Isolation of two early B lymphocyte progenitors from mouse marrow: A committed Pre-Pre-B cell and a clonogenic Thy-1lo hematopoietic stem cell

Two novel early B lymphocyte precursor populations have been identified by their capacity to differentiate in Whitlock-Witte bone marrow cultures. Cells expressing neither the B lineage antigen B220 nor Thy-1 contain committed B cell precursors which differentiate in short-term culture into pre-B and B cells. The other population expresses low levels of Thy-1, and lacks B220 as well as the T cell markers L3T4 and Lyt-2. The Thy-1+ cells which initiate long-term B cell cultures contain clonogenic B cell precursors at a frequency of 1 in 11, a 100-fold enrichment over unseparated bone marrow. Thy-1+ cells are also highly enriched for myeloid-erythroid precursors (CFU-S). Thy-1+ cells allow long-term survival of lethally irradiated mice and fully reconstitute the hematopoietic system, including T and B lymphocyte compartments. These results indicate that this population (∼0.1% of bone marrow) may contain the pluripotent hematopoietic stem cell.