Ontogeny of thymic cortical epithelial cells expressing the thymoproteasome subunit β5t

Proteasomes are responsible for generating peptides presented by class I MHC molecules of the immune system. beta 5t, a recently identified proteasome component, is specifically expressed in thymic cortical epithelial cells (cTECs) and plays a pivotal role in generating an immunocompetent repertoire of class I MHC-restricted CD8(+) T cells. Here, we report that beta 5t is detectable in the thymus as early as E12.5 mouse embryos. We also found that beta 5t expression in cTECs was detectable in mice deficient for RelB or Rag2, indicating that beta 5t in cTECs is expressed in the absence of thymic medulla formation or thymocyte development beyond the CD4(-) CD8(-) stage. beta 5t expression in the embryonic thymus was not detectable in Foxn1-deficient nude mice, although its expression was not reduced in mice deficient for both CCR7 and CCR9, in which fetal thymus colonization by leukocytes is defective. These results indicate that beta 5t expression in cTECs is dependent on Foxn1 but independent of thymocyte crosstalk or thymic medulla formation.