Limiting dilution clonal assay of human bone marrow hematopoietic progenitors (granulocytes-macrophages).

We have developed a limiting dilution assay for human bone marrow hematopoietic precursor (granulocytes-macrophages) in microwells. Bone marrow cells were fractionated by discontinuous Percoll gradients and diluted in culture medium containing colony-stimulating factors. They were diluted and cultured in medium containing colony-stimulating factor and 10(-5) M hydrocortisone in microwells over a range of cell densities that allowed calculation of the frequency of growth-inducible precursors. After 10 days, the wells were examined for clonal growth. Clonal proliferation followed the single-hit model of the Poisson distribution. More progenitors were detected in this assay than in simultaneous methylcellulose colony assays or agar cultures. Thymidine suicide experiments led to an increase in the frequency of progenitors detected in this assay, but a decrease in the frequency of methylcellulose colonies. This system may detect additional, less mature progenitors than are detected in semisolid culture systems.