Ex Vivo Proliferation and Differentiation of CD34⁺ Cells into Myeloid Progenitor Cells: Cord Blood Versus Mobilized Peripheral Blood.

The purpose of our study was to evaluate the capacities of cord blood (CB) CD34⁺ cells to proliferate and differentiate ex vivo into myeloid lineage in response to cytokines and to compare them with mobilized peripheral blood (MPB) cells. Briefly, 2.5 × 10⁴ CD34⁺ cells, isolated from CB (n=10) and MPB (n= 9), were cultured in 5 ml MacoBiotech HP01 (Macopharma) with SCF, Flt3-L, IL-3 and G-CSF. At day 9, 10⁶ cultured cells were replated for 5 additional days. Cells were counted and evaluated for their CD34, CD13 and CD15 expression. Differentiation into myeloid compartment was assessed by CD11b and CD16 coexpression on CD15⁺ cells. We observed that

1. leucocyte expansion was significantly higher in CB than in MPB at day 9 (24.3±3.8 vs 15.2±1.9) and at day 14 (224.7±54.2 vs 72.9±20.0). A similar difference was observed for CD34⁺ cell expansion (8.7±1.4 vs 3.4±0.5 at day 9 and 31.3±4.6 vs 7.6±2.4 at day 14).

2. at day 9, despite superior CB leucocyte expansion, CD13⁺ and CD15⁺ cell number produced per CD34⁺ cell seeded at day 0 were similar in CB and in MPB (18.5±2.4 vs 14.4±2.5 for CD13⁺ and 7.1±1.3 vs 6.0±1.2 for CD15⁺). Increasing the culture period led to higher numbers of CD13⁺ and CD15⁺ cells in CB than in MPB. This increase was due to a total leucocyte expansion rather than to high CD13⁺ and CD15⁺ cell percentage.

3. The distribution of CD11b⁻CD16⁻, CD11b⁺CD16⁻ and CD11b⁺CD16⁺ subpopulations in CD15⁺ cells was comparable in CB and in MPB after 9 days of culture, with a majority of relatively immature CD11b⁻CD16⁻ myeloid progenitor cells.

However, after 5 additionnal days of culture, MPB CD15⁺ cells expressed a more mature phenotype than CB CD15⁺ cells, with a dramatic increase of CD11b⁺CD16⁻ cells (promyelocytes and myelocytes). In conclusion, our study suggests that, despite the high CB cell capacity of expansion in our culture conditions, CB CD34⁺ cell differentiation process into myeloid lineage appears to be slower. This difficulty of CB cells to reach maturation in vitro is likely to be related with the longer delay of neutrophil recovery after CB transplantation.