Mechanisms of NET formation

1 6 1 2 jci.org Volume 126 Number 5 May 2016 Introduction Neutrophil extracellular traps (NETs) are extracellular strands of decondensed (unwound) DNA in complex with histones and neutrophil granule proteins. NETs were discovered more than a decade ago (1) in a study demonstrating that they are generated in vitro after stimulation of isolated neutrophils with IL-8, a major neutrophil chemoattractant; LPS, a component of Gram-negative bacteria; or PMA, a potent activator of PKC. Further, these in vitro generated NETs possessed antibacterial activity, which was ascribed to the associated histones (2), proteolytic enzymes from granules that might degrade bacterial virulence factors, and enzymatically active myeloperoxidase (MPO). Notably, the antibacterial activity of NETs is abrogated by DNase (1). Induction of NETs by IL-8 and LPS indicates that NETs are formed during inflammation and infection, and NETs are found in vivo during bacterial infections such as appendicitis (1). This seminal publication demonstrating the existence of NETs showed that neutrophils may undergo an alternative death pathway, termed NETosis, which allows them to serve in innate immune defense even after their death. However, NETosis as a neutrophil death mechanism may largely be an in vitro phenomenon, and NETs seem to be generated in vivo by mechanisms different from those described in vitro. The purpose of this Review is to contrast the fundamental mechanisms for NET formation in vitro and in vivo and to discuss the biological relevance of the latter.