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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 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 and 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. View Publication