The thymus generates a lineage-committed subset of Regulatory T-cells (Tregs), best identified by the expression of the transcription factor FOXP3. The development of thymus-derived Tregs is known to require high avidity interaction with MHC-self peptides leading to the generation of self-reactive Tregs fundamental for the maintenance of self-tolerance. Notwithstanding their crucial role in the control of immune responses, human thymic Treg differentiation remains poorly understood. In this mini-review we will focus on the developmental stages at which Treg lineage commitment occurs, and their spatial localization in the human thymus, reviewing the molecular requirements, including TCR and cytokine signalling, as well as the cellular interactions involved. An overview of the impact of described thymic defects on the Treg compartment will be provided, illustrating the importance of these in vivo models to investigate human Treg development.

Treg cells, best identified by the expression of the transcription factor FOXP3, play a crucial role in maintaining self‐tolerance. Natural Treg cells constitute an independent thymus‐derived T‐cell lineage whose developmental program in humans is still ill‐defined. Here, we provide evidence of a Treg‐cell differentiation pathway at the double positive (DP) stage, prior to commitment to the CD4⁺ or CD8⁺ lineage, in pediatric thymuses. FOXP3⁺ DP cells displayed a functional IL‐7 receptor and increased Bcl‐2 levels that may protect them from cell death/negative selection, and an activated/suppressive phenotype that was lost as CD4 single positive (SP) cells matured and acquired egress markers. A subpopulation of FOXP3⁺ DP thymocytes expressing CD103 likely represents the precursor of FOXP3⁺ CD8SP cells, which homogeneously expressed this mucosal‐homing molecule. Finally, co‐cultures of DP thymocytes with primary thymic epithelial cells and multiple linear regression analyses support that FOXP3⁺ SP cells are largely derived from FOXP3⁺ DP thymocytes. Overall, our data suggest that human Treg‐cell lineage commitment significantly occurs at the DP stage with possible implications for the diversity and autoreactivity of the natural Treg‐cell repertoire.

The thymus generates a T‐cell lineage dedicated to immune regulation, ‘naturally occurring’ regulatory T cells, best specified by the forkhead family transcription factor Foxp3. Here, we have conducted a parallel study in humans and mice where we have dissected the earliest stages of Foxp3 induction during thymocyte development. By analyzing a large collection of 21 human thymuses we show that Foxp3 can be consistently detected in CD4 immature single positive thymocytes that precede the CD4(⁺)CD8(⁺) (double positive, DP) stage. The reduced levels of CD3 expression found at this stage of human thymocyte development raise the question of TCR (T‐cell receptor) requirement for Foxp3 induction. We further show that, in mice, Foxp3 expression was also detected in pre‐DP thymocytes of TCRα‐sufficient but not in TCRα‐deficient animals, genetically showing the TCR dependence of Foxp3 expression at pre‐DP stages of T‐cell development.

The contribution of naive CD4⁺ T cells to the pool of HIV-infected cells remains poorly described. This study aimed at evaluating HIV infection in naive T-cell subsets in viremic and HAART-treated patients, together with various parameters implicated in naive T-cell homeostasis, in order to better understand infection in these subsets. HIV provirus was quantified in various FACS-sorted CD4/CD8 T-cell subsets [recent thymic emigrants (RTEs), non-RTE naives and memory T cells] purified from peripheral blood cells of untreated viremic and HAART-treated aviremic HIV-infected patients. HIV proviral DNA was quantified using a highly sensitive real-time PCR assay allowing detection of one HIV copy in 10⁵ cells. Intrathymic precursor T-cell proliferation and circulating T-cell cycling were, respectively, evaluated through measurement of the sj/βTREC ratio (signal joint T-Cell Receptor Excision Circle frequency divided by DβJβTREC frequency) and Ki-67 expression. Plasma interleukin (IL)-7 concentrations were measured by ELISA. RTEs and non-RTEs were equally HIV infected. Altogether, naive CD4⁺ T cells represented 0.24%-60% of the infected cells. In contrast, HIV DNA was undetectable in naive CD8⁺ T cells. RTE infection rate directly correlated with IL-7 plasma levels (r = 0.607, P = 0.0035) but was independent from plasma viral load, peripheral T-cell cycling and intrathymic precursor T-cell proliferation. We demonstrated that RTEs are effectively HIV infected. The similar infection rate observed in RTEs and other naive T cells, its relationship with plasma IL-7 levels, together with the lack of correlation between RTE infection and either thymic or peripheral proliferation, strongly suggests that RTE infection occurs either late during thymopoiesis or early on during their extrathymic maturation.

Clinical evaluation of genetically modified rodent malaria parasites for whole-sporozoite immunization against P. falciparum in healthy volunteers.