Microbial dysbiosis sculpts a systemic ILC3/IL-17 axis governing lung inflammatory responses and central hematopoiesis

Advancements in vaccination and sanitation have significantly reduced the prevalence and burden of infectious diseases; however, these benefits have coincided with a marked rise in autoimmune and allergic disorders. Recent studies have investigated these linked trends through the lens of host–microbiome alterations, proposing these shifts as a potential explanatory mechanism. Previously, we demonstrated that vancomycin-induced depletion of short-chain fatty acid (SCFA) producing bacteria results in hyperactivation of ILC2s and exacerbated allergic responses. Here we investigate the effects of low-dose streptomycin on innate and adaptive immune cell populations and their activation states. Although streptomycin-treated mice exhibit normal allergic responses, they display heightened susceptibility to Th1/Th17-mediated disease, specifically hypersensitivity pneumonitis (HP). This is characterized by a two-fold increase in ILC3s and Th17 cells in the lungs, alongside activation of antigen-presenting cells (APCs) at steady state, an effect that is further amplified upon exposure to HP-inducing agents. Shotgun metagenomic analysis revealed that streptomycin-induced dysbiosis reduces microbial diversity, depletes bile acid–metabolizing bacteria, and enriches for metabolic pathways involved in branched-chain amino acid biosynthesis, including leucine, a known activator of mTORC1. Strikingly, administration of the secondary bile acid metabolite isolithocholic acid (an inverse agonist of RORγt), or an IL-23 neutralizing antibody, reverses the enhanced susceptibility to HP. Inhibition of mTORC1 also significantly reduced Th17/ILC3 responses and histopathology. Our findings underscore microbial equilibrium as a key determinant of susceptibility to HP and uncover a positive feedback loop between IL-23-producing APCs and ILC3/Th17 cells that mechanistically links dysbiosis to sustained type 3 inflammation; and we identify a simple, actionable means of intervention.