β-Glucan protects neutrophil extracellular traps against degradation by Aeromonas hydrophila in carp (Cyprinus carpio)

A novel host innate immune defence mechanism against invading pathogens, namely the formation of neutrophil extracellular traps (NETs), has recently been discovered. These NETs are described as DNA fibres released by dying neutrophils, which are able to entrap and kill various microbes. Here we studied the effect of the feed additive β-glucan, namely MacroGard®, on the degradation of NETs by the important fish pathogen Aeromonas hydrophila. Therefore, common carp (Cyprinus carpio) head kidney cells consisting of approximately 45% neutrophils were isolated and treated with or without β-glucan. The degradation of NETs after co-incubation with A. hydrophila was analysed by immunofluorescence microscopy. The data show that A. hydrophila is able to degrade NETs and that treatment of cells with β-glucan significantly protects the NETs against bacterial degradation. Control experiments revealed that β-glucan augments nuclease activity of the bacteria at the same time while protecting the NETs against its degradation. In conclusion the data indicate that β-glucan might affect the composition and stabilisation of NETs and thereby protecting them against degradation by A. hydrophila nuclease.