Elevated amount of Toll-like receptor 4 mRNA in bronchial epithelial cells is associated with airway inflammation in horses with recurrent airway obstruction.

This research investigates the role of Toll-like receptor 4 (TLR4) in the inflamed airways of horses suffering from recurrent airway obstruction (RAO), a condition similar to human asthma. The results suggest that increased TLR4 expression and lack of upregulation of the regulatory protein A20 may heighten inflammation in the airways of these horses, particularly when exposed to dust abundant in stables.

Background and Hypothesis

• This study revolves around RAO, a lung disease in horses that is similar to human asthma. This condition causes inflammation and obstruction in horses’ airways and it often worsens when horses are stabled and therefore exposed to more dust.
• The dust within stables is rich in endotoxins that are recognized by Toll-Like Receptor 4 (TLR4), a component of the immune system that acts to recognize pathogens. In humans, stimulation of TLR4 in bronchial epithelial cells increases the expression of IL-8 mRNA, a molecule involved in inflammation.
• The team posited that TLR4 and IL-8 mRNA levels, along with neutrophil numbers, are elevated, and that A20 mRNA (a negative regulator of inflammation) is not increased in horses with RAO during stabling. These changes could potentially explain the heightened inflammatory response seen in horses affected by RAO.

Methodology

• The researchers studied twelve horses in total, half of them with RAO and the other half in control. The conditions for each pair of horses (one RAO-affected and one control) were carefully monitored both while the RAO-affected horse had airway obstruction following stabling and at various time points during pasture.
• Inflammatory cell counts were determined using bronchoalveolar lavage fluid (BAL) as well as measuring the maximum change in pleural pressure (DeltaPpl(max)). The team also quantified TLR4, IL-8, and A20 mRNA in bronchial epithelium using a technique called quantitative RT-PCR.

Findings

• Stabling increased BAL neutrophils, DeltaPpl(max), and TLR4 significantly in RAO horses compared to both healthy horses and RAO horses kept on pasture, indicating an increased inflammatory response.
• TLR4 correlated strongly with IL-8, reinforcing the idea that TLR4 activation could lead to heightened inflammation through IL-8 production.
• While IL-8 levels increased in all horses in response to stabling, levels of the anti-inflammatory A20 remained constant, suggesting that the lack of A20 upregulation could contribute to continued inflammation.
• The findings suggest that high TLR4 expression and a lack of A20 upregulation in bronchial epithelial cells from RAO-affected horses may contribute to increased IL-8 production and thus to an exaggerated inflammatory response to stable dust inhalation.