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

Neonatal alloimmune thrombocytopenia (NAIT) results from maternal alloimmunization to paternally inherited human platelet-specific alloantigens (HPA) expressed on the surface of fetal platelets (PLTs)1. The transplacental passage of maternal HPA-1a antibodies leads to destruction of fetal PLTs. Of the 16 alloantigen systems identified to date, HPA-1a antibodies are the most frequent cause of severe thrombocytopenia in Caucasian persons2,3. A single Leu33Pro amino acid polymorphism is responsible for the HPA-1a alloimmunization4. The manifestations of NAIT range from subclinical thrombocytopenia to intracranial hemorrhage, the latter of which occurs in 10 to 20 percent of the NAIT cases1. Although most intracranial hemorrhage cases are reported to occur in utero, NAIT-affected newborns are also at risk to develop cerebral hemorrhage, especially in the first 24 to 48 hours postpartum1. Owing to the absence of a routine screening program to predict maternal HPA alloimmunization, first-pregnancy NAIT cases are usually identified postnatally. These cases require immediate management of severe thrombocytopenia to achieve a rapid correction of PLT count to prevent intracranial hemorrhage in the neonates. Currently, HPA-1a–negative PLT transfusion is the treatment of choice1. Because only 2.5 percent of Caucasian persons are HPA-1bb, the availability of HPA-1a–negative PLTs is restricted to major transfusion centers3. Other therapeutic options such as high-dose intravenous globulin and random PLT transfusion have some limitations, like delay of action and refractoriness1,5,6. Therefore, there is a need for novel therapeutic approaches that might efficiently prevent clearance of newborn PLTs. SZ21, a monoclonal antibody (MoAb) directed against PLT glycoprotein (GP) IIIa, binds at or near the HPA-1a epitope7-9. Recently, we developed an in vivo model system in which human PLTs are injected into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice and allowed to circulate for up to 24 hours10. Using this model, it is possible to investigate the fate of human PLTs in the presence of PLT-reactive antibodies and to examine the potential for novel therapeutics to prevent PLT clearance. In this study, we show that coinjection of divalent F(ab′)2 fragments of SZ21 prevents anti-HPA-1a– mediated human PLT clearance from the circulation.