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.

In DN patients, the complement system is activated and, of the three possible complement pathways, activation of the lectin and alternative pathways is associated with renal damage.

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