GENETIC-CONTROL OF MURINE HEMATOPOIETIC STEM-CELL POOL SIZES AND CYCLING KINETICS

Bone marrow from each of two inbred mouse strains, C57BL/6J and DBA/2J, was highly enriched for stem cells using flow cytometry and was divided into two stem cell subpopulations using the mitochondrial dye rhodamine 123 (Rh-123). The Rh-123lo population was determined to be more primitive than Rh-123hi based on the expression of stem cell markers such as the c-kit protooncogene (stem cell factor receptor) and the Ly-6A/E stem cell antigen (Sca-1) as well as the lack of in vitro colony-forming ability. Compared to DBA/2J mice, marrow from the C57BL/6J strain consistently showed a higher proportion of "very primitive" (Rh-123lo) cells, suggesting that the sizes of functionally distinct stem cell subpopulations are maintained under precise genetic control. Marrow from both strains exposed to the cytotoxic drug 5-fluorouracil showed a dramatic increase in the proportion of Rh-123lo cells within 2 days as repopulation began. Marrow subpopulations returned to pretreatment proportions by the eighth day in DBA/2J mice but not until 14 days in C57BL/6J mice. This intrinsic difference in 5-fluorouracil recovery time was attributed to an increased rate of stem cell cycling in DBA/2J relative to C57BL/6J mice. When stem cell factor was injected into a C57BL/6J<->DBA/2J allophenic mouse, blood cell chimerism shifted markedly but transiently toward the DBA/2J genotype, suggesting that the DBA/2J target population, because of an inherent kinetic advantage, was able to respond faster to the cytokine. A model is proposed that is based on these and our earlier observations to explain this strain-specific stem cell behavior and offer new insights into the genetic control of stem cell cycling and population dynamics.