Recreation of Fetal Neonatal Alloimmune Thrombocytopenia in CRISPR/Cas9 Gene-Edited Mice Expressing the Human Platelet Alloantigen, HPA-1a

Fetal neonatal alloimmune thrombocytopenia (FNAIT) is a life-threatening alloimmune disorder caused by maternal antibodies directed against fetal platelet alloantigens. The human platelet alloantigens, (HPA)-1a/HPA-1b (also known as the Pl^A1 alloantigen system), residing on the integrin β3 subunit (= platelet membrane glycoprotein IIIa), is the most frequently implicated HPA for causing FNAIT in Caucasians, and a single Leu33Pro amino acid polymorphism is responsible for generating the HPA-1a and HPA-1b epitopes. Neither human nor mouse anti-HPA-1a antibodies recognize murine GPIIIa due to amino acid differences both within and surrounding the polymorphic amino acid, which lies within a long flexible loop in PSI domain of the molecule. As a result, there are currently no authentic mouse models of FNAIT capable of recapitulating the human alloimmune response to this clinically-important platelet antigen. We employed CRISPR/Cas9 gene editing technology to generate mice expressing the human HPA-1a allogeneic epitope. Four critical amino acid substitutions (A₃₀P₃₂L₃₃D₃₉) were introduced into the PSI domain of murine GPIIIa. The resulting humanized form of murine GPIIIa, termed APLD, reacted in a variety of immunoassays with both murine monoclonal (mAb) and human maternal polyclonal antibodies specific for the human HPA-1a epitope. Intraperitoneal injection of an anti-HPA-1a mAb induced thrombocytopenia in APLD, but not wild-type, mice. Importantly, wild-type mice immunized with murine APLD transgenic platelets generated a high-titer HPA-1a-specific antibody response, demonstrating that the humanized form of murine GPIIIa is immunogenic in mice. Finally, pre-immunized female wild-type mice bred with APLD males gave birth to neonatal mice that exhibited profound thrombocytopenia, with the maternal titer directly related to the degree of thrombocytopenia in the neonates. Recapitulating the essential features of FNAIT in mice will facilitate the investigation of a wide-range of clinically-important issues in the diagnosis, treatment, and management of platelet alloimmune disorders.