Autonomous behavior of hematopoietic stem cells.

Mechanisms that affect the function of primitive hematopoietic stem cells with long-term proliferative potential remain largely unknown. Here we assessed whether properties of stem cells are cell-extrinsically or cell-autonomously regulated. We developed a model in which two genetically and phenotypically distinct stem cell populations coexist in a single animal. Chimeric mice were produced by transplanting irradiated B6D2F1 (BDF1) recipients with mixtures of DBA/2 (D2) and C57BL/6 (B6) day-14 fetal liver cells. We determined the mobilization potential, proliferation, and frequency of D2 and B6 stem and progenitor cells in animals with chimeric hematopoiesis. After granulocyte colony-stimulating factor (G-CSF) administration, peripheral blood D2 colony-forming units granulocyte-macrophage were fourfold to eightfold more numerous than B6 progenitors. We determined that D2 and B6 progenitors maintained their genotype-specific cycling activity in BDF1 recipients. Chimeric marrow was harvested and D2 and B6 cell populations were separated by flow cytometry. Cobblestone area-forming cell (CAFC) analysis of sorted marrow showed that the number of late appearing CAFC subsets within the D2 cell population was approximately threefold higher than within the B6 fraction. We performed secondary transplantation using unfractionated chimeric marrow, which was given in limiting doses to lethally irradiated BDF1 recipients. Comparison of the proportion of animals possessing D2 and/or B6 leukocytes 5 months after transplant revealed that the frequency of D2 LTRA was approximately 10-fold higher than B6 LTRA numbers. Our data demonstrate that genetically distinct stem cell populations, coexisting in individual animals, independently maintain their parental phenotypes, indicating that stem cell properties are predominantly regulated cell-autonomously.