Differential expression of CLIP: MHC class II and conventional endogenous peptide: MHC class II complexes by thymic epithelial cells and peripheral antigen‐presenting cells

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Abstract
Major histocompatibility complex (MHC) class II molecules expressed by thymic epithelial cells are involved in positive selection of CD4 T cells, whereas the high‐avidity interaction of T cell receptors with the endogenous peptide : MHC class II complexes expressed on bone marrow (BM)‐derived antigen‐presenting cells (APC) and, to a lesser extent, on thymic epithelial cells mediate negative selection. To understand better the generation of the CD4 T cell repertoire both in the thymus and in the periphery we analyzed relative levels of expression of specific endogenous peptide: MHC class II complexes in thymic epithelial cells (TEC) and peripheral APC. Expression of Eα52–68: I‐Ab and class II‐associated invariant chain peptide (CLIP): I‐Ab complexes in thymic epithelial cells and in the bone‐marrow derived splenic APC, i.e. B cells, was studied using YAe and 30‐2 monoclonal antibodies which are specific for the corresponding complexes. To distinguish between expression of both complexes in radioresistant thymic epithelial elements and radiation sensitive BM‐derived APC, radiation BM chimeras were constructed. Using immunohistochemical and immunochemical approaches we demonstrated that the level of expression of Eα52–68:I‐Ab complexes in thymic epithelial cells is approximately 5–10 % of that seen in splenic cells whereas total class II levels were comparable. In contrast, CLIP: I‐Ab complexes are expressed at substantially higher levels in TEC vs. splenic APC. This result demonstrates quantitative differences in expression of distinct peptide: MHC class II complexes in thymic epithelial cells and peripheral splenic APC.