Recent thymic emigrants are distinct from most medullary thymocytes

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Abstract
In the mouse thymus, newly formed single positive (SP) cells spend an average of 14 days in the thymic medulla. During this time, phenotypic and functional maturation occurs with down‐regulation of CD69 and heat stable antigen (HSA), and up‐regulation of Qa‐2. Very little is known about the final steps that allow or direct these T cells to emigrate and join the recirculating peripheral T cell pool. Currently available data suggest that not all recent thymic emigrants (RTE) complete this maturational sequence in the medulla and that emigration may occur at any time during the medullary maturation stage. In this study, we have compared adhesion and activation marker expression on SP thymocytes, RTE and peripheral T cells to determine more precisely which SP medullary thymocytes are exported. Although RTE were heterogeneous for HSA and Qa‐2 expression, they were quite uniform with regard to the expression of other molecules. In contrast to medullary SP thymocytes, most RTE were L‐selectinhigh and CD69−. In addition, CD4+ CD8− and CD4− CD8+ RTE were phenotypically distinct from each other in that the former were β7 integrin−/low, CD45RBintermediate and CD45RC−, while the latter were β7 integrinhigh, CD45RBhigh and CD45RClow. These phenotypes were comparable to only a minor (as little as 6 %) subpopulation of medullary SP thymocytes. Overall, the data indicate that export of cells from the medullary pool of SP thymocytes is not random, but that a series of maturational events within the SP stage are necessary before export can occur.