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<p>Diversity of T cell receptors (TCR) and immunoglobulins (Ig) is generated by V(D)J recombination of antigen receptor (AgR) loci. The Tcra-Tcrd locus is of particular interest because it displays a nested organization of Tcrd and Tcra gene segments and V(D)J recombination follows an intricate developmental program to assemble both TCRδ and TCRα repertoires. However, the mechanisms that dictate the developmental regulation of V(D)J recombination of the Tcra-Tcrd locus remain unclear. </p><p>We have previously shown that CCCTC-binding factor (CTCF) regulates Tcra gene transcription and rearrangement through organizing chromatin looping between CTCF- binding elements (CBEs). This study is one of many showing that CTCF functions as a chromatin organizer and transcriptional regulator genome-wide. However, detailed understanding of the impact of specific CBEs is needed to fully comprehend the biological function of CTCF and how CTCF influences the generation of the TCR repertoire during thymocyte development. Thus, we generated several mouse models with genetically modified CBEs to gain insight into the CTCF-dependent regulation of the Tcra-Tcrd locus. We revealed a CTCF-dependent chromatin interaction network at the Tcra-Tcrd locus in double-negative thymocytes. Disruption of a discrete chromatin loop encompassing Dδ, Jδ and Cδ gene segments allowed a single Vδ segment to frequently contact and rearrange to diversity and joining gene segments and dominate the adult TCRδ repertoire. Disruption of this loop also narrowed the TCRα repertoire, which, we believe, followed as a consequence of the restricted TCRδ repertoire. Hence, a single CTCF-mediated chromatin loop directly regulates TCRδ diversity and indirectly regulates TCRα diversity. In addition, we showed that insertion of an ectopic CBE can modify chromatin interactions and disrupt the rearrangement of particular Vδ gene segments. Finally, we investigated the role of YY1 in early T cell development by conditionally deleting YY1 in developing thymocytes. We found that early ablation of YY1 caused severe developmental defects in the DN compartment due to a dramatic increase in DN thymocyte apoptosis. Furthermore, late ablation of YY1 resulted in increased apoptosis of DP thymocytes and a restricted TCRα repertoire. Mechanistically, we showed that p53 was upregulated in both DN and DP YY1-deficient thymocytes. Eliminating p53 in YY1-deficient thymocytes rescued the survival and developmental defects, indicating that these YY1-dependent defects were p53-mediated. We conclude that YY1 is required to maintain cell viability during thymocyte development by thwarting the accumulation of p53.</p><p>Overall, this thesis work has shown that CTCF-dependent looping provides a central framework for lineage- and developmental stage-specific regulation of Tcra-Tcrd gene expression and rearrangements. In addition, we identified YY1 as a novel regulator of thymocyte viability.</p> / Dissertation

The aetiology of the inflammatory bowel diseases ulcerative colitis and Crohn’s disease remains uncertain. Genetic studies and model systems strongly implicate components of the IL-23/type-17 axis in the pathogenesis of disease, but the cellular and molecular mediators are uncertain. Using an IL-23R^gfp reporter mouse we analysed the cellular expression of IL-23R in homeostasis and disease. Whereas steady state expression in the intestine was dominated by a collection of unconventional lymphoid cells including ;gamma& ;delta& T-cells, we found rapid accumulation of IL-23R⁺CD4<sup>+</sup T-cells occurred in evolving colitis, and demonstrate an important role for IL-10 in the regulation of IL-23R specifically upon intestinal CD4+ T-cells. Examining the role of ;gamma& ;delta& T-cells in a model of IL-23-dependent colitis, we demonstrate apparent redundancy of such cells for the development of the adaptive CD4+ Th17 response. Furthermore, treatment with FTY720 which is known to inhibit lymphocyte recirculation did not attenuate disease nor reduce intestinal Th17 cell accumulation, suggesting the mechanisms of accumulation of Th17 cells in the intestine may differ from other anatomical sites. Next, we addressed the role of IL-21, a cytokine implicated in the development and effector functions of the IL-23/Th17 axis. Remarkably, we found that although IL-21 was pathogenic in models of chronic colitis, its effects on effector T-cell subsets were model-specific and included Th17 and Th1 cells. However, increased regulatory T-cell populations and reduced Ccl5 expression were common effects between models. Paradoxically, in a model of enteric infection, IL-21 was required for host defence, with IL-21R^-/- mice developing increased bacterial colonisation and severe colitis, shown to be driven by increased Th1/IFN-;gamma& responses. These studies provide novel insights into aspects of IL-23 driven cellular and molecular pathways in homeostasis and inflammation in the intestine, with implications for future therapeutic approaches to IBD.

Bien que le rejet humoral en transplantation rénale soit de mieux en mieux caractérisé, des difficultés diagnostiques persistent et son pronostic reste sombre. Objectifs : dans un tel contexte, nous avons privilégié 2 objectifs : (1) préciser les mécanismes physiopathologiques en cause lorsque des signes d’inflammation microvasculaire tels qu’une glomérulite sont observés isolément sur biopsie systématique ; (2) évaluer le rôle de l’autoimmunité au cours de la transplantation dans le but d’identifier d’éventuels marqueurs prédictifs d’une évolution particulière au cours de la greffe. Méthodes : dans un premier temps, nous avons effectué une analyse clinico-pathologique d’une cohorte de 20 patients avec glomérulite isolée sur biopsie systématique à 3 mois de la greffe, couplée à un phénotypage par analyse transcriptomique. Après cette première étape, les distorsions du répertoire B induites par la greffe ont été évaluées de manière séquentielle par technique d’immuno-empreinte chez 43 patients transplantés rénaux dans l’optique de la caractérisation éventuelle de nouveaux biomarqueurs à valeur diagnostique et pronostique. Résultats et conclusion : il n’y avait pas de différence significative à 3 ans de la transplantation entre le groupe de patients avec glomérulite isolée et le groupe témoin.Cependant, la cohorte de patients étudiée est hétérogène puisqu’une évolution péjorative a été observée chez une minorité de patients, pour lesquels des anticorps anti-HLA (non spécifiques du donneur) étaient plus souvent présents et le grade lésionnel plus élevé. L’étude du répertoire B illustre l’importante hétérogénéité interindividuelle des profils de réactivitévis à vis du tissu rénal. Après transplantation, l’apparition de bandes de réactivité additionnelles était notée chez 19/43 patients, et ce dans toutes les catégories anatomocliniques représentées. L’identification des cibles antigéniques est un complément indispensable de cette approche. / X

Aging-related decline in immunity is believed to be the main driver behind decreased vaccine efficacy and reduced resistance to infections in older adults. Unrepaired DNA damage is known to precipitate cellular senescence, which was hypothesized to be the underlying cause of certain age-related phenotypes. Consistent with this, some hallmarks of immune aging were more prevalent in individuals exposed to whole-body irradiation (WBI), which leaves no anatomical repository of undamaged hematopoietic cells. To decisively test whether and to what extent WBI in youth will leave a mark on the immune system as it ages, we exposed young male C57BL/ 6 mice to sublethal WBI (0.5-4 Gy), mimicking human survivor exposure during nuclear catastrophe. We followed lymphocyte homeostasis thorough the lifespan, response to vaccination, and ability to resist lethal viral challenge in the old age. None of the irradiated groups showed significant differences compared with mock-irradiated (0 Gy) animals for the parameters measured. Even the mice that received the highest dose of sublethal WBI in youth (4 Gy) exhibited equilibrated lymphocyte homeostasis, robust T-and B-cell responses to live attenuated West Nile virus (WNV) vaccine and full survival following vaccination upon lethal WNV challenge. Therefore, a single dose of nonlethal WBI in youth, resulting in widespread DNA damage and repopulation stress in hematopoietic cells, leaves no significant trace of increased immune aging in a lethal vaccine challenge model.