ER-MP12 antigen, a new cell surface marker on mouse bone marrow cells with thymus-repopulating ability: II. 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 into sublethally irradiated mice thymus-repopulating ability was restricted to ER-MP20^- BM cells expressing either high or intermediate levels of the ER-MP12 antigen (1-2% and -30% of BM nucleated cells respectively). The highest frequency of thymus-repopulating cells was found in the minor subset of ER-MP12⁺⁺20^- BM cells. In the present study we demonstrate that upon intravenous transfer, thymus-homing and-repopulating BM cells are exclusively confined to the ER-MP12⁺⁺20^- and ER-MP12⁺20^- subpopulations, the highest frequency being detected among ER-MP12⁺⁺20^- BM cells. Analysis of the peripheral blood leucocytes of reconstituted mice showed that not only prothymocytes but also progenitorcells of the B cell lineage as well as the myelold lineage were present within both subsets. Three-colour flow cytometric analysis revealed that ER-MP12⁺⁺20^- BM cells in particular were phenotyplcally heterogeneous with respect to the expression of the cell surface markers Thy-1, Sca-1, CD44, B220 and c-kit. Taken together our data demonstrate that ER-MP12 positively identifies BM cells with the ability to home to and repopulate the thymus. The phenotypic heterogeneity displayed 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 activities in the BM of the mouse.