The effects of Peg-IFNα 2a and ABT-737 alone and in combination on hematopoietic colony formation by JAK2V617F-positive PV CD34⁺ cells. (A) CFU-GM and BFU-E derived colony formation by CD34⁺ cells isolated from 6 different CB, 6 different bone marrow samples, and peripheral blood from 14 different patients with PV in the presence of 200 U/mL Peg-IFNα 2a and 250 nM of ABT-737 alone or in combination; hematopoietic colony numbers were enumerated after 14 days of incubation. Data were analyzed by the Student t test (*P < .05, **P < .01; ***P < .001; n = 14). (B) Effect of 200 and 500 U/mL Peg-IFNα 2a and 250 nM ABT-737 alone or combination treatment on the JAK2 V617F-positive colony formation by PV HPC. Individual hematopoietic colonies were randomly plucked from the cultures; JAK2V617F was detected using a nested allele-specific PCR. At least 50% of colonies were examined if the number of colonies exceeded 50 in an individual dish. If the number of colonies in an individual dish were less than 50, all colonies were plucked and examined. In these experiments, more than 78% of the total number of colonies were plucked and analyzed. Data were analyzed by the Student t test (**P < .01; n = 14). (C) Treatment with 200 U/mL Peg-IFNα 2a combined with 250 nM of ABT-737 significantly decreased the number of JAK2V617F homozygous hematopoietic colonies compared with treatment with Peg-IFNα 2a alone (*P < .05; n = 9). In panels B and C, different symbols stand for the different samples.