglobulins, Intravenous; D017452:Receptors, IgG; D016173:Macrophage Colony-Stimulating Factor; D016178:Granulocyte-Macrophage Colony-Stimulating Factor; D019158:N-Acetylneuraminic Acid