FLT3 FLK-2 AND C-KIT ARE NOT ESSENTIAL FOR THE PROLIFERATION OF B-LYMPHOID PROGENITOR CELLS IN THE BONE-MARROW OF THE ADULT-MOUSE/

The receptor tyrosine kinase Flk-2/Flt3 was originally cloned from hem atopoietic stem cell-enriched fetal liver and placenta and is believed by some investigators to play a role in the regulation of the hematop oietic stem cell. However, targeted disruption of the flt3 gene result s in a specific deficiency in early B cell progenitors. Using an antag onistic monoclonal antibody developed against the extracellular domain of Flt3, we investigated the expression and function of the molecule on B lymphoid lineage cells in the bone marrow (BM) of adult mice. App roximately 10% of B220(+) cells in the BM expressed Flt3 on the cell s urface, and most of the cells belonged to a pro-B cell fraction when j udged by an expression pattern of CD43, heat-stable antigen, and BP-1. However, B lymphoid precursor cells that are clonable in vitro could not be enriched in the B220(+)/Flt3(+) cell fraction sorted by flow cy tometry. Furthermore, proliferation of B lymphoid precursor cells in t he adult BM was not blocked by administration of the antagonistic mono clonal antibodies against Flt3 and c-Kit, suggesting that signalings m ediated by Flt3 and c-Kit receptors are not essential for the prolifer ation of B cell progenitors in adult mouse BM.