The supply of naive T cells by the thymus normally requires precursor T cell proliferation within the thymus and would be particularly important in the setting of HIV infection when both naive and memory T cells are progressively depleted. As a robust, quantitative index of intrathymic proliferation, the ratio of different T cell receptor excision circles (TRECs), molecular markers of distinct T cell receptor rearrangements occurring at different stages of thymocyte development, was measured in peripheral blood-mononuclear cells (PBMCs). This ratio has the virtue that it is a "signature" of thymic emigrants throughout their entire life and, thus, can be measured in peripheral cell populations that are easy to obtain. Using the new assay, we evaluated the effect of HIV infection on intrathymic precursor T cell proliferation by longitudinal analysis of PBMCs from recently infected individuals. Our findings reveal a substantial reduction in intrathymic proliferation. The analysis also indicates the existence of a compensatory mechanism acting to sustain the numbers of recent thymic emigrants (RTEs) in the periphery.

Although the thymus constitutes a target organ for most protein and steroid hormones, it has been quite difficult to determine the precise control exerted in vivo by the endocrine system upon thymic function. The biological role of the thymus is to ensure the generation of a diversified population of peripheral T cells able to respond to non-self-antigens but nevertheless tolerant to selfantigens. For a long time, thymic function could not be monitored, as a consequence of the absence of adequate technology to differentiate recent thymic emigrants from naive T cells. The generation of T cell receptor (TCR) diversity occurs in the thymus through recombination of gene segments encoding the variable parts of the TCR and chains. During these processes, by-products of the rearrangements are generated in the form of TCR excision circles (TRECs). As these molecules are lost upon further cell division, their quantification is actually considered as a very valuable tool to estimate thymic function. The most appropriate TREC is Rec-J TREC or signal joint TREC resulting from Rec-J rearrangement (TCRD deletion) that occurs late during thymopoiesis, before V -J rearrangement. Here we describe how TREC quantification is a powerful and reliable method to evaluate the impact of hormones and endocrine disorders upon thymic function.

In chronic HIV infection, most untreated patients lose naive CD4 ؉ and CD8 ؉ T cells, whereas a minority preserve them despite persistent high viremia. Although antiretroviral therapy (ART)-mediated viral suppression generally results in a rise of naive and total CD4 ؉ T cells, certain patients experience very little or no T-cell reconstitution. High peripheral T-cell activation has been linked to poor clinical outcomes, interfering with previous evaluations of thymic function in disease progression and therapy-mediated T-cell recovery. To circumvent this, we used the sj/␤TREC ratio, a robust index of thymopoiesis that is independent of peripheral T-cell proliferation, to evaluate the thymic contribution to the preservation and restoration of naive CD4 ؉ T cells. We show that the loss of naive and total CD4 ؉ T cells is the result of or is exacerbated by a sustained thymic defect, whereas efficient thymopoiesis supports naive and total CD4 ؉ T-cell maintenance in slow progressor patients. In ART-treated patients, CD4 ؉ T-cell recovery was associated with the normalization of thymopoiesis, whereas the thymic defect persisted in aviremic patients who failed to recover CD4 ؉ T-cell counts. Overall, we demonstrate that efficient thymopoiesis is key in the natural maintenance and in therapymediated recovery of naive and total CD4 ؉ T cells. (Blood. 2007;109:2912-2920)