title: Dimerization of glycoprotein Ibα is not sufficient to induce platelet clearance.
abstract: ESSENTIALS: Many anti-glycoprotein (GP)Ibα antibodies induce platelet clearance in a dimer-dependent manner. Characterization of monoclonal antibodies that bind the mechanosensitive domain (MSD) of GPIbα. An anti-MSD antibody binds two copies of GPIbα in platelets but does not induce platelet clearance. The prevailing clustering model of GPIbα signaling is incorrect or needs revision.
  
  
  BACKGROUND
  The mechanism of platelet clearance is not clear. Many antibodies binding the membrane-distal ligand-binding domain of glycoprotein (GP)Ibα induce rapid clearance of platelets and acute thrombocytopenia, which requires the bifurcated antibody structure. It was thought that binding of these antibodies induced lateral dimerization or clustering of GPIbα in the plasma membrane, which leads to downstream signaling and platelet clearance. However, many antibodies targeting GPIbβ and GPIX, which are associated with GPIbα in the GPIb-IX complex, do not induce platelet clearance, which is in contradiction to the clustering model.
  
  
  OBJECTIVE
  To test whether dimerization or clustering of GPIbα is sufficient to transmit the signal that leads to platelet clearance.
  
  
  METHODS
  We have recently raised several mAbs targeting the mechanosensitive domain (MSD) of GPIbα. Binding of these anti-MSD antibodies was characterized with biochemical methods. Their ability to stimulate platelets and induce platelet clearance in mice was assessed.
  
  
  CONCLUSIONS
  Infusion of anti-MSD antibodies does not cause thrombocytopenia in mice. These antibodies show no detectable effects on platelet activation and aggregation in vitro. Further biochemical investigation showed that the anti-MSD antibody 3D1 binds two copies of GPIbα on the platelet surface. Therefore, lateral dimerization of GPIbα induced by antibody binding is not sufficient to initiate GPIb-IX signaling and induce platelet clearance. Our results suggest that a factor other than or in addition to clustering of GPIbα is required to induce platelet clearance.
authors.affiliation: Department of Pediatrics, Aflac Cancer and Blood Disorders Center, Emory University School of Medicine, Atlanta, GA, USA.
authors.forename: X
authors.initials: X
authors.lastname: Liang
authors.affiliation: Department of Pediatrics, Aflac Cancer and Blood Disorders Center, Emory University School of Medicine, Atlanta, GA, USA.
authors.forename: A K
authors.initials: AK
authors.lastname: Syed
authors.affiliation: Department of Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, AR, USA.
authors.forename: S R
authors.initials: SR
authors.lastname: Russell
authors.affiliation: Department of Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, AR, USA.
authors.forename: J
authors.initials: J
authors.lastname: Ware
authors.affiliation: Department of Pediatrics, Aflac Cancer and Blood Disorders Center, Emory University School of Medicine, Atlanta, GA, USA.
authors.forename: R
authors.initials: R
authors.lastname: Li
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D005838:Genotype; D006801:Humans; D008822:Mice, Transgenic; D010641:Phenotype; D010976:Platelet Count; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000737:chemistry / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D011487:Protein Conformation; D055503:Protein Multimerization; D015398:Signal Transduction; D013329:Structure-Activity Relationship
chemical_list: D000911:Antibodies, Monoclonal; D019038:Platelet Glycoprotein GPIb-IX Complex; C094990:adhesion receptor
keywords: antibody-binding sites; blood platelets; glycoprotein receptor GPIb-IX; platelet activation; thrombocytopenia