Bruce Vallance

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Mar 2019)
Intestinal homeostasis and host defense as promoted by commensal bacteria and the colonic mucus layer (2019)

No abstract available.

The intestinal epithelium during times of dynamic change : development and enteric infection (2017)

Enteric infections and neonatal intestinal development both represent times of significant change in the intestine. Mode of delivery as well as food source, i.e. breast milk or formula, may impact later susceptibility to the development of allergies, asthma and other inflammatory diseases. Similarly, the development of pathological conditions such as irritable bowel syndrome and inflammatory bowel disease have been reported as long-term sequelae of infection by various enteric pathogens. The single cell layer of epithelial cells lining the intestine are in closest proximity to the luminal changes that occur, be it interaction with a bacterial pathogen or the introduction of breast milk. This positions intestinal epithelial cells (IEC)s to serve as key players in shaping responses during these times. To address the importance of IEC responses during enteric infections, we first examined the intricacies of IEC innate immune-mediated responses (MyD88-dependent) using in vivo models. Following infection with the common food poisoning pathogen Salmonella Typhimurium, IEC-specific MyD88 signalling was required to limit pathogen penetration of intestinal crypts and mediate goblet cell/antimicrobial responses. IECs lacking MyD88 also displayed decreased bactericidal capacity against Salmonella and Citrobacter rodentium, a close relative of enteropathogenic Escherichia coli. Thus, IEC MyD88 signalling promotes early, protective responses during enteric infection. Next, IEC changes during intestinal development, as well as the impact of food source during this process (i.e. mother’s milk versus formula) were explored using a neonate pup-in-a-cup model. Specifically, the role of milk fats found in mammalian milk (and currently not widely used in formulas), referred to as milk fat globule membrane (MFGM), were assessed for their ability to normalize intestinal development to that seen with mother’s milk. Interestingly, MFGM addition to formula resulted in similar villus and crypt development, as well as goblet cell, Paneth cell and enterocyte numbers and/or expression to that of mother’s milk fed pups. Further, addition of MFGM protected the formula fed neonate from intestinal damage by bacterial toxins. These studies highlight IECs as key players in shaping beneficial responses during enteric infection and intestinal development and have important implications for newborn health as well as for populations vulnerable to enteric infections.

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Mucus-bacteria interactions in the gut : investigating the role of the mucin Muc2 and its glycosylation in host defense during enteric bacterial infections (2016)

The intestinal mucus layer, which is largely composed of the secreted mucin Muc2 provides a first line of defense in the intestine. Muc2 is a heavily O-glycosylated protein with core 1 and core 3 derived O-glycans as primary constituents. It plays an important role in host defense against the attaching/effacing (A/E) pathogen Citrobacter rodentium. However whether it provides protection against the invasive human pathogen Salmonella is still unclear. Furthermore, the role of O-glycosylation in mediating the protective role played by the Muc2 mucin against enteric pathogens has not been investigated. Likewise, although almost all enteric bacterial pathogens must cross the overlying mucus layer to infect the intestinal epithelium, there is very little known about mucus-enteric bacterial interactions and virulence strategies used to accomplish this feat. We began our investigations by comparing Salmonella-induced colitis and mucus dynamics in Muc2-deficient (Muc2 -/-), C3GnT -/-, and C57BL/6 (WT) mice. While absence of core 3 derived O-glycosylation only impacted epithelial barrier integrity, absence of Muc2 resulted in significantly higher barrier disruption, host mortality rates, and increased colonic and systemic Salmonella burdens. Likewise, absence of core 1 derived O-glycans (C1galt1 -/- mice) resulted in heightened susceptibility to C. rodentium, characterized by impaired mucus levels in the lumen, and bacterial aggregation in close proximity to the intestinal epithelial surface, phenotypes not seen in WT or C3GnT -/- counterparts. To understand if the non-motile pathogen C. rodentium used bacterial proteases/mucinases as a mucus degrading strategy to gain access to the underlying epithelium, we investigated the role of a putative mucinase and a class 2 SPATE PicC. While PicC did not affect C. rodentium’s ability to colonize the colon, it appeared to have an unprecedented role in regulating C. rodentium’s activation of the innate receptor TLR2, suggesting that despite its mucinase activity, PicC's major roles in vivo may be to limit C. rodentium aggregation and its recognition by the host's innate immune system. Overall these studies highlight a novel protective role of Muc2 and its O-linked glycosylation in host defense against enteric infections and the importance of Muc2-mediated regulation of pathogen burdens at the intestinal epithelial surface.

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Diet and its modulatory effects on inflammatory bowel disease : a focus on vitamin D and infectious colitis (2015)

Vitamin D deficiency increases the risk of developing inflammatory bowel disease (IBD), a disease characterized by exaggerated immune responses to luminal bacteria. While it is unclear how vitamin D impacts IBD development, it is recognized that vitamin D plays an important role in host defense against pathogenic microbes. However, the mechanisms underlying vitamin D’s ability to affect a host’s susceptibility to infection is poorly understood. Escherichia coli is a pathobiont associated with IBD. Intestinal mucosa associated E. coli have been observed in greater numbers in patients with IBD compared to healthy controls, and these bacteria have been shown to play a role in driving intestinal inflammation. Since clinically important strains of E. coli generally do not colonize mice, researchers often rely on the related but mouse-specific attaching and effacing bacterial pathogen Citrobacter rodentium. This thesis explores the impact of vitamin D in modulating host defenses and intestinal homeostasis during infection with C. rodentium. In chapter 2, I describe how treatment with active vitamin D, calcitriol worsens colitis during C. rodentium infection. Surprisingly, calcitriol treatment of infected mice led to increased pathogen burdens, exaggerated tissue pathology and mucosal erosions. In association with their increased susceptibility, calcitriol-treated mice had substantially reduced numbers of Th17 T-cells within their infected colons and defects in their production of the antimicrobial peptide RegIIIγ. In chapter 3, I describe how dietary induced vitamin D3 deficiency also increases susceptibility to C. rodentium infection. Vitamin D3 deficient mice carried higher C. rodentium burdens, developed worsened histological damage and had higher inflammatory tone. Notably, these exaggerated inflammatory responses accelerated the loss of commensal microbes and were associated with an impaired ability to detoxify bacterial lipopolysaccharide. Together, these studies show that vitamin D plays an important role in regulating host response during enteric infection. Vitamin D deficiency impairs host defense, yet treatment with the active vitamin D also suppresses Th17 T-cell responses in vivo, and may impair mucosal host defense against bacteria. These findings have important implications for patients with IBD who suffer from overactive immune responses, yet are also have a high risk of enteric infection.

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Host and pathogen regulation of intestinal epithelial inflammatory responses during bacterial infections (2013)

The attaching and effacing (A/E) bacterial pathogen enteropathogenic Escherichia coli (EPEC), targets the intestinal epithelial cells (IEC) lining the gastrointestinal tract, causing severe diarrhea and potentially death. Although IEC express Toll like receptors (TLRs), they are hypo-responsive to most bacterial products, thereby preventing overt inflammatory responses against commensal bacteria. Single Ig IL-1-Related Receptor (SIGIRR) is a negative regulator of interleukin (IL)-1 and TLR expressed by IECs. Its expression by IEC may limit their ability to respond to invading pathogens, potentially increasing host susceptibility to infection. To address whether SIGIRR expression influences host defense against enteric pathogens, Sigirr deficient (-/-) mice were infected with the mouse adapted A/E pathogen Citrobacter rodentium. Sigirr -/- mice responded with accelerated IEC proliferation, and strong pro-inflammatory and antimicrobial responses in this study. Yet, they were highly susceptible to infection. Exaggerated IEC response of Sigirr -/- mice were primarily dependent on IL-1R signaling leading to the rapid and dramatic loss of competing commensal microbes from the infected intestine. Thus, SIGIRR promotes commensal-based resistance to pathogen colonization despite limiting IEC responses to infection. Besides the host, A/E pathogens also actively suppress IEC inflammatory and anti-microbial responses using a type 3 secretion system to deliver bacterial effector proteins into infected IEC. To identify these effector(s), I tested an array of EPEC mutants and identified that non-LEE encoded effector (Nle)C suppressed the release of the chemokine IL-8 from infected IEC in vitro. NleC localized to EPEC-induced pedestals and inhibited both NF-ΚB and p38MAP kinase activation. Comparison between mice infected by ΔnleC to wildtype C. rodentium demonstrated that loss of NleC did not impact pathogen burdens but did result in more severe colitis. Furthermore, ΔnleC compared to wildtype C. rodentium induced significantly greater chemokine responses. Thus, innate IEC responses are actively suppressed by the host in an attempt to prevent enteric infections. However, once A/E pathogens succeed in infecting their host, they try to suppress IEC responses to prolong the infection. These studies highlight the importance of IEC as the key player in controlling host susceptibility to pathogens and in maintaining a mutualistic relationship with the intestinal commensal microbiota.

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The role of colonic goblet cells in host defense against attaching and effacing bacterial pathogens (2011)

The non-invasive attaching and effacing (A/E) pathogens Enteropathogenic and Enterohemorrhagic Escherichia coli (EPEC and EHEC, respectively) are a prominent subgroup of diarrheagenic E. coli, and remain an important cause of morbidity and mortality worldwide. A/E pathogens intimately attach to the surface of intestinal epithelial cells, and efface or destroy their microvilli. The intestinal epithelium is the first line of defense against A/E and all pathogens, and evidence is implicating epithelial secretory cells as playing a key role in this regard. Goblet cells are specialized secretory epithelial cells that are the sole producers of the mucus barrier lining the intestinal tract through the release of the polymeric Muc2 mucin. Goblet cells also secrete the small peptide Resistin-like Molecule-{beta} (RELMβ), which plays a direct role in host defense against parasitic helminths. Despite the abundance of which these molecules are released, little is known of their role in host-protection against A/E pathogens. I hypothesized that goblet cells play a critical role in host defense against A/E bacteria by secretion of Muc2/mucus and RELMβ into the intestinal tract. Using Citrobacter rodentium, a murine A/E pathogen that is an established model of EPEC and EHEC, my results demonstrate that goblet cells are a critical component of innate host-defense against an A/E pathogen. Studies with Muc2⁻/⁻ mice show that Muc2/mucus production was critical for limiting luminal burdens, and for flushing away pathogenic bacteria as well as commensal bacteria from the mucosa. Moreover, RELMβ was highly induced and secreted into the lumen during the first week of infection. Studies with Retnlb⁻/⁻ mice demonstrated that RELMβ production limited cecal burdens, deep penetration of colonic crypts, and severe inflammatory damage following infection. Lastly, adaptive immunity plays a role in modulating goblet cell function, by promoting a down-regulation of goblet cell-specific gene expression and protein production, including Muc2. This effect appears to reflect a generalized adaptive immunity-mediated epithelial proliferative response and is associated with clearance of surface-associated pathogens. Thus, goblet cells are critical for managing infection by an A/E bacterial pathogen. These studies highlight a novel and previously unappreciated function of goblet cells in host defense against enteric bacterial infection.

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Characterization of innate immune responses to enteric bacterial pathogens in intestinal epithelial cells (2010)

Enteropathogenic Escherichia coli (EPEC) belongs to the attaching and effacing (A/E) family of bacterial pathogens, which infect intestinal enterocytes with type three secretion system (T3SS), leading to diarrheal disease. While A/E bacteria also express flagellin (FliC) which inducessecretion of pro-inflammatory interleukin (IL)-8 from intestinal epithelial cells (IECs), it is unclear if flagellin is the sole trigger of epithelial responses. IECs express innate toll-like receptors (TLRs)to recognize flagellin and trigger inflammatory responses. Activation of TLRs is strictly controlled in IECs producing a state of hyporesponsiveness which prevents unwanted inflammation. The Single IgG IL-1 related receptor (Sigirr) is described as a negative regulator of IL-1β and TLR4 responses. While IECs express Sigirr, it is unknown if Sigirr inhibits innate responses to bacterial flagellin. The aims of this study were to characterize the role of flagellin in innate responses to EPEC infection and determine how Sigirr regulates these responses. Following infection of Caco-2 IECs by wild type and △fliC EPEC, activation of several proinflammatorygenes including IL-8, MCP-1 and MIP3α occurred in a FliC-dependent manner. Atlater time points, a subset of these pro-inflammatory genes (IL-8, MlP3α) was also induced in cells infected with △fliC EPEC. The mouse adapted A/E pathogen Citrobacter Rodentium, triggered asimilar innate response through a TLR5-independent but partially NF-kB-dependent mechanism. Moreover, the EPEC F1iC-independent responses increased in the absence of T3SS, suggesting that translocated bacterial effectors attenuated these response. While exploring regulatory mechanisms, we found that exposure of non-transformed TECs to bacterial flagellin transiently downregulated Sigirr expression correlating with IL-8 response. Transient silencing of the Sigirr gene augmented IL-S responses to flagellin, whereas stable over-expression of Sigirr diminished theNF-kB mediated IL-8 response to TLR ligands and inflammatory cytokines. The expression of Sigirr increased as IECs differentiated in tissue culture. Similarly, Sigirr expression was prominent in differentiated cells on the apex while diminished at the base of intestinal crypts in human colonic tissues. Thus, we demonstrate that A/E pathogens trigger pro-inflammatoryresponses through both FliC-dependent and -independent pathways that are regulated by Sigirr in differentiated human IEC, with clinical implications for infectious and inflammatory bowel diseases.

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The role of natural resistance-associated macrophage protein 1 (Nramp1) in salmonellosis (2009)

Salmonellosis poses a global threat to human health. Host resistance against Salmonella enterica serovar Typhimurium (S. Typhimurium) in the murine model is mediated by Natural resistance-associated macrophage protein 1 (Nramp1/Slc11a1). Nramp1 is critical for host defense, as mice lacking Nramp1 fail to control bacterial replication and succumb to low doses of S. Typhimurium. Despite this critical role, the mechanisms underlying Nramp1’s protective effects are unclear. This thesis presents a detailed analysis of Nramp1 expression in the murine gastrointestinal tract and its impact on S. Typhimurium infection following oral infection. Dendritic cells (DCs) that sample the intestinal lumen are among the first cells encountered by S. Typhimurium and play an important role in Salmonella pathogenesis. Intestinal, splenic and bone marrow derived DCs (BMDCs) all expressed Nramp1 protein. In intestinal DCs, Nramp1 expression is restricted to a discrete subset of DCs (CD11c⁺ CD103-) that express elevated levels of pro-inflammatory cytokines in response to bacterial products. While Nramp1 expression did not affect S. Typhimurium replication in DCs, infected Nramp1⁺/⁺ DCs secreted more inflammatory cytokines (IL-6, IL-12 and TNF-α) than Nramp1-/- DCs. This suggests that Nramp1 expression promotes accelerated inflammatory responses to S. Typhimurium. This hypothesis was tested using the Salmonella-induced colitis model, where pre-treatment of mice with antibiotics enhances colonization of the cecum/colon and induces massive inflammation. We found that Nramp1⁺/⁺ mice mounted a faster and more robust inflammatory response characterized by elevated pro-inflammatory cyto/chemokines (IFN-γ, TNF-α and MIP1-α) and recruitment of neutrophils and macrophages, thereby limiting spread of S. Typhimurium to systemic sites and ultimately protecting the host.Nramp1⁺/⁺ mice also developed a chronic Salmonella infection of the gastrointestinal tract that led to severe tissue fibrosis. Intestinal fibrosis is a serious complication of Crohn’s disease, often requiring surgical intervention but the mechanisms underlying its development are poorly understood due to the lack of relevant animal models. A novel model of severe and persistent intestinal fibrosis caused by chronic bacterial induced colitis was developed. Since the pathology closely resembles human fibrosis, we present a valuable tool for investigating host and bacterial contributions to inflammatory bowel diseases, as well as infectious colitis.

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Master's Student Supervision (2010-2017)
Immunomodulation of intestinal epithelial cell proliferation and function as a novel host defense mechanism in infectious colitis (2014)

Although epithelial cells represent the primary site of host contact for attaching and effacing pathogens, their contribution to host defense is relatively unrecognized. Both idiopathic and infectious forms of colitis disrupt normal intestinal epithelial cell (IEC) proliferation, differentiation and function, although the mechanisms involved remain unclear. Infection by the attaching and effacing murine pathogen, Citrobacter rodentium leads to significant colonic hyperplasia but also a reduction in colonic goblet cell numbers (goblet cell depletion). This pathology depends on T and/or B cells as Rag1 -/- mice do not suffer this depletion during infection, instead suffering high mortality rates. Reconstitution studies reveal that both CD4+ and CD8+ T cell subsets greatly increase survival of Rag1 -/- mice. However, while mice receiving CD8+ T cells develop exaggerated colonic tissue damage and ulcers, mice receiving CD4+ T cells develop goblet cell depletion in concert with exaggerated IEC proliferation preventing deep pathogen penetration of colonic crypts. Studies with Ifn-γ receptor -/- mice and wildtype mice given IL-17A neutralizing antibodies identify IFN-γ signaling as a critical cytokine required for both goblet cell depletion and increased IEC proliferation. Finally, studies inhibiting notch signaling and thus vastly increasing goblet cell numbers greatly increased pathogen burdens and mortality rates. These studies thus demonstrate that goblet cell depletion reflects host immunomodulation of IEC homeostasis and reflects a novel host defense mechanism against mucosal adherent pathogens.

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Current Students & Alumni

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