Reversible Cell Surface Expression of CD38 on CD34-positive Human Hematopoietic Repopulating Cells

Objective: Although increased expression of CD38 on the surface of human CD34(+) cells is associated with differentiation, we reported recently that both lineage-negative (Lin(-)) CD34(+)CD38(-) and Lin(-)CD34(+)CD38(lo) fractions of cord blood contain primitive severe combined immunodeficient (SCID)-repopulating cells (SRC). Thus, it is important to determine if a hierarchical relationship exists between the SRC from these two populations or if CD38 is reversibly expressed.

Materials And Methods: To determine if SRC from the CD34(+)CD38(-) and CD34(+)CD38(lo) cell fractions could generate SRC of the same and/or alternate CD38 expression, cells from primary nonobese diabetic/SCID mice transplanted with CD34(+)CD38(-) cells were resorted into both CD34(+)CD38(-) and CD34(+)CD38(lo) fractions and injected into separate secondary recipients, which were evaluated for human cell engraftment 7 to 10 weeks later. As primary mice transplanted with CD34(+)CD38(lo) cells also contained cells of both immunophenotype, these cells were also resorted and transplanted into separate secondary recipients. The cell-cycle status of various CD34(+) SRC fractions were evaluated using Hoechst 33342 and Pyronin Y staining in order to determine if CD38 expression was coordinated with divisional activation.

Results: Each cell fraction obtained from primary recipients was able to reconstitute secondary mice, indicating that CD38 expression reversibly oscillates between negative and low levels on CD34(+) repopulating cells. CD38 expression on repopulating cells correlated with a transition between the G(0) and G(1) phases of the cell cycle.

Conclusion: CD38 is reversibly expressed on CD34(+) SRC between negative and low levels and corresponds to a change in the cell-cycle state. These observations establish a foundation to uncover the molecular program of stem cell regulation and underscore the importance of functional assessments when isolating and characterizing human hematopoietic stem cells.