Neutrophil extracellular traps and syndecan-1 in equine sepsis and inflammation

Sepsis is a common cause of death in young foals. It is generally seen as a dysregulated inflammatory response of the host to infection, however there is no consensus definition of sepsis. The diagnosis of sepsis is challenging because of its vague clinical signs and the lack of a highly sensitive and specific test. The identification of new biomarkers could assist in a more timely sepsis diagnosis and contribute to a better understanding of the pathophysiology of sepsis.

This research focused on neutrophil extracellular traps (NETs), nucleosomes, and syndecan-1 to determine their utility for identifying septic processes in horses, particularly in septic foals. Neutrophil extracellular traps are an important part of the innate immune system that are released by neutrophils to entrap microorganisms. Excessive NET release can cause tissue injury, including endothelial glycocalyx damage, to the host. The endothelial glycocalyx is a gel-like structure that lines the luminal surface of blood vessels and plays an important role in vascular homeostasis. It is a fragile structure that is susceptible to injury from various insults, and syndecan-1 is a biomarker of endothelial glycocalyx damage. Increased NET release and related biomarkers such as nucleosomes, have been identified as associated with sepsis in humans and some other animals. There are only a small number of NET and endothelial glycocalyx studies in horses, and their involvement in equine neonatal sepsis is unknown.

This study involved the analysis of clinical and laboratory data, and samples collected from foals and adult horses under the care of a university hospital. Immunofluorescence was used to assess for the presence of NETs in septic synovial fluid (SF) and peritoneal fluid (PF) samples to determine if they may be associated with infection or septic inflammation. Multiple sepsis criteria incorporating clinical and laboratory data, were utilised to evaluate foals for sepsis, and compared in terms of their sensitivity and specificity. Enzyme-linked immunosorbent assays were used to measure nucleosomes and syndecan-1 in plasma samples from septic, sick non-septic and clinically healthy foals.

Neutrophil extracellular traps were visualised and quantified in septic SF and PF samples from foals and adult horses. Nucleosomes were detected in plasma samples from unwell foals, with significant increases occurring in foals with severe sepsis. There was a large degree of overlap in nucleosome levels for septic, sick non-septic and clinically healthy foals. Syndecan-1 concentrations from septic and sick non-septic foals were low.

The findings of this research indicate that NETs and associated biomarkers may be useful for investigating and understanding sepsis and septic inflammation in horses. Nucleosomes showed potential as an early marker of severe sepsis in foals. A single perfect test for sepsis is unlikely given the complex and heterogenous nature of the syndrome. Neutrophil extracellular traps and related biomarkers may be useful adjuncts for the assessment of sepsis and infectious disease processes when they are interpreted with clinical data and other inflammatory markers.