Fig. 2
Fc hexamers block antibody-mediated phagocytosis by human macrophages in vitro and inhibit in vivo mouse models of ITP. a Inhibition of phagocytosis. Human macrophages were co-cultured with anti-CD20 opsonized target B cells and IgG1 (red triangles), IgG4 (blue circles) Fc hexamers (L309) or IVIG (open black boxes) were added at the indicated concentrations. Disappearance of target cells was measured by FACS after 18 h and % inhibition of phagocytosis calculated. Data are mean of n = 5 individual donors ± SEM. b Acute ITP model. ITP was induced by intraperitoneal administration of anti-CD41. IVIG (1000 mg/kg) or Fc hexamers (10 mg/kg) hexamers were administered i.v. 1 h prior to anti-CD41. Data are expressed as % of baseline platelet numbers with mean ± SEM and is a compilation of five individual experiments, n = 18–28/group. Statistical analysis was carried out by two-way ANOVA (experiments and treatment) and Dunnetts multiple comparison test (***p < 0.01) vs PBS controls. c Chronic ITP model. ITP was induced by subcutaneous implantation of an osmotic minipump containing anti-CD41. IgG1 (red triangles) or IgG4 (blue circles) Fc hexamer (L309C) was dosed at 10 mg/kg and IVIG (open black boxes) at 1000 mg/kg i.v. on the indicated day/s. Data are expressed as mean ± SEM of the % of baseline platelet numbers remaining, n = 6/group. Statistical analysis was carried out separately for days 4–7 by one-way ANOVA and Dunnetts multiple comparison test. The increase in platelet numbers for IgG1 Fc hexamer vs PBS controls (open black circles) was significant on days 5–7 (n.s. day 4, p < 0.01 day 5, p < 0.0001 days 6 and 7). The increase in platelet numbers for IgG4 Fc hexamer vs PBS controls was significant on days 6 and 7 (n.s. days 4 and 5, p < 0.05 day 6, p < 0.01 day 7). The increase in platelet numbers for IVIG vs PBS controls was significant on days 4–6 (p < 0.001 days 4 and 5, p < 0.01 day 6, n.s. day 7)
