Differentiation kinetics and globin gene expression by circulating human BFUe in suspension cultures

We studied the kinetics of erythroid differentiation and the globin synthetic patterns of circulating early erythroid progenitors (erythroid burst-forming units, BFUe) stimulated to differentiate in suspension cultures in the presence of interleukin 3 (IL-3) and erythropoietin. Erythroid progenitor cells present at the onset of culture and on successive days (2-12) thereafter were quantitatively assessed by clonal assays, whereas globin synthesis was measured sequentially in aliquots from the suspension culture. Although BFUe numbers increased to a peak value by day 4, the number of progenitors generating larger bursts was progressively decreasing with a concomitant increase in the number of smaller sized bursts. Erythroid colony-forming units (CFUe) and erythroid clusters were first detected by day 4 and peaked on day 6. Proerythroblasts were morphologically identifiable on day 4, and they progressively increased in number and maturity so that, at culture days 10 and 12, 51% and 59% of the culture cells were erythroblasts, respectively. In keeping with the morphologic changes during the liquid culture, globin mRNA was first detected on day 4. gamma/gamma + beta mRNA ratios were highest on days 4 and 6 and declined thereafter. Our results show that circulating BFUe (at least the majority of them) can differentiate and mature as a cohort in suspension cultures, providing terminal progeny with accelerated kinetics compared to semisolid, clonal cultures. In this system the same cohort of cells can be easily sampled throughout the culture for molecular studies on erythroid differentiation.