Clinical stem-cell sources contain CD8+CD3+ T-cell receptor-negative cells that facilitate bone marrow repopulation with hematopoietic stem cells

Clinical observations in patients undergoing bone marrow transplantation implicate the involvement of CD8⁺ cells in promoting the stem-cell engraftment process. These findings are supported by mouse transplant studies, which attributed the engraftment-facilitating function to subpopulations of murine CD8⁺ cells, but the analogous cells in humans have not been identified. Here, we report that clinical stem-cell grafts contain a population of CD8α⁺CD3ε⁺ T-cell receptor-negative cells with an engraftment facilitating function, named candidate facilitating cells (cFCs). Purified cFC augmented human hematopoiesis in NOD/SCID mice receiving suboptimal doses of human CD34⁺ cells. In vitro, cFCs cocultured with CD34⁺ cells increased hematopoietic colony formation, suggesting a direct effect on clonogenic precursors. These results provide evidence for the existence of rare human CD8⁺CD3⁺TCR^- cells with engraftment facilitating properties, the adoptive transfer of which could improve the therapeutic outcome of stemcell transplantation.