APRIL promotes MK expansion and platelet production. (A) Cycling cord blood CD34⁺ cells were transduced with empty vector (P), with APRIL (APRIL) or BLyS (BLyS) cDNA and analyzed by immunoblot for APRIL and BLyS expression after cytofluorimetric sorting based on GFP⁺ cells. 293 cells transfected with empty vector (P) or with APRIL (APRIL) or BLyS (BLyS) cDNA were used as controls. (B) Cell growth of cells transduced and sorted as described in panel A and cultured in unilineage MK system from day 0 to day 3 and from day 5 to day 8. (C) Cell growth of cord blood CD34⁺ cells cultured in standard unilineage MK system (control) or in the presence of TACI-Fc or Fn14-Fc recombinant soluble proteins (Alexis). (D) Platelet production from unilineage MK culture or in the presence or in the absence of recombinant soluble TACI-Fc. Error bars indicate ± SD. ^*P < .01, ^**P < .001.