The role of neutrophils and NETosis in autoimmune and renal diseases

Abstract

Systemic autoimmune diseases are a group of disorders characterized by a failure in self-tolerance to a wide variety of autoantigens. In genetically predisposed individuals, these diseases occur as a multistep process in which environmental factors have key roles in the development of abnormal innate and adaptive immune responses. Experimental evidence collected in the past decade suggests that neutrophils - the most abundant type of white blood cell - might have an important role in the pathogenesis of these diseases by contributing to the initiation and perpetuation of immune dysregulation through the formation of neutrophil extracellular traps (NETs), synthesis of proinflammatory cytokines and direct tissue damage. Many of the molecules externalized through NET formation are considered to be key autoantigens and might be involved in the generation of autoimmune responses in predisposed individuals. In several systemic autoimmune diseases, the imbalance between NET formation and degradation might increase the half-life of these lattices, which could enhance the exposure of the immune system to modified autoantigens and increase the capacity for NET-induced organ damage. This Review details the role of neutrophils and NETs in the pathophysiology of systemic autoimmune diseases, including their effect on renal damage, and discusses neutrophil targets as potential novel therapies for these diseases.

Figure 1. NETosis pathways and potential therapeutic targets

Stimulation of neutrophil receptors by microorganisms or sterile stimuli leads to release of calcium (Ca⁺) from the endoplasmic reticulum (ER), which results in activation of protein kinase C (PKC) and assembly of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex and/or mitochondrial activation, leading to generation of reactive oxygen species (ROS). Other cellular sources might also be important contributors of ROS. This ROS generation is followed by myeloperoxidase (MPO)-dependent migration of granular neutrophil elastase (NE) to the nucleus where it cleaves histones. In addition, peptidylarginine deiminase (PAD) 4 is activated and induces histone citrullination, which contributes to chromatin decondensation. Finally, the nuclear membrane is degraded and a mixture of chromatin and granular proteins is extruded from the cell. Extracellular DNase eventually degrades neutrophil extracellular traps (NETs). Modulation of critical steps in NET formation and degradation (shown by blocking arrows) might be beneficial for the treatment of autoimmune disorders. C5aR, C5a receptor; G-CSF: granulocyte colony stimulating factor; G-CSF-R, G-CSF receptor; FcγR, Fc γ receptor; TLR, Toll-like receptor.

Figure 2. NETs in low-density granulocytes

Low-density granulocytes from a patient with systemic lupus erythematosus show spontaneous formation of neutrophil extracellular traps (NETs). Immunofluorescence imaging reveals NETs as strands of DNA (blue (labelled with Hoechst stain)) with neutrophil granular proteins (red (myeloperoxidase)). Image obtained by Monica Purmalek, Systemic Autoimmunity Branch, National Institute of Arthritis and Musculoskeletal and Skin Diseases, NIH, USA.

Figure 3. Neutrophils and NETosis in the pathogenesis of autoimmune and renal diseases

Upon exposure to various infectious and ‘sterile’ stimuli, neutrophils (and low density granulocytes (LDGs) in the setting of systemic lupus erythematosus (SLE)) release neutrophil extracellular traps (NETs). These NETs externalize granular peptides (such as LL-37) in complex with DNA, which, in turn, activate plasmacytoid dendritic cells (pDCs) to synthetize type I interferons (IFN), which have critical roles in SLE pathogenesis. Neutrophils also release inflammatory cytokines that activate T cells and B cells, which produce autoantibodies, potentially contributing to the development of systemic autoimmune and renal diseases. NETs promote the expression of tissue factor, which activates platelets and coagulation factors, thereby promoting thrombosis. This process might contribute to the development of renal diseases, particularly antiphospholipid antibody syndrome (APS) and ANCA-associated vasculitis (AAV). NET-bound proteins, such as metalloproteinases (MMPs) and histones, promote vasculopathy — a feature of renal disease, AAV, SLE and APS — by damaging endothelial cells. Autoantibodies, immune complexes, autoantigens, complement activation factors and cytokines can in turn cause NETosis. NET peptides stimulate NLRP3 inflammasome activation in macrophages, leading to release of IL-1 and IL-18, which promote neutrophil activation and NET formation.