Colonization With Distinct Proteobacteria Provocateurs Induces Multifactorial Gastrointestinal Inflammation That Is Regulated by CD4+ T Cells: P-194

Abstract

An abundance of clinical, epidemiologic and animal model studies indicates that IBD results from complex interactions between a genetically susceptible host and environmental factors that induce an aberrant immune response against resident members of the gastrointestinal microbiota. Although many gene polymorphisms have been associated with an increased risk of IBD, they only explain a small percentage of the overall disease risk. Initiation of IBD may also be explained by environmental factors, including persistent colonization with bacterial provocateurs. Our laboratory has developed a novel model of multifactorial gastrointestinal inflammation that allows us to study the role of bacterial provocation on the onset of disease. By utilizing immunocompetent C3H/HeN:Tac mice harboring a defined microbiota (the altered Schaedler flora or ASF), we can evaluate the induction of immune responses to antigens derived from the resident microbiota. Prior colonization of these defined microbiota (DM) mice with Helicobacter bilis increased host susceptibility to the onset of colitis following a subsequent inflammatory insult with a very low dose (1.5%) of the inflammatory trigger, dextran sulfate sodium (DSS). This dose of DSS alone was not sufficient to elicit colitis. In addition, prior colonization with non-pathogenic Escherichia coli SWW33 failed to affect disease severity. Interestingly, both H. bilis- and E. coli-colonized mice treated with DSS upregulated mucosal expression of IL-17A mRNA; however, expression was higher in E. coli-colonized mice compared to H. bilis-colonized mice at 3 weeks post-infection. This pattern reversed at 12 weeks post-infection with the antigen-specific IL-17A protein production from CD4+ T cells following a similar pattern. Although the induction of IL-17A was demonstrable in mice colonized with either provocateur, only mice colonized with H. bilis were more susceptible to low-dose DSS-induced colitis. Together, these data suggest that H. bilis and E. coli differentially induce pathogenic or beneficial mucosal immune responses to antigens derived from the resident microbiota. We subsequently hypothesized that the dichotomous immune responses induced following colonization with these provocateurs would be reflected in the differential, functional capacity of the antigen-specific CD4+ T cells. Following colonization with either H. bilis or E. coli for 3 weeks, DM mice were administered either a monoclonal antibody to deplete CD4+ T cells or an isotype control antibody. Mice then received an inflammatory insult in the form of 1.5% DSS. Unexpectedly, depletion of CD4+ T cells in H. bilis-colonized mice prior to the DSS treatment period exacerbated disease as evidenced by marked signs of clinical disease as well as severe colonic shortening, cecal atrophy, edema and luminal blood. Severe histopathological inflammation was also observed in anti-CD4 treated, H. bilis-colonized mice, characterized by significant edema, vasculitis, glandular drop out, necrosis of the progenitor epithelium and stromal collapse. In contrast, depletion of CD4+ T cells from E. coli-colonized mice caused only minor increases in disease severity, including mild histopathological changes of edema, inflammatory cell infiltrate and crypt hyperplasia. These findings indicate that colonization with distinct Proteobacteria provocateurs altered the character of the host's adaptive immune response directed against the resident microbiota following a multifactorial inflammatory insult.