The APLD humanized murine PSI domain supports the binding of many, but not all, human anti–HPA-1a alloantisera. (A) Flow cytometric analysis of the binding of the HPA-1a–selective murine mAb SZ21 to human and mouse platelets. Note that SZ21 binds to human HPA-1a⁺ but not HPA-1b⁺ human platelets, demonstrating its alloselectivity, and to APLD but not WT murine platelets. The PSI domain–specific mAb PSIB1 was used as a positive control for expression of GPIIb-IIIa and, as shown, binds all PSI domains, regardless of species or HPA allotype. (B) Antigen capture enzyme-linked immunosorbent assay of anti–HPA-1a maternal alloantisera binding to human and murine forms of GPIIb-IIIa. Five different human FNAIT alloantisera were incubated with human or murine platelet of the indicated phenotype. Platelet/antibody complexes were then detergent lysed and added to microtiter wells that had been coated with either anti-mouse CD41 to capture immune complexes from mouse platelets or mAb AP2 to capture immune complexes from human platelets. Note that human alloantisera 2, 3, and 4 react similarly with human GPIIb-IIIa and APLD murine GPIIb-IIIa, whereas alloantisera 1 and 5 do not react with murine APLD GPIIb-IIIa, suggesting that the preponderance of the HPA-1a–specific alloantibodies present in these polyclonal sera have more complex epitope requirements. None of the FNAIT alloantisera react with WT murine GPIIb-IIIa, as expected.