The NOD-like receptors (NLRs) are a specialized group of intracellular receptors that represent a key component of the host innate immune system. Since the discovery of the first NLR almost 10 years ago, the study of this special class of microbial sensors has burgeoned; consequently, a better understanding of the mechanism by which these receptors recognize microbes and other danger signals and of how they activate inflammatory signaling pathways has emerged. Moreover, in addition to their primary role in host defense against invading pathogens, their ability to regulate nuclear factor–kappa B (NF-κB) signaling, interleukin-1-beta (IL-1β) production, and cell death indicates that they are crucial to the pathogenesis of a variety of inflammatory human diseases.

Recent studies have shown that individuals with colorectal cancer have an altered gut microbiome compared to healthy controls. It remains unclear whether these differences are a response to tumorigenesis or actively drive tumorigenesis. To determine the role of the gut microbiome in the development of colorectal cancer, we characterized the gut microbiome in a murine model of inflammation-associated colorectal cancer that mirrors what is seen in humans. We followed the development of an abnormal microbial community structure associated with inflammation and tumorigenesis in the colon. Tumor-bearing mice showed enrichment in operational taxonomic units (OTUs) affiliated with members of theBacteroides,Odoribacter, andAkkermansiagenera and decreases in OTUs affiliated with members of thePrevotellaceaeandPorphyromonadaceaefamilies. Conventionalization of germfree mice with microbiota from tumor-bearing mice significantly increased tumorigenesis in the colon compared to that for animals colonized with a healthy gut microbiome from untreated mice. Furthermore, at the end of the model, germfree mice colonized with microbiota from tumor-bearing mice harbored a higher relative abundance of populations associated with tumor formation in conventional animals. Manipulation of the gut microbiome with antibiotics resulted in a dramatic decrease in both the number and size of tumors. Our results demonstrate that changes in the gut microbiome associated with inflammation and tumorigenesis directly contribute to tumorigenesis and suggest that interventions affecting the composition of the microbiome may be a strategy to prevent the development of colon cancer.IMPORTANCEThe trillions of bacteria that live in the gut, known collectively as the gut microbiome, are important for normal functioning of the intestine. There is now growing evidence that disruptive changes in the gut microbiome are strongly associated with the development colorectal cancer. However, how the gut microbiome changes with time during tumorigenesis and whether these changes directly contribute to disease have not been determined. We demonstrate using a mouse model of inflammation-driven colon cancer that there are dramatic, continual alterations in the microbiome during the development of tumors, which are directly responsible for tumor development. Our results suggest that interventions that target these changes in the microbiome may be an effective strategy for preventing the development of colorectal cancer.

The nucleotide-binding oligomerization domain-like receptor (NLR) family member, Nlrp6, has been implicated in inflammasome signaling to activate caspase-1, which is essential for the production of mature IL-1β and IL-18. However, a function for Nlrp6 in vivo has never been demonstrated. Due to the relative high expression of Nlrp6 in intestinal tissue, we hypothesized that Nlrp6 has a role in intestinal homeostasis. Indeed, Nlrp6-deficient mice are more susceptible to chemically induced colitis as well as colitis-induced tumorigenesis than wild-type (WT) mice. Nlrp6-deficient mice exhibited significantly more inflammation within the colon than WT mice after dextran sulfate sodium treatment. Their inability to resolve inflammation and repair damaged epithelium as efficiently as WT mice resulted in prolonged increases in epithelial proliferative activity that likely underlie the increased propensity for tumors in these mice during chronic inflammation. We further show that the activity of Nlrp6 in hematopoietic cells is critical for protection against inflammation-related colon tumorigenesis. This study highlights the importance of NLR function in maintaining intestinal homeostasis to prevent the development of aberrant inflammation and tumor development within the colon.

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.