Ability of NGM-SZ21 to prevent anti–HPA-1a–mediated phagocytosis of HPA-1a platelets. The effect of antibody deglycosylation on the phagocytic activity was investigated by opsonizing CMFDA (FITC)–labeled platelets with SZ21 or NGM-SZ21 before adding the monocytes. Monocytes were then gated and the percentage of FITC-positive monocytes, that is, those that ingested labeled platelets, was identified as the phagocytic activity (%). Note that SZ21 but not NGM-SZ21 was capable of inducing platelet phagocytosis (white and gray bars, respectively). To assess the protection property, labeled platelets were incubated with NGM-SZ21 (black bars) or with NGM-AP2 as an isotype-matched control (gray bars) before adding maternal anti–HPA-1a IgG. Then monocytes were added and phagocytosis of the opsonized platelets was assessed and expressed as phagocytic activity. White bars represent the native phagocytic activity of the maternal anti–HPA-1a antibodies. Gray bars show that the control antibody NGM-AP2 did not inhibit phagocytosis, while the black bars show inhibition of the phagocytic activity by NGM-SZ21 (median of inhibition: 47% vs 0%, respectively, P = .008).