ER-MP12 antigen, a new cell surface marker on mouse bone marrow cells with thymus-repopulating ability: I. Intrathymic repopulating ability of ER-MP12-positive bone marrow cells

We searched for new cell surface markers that allow a positiveidentification of thymusrepopulating cells in the bone marrow(BM) of the mouse. Recently we raised two rat monocional antibodies(ER-MP12 and ER-MP20) that recognize cell surface antigens expressedby mouse haematopoietic progenitor cells, among which are progenitorcells of the macrophage lineage. Here we show that the ER-MP12antigen, but not the ER-MP20 antigen, is also expressed by BMcells with thymus-repopulating ability. Using ER-MP12 and ER-MP20in two-colour immunofluorescence analysis six subpopulatlonsof BM cells can be identified. The thymusrepopulating abilityof each BM subpopulation was assessed after fluorescence-activatedcell sorting and subsequent intrathymic injection into sublethallyirradiated Thy-1 congenic recipient mice. Thymus-repopulatingactivity appeared to be exclusively confined to two subsetsof BM cells expressing either high or intermediate levels ofthe ER-MP12 antigen, but lacking ER-MP20 antigen expression.These BM subsets comprised 1–2% and 30% of total nucleatedBM cells respectively. The frequency of thymus-repopulatingcells was maximal in the minor BM subpopulation with the highestlevel of ER-MP12 antigen expression. We conclude that ER-MP12detects a hitherto unknown cell surface marker expressed byBM cells with thymus-repopulating ability.