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

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 E alpha 52-68:I-A(b) and class II-associated invariant chain peptide (CLIP):I-A(b) 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 E alpha 52-68:I-A(b) 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-A(b) 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.