Impact of thymectomy on the peripheral T cell pool in rhesus macaques before and after infection with simian immunodeficiency virus.

The goal of this study was to define, by surgical removal of the thymus in juvenile rhesus macaques, the role of the thymus in peripheral T cell homeostasis and to assess the significance of thymic output in SIV infection. By monitoring the changes in phenotypic T cell markers as well as in the numbers of TCR excisional circles--a recently described marker for recent thymic emigrants--following thymectomy, we present evidence that surgical thymectomy in juvenile macaques results in a faster decay of peripheral CD4(+) cells, but does not cause a substantial shift in CD45RA(+) and CD45RA(-) populations. We were able to measure a thymic output of 0.32% and 0.21% per day of CD4(+) and CD8(+) cells, respectively. No compensatory extra-thymic source was detected in lymphoid tissues, although there was a small compensatory increase in T cell proliferation in the peripheral T cell pool. After SIV infection, thymectomized animals did not have higher viral loads, greater T cell decay, or faster disease progression. We therefore conclude that peripheral destructive processes, rather than a loss of thymic output, appear to be the main causes of T cell depletion in SIV infection.