Bone marrow clones representing an intermediate stage of development between hematopoietic stem cells and pro-T-lymphocyte or pro-B-lymphocyte progenitors.

We have established in culture several nontransformed bone marrow clones (called PR) that show phenotypic and genotypic characteristics that distinguish them from totipotent stem cells and lineage-restricted Pro-T lymphocytes, Pro-B lymphocytes, and myeloid cell progenitors. In vivo and/or in vitro the PR clones give rise to T lymphocytes, B lymphocytes, and some myeloid-lineage cells, but they appear not to be able to generate cells of the erythroid lineage, nor can they rescue mice from a lethal dose of irradiation. We conclude that the PR clones are precursor cells representing an intermediate stage of development between the totipotential stem cell and lineage-restricted progenitor cells. The results described here support a model of blood cell formation in which stem cell differentiation is a progressive process marked by the stepwise loss of self renewal and functional potential. In addition, they provide evidence that cytokines and specialized microenvironments can direct the fate of the developing multipotent progenitor cells.