Erythropoietin-dependent and erythropoietin-independent enhancement of colony formation by immature erythroid progenitors (BFUe).

The erythropoietin (epo)-dependent burst forming activities (BFA) of foetal calf serum (FCS), mouse bone marrow cells (BFA-cell) and lectin-stimulated mouse spleen conditioned medium (BFA-mscm) were investigated. Burst enhancement by BFA-mscm was independent of FCS concentration above 5% FCS. The activity of BFA-cell was directly proportional to FCS concentration. There was a high correlation for the relationship between the logarithm of total cell concentration and the logarithm of burst number for each concentration of FCS. Burst formation in the presence of BFA-cell showed a high epo requirement whereas plateau numbers of bursts were evident at concentrations of 1 to 2 units of epo/ml in cultures containing BFA-mscm. The epo-independent developmental activities (EIDA) of BFA-cell and BFA-mscm were examined in methylcellulose cultures to which the addition of epo was delayed for up to four days. Under all conditions examined, the number of bursts formed in the presence of BFA-cell declined with increasing delay of epo addition. Studies on the kinetics of haemoglobin synthesis in such experiments demonstrated that this decline in colony number was associated with a delay in haemoglobin synthesis. Burst formation in cultures containing various levels of BFA-mscm was independent of the time of epo addition. These results indicate that the burst enhancing ability of BFA-mscm is directly related to its EIDA whereas bone marrow cells appear to lack this activity.