PROLIFERATION OF SPLEEN COLONY-FORMING-UNITS (CFU-S8, CFU-S13) AND CELLS WITH MARROW REPOPULATING ABILITY

The practicalities of gene transfer therapy using retroviral vector sy stems require both that host cells be as primitive as possible and tha t those cells be proliferating. Here, the kinetics of hemopoietic stem cells with marrow repopulating ability (MRA) have been studied with a view to defining the timescale over which these normally quiescent ce lls can be triggered into cell cycle. Mice were injected with hydroxyu rea (1 g/kg) four times over a period of 26 h and assayed at intervals up to eight days for 8-day and 13-day spleen colony-forming units (CF U-S) and for generation of 12-day CFU-S in the bone marrow (MRA assay) . The proliferative activity of these cell populations was measured by in vitro tritiated thymidine assays. CFU-S were reduced rapidly to 11 % of normal and induced into cycle. Their number and proliferative qu iescence recovered by four to five days. Cells with MRA reached their nadir after four days and only then started to proliferate. For each o f these progenitor cell subclasses, the proliferative activity inverse ly reflects their numbers and indicates regulation by negative feedbac k processes.