The proliferation kinetics and clonogenic activity of CD34(+)/38(hl) (CD38(hi)) and CD34(+)/38(lo) (CD38(lo)) human marrow cells were measured before and after culturing the cells in vitro over a 6-day period in serum-deprived medium containing recombinant growth factors (interleukin-1 [IL-1], IL-3, IL-6, granulocyte colony-stimulating factor [C-CSF], granulocyte-macrophage [GM]-CSF, kit ligand, and erythropoietin). Before in vitro culture, 3% +/- 3% of the CD38(lo) and 13% +/- 2% of the CD38(hi) cells were in the S-phase of the cell cycle. The clonogenic activity of CD38(hi) cells was twofold greater than that of the CD38(lo) cells, as measured by colony-forming units (CFU) in short-term assays. However, CD38(hi) cells contained fewer pre-CFU than did the CD38(lo) cells, generating only 3 +/- 2 colonies per 1,000 cells after 4 weeks of culture on competent stromal layers, compared with 107 +/- 46 colonies per 1,000 cells from the CD38(lo) population. CD38(hi) and CD38(lo) cells exhibited distinctly different responses when cultured in serum-deprived medium supplemented with recombinant growth factors. After culturing cells for 24 hours, CD38(lo) cells essentially remained a noncycling population with only 5.1% +/- 3.0% of the cells cycling, whereas 44.2% +/- 6.9% of the CD38(hi) cells were in DNA synthesis. Gradually CD38(lo) cells were recruited into cycle, such that by 72 hours, approximately 28% of the CD38(lo) cells were in S-phase. However, during 6 days of culture, the percentage of cycling CD38(lo) cells never exceeded the proliferative response observed for CD38(hi) cells. Phenotype analysis conducted at day 6 indicated that 86% of the CD38(hi) population were no longer phenotypically CD34(+)/38(hi), while 60% of CD38(lo) cells maintained a CD34(+)/38(lo) phenotype. Long-term cultures initiated with B-day in vitro-expanded CD38(lo) cells showed approximately a twofold decrease in clonogenic activity attributable to a loss of erythroid precursors and a decrease in GM colonies. Thus, a proportion of CD38(lo) cells capable of generating CFU was maintained even after exposure to growth factors. (C) 1995 by The American Society of Hematology.
