Presentation of antigen, foreign major histocompatibility complex proteins and self by thymus cortical epithelium

Abstract

IN mouse and man most peripheral T cells bear clonally variable receptors made up of α- and β-chains¹ which bind ligands on target cells consisting of peptide fragments of foreign antigens, complexed with cell surface proteins encoded by the major histocompatibility complex (MHC) of the individual^2–4. In the thymus, developing T cells are selected to mature only if their receptors will be able to participate in self-MHC plus antigen recognition in the periphery^5–8. This positive selection occurs in the presence of self-MHC, but In the apparent absence of antigen, leading to the paradoxical conclusion that developing thymocytes must be positively selected by engagement of their receptors and self-MHC alone, although thymocytes that react too well with self-MHC are eliminated⁹. To account for this, it has been suggested that MHC molecules in the thymus are not identical to those found elsewhere¹. To test this and other hypotheses, we have examined the ability of the presumed selecting cells, those of the thymus cortical epithelium^10,11, to present various MHC complexes to T cells. Our results indicate that MHC molecules on thymus epithelium are not always the same as those found elsewhere.