Expression of haem oxygenase-1 and inducible nitric oxide synthase in horses with lower airway inflammation

The heat shock protein haem oxygenase-1 (HO-1) and the enzyme inducible nitric oxide synthase (iNOS) are both induced during conditions of oxidative stress and by pro-inflammatory mediators. Both HO-1 and iNOS may therefore be induced by a range of disease states that result in tissue inflammation. Both have been demonstrated to have potentially beneficial anti-inflammatory effects. Both HO-1 and iNOS and their respective gaseous products carbon monoxide and nitric oxide have being the subject of investigation as potential therapeutic targets for human respiratory disease, most notably asthma. The pathogenesis of inflammatory diseases of the equine lower respiratory tract has many similarities with human asthma and both HO-1 and iNOS have been demonstrated in fixed equine pulmonary tissue. The aim of the study was to investigate the expression of HO-1 and iNOS in the equine respiratory tract. The primary objective was to investigate whether HO-1 and iNOS were expressed within leukocytes from equine BALF and to semi-quantify expression in different cell types. If it proved possible to characterise and quantify expression, additional aims were to investigate associations between HO-1 expression, iNOS expression, clinical signs of respiratory disease and markers of oxidative stress in exhaled breath condensate. We were successful in demonstrating the expression of both HO-1 and iNOS in leukocytes from equine BALF. Expression was most intense in macrophages and neutrophils which was consistent with previous studies performed in other species. We did not identify an association between the degree of HO-1 or iNOS staining in leukocytes from BALF and the severity of clinical respiratory disease. We also failed to identify an association between expression of HO-1 and iNOS in leukocytes from BALF and markers of oxidative stress in exhaled breath condensate. Further investigations ought to be performed into the expression and activity of both HO-1 and iNOS in the equine respiratory tract