Refining the Migration and Engraftment of Short-term and Long-term HSCs by Enhancing Homing-specific Adhesion Mechanisms

In contrast to the short-term (ST) CD34+ stem cells, studies have suggested that long-term (LT) hematopoietic stem cells (HSCs) found in the CD34- stem cell pool have trouble migrating and engrafting when introduced through IV. To understand why these deficiencies exist, we set out to fully elucidate the adhesion mechanisms used by ST and LT-HSCs to migrate to the bone marrow(BM). Specifically focusing on murine ST-HSCs (Flk2-CD34+) and LT-HSCs (Flk2-CD34-), we observed a distinctive expression pattern of BM homing effectors necessary for the first step, namely sialyl Lewis-X (sLex) (ligand for E-selectin), and the second step, namely CXCR4 chemokine receptor (receptor for SDF-1). sLex expression was higher on Flk2-CD34+ ST-HSCs (>60%) compared with Flk2-CD34- LT-HSCs (<10%), which correlated to binding to E-selectin. Higher concentrations of CXCR4 were observed on Flk2-CD34+ ST-HSCs compared with Flk2-CD34- LT-HSCs. Interestingly, the expression of CD26, a peptidase known to deactivate chemokines (ie, SDF-1), was higher on Flk2-CD34- LT-HSCs. Given that both E-selectin-binding and CXCR4-mediated migration are compromised in Flk2-CD34- LT-HSCs, we aimed to enhance their ability to migrate using recombinant human fucosyltransferase 6 (rhFTVI) and the CD26 inhibitor, Dip A (diprotin A). To this end, we observed that although LT-HSCs expressed low concentrations of sLex, they were able to engraft when transplanted into recipient mice. Moreover, although both CD26 inhibition and fucosylation enhanced migration of both HSC populations in vitro, only pretreatment of LT-HSCs with Dip A enhanced engraftment in vivo after transplantation into recipient mice. Remarkably, fucosylation of Flk2-CD34+ ST-HSCs consistently led to their ability to transplant secondary recipients. These data suggest that using fucosylation and Dip A to overcome the molecular disparity in adhesion mechanisms among ST-HSCs and LT-HSCs differentially influences their abilities to migrate and engraft in vivo and promotes the ability of ST-HSCs to engraft secondary recipient mice, the gold standard for testing functionality of LT-HSCs.