RECENT THYMIC EMIGRANTS ARE DISTINCT FROM MOST MEDULLARY THYMOCYTES

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 hea t stable antigen (HSA), and up-regulation of Qa-2. Very little is know n about the final steps that allow or direct these T cells to emigrate and join the recirculating peripheral T cell pool. Currently availabl e data suggest that not all recent thymic emigrants (RTE) complete thi s maturational sequence in the medulla and that emigration may occur a t any time during the medullary maturation stage. In this study, we ha ve compared adhesion and activation marker expression on SP thymocytes , RTE and peripheral T cells to determine more precisely which SP medu llary thymocytes are exported. Although RTE were heterogeneous for HSA and Qa-2 expression, they were quite uniform with regard to the expre ssion of other molecules. In contrast to medullary SP thymocytes, most RTE were L-selectin(high) and CD69(-). In addition, CD4(+) CD8(-) and CD4(-) CD8(+) RTE were phenotypically distinct from each other in tha t the former were beta(7) integtin(-/low), CD45RB(intermediate) and CD 45RC(-), while the latter were beta(7) integrin(high), CD45RB(high) an d CD45RC(low). These phenotypes were comparable to only a minor (as li ttle as 6 %) subpopulation of medullary SP thymocytes. Overall, the da ta indicate that export of cells from the medullary pool of SP thymocy tes is not random, but that a series of maturational events within the SP stage are necessary before export can occur.