HIGH-LEVEL EXPRESSION OF THE ER-MP58 ANTIGEN ON MOUSE BONE-MARROW HEMATOPOIETIC PROGENITOR CELLS MARKS COMMITMENT TO THE MYELOID LINEAGE

Studies on the early events in the differentiation of the nonspecific immune system require the identification and isolation of myeloid-comm itted progenitor cells. Using the monoclonal antibodies (mAb) ER-MP12 and ER-MP20, generated against immortalized macrophage precursors, we have shown previously that the earliest macrophage colony-stimulating factor (M-CSF)-responsive cells in the bone marrow have the ER-MP12(hi )20(-) phenotype. In addition, we found that the ER-MP12(hi)20(-) subs et (comprising about 2% of total nucleated marrow) contains progenitor cells of all hematopoietic lineages. Aiming at the identification and purification of the myeloid progenitor cells within the ER-MP12(hi)20 (-) subset, we used ER-MP58, a marker expressed at high level by all M -CSF-responsive bone marrow progenitors. With this marker the ER-MP12( hi)20(-) cell population could be divided into three subfractions: one with absent or low level ER-MP58 expression, one with intermediate, a nd one with high ER-MP58 expression. These subfractions were isolated by fluorescence-activated cell sorting and tested in vitro and in vivo for their differentiation capacities. In addition, the expression of ER-MP58 on stem cell subsets was examined in the cobblestone area-form ing cell (CAFC) assay. Our data indicate that in the ER-MP12(hi)20(-) subpopulation myeloid-committed progenitors are characterized by high- level expression of the ER-MP58 antigen, whereas cells with other or b roader differentiation capacities have an ER-MP58 negative/low or inte rmediate phenotype. These myeloid-committed progenitors have no signif icant repopulating ability in vivo, in contrast to the ER-MP58 interme diate cells. Primitive CAFC-28/35, corresponding to cells providing lo ng-term hematopoietic engraftment in vivo, also did not express the ER -MP58 Ag at a high level. Thus, cells committed to the myeloid lineage can be separated from progenitor cells with other differentiation cap acities by means of multiparameter cell sorting using ER-MP58 in combi nation with ER-MP12 and ER-MP20.