NGM-SZ21 prevents anti–HPA-1a antibody-mediated platelet destruction in a NOD/SCID mouse model of alloimmune thrombocytopenia. Resting human platelets (HPA-1ab) were injected retroorbitally into NOD/SCID mice and the survival of platelets was analyzed. Results are shown as a median and range of experiments that were performed in duplicate with anti–HPA-1a antibodies from 4 FNAIT cases. (A) Epitope blockade: When NGM-SZ21 was injected before the maternal anti–HPA-1a antibodies, the elimination of platelets was largely inhibited (black symbols), while the control antibody NGM-AP2 did not prevent rapid elimination by human anti–HPA-1a antibodies (gray symbols). NGM-SZ21 alone did not induce relevant platelet destruction (white symbols). (B) Antibody competing: To simulate the situation of advanced pregnancy, maternal anti–HPA-1a antibodies were injected 30 minutes before giving NGM-SZ21 or NGM-AP2. The injection of maternal anti–HPA-1a alone resulted in a rapid destruction of circulating human HPA-1a platelets (white symbols). Note that NGM-SZ21 (black symbols) but not NGM-AP2 (gray symbols) was able to ameliorate platelet destruction induced by the circulation of maternal anti–HPA-1a antibodies.