Fetal and Neonatal Alloimmune Thrombocytopenia: Lessons Learned from Animal Models

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a severe alloimmune disorder that results from fetal/neonatal platelet opsonization by maternal antibodies, which cross the placenta and result in fetal/neonatal platelet destruction. The frequency of FNAIT has been estimated at 0.5-1.5/1,000 liveborn neonates. However, this number does not include fetuses that die from this disease, since the incidence of FNAIT miscarriage has not been adequately studied. Analogous to autoimmune thrombocytopenia (ITP), the major target antigens in FNAIT are the platelet GPIIIa (β₃ integrin) and GPIbα. However, severe bleeding is much more frequent in FNAIT, particularly the occurrence of intracranial hemorrhage (ICH). It is not known why the reported incidence of FNAIT mediated by anti-GPIbα antibodies is at a much lower frequency when compared to ITP, and whether the severe bleeding tendency in FNAIT is due to β₃ integrin expression on angiogenic vessels in the developing fetus, which are targeted by cross-reacting maternal anti-β₃ integrin antibodies. To study the pathogenesis and to develop new strategies for prevention and treatment, we have established murine models of FNAIT using β₃ integrin and GPIbα deficient (^-/-) mice. We first transfused these deficient mice with wild-type (WT) platelets to induce anti-β₃ or anti-GPIbα antibody responses; we then bred these immunized female mice with WT males, causing FNAIT in the offspring. We found that maternal antiplatelet antibody titer correlated with the severity of FNAIT. These two murine models have revealed fundamental differences between the pathogenesis of anti-β₃ and anti-GPIbα-mediated FNAIT. In anti-β₃-mediated FNAIT, we found severe thrombocytopenia, ICH, and miscarriage. We also found the impairment of angiogenesis, which may contribute to ICH and intrauterine growth retardation. In contrast, the anti-GPIbα-mediated model revealed a nonclassical form of FNAIT (e.g., miscarriage but not bleeding disorders in neonates). We found that anti-GPIbα antibodies caused thrombosis in the placentas and miscarriage in most pregnant mice, which may partially explain the rarity of anti-GPIbα-mediated FNAIT reported in humans. Despite these substantial differences, there are also similarities between anti-β₃ and anti-GPIbα-mediated FNAIT, such as the role of the neonatal Fc receptor (FcRn). FcRn is important for serum IgG homeostasis and for IgG transportation across the placenta. Using FcRn^-/- mice, we demonstrated that fetal (but not maternal) FcRn is required to transport maternal antibodies to the fetal circulation and is indispensable for FNAIT. Blocking FcRn with an anti-FcRn antibody markedly reduced the severity of both anti-β₃ and anti-GPIbα-mediated FNAIT. This important finding may lead to the development of new therapies (e.g., anti-FcRn antibody) against FNAIT and other maternal pathogenic antibody-mediated fetal/neonatal diseases. These models of FNAIT have allowed us to investigate the efficacy and mechanism of action of several therapies, including the aforementioned anti-FcRn antibody as well as antibody-mediated immune suppression (AMIS) and intravenous IgG (IVIG). In β₃^-/- mice, prophylactic administration of anti-HPA-1a antibody or murine β₃ antisera induced AMIS against human HPA-1a-positive or murine WT platelets, respectively. Importantly, AMIS induced by β₃ antisera suppressed the antibody response, thrombocytopenia, and miscarriage in FNAIT mice. These findings support the hypothesis that anti-HPA-1a antibody administration to HPA-1a-negative women after delivery of an HPA-1a-positive child may prevent FNAIT in subsequent pregnancies. The efficacy of IVIG is inconsistent, sources are limited, and the mechanism of action is not fully understood. We demonstrated that IVIG ameliorated both anti-β₃ and anti-GPIbα-mediated FNAIT. IVIG decreased antiplatelet antibodies in both maternal and neonatal circulations, fetal platelet clearance, bleeding, and fetal mortality. In summary, we have uncovered fundamental differences in the pathogenesis of anti-β₃ and anti-GPIbα-mediated FNAIT and have greatly enhanced our understanding of emerging and existing therapies. We will continue to investigate the pathogenesis of FNAIT so that we may develop more tailored and accessible therapeutic strategies for patients.