Neutrophil extracellular traps: double-edged swords of innate immunity

Abstract

Spectacular images of neutrophils ejecting nuclear chromatin and bactericidal proteins, in response to microbes, were first reported in 2004. As externalized chromatin could entangle bacteria, these structures were named neutrophil extracellular traps (NETs). Subsequent studies identified microorganisms and sterile conditions that stimulate NETs, as well as additional cell types that release extracellular chromatin. The release of NETs is the most dramatic stage in a cell death process called NETosis. Experimental evidence suggests that NETs participate in pathogenesis of autoimmune and inflammatory disorders, with proposed involvement in glomerulonephritis, chronic lung disease, sepsis, and vascular disorders. Exaggerated NETosis or diminished NET clearance likely increases risk of autoreactivity to NET components. The biological significance of NETs is just beginning to be explored. A more complete integration of NETosis within immunology and pathophysiology will require better understanding of NET properties associated with specific disease states and microbial infections. This may lead to the identification of important therapeutic targets.

Figure 1

Representative images of NETs induced in vitro by LPS in human neutrophils. NETs are visualized by costaining of neutrophil elastase (green) and nuclear material (DAPI, blue). Magnification is 40X.

Figure 2. Putative stimuli and steps in NETosis

A. Microbes and their products, immune complexes, autoantibodies, cytokines and other stimuli (IL8, TNF, and type I and II IFNs) can induce NETosis. Binding via TLRs, Fc receptors or complement receptors have been implicated in NETs’ induction. B. NETosis is initiated by binding of neutrophil surface receptors (R) to microbes or microbial breakdown products, inflammatory stimuli, or endogenous inducers. Binding to receptor(s) (exemplified in diagram as R1 and R2) induces ER calcium store release and opening of membrane channels that lead to cytoplasmic calcium increases. Elevated calcium stimulates PKC activity, phosphorylation of gp91phox, and assembly of functional NADPH Oxidase leading to reactive oxygen species (ROS) and nitric oxide (NO) production. Morphological changes observed during NETosis include breakdown of nuclear and granule membranes and the mixing of nuclear, granular and cytoplasmic contents. Deimination and proteolytic cleavage of histones may initiate before nuclear breakdown, and contribute to chromatin decondensation. A rupture in the plasma membrane allows the release of extracellular chromatin traps.