Effect of death receptor activation on the proliferation and survival of PV erythroid precursors. CD34⁺ cells purified from the peripheral blood of healthy donors and patients with PV were cultivated in standard erythroid medium. From day 5 of erythroid culture, cells were treated with the indicated stimuli in medium containing 0.5 U/mL EPO. Anti-CD95 or TRAIL was maintained at the indicated concentrations throughout the time of culture. (A) Growth of normal (N, ▪) and PV (PV, ▴) erythroblasts in the presence of 50 ng/mL anti-CD95 antibody. Data are expressed as mean ± SD of experiments performed with erythroblasts derived from 5 healthy donors and 10 patients with PV. (B) Percentage of apoptotic cells in cultures of normal (N) and PV erythroblasts (PV) at day 14 in the presence of 50 ng/mL anti-CD95 antibody from day 5 of culture. Data represent the mean ± SD of experiments performed with cells derived from 13 patients (**P < .01). (C) Growth inhibition exerted by 50 ng/mL anti-CD95 antibody (left) or by 200 ng/mL TRAIL (right) on normal (N) and PV (PV) erythroblasts in relation to the presence of JAK2 V617F mutation. WT indicates wild-type JAK2; monoallelic, monoallelic JAK2 V617F mutation; biallelic, biallelic JAK2 V617F mutation. The percentage of growth inhibition was calculated on cells at day 18 of culture, as described in “Materials and methods.” P ≤ .001 for anti-CD95 antibody; P ≤ .005 for TRAIL. Horizontal bars indicate the mean value of the N and PV percentage growth inhibition.