THE FLT3 LIGAND PROMOTES THE SURVIVAL OF PRIMITIVE HEMATOPOIETIC PROGENITOR CELLS WITH MYELOID AS WELL AS B-LYMPHOID POTENTIAL - SUPPRESSION OF APOPTOSIS AND COUNTERACTION BY TNF-ALPHA AND TGF-BETA

The recently cloned flt3 ligand (FL) potently enhances hemopoietic gro wth factor-induced growth of primitive hemopoietic progenitors, In agr eement with previous reports, we found FL alone to be a weak mitogen f or primitive Lin(-)Sca-1(+) murine bone marrow progenitors, Using dela yed addition of growth-promoting cytokines, we demonstrate that FL pot ently promotes the in vitro survival of Lin(-)Sca-1(+) progenitors res ponsive to a potent myeloid growth factor combination (FL, stem cell f actor (SCF), granulocyte CSF (C-CSF), and IL-1 alpha). Whereas no such progenitors survived in cultures supplemented with medium alone, 33% survived in FL compared with 75 and 13% in the presence of SCF and IL- 1 alpha; respectively. These results were obtained when cells were pla ted individually, suggesting that the viability-promoting effect of FL is mediated directly on the progenitors. Whereas SCF was superior to FL in promoting the survival of FL-, SCF-, G-CSF-, and IL-1 alpha-stim ulated Lin(-)Sca-1(+) progenitors, FL was more efficient than SCF at p romoting the survival of progenitors with a B cell potential, as measu red by their ability to produce B220(+) cells in response to delayed a ddition of FL, SCF, and IL-7. Seventy-one percent of the B220(+) cell production could be recovered following 40-h incubation with FL compar ed with 2% in response to SCF, Analysis of day 12 spleen CFU content a fter 40-h preincubation of Lin(-)Sca-1(+) cells in FL or SCF demonstra ted that SCF maintained 64% of the day 12 spleen CFU, whereas only 16% survived in the presence of FL. Finally, there was no significant dif ference between the ability of FL and SCF to maintain the viability of long-term culture-initiating cells (25 and 32%, respectively). The ab ility of FL to promote the survival of Lin(-)Sca-1(+) progenitor cells was reflected by the finding that FL also suppressed apoptosis. Final ly, TGF-beta abrogated and TNF-alpha potently counteracted the surviva l-promoting effect of FL. Thus, FL promotes the survival of primitive hemopoietic progenitor cells, in particular those with an inherent B l ymphoid potential.