Effect of IVIG on anti-β3 integrin IgG levels in immunized β3^−/−FcRn^+/+ and β3^−/−FcRn^−/− mice. (A) Platelet counts in pups from β3^−/−FcRn^+/+ females bred with β3^+/+FcRn^+/+ males. IVIG (1 g/kg) was injected into pregnant immunized β3^−/−FcRn^+/+ mice once a week until delivery. Albumin (1 g/kg) was used as a control. n = 11-25 for each group. (B) Circulating anti-β3 integrin and platelet-associated IgG were undetectable in β3^−/+FcRn^−/− pups delivered from immunized β3^−/−FcRn^−/− mothers after IVIG treatment, as assessed by flow cytometry. (C) IVIG down-regulated maternal anti-β3 integrin antibodies in β3^−/−FcRn^−/− mice. Sera from female β3^−/−FcRn^−/− mice after immunization (black line), during pregnancy (gray line), and after delivery (dotted line) were incubated with 10⁶ β3^+/+FcRn^−/− platelets at a final dilution of 1:100 for 1 hour. Anti-β3 integrin IgG was detected via flow cytometry. This result is representative of at least 3 independent experiments. (D) IVIG also down-regulated anti-β3 integrin IgG in nonpregnant β3^−/−FcRn^−/− mice immunized twice with β3^+/+ platelets. IVIG (2 g/kg) was injected into immunized β3^−/−FcRn^−/− mice. After 1 week, sera were collected, and anti-β3 integrin IgG was detected via flow cytometric analysis. Human albumin (2 g/kg) was used as a control. MFI of anti-β3 integrin IgG before injection with IVIG or albumin was set as 100%. n = 3 for each group.