title: Blockade of maternal anti-HPA-1a-mediated platelet clearance by an HPA-1a epitope-specific F(ab') in an in vivo mouse model of alloimmune thrombocytopenia.
abstract: BACKGROUND
  Neonatal alloimmune thrombocytopenia (NAIT) is most commonly caused by transplacental passage of maternal human platelet-specific alloantigen (HPA)-1a antibodies that bind to fetal platelets (PLTs) and mediate their clearance. SZ21, a monoclonal antibody (MoAb) directed against PLT glycoprotein IIIa, competitively inhibits the binding of anti-HPA-1a alloantibodies to PLTs in vitro. The purpose of this investigation was to determine whether SZ21 F(ab')(2) fragments might be therapeutically effective in inhibiting or displacing maternal HPA-1a antibodies from the fetal PLT surface and preventing their clearance from circulation.
  
  
  METHODS
  Resting human PLTs from HPA-1ab heterozygous donors were injected into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. Purified F(ab')(2) fragments of SZ21 or control immunoglobulin G (IgG) were injected intraperitoneally 30 minutes before introduction of HPA-1a antibodies. Blood samples were taken periodically and analyzed by flow cytometry to determine the percentage of circulating human PLTs.
  
  
  RESULTS
  Anti-HPA-1a IgG from NAIT cases were able to efficiently clear HPA-1a-positive PLTs from murine circulation. Administration of SZ21 F(ab')(2) fragments not only inhibited binding of HPA-1a antibodies to circulating human PLTs, preventing their clearance, but also displaced bound HPA-1a antibodies from the PLT surface.
  
  
  CONCLUSIONS
  F(ab')(2) fragments of HPA-1a-selective MoAb SZ21 effectively inhibit anti-HPA-1a-mediated clearance of human PLT circulating in an in vivo NOD/SCID mouse model. These results suggest that agents that inhibit binding of anti-HPA-1a to PLTs may have therapeutic potential in the treatment of NAIT.
authors.affiliation: Blood Research Institute, BloodCenter of Wisconsin, Milwaukee, 53201, USA.
authors.forename: Tamam
authors.initials: T
authors.lastname: Bakchoul
authors.forename: Brian
authors.initials: B
authors.lastname: Boylan
authors.forename: Ulrich J H
authors.initials: UJ
authors.lastname: Sachs
authors.forename: Gregor
authors.initials: G
authors.lastname: Bein
authors.forename: Changgeng
authors.initials: C
authors.lastname: Ruan
authors.forename: Sentot
authors.initials: S
authors.lastname: Santoso
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Newman
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology / Q000378:metabolism; D016824:Antigens, Human Platelet / Q000276:immunology*; D001782:Blood Donors; D001792:Blood Platelets / Q000276:immunology* / Q000378:metabolism; D004195:Disease Models, Animal; D006579:Heterozygote; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000008:administration & dosage / Q000276:immunology* / Q000302:isolation & purification; D039661:Integrin beta3; D051379:Mice; D016688:Mice, Inbred NOD; D016513:Mice, SCID; D009035:Mothers; D011485:Protein Binding; D054098:Thrombocytopenia, Neonatal Alloimmune / Q000276:immunology*
chemical_list: D000911:Antibodies, Monoclonal; D016824:Antigens, Human Platelet; C061248:ITGB3 protein, human; D007140:Immunoglobulin Fab Fragments; D039661:Integrin beta3