NLRP6 Protects Il10-/- Mice from Colitis by Limiting Colonization of Akkermansia muciniphila

Abstract

Dysfunction in host immune responses and pathologic alterations in the gut microbiota, referred to as dysbiosis, can both contribute to the development of inflammatory bowel disease (IBD). However, it remains unclear how specific changes in host immunity or the microbiota cause disease. We previously demonstrated that the loss of the innate immune receptor NLRP6 in mice resulted in impaired production of interleukin-18 (IL-18) and increased susceptibility to epithelial-induced injury. Here, we show that NLRP6 is important for suppressing the development of spontaneous colitis in the Il10^-/- mice model of IBD and that NLRP6 deficiency results in the enrichment of Akkermansia muciniphila. A. muciniphila was sufficient for promoting intestinal inflammation in both specific-pathogen-free and germ-free Il10^-/- mice. Our results demonstrate that A. muciniphila can act as a pathobiont to promote colitis in a genetically susceptible host and that NLRP6 is a key regulator of its abundance.

Keywords: Akkermansia; IL-10; Microbiota; NLRP6; NLRs; colitis; dysbiosis; inflammation; intestine; pathobiont.

Figure 1. IL10^−/−Nlrp6^−/− mice develop significant spontaneous inflammation in the colon

(A) Representative photographs and histological scores of colon sections from 8-week old WT, IL10^−/−, Nlrp6^−/− and IL10^−/−Nlrp6^−/− mice. Inflammation and epithelial hyperplasia are indicated by arrows. Original magnification = 200x. (B) Incidence of rectal prolapse in WT (n=116), Nlrp6^−/−(n=89), IL10^−/− (n=51), and IL10^−/−Nlrp6^−/− (n=58). (C) Spleen weights, (D) fecal lipocalin-2 levels, (E) relative levels of total bacteria/MLN in 14 week old IL10^−/− or IL10^−/−Nlrp6^−/− mice (n=7 mice/group), and (F) mRNA expression of pro-inflammatory mediators relative to β-actin in the colons of WT (n=13), IL10^−/− (n=11), Nlrp6^−/− (n=12), and IL10^−/−Nlrp6^−/− (n=14) mice. Data are representative of three independent experiments. * - p<0.05, as compared to all other genotypes. See also Figure S1.

Figure 2. Increased colitis susceptibility in IL10^−/−Nlrp6^−/− mice does not transfer to IL10^−/− mice upon cohousing

(A) Representative micrographs and histological scoring of 12 week old IL10^−/− (n=6) and IL10^−/−Nlrp6^−/− (n=8) mice after 2 months of cohousing. Arrows point to Inflammation and hyperplasia. Original magnification = 200x. (B) Spleen weights and (C) fecal lipocalin-2 levels from cohoused IL10^−/− and IL10^−/−Nlrp6^−/− mice. (D) Relative mRNA expression of pro-inflammatory mediators in the colon of cohoused (COH) or non-cohoused IL10^−/− and IL10^−/−Nlrp6^−/− mice (n=8). Data are representative of two independent experiments. * - p<0.05 as compared to IL10^−/− mice. See also Figure S2.

Figure 3. Inflammatory phenotype of IL10^−/−Nlrp6^−/− mice correlates with altered microbiome not corrected by cohousing

(A) Inverse Simpson’s α-diversity index and observed community richness as measured by number of operational taxonomic units (OTUs), are shown for non-cohoused, cohoused (at 35 and 60 days), and littermate IL10^−/− and IL10^−/−Nlrp6^−/− mice. LEfSe analysis shows bacteria that were most differentially abundant between (B) non-cohoused or (C) cohoused IL10^−/− and IL10^−/−Nlrp6^−/− mice and indicate the effect size of differentially abundant OTUs in the colon. Data are representative of two independent experiments, n=8 for non-cohoused groups, n=6 and n=8 for cohoused IL10^−/− and IL10^−/−Nlrp6^−/− mice respectively. * - p<0.05 as compared to IL10^−/− group. See also Figure S2.

Figure 4. Conventionalized GF Nlrp6^−/− mice have altered microbiota associated with reduced α-diversity and richness and dramatically increased A. muciniphila colonization

(A) Time course for inverse Simpson’s α-diversity index and (B) observed community richness are shown for gWT and gNlrp6^−/− mice. (C) Average θ_yc distance within or between groups of gWT and gNlrp6^−/− mice. (D) LEfSe results show most differentially abundant OTUs. (E) Venn diagram illustrating the number of bacterial OTUs that were differentially abundant (LDA score > 2) between the indicated mice. (F) Relative abundance of A. muciniphila in the colons of 12-week old non-cohoused or cohoused IL10^−/− and IL10^−/−Nlrp6^−/− mice normalized to total bacteria. (G) Relative abundance of A. muciniphila in colons of gWT and gNlrp6^−/− mice (day 15 after conventionalization); n=8 for non-cohoused groups, n=6 and n=8 for cohoused IL10^−/− and IL10^−/−Nlrp6^−/− mice respectively; n=5, n=8 for gWT and gNlrp6^−/− mice respectively. * - p<0.05 as compared to corresponding gWT group (A–C) or to IL10^−/− group (F). See also Figure S3.

Figure 5. rIL18 is sufficient to reduce A. muciniphila colonization in Nlrp6^−/− mice

(A) Relative abundance of A. muciniphila from fecal samples collected from adult WT, IL18^−/−, IL18R^−/− (n=8 mice/genotype), IL1β^−/−, IL1R^−/−, IL10^−/−, Nlrp6^−/− and IL10^−/−Nlrp6^−/− mice (n=5 mice/genotype) and (B) Nlrp6^−/− mice before and after treatment with rIL-18 (treatment days indicated by arrows). Data are representative of two independent experiments. * - p<0.05 compared to WT. See also Figure S4.

Figure 6. A. muciniphila gavaged into SPF IL10^−/− mice triggers colitis

SPF IL10^−/− mice were left untreated or gavaged weekly with 2×10⁸ CFU/ml of A. muciniphila or B. acidifaciens. (A) Percent weight change of age- and sex-matched groups of mice. (B) Representative micrographs and histological scores of colon sections from IL10^−/− + no gavage, IL10^−/− + A. muciniphila or IL10^−/− + B. acidifaciens mice (200x). (C) Spleen weights (left) and size (right), (D) colon inflammation index (weight/length), (E) fecal lipocalin-2 levels as measured by ELISA, (F) levels of total bacteria/MLN and (G) relative mRNA expression of various pro-inflammatory mediators in the colons from IL10^−/− + no gavage, IL10^−/− + A. muciniphila or IL10^−/− + B. acidifaciens mice as determined by qPCR with β-actin used as the housekeeping gene control. Data are representative of three independent experiments, n=11, n=13, n=12 for IL10^−/− control, IL10^−/− + A. muciniphila or IL10^−/− + B. acidifaciens groups of mice respectively. * - p<0.05 as compared to IL10^−/− + B. acidifaciens group. See also Figure S5.

Figure 7. A. muciniphila is sufficient to trigger inflammation in GF IL10^−/− mice

(A) GF IL10^−/− mice were monocolonized with A. muciniphila or B. acidifaciens and specific colonization confirmed by qPCR. (B) Percent weight change, (C) fecal lipocalin-2 levels, (D) histological inflammatory scores, and (E) normalized levels of total bacteria/MLN in monoassociated GF IL10^−/− mice. (F) mRNA expression of various pro-inflammatory mediators relative to β-actin in the colons of the indicated mice. n=11 for GF IL10^−/− + A. muciniphila and GF IL10^−/− + B. acidifaciens groups, and n=6 for GF IL10^−/− mice. * - p<0.001 compared to GF IL10^−/− + B. acidifaciens (B, C) or to GF IL10^−/− (F).