Antigen-presenting cells in the thymus that can negatively select MHC class II-restricted T cells recognizing a circulating self antigen.

We investigated Ag presentation of an extracellular self Ag (C5), which only reaches the thymus via the blood circulation, for negative selection of MHC class II-restricted, C5-specific T cells. Thymic APC were introduced into fetal thymic reaggregation culture with thymocytes from C5-specific TCR transgenic mice to follow the development of C5-specific T cells in the presence or the absence of self Ag presented by various APC. To mimic the physiologic distribution of C5 peptide/MHC class II complexes on thymic APC as closely as possible, they were isolated from thymi of C5+ mice, so that the amount of C5 peptide bound to MHC class II on their surface would reflect the amount of self Ag they have access to and process normally in vivo. This circumvented the problems related to artificially high doses of Ag or peptide in vivo or in vitro, that might obscure physiologic differences such as the capacity to internalize and process Ag. The results show that not only thymic dendritic cells, but also cortical and medullary epithelial cells were able to induce negative selection of C5-specific thymocytes with similar efficiency. In contrast, thymic macrophages were unable to influence the development of C5-specific T cells. Their failure to present exogenous self Ag for negative selection suggests that macrophages concentrate on their primary function in the thymus, the disposal of dying thymocytes.