ER-MP12 ANTIGEN, A NEW CELL-SURFACE MARKER ON MOUSE BONE-MARROW CELLSWITH THYMUS-REPOPULATING ABILITY .1. INTRATHYMIC REPOPULATING ABILITYOF ER-MP12-POSITIVE BONE-MARROW CELLS

We searched for new cell surface markers that allow a positive identif ication of thymus-repopulating cells in the bone marrow (BM) of the mo use. Recently we raised two rat monoclonal antibodies (ER-MP12 and ER- MP20) that recognize cell surface antigens expressed by mouse haematop oietic progenitor cells, among which are progenitor cells of the macro phage lineage. Here we show that the ER-MP12 antigen, but not the ER-M P20 antigen, is also expressed by BM cells with thymus-repopulating ab ility. Using ER-MP12 and ER-MP20 in two-colour immunofluorescence anal ysis six subpopulations of BM cells can be identified. The thymus-repo pulating ability of each BM subpopulation was assessed after fluoresce nce-activated cell sorting and subsequent intrathymic injection into s ublethally irradiated Thy-1 congenic recipient mice. Thymus-repopulati ng activity appeared to be exclusively confined to two subsets of BM c ells expressing either high or intermediate levels of the ER-MP12 anti gen, but lacking ER-MP20 antigen expression. These BM subsets comprise d 1 - 2% and 30% of total nucleated BM cells respectively. The frequen cy of thymus-repopulating cells was maximal in the minor BM subpopulat ion with the highest level of ER-MP12 antigen expression. We conclude that ER-MP12 detects a hitherto unknown cell surface marker expressed by BM cells with thymus-repopulating ability.