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A diverse repertoire among peripheral T cells is established in early life by thymic export when the naive T cell pool is first formed. In contrast, during adult life the thymus has been thought to play only a minor role in T cell homeostasis. As individuals age there is an increasing proportion of peripheral T cells bearing a memory phenotype, as well as a corresponding decrease in the number of naive T cells. The change in the composition of the peripheral T cell pool with age is thought to occur as a result of reduced or completely curtailed thymic export following thymic involution at puberty together with the antigen-driven expansion of naive T cells in the periphery. We examined thymic export throughout life in fetal, neonatal and aged sheep. We found that the thymus in adult animals showed an efficiency of production and export on a per gram basis equivalent to that observed for much younger animals, and continued to export substantial numbers of T cells long after puberty. The data demonstrate that naive T cells constantly enter the peripheral T cell pool at the same rate throughout fetal, neonatal and adult life, and that one in every 50 T cells in the peripheral lymphoid tissues of aged sheep had emigrated from the thymus in the previous 24 h. The data suggest that restoration by the thymus of a normal peripheral T cell repertoire in chronic T cell-depleting conditions should be possible in adult patients, provided the thymus is not damaged by disease or therapy.

SUMMARYWe have studied the appearance and phenotype of recent thymic emigrants in blood, spleen and lymph nodes (LN) of neonatal lambs. Using in situ labelling of thymocytes with¯uoroscein isothiocyanate (FITC), we examined the expression of the LN homing receptor L-selectin on ab and cd subsets of recent thymic emigrants 24 hr after labelling. There were marked differences in the proportions of CD4 + , CD8 + and cd T-cell receptor (TCR + ) cells exported from the thymus to spleen compared to lymph nodes. Spleen was enriched in CD8 + and cd TCR + emigrants while LN were enriched in CD4 + emigrants. There were also marked differences in the expression of L-selectin by emigrants homing to spleen compared with those homing to lymph nodes. While the majority of thymic emigrants in LN expressed L-selectin, considerably fewer emigrants in spleen wereThe presence of large numbers of CD8 + L-selectin ± and cd TCR + L-selectin ± thymic emigrants homing to spleen raises the possibility that unique homing receptor speci®cities underpin the migration of T cells to spleen as distinct from lymph nodes.