Since the discovery and definition of neutrophil extracellular traps (NETs) 14 years ago, numerous characteristics and physiological functions of NETs have been uncovered. Nowadays, the field continues to expand and novel mechanisms that orchestrate formation of NETs, their previously unknown properties, and novel implications in disease continue to emerge. The abundance of available data has also led to some confusion in the NET research community due to contradictory results and divergent scientific concepts, such as pro- and anti-inflammatory roles in pathologic conditions, demarcation from other forms of cell death, or the origin of the DNA that forms the NET scaffold. Here, we present prevailing concepts and state of the science in NET-related research and elaborate on open questions and areas of dispute.

Nasal polyps, which often occur in association with allergic rhinitis and asthma, are characterized by a marked infiltration of eosinophils. Using a method for detecting eosinophils with DNA strand breaks, we found direct evidence for inhibition of eosinophil apoptosis in this model of tissue eosinophilia. By using Southern blot analysis linked to reverse transcription-PCR, we detected a mRNA signal specific for IL-5 in all nasal polyps. The identification of IL-5 as a major eosinophil survival factor was confirmed by ELISA measurements using tissue homogenates. Moreover, immunohistochemical analysis of the nasal polyp tissues demonstrated that IL-5 was localized in lymphocytes, mast cells, and eosinophils. Treatment of the eosinophil-infiltrated tissue with neutralizing anti-IL-5 mAb induced eosinophil apoptosis and decreased tissue eosinophilia. Therefore, IL-5 may represent an important cytokine responsible for the delay of the death process in eosinophils in nasal polyps. In addition, a previously suggested IL-4-dependent specific recruitment of eosinophils into the inflamed tissue could be excluded by our studies. Taken together, these findings suggest a novel mechanism by which eosinophils specifically accumulate in pathologic human tissues.