Effect of the in vivo priming regimen for peripheral blood stem cells (PBSC) mobilization on in vitro generation of cytotoxic effectors by IL-2 activation of PBSC in a murine model.

Priming of patients with different PBSC mobilizing regimens leads to an increase by several fold in circulating hematopoietic progenitors in peripheral blood. However, the effect of these mobilizing regimens on lymphoid cells contained within the harvested PBSC population is not well understood. We have studied the effect of CY and/or G-CSF +/- IL-2 containing regimens on lymphoid cells, and their capacity to give rise to cytotoxic effectors on subsequent in vitro IL-2 activation in a murine model of PBSC mobilization. C57B1/6 mice were given CY 100 or 200 mg/kg on day 0 followed 48 h later by G-CSF 125 micrograms/kg twice a day and/or IL-2 60000 i.u. twice a day in different schedules. Mice were sacrificed on day 4, 6, 8 and 10 following CY and the number of hematopoietic progenitors mobilized to the spleens of these mice was assessed by CFC assay and cytotoxicity was evaluated by 4 h 51Cr release assay against both NK-sensitive (Yak-1), and NK-resistant (B16, C1498) cell lines after 24 h in vitro IL-2 activation in the presence of 6000 i.u./ml of IL-2. Peak numbers of CFC in the splenic PBSC population were seen on day 6 following CY. Administration of CY 200 mg/kg + GCSF, the most potent regimen for CFC mobilization, led to a marked decrease in proportion of CD3+ cells in day 6 PBSC as compared to controls (17.7% vs 3.9%) and was associated with a significant decrease in generation of cytotoxic cells after IL-2 activation. Combining IL-2 to CY + G-CSF prevented the marked loss in cytotoxicity associated with this regimen without any decrease in number of CFC mobilized. When IL-2 was combined with CY without G-CSF, the number of CFC mobilized was comparable to that seen with CY + G-CSF and these CY + IL-2 mobilized PBSC generated potent cytotoxic effectors after in vitro IL-2 activation. Thus our results indicate that combining IL-2 with a PBSC mobilizing regimen can avert a decrease in the cytotoxic potential of mobilized cells without compromising the number of hematopoietic progenitors.