Methanogenic Archaea: Emerging Partners in the Field of Allergic Diseases.

Archaea, which form one of four domains of life alongside Eukarya, Bacteria, and giant viruses, have long been neglected as components of the human microbiota and potential opportunistic infectious pathogens. In this review, we focus on methanogenic Archaea, which rely on hydrogen for their metabolism and growth. On one hand, methanogenic Archaea in the gut are functional associates of the fermentative digestion of dietary fibers, favoring the production of beneficial short-chain fatty acids and likely contributing to the weaning reaction during the neonatal window of opportunity. On the other hand, methanogenic Archaea trigger the activation of innate and adaptive responses and the generation of specific T and B cells in animals and humans. In mouse models, lung hypersensitivity reactions can be induced by inhaled methanogenic Archaea mimicking human professional exposure to organic dust. Changes in methanogenic Archaea of the microbiota are detected in an array of dysimmune conditions comprising inflammatory bowel disease, obesity, malnutrition, anorexia, colorectal cancer, and diverticulosis. At the subcellular level, methanogenic Archaea are activators of the TLR8-dependent NLRP3 inflammasome, modulate the release of antimicrobial peptides and drive the production of proinflammatory, Th-1, Th-2, and Th-17 cytokines. Our objective was to introduce the most recent and major pieces of evidence supporting the involvement of Archaea in the balance between health and dysimmune diseases, with a particular focus on atopic and allergic conditions.