LONG-TERM ERYTHROPOIESIS FROM CONSTANT NUMBERS OF CD34+ CELLS IN SERUM-FREE CULTURES INITIATED WITH HIGHLY PURIFIED PROGENITOR CELLS FROM HUMAN BONE-MARROW

To directly study the biological properties of purified hematopoietic colony-forming cell precursors, cells with a CD34+ CD45RA(lo) CD71lo phenotype were purified from human bone marrow using density separation and fluorescence-activated cell sorting, and were cultured in serum-free culture medium supplemented with various cytokines. In the presence of interleukin 3 (IL-3), IL-6, erythropoietin, and mast cell growth factor (a c-kit ligand), cell numbers increased approximately 10(6)-fold over a period of 4 wk, and the percentage of cells that expressed transferrin receptors (CD71) increased from < 0.1% at day 0 to > 99% at day 14. Interestingly, the absolute number of CD34+ CD71lo cells did not change during culture. When CD34+ CD71lo cells were sorted from expanded cultures and recultured, extensive cell production was repeated, again without significant changes in the absolute number of cells with the CD34+ CD71lo phenotype that were used to initiate the (sub)cultures. These results document that primitive hematopoietic cells can generate progeny without an apparent decrease in the size of a precursor cell pool.