Cell Complexes in Murine

T lymphocyte differentiation is a complex developmental process involving the interaction of lymphoid cells with thymic stromal elements. Humoral factors are known to effect certain steps in intrathymic T cell differentiation (1). There is, however, little doubt that other developmental steps require direct cell contacts between differentiating thymocytes and thymic stroma cells. This is particularly true for selection processes, which are presumably involved in the generation of T lymphocyte specificity and diversity (2-5). The mechanism of intrathymic T lymphocyte differentiation is mostly unknown. We do not even know, where in the thymus these differentiation events take place. One possible site of thymocyte differentiation is a cell type, which we have recently discovered. 1 These cells are stromal elements of extraordinary size, which may contain 25 or more smallto medium-sized lymphoid thymocytes. The engulfed thymocytes are not degraded. In contrast, they appear to encounter microenvironmental conditions, which facilitate lymphocyte survival and differentiation. We have termed these specialized stroma cells "thymie nurse cells," (TNC). 2 In this contribution, we present ultrastructural data, which identify TNC as epithelial cells. TNC contain lymphocytes, which appear to be fully intact and metabolically active, and display high mitotic activity. The lymphocytes are completely enclosed by intact TNC membranes. It appears that specialized contact areas exist that connect internalized lymphocytes with their surrounding TNC membranes. We furthermore present the results of experiments where we applied monoclonal antibodies against products of various subregions of the H-2 system and against a number of (T) lymphocyte differentiation markers. Antibody binding was assayed by indirect immunofluorescence with parallel flow cytofluorimetry and fluorescence microscopy. TNC express products of the K / D regions as well as of the I-A and I-E/C regions. They lack, however, lymphocytic differentiation markers, such as Thy-1, Ly-1, Ly-2,