CD34-negative hematopoietic stem cells show distinct expression profiles of homing molecules that limit engraftment in mice and sheep

We and others have reported that human hematopoietic stem cells (HSCs) are also present in the CD34-negative (CD34⁻) fraction of human cord blood (CB). Here, we examined the hematopoietic engraftment potential of 13 or 18 lineage-negative (13Lin⁻ or 18Lin⁻) CD34^+/− cells from human CB in mice and sheep. Both 13Lin⁻ and 18Lin⁻ CD34⁺ cells efficiently engrafted in mice irrespective of transplantation route, be it by tail-vein injection (TVI) or by intra-bone marrow injection (IBMI). These cells also engrafted in sheep after in utero fetal intra-hepatic injection (IHI). In contrast, neither 13Lin⁻ nor 18Lin⁻ CD34⁻ cells engrafted in either mice or sheep when transplanted by regular routes (i.e., TVI and fetal IHI, respectively), although both 13Lin⁻ and 18Lin⁻ CD34⁻ cells engrafted in mice when transplanted by IBMI and exhibited multilineage reconstitution ability. Thus, the homing ability of CD34⁻ HSCs is significantly more limited than that of CD34⁺ HSCs. As for 18Lin⁻, CD34⁻ HSCs are characterized by low expression of the tetraspanin CD9, which promotes homing, and high expression of the peptidase CD26, which inhibits homing. This unique expression pattern homing-related molecules on CD34⁻ HSCs could thus explain in part their reduced ability to home to the BM niche.