Mariana J. Kaplan

Marko Radic

NETosis is a unique form of cell death that is characterized by the release of decondensed chromatin and granular contents to the extracellular space. The initial observation of NETosis placed the process within the context of the innate immune response to infections. Neutrophils, the most numerous leukocytes that arrive quickly at the site of an infection, were the first cell type shown to undergo extracellular trap formation. However, subsequent studies showed that other granulocytes are also capable of releasing nuclear chromatin following stimulation. The extracellular chromatin acts to immobilize microbes and prevent their dispersal in the host. Bacterial breakdown products and inflammatory stimuli induce NETosis and the release of NETs requires enzyme activities. Histones in NET chromatin become modified by peptidylarginine deiminase 4 (PAD4) and cleaved at specific sites by proteases. NETs serve for attachment of bactericidal enzymes including myeloperoxidase, leukocyte proteases, and the cathelicidin LL-37. While the benefit of NETs in an infection appears clear, NETs also figure prominently at the center of various pathologic states. Therefore, it is important for NETs to be efficiently cleared; else digestive enzymes may gain access to tissues where inflammation takes place. Persistent NET exposure at sites of inflammation may lead to a further complication: NET antigens may provoke acquired immune responses and, over time, could initiate autoimmune reactions. Recent studies identified aberrant NET synthesis and/or clearance in inflammatory/autoimmune conditions such as systemic lupus erythematosus (SLE), psoriasis, ANCA-positive vasculitis, gout and Felty’s syndrome. In the case of SLE, for example, it appears that LL-37 exposed in the NETs may be a significant trigger of type I Interferon responses in this disease. Recent evidence also implicates aberrant NET formation in the development of endothelial damage, atherosclerosis and thrombosis. NETosis is thus of interest to researchers who investigate innate immune responses, host-pathogen interactions, chronic inflammatory disorders, cell and vascular biology, biochemistry, and autoimmunity. As we approach the 10-year-anniversary of the initial discovery of NETosis, it is useful and timely to review the so far identified mechanisms and pathways of NET formation, their role in bacterial and fungal defense and their putative importance as inducers of autoimmune responses. We look forward to a rich and rigorous discussion of these and related issues that benefit from interdisciplinary approaches, collaborations and exciting discoveries.

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