Changes in reticulocyte fractions during peripheral stem cell harvesting: role in monitoring stem cell collection

We prospectively evaluated the changes in immature reticulocyte fractions during peripheral blood stem cell mobilization to determine any possible relationship with mobilization of stem cells into the peripheral blood. Circulating neutrophils, immature reticulocyte fractions (% of HFR + MFR) (HMFR) (measured by flow cytometry), circulating CD34+ cells (measured by flow cytometry) and CFU-GM (measured by semisolid media assay in the apheresis fluid) were closely monitored following priming with chemotherapy and colony-stimulating factors in 15 patients with hematological or solid tumors (group I). Day 0 was defined as the day on which the neutrophil count fell below 0.5 x 10(9)/l. In a second group of nine patients (group II) reticulocyte fractions and CD34+ cells were measured directly on the days on which they were predicted to increase using the data from group I. Reticulocyte counts and HMFR were also monitored in 18 patients who were mobilized with G-CSF alone. In group I, a significant rise in HMFR and CD34+ cells occurred on days 2, 4 and 6, and a linear correlation between HMFR on day 2 and CD34+ cells on day 4 was demonstrated (P = 0.0068, r = 0.74). In group II similar patterns of recovery were found. During mobilization with G-CSF alone HMFR significantly increased on days 2, 4 and 6 of treatment with respect to baseline values, and a multiplicative relationship between the increase of HMFR and neutrophils was observed (r = 0.707, P < 0.00001). Unfortunately, patients who did not mobilize CD34+ cells (one in groups I and II and three in the G-CSF group) showed similar HMFR kinetics to those who mobilized CD34+ cells. An increase in immature reticulocyte fractions precedes the presence of circulating CD34+ cells by about 2 days in patients mobilized with chemotherapy and growth factors, and it could thus serve as an indirect surrogate marker for monitoring the timing of stem cell harvesting. An unexpected increase of HMFR during treatment with G-CSF alone was found, indicating an effect of this factor on erythropoiesis in vivo.