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

Abstract

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.