The sphingosine 1-phosphate receptor agonist FTY720 supports CXCR4-dependent migration and bone marrow homing of human CD34+ progenitor cells

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Abstract
The stem cell pool can be fractionated by using the mitochondrial dye, rhodamine-123, into Rholow hematopoietic stem cells and Rhohigh progenitors. Rholow stem cells permanently engraft all lineages, whereas Rhohighprogenitors transiently produce erythrocytes, without substantial platelet or granulocyte production. We hypothesized that the inability of the Rhohigh cells to produce platelets in vivo was due to the fact that these cells preferentially engraft in the spleen and lack marrow engraftment. Initially, we demonstrated that Rhohigh progenitors produced more megakaryocytes in vitro than Rholow stem cells did. To study the activity of the Rholow and Rhohighsubsets in vivo, we used mice allelic at the hemoglobin and glucose phosphate isomerase loci to track donor-derived erythropoiesis and thrombopoiesis. Rholow stem cells contributed to robust and long-term erythroid and platelet engraftment, whereas Rhohigh progenitors contributed only to transient erythroid engraftment and produced very low numbers of platelets in vivo. Donor-derived megakaryopoiesis occurred at higher densities in the spleen than in the bone marrow in animals receiving Rholowstem cells and peaked around day 28. Blockade of splenic engraftment using pertussis toxin did not affect the peak of splenic megakaryopoiesis, supporting the hypothesis that these megakaryocytes were derived from progenitors that originated in the bone marrow. These data emphasize that in vitro behavior of hematopoietic progenitor cell subsets does not always predict their behavior following transplantation. This study supports a major role for the spleen in thrombopoiesis following engraftment of transplanted stem cells in irradiated mice.

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