A central role for thymic emigrants in peripheral T cell homeostasis

After initial seeding by thymic emigrants, homeostatic regulation of the T cell pool has been thought to occur entirely within the periphery. Here we report that the degree of thymic emigration directly affects the number and the CD4/CD8 ratio of peripheral T cells. We demonstrate that the increase in T cell pool size caused by the engraftment of 2, 6, or 9 thymic lobes correlates almost exactly with the number of emigrants exported from those grafts in the previous 3 weeks, regardless of how long the graft has been in place. The extent of the increase supports the concept of a 3-week period after thymic export in which emigrant T cells are exempt from peripheral T cell homeostasis. This apparent exclusion of recent thymic emigrants from the niche-based regulation of peripheral T cell numbers ensures repertoire turnover throughout adult life and provides the basis of a direct and previously unrecognized role for the thymus in the regulation of peripheral T cell homeostasis.