Role of peptidyl arginine deiminase 4 (PAD4) in anti-β2GP1/β2GP1 complex-mediated neutrophil extracellular trap formation

Objective To investigate the role of peptidyl arginine deiminase 4 (PAD4) in anti-β2GP1/β2GP1 complex-induced formation of neutrophil extracellular trapping networks (NETs). Methods Peripheral blood neutrophils were isolated from healthy humans by density gradient centrifugation. PAD4 expression was detected by Western blot after the neutrophils were incubated with anti-β2GP1/β2GP1 complex (100 μg/ml) for a certain period of time. PAD4 inhibitor Cl-amidine (10 μmol/L) was used to pretreat neutrophils. Changes in the expression of citrullinated histone 3 (CitH3) at protein level and the relative content of myeloperoxidase (MPO)-DNA were detected by Western blot and ELISA, respectively. A mouse thrombus model of antiphospholipid syndrome (APS) was established by inferior vena cava stenosis. Intervention experiments were performed by intraperitoneal injection of Cl-amidine (50 mg/kg). The expression of CitH3 at protein level in plasma was detected by Western blot. The concentration of circulating free DNA (cf-DNA) in plasma was measured with fluorescent staining. Thrombus in inferior vena cava was collected and weighted to evaluate whether inhibiting the activity of PAD4 would suppress the APS-IgG-induced formation of NETs and thrombosis. Differences among groups were analyzed by t test or one-way analysis of variance (ANOVA). Results The expression of PAD4 induced by anti-β2GP1/β2GP1 complex was significantly down-regulated in the cytoplasm, but increased in the nucleus [(3.67±0.32) vs (1.47±0.19), t=10.22, P<0.05; (0.57±0.19) vs (2.97±0.31), t=11.49, P<0.05]. Cl-amidine significantly inhibited the anti-β2GP1/β2GP1 complex-induced expression of CitH3 protein by neutrophils [(2.46±0.47) vs (0.46±0.13), t=12.24, P<0.01], and reduced the MPO-DNA content in the culture supernatants [(4.09±0.94) vs (2.80±0.57), t=4.23, P<0.05]. In vivo, Cl-amidine significantly inhibited the expression of CitH3 protein [(3.97±0.56) vs (1.09±0.45), t=11.83, P<0.01] and decreased the content of cf-DNA [(2 685.0±735.8) vs (1 784.0±577.0), t=3.93, P<0.05] in plasma of APS mice. Compared with the experimental APS mice in the control group, the weight of thrombus in the APS mice pretreated with Cl-amidine was significantly reduced [(8.22±3.06) vs (4.89±1.90), t=2.27, P<0.05]. Conclusions PAD4 was involved in the formation of NETs induced by anti-β2GP1/β2GP1 complex, which might play an important role in APS thrombosis. Key words: Antiphospholipid syndrome; Anti-β2GP1/β2GP1 complex; Neutrophil extracellular traps; Peptidyl arginine deiminase 4