The in vitro growth of a cord blood-derived cell population enriched for CD34+ cells is influenced by its cell cycle status and treatment with hydroxyurea

Objective: Cell cycle plays a fundamental role in the physiology of hematopoietic stem and progenitor cells. In the present study we used a negative selection system to obtain an immature cell population enriched for cord blood derived CD34(+) cells-and we determined its proliferation, expansion and differentiation patterns as a function of the cell cycle status. The effects of hydroxyurea (HU) were also assessed. Results: As compared with cells in synthesis (S)/Gap2 (G2)/mitosis (M), cells in quiescent state (G0)/Gap1 (G1) showed a higher proliferation potential in vitro. At culture onset, G0, G1 and S/G2/M cells corresponded with 63%, 33% and 4%, respectively. Treatment with HU before culture resulted in an increase in the proportion of cells in G1 with a concomitant decrease in S/G2/M cells, without affecting the proportion of cells in G0. After 3 days of culture in the presence of recombinant cytokines, the vast majority of the cells (90%) were in G1, and by day 8, G0, G1 and S/G2/M cells corresponded with 18%, 67% and 15%, respectively. HU also induced an increase in colony-forming cell (CFC) frequency, in the proliferation and expansion capacities of cultured cells under myeloid conditions, and favored the development of the erythroid lineage. Conclusion: Our results show that the in vitro proliferation, expansion and differentiation potentials of immature hematopoietic cells are determined, at least in part, by their cell cycle status and that the cell cycle modifier HU significantly influences the growth of human hematopoietic cells. These results are of potential relevance for the development of ex vivo expansion protocols.