FLK2 FLT3 LIGAND IS A POTENT COFACTOR FOR THE GROWTH OF PRIMITIVE B-CELL PROGENITORS/

We have investigated the role of flk2/flt3 ligand (FL) in B cell lymph opoiesis, The ability of FL to stimulate the growth of immature B cell s was assessed using distinct populations: CD43(low)B220(+) pre-B cell s, CD43(+)B220(+) pro-B cells, and CD43(+)B220(low) progenitors, FL fa iled to affect the growth of the pro-B or pre-B cells whether used alo ne or in combination with stem cell factor (SCF) or IL-7. In striking contrast, FL was a potent cofactor for the CD43(+)B220(low) progenitor cells, interacting with either IL-7 and/or SCF to stimulate their gro wth, The combination of FL with IL-7 plus SCF stimulated maximum expan sion of these cells, albeit, less than that stimulated in stromal cell cultures, When the CD43(+)/B220(low) progenitors were divided based o n expression of heat stable Ag (CD24) into a CD24(-) and a CD24(+) sub set, the FL-responsive cells were contained only within the CD24(-) su bset. FL interacted with SCF or with IL-7 to stimulate their growth re sulting in a 20- and 50-fold increase in cellularity, respectively, Si nce the CD24(-) subset was the most immature of the B cell populations studied, our data suggest that FL costimulates the expansion of very primitive B cell progenitors.