Human FLT3/FLK2 Targets Hematopoietic Stem Cells and Granulocyte/Macrophage Progenitors.

FLT3/FLK2, a member of the receptor tyrosine kinase family, plays a critical role in maintenance of hematopoietic homeostasis, and the constitutively active form of the FLT3 mutation is one of the most common genetic abnormalities in acute myelogenous leukemia. In murine hematopoiesis, Flt3 is not expressed in self-renewing long-term hematopoietic stem cells (LT-HSCs), but its expression is restricted to the multipotent and the lymphoid progenitor stages at which cells are incapable of self-renewal. In order to test whether Flt3 expression can delineate such a developmental pathway also in human hematopoiesis, we have analyzed the expression of human Flt3 (hFlt3) in prospectively-purified human stem and progenitors (PNAS 2002) by utilizing 7-color FACS and a highly efficient xenograft systems. We have found that Flt3 expression in early hematopoiesis is completely different between human and mice: hCD34+hCD38-hCD90+Lin-LT-HSCs capable of long-term reconstitution in xenogeneic hosts uniformly express hFlt3, and its expression is upregulated through hCD34+hCD38+hCD45RA-hCD123+Lin-common myeloid progenitors (CMPs) to hCD34+hCD38+hCD45RA+hCD123+Lin-granulocyte/macrophage progenitors (GMPs), but hCD34+hCD38+hCD45RA-hCD123-Lin- megakaryocyte/erythrocyte progenitors (MEPs) shut off its expression in human. Furthermore, we have also demonstrated that hFlt3 signaling can prevent stem and progenitors from apoptotic cell death in vitro without any effects on lineage fate decision. Next, we tried to find key molecules for Flt3-Flt3 ligand (FL)-mediating anti-apoptotic effect. First, we tested expression pattern of anti-apoptotic Bcl-2 family genes in HSCs, CMPs, GMPs, MEPs and common lymphoid progenitors (CLPs) in human hematopoiesis. Mcl-1, an indispensable survival factor for murine hematopoiesis (Science, 2005), was also expressed at the highest level in human HSCs, whereas Bcl-2 and Bcl-xL was highly expressed in GMPs and MEPs, respectively. Next, we examined whether FL stimulation can upregulate the expression of Bcl-2 family genes in human purified HSCs and progenitors. FL significantly upregulated the expression of Mcl-1, but not of Bcl-2 or Bcl-xL in HSCs as well as CMPs and GMPs. In conclusion, our data show that the distribution of Flt3 is quite different in mouse and human hematopoeisis. Human Flt3 targets LT-HSCs and myeloid progenitors except for MEPs. Flt3 signaling might support cell survival in early hematopoiesis including the HSC and the myeloid progenitor stages through upregulation of Mcl-1. This is a striking example that the expression pattern of key molecules could be significantly different between human and mouse.