The APLDQ murine form of GPIIIa is immunogenic. (A) Seven WT female BALB/c mice were injected intraperitoneally (IP) with a Sigma adjuvant mixed with 1 × 10⁸ washed platelets derived from APLDQ C57BL/6 mice on days 0 and 7. Note that all 7 recipients developed an IgG immune response specific for APLDQ platelets, although the titers varied approximately fourfold. There was little reactivity against WT C57BL/6 or BALB/c platelets (supplemental Figure 2). (B) IgM as well as all IgG subclasses were represented. (C) Five mouse plasmas were examined for alloantigen specificity and were demonstrated to bind GPIIIa derived from human HPA-1a⁺ platelets and GPIIIa from APLDQ platelets, but not WT murine platelets. Two different anti-HPA-1mAbs (SZ21 and 26.4) were used as controls, along with a broadly reactive mAb (kindly supplied by Heyu Ni, PhD, University of Toronto) that binds both the human and murine PSI domain irrespective of HPA allotype. MFI, mean fluorescent intensity.