β5t-containing thymoproteasome: specific expression in thymic cortical epithelial cells and role in positive selection of CD8+ T cells

► The thymoproteasome contains the vertebrate-specific catalytic subunit β5t. ► The thymoproteasome is specifically expressed by thymic cortical epithelial cells. ► The thymoproteasome is essential for positive selection of a functionally competent repertoire of CD8+ T cells.