CD34 Antigen and the MPL Receptor Expression Defines a Novel Class of Human Cord Blood-Derived Primitive Hematopoietic Stem Cells

In the murine hematopoietic stem cell (HSC) compartment, thrombopoietin (THPO)/MPL (THPO receptor) signaling plays an important role in the maintenance of adult quiescent HSCs. However, the role of THPO/MPL signaling in the human primitive HSC compartment has not yet been elucidated. We have identified very primitive human cord blood (CB)-derived CD34– severe combined immunodeficiency (SCID)-repopulating cells (SRCs) using the intra-bone marrow injection method. In this study, we investigated the roles of the MPL expression in the human primitive HSC compartment. The SRC activities of the highly purified CB-derived 18Lin–CD34+/–MPL+/– cells were analyzed using NOG mice. In the primary recipient mice, nearly all mice that received CD34+/–MPL+/– cells were repopulated with human CD45+ cells. Nearly all of these mice that received CD34+MPL+/– and CD34–MPL– cells showed a secondary repopulation. Interestingly, the secondary recipient mice that received CD34+/–MPL– cells showed a distinct tertiary repopulation. These results clearly indicate that the CD34+/– SRCs not expressing MPL sustain a long-term (LT) (>1 year) human cell repopulation in NOG mice. Moreover, CD34– SRCs generate CD34+CD38–CD90+ SRCs in vitro and in vivo. These findings provide a new concept that CD34–MPL– SRCs reside at the apex of the human HSC hierarchy.