RNA-seq evaluation of equine alveolar macrophages and monocyte-derived macrophages exposed to an inflammatory stimulus (short communication)

Macrophage populations in the lung, including resident alveolar macrophages (AMs) and recruited monocyte-derived macrophages (MDMs), recognize the inhaled particulates in barn dust that cause severe equine asthma and orchestrate an immune response though the cytokines they produce. Despite their importance, the specific contributions of these macrophage subsets to lower airway inflammation remain poorly understood. This exploratory in vitro study investigated the likely contributions of AMs and MDMs from healthy horses to the early inflammatory response using RNA-seq. If biologically important contrasts were identified, future studies to elucidate differences in AM and MDM function in severely asthmatic horses could be undertaken. AMs and MDMs from six healthy female Standardbred horses were exposed for 6 h to a mixture of fungal spores, lipopolysaccharide, and silica microspheres (FLS), which represents the major components of barn dust, with serum-free cell culture medium as the control. Equine AMs and MDMs exposed to FLS showed broadly similar transcriptional responses, with cytokine signalling emerging as the dominant biological theme in both cell types. At an exploratory FDR threshold of 0.25, gene set enrichment analysis indicated that “JAK-STAT/IL-15 signalling pathway”, “NOD-like receptor signalling pathway”, and “infection/C-type lectin receptor” had greater enrichment in FLS-exposed MDMs than AMs. These exploratory data indicate that equine AMs and MDMs mount largely overlapping cytokine responses to the elements that contribute to exacerbation of severe equine asthma, but also exhibit cell-type specific pathway differences. This work establishes the baseline for future studies testing the hypothesis that in horses with severe equine asthma, AMs and MDMs each have altered function that contribute uniquely to disease pathogenesis.