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

Abstract

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 + platelets and substantially inhibited binding of clinical HPA-1a-specific sera to HPA-1a + 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 + platelets, concomitant with transient thrombocytopenia. As the Δnab constant region is uninformative in mice, F(ab′) 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.

Relationship of Proximal Renal Tubular Dysgenesis and Fetal Liver Injury in Neonatal Hemochromatosis

“…Additionally, in contrast to the liver, the kidney in these cases shows no injury-no inflammation, no fibrosis, etc-and thus, the paucity of tubules does not seem to result from antibody-mediated destruction of existing tubules (as is evident in hepatocyte loss in the liver). Furthermore, to actually perform a blocking function in the development of fetal disease (as in blocking angiotensin receptors to hypothetically cause failed tubule development), the antibody would need to be in the IgG4 subclass, as only IgG4 engages the Fc receptor well enough to be escorted across the placenta by FcRn and does not activate complement (30). Thus, a cross-reacting antibody could not do both things: injure the liver and block development of the renal tubule.…”

Fc receptors in immune thrombocytopenias: a target for immunomodulation?

“…In 19 of 20 maternal sera tested in vitro, up to 95% inhibition of anti-HPA-1a-binding to platelets was achieved using the B2G1Δnab antibody (6). Murine studies confirmed that this recombinant antibody abrogated FcγR-mediated antibodycoated platelet clearance, encouraging further exploration of the feasibility of this approach in patients.…”

“…In the first study, Asahi et al examined the changes in the balance of FcRs expressed by patients with immune thrombocytopenia purpura (ITP) and Helicobacter pylori infection in order to explore the mechanism of platelet recovery that has been observed in these individuals following treatment to eradicate H. pylori (5). In the second study, Ghevaert et al report the development and preclinical testing of a recombinant antibody designed to prevent FcR-mediated alloimmune destruction of platelets, which may have potential as a treatment approach for fetomaternal alloimmune thrombocytopenia (FMAIT) (6).…”

“…The report by Ghevaert et al (6) in this issue of the Journal exploits the FcγR pathway by a totally different approach from that of Asahi et al (5), this time with the aim of treating FMAIT. FMAIT results from transplacental transfer of maternal antibodies that develop in response to alloimmunization against paternal human platelet antigens (HPAs) expressed on fetal platelets.…”

Et nyfødt barn med petekkier

“…Innføring av slik profylakse vil kreve HPA-1-typing av alle gravide for å identifisere behandlingskandidatene. I tillegg pågår forskning med tanke på å ufarliggjøre antistoffer hos allerede immuniserte mødre (19).…”