Card9 mediates susceptibility to intestinal pathogens through microbiota modulation and control of bacterial virulence.

Abstract

In association with innate and adaptive immunity, the microbiota controls the colonisation resistance against intestinal pathogens. Caspase recruitment domain 9 (<i>CARD9</i>), a key innate immunity gene, is required to shape a normal gut microbiota. <i>Card9</i>^-/- mice are more susceptible to the enteric mouse pathogen <i>Citrobacter rodentium</i> that mimics human infections with enteropathogenic and enterohaemorrhagic <i>Escherichia coli</i>. Here, we examined how <i>CARD9</i> controls <i>C. rodentium</i> infection susceptibility through microbiota-dependent and microbiota-independent mechanisms. <i>C. rodentium</i> infection was assessed in conventional and germ-free (GF) wild-type (WT) and <i>Card9</i>^-/- mice. To explore the impact of <i>Card9</i>^-/-microbiota in infection susceptibility, GF WT mice were colonised with WT (WT→GF) or <i>Card9</i>^-/- (<i>Card9^-/-</i> →GF) microbiota before <i>C. rodentium</i> infection. Microbiota composition was determined by 16S rDNA gene sequencing. Inflammation severity was determined by histology score and lipocalin level. Microbiota-host immune system interactions were assessed by quantitative PCR analysis. <i>CARD9</i> controls pathogen virulence in a microbiota-independent manner by supporting a specific humoral response. Higher susceptibility to <i>C. rodentium</i>-induced colitis was observed in <i>Card9^-/-</i> →GF mice. The microbiota of <i>Card9</i>^-/- mice failed to outcompete the monosaccharide-consuming <i>C. rodentium</i>, worsening the infection severity. A polysaccharide-enriched diet counteracted the ecological advantage of <i>C. rodentium</i> and the defective pathogen-specific antibody response in <i>Card9</i>^-/- mice. CARD9 modulates the susceptibility to intestinal infection by controlling the pathogen virulence in a microbiota-dependent and microbiota-independent manner. Genetic susceptibility to intestinal pathogens can be overridden by diet intervention that restores humoural immunity and a competing microbiota.