Fetal Maternal Hemorrhage As a Trigger for Fetal/Neonatal Alloimmune Thrombocytopenia: Evidence from a Murine Model

Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is a severe bleeding disorder caused by maternal alloantibodies targeting fetal platelet alloantigens. Among the 37 human platelet alloantigens (HPAs) identified, the HPA-1a/HPA-1b polymorphism, involving a Leu33Pro amino acid substitution in the integrin β3 subunit, is the most common cause of severe FNAIT, particularly in Caucasians. Leu33 serves as a key target for alloantibody binding, leading to fetal and neonatal thrombocytopenia and, in severe cases, spontaneous or trauma-induced intracranial hemorrhage. FNAIT shares a pathophysiologic basis with Hemolytic Disease of the Fetus and Newborn (HDFN), which stem from maternal immunization to alloantigens on fetal platelets and red blood cells (RBCs), respectively. While alloimmunization against RBC alloantigens typically occurs in response to fetal maternal hemorrhage (FMH), a clear link between FMH and maternal alloimmunization against HPA-1a on circulating fetal platelets has remained elusive. To assess whether FMH can induce platelet alloimmunization and lead to the development of FNAIT during pregnancy, we utilized recently developed transgenic mice expressing the HPA-1a allogeneic epitope on a murine GPIIIa backbone. To simulate varying degrees of FMH, we transfused different doses of HPA-1a platelets into wild-type (WT), non-pregnant female mice. We found that transfusing as few as 1x10⁷ HPA-1a-positive platelets (equivalent to a FMH of approximately 30 ml in humans) resulted in production of high-titer HPA-1a-specific alloantibodies. Since >25% of the cases of FNAIT in humans are thought to occur in what are experienced as first pregnancies, we further investigated whether exposure to HPA-1a in pregnant female mice could still induce FNAIT. We found that when WT female mice pregnant with HPA-1a-positive fetuses were transfused with HPA-1a-positive platelets, they consistently developed HPA-1a-specific antibodies with similar titers as observed in non-pregnant females and delivered thrombocytopenic pups with FNAIT. Taken together, these findings demonstrate that exposure to HPA-1a-positive platelets in the maternal circulation during pregnancy is sufficient to trigger FNAIT, and that maternal tolerance, normally increased during pregnancy, does not play a significantly immunosuppressive role to prevent alloimmunization to the HPA-1a epitope. These findings support the notion that FMH may be a possible cause of alloimmunization in humans, even during a first pregnancy. Finally, the result of this study further validates the usefulness of this HPA-1a alloantigen-specific humanized transgenic mouse model to explore the etiology and pathophysiology of FNAIT. Further studies should significantly advance preventative and therapeutic strategies for this life-threatening immunologic disorder.