Transcriptomic response of equine bone marrow-derived macrophages cultured in macrophage colony-stimulating factor (CSF-1) to lipopolysaccharide (LPS)

Introduction: Endotoxaemia, the accumulation of endotoxin in the bloodstream due to Gram negative infection, is a leading cause of death in horses. The binding of lipopolysaccharide (LPS), a Gram-negative marker, to the CD14/MD2/TLR4 complex on macrophages results in the release of cytokines, including TNF, IL-1 and IL-6. Whilst the macrophage response to LPS is similar across species, species-specificity exists with respect to the sensitivity to LPS; horses and humans are particularly sensitive in comparison to rodents. Using RNA-Seq data from LPS-stimulated equine bone marrow-derived macrophages (eqBMDMs), we analysed the transcriptomic response of eqBMDMs to LPS.
Material and Methods: Bone marrow from ribs of adult horses was isolated and cells cultured in CSF-1 to generate eqBMDMs. Once differentiated, eqBMDMs were stimulated with LPS for 7 hours. RNA was extracted and the LPS response was validated with qRT-PCR prior to RNA-Seq analysis.
Results: Macrophages stimulated with LPS strongly upregulated genes including Il-1B, IL-6, TNF and CXCL8. One of the most highly expressed genes, CXCL6 (granulocyte chemoattractant protein-2), mediates neutrophil recruitment and is upregulated alongside other chemokines involved in neutrophil recruitment. In notable contrast to rodent macrophages, eqBMDMs failed to metabolise arginine in response to LPS, instead metabolising tryptophan.
Conclusion: Our data shows species-specific variation in innate immune biology. In particular, CXCL6 upregulation signals the recruitment of neutrophils, suggesting the horse may be biased towards a strong neutrophilic response. Consequently, it would be interesting to make comparisons with equivalent genes in other species.