Differential effects of age on mitotically active and inactive bone marrow stem cells and splenic T cells in mice

Young and old BALB/c mice and thymectomized young mice were subjected to continuous exposure of 6-thioguanine (6-TG), and the numbers of their bone marrow spleen colony-forming units (CFU-S) and in vitro culture colony-forming units (CFU-C) and the number and mitogen-induced proliferative activity of their splenic T cells were determined at various time intervals. The results indicated that (a) old mice have seven times more 6-TG-resistant (6-TGr) CFU-S than young mice, (b) the mitogen-induced proliferative activity of 6-TGr T cells is comparable to that of 6-TG-sensitive (6-TGs) T cells of both young and old mice, and (c) 6-TGr CFU-S and T cells are resistant to 6-TG because they are mitotically inactive and not because they are drug-resistant mutants.