Contact with the bone marrow microenvironment readdresses the fate of transplanted hematopoietic stem cells.

Objective. Despite enormous advancements in our comprehension of molecular mechanisms governing hematopoietic stem cells (HSCs) engraftment in the bone marrow, current clinical protocols of intravenous (IV) transplantation suffer from a relatively low seeding efficiency. To solve this problem, intrabone (IB) injection of HSCs has been proposed. However, the mechanisms underlying the benefit provided by this procedure remain unknown. This study aims to evaluate the effect of IB on HSCs trafficking and homing features in the living rat. Materials and Methods. A total of 35 Lewis rats underwent IB or IV administration of HSCs harvested from syngeneic animals and purified according to CD90 expression. These cells were labeled with 37 MBq 99mTc-exametazime and injected either IV or LB. Cell trafficking and distribution in heart, lung, spleen, liver, and forelimb was evaluated by dynamic radionuclide imaging. Logan graphical approach was used to estimate tissue recruitment of HSCs. Results. More than 90% of cells escaped from the injected bone to the bloodstream in <15 seconds. However, this short contact profoundly modified HSCs kinetics, reducing their lung sequestration and shortening their blood persistence with respect to IV. More importantly, IB passage resulted in reduced lung uptake and in a fourfold increase in homing of remote bone marrow sites. CD90(+) cells transplantation restored hematopoiesis in eight further rats previously exposed to lethal irradiation. Conclusion. The first-entry contact with the hematopoietic microenvironment immediately readdresses the fate of transplanted HSCs, providing them with "the final destination stamp" to define their bone marrow homing.