Invariant NKT Cells and Control of the Thymus Medulla.

Most αß T cells that form in the thymus are generated during mainstream conventional thymocyte development and involve the generation and selection of a diverse αß TCR repertoire that recognizes self-peptide/MHC complexes. Additionally, the thymus also supports the production of T cell subsets that express αß TCRs but display unique developmental and functional features distinct from conventional αß T cells. These include multiple lineages of CD1d-restricted invariant NKT (iNKT) cells that express an invariant αß TCR, branch off from mainstream thymocytes at the CD4+CD8+ stage, and are potent producers of polarizing cytokines. Importantly, and despite their differences, iNKT cells and conventional αß T cells share common requirements for thymic epithelial microenvironments during their development. Moreover, emerging evidence suggests that constitutive cytokine production by iNKT cells influences both conventional thymocyte development and the intrathymic formation of additional innate CD8+ αß T cells with memory-like properties. In this article, we review evidence for an intrathymic innate lymphocyte network in which iNKT cells play key roles in multiple aspects of thymus function.