title: Low-affinity FcγR interactions can decide the fate of novel human IgG-sensitised red blood cells and platelets.
abstract: G1Δnab is a mutant human IgG1 constant region with a lower ability to interact with FcγR than the natural IgG constant regions. Radiolabelled RBCs and platelets sensitised with specific G1Δnab Abs were cleared more slowly from human circulation than IgG1-sensitised counterparts. However, non-destructive splenic retention of G1Δnab-coated RBCs required investigation and plasma radioactivities now suggest this also occurred for platelets sensitised with an IgG1/G1Δnab mixture. In vitro assays with human cells showed that G1Δnab-sensitised RBCs did not cause FcγRI-mediated monocyte activation, FcγRIIIa-mediated antibody-dependent cell-mediated cytotoxicity (ADCC) or macrophage phagocytosis although they did adhere to macrophages. Thus, FcγRII was implicated in the adhesion despite the Δnab mutation reducing the already low-affinity binding to this receptor class. Additional contacts via P-selectin enhance the interaction of sensitised platelets with monocytes and this system provided evidence of FcγRII-dependent activation by G1Δnab. These results emphasise the physiological relevance of low-affinity interactions: It appears that FcγRII interactions of G1Δnab allowed splenic retention of G1Δnab-coated RBCs with inhibitory FcγRIIb binding preventing RBC destruction and that FcγRIIb engagement by G1Δnab on IgG1/G1Δnab-sensitised platelets overcame activation by IgG1. Considering therapeutic blocking Abs, G1Δnab offers lower FcγR binding and a greater bias towards inhibition than IgG2 and IgG4 constant regions.
authors.affiliation: Department of Pathology, University of Cambridge, Cambridge, UK.
authors.forename: Kathryn L
authors.initials: KL
authors.lastname: Armour
authors.forename: Cheryl S
authors.initials: CS
authors.lastname: Smith
authors.forename: Craig P
authors.initials: CP
authors.lastname: Turner
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Kirton
authors.forename: Anthony M
authors.initials: AM
authors.lastname: Wilkes
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Hadley
authors.forename: Cedric
authors.initials: C
authors.lastname: Ghevaert
authors.forename: Lorna M
authors.initials: LM
authors.lastname: Williamson
authors.forename: Michael R
authors.initials: MR
authors.lastname: Clark
mesh_terms: D016824:Antigens, Human Platelet / Q000276:immunology; D001792:Blood Platelets / Q000276:immunology* / Q000378:metabolism*; D002470:Cell Survival / Q000276:immunology / Q000528:radiation effects; D004912:Erythrocytes / Q000276:immunology* / Q000378:metabolism*; D006801:Humans; D007074:Immunoglobulin G / Q000276:immunology* / Q000378:metabolism; D039661:Integrin beta3; D009000:Monocytes / Q000276:immunology; D009687:Nuclear Proteins / Q000276:immunology; D011485:Protein Binding; D017452:Receptors, IgG / Q000378:metabolism*; D012204:Rh-Hr Blood-Group System / Q000276:immunology; D014157:Transcription Factors / Q000276:immunology
chemical_list: D016824:Antigens, Human Platelet; C061248:ITGB3 protein, human; D007074:Immunoglobulin G; D039661:Integrin beta3; D009687:Nuclear Proteins; D017452:Receptors, IgG; D012204:Rh-Hr Blood-Group System; C026410:Rho(D) antigen; D014157:Transcription Factors; C498432:ZFPM1 protein, human
keywords: Blocking antibody; Fc engineering; IgG effector function; Low-affinity Fc receptors