Airway Hyperresponsiveness, but Not Bronchoalveolar Inflammatory Cytokines Profiles, Is Modified at the Subclinical Onset of Severe Equine Asthma

SIMPLE SUMMARY: Non-septic airway inflammatory disease in horses has recently been renamed to equine asthma syndrome in reference to human asthma. Airway hyperresponsiveness and inflammation are two closely related components of both human and equine conditions. The understanding of their relationship is crucial for diagnosing, managing, and treating this common disease. One of the shared characteristics of equine and human asthma is the absence of clinical signs in asymptomatic patients. This study aimed to determine the relationship between airway inflammation and hyperresponsiveness at the onset of severe equine asthma. The repeatability of the bronchoprovocation test using methacholine and impulse oscillometry system was first showed in healthy and asymptomatic asthmatic horses. While no clinical or ancillary examination allowed asthmatic horses in clinical remission to be distinguished from healthy ones, a low-dust environmental challenge did not change the results of clinical or conventional ancillary examinations. The methacholine bronchoprovocation test, however, allowed both groups to be distinguished after a 7-day low-dust environmental challenge. The gene expression and the concentrations of pro-inflammatory cytokines showed no change in the bronchoalveolar lavage fluid. No association was found between airway inflammation and hyperresponsiveness, suggesting airway hyperresponsiveness may appear earlier than inflammation at the very early onset of severe equine asthma. ABSTRACT: Airway hyperresponsiveness (AHR) and inflammation are both observed in human and equine asthma. The aim of this study was to assess the timeline and relationship of both features at the subclinical onset of severe equine asthma (SEA). First, the repeatability of the pulmonary function test (PFT) using impulse oscillometry system, and the methacholine bronchoprovocation test (BPT) were assessed at a 1-day interval on six SEA horses in clinical remission and six control horses. Then, clinical and ancillary tests were performed before and after a 1-week low-dust environmental challenge, including weighted clinical score, respiratory endoscopy, bronchoalveolar fluid cytology, PFT, and BPT. Both PFT and BPT showed acceptable repeatability. No test allowed SEA horses in clinical remission to be distinguished from control, unlike in human patients. Because of the low-dust environment, no significant difference was observed in the results of clinical and conventional ancillary examinations after the challenge. However, SEA horses showed increased AHR after the environmental challenge. At that stage, no signs of inflammation or changes in pro-inflammatory cytokines profiles (quantification and gene expression) were observed, suggesting AHR is present at an earlier stage of equine asthma than airway inflammation. This feature indicates SEA could present in a different disease pathway than neutrophilic human asthma.