The bacterial lysate OM-85 mitigates HDM-induced allergic inflammation through airway epithelial cell- macrophage crosstalk

<bold>Introduction:</bold> The rise in allergic diseases in industrialized countries highlights the need for interventions recreating diverse microbial exposures to support tissue balance and immune homeostasis. Microbial-based therapies, like the bacterial lysate OM-85, show potential for immune modulation in allergic asthma. <bold>Objective:</bold> We hypothesize that intranasal OM-85 administration attenuates house dust mite (HDM)-induced allergic airway inflammation (AAI) by modulating alveolar epithelial cell-macrophage interactions. <bold>Methods:</bold> Eight-week-old female BalbC/J mice received intranasal (i.n.) OM-85 (1 mg/kg in 40μl) or vehicle every 48-72 hours, starting 5 days before and continuing through HDM sensitization and challenge (25 μg HDM every 48-72 hours for 2 weeks). Bronchoalveolar lavage (BAL) fluid was collected 96 hours post-challenge for cell counts. Lung immune cell composition and kinetics were analysed by flow cytometry, respiratory function assessed with FlexiVent™, and bulk RNA-sequencing on CD45⁺ and CD45^- lung cells. <bold>Results:</bold> I.n. OM-85 reduced HDM-induced AAI, decreasing BAL eosinophilia, airway mucus hypersecretion, and improving respiratory function. Type II alveolar epithelial cells (ATII) and distinct macrophage subsets, including alveolar macrophages, increased following OM-85 treatment in both control and HDM-exposed groups. GM-CSF-mediated ATII-macrophage interactions were observed. RNA-sequencing revealed OM-85-driven modulation of immune pathways, particularly those resolving inflammation. <bold>Conclusions:</bold> Intranasal OM-85 reduces HDM-induced AAI by modulating immune and epithelial cells, promoting inflammation resolution, and enhancing lung function.