Recombinant HPA-1a antibody therapy for treatment of fetomaternal alloimmune thrombocytopenia: proof of principle in human volunteers

Abstract

Key Points• Recombinant antibody B2G1Dnab protects platelets from destruction by anti-HPA-1a in the circulation of HPA-1a1b human volunteers.• B2G1Dnab is a potential therapeutic agent for antenatal treatment of fetomaternal alloimmune thrombocytopenia due to HPA-1a antibodies.Fetomaternal alloimmune thrombocytopenia, caused by the maternal generation of antibodies against fetal human platelet antigen-1a (HPA-1a), can result in intracranial hemorrhage and intrauterine death. We have developed a therapeutic human recombinant high-affinity HPA-1a antibody (B2G1Dnab) that competes for binding to the HPA-1a epitope but carries a modified constant region that does not bind to Fcg receptors. In vitro studies with a range of clinical anti-HPA-1a sera have shown that B2G1Dnab blocks monocyte chemiluminescence by >75%. In this firstin-man study, we demonstrate that HPA-1a1b autologous platelets (matching fetal phenotype) sensitized with B2G1Dnab have the same intravascular survival as unsensitized platelets (190 hours), while platelets sensitized with a destructive immunoglobulin G1 version of the antibody (B2G1) are cleared from the circulation in 2 hours. Mimicking the situation in fetuses receiving B2G1Dnab as therapy, we show that platelets sensitized with a combination of B2G1 (representing destructive HPA-1a antibody) and B2G1Dnab survive 3 times as long in circulation compared with platelets sensitized with B2G1 alone. This confirms the therapeutic potential of B2G1Dnab. The efficient clearance of platelets sensitized with B2G1 also opens up the opportunity to carry out studies of prophylaxis to prevent alloimmunization in HPA-1a-negative mothers. (Blood. 2013;122(3):313-320)

Characterization of a Human Platelet Antigen-1a–Specific Monoclonal Antibody Derived from a B Cell from a Woman Alloimmunized in Pregnancy

“…In support of the feasibility of Ab prophylaxis with HPA-1a-specific mAbs, we have demonstrated in vitro that both IgG1 and IgG3 subclasses of recombinantly expressed mAb 26.4 are able to induce phagocytosis of HPA-1a + platelets. Also, rapid clearance of autologous HPA-1ab platelets that were sensitized ex vivo with the HPA-1a-specific human mAb B2G1 was demonstrated in a recent study (35). Furthermore, our demonstration that mAb 26.4 can efficiently block maternal polyclonal HPA-1a-specific IgG from various donors from binding platelets suggests that the mAb could also interfere with binding to receptors on HPA-1a-specific B cell clones in women susceptible to immunization.…”

“…This may be achieved by protecting fetal tissues and platelets from potentially damaging anti-HPA-1a Abs with Abs that compete for binding to HPA-1a but lack the ability to activate immune effector functions. This is not a new concept and has been proved to function in principle with HPA-1a-specific mAbs both in a murine model and in human volunteers (35,36). The stable binding of mAb 26.4 to both platelet-derived and trophoblast-derived HPA1a, as demonstrated in this study, could be an additional advantage for therapeutic purposes because anti-HPA-1a Abs have been reported to have possible negative effects on fetal growth, in addition to causing thrombocytopenia in the fetus (5,37).…”

How do we treat fetal and neonatal alloimmune thrombocytopenia?

“…Anti‐FcγR immunoglobulins may have the capacity to reduce PLT destruction . Another approach consists of preventing PLT destruction using recombinant HPA‐1a–specific antibodies with abrogated Fcγ receptor binding . These antibodies have the capacity to saturate available HPA‐1a–binding sites from fetal PLTs without activating FcγR signaling.…”

Neonatal Thrombocytopenia and Platelet Transfusion - A UK Perspective

“…For the future, a recent proof-of-principle study has shown that a recombinant anti-HPA-1 antibody can, in vivo , block platelet destruction by immune HPA-1 IgG antibodies [33]. …”