In vivo maintenance of T-lymphocyte unresponsiveness induced by thymic medullary epithelium requires antigen presentation by radioresistant cells.

The T-cell repertoire developing in the thymus is rid of autospecific cells by the process of thymic negative selection. Recognition of major histocompatibility complex (MHC)/self-peptide complexes expressed by thymic antigen-presenting cells (APC) of bone marrow origin leads to induction of apoptotic death of autospecific thymocytes. Induction of tolerance to self-antigens not presented by thymic APC is mediated by medullary thymic epithelial cells (mTEC) which express a very wide range of proteins, e.g. inducible and tissue-specific proteins. The main type of tolerance induced by mTEC is non-deletional and the issue of how it is maintained outside the thymus is therefore of crucial interest. We have previously shown that the non-T-cell receptor (TCR) -transgenic T-cell repertoire developing in conditions in which tolerance to self-MHC/peptide ligands is exclusively induced by mTEC is tolerant to syngeneic targets in vivo but lyses such targets in vitro. Here we report that this non-deletional in vivo self-tolerance is not due to active tolerance assured by known naturally occurring regulatory or immune-modulating T lymphocytes. Importantly, we show that in vivo maintenance of this therefore probably anergic state requires continued interaction of autospecific T cells with self-MHC/peptide ligands expressed by radioresistant cells while APC are incapable of maintaining the tolerant state. Therefore, maintenance of non-deletional T-lymphocyte tolerance to the wide range of self-antigens expressed by mTEC depends on continued interaction with radioresistant cells that very probably express a much more limited repertoire of antigens. Our data may therefore have important consequences for tolerance to tissue-specific and inducible self-antigens.