THYMIC T-CELL EXPORT IS NOT INFLUENCED BY THE PERIPHERAL T-CELL POOL

The peripheral T cell pool is maintained both by export of naive T cel ls from the thymus and by post-thymic expansion of activated/memory T cells. However, it is not known whether the thymus can alter its outpu t following peripheral T cell depletion. Using intrathymic injection o f fluorescein isothiocyanate to detect recent thymic emigrants (RTE), we directly tested whether the thymus is able to alter the number of R TE or the CD4:CD8 ratio of RTE emigrating to the periphery in response to in vivo depletion of total peripheral T cells or CD4 T cells, resp ectively. Depletion of peripheral T cells was achieved with anti-Thy-1 or anti-CD4, at doses that did not affect thymocyte numbers. Depletio n of greater than 70% of peripheral T cells by treatment with anti-Thy -1 in vivo did not alter the number or cell cycle status of RTE traffi cking to lymph nodes or spleen during the peripheral reconstitution ph ase (6, 9, 12 days). Similarly, depletion of the majority of CD4 T cel ls, which significantly reduced the peripheral CD4:CD8 T cell ratio, d id not alter the total number or the proportion of CD4(+) CD8(-) RTE i n peripheral lymphoid organs. These data clearly indicate that thymic output is not influenced by downstream alterations in peripheral T cel l pool size or CD4:CD8 ratio. Rather we contend that thymic T cell exp ort is internally regulated by as yet undefined mechanisms.