Citrobacter rodentium: a model enteropathogen for understanding the interplay of innate and adaptive components of type 3 immunity

Abstract

Citrobacter rodentium is a natural murine intestinal pathogen that shares a core set of virulence factors with the related human pathogens enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC). C. rodentium is now the most widely used small animal model for studying the molecular underpinnings of EPEC and EHEC infections in vivo, including: enterocyte attachment; virulence; colonization resistance; and mucosal immunity. In this review, we discuss type 3 immunity in the context of C. rodentium infection and discuss recent publications that use this model to understand how the innate and adaptive components of immunity intersect to mediate host protection against enteric pathogens and maintain homeostasis with the microbiota.

Fig. 1. Enteropathogenic infection leads to dysbiosis of the microbiota and activation of type 3 immunity

C. rodentium attachment to the colonic epithelium is associated with the development and regulation of type 3 immune responses in the lamina propria at early (d1–4; left panel), middle (d5–12; middle panel), and late (d13–21; right panel) phases of the infection. Initially, DCs capture luminal C. rodentium (red), either directly or from microfold (M) cells overlying ILFs and colonic patches that translocate bacteria from the lumen (not shown), and upregulate production of IL-1 and IL-23, which induce IL-22 expression from ILC3s. Attachment of host-adapted C. rodentium (green) to enterocytes (attaching and effacing, or A&E, lesions) upregulates epithelial production of SAA (serum amyloid A), which induces the production of IL-6 from intestinal macrophages and promotes the expansion of Th17 and Th22 cells. C. rodentium attachment induces epithelial hyperplasia and reduces the diversity of the microbiota, which is restored during the resolution of C. rodentium infection. Clearance of C. rodentium is associated with T-dependent antibody production, neutrophil recruitment, and an increase in IFN-γ and IL-21 production.

Fig. 2. IL-22-dependent actions that contribute to host defense during C. rodentium infection

During type 3 immune responses to C. rodentium infection IL-22 promotes host protection by upregulating epithelial wound repair and antimicrobial peptide production. In particular, IL-22 enhances the mRNA expression of the gap junction protein claudin-1 and the antimicrobial peptides RegIIIβ, RegIIIγ, S100A8, S100A9, and lipocalin-2 in the colon. S100A8 and S100A9 heterodimerize to form calprotectin, which sequesters zinc and manganese from pathogens and commensals. Although not directly bactericidal against C. rodentium, RegIIIγ likely enhances survival of WT mice by preventing the dissemination of mucosa-associated Gram-positive pathobionts, such as E. faecalis, which occurs in Il22^−/− mice. IL-22 further regulates the fucosylation of mucins forming the gylcocalix and production of the C3 component of complement that contribute to alteration of the microbiota composition and prevent systemic dissemination of pathobionts during enteric infection^,.

Fig. 3. IL-21 enhances humoral immunity to enteropathogens

During type 3 immune responses IL-6 induces IL-21 production by CD4⁺ T cells and promotes the clearance of C. rodentium infection. IL-21 is highly produced by both Tfh cells and Th17 cells, inhibits Foxp3 expression, and alters the Tfh to Tfr balance via undefined mechanisms^,. IL-21 enhances plasma cell differentiation and IgG and IgA antibody class switch recombination in the intestine but does not modulate fecal microbiota composition or systemic bacteria dissemination during C. rodentium infection (Silberger et al., manuscript in preparation).