Intestinal intraepithelial T lymphocytes (IELs) are likely to play a key role in host mucosal immunity and, unlike other T cells, have been proposed to differentiate from local precursors rather than from thymocytes. We show here that lELs expressing the α β T cell receptor are derived from precursors that express RORγt, an orphan nuclear hormone receptor detected only in immature CD4~+CD8~+ thymocytes, fetal lymphoid tissue-inducer (LTi) cells, and LTi-like cells in crypto-patches within the adult intestinal lamina propria. Using cell fate mapping, we found that all intestinal α β T cells are progeny of CD4~+CD8~+ thymocytes, indicating that the adult intestine is not a significant site for α β T cell development. Our results suggest that intestinal RORγt~+ cells are local organizers of mucosal lymphoid tissue.
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