Expression change of Flk-2/Flt-3 on murine hematopoietic stem cells in an activating state

The receptor tyrosine kinase Flk-2/Flt-3 (Flt-3) represents an important molecule involved in early hematopoiesis. Murine hematopoietic stem cells (HSCs) have been shown to be negative for the expression of Flt-3. We now present clear evidence for the expression change of Flt-3(-) HSCs in an activating state, and the reversibility of Flt-3 expression by HSCs in vivo.

Bone marrow cells isolated from Ly5.1 mice were sorted on the basis of Flt-3 expression and transplanted into lethally irradiated Ly5.2 recipients. After 24 weeks, peripheral blood was analyzed for donor contribution by flow cytometry.

Although long-term engraftment was predominantly detected in Flt-3(-) populations as previously described, a 6-day cultivation of Lin(-/low)c-kit(+)Sca-1(+) Flt-3(-) bone marrow cells with stem cell factor and interleukin-11 resulted in the generation of Flt-3(+) HSCs with long-term engraftment capabilities. However, the Flt-3 ligand had no significant effect on self-renewal of the Flt-3(+) HSCs. Next, to examine reversible expression of this receptor molecule, Flt-3(+) cells converted in vitro from Ly5.1 Lin(-/low)c-kit(+)Sca-1(+) Flt-3(-) bone marrow cells were isolated and transplanted into Ly5.2 primary recipients. After 24 weeks, Ly5.1 Lin(-/low) bone marrow cells were again separated into Flt-3(-) and Flt-3(+) cells and retransplanted into Ly5.2 secondary recipients. The majority of donor HSCs with long-term engraftment capabilities were detected in the Flt-3(-) populations, indicating the reversion of Flt-3(+) to Flt-3(-) HSCs.

These observations suggest that Flt-3 is a useful cell-surface marker of HSC activation and that this phenotypic change is reversible.