Long-term follow-up study on the engraftment of human hematopoietic stem cells in sheep

Advancements in medical research using fetal sheep: Implications for human health and treatment methods

Sheep are typically considered as industrial animals that provide wool and meals. However, they play a significant role in medical research in addition to their conventional use. Notably, sheep fetuses are resistant to surgical invasions and can endure numerous manipulations, such as needle puncture and cell transplantation, and surgical operations requiring exposure beyond the uterus. Based on these distinguishing characteristics, we established a chimeric sheep model capable of producing human/monkey pluripotent cell-derived blood cells via the fetal liver. Furthermore, sheep have become crucial as human fetal models, acting as platforms for developing and improving techniques for intrauterine surgery to address congenital disorders and clarifying the complex pharmacokinetic interactions between mothers and their fetuses. This study emphasizes the significant contributions of fetal sheep to advancing human disease understanding and treatment strategies, highlighting their unique characteristics that are not present in other animals.

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.