Expression of antigen-specific, major histocompatibility complex-restricted receptors by cortical and medullary thymocytes in situ.

We have examined the distribution of the antigen-specific, major histocompatibility complex-restricted receptor on mouse thymocytes in situ, using immunohistochemical techniques and the monoclonal antibody KJ16-133. This antibody reacts with the beta chain of the receptors on about 20% of peripheral murine T cells. Of the cortical thymocytes reacting with KJ16-133, cells with only cytoplasmic staining were most frequently observed. Such cytoplasmic staining was not observed in the medulla. Occasional cortical cells had low levels of surface expression, which was almost invariably patched in the region of contact with epithelial cell processes. KJ16-133+ medullary thymocytes had high levels of uniform surface labeling. These results suggest that thymic selection of MHC restriction and/or tolerance may occur in the cortex, where the receptors on maturing thymocytes interact with MHC proteins on epithelial cells.