Hematopoietic Stem - and Progenitor Cells Are Differentially Mobilized in Response to Flt3-Ligand

Flt3 is a tyrosine kinase receptor expressed mainly on primitive hematopoietic cells and its ligand, Flt3-ligand (FL), is a slowly mobilizing agent in mice when administered for 5–10 days. This provides a time-frame to study the HPC and HSC migration kinetics in detail. BALB/c mice were injected for 3, 5, 7 and 10 days with FL (10 ug/day, intraperitoneally). Mobilization of hematopoietic stem- and progenitor cells was studied using colony-forming-unit granulocyte/monocyte (CFU-GM) and cobblestone-area-forming-cell (CAFC) assays. The radioprotective capacity of mobilized peripheral blood mononuclear cells (PBMC) was studied by transplantation of 1.5 × 10⁶ FL-mobilized PBMNC into lethally irradiated (9.5 Gy) recipients. Hematopoietic progenitor cell mobilization was detected from day 3 onwards and prolonged administration of FL showed a steady increase in mobilized progenitor cells (CFU-GM; 23.1±20.1 [PBS], 219.0±213.7 [3 days FL, n=10], 1,007.8±742 [5 days FL, n=21], 3,526.6±1,406 [7 days FL, n=10] and 19,149±2,338 [10 days FL, n=10]). Administration of FL for 10 days lead to a 5.5-fold increase in CAFC-week 4, compared to FL administration for 5 days (0.69 vs 0.13 CAFC per 10⁵ PBMC respectively) and a 5.0-fold increase in CAFC-week 5 (0.27 vs 0.05 CAFC per 10⁵ PBMC respectively). No CAFC week 4/5 were found in PBMC obtained from PBS-injected control mice. Transplantation of 5-day FL-mobilized PBMC did not radioprotect lethally irradiated recipients (mean survival time (MST): 66 days; MST PBS: 13 days). In contrast, transplantation of PBMC obtained from a 10-day FL mobilization regime radioprotected 100% of the recipients (Table 1). These results indicate that HPC and HSC show different mobilization kinetics in response to FL, resulting in preferential mobilization of HPC at day 5, followed by HSC mobilization at day 10. We speculate that this difference in mobilization kinetics might be due to the selective recruitment of HPC and HSC from their respective bone marrow compartments.

Table 1. HPC and HSC frequencies in peripheral blood following Flt3-ligand-induced mobilization