Cytokine mRNA expression of pulmonary macrophages varies with challenge but not with disease state in horses with heaves or in controls.

The research studied whether pulmonary macrophages in horses with heaves, a breathing disorder, are associated with airway inflammation. The study found that while these macrophages do contribute to airway inflammation, they are not likely the main cause of the significant airway inflammation seen in horses with heaves.

Study Design and Methods

• The researchers focused on cytokines and chemokines (proteins important for cell signaling) produced by pulmonary macrophages, including TNF-α, IL-1β, IL-8, and MIP-2. They hypothesized that these proteins may lead to neutrophilic inflammation in horses with heaves, a condition characterized by reversible airflow obstruction and lower airway inflammation.
• Macrophages from bronchoalveolar lavage cells were isolated and the mRNA expression of these chemokines was quantified using real-time RT-PCR. This helped the researchers understand the role of these proteins in inflammation.
• The researchers studied 6 horses with heaves and 6 control horses. Studies were conducted after allowing the horses to graze for 3 months (which tends to reduce symptoms), and then after introducing a ‘natural antigen challenge’ – consisting of hay feeding and straw bedding – for 24 hours and 9 days.

Findings

• The study found that airway obstruction occurred in horses with heaves, but not in control horses, upon exposure to the natural antigen challenge. Airway neutrophilia (increase in a type of white blood cell) was observed in both groups of horses.
• Stabling led to a significant increase in expression of IL-8/ß-actin and MIP-2/ß-actin in both groups of horses after 24 hours. IL-8 and MIP-2 are vital for neutrophil recruitment and activation, indicatives of inflammation.
• However, the increase in these chemokines did not keep up over the longer term. By day 9, IL-8/ß-actin expression had significantly decreased in the second year of the study. No significant differences were observed for the ratios of IL-1β/ß-actin, and TNF-α/ß-actin – proteins related to inflammation and cell death respectively.

Implications

• The studies concluded that while macrophages do contribute to airway inflammation in horses, they are not solely responsible for the significant airway neutrophilia seen in heaves.
• The variation in the expression of IL-8 and MIP-2 by alveolar macrophages across the two study years underscores the complexity of the inflammatory pathways involved in airway inflammation.
• This study highlights the need for future research to further unpack the contributions of different cellular processes to the development of heaves, in order to improve treatments for this condition in horses.