The Recurrent Airway Obstruction Restricted Lung Transcriptome Identifies Molecular Signatures of Airway Hyperresponsiveness and Neutrophilic Inflammation

Airway hyper-responsiveness (AHR) is a hallmark of Recurrent Airway Obstruction (RAO) and Inflammatory Airway Disease (IAD), diseases that account for a majority of equine respiratory disease. Despite the clear importance of airway homeostasis to equine health and athletic potential, molecular mechanisms by which inhaled stimuli result in AHR and bronchoconstriction remain poorly characterized in horses. By contrast, molecular mechanisms mediating AHR have been extensively investigated in human asthmatics and in asthma models. To address this disparity, we screened for molecular signatures of asthmatic AHR in horses with Summer Pasture Associated Recurrent Airway Obstruction (SPARAO, aka heaves). As neutrophilic airway inflammation is a feature of SPARAO and severe asthma, and inflammation is presumed to contribute to bronchoconstriction, genes influencing neutrophil functions were also interrogated. RNA sequencing was performed on serially sampled lung biopsies acquired from SPARAO-affected and non-diseased horses during seasonal disease exacerbation and remission. Differentially expressed genes associated with season were identified at a 5% false discovery rate and 180 genes were differentially expressed (DE) in association with disease. Ingenuity Pathway Analysis identified inflammatory disease and respiratory disease as the most significantly represented disease processes from the differentially expressed genes. The AHR pathway was up-regulated during disease exacerbation, reflecting significantly increased expression of IL-1, IL-13, TNF-α, and hyaluronan synthase 2.  DE genes expressed during disease collectively increased neutrophil migration, recruitment, accumulation, and infiltration, while decreasing neutrophil apoptosis. Our results demonstrate that network analysis can be employed to identify molecular signatures of disease and to characterize complex molecular events in genome scale datasets derived from clinically relevant equine disease.