Threshold levels of Flt3-ligand are required for the generation and survival of lymphoid progenitors and B-cell precursors

The generation of B-cell precursors (BCP) from lymphohematopoietic progenitors (LHP) in bone marrow is dependent on signals provided by the receptor tyrosine kinase Flt3 and its ligand, Flt3-ligand (FL). Mice deficient in FL exhibit striking reductions in LHP and BCP. Currently, the mechanism by which Flt3 regulates lymphoid lineage/B-cell development is unknown. Here, we show that haploinsufficiency of FL (FL^+/-) reduced the numbers of LHP, common lymphoid progenitors, and pro-B cells, suggesting that FL levels set a threshold for B lymphopoiesis. Limiting dilution analysis confirmed reduced BCP frequency in FL^+/- mice. Real-time PCR of LHP from FL^+/- animals showed increased transcripts of the B lineage inhibitor id1. However, targeted deletion of id1 did not restore the lymphoid/B lineage deficiencies in FL^-/- mice, supporting Id1-independent mechanisms. BrdU incorporation studies established that FL is not essential for the proliferation of Flt3⁺ multipotential progenitors. Analysis of FL^-/- progenitors expressing low levels of Flt3 revealed decreased levels of the pro-survival factor Mcl1. Consequently, the Flt3⁺ LHP progeny of Flt3^low LSK⁺ cells exhibited increased Annexin V staining. Together, these data suggest that Flt3 signaling initiates a cascade of events in Flt3^low precursors that promote the survival of LHP from which BCP are derived.