CD8 + recent thymic emigrants home to and efficiently repopulate the small intestine epithelium Tracy L Staton 1 , Aida Habtezion 2 , Monte M Winslow 1 , Tohru Sato 2 , Paul E Love 4 & Eugene C Butcher 1–3 Prevailing knowledge dictates that naive ab T cells require activation in lymphoid tissues before differentiating into effector or memory T cells capable of trafficking to nonlymphoid tissues. Here we demonstrate that CD8 + recent thymic emigrants (RTEs) migrated directly into the small intestine. CCR9, CCL25 and a 4 b 7 integrin were required for gut entry of CD8 + RTEs. After T cell receptor stimulation, intestinal CD8 + RTEs proliferated and acquired a surface phenotype resembling that of intraepithelial lymphocytes. CD8 + RTEs efficiently populated the gut of lymphotoxin-a-deficient mice, which lack lymphoid organs. These studies challenge the present understanding of naive ab T cell trafficking and suggest that RTEs may be involved in maintaining a diverse immune repertoire at mucosal surfaces. The immune system is compartmentalized into primary, secon- dary and tertiary organs 1 . Mature lymphocytes are generated in primary lymphoid organs. After export, these naive lymphocytes are thought to recirculate in the secondary lymphoid organs, where they can be activated by cognate antigen. Once activated, lymphocytes can enter tertiary nonlymphoid sites, such as the skin and intestine, where they can function in clearing infection. Present knowledge suggests that naive ab T cell circulation is restricted to secondary lymphoid tissues 1,2 . The small intestine tertiary site is important in host defense. The gut epithelium forms a border between the outside environment and the body interior and is a potential entry site for antigens 3 . Of the immune cells that take up residence in the epithelium (called ‘intraepithelial lymphocytes’ (IELs)), more than 90% are CD8 + T cells. Because of the large surface area of the small intestine, IELs comprise a considerable fraction of the body’s T cells, and approxi- mately 1  10 9 IELs can be found in the human small intestine 4–6 . IELs have an activated or memory phenotype 7 and, once associated with the epithelium, do not recirculate throughout the body 8 . CD8 + IELs have high expression of a E integrin, which interacts with E-cadherin on epithelial cells and allows retention of IELs in the gut epithelium 9 . The CD8 + IEL compartment contains CD8aa + gd and ab T cells as well as conventional CD8ab + ab T cells 10–12 . Studies suggest that CD8ab + ab T cell receptor (TCR)–positive IELs originate from the thymus 13,14 , specifically via naive T cells that are activated in the Peyer’s patch that then migrate to the gut epithelium 15–17 . Recent thymic emigrants (RTEs), a distinct subpopulation of naive ab T cells, have ‘preferential’ access to survival factors and niches and are ‘preferentially’ incorporated into the peripheral naive ab T cell pool 18 . Splenic RTEs are less responsive than other naive ab T cells to TCR stimulation 19 and may be especially susceptible to tolerization 20 . CD8 + RTEs have unique adhesive and chemotactic properties. Unlike other naive ab T cells, RTEs express a E integrin and CCR9, which is involved in recruiting memory T cells to the intestines 21,22 . CCR9 is a chemoattractant receptor expressed on small intestine–specific T cells, and its ligand, CCL25, is expressed by small intestine epithelial cells 23 . CD8 + RTEs also express a 4 b 7 integrin, which is the receptor for the intestinal vascular addressin MAdCAM-1. Those features support the hypothesis that CD8 + RTEs might home to the mucosal surface of the small intestine without prior activation and ‘reprogramming’ of homing properties in peripheral lymphoid tissues. Moreover, because RTEs establish and contribute to the TCR diversity of the peripheral ab T cell pool, we reasoned that RTEs might also maintain the TCR diversity of the IEL population 24 . Here we demonstrate that CD8 + RTEs homed to the gut and show that this migration required a 4 b 7 integrin, CCR9 and CCL25. RTE gut homing occurred independently of prior antigen recognition in peripheral lymphoid tissues. After entry into the gut wall, CD8 + RTEs proliferated and differentiated, ultimately acquiring the pheno- type of resident IELs. CD8 + RTEs populated the IEL compartment of lymphopenic mice more efficiently than naive non-RTE CD8 + T cells. In the steady state, CD8ab + TCRab + RTEs were readily recovered from the resident IEL population and expressed a diverse TCR repertoire. Our data indicate that trafficking of naive CD8 + ab T cells, particularly RTEs, is not limited to secondary lymphoid tissues. Instead, CD8 + RTEs can home directly into the intestinal wall, where they might help to maintain the TCR diversity of the mucosal CD8ab + ab T cell pool. Received 21 November 2005; accepted 14 February 2006; published online 2 April 2006; corrected after print 2 May 2006 (details online); doi:10.1038/ni1319 1 Program in Immunology and 2 Laboratory of Immunology and Vascular Biology, Department of Pathology, Stanford University School of Medicine, Stanford, California 94305, USA. 3 Center for Molecular Biology and Medicine, Veterans Affairs Palo Alto Health Care System, Palo Alto, California 94304, USA. 4 Laboratory of Mammalian Genes and Development, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA. Correspondence should be addressed to E.C.B. (ebutcher@stanford.edu). NATURE IMMUNOLOGY ADVANCE ONLINE PUBLICATION 1 ARTICLES © 2006 Nature Publishing Group http://www.nature.com/natureimmunology