Expression of HLA Antigens by Human Thymic Epithelial Cells Robert V. Rouse, Peter Parham, F. Carl Grumet, and Irving L. Weissman ABSTRACT: Human thymuses were examined by tissue section staining with antibodies sp~+ fez monomorphic and polymorphic HLA-A. B. C. and DR determinants. The principal call (~+p* expressing high levels of HLA antigens has the distribution of epithelial cells, lmmuna~l, ctr+~ microscopy confirnted their epithelial nature. As in the mouse, both medullary and cortical epi- thelial cells express high levels of class II (DRJ antigens, a finding that is remarkable in t/m~ these antigens were originally thought to be restricted to lyntphoid and accessory cells. Class I ¢A. B, and C) antigens are also present on thymic epithelial cells. They are easily detectab[¢ +n medallary epithelial cells, but two distinct patterns of cortical staining were observed. One gr+u,~ of antibodies produced intense dendritic staining throughout the cortex: the other group pmducd only faint or no cortical dendritic staining at all. These different staining patterns do not corrddf, with known properties of the antibodies and thus appear to be due to intrinsic properties of t~" different A, B. and C antigens. ABBREVIATIONS MHC major histocompatability gene lg immunoglobulin complex PBS phosphate-buffered saline HLA human MHC IEM immunoelectron microscopy DPBS Dulbecco's phosphate-buffered H-2 mouse MHC saline, pH 7.2 I NTRODUCTIO N The thymus plays an important role in the differentiation and developmem ofT lymphocytes. Rare circulating thymic lymphocyte precursors enter the thymus and undergo an extensive proliferation and selection process produdng numbers of thymic emigrants already differentiated into functional subtypes [ 1,2]. The thymus is divided into a cortex and a medulla. The framework of the cortex is an interconnecting network of long, thin dendritic processes of thymic epithe~ cells. The spaces between these processes are filled with large numbers of |ym- phocytes. The framework of the medulla is also formed by epithelia] cells ~,h~:h range from ce~is with short dendritic processes to cohesive clusters of flattened cells (Hassal's corpuscles). Since the spaces between medullary epithelial cell processes are smaller than in the cortex, lymphocytes are present there only in From the Lal oratory of Experimental Ontology. Departmentof Pathologo,eRVR. FCG. ILW~ a ~ + tk~ Department of Structural Biology (PPL Stanford University Schoolof Medicine. Stanf6rd. Califer~. Address requests for reprints to Dr. Robert V Rouse. Department of Patho/ogr. Stanford Uni~.,~+it~ Schoolof Medicine. Stanford. CA 94305. ReceiredNorember 2. 1981; acceptedFebruary 19. 1982. I+lum~-i Immunology 5.21-34 ( ! 982) ~ El~'v~r Sch:nce Publishing Co., Inc., 1982 52 Vanderbilt Ave.,New York,NY 10017 21 0198-8859t82i05f3~2 l-i" ~S2.-5