Delayed Microbial Maturation Durably Exacerbates Th17 Driven Asthma in Mice.

Microbial maturation disrupted by early life dysbiosis has been linked with increased asthma risk and severity, however the immunological mechanisms underpinning this connection are poorly understood. We sought to understand how delaying microbial maturation drives worsened asthma outcomes later in life and its long-term durability. Drinking water was supplemented with antibiotics on postnatal days 10-20. To assess the immediate and long-term effects of delaying microbial maturation on experimental asthma, we initiated house dust mite exposure when bacterial diversity was either at a minimum or had recovered. Airway hyperresponsiveness, histology, pulmonary leukocyte recruitment, flow cytometric analysis of cytokine producing lymphocytes, and assessment of serum IgG1 and IgE levels were performed. RT-PCR was used to measure IL-13-induced gene expression in sequentially sorted mesenchymal, epithelial, endothelial and leukocyte cell populations from the lung. Delayed microbial maturation increased allergen driven airway hyperresponsiveness and Th17 frequency compared to allergen exposed control mice, even when allergen exposure began after bacterial diversity recovered. Blockade of IL-17A reversed the airway hyperresponsiveness phenotype. Additionally, allergen exposure in animals that experienced delayed microbial maturation showed signs of synergistic signaling between IL-13 and IL-17A in the pulmonary mesenchymal compartment. Delaying microbial maturation in neonates promotes the development of more severe asthma by increasing Th17 frequency, even if allergen exposure is initiated weeks after microbial diversity is normalized. Additionally, IL-17A-aggravated asthma is associated with increased expression of IL-13-induced genes in mesenchymal, but not epithelial cells.