Neutrophil Extracellular Traps in Host Defense

Abstract

Neutrophils are produced in the bone marrow and then patrol blood vessels from which they can be rapidly recruited to a site of infection. Neutrophils bind, engulf, and efficiently kill invading microbes via a suite of defense mechanisms. Diverse extracellular and intracellular microbes induce neutrophils to extrude neutrophil extracellular traps (NETs) through the process of NETosis. Here, we review the signaling mechanisms and cell biology underpinning the key NETosis pathways during infection and the antimicrobial functions of NETs in host defense.

Figure 1.

NETosis is a distinct cellular program in neutrophils. Neutrophil NETosis involves sequential morphological changes: (1) The characteristic lobulated architecture of the nucleus (purple) is lost, (2) the membranes of the nucleus and granules (pink) become permeable, (3) histone inactivation by clipping and/or citrullination (Cit-histones) leads to chromatin expansion into the cytosol, (4) chromatin mixes with granule content, (5) the loss of internal membranes leads to the disappearance of cytosolic organelles, and (6) the plasma membrane breaks and neutrophil extracellular traps (NETs) are released into the extracellular space.

Figure 2.

Suicidal NETosis. Phorbol-12-myristate-13-acetate (PMA) or extracellular microbes induce suicidal NETosis by inducing the extracellular signal-regulated kinase (ERK)-dependent assembly of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which generates reactive oxygen species (ROS) and the cytoplasmic release of granule proteins, and leads to chromatin decondensation in the nucleus. Chromatin is then expelled into the extracellular space via gasdermin D (GSDMD) pores or GSDMD-driven membrane tears, resulting in neutrophil death. (NE) neutrophil elastase, (MPO) myeloperoxidase, (NET) neutrophil extracellular trap.

Figure 3.

Noncanonical NETosis. The lipopolysaccharide (LPS) of cytosolic Gram-negative bacteria triggers the assembly of the noncanonical inflammasome, leading to caspase-4/11 activation and gasdermin D (GSDMD) cleavage to p30. GSDMD-p30 pores rupture the nuclear and granule membranes and, eventually, the plasma membrane. Caspase-11 gains access to the nucleus, where it cleaves histones to mediate DNA decondensation. Chromatin is then expelled into the extracellular space during plasma membrane rupture, resulting in neutrophil death. During NETosis, GSDMD-driven NLRP3 inflammasome activation triggers the release of interleukin (IL)-1β to induce neutrophil recruitment, activation, and killing of the neutrophil extracellular trap (NET)-entrapped bacterium.

Figure 4.

Vital NETosis. Microbial products or activated platelets stimulate vital NETosis through cell-surface receptors (e.g., TLR2, TLR4, complement receptor-3 [CR3]). The resulting spike in cytosolic calcium activates neutrophil PAD4, triggering histone citrullination and DNA decondensation. Chromatin derived from nuclear blebs or the mitochondria is expelled to generate a neutrophil extracellular trap (NET) while the neutrophil is still able to perform cellular functions such as cell migration.