Limited role for the thymus in SIV pathogenesis

The role of the thymus in the pathogenesis of AIDS is a frequently discussed and controversial topic. Tuttleton Arron et al. studied the role of thymic output in SIV infection directly, by comparing the dynamics of TCR excision circles and CD4+ and CD8+ T cell numbers in healthy and in SIV‐infected euthymic and thymectomized rhesus macaques. In this issue of the European Journal of Immunology, they report that complete abrogation of thymic output in juvenile rhesus macaques has very little impact on the peripheral T cell compartment, both in healthy and in SIV‐infected macaques. Their data therefore suggest that the main cause of CD4+ T cell loss during SIV infection is the peripheral effect of SIV, and not its effect on thymic output.