Modest cortex and promiscuous medulla for thymic repertoire formation.

Recent advances in the understanding of medullary thymic epithelial cell biology, and especially of their promiscuous gene expression, have highlighted the indispensable role of thymic medulla in shaping a self-tolerant T-cell repertoire. Additionally, our recent results have shown that cortical thymic epithelial cells possess a unique proteasome (the so-called thymoproteasome), which seems to possess limited protein degradation capability for generating peptides that are loaded onto class I major histocompatibility complex molecules. We discuss here the unique role of thymoproteasomes in the development and repertoire formation of CD8+ T cells, focusing on the stepwise and contrasting roles of cortical epithelial cells and medullary epithelial cells. These results could offer fundamental new insights into the molecular mechanisms of T-cell repertoire selection.