Reduction of viable Mks and induction of apoptosis on QITP antibody treatment. 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). The total viable cell number was determined after 4 days of treatment by Trypan blue exclusion. Cells were treated with serum (A) or with purified IgG or WJ serum preabsorbed on platelets to remove auto-antibodies (WJpa) (B). (C) Treated Mks stained with anti-GPIIb and PI. The percentage of GPIIb⁺/PI⁺ cells was determined by flow cytometry. (D) Viable cells enumerated by Trypan blue exclusion and stained with anti-GPIIb, anti-GPIX, and anti-GPIbα antibodies. The percentage of GPIIb⁺ (empty bars), GPIX⁺ (gray bars), and GPIbα⁺ (solid bars) Mks present in the culture was determined by flow cytometry. The total cell number was calculated by multiplying the number of cells (direct counting) by the percentage of Mks positive for each marker. (E) Cells treated for 20 hours with the indicated sera or with 25μM paclitaxel followed by staining with Annexin V–FITC. The percentage of Annexin V⁺ cells in the control culture (NS + Qn) was normalized to 1 and the -fold increase in Annexin V⁺ cells on treatment plotted (n = 4). (F) Cells were treated as in panel E, stained with anti activated caspase 3-FITC antibody, and the -fold increase in activated caspase 3⁺ cells was plotted (n = 3). The mean ± SD is shown in all cases.