THE ROLE OF THE THYMUS AND RECENT THYMIC MIGRANTS IN THE MAINTENANCE OF THE ADULT PERIPHERAL LYMPHOCYTE POOL

The thymus is essential for the initial seeding of T cells to the peri phery, but its role in maintaining the adult T cell pool remains poorl y defined. We investigated whether changes to the rate of T cell expor t could form part of the mechanism(s) controlling the homeostatic regu lation of the size and composition of the peripheral T cell pool. Usin g neonatal thymi grafted under the kidney capsule, we found that irres pective of whether the pool was oversupplied (by thymic grafts) or und ersupplied (due to neonatal thymectomy), the thymic export rate was co nstant from both the host and graft thymus, and the periphery remained constant in size. Recent thymic emigrants (RTE) were also tracked to determine the extent of their acceptance into the T cell pool of a nor mal mouse. As a population, RTE are phenotypically mature, but were di stinct from resident T cells in the periphery, being released in a CD4 /CD8 ratio approximately twice that of established peripheral T cells. This export ratio is similar to that of T cells in the mature thymic compartment, but soon after entry into the periphery, the ratio tills, indicating separate thymic and peripheral regulation of the CD4/CD8 r atio. RTE may also be preferentially incorporated into the periphery, causing displacement of resident T cells, thus maintaining the size of the peripheral pool. Although not vital for the maintenance of a func tional T cell pool, the acceptance of RTE in a ''full'' peripheral poo l would ensure that the T cell receptor repertoire is kept diverse and that the T cell population encompasses a broad range of naive as well as memory T cells.