NETosis markers: Quest for specific, objective, and quantitative markers

More than 10 years have passed since the discovery of neutrophil extracellular traps (NETs) in 2004. NETs are extracellular web-like DNA decorated with antimicrobial proteins, which are released from activated neutrophils. The state of neutrophils with NET formation is called NETosis. It has been realized that NETosis includes suicidal NETosis and vital NETosis. The former state means cell death of neutrophils, whereas the latter state preserves living neutrophilic functions. Although both suicidal and vital NETosis play essential roles in elimination of microorganisms, excessive formation of NETs, especially the ones derived from suicidal NETosis, can harm the hosts. Therefore, the discovery of NETosis markers and development of evaluation methods are important. In this review, we compare the methods for evaluating NETosis, including immunocytological and immunohistological detection of co-localized neutrophil-derived proteins and extracellular DNA, and citrullinated histones, detection of NET remnants in fluid samples, and flow cytometric detection of cell-appendant NET components, with focus on the specificity, objectivity, and quantitativity. Since the gold standard marker of NETosis or method of NET detection has not been established yet, researchers should choose the most appropriate marker or method in each situation based on the knowledge of the respective virtues and faults.