Autonomous behavior of hematopoietic stem cells

Objective. Mechanisms that affect the function of primitive hematopoietic s tem cells with long-term proliferative potential remain largely unknown. He re we assessed whether properties of stem cells are cell-extrinsically or c ell-autonomously regulated. Materials and Methods. We developed a model in which two genetically and ph enotypically distinct stem cell populations coexist in a single animal. Chi meric mice were produced by transplanting irradiated B6D2F1 (BDF1) recipien ts with mixtures of DBA/2 (D2) and C57BL/6 (B6) day-14 fetal liver cells. Results. We determined the mobilization potential, proliferation, and frequ ency of D2 and B6 stem and progenitor cells in animals with chimeric hemato poiesis. After granulocyte colony-stimulating factor (G-CSF) administration , peripheral blood D2 colony-forming units granulocyte-macrophage were four fold to eightfold more numerous than B6 progenitors. We determined that D2 and B6 progenitors maintained their genotype-specific cycling activity in B DF1 recipients. Chimeric marrow was harvested and D2 and B6 cell population s were separated by flow cytometry. Cobblestone area-forming cell (CAFC) an alysis of sorted marrow showed that the number of late appearing CAFC subse ts within the D2 cell population was similar to threefold higher than withi n the B6 fraction. We performed secondary transplantation using unfractiona ted chimeric marrow, which was given in limiting doses to lethally irradiat ed BDF1 recipients. Comparison of the proportion of animals possessing D2 a nd/or B6 leukocytes 5 months after transplant revealed that the frequency o f D2 LTRA was similar to 10-fold higher than B6 LTRA numbers. Conclusion. Our data demonstrate that genetically distinct stem cell popula tions, coexisting in individual animals, independently maintain their paren tal phenotypes, indicating that stem cell properties are predominantly regu lated cell-autonomously. (C) 2000 International Society for Experimental He matology. Published by Elsevier Science Inc.