Background

Equine asthma (EA) is characterised by hyperresponsive airways and chronic lower airway inflammation of variable degree. Severe EA is associated with structural changes in the airways and apparent clinical signs, such as frequent coughing and respiratory effort at rest, while in mild/moderate EA the inflammation is milder without marked structural changes, and it is less distinct both in diagnostics and clinical signs. Typical clinical signs in mild/moderate EA are decreased performance and chronic, intermittent or occasional cough usually observed during exercise [1].

EA and human asthma share some etiological, pathophysiological, and immunological similarities, such as hyperresponsive airways, sensitivity to certain antigens and endotoxins, and pulmonary remodelling. Thus, horses can provide a naturally occurring translational model for human asthma [2]. While about 50% of severe human asthma is characterised by eosinophilia, this is unusual for severe EA, which typically shows neutrophilic response, and mild/moderate EA displays mildly increased neutrophils, eosinophils, or metachromatic cells [1]. Analysing histopathological findings in bronchial biopsies, horses with naturally occurring EA showed bronchial epithelial and submucosal inflammation, thickening of the basal membrane, airway smooth muscle hypertrophy, and fibrosis, expanding the understanding of the pathophysiology of EA [3,4,5,6].

Hyaluronic acid (HA) is a critical component of the healthy, effectively working extracellular matrix (ECM) with dual functions in inflammatory processes. HA is a non-sulfated glycosaminoglycan, produced in the inner leaflet of plasma membrane by the activity of HA synthases [7]. HA synthases and hyaluronidases are responsible for the diversity of HA molecular size. Different HA synthases produce HA molecules of various sizes, and hyaluronidases degrade HA to smaller fragments [8]. The biological activity of HA depends on its size: low-molecular-weight (LMW) HA (100–500 kDa) stimulates cell migration and inflammatory protein production, while high-molecular-weight (HMW) HA (1000–6000 kDa) has anti-inflammatory and tissue protecting functions [7, 9]. HA serves as a critical component in the ECM of most tissues, including airways and lungs [7, 10]. Elevated levels of HA in bronchoalveolar lavage fluid (BALF) have been detected in asthma in animal and human studies, and the presence of LMW HA has been suggested to indicate inflammation [11,11, 13, 14, 26,27,28,29]. In these previous studies, the increased HA was suggested to derive from its increased production in lung interstitium fibroblasts, epithelial cells, and alveolar cells, and to be associated with tissue fibrosis. This could indicate an attempt of the organism to protect the tissues and to provide hydration during chronic inflammation. However, in this study, increased HA content was not associated with fibrosis. HA is synthesised in the inner surface of cell membrane by HA synthases, and after synthesis it is transported to the extracellular space [30]. Therefore, as shown in the present study, HA is abundant in cell membranes, extracellular structures, and connective tissue.

In this study, the tissue distribution of HA, the intensity of HA staining, and plasma HA concentration were similar between the groups, unlike in a study on human patients with induced asthma exacerbation [31]. Lauer et al. [31] found that controls had less HA in lung tissues due to lack of smooth muscle hypertrophy and basement membrane thickening. Additionally, in their study serum HA concentration peaked in asthmatic patients five hours after an allergen challenge, which was not performed in our study. Moreover, human allergic asthma is often eosinophilic, while we studied neutrophilic airway inflammation [32].

HA molecules can have both pro- and anti-inflammatory actions depending on their size, and during inflammation the production of LMW HA increases, which leads to the activation of inflammatory pathways and intensive cytokine production [7, 33]. It has been suggested that local production of LMW HA contributes to airway bronchoconstriction, inflammation, and fibrosis, and is associated with chronic asthma and remodelling [41].

Conclusion

In this study, we show for the first time that HA concentration increases in BALF in naturally occurring neutrophilic EA. However, the HA tissue distribution and plasma concentration remained similar in control horses and horses with EA. Airway remodelling was the highest in horses with severe NAI, but it was not associated with airway HA content. Identifying the association between EA and increased HA concentration in lungs can offer new insights to EA therapy modalities and further research. In the future, the roles of HMW and LMW HA in EA need further investigations with the focus on therapy.