The thymic stroma primarily comprises epithelial, mesenchymal and endothelial cells, interspersed with those of haematopoietic origin. Thymic epithelial cells (TECs) are highly heterogeneous, but can be divided into two broad lineages, cortical and medullary, based on phenotype, functionality and location. A population of Plet1+ TEC progenitors have been identified which, when isolated from mouse E12.5 or E15.5 fetal thymus, reaggregated, and grafted, can produce a functional thymus. However, the potential of individual progenitors to form cortex and/or medulla is undefined. The main aim of this thesis was to use retrospective clonal analysis to ascertain the point during thymus ontogeny at which the cortical and medullary lineages diverge. To this end, I used transgenic mice carrying a ubiquitous ROSA26laacZ reporter gene (where a duplication within lacZ encodes non-functional b-galactosidase). Here, rare, random laacZ-lacZ genetic recombinations result in heritable expression of functional b-gal, producing labelled clones. As this occurs at a known frequency, determination of TEC numbers would enable calculation of the expected number of TEC clones present throughout ontogeny. Due to the lack of quantitative data on all thymic cell populations, I determined the size not only of TEC (lin-EpCAM+), but also haematopoietic (CD45+), mesenchymal (lin-PDGFRa+ and/or lin-PDGFRb+) and endothelial (lin-CD31+) populations from E12.5 until E17.5. I then showed that the absolute number of Plet1+ TECs remains constant during this time, although the proportion of Plet1+ cells in cycle decreases. From these collective data, I propose a model for the role of the Plet1+ population in thymus development, in which Plet1+ cells continually give rise to Plet1- TECs in a self-renewing manner. Finally, I present a ‘dual origin coefficient’ strategy for analysis of a library of prospective TEC clones. I calculated the number of TEC lacZ+ clones expected to be present throughout thymus ontogeny, selecting an appropriate developmental stage for analysis. Although I observed several clones of apparent mesenchymal origin, supporting a single origin for intrathymic and capsular mesenchyme at E15.5, I observed no TEC clones in this extensive analysis. The CpG content of the ROSA26 promoter suggests a possibility of methylation-induced silencing brought about by de novo methylation of the lacZ reporter gene.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal.

The genetic mechanisms underlying the specification of thymic epithelial (TE) lineage cells are poorly understood. Foxn1 is an early specific marker of thymic epithelial cells (TECs) in the third pharyngeal pouch (3PP) and is required for development of all mature TE lineage cells but does not specify the TE lineage. The upstream regulators of Foxn1 are currently unknown, however evidence points to a potential role for Pax1 and Pax9. While the thymus phenotypes of the Pax1-/- and Pax9-/- mutant mice have been investigated in detail and TECs in these mice are known to express Foxn1, the possibility of functional redundancy exists and the compound mutants of these genes have not been studied. Therefore, the aim of this thesis was to test the hypothesis that Pax1 and Pax9 are required for TE lineage specification and regulation of Foxn1 expression. This hypothesis was addressed by analysis of thymus development and TEC function in Pax1/Pax9 compound mutant mice. The data presented in this thesis indicates that prenatally, Pax1 and Pax9 cooperatively regulate thymus organogenesis, such that the size, structure and location of the thymus is affected in a Pax1/Pax9 gene dosage-dependent manner, and the Pax1unex/unexPax9lacZ/lacZ embryo is functionally athymic. Furthermore, they establish that the thymic rudiment of Pax1unex/unexPax9lacZ/lacZ embryos does not express Foxn1, establishing that Pax1 and Pax9 are required together for the initiation of Foxn1 and suggesting they are required to specify the TEC lineage. Postnatally, enlarged blood vessels observed in the Pax1unex/unex thymus suggested a role for Pax1 in vascularisation of the thymus. In addition, the effect of loss of one or more Pax1/Pax9 alleles on the expression of Foxn1 and other genes known to regulate TEC development or function was assessed. These data demonstrate that Pax1 and Pax9 co-operate to regulate Foxn1 in a dosage-dependent manner. Furthermore, Pax1 and Pax9 appear to negatively regulate both Hoxa3 and Vegfa, providing a possible explanation for the enlarged blood vessels in the postnatal Pax1unex/unex thymus. Finally, an inducible and reversible recombinase-mediated cassette exchange system that will allow the knockdown of Pax1 and Pax9 at defined time points during development has been established, that has the potential to test the function of these genes during thymus organogenesis and in the postnatal thymus.

The development and function of T helper (Th) cells and regulatory T cells (Tregs) are plastic processes that are regulated by cytokines. In our project we first analyzed the effect of different cytokines on the development of induced (i) Tregs. It has been demonstrated that iTregs arise from CD4+ CD25- T cells upon stimulation with alloantigen in the presence of transforming growth factor β (TGF-β). The development of these Tregs and their proliferation were inhibited by interleukin (IL)-4 and IL-12. The aquired results also demonstrated distinct responses of naturally occuring (n) Tregs and iTregs to the regulatory action of IL-4 and an opposite role of IL-4 in maintenance of nTregs and iTregs phenotype. An important role in the induction of T cell subsets may play also mesenchymal stem cells (MSCs) which can, under specific conditions, produce TGF-β and IL-6. Depending on the current production of TGF-β or IL-6, MSCs can qualitatively regulate the ration between Tregs and Th17 cells. Anti-inflammatory Tregs and pro-inflammatory Th17 cells are induced upon stimulation in the presence of TGF-β and TGF-β and IL-6, respectively. In addition to our previous work we studied the role of IL-12 in the development of Tregs and Th17 cells. It was shown that Treg and also Th17 cell differentiation was...

O timo é um órgão essencial para a maturação, diferenciação e seleção de linfócitos T, consequentemente fundamental para o desenvolvimento da imunidade do organismo. Além do papel na manutenção da integridade tecidual, os vasos sanguíneos tímicos desempenham função no processo de migração de células precursoras de linfócitos para o interior do órgão. Sua arquitetura típica é caracterizada por grandes vasos localizados na junção corticomedular e uma fina cadeia de ramificações e anastomoses no interior do córtex, porém são pouco conhecidas as bases moleculares que induzem a formação desta particular vascularização, bem como os exatos mecanismos que desencadeiam a involução do órgão, evento relacionado a alterações nos padrões vasculares. Entretanto, tem sido demonstrado que as interações entre o estroma, que inclui o endotélio vascular, e o compartimento hematopoiético tímico desempenham papéis cruciais na funcionalidade do órgão. Sendo o VEGF um fator angiogênico essencial na formação do leito vascular tecidual e na modulação de funções diretamente relacionadas à vascularização, objetivamos neste estudo avaliar a expressão gênica e protéica deste fator de crescimento durante estágios de desenvolvimento e involução do órgão. Foram utilizadas amostras de timo suíno em 3 diferentes estágios de desenvolvimento fetal (65, 85 e 111 dias) e 2 estágios de vida pós-natal, um compreendendo fase de maturação (5 meses) e outro de involução do órgão (2 anos), compreendendo um total de 25 animais, divididos em 5 grupos (n = 5). A análise da expressão relativa do mRNA do sistema VEGF-A, acessada por meio de PCR em tempo real, apresentou modificações tempo-dependentes significativas (P<0,05) em relação ao controle endógeno GAPDH. Também foram observadas diferenças significativas (P0,05) na quantificação vascular por meio de estereologia entre as idades. A imunolocalização da proteína do VEGF e de seus receptores Flt-1 e KDR foi identificada no timo de todos os grupos experimentais e variou entre os grupos pré e pós-natais, com expressão mais rara nestes últimos, o que sugere diferenças temporais na modulação do mRNA no órgão. Assim como as células endoteliais, as células epiteliais tímicas demonstraram modulação pelo sistema VEGF-A, indicando ação parácrina e/ou autócrina deste fator de crescimento no timo. Os valores das variáveis estereológicas Sv[m] e Vv[m] do timo fetal em período próximo ao nascimento mantiveram-se maiores do que no córtex e se correlacionaram positivamente com a expressão do mRNA do KDR e negativamente com a do Flt-1 até a idade adulta. Ao contrário da expressão protéica, a expressão gênica do sistema VEGF-A pode se correlacionar negativamente com aspectos quantitativos da vascularização do timo pós-natal, como observado para o mRNA do VEGF e seu receptor Flt-1 a partir dos 5 meses de idade, quando os valores das variáveis Vv[vasos,timo], Sv[vasos,timo] e Lv[vasos,timo] diminuíram significativamente (P0,05), o que sugere uma resposta de compensação ou manutenção de uma condição hipóxica instalada no órgão. / Crucial for maturation, differentiation and selection of T lymphocytes, the thymus is an essential organ to the development of immunity. In addition to its role in the maintenance of tissue integrity, thymus blood vessels play a role in migration of T-cell precursors into the organ, and show a typical architecture characterized by the presence of large blood vessels at the corticomedullary junction and fine network of branching vessels and anastomosing arcades extending into the cortex. The molecular basis of this typical vasculature and the exact mechanisms that trigger thymus involution are both not well understood. However, it has been shown that the interactions between thymus stroma, including vascular endothelium and the hematopoietic compartment play crucial roles for normal organ function. Thus, because VEGF is an essential angiogenic factor for induction of the vascular bed and modulation of functions directly related with vascularization, the aim of this study was to assess the temporal protein and mRNA expression of VEGF and its receptors Flt-1 and KDR in the thymus during early stages of its development and involution. Samples of thymus from 3 different stages of fetal development (65, 85, and 111 days) and 2 stages of postnatal life of pigs, including one stage of maturation (5 months) and another of involution (2 years) comprising a total of 25 animals that were divided into 5 groups (n = 5). Relative mRNA expression of VEGF-A system, accessed by real-time PCR, showed temporal significant changes (P<0.05) compared to endogenous control GAPDH. Quantification of blood vessels by means of stereology also showed significant differences (P0.05) between ages. Positive immunostaining for VEGF and its receptors Flt-1 and KDR was identified in the thymus of all groups and it varied between the pre and postnatal groups with rare expression in the latter, suggesting temporal differences in the modulation of mRNA. Endothelial and epithelial cells showed modulation by the VEGF-A system, suggesting paracrine or alternatively autocrine activity of this growth factor. The values of the stereological parameters Sv[m] and Vv[m] of the thymus in fetal period around the birth remained higher than in the cortex and upregulated with mRNA expression of KDR and downregulated with mRNA expression of Flt-1 until adulthood. Unlike protein expression, gene expression gene expression of VEGF-A system can be downregulated with quantitative aspects of the postnatal thymus vascularization as observed for the mRNA of VEGF and its receptor Flt-1 from the age of 5 months, when the values of the stereological parameters Vv[vessels,thymus], Sv[vessels,thymus] and Lv[vessels,thymus] decreased significantly (P0.05), which suggests a compensatory or maintenance response to an hypoxic condition in the organ.

Regulatorische T-Lymphozyten differenzieren sich im fetalen Thymus unter dem Einfluss des Transkriptionsfaktors FoxP3. Sie sind für die Aufrechterhaltung des Gleichgewichts des Immunsystems wichtig. Es wurde untersucht ob eine Chorioamnionitis, induziert durch intraamniotische Endotoxingabe, die fetale Thymusentwicklung beeinflusst. Den Mutterschafen wurde fünf Tage, zwei Tage, einen Tag oder fünf Stunden vor der Sectio cesarea 10mg Endotoxin intraamniotisch verabreicht. Die Sectio cesarea wurde bei einem Gestationsalter von 123 Tagen durchgeführt. Der entnommene Thymus wurde gewogen, Nabelschnurblutlymphozyten und Plamakortisolwerte wurden bestimmt. Glukokortikoidrezeptoren, aktivierte Caspase-3-, Ki67-, PCNA-, NFkB- und FoxP3-positive Zellen wurden immunohistochemisch nachgewiesen. Das Thymusgewicht war im Verhältnis zum Körpergewicht der Lämmer nach intraamniotischer Endotoxingabe zu allen gemessenen Zeitpunkten verringert. Die zirkulierenden Lymphozyten im Nabelschnurblut nahmen einen Tag nach Endotoxingabe um 40% ab. Die Endotoxingabe führte zu einem vorübergehenden Anstieg der Plasmakortisolwerte, zu einer Verdoppelung NFkB positiver Zellen und zu einer Abnahme Foxp3 positiver Zellen in der Thymusrinde einen Tag nach Endotoxingabe. Die intraamniotische Verabreichung eines Endotoxins führte im Schafmodell zu Veränderungen im fetalen Thymus. / Regulatory T-lymphocytes differentiate in the fetal thymus under the control of the transcription factor FoxP3. T-lymphocytes mediate homeostasis of the immune system. The objective was whether chorioamnionitis, caused by endotoxin,would modulate fetal thymus development. An intaamniotic injection of 10mg endotoxin was given to the sheep five days, two days, one day or five hours before delivery at 123 gestation days. Thymus weight, cord blood lymphocytes and plasma cortisol were measured. Glucocorticoid receptor-, activated caspase-3-, Ki67-, proliferating cell nuclear antigen-, nuclear factor kB-, and FoxP3-positive cells were immunohistochemically evaluated. Thymus-to-body weight ratios were reduced in all endotoxin groups. There was a decrease of circulation lymphoctes after intraamniotic endotoxin exposure by 40% after one day. Plasma cortisol concentration increased transiently, nuclear factor kB positive cells in thymic cortex doubled and FoxP3 positive cells were reduced one day after endotoxin exposure. Intraamniotic exposure to endotoxin induced thymic changes in fetal sheep.

Rationale: Eosinophils are “end cell” leucocytes, associated with allergy, asthma and helminthiasis. At sites of inflammation, eosinophils may modulate immune response through expression of the extra-hepatic tryptophan-catabolising enzyme, Indoleamine 2, 3-dioxygenase (IDO). Kynurenines, products of tryptophan cleavage, induce apoptosis of T-cells, including thymocytes. Eosinophils naturally home to the thymi in mammals. Thymus is a primary lymphoid organ, where T-cells develop and undergo selection. My hypothesis is that eosinophils homing to the thymi participate in T-cell development through their expression of IDO. Methods: Immunohistochemistry revealed eosinophils in thymic tissue. Immunocytochemistry and flow cytometry were used to locate IDO protein expression in the thymus particularly in thymic eosinophils. RT-PCR and real-time PCR determined the presence of IDO mRNA in the thymus. Results: thymic eosinophils express IDO and infiltrate compartments associated with negative selection. The highest IDO transcription correlated with the influx of eosinophils and prevalence of immature thymocytes. / Experimental Medicine

Early T-cell development is regulated by a complex interplay between transcription factors and developmental cues which ensure that functional T-cells are produced within the thymus. Early thymocytes integrate these signals in a step-wise fashion that progressively restricts their lineage potential as they transition through the early stages of T-cell development. Gene knockout studies have shown that the E-protein transcription factor HEB is required for normal thymocyte development. Furthermore, many additional key regulators such as Notch1 have been identified, but the connections among them and their specific roles in early T-cell development have not been well established. In this thesis, I set out to determine the specific roles of HEB at the beta-selection checkpoint and to establish connections between HEB and the key regulators within the gene regulatory network that orchestrates early T-cell development. To facilitate these studies, I generated a series of new mouse models including HEBAlt transgenic mice that express a short form of HEB called HEBAlt, which enabled me to answer specific questions and examine rare populations. First, my studies of HEB-/- mice allowed me to identify an early block in T-cell development, which was alleviated upon the addition of an HEBAlt transgene. Furthermore, I identified pTa and CD3e signalling as specific targets of HEBAlt during -selection. Second, my studies on HEB-/- mice revealed that they have a defect in T-cell commitment, with compromised Notch1 function and a tendency to become DN1-like cells. Moreover, the DN1-like cells could be induced to differentiate into thymic NK cells, revealing a role for HEB in the T/NK cell lineage decision. This study has revealed a new set of interactions among HEB, Notch1, and GATA3 that regulate the T-cell fate choice in developing thymocytes. Unexpectedly, my studies have also provided evidence for a role of HEBAlt in lymphomagenesis, highlighting the strict regulation of E-protein function that is necessary to ensure normal T-cell development.

Early T-cell development is regulated by a complex interplay between transcription factors and developmental cues which ensure that functional T-cells are produced within the thymus. Early thymocytes integrate these signals in a step-wise fashion that progressively restricts their lineage potential as they transition through the early stages of T-cell development. Gene knockout studies have shown that the E-protein transcription factor HEB is required for normal thymocyte development. Furthermore, many additional key regulators such as Notch1 have been identified, but the connections among them and their specific roles in early T-cell development have not been well established. In this thesis, I set out to determine the specific roles of HEB at the beta-selection checkpoint and to establish connections between HEB and the key regulators within the gene regulatory network that orchestrates early T-cell development. To facilitate these studies, I generated a series of new mouse models including HEBAlt transgenic mice that express a short form of HEB called HEBAlt, which enabled me to answer specific questions and examine rare populations. First, my studies of HEB-/- mice allowed me to identify an early block in T-cell development, which was alleviated upon the addition of an HEBAlt transgene. Furthermore, I identified pTa and CD3e signalling as specific targets of HEBAlt during -selection. Second, my studies on HEB-/- mice revealed that they have a defect in T-cell commitment, with compromised Notch1 function and a tendency to become DN1-like cells. Moreover, the DN1-like cells could be induced to differentiate into thymic NK cells, revealing a role for HEB in the T/NK cell lineage decision. This study has revealed a new set of interactions among HEB, Notch1, and GATA3 that regulate the T-cell fate choice in developing thymocytes. Unexpectedly, my studies have also provided evidence for a role of HEBAlt in lymphomagenesis, highlighting the strict regulation of E-protein function that is necessary to ensure normal T-cell development.

Thèse doctorat : Sciences de la vie et de la santé : Paris 12 : 2003. / Titre provenant de l'écran-titre.