Blockade of maternal anti‐HPA‐1a–mediated platelet clearance by an HPA‐1a epitope–specific F(ab′) 2 in an in vivo mouse model of alloimmune thrombocytopenia

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. STUDY DESIGN AND 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. CONCLUSION: 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.