Regulation of the immune responses to Citrobacter rodentium

© 2016 Dr. Tania RahmanPathogenic strains of E. coli including enteropathogenic E. coli (EPEC), enterohemorrhagic E. coli (EHEC), Enterotoxigenic E. coli (ETEC) are principle causes for diarrhoea in many parts of the globe. Citrobacter rodentium (C. rodentium), a gram negative bacterium, is a murine pathogen that also utilizes type III secretion system and similar virulence factors to EPEC and EHEC and forms comparable attaching/effacing lesions in the intestines as EPEC and EHEC. The infection caused by C. rodentium in mice is usually self-limiting and results in only minor systemic effects with higher mortality in some susceptible mouse strains. All these characteristics have made the bacteria a commonly used model to study host immune responses to pathogenic E. coli infection. Regulatory T cells (Treg), a component of adaptive immunity, are well known for their immunosuppressive roles and their ability to maintain the balance between the immunological and pathological reactions. Treg have been shown to provide protective responses and their depletion has resulted more severe infection in some pathogen infections. The work presented in Chapter 3 demonstrates that Treg act as a protective mediator to restrain accelerated immune response and tissue pathology during C. rodentium infection. This was supported by the findings that in vivo depletion of Treg resulted in increased bacterial loads accompanied by significant weight loss and pronounced histopathological changes in the gut including increased enterocyte hyperplasia and disrupted crypt architecture. In addition, the gut mucosal barrier of Treg depleted mice was more permeable to FITC-dextran and there was an increased level of proinflammatory cytokines in serum. Importantly, Treg depleted mice exhibited reduced expression of inflammatory cytokines and antimicrobial peptides in the colon, which have been found to be protecting against C. rodentium infection and essential for regulating epithelial integrity. Furthermore, Treg depletion was associated with differential pattern of effector cell recruitment between draining lymph nodes and colon. For instance, in the absence of Tregs, B cells and neutrophils increased in draining lymph nodes though their numbers did not change in colon. Together, these data suggest Treg mediated suppression might be a valuable component in the early protective response against C. rodentium, which might provide novel therapeutic approaches for vaccination against EPEC, EHEC and for IBD. Plasmacytoid dendritic cells (pDC) are well known for their ability to combat viral infections by the production of type I interferons but have also been implicated in the defence against some bacterial infections. Work presented in Chapter 4 demonstrates that pDC contribute to protection against the gut pathogen C. rodentium. After infection of mice depleted of pDC with C. rodentium, significantly higher bacterial C. rodentium loads were found from 7 days after infection. Substantial weight loss necessitating euthanasia of the mice. pDC depletion was associated with increased gut pathology and the gut mucosal barrier of pDC-depleted mice was more permeable to FITC-dextran and there was an increased level of C. rodentium bacteria observed in liver and spleen. Increased levels of inflammatory cytokines and antimicrobial peptide were also found in colons and systemically in mice depleted of pDC. An increased recruitment of neutrophils and macrophages were observed in colon following pDC depletion during C. rodentium which was exhibited by increased spleen weight and exacerbated colon tissue damage observed in these mice. This work demonstrates that pDC play a significant role in combatting gut pathogen and may also regulate the systemic response to bacteria that leak across the compromised gut mucosal barrier. Type I IFNs are recognized for their induction of robust antiviral, antibacterial, antiproliferative and immunomodulatory properties and large amount of type I interferon are produced by pDC. Work presented in Chapter 5 demonstrates that the ability of pDC to combat C. rodentium is not dependent on type I interferon. Also type I interferon does not appear to be required to limit C. rodentium replication during infection. This was supported by the findings that following infection IFNAR2-/- mice had comparable bacterial load in faeces, weight loss, and epithelial permeability to dextran and colonic pathology to C57BL/6 mice. The investigations in this thesis have unravelled the potential of two regulatory cells in the immune system including Treg and pDC in the pathogenesis of C. rodentium. These findings might help set up future avenues of research to elucidate a key mechanism of action of these cells and provide new therapeutic insights for EPEC/EHEC/ETEC pathogenesis and also for broader antibacterial/antiviral/antiproliferative immunity