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

Cortical thymic epithelial cells (cTECs) express a unique thymoproteasome subunit-beta 5T that plays an essential role in the development of CD8 T cells. In contrast, the immunoproteasome subunit-beta 5i 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 peptide repertoires in cTECs vs. other APCs without expressing beta 5T, by generating beta 5t(5i) knockin mice, in which beta 5i replaced beta 5T in cTECs. When such animals were crossed to beta 5i(-/-) mice, beta 5i was exclusively expressed in cTECs, whereas beta 5 was expressed in other cells. However, this mouse did not support normal positive selection, suggesting that beta 5T generates peptides that are intrinsically better for positive selection (i.e., beta 5i could not replace beta 5T) 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 beta 5T have higher reactivity to self, generating predominantly CD44(hi) memory phenotype 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, naive T-cell clones.