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

Background Severe equine asthma is common and analogous to neutrophilic asthma in humans. Caused by exposure to organic and inorganic environmental particulates, the disease manifests in mature horses as hyperreactive airways and severe neutrophilic lower airway inflammation. Macrophage populations in the lung, including resident alveolar macrophages (AMs) and recruited monocyte-derived macrophages (MDMs), recognize these barn dust particulates, and orchestrate an immune response thought the cytokines they produce. Despite their importance, the specific contributions of these macrophage subsets to equine asthma remain poorly understood. Our work aimed to investigate the contributions of AMs and MDMs to the early inflammatory response using RNA-seq. Therefore, we undertook a 6-hour exposure of AMs and MDMs from six healthy female Standardbred horses to a mixture of fungal spores, lipopolysaccharide, and silica microspheres (FLS), as these form the major components of barn dust, with tissue culture medium as control. We hypothesized that AMs and MDMs would have differing transcriptional responses to FLS. Results From our RNA-seq analyses, we identified differentially expressed genes and associated biological pathways. “Cytokine signaling” was identified as the major biological process activated by FLS in both cell types. Pathways including JAK-STAT/IL-15, TNF receptor binding, and IFN signaling were more highly upregulated in MDMs than AMs, suggesting that the two cell types have unique signalling pathways and inflammatory responses. Conclusions These results indicate that equine AMs and MDMs have distinct responses to common inflammatory signals, and therefore, provide differing contributions to the early inflammatory response. These insights provide a foundation for future investigations of the role of equine AMs and MDMs to the pathogenesis of severe equine asthma.