ER-MP12 ANTIGEN, A NEW CELL-SURFACE MARKER ON MOUSE BONE-MARROW CELLSWITH THYMUS-REPOPULATING ABILITY .2. THYMUS-HOMING ABILITY AND PHENOTYPIC CHARACTERIZATION OF ER-MP12-POSITIVE BONE-MARROW CELLS

In the accompanying paper we showed that six distinct subsets of bone marrow (BM) cells can be identified using the mAb ER-MP12 and ER-MP20 in two-colour immunofluorescence analysis. Upon intrathymic transfer i nto sublethally irradiated mice thymus-repopulating ability was restri cted to ER-MP20- BM cells expressing either high or intermediate level s of the ER-MP12 antigen (1 - 2% and approximately 30% of BM nucleated cells respectively). The highest frequency of thymus-repopulating cel ls was found in the minor subset of ER-MP12++20- BM cells. In the pres ent study we demonstrate that upon intravenous transfer, thymus-homing and -repopulating BM cells are exclusively confined to the ER-MP12++2 0- and ER-MP12+20- subpopulations, the highest frequency being detecte d among ER-MP12++20- BM cells. Analysis of the peripheral blood leucoc ytes of reconstituted mice showed that not only prothymocytes but also progenitor cells of the B cell lineage as well as the myeloid lineage were present within both subsets. Three-colour flow cytometric analys is revealed that ER-MP12++20- BM cells in particular were phenotypical ly heterogeneous with respect to the expression of the cell surface ma rkers Thy-1, Sca-1, CD44, B220 and c-kit. Taken together our data demo nstrate that ER-MP12 positively identifies BM cells with the ability t o home to and repopulate the thymus. The phenotypic heterogeneity disp layed by the ER-MP12++20- BM subset, containing the highest frequency of thymus-homing and -repopulating cells, provides a basis for further separation of prothymocyte activity from other haematopoietic activit ies in the BM of the mouse.