Identification of a human recent thymic emigrant phenotype

The ability to measure human thymic output would be an invaluable tool for the study of the development of the naive T cell repertoire, as well as naive T cell regeneration after intensive cytotoxic chemotherapy or effective antiretroviral therapy of progressive HIV infection. We and others have demonstrated previously that quantification of T cell receptor rearrangement excision circles (TREC) within peripheral T cell populations provides insight into the frequency of recent thymic emigrants (RTE) and, therefore, into thymic function. However, measurement of RTE by this approach is complicated by the fact that TREC levels also are determined by turnover within the naive T cell compartment. Here, we report a phenotypic approach to RTE measurement. We demonstrate that αE integrin (CD103) expression is up-regulated very late in thymic development on a subset of CD8⁺/CD4^- thymocytes and also defines a distinct subset of naive CD8⁺ T cells in the periphery. The latter subset is differentiated from circulating CD103⁺ mucosa-associated memory T cells by its naive T cell phenotype (CD45RO-, CD62L(bright), CD27(bright), CD11a(dim), CD95(dim) and its high concentration of TREC. Indeed, sorted CD103⁺ naive CD8⁺ cells display higher levels of TREC than their CD103^- naive counterparts, and these cells demonstrate an age-related decline in frequency that is enhanced significantly by thymectomy. The thymic dependence of this subset and the cells' relatively evanescent presence in the periphery suggest that these cells are a population of RTE and that quantification of their frequency in peripheral blood provides an estimate of the level of ongoing thymopoiesis.