The role of alveolar macrophages in the pathogenesis of recurrent airway obstruction in horses.
Authors: Laan Tamarinde T J M, Bull Sarah, Pirie R, Fink-Gremmels Johanna
Journal: Journal of veterinary internal medicine
Summary
Recurrent airway obstruction (RAO) in horses involves complex immune dysregulation, yet the role of innate immunity—specifically alveolar macrophage responses—remained poorly characterised prior to this 2006 investigation. Researchers exposed seven RAO-susceptible and five nonsusceptible horses to aerosolised saline, lipopolysaccharide (LPS), hay dust solution, and *Aspergillus fumigatus*, then measured bronchoalveolar lavage cytology, lung function, and alveolar macrophage cytokine expression (TNF-α, IL-1β, IL-6, IL-8, and IL-10) at 6 and 24 hours post-challenge using real-time PCR. Susceptible horses demonstrated significantly elevated neutrophil ratios, altered total cell counts, and—crucially—exaggerated macrophage production of pro-inflammatory mediators (TNF-α, IL-1β, IL-8) compared with resistant animals; conversely, nonsusceptible horses mounted a more protective IL-6 response within 6 hours of allergen exposure. These findings fundamentally reframe RAO as a disorder rooted in dysregulated innate immune activation rather than adaptive immunity alone, suggesting that early therapeutic targeting of macrophage-driven inflammation—rather than waiting for neutrophil infiltration—may offer a preventative window for at-risk animals, particularly through environmental control and anti-inflammatory protocols initiated before clinical signs develop.
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Practical Takeaways
- •RAO susceptibility appears to involve an intrinsic difference in how individual horses' alveolar macrophages respond to airway irritants—some horses are genetically predisposed to mount excessive pro-inflammatory responses
- •Understanding these innate immune differences may eventually enable identification of high-risk horses earlier and guide development of targeted therapies beyond standard environmental management
- •Current management focusing solely on reducing allergen exposure may be insufficient for susceptible horses; future treatments may need to modulate macrophage immune function directly
Key Findings
- •RAO-susceptible horses showed significantly higher expression of pro-inflammatory cytokines (TNF-alpha, IL-1beta, IL-8) by alveolar macrophages compared to nonsusceptible horses after challenge
- •Nonsusceptible horses demonstrated higher IL-6 expression (anti-inflammatory) 6 hours post-challenge, suggesting a protective immune phenotype
- •Differences in RAO susceptibility originate from innate immune responses at the macrophage level, not solely from adaptive immunity
- •Neutrophil ratios and total cell counts in bronchoalveolar lavage fluid were significantly elevated in RAO-susceptible horses across all challenge types