Nature and causes of mucus accumulation in equine lower airway disease

The purpose of my work was to quantify airway mucus accumulation and investigate its causes in RAO-affected horses and clinically healthy controls. I hypothesized that mucus accumulation is a function of the interaction of disease status (RAO), environment (dust exposure) and age of the horse. My investigations showed that endoscopic scoring is a reliable tool to quantify mucus accumulation. Using endoscopic scoring to compare RAO-affected and clinically healthy horses, I found very large mucus accumulations to be limited to RAO-affected horses, especially during acute exacerbation. There was less mucus accumulation in controls, and I found no difference between age groups. Variation between individuals was high, however, with many clinically healthy horses showing signs of IAD. Overall, mucus accumulation was associated with neutrophilic airway inflammation. I then proposed that increased accumulation is a consequence of unfavorable rheological changes and/or increased mucin production. I found that increases of mucus accumulation in RAO exacerbation are accompanied by a large rise of mucus viscoelasticity, which in turn is at least partly due to F-actin fibers, likely released from degenerate neutrophils. These rheological changes could not explain the increased mucus accumulation in RAO remission or the overall high variability, however. Therefore, I identified equine homologues of gel-forming mucin genes. EqMUC5AC, but not egMUC2 showed robust expression in large and small airway generations of both RAO-affected and clinically healthy horses. A preliminary semi-quantitative study on pooled samples suggested that eqMUC5AC expression is increased in RAO. I identified equine homologues of hCACC1 and EGFR, key signaling elements in mucin gene regulation, and developed quantitative RT-PCR assays for eqMUC5AC, equine hCACC1-like, EGFR and EGF. This allowed me to accurately determine mRNA levels in individual samples and investigate the association of eqMUC5AC expression with the expression of these key signaling elements, with stored intraepithelial mucosubstance and with neutrophilic inflammation. My findings did not support the hypothesis that eqMUC5AC is up-regulated and intraepithelial mucosubstances are increased in RAO-affected compared to clinically healthy control horses. EqMUC5AC, equine hCACC1-like, EGFR or EGF mRNA levels showed remarkable consistency across different lung lobes within individuals, indicating that the underlying pathogenesis is a diffuse process. (Abstract shortened by UMI.)...