Murine CD8⁺ recent thymic emigrants are α_E integrin-positive and CC chemokine ligand 25 responsive

Recent thymic emigrants (RTE) are an important subpopulation of naive CD8⁺ T cells because of their ability to reconstitute a diverse immune system after periods of T cell depletion. In neonatal mice, the majority of peripheral T lymphocytes are RTE, cells that have recently left the thymus to populate the periphery. Postulating that these cells could have unique trafficking mechanisms, we compared adhesion molecule and chemokine receptor expression of neonatal RTE with mature adult lymphocytes. Neonatal CD8 ⁺ splenocytes uniformly express α_E integrin and exhibit a high responsiveness to CC chemokine ligand (CCL25) (as compared with adult CD8⁺ splenocytes). Mature CD8⁺ thymocytes have a similar α_E integrin⁺ CCL25 responsive phenotype, as do adult CD8⁺ RTE identified by intrathymic FITC injection. With increasing age, the frequency of CD8⁺ αE integrin⁺ splenocytes decreases, roughly correlating with thymic involution. Moreover, halting thymic output by thymectomy accelerates the age-dependent decline in peripheral CD8⁺ α_E integrin⁺ RTE phenotype cells. Low expression of CD44 distinguishes these CD8⁺ RTE from a population of memory phenotype α_E integrin⁺ CD8⁺ cells that are CD44^high. We conclude that CD8 ⁺ RTE have unique adhesive and chemotactic properties that distinguish them from naive CD8⁺ T cells. These properties may enable specialized microenvironmental and cell-cell interactions contributing to the fate of RTE in the periphery during the early post-thymic period. This phenotype will also facilitate the identification and isolation of RTE for further studies.