IMMUNOELECTRON MICROSCOPY OF THE THYMIC EPITHELIAL MICROENVIRONMENT

Normal T cell development depends upon interactions between progenitor cells and the thymic microenvironment. Monoclonal antibodies (Mabs) h ave been used to define subtypes of thymic epithelium by light microsc opy (clusters of thymic epithelial staining [CTES]). We have now used a range of these Mabs together with gold-coupled reagents in immuno-el ectron microscopy to study the fine cellular distribution of the molec ules to which the antibodies bind. Anti-cytokeratin antibodies were us ed 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 surfac e staining of thymocytes, medullary macrophages, and interdigitating c ells. Mab 8.18 (CTES V) also labels a cell surface molecule; this is p resent on Hassall's corpuscles and associated medullary epithelial cel ls. The molecules detected by Mabs MR6 and 8.18 are therefore located in a position where they are available to interact with external cellu lar and soluble signals within the thymus. In contrast, Mabs MR10 and MR19 (CTES II) recognise intracellular molecules within subcapsular, p erivascular, and medullary epithelium. A similar distribution was seen with Mab 4 beta, directed against the thymic hormone thymulin, althou gh, in addition to the expected intracellular epithelial staining, lar ge lymphoblasts in the subcapsular zone showed surface positivity, ind icating the presence of thymulin bound to surface receptors on these e arly lymphoid cells. As expected, MHC class IE molecules were expresse d on some medullary and essentially all cortical epithelial cells. How ever, 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. Interesti ngly, some cortical epithelial cells surrounding capillaries were posi tive for both MR6 (CTES III) and for MR10, MR19, and 4 beta (CTES II). Double-labelling experiments, using MR6 and MR19 simultaneously, reve aled a double-positive perivascular epithelial cell population in the thymic cortex. The possibility that these cells represent a thymic epi thelial progenitor population is discussed. (C) 1997 Wiley-Liss, Inc.