Immuno-electron microscopy of the thymic epithelial microenvironment.

Normal T cell development depends upon interactions between progenitor cells and the thymic microenvironment. Monoclonal antibodies (Mabs) have been used to define subtypes of thymic epithelium by light microscopy (clusters of thymic epithelial staining [CTES]). We have now used a range of these Mabs together with gold-coupled reagents in immuno-electron microscopy to study the fine cellular distribution of the molecules to which the antibodies bind. Anti-cytokeratin antibodies were used to identify all thymic epithelial cells, while the distribution of MHC class II molecules was revealed with Mabs to shared nonpolymorphic determinants. MR6, a CTES III Mab, shows strong surface labelling of cortical epithelial cells and thymic nurse cells and very weak surface staining of thymocytes, medullary macrophages, and interdigitating cells. Mab 8.18 (CTES V) also labels a cell surface molecule; this is present on Hassall's corpuscles and associated medullary epithelial cells. The molecules detected by Mabs MR6 and 8.18 are therefore located in a position where they are available to interact with external cellular and soluble signals within the thymus. In contrast, Mabs MR10 and MR19 (CTES II) recognise intracellular molecules within subcapsular, perivascular, and medullary epithelium. A similar distribution was seen with Mab 4beta, directed against the thymic hormone thymulin, although, in addition to the expected intracellular epithelial staining, large lymphoblasts in the subcapsular zone showed surface positivity, indicating the presence of thymulin bound to surface receptors on these early lymphoid cells. As expected, MHC class II molecules were expressed on some medullary and essentially all cortical epithelial cells. However, although most subcapsular epithelium was class II-negative, some cells did express these MHC molecules on their apical surface and on the surface of their cytoplasmic extensions into the cortex. Interestingly, some cortical epithelial cells surrounding capillaries were positive for both MR6 (CTES III) and for MR10, MR19, and 4beta (CTES II). Double-labelling experiments, using MR6 and MR19 simultaneously, revealed a double-positive perivascular epithelial cell population in the thymic cortex. The possibility that these cells represent a thymic epithelial progenitor population is discussed.