Introduction: 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.

Objective: We hypothesize that intranasal OM-85 administration attenuates house dust mite (HDM)-induced allergic airway inflammation (AAI) by modulating alveolar epithelial cell-macrophage interactions.

Methods: Eight-week-old female BalbC/J mice received intranasal (i.n.) OM-85 (1 mg/kg in 40?l) or vehicle every 48-72 hours, starting 5 days before and continuing through HDM sensitization and challenge (25 ?g 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.

Results: 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.

Conclusions: Intranasal OM-85 reduces HDM-induced AAI by modulating immune and epithelial cells, promoting inflammation resolution, and enhancing lung function.