Exploring the heterogeneity of the hematopoietic stem and progenitor cell pool in cord blood: simultaneous staining for side population, aldehyde dehydrogenase activity, and CD34 expression.

Background: The stem cell content in cord blood (CB) units is routinely assessed regarding nucleated cells, CD34+ cell count, and number of colony-forming units (CFUs). Efforts are made toward finding better ways of defining stemness of CB units. Side population (SP) phenotype and activity of aldehyde dehydrogenase (ALDH) are functional markers of stemness that can be assayed using flow cytometry.

Study Design And Methods: We have developed a protocol for simultaneous determination of CD34+, SP, and ALDH+ populations in relation to immature white blood cells (CD45dim) in CB. Viable nucleated cells were consecutively stained for SP and ALDH activity and with antibodies against the CD45, CD34, and CD117 antigens.

Results: The SP and ALDH+ populations could reliably be measured simultaneously. The median sizes of the SP and the ALDH+ populations were 0.85 and 3.3% of CD45dim cells, respectively. There was no overlap between the SP and ALDH+ populations. Cells that were ALDH+ expressed CD34 and CD117, but SP cells were negative for these markers. The ALDH+ cell content correlated with CD34+ cell content (p < 0.001) and with CFU-granulocyte-macrophage (GM; p = 0.03) but not with total CFUs. SP did not correlate with CD34+, CFU-GM, or total CFU.

Conclusions: We show that simultaneous detection of the CD34, SP, and ALDH+ cells is clearly feasible using only small amounts of CB. In CB, ALDH+, and CD34+ cells are overlapping populations distinctly separated from the SP population. The difference in relation to the capacity for colony growth between ALDH+ and SP underlines that they define different cell populations.