Developing recombinant HPA-1a–specific antibodies with abrogated Fcγ receptor binding for the treatment of fetomaternal alloimmune thrombocytopenia

Fetomaternal alloimmune thrombocytopenia (FMAIT) is caused by maternal generation of antibodies specific for paternal platelet antigens and can lead to fetal intracranial hemorrhage. A SNP in the gene encoding integrin β3 causes a clinically important maternal-paternal antigenic difference; Leu33 generates the human platelet antigen 1a (HPA-1a), whereas Pro33 generates HPA-1b. As a potential treatment to prevent fetal intracranial hemorrhage in HPA-1a alloimmunized pregnancies, we generated an antibody that blocks the binding of maternal HPA-1a--specific antibodies to fetal HPA-1a1b platelets by combining a high-affinity human HPA-1a--specific scFv (B2) with an IgG1 constant region modified to minimize Fcγ receptor--dependent platelet destruction (G1Δnab). B2G1Δnab saturated HPA-[1a.sup.+] platelets and substantially inhibited binding of clinical HPA-1a--specific sera to HPA-[1a.sup.+] platelets. The response of monocytes to B2G1Δnab-sensitized platelets was substantially less than their response to unmodified B2G1, as measured by chemiluminescence. In addition, B2G1Δnab inhibited chemiluminescence induced by B2G1 and HPA-1a--specific sera. In a chimeric mouse model, B2G1 and polyclonal Ig preparations from clinical HPA-1a--specific sera reduced circulating HPA-[1a.sup.+] platelets, concomitant with transient thrombocytopenia. As the Δnab constant region is uninformative in mice, F[(ab').sub.2] B2G1 was used as a proof of principle blocking antibody and prevented the in vivo platelet destruction seen with B2G1 and polyclonal HPA-1a--specific antibodies. These results provide rationale for human clinical studies.