My dissertation research focused on studying mechanisms of immunomodulation by probiotic lactobacilli on innate and T cell immune responses induced by rotavirus infection and vaccines in a gnotobiotic pig model of human rotavirus (HRV) infection and vaccination. We first studied the effects of probiotics on antigen-presenting cells (APCs) through TLR activation. We found that a mixture of Lactobacilli acidophilus strain NCFM (LA) and L. reuteri (ATCC# 23272) induced strong TLR2-expressing APC responses and virulent HRV induced a TLR3 response. Probiotics and HRV had an additive effect on TLR2- and TLR9-expressing APC responses, consistent with the adjuvant effect of lactobacilli. Dose effects of LA on T cell immune responses were investigated. We found that low dose LA significantly enhanced frequencies of HRV-specific IFN-γ producing CD4⁺ and CD8+ T cells whereas high dose LA reduced frequencies of HRV-specific IFN-γ producing CD4+ T cells. Low dose LA reduced frequencies of induced regulatory (iTreg) cells and TGF-β expression in the iTreg cells whereas high dose LA increased frequencies of iTreg cells and IL-10 expression in the iTreg cells. The dose effects of LA were independent of HRV infection/vaccination. In addition, we demonstrated that TCR-γδ T cells play an important role in modulating immune responses to rotavirus infections. All three γδ T cell subsets showed evidence of activation after HRV infection by increasing TLR2, TLR3, TLR9 expression and IFN-γ production during the acute phase of infection. There was an additive effect between lactobacilli and HRV in inducing total γδ T cell expansion in ileum and in recruiting the cells from blood. HRV infection induced a significant expansion of the CD2+CD8+ γδ T cell subset in the ileum. This subset mainly exerts regulatory functions as evident by expressing FoxP3, secreting TGF-β and IL-10 or increasing production of the anti-inflammatory cytokines by CD4+ and/or CD8+ αβ T cells in the co-cultures. CD2+CD8- and CD2-CD8- γδ T cell subsets have mainly pro-inflammatory and anti-viral functions as evident by secreting IFN-γ or promoting CD4+ αβ T cell proliferation and IFN-γ production. This knowledge will facilitate the development of more effective vaccination and therapeutic strategies to protect children and young animals against rotavirus gastroenteritis. / Ph. D.

Le récepteur activé par les proliférateurs de peroxysomes (PPAR) b/d est un facteur de transcription impliqué dans l’activation de la voie d’oxydation des lipides qui possède également une fonction anti-inflammatoire. Étudié chez les macrophages, son rôle reste très peu connu dans d’autres cellules immunitaires comme les lymphocytes T. Nous avons généré un nouveau modèle murin où PPARb/d est surexprimé spécifiquement dans les lymphocytes T (souris Tg T-PPARb), afin d’étudier l’effet de sa surexpression sur le développement ainsi que la fonction des lymphocytes T grâce à deux types de challenges : métabolique et immunitaire. Nos résultats ont permis de mettre en évidence un rôle de PPARb/d dans le développement des lymphocytes T ab (blocage de leur développement) mais pas des lymphocytes T gd. On observe alors une diminution de 70% du nombre de lymphocytes T ab dans les organes lymphoïdes conduisant à une diminution du ratio de Lymphocytes T ab/gd. Les lymphocytes T ab qui arrivent tout de même à se développer ne surexpriment pas PPARb/d. Au cours d’un challenge métabolique (régime hyperlipidique), nous avons observé que les souris Tg T-PPARb sont partiellement protégées contre l’obésité. Elles présentent également une amélioration de leur phénotype métabolique (sensibilité à l’insuline et au glucose, stéatose hépatique) et inflammatoire (diminution de l’inflammation des dépôts de tissus adipeux). Au cours d’un challenge immunitaire (injection de LPS), nous observons une diminution du nombre de macrophages pro-inflammatoires M1 dans la cavité péritonéale des souris Tg T-PPARb dès 1h post-injection. Chez les souris contrôles, ce phénomène est visible à partir de 3h postinjection de LPS. Il semble ainsi que la réponse immunitaire des souris Tg T-PPARb soit plus précoce que celle des souris contrôles en réponse à un challenge immunitaire. En conclusion, la surexpression de PPARb spécifiquement dans les lymphocytes T semble provoquer une altération des populations de lymphocytes T ainsi qu’une potentielle modification de leur fonction qui pourraient expliquer que les souris Tg T-PPARb réagissent mieux que les souris contrôles lorsqu’elles sont soumises à différents types de challenges (immunitaire ou métabolique). / The Peroxisome Proliferator Activated Receptor (PPAR) b/d is a transcription factor involved in the activation of the lipid oxidation pathway that also has an anti-inflammatory function. While well-studied in macrophages, its role in other immune cells like in T cells remains largely unknown. We have generated a new mouse model in which PPARb/d is specifically overexpressed in T cells (Tg T-PPARb mice) and studied the effect of its overexpression on the development and the function of T cells through two types of challenge: metabolic and immune. Our results show a role of PPARb/d in the development of ab T cells (blocking their development) but not gd T cells. There is then a 70% decrease in the number of ab T cells in lymphoid organs leading to a decrease in the ab/gd T cell ratio. ab T cells that are still able to develop do not overexpress PPARb/d. During a metabolic challenge (high fat diet), we observed that Tg T-PPARb mice are partially protected against high fat diet inducedobesity. They also show an improvement in their metabolic (insulin and glucose sensitivity, hepatic steatosis) and inflammatory phenotype (decrease in inflammation in adipose tissue depots). During an immune challenge (LPS injection), we observed a decrease in pro-inflammatory M1 macrophage number in the peritoneal cavity of Tg TPPARb mice at 1h post-injection. In control mice, this phenomenon is seen at 3h post-injection of LPS. Thus, it appears that the immune response in Tg T-PPARb mice is faster than the one in control mice in response to an immune challenge. In conclusion, PPARb/d overexpression specifically in T cells appears to cause an alteration in T cell population as well as a potential change in their function which could explain that Tg T-PPARb mice respond better than control mice when they are subjected to different types of challenges (immune or metabolic).