Human CD34- hematopoietic stem cells:: Basic features and clinical relevance

CD34 has been widely used as a stem and progenitor cell marker, and clinical CD34 stem cell transplantation (CD34(+) SCT) has been performed for tumor purging and for prevention of graft-versus-host disease. Recently CD34-neoative hematopoietic stem cells (CD34(-) HSCs) were identified in mice and humans. Xenogeneic chimera engraftment assays with immunodeficient mice or preimmune fetal sheep resulted in identification of human CD34(-) HSCs in cord blood, bone marrow, and granulocyte colony-stimulating factor-mobilized peripheral blood, although no significant clonogenic activity was detected in vitro. These characteristics of CD34(-) HSCs make the assessment of clinical samples difficult. The generation of CD34(-) HSCs from CD34(-) cells in vitro may be a surrogate assay for detecting CD34(-) HSC activity. This approach was used in recipients of CD34(+) SCT and revealed the absence of a CD34(-) precursor population. The identification of a positive marker in CD34(-) HSCs and the development of a simpler and more efficient in vivo assay for CD34(-) HSCs may allow the diagnostic evaluation of human CD34(-) HSCs in various clinical procedures and diseases. Int J Hematol. 2002:75:370-375. (C)2002 The Japanese Society of Hematology.