Fish cast NETs: neutrophil extracellular traps are released from fish neutrophils

Neutrophil extracellular traps (NETs), which are extracellular DNA structures released from neutrophils, are described and characterized for the first time in fish using fluorescent confocal microscopy. Confocal images of fish neutrophil suspensions stained with 6'-diamino-2-phenylindole, dihydrochloride DNA fluorescent stain (DAPI) revealed the presence of NETs which appeared as fibrous structures connecting several cells. Co-localization of NETs with neutrophil granular proteins and actin was investigated using specific antibodies and probes. Double staining of neutrophils with SYTOX green and DAPI revealed that SYTOX stain applied to living cells stained extracellular DNA, but not nuclei. NETs are actively released from stimulated living cells, associated with granular proteins, but not with cytoskeleton, and are not a product of nuclear degradation seen in late apoptotic stages. Additionally, a fluorometric microtiter plate assay to quantify the release of NETs was adopted for use with fish neutrophils, and the effect of stress on NETs release was studied. This assay detected the inhibition of DNA release during stress conditions. In summary, NETs were released from living fish kidney neutrophils upon stimulation, characterized using fluorescence DNA-binding dyes, specific antibodies and probes, and quantified using a microtiter plate fluorometric assay that can rapidly measure a large number of samples. Detection of NETs can be used as an additional assay to an existing battery of functional tests, and as a new research model to study the effects of stress, immunomodulators, and diseases.