Thymoproteasome subunit-β5T generates peptide-MHC complexes specialized for positive selection

Cortical thymic epithelial cells (cTECs) express a unique thymopro-teasome subunit-β5T that plays an essential role in the development of CD8 T cells. In contrast, the immunoproteasome subunit-β5i is expressed in other thymic antigen-presenting cells (APCs). The thymoproteasome may generate peptides that are specialized for positive selection, or it may simply serve to generate peptides that are distinct from other APCs that cause negative selection, thereby promoting an overall larger number of surviving clones to mature and function in the immune system. To distinguish these models, we genetically engineered mice to express distinct pep-tide repertoires in cTECs vs. other APCs without expressing β5T, by generating β5t⁵ⁱ knockin mice, in which β5i replaced β5T in cTECs. When such animals were crossed to β5i^-/- mice, β5i was exclusively expressed in cTECs, whereas β5 was expressed in other cells. However, this mouse did not support normal positive selection, suggesting that β5T generates peptides that are intrinsically better for positive selection (i.e., β5i could not replace β5T) and not merely because these peptides are distinct from peptides presented by other APCs. Finally, using an Nur77^GFP reporter, we show that the T cells generated in the absence of β5T have higher reactivity to self, generating predominantly CD44^hi memory phe-notype peripheral CD8⁺ T cells. Altogether, our results suggest that the thymoproteasome supports positive selection by generating peptides that are optimized for the selection of weakly self-reactive, naïve T-cell clones.