Inhibition of GPIX and GPIbα expression on treatment of CD34⁺ cells with QITP sera or QITP IgG. Cultured CD34⁺ cells were treated on day 4 with QITP serum or purified IgG plus Qn or with controls (QITP serum or QITP IgG − Qn or NS or NS IgG + Qn) and analyzed after 4 days in culture. (A) Left panels: flow cytometric data for GPIIb⁺, GPIX⁺, and GPIbα⁺ Mks treated with NS + Qn (blue), QITP serum − Qn (green), and QITP serum + Qn (red). Right panels: flow cytometric data for GPIIb⁺, GPIX⁺, and GPIbα⁺ Mks treated with NS IgG + Qn (blue), QITP IgG − Qn (green), and QITP IgG + Qn (red). Treatment with QITP serum or QITP IgG + Qn greatly decreased the expression of GPIX and GPIbα. (B) Mature Mks were stained with QITP serum + Qn or with controls (QITP serum − Qn or NS + Qn) for 30 minutes, followed by staining with PE-conjugated anti-human IgG antibody and anti-GPIX antibody conjugated with Alexa Fluor 647. The flow cytometric data show that the binding of the anti-GPIX antibody was not affected by prior incubation with QITP serum + Qn. (C) Number of receptors present on Mks determined as described in the supplemental “Methods.” The number of GPIX and GPIbα molecules on the cell surface was significantly reduced after QITS + Qn treatment. The mean ± SD is shown.