Akkermansia muciniphila is a Gram-negative anaerobic mucus-layer-degrading bacterium that colonizes the intestinal mucosa of humans and rodents. Metagenomic data have shown an inverse correlation between the abundance of A. muciniphila and diseases such as inflammatory bowel disease (IBD), obesity, and diabetes. Thus, in recent decades, the potential of this bacterium as an immunomodulatory probiotic for autoimmune and chronic inflammatory diseases has been explored in experimental models. Corroborating these human correlation data, it has been reported that A. muciniphila slows down the development and progression of diabetes, obesity, and IBD in mice. Consequently, clinical studies with obese and diabetic patients are being performed, and the preliminary results are very promising. Therefore, this mini review highlights the main findings regarding the beneficial roles of A. muciniphila and its action mechanisms in autoimmune and chronic inflammatory diseases.

Type 1 diabetes (T1D) is an autoimmune disease that is triggered by both genetic and environmental factors, resulting in the destruction of pancreatic β cells. The disruption of the intestinal epithelial barrier and consequent escape of microbial products may be one of these environmental triggers. However, the immune receptors that are activated in this context remain elusive. We show here that during streptozotocin (STZ)-induced T1D, the nucleotide-binding oligomerization domain containing 2 (NOD2), but not NOD1, participates in the pathogenesis of the disease by inducing T helper 1 (Th1) and Th17 cells in the pancreatic LNs (PLNs) and pancreas. Additionally, STZ-injected wild-type (WT) diabetic mice displayed an altered gut microbiota compared with vehicle-injected WT mice, together with the translocation of bacteria to the PLNs. Interestingly, WT mice treated with broad-spectrum antibiotics (Abx) were fully protected from STZ-induced T1D, which correlated with the abrogation of bacterial translocation to the PLNs. Notably, when Abx-treated STZ-injected WT mice received the NOD2 ligand muramyl dipeptide, both hyperglycemia and the proinflammatory immune response were restored. Our results demonstrate that the recognition of bacterial products by NOD2 inside the PLNs contributes to T1D development, establishing a new putative target for intervention during the early stages of the disease.

Aldosterone excess aggravates endothelial dysfunction in diabetes and hypertension by promoting the increased generation of reactive oxygen species, inflammation, and insulin resistance. Aldosterone activates the molecular platform inflammasome in immune system cells and contributes to vascular dysfunction induced by the mineralocorticoid hormone. It is unclear as to whether the NLRP3 inflammasome associated with the mineralocorticoid receptor contributes to vascular dysfunction in diabetic conditions. Here, we tested the hypothesis that an excess of aldosterone induces vascular dysfunction in type 2 diabetes, via the activation of mineralocorticoid receptors (MR) and assembly of the NLRP3 inflammasome. Mesenteric resistance arteries from control (db/m) and diabetic (db/db) mice treated with vehicle, spironolactone (MR antagonist) or an NLRP3 selective inhibitor (MCC950) were used to determine whether NLRP3 contributes to diabetes-associated vascular dysfunction. Db/db mice exhibited increased vascular expression/activation of caspase-1 and IL-1β, increased plasma IL-1β levels, active caspase-1 in peritoneal macrophages, and reduced acetylcholine (ACh) vasodilation, compared to db/m mice. Treatment of db/db mice with spironolactone and MCC950 decreased plasma IL-1β and partly restored ACh vasodilation. Spironolactone also reduced active caspase-1-positive macrophages in db/db mice, events that contribute to diabetes-associated vascular changes. These data clearly indicate that MR and NLRP3 activation contribute to diabetes-associated vascular dysfunction and pro-inflammatory phenotype.

The intestinal microbiome maintains a close relationship with the host immunity. This connection fosters a health state by direct and indirect mechanisms. Direct influences occur mainly through the production of short-chain fatty acids (SCFAs), gastrointestinal hormones and precursors of bioactive molecules. Indirect mechanisms comprise the crosstalk between bacterial products and the host's innate immune system. Conversely, intestinal dysbiosis is a condition found in a large number of chronic intestinal inflammatory diseases, such as ulcerative colitis and Crohn's disease, as well as in diseases associated with low-grade inflammation, such as obesity, type 1 and 2 diabetes mellitus and cardiovascular diseases. NOD-Like receptors (NLRs) are cytoplasmic receptors expressed by adaptive and innate immune cells that form a multiprotein complex, termed the inflammasome, responsible for the release of mature interleukin (IL)-1β and IL-18. NLRs are also involved in the recognition of bacterial components and production of antimicrobial molecules that shape the gut microbiota and maintain the intestinal homeostasis. Recent novel findings show that NLRs may act as positive or negative regulators of inflammation by modulating NF-κB activation. This mini-review presents current and updated evidence on the interplay between NLRs and gut microbiota and their dual role, contributing to progression or conferring protection, in diabetes and other inflammatory diseases.