Relationship between Thy‐1 expression and cell‐cycle distribution in human bone marrow hematopoietic progenitors

Analysis of the relationship between Thy‐1 expression and cell‐cycle distribution of hematopoietic stem cells (HSCs) showed that freshly isolated Thy‐1 + and Thy‐1 − subsets of the CD34 high CD38 − flt‐3 − Lin − population were predominantly in G 0 /G 1 phase and remained essentially quiescent, whereas after 6 days of cytokine stimulation, the Thy‐1 + subset of the population entered the cycling state while the Thy‐1 − subset again remained quiescent. Expression of Thy‐1 antigen resulted in a drastic increase in the percentage of cycling cells in CD34 high CD38 ‐ flt‐3 ‐ Lin ‐ Thy‐1 + ‐ as well as CD34 high CD38 − flt‐3 − Lin −  Thy‐1 − ‐cell‐initiated cultures. The Thy‐1 + subset of the CD34 high CD38 − flt‐3 − Lin − population exists in the freshly isolated CD34 high CD38 − flt‐3 − Lin −  Thy‐1 + fraction, loses Thy‐1 expression during 6 days, and re‐expresses Thy‐1 for an additional 2 days. Cell‐cycle analysis demonstrated that this unique subset contains abundant S/G 2 M cells. Thus, Thy‐1 expression appears to be an indicator of cell‐cycle phase in targeting HSC, which might serve in the cell subset best suited for gene transfer. Am. J. Hematol. 79:187–193, 2005. © 2005 Wiley‐Liss, Inc.