Neutrophil extracellular traps can activate alternative complement pathways

Abstract

SummaryThe interaction between neutrophils and activation of alternative complement pathway plays a pivotal role in the pathogenesis of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). ANCAs activate primed neutrophils to release neutrophil extracellular traps (NETs), which have recently gathered increasing attention in the development of AAV. The relationship between NETs and alternative complement pathway has not been elucidated. The current study aimed to investigate the relationship between NETs and alternative complement pathway. Detection of components of alternative complement pathway on NETs in vitro was assessed by immunostain and confocal microscopy. Complement deposition on NETs were detected after incubation with magnesium salt ethyleneglycol tetraacetic acid (Mg-EGTA)-treated human serum. After incubation of serum with supernatants enriched in ANCAinduced NETs, levels of complement components in supernatants were measured by enzyme-linked immunosorbent assay (ELISA). Complement factor B (Bb) and properdin deposited on NETs in vitro. The deposition of C3b and C5b-9 on NETs incubated with heat-inactivated normal human serum (Hi-NHS) or EGTA-treated Hi-NHS (Mg-EGTA-Hi-NHS) were significantly less than that on NETs incubated with NHS or EGTA-treated NHS (Mg-EGTA-NHS). NETs induced by ANCA could activate the alternative complement cascade in the serum. In the presence of EGTA, C3a, C5a and SC5b-9 concentration decreased from 800Á42 6 244Á81 ng/ml, 7Á68 6 1Á50 ng/ml, 382Á15 6 159Á75 ng/ml in the supernatants enriched in ANCA induced NETs to 479Á07 6 156Á2 ng/ml, 4Á86 6 1Á26 ng/ml, 212Á65 6 44Á40 ng/ml in the supernatants of DNase I-degraded NETs (P < 0Á001, P = 0Á008, P < 0Á001, respectively). NETs could activate the alternative complement pathway, and might thus participate in the pathogenesis of AAV.

Factor H Binds to Extracellular DNA Traps Released from Human Blood Monocytes in Response to Candida albicans

“…This is in agreement with recent reports of NETs (48, 49) and suggested that complement is regulated on the NETs to allow opsonization and phagocytosis to terminate the immune response. Clearance of extracellular traps is achieved with the help of DNase I, which cleaves the extracellular DNA and facilitates uptake and clearance of DNA by macrophages (28).…”

Complement in disease: a defence system turning offensive

“…For example, a positive feedback mechanism between complement and neutrophil activation has been described in vitro : C5a can activate neutrophils to secrete properdin that, in turn, activates complement on released neutrophil extracellular traps to generate more C5a. This perpetuation of neutrophil-derived inflammation signals might enhance defence mechanisms but also have pathological consequences in diseases such as antineutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis (see below) 38,39 . By contrast, C3a-mediated signalling demonstrated a protective effect in a mouse model of ischaemia–reperfusion injury (IRI), constraining neutrophil mobilization in response to injury 40 .…”

Decreased Neutrophil Extracellular Trap Degradation in Shiga Toxin-Associated Haemolytic Uraemic Syndrome

“…NETs can serve as scaffolding that traps platelets [25] and express the pro-thrombotic tissue factor that initiates coagulation [26,27] . We and others have also previously reported that NETs can activate complement [19,28] and this could enhance the Stx-induced complement activation leading to C3b deposition in the glomeruli and on platelets [29,30] , inducing microvascular thrombosis and kidney injury [31] . These adverse procoagulant, pro-inflammatory and complement-activating effects of NETs may be particularly severe in kidneys and fit well the known pathology of STEC-HUS.…”