The contribution of circulating lymphocytes to the small intestine epithelium is thought to be limited at steady-state, because the niches are occupied by long-lived resident γδ T cells and unconventional αβ T cells. In The Journal of Experimental Medicine, Guy-Grand et al. show that circulating cells continuously colonize the small intestine and contribute to the dynamics of resident intraepithelial lymphocytes (IELs). Whereas conventional recent thymic emigrants (RTE) circulate preferentially to lymphoid organs, unconventional RTEs express high amounts of both α4β7 and CD62L, which makes them equally tropic for the secondary lymphoid organs or the small intestine. Circulation through the gut associated lymphoid tissue (GALT) induces high proliferative and cytotoxic potential in unconventional RTEs, whereas those directly entering the small intestine do not proliferate in situ. The calculated average lifespan for γδ T cells in the small intestine epithelium is 40 days, indicating continuous thymic input of unconventional T cells to this site.

J. Exp. Med. (5 August 2013) doi:10.1084/jem.20122588