Mechanisms underlying the progressive loss of peripheral CD4 T cells in HIV and SIV infections include a possible impairment of thymic T-cell production. As we lack assays to measure thymic function directly, different factors, discussed here, have confounded interpretations regarding the role of the thymus during HIV infection. In a recent issue of Science, Terszowski et al.  reported the existence of a regular second thymus in the mouse, and suggested reconsidering the effect of thoracic thymectomy on de novo T-cell production. Our novel finding of an ectopic functional thymus in an SIV-infected adult monkey thus identifies a second thymus as yet another confounder regarding thymectomy experiments, and could reinstate the thymus as a major player in HIV pathogenesis.
The measurement of T-cell receptor excision circles (TREC), DNA circles produced during intrathymic T-cell development (Fig. 1a), was used to assess thymic output. After the early excitement of the discovery that the thymus is functional throughout life, and that its function can be restored in HIV infection , the thymus fell into relative obscurity. TREC are stable over time but not replicated during mitosis. Because HIV/SIV infections provoke an increase in immune activation, the resulting dilution of TREC by the subsequent division of peripheral T cells confounds the estimation of thymic output. This has contributed to the controversy surrounding the assessment of the proportion of peripheral TREC as the sole measure of thymic output, and the refusal to acknowledge a role of the thymus in HIV pathogenesis . However, absolute numbers of naive CD4+CD45RA+TREC+ and CD8+CD45RA+TREC+ T cells/μl of blood, and not frequencies of TREC in bulk populations, more accurately reflect thymic output (Fig. 1b). Using these absolute counts, we previously demonstrated in a unique study of long-term infection with an attenuated SIV, which induces much less immune activation than wild-type virus , that thymic output correlated with the rate of disease progression. More importantly, we found that thymic T-cell production was substantially increased in monkeys that did not progress towards AIDS .
Another way to assess thymic output is to compare thymectomized versus non-thymectomized individuals. Some studies concluded that the thymus plays a minor role in SIV pathogenesis  and immune reconstitution in humans on HAART . However, these studies assumed that the thymectomies were complete. After the publication describing a regular second thymus in mice , we report here that a cervical thymus also exists in the adult rhesus macaque and is functional during SIV infection. We compared the immunophenotype of thymocytes from two animals infected with attenuated SIV . The thoracic thymus in animal no. 76 showed a typical lobal structure, and was large with a cellularity of 4 × 108 cells. We did not search for an ectopic thymus in this animal. In animal no. 72, the thoracic thymus was completely involuted, but we detected an ectopic thymus in the neck around the trachea. The cervical thymus was collected and analysed independently from the thoracic thymic remnant. Despite the acellularity of the thoracic thymus (data not shown), the cervical thymus contained 1.5 × 107 cells. The development of T cells in the cervical thymus of animal no. 72 progressed appropriately, with normal percentages of CD4+CD8+, CD4 and CD8 T cells, as well as normal T-cell receptor (CD3) expression in the different subpopulations similar to the thoracic thymus of animal no. 76 (Fig. 1c).
During attenuated SIV infection, the thymic output is highly correlated with the non-progression to AIDS , and this T-cell production can be caused by a cervical thymus alone. Moreover, studies showed an increase in thymic size  and naive T cells in HIV-1-infected patients after the initiation of HAART . In addition, a higher naive TREC-positive T-cell number is associated with a good immunological response to antiretroviral treatment . In this context, even TREC frequencies suggest a rise in thymic output . In spite of all this indirect evidence, numerous studies have failed clearly to implicate the thymus in the complex and multifactorial HIV pathogenesis. During wild-type SIV or non-treated HIV infections, high levels of immune activation occur and can thus mask an increase in thymic output as measured by TREC proportions alone . Moreover, because reports of cervical thymus in humans mainly involved pathological masses , the prevalence of a second thymus may be underestimated as it can be small and located deep in the neck . This second thymus may thus obscure the impact of thymectomy on the course of HIV infection. In this new light, the active role of the thymus in HIV pathogenesis can no longer be excluded.
This paper is dedicated to the memory of Bruno Hurtrel who initiated this study.
The authors would like to thank F. Barré-Sinoussi and B.D. Jamieson for insightful comments.
Sponsorship: This study was supported by the Agence Nationale de Recherches sur le Sida and the Institut Pasteur.
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