Chronic Salmonella Infection Induced Intestinal Fibrosis

Ulcerative colitis (UC) and Crohn’s disease (CD) are the two major forms of IBD and are characterized as chronic and relapsing inflammatory disorders of the gastrointestinal tract ^1,2. These disorders have a major impact on the quality of life of patients. Symptoms of IBD include abdominal pain, diarrhea, nausea, weight loss, fever, and fatigue³. Recent studies have identified genetic and environmental factors that contribute to disease pathogenesis; it is thought that such risk factors contribute to the disruption of the epithelial barrier resulting in the translocation or oversampling of luminal antigens⁴. As a consequence, this initiates an aberrant inflammatory response to the commensal flora mediated by intestinal immune cells⁴. Features of IBD-associated complications may extend to sites beyond the GI tract affecting various organs including joints, skin, and liver^1,2. Hallmarks of UC include severe and diffuse inflammation typically localized in the colon¹. Disease pathology affects the mucosa and submucosa of the bowel resulting in superficial mucosal ulcerations¹. In contrast, CD can affect any part of the GI tract although evidence of disease is commonly found in the colon and distal ileum². Moreover, the inflammation in CD is transmural, affecting all layers of the bowel wall².

Several IBD susceptibility genes that have been identified would indicate that dysregulation of the epithelial barrier or immunity are critical contributors to disease progression⁵. Mutations in nucleotide oligomerization domain 2 (NOD2) expressed by monocytes was found to be associated with increased susceptibility to CD; this highlights a link between altered innate immune detection of bacterial components and the disease⁶. More recent genome-wide association studies (GWAS) have revealed additional pathways potentially involved in the pathogenesis of IBD including genetic variations in: STAT1, NKX2-3, IL2RA, IL23R dependent pathways linked to adaptive immunity, MUC1, MUC19, and PTGER4 in intestinal barrier maintenance, and ATG16L-mediated autophagy^7,8,9. While these population-based genetics studies have enhanced our understanding of IBD, susceptibility alleles alone are likely insufficient in initiating and sustaining chronic disease³. Other non-genetic factors including alterations in gut microbiome composition and a reduction in diversity have been associated with intestinal inflammation. However, it is unclear whether gut dysbiosis precedes or is the consequence of dysregulated immune responses³. Although the etiology of IBD remains unclear, our understanding of the pathogenesis of the disease has been enhanced by experimental mouse models of intestinal inflammation^10,11. These models individually do not fully represent the complexity of the human disease, but they are valuable for elucidating pathophysiological pathways that could be relevant to IBD and for the validation of tentative therapeutic strategies^10,11. Such mouse models typically rely on the initiation of inflammation by chemical induction or infection, immune cell transfer, or genetic manipulation. Moreover, these strategies often involve perturbations in epithelial integrity or modulation of innate or adaptive immunity.

Salmonella enterica serovars are intestinal pathogens that can infect humans and mice. After ingestion, Salmonella can colonize the gut by direct invasion of epithelia, M cells, or antigen presenting cells¹². Mice infected orally with S. Typhimurium results in the colonization primarily of systemic sites such as the spleen and mesenteric lymph nodes with relatively low abundance in the GI tract¹². However, pretreatment of mice with streptomycin enhances the efficiency of Salmonella colonization of the gut by diminishing the host protective effects of the normal microbiota¹³. Pathological features of this model include the disruption or ulceration of the epithelial barrier, granulocyte recruitment, and severe edema¹³. Alternatively, infection with the vaccine grade S. Typhimurium ΔAroA mutant leads to chronic colonization of the cecum and colon that persists up to day 40 after infection¹⁴. The S. Typhimurium ΔAroA strain has a defect in the biosynthesis of aromatic amino acids; this renders the mutant strain avirulent and can be utilized as a highly effective vaccine¹⁵. Oral infection in mice leads to a Th1- and Th17-cytokine associated inflammatory response, extensive tissue remodeling, and collagen deposition. Tissue pathology is associated with elevated levels of pro-fibrotic factor such as TGF-β1, CTGF, and IGF¹⁴. The transmural fibrotic scarring reported in this model is reminiscent of stricture formations often observed in IBD. The induction of fibrosis by Salmonella requires virulence encoded by Salmonella pathogenicity islands (SPI)-1 and 2 ¹². Importantly, this S. Tymphimurium ΔAroA infection model is a useful system for the study of fibrotic responses in mutant mice maintained on a C57/Bl6 background. The C57/Bl6 strain is extremely sensitive to S. Typhiumurim SL1344 infection due to a loss-of-function mutation in the gene encoding the natural resistance-associated macrophage protein (NRAMP)-1^16,17. We have found that IL-17A and RORα-dependent innate lymphoid cells are important contributors to pathogenesis in this model¹⁸.

A major complication of CD is the dysregulated and excessive deposition of extracellular matrix (ECM) including collagen^2,19. Although the GI tract has a relatively high capacity for regeneration, fibrotic scarring can arise due to unresolved wound healing responses that are associated with chronic and severe inflammation^20,21. In CD, this results in deleterious effects on tissue architecture leading to significant organ impairment^21,22. The transmural nature of the inflammation observed in CD ultimately precedes the thickening of the bowel wall associated with symptomatic stenosis or stricture formation²¹. About a third of CD patients require intestinal resection for this complication²². There are no effective anti-fibrotic therapies in IBD given that the use of immunosuppressants such as azathioprine or anti-TNFα biologics have no impact or only modestly reduced the requirement of surgical interventions^19,23. While fibrosis is thought be the consequence of chronic inflammation, cells of mesenchymal origin such as fibroblasts and pericytes are thought to be the primary cellular sources of ECM in fibrotic scarring^21,24. Chronic S. Typhimurium ΔAroA infection is a robust mouse model of intestinal fibrosis that can offer insights into the pathogenesis of CD-like features.