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+CD38lo 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+CD38lo 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+CD38lo 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+CD38lo 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 G0 and G1 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.