Identification of acid cysteine proteinase inhibitor (cystatin A) in the human thymus

Background: Acid cysteine proteinase inhibitor (ACPI, also called cystatin A) is a protein that is present in the epithelial cells of the skin and in the dendritic reticulum cells of lymphoid tissues. In this study the presence and cellular localization of ACPI in the thymus was investigated. Methods: The cellular and topographical location of ACPI was immunohistochemically demonstrated in the normal thymus of man. Results: ACPI was found in the cells of the-Hassall's corpuscles and in many medullary cells. Most of these cells were epithelial cells, as shown by the results of immunohistochemical cytokeratin and epithelial membrane antigen stainings. Also, some individual cytokeratin negative but S-100 positive medullary reticular dendritic cells were stained with ACPI. Conclusions: The finding that ACPI is constantly present in the thymus at restricted and specific cellular locations leads to the suggestion that protease inhibitors may play a role in specific thymic functions. © 1994 Wiley-Liss, Inc.