Background
Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is an autoimmune disease characterized by serum-positive ANCA which mainly recognizes myeloperoxidase (MPO) or proteinase 3 (PR3) and the rapidly progressive glomerulonephritis which shows pauci-immune complex deposition in pathogenic biopsy [
1]. AAV encompasses microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA), and eosinophilic granulomatosis with polyangiitis (EGPA) [
2]. In AAV, the ANCA-activated neutrophils (polymorphonuclear lymphocytes, PMNs) extrude neutrophil extracellular traps (NETs), which are decorated by histones, MPO, PR3, neutrophil elastase (NE), as well as other cytoplasmic proteins [
3].
Previous studies demonstrated that activated neutrophils could also extrude damage-associated molecular pattern (DAMP) proteins such as high mobility group box chromosomal protein 1 (HMGB-1) and some S100 family proteins [
4]. HMGB-1 has been reported to take part in the pathogenesis of AAV [
5,
6]. However, the role of the S100 family proteins in AAV has not been clarified. S100A8/A9 and S100A12 belong to the S100 protein family that was first extracted from cow brain by Blake W. Moore and his colleagues in 1965 [
7]. Literature reported that S100A8/A9 could stimulate renal mesangial cells to release IL-6, TNF-α, and CXCL1 [
8], while S100A12 enhanced cytokine expression in a dose-dependent manner and promoted the secretion of chemokines and cell adhesion molecules in the normal bronchial epithelial cells [
9]. It has been reported that the serum levels of S100A8/A9 and S100A12 in AAV were elevated [
10‐
12]. However, the exact pathogenic functions of S100A8/A9 and S100A12 in AAV with MPO-ANCA have no further refining study. In the current study, we tried to investigate the role of S100A8/A9 and S100A12 in MPO-ANCA-positive vasculitis.
Discussion
As members of DAMPs, S100A8/A9 and S100A12 play crucial roles in various diseases. Previous studies have reported high levels of S100A8/A9 and S100A12 in AAV patients, and the serum level of S100A8/A9 was related to disease relapse in PR3-AAV [
16,
17]. These findings prompted us to clarify the significance of S100A8/A9 and S100A12 in AAV. In the current study, we verified the association between clinical parameters and serum or urine levels of S100A8/A9 and S100A12 in patients with active MPO-AAV. Furthermore, the possible pathogenic roles of S100A8/A9 and S100A12 in MPO-AAV were unveiled with the in vitro experiments.
In the current study, the serum levels of both S100A8/A9 and S100A12 were positively correlated with the serum MPO-ANCA levels. This result differs from the study reported by Pepper RJ [
17]. Neutrophils play an important role in the pathophysiology of AAV [
18]. It has been demonstrated that S100A8/A9 and S100A12 are primarily released from activated or necrotic neutrophils and are involved in the pathogenesis of various diseases [
19]. Since ANCA is an activator of neutrophils, it is reasonable to speculate that the ANCA-activated neutrophils are important sources of the increased serum S100A8/A9 and S100A12 in AAV. We demonstrated that the MPO-ANCA-containing IgG could stimulate neutrophils to release S100A8/A9 and S100A12 dose-dependently.
ANCA-activated neutrophils migrate across endothelial cells and cause inflammation in AAV [
20]. Thus, all factors that can enhance the chemotaxis and migration of neutrophils might increase the severity of the disease. Previous studies have reported S100A8/A9 and S100A12 induced neutrophil chemotaxis and adhesion [
21,
22]. In the present study, S100A8/A9 and S100A12 dramatically enhanced the chemotaxis of ANCA-activated neutrophils in the transwell experiment. Moreover, we verified that S100A8/A9 and S100A12 promoted the release of IL-8 in neutrophils, which is a potent neutrophil chemotactic factor [
23]. This result indicated that in MPO-AAV, S100A8/A9 and S100A12 in the local area of tissue damage would attract more ANCA-activated neutrophils and further amplify the inflammatory response.
Dysregulation of neutrophils’ life span may contribute to the pathogenesis of AAV. Some studies have reported the infiltration and accumulation of unscavenged apoptotic neutrophils in the perivascular tissues and the delayed spontaneous apoptosis of neutrophils in AAV [
24,
25]. However, the exact mechanisms have not been clarified. We found that S100A8/A9 and S100A12 prolong survival and decrease the apoptosis of neutrophils. Therefore, S100A8/A9 and S100A12 might contribute to the neutrophil accumulation in regions of inflammation by inhibiting neutrophil apoptosis in AAV.
IL-1β plays a vital role in autoimmune disease as an important pro-inflammatory cytokine. Former literature showed that ANCA could stimulate neutrophils to express mRNA and protein of IL-1β [
26]. On the other hand, S100A8/A9 was once reported to stimulate peripheral blood mononuclear cells (PBMCs) to produce IL-1β [
27]. Similarly, we found that S100A8/A9 and S100A12 could exaggerate the release of IL-1β through binding to TLR4 and RAGE on neutrophils. This result signified that S100A8/A9 and S100A12 had a pro-inflammatory function in MPO-AAV.
The activation of the alternative pathway of the complement system plays a crucial role in the development of AAV [
28,
29]. The raised levels of complement factors in the supernatant and the increased C5 expression of ANCA-activated neutrophils demonstrated that S100A8/A9 and S100A12 could promote the activation of the alternative complement pathway in MPO-ANCA-positive vasculitis. The effects of inhibitors of TLR4 and RAGE further confirmed that the TLR4/RAGE axis is involved in the pathogenic effects of S100A8/A9 and S100A12 in MPO-AAV. It was worth noting that S100A8/A9 and S100A12 also increased the expression of TLR4 and RAGE on ANCA-stimulated neutrophils. This result was consistent with the research of T. H. Page, who demonstrated that the TLR4/RAGE axis is a common pathogenic pathway in AAV [
17].
According to previous studies, activations of TLR4 and RAGE can activate the MAPKs and NF-κB, and subsequently enhance the transcription of pro-inflammatory mediators [
30,
31]. MAPKs are crucial regulators of a series of cellular processes, such as the proliferation and differentiation of cells [
32]. Our data showed that S100A8/A9 and S100A12 increased the expression of phosphorylated p38 MAPK and NF-κB p65, which was reduced by the blockade of TLR4 and RAGE. Therefore, TLR4/RAGE-p38 MAPK-NF-κB p65 signaling pathways were involved in the effects of S100A8/A9 and S100A12 on neutrophils.
A previous report demonstrated that S100A8 reduced reactive oxygen species generated by activated leukocytes through its thiol-scavenging capacity [
33]. In the current study, S100A8/A9 inhibited the MPO-ANCA-induced ROS generation of neutrophils. Correspondingly, S100A8/A9 tended to inhibit the ROS-dependent generation of NETs. However, S100A12 did not inhibit the ANCA-induced ROS generation of neutrophils and did not tend to inhibit the generation of NETs.
Some limitations of this study should be mentioned. First, our samples of patients are relatively small, so a larger cohort will be needed to explore the relationship between the level of S100A8/A9 and S100A12 and the prognosis of the AAV. Second, due to the characteristics of AAV in the Chinese population, all patients enrolled in our study were MPO-ANCA positive. Third, all our experiments were done in vitro, so further in vivo study is necessary in the future.
In conclusion, the serum and urine levels of S100A8/A9 and S100A12 in patients with active MPO-ANCA-positive vasculitis were elevated and correlated with the severity of the disease. Besides, S100A8/A9 and S100A12 might take part in the pathogenesis of the disease. Both S100A8/A9 and S100A12 can exaggerate the inflammatory effects of MPO-ANCA in a ROS-independent manner.
Materials and methods
Patients
Patients’ serum and urine were obtained from 34 patients with active AAV and positive MPO-ANCA and 8 AAV patients in remission. These 42 patients diagnosed in Tianjin Medical University General Hospital fulfilled the 2012 revised International Chapel Hill Consensus Conference Nomenclature of Vasculitides [
2]. Clinical characteristics and laboratory parameters were recorded on the day of sample collection. Disease activity was assessed using version 3 of BVAS. Active patients were defined as those with a BVAS score of more than zero. Patients who got a BVAS score of 0 were identified in remission. Serum and urine from 10 healthy donors were obtained as normal controls. The research complied with the declaration of Helsinki, and the institutional review board of Tianjin Medical University General Hospital approved the protocol (IRB2018-202-01). Informed consent was obtained from all individual participants included in the study.
Clinical and laboratory data
Clinical data included the following: gender, age, BVAS, disease duration, and organ involvement. Laboratory data included as following: hemoglobin, serum creatinine level, the level of MPO-ANCA, CRP, C3 and C4, ESR, ferritin, D-dimer, RF, albumin, proteinuria, hematuria, urinary NGAL.
Immunoassays of S100A8/A9 and S100A12
Concentrations of S100A8/A9 (439707, Biolegend) and S100A12 (CSB-E13095h, Cusabio) in serum and urine from participants were measured by ELISA according to the manufacturer’s instructions.
Neutrophils isolation of peripheral blood
Neutrophils from healthy donors were isolated by density centrifugation [
34]. Briefly, a double gradient was formed by layering Histopaque 1077 (10771, Sigma-Aldrich) on an equal volume of Histoque 1119 (11191, Sigma-Aldrich). Heparinized blood donated by healthy volunteers was carefully layered onto the upper gradient. After 20 min centrifugation at 300×
g, neutrophils between two Histopaque mediums were carefully collected. Cells were washed by adding 10 ml of isotonic phosphate-buffered saline (PBS). The neutrophil pellet was resuspended with red blood cell lysis buffer (R1010, Solarbio) to lyse red blood cells. Neutrophils were then washed twice and resuspended in an appropriate volume of RPMI1640 medium. The concentration of neutrophils was adjusted to 1 × 10
6/ml.
Production of S100A8/A9 and S100A12 by neutrophils stimulated with ANCA
Neutrophils (1 × 106/ml) were primed with 2 ng/ml TNF-α (H8916, Sigma-Aldrich) at 37 °C for 15 min and cultured with normal control IgG from healthy donors and different concentrations of ANCA-containing IgG purified from AAV patients at 37 °C for 24 h. The concentrations of S100A8/A9 and S100A12 were detected by ELISA as above.
TLR4 and RAGE expression on neutrophils
Isolated neutrophils were primed with TNF-α and cultured with various stimulators for 2 h. The expressions of TLR4 and RAGE on neutrophils were performed by flow cytometry. The cells were stained with mouse anti-TLR4 (ab105950, Abcam) and rabbit anti-RAGE (ab228861, Abcam) monoclonal antibodies for 30 min. After washing three times with PBS, neutrophils were stained with PE-conjugated donkey anti-mouse IgG and FITC-conjugated goat anti-rabbit IgG antibodies for 30 min. The cells were resuspended in PBS following washing and detected with the flow cytometer.
Neutrophil migration and chemotaxis
A sterile 24-well transwell plate with a pore size of 3 μm was used. Cells were pre-stimulated with ANCA-containing IgG (1 mg/ml) for 1 h and were inoculated in the upper chamber. Then the upper chamber was moved to an exploratory well with fMLP (Solarbio), different concentrations of S100A8/A9 (Sino Biological) and S100A12 (Cusabio) in a 37 °C, 5% CO2 cell incubator for 2 h. Five fields of view were randomly selected to take pictures under a 100× optimal microscope. The CI was calculated by dividing the total number of cells in the lower chamber of each experimental group by the total number of cells in the lower chamber of the blank control group. The supernatant concentration of IL-8 was detected by ELISA (ab214030, Abcam).
Apoptosis of neutrophils
Isolated neutrophils from healthy donors were primed with TNF-α and treated with ANCA, S100A8/A9, or S100A12 for 12 h at a cell incubator. Cells were collected at 1000 rpm and washed twice with PBS. The PMNs pellet was resuspended with binding buffer (10 mM HEPES/NaOH, pH 7.4, 140 mM NaCl, 2.5 mM CaCl2). Flow cytometry assessed apoptosis of neutrophils with Annexin V-FITC/PI-PE Kit (FX018, 4A Biotech). The results were analyzed by FlowJo 10.4.0.
Assessment of IL-1β and complement factors released by neutrophils
TNF-α-primed neutrophils were stimulated for 2 h with normal control IgG, ANCA-containing IgG, S100A8/A9, S100A12, S100A8/A9 + ANCA, S100A12 + ANCA, respectively. In the groups blocking TLR4, RAGE, or TLR4 + RAGE, the antibody of TLR4 (312802, Biolegend) or RAGE (ab37647, Abcam) was added and incubated for 30 min before adding S100A8/A9 and S100A12. The supernatant was collected to detect the concentration of IL-1β (437007, Biolegend), complement 5a (C5a) (JL10644, Shanghai Jianglai Biological Technology Co., Ltd.), complement Bb (CBb) (JL19313, Shanghai Jianglai Biological Technology Co., Ltd.) and soluble complement 5b-9 (sC5b-9) (JL18355, Shanghai Jianglai Biological Technology Co., Ltd.).
RNA extraction and real-time PCR
Total RNA was extracted from the isolated neutrophils using TRIzol reagent. The quality and integrity of RNA were detected using a NanoDrop ND1000 (Thermo Fisher, USA) and determined via the A260/A280 ratio. Next, the total RNA was reversely transcripted to cDNA using 1st Strand cDNA Synthesis SuperMix (11141ES10, Yeasen) following the manufacturer’s instructions. Real-time quantitative PCR was performed with specific primers by the CFX Manager™ Real-time PCR system (Bio-Rad, USA). Relative changes in mRNA levels were calculated by the 2 − ΔΔCt method. Primer sequences are as follows:
Human GAPDH: forward 5′-GGAGCGAGATCCCTCCAAAAT-3′,
reverse 5′-GGCTGTTGTCATACTTCTCATGG-3′;
Human complement C5: forward 5′-ACAGTCATAGAGTCTACAGGTGG-3′,
reverse 5′-CCAACTGGTCAAGCGAATCTT-3′.
Western blot analysis
Neutrophils were collected and added to the RIPA lysis solution and the protease inhibitor PMSF. After 30 min incubation on ice, supernatants were extracted with a 10,000 rpm centrifugation. Denatured PMNs protein extract was subjected to SDS-PAGE and transferred to nitrocellulose membranes, which were then blocked for 1 h at room temperature with 5% skim milk. After incubated with primary antibodies against C5/C5a, p-p38 MAPK/t-p38 MAPK, NF-κB p65 (Abcam, Cambridge, USA), and GAPDH (A19056, ABclonal) overnight at 4 °C, the horseradish peroxidase-conjugated goat anti-mouse or rabbit monoclonal antibody was used to detect the bound primary antibodies. The membranes were exposed with a chemiluminescence imaging system, and the results were performed with the Image J software system (NIH, USA).
ELISA for the level of C5 protein in PMN lysates
Neutrophils were incubated with different stimulants and collected at 1000 rpm. The lysates of neutrophils were extracted by adding a RIPA lysis solution. The concentration of complement C5 was detected according to the manufacturer’s instruction (ab125963, Abcam).
Flow cytometry for the oxidative respiratory burst of neutrophils
The measurement of oxidative activation of neutrophils was based on ROS-dependent oxidation of dihydrorhodamine 123 (DHR123) to rhodamine 123 (R123), which is a cationic green fluorescent dye and can derive the uncharged non-fluorescent dye DHR123 [
35]. DHR123 was added to the TNF-α primed neutrophils suspension to the final concentration of 5 μg/ml. Neutrophils were then incubated with ANCA-containing IgG (1 mg/ml), normal control IgG (1 mg/ml), S100A8/A9 + ANCA IgG and S100A12 + ANCA IgG at 37 °C for 1 h. The samples were assessed by flow cytometry analysis, and the production of ROS was represented by the mean fluorescence intensity (MFI) of the FITC gating channel.
Induction of netting neutrophils by S100A8/A9 and S100A12 with ANCA
Neutrophils (1 × 106/ml) were primed with 2 ng/ml TNF-α at 37 °C for 15 min, then incubated with normal control IgG (1 mg/ml), ANCA-containing IgG (1 mg/ml), S100A8/A9 heterodimer protein or S100A12 protein at 37 °C for 24 h. Neutrophils were centrifuged for 5 min at 1500 rpm, and the supernatant was collected. The concentration of NE (JL12352, Shanghai Jianglai Biological Technology Co., Ltd.) was determined by ELISA.
Statistical analysis
Different experiments were performed at least three times. The normal distribution of quantitative data was tested by the Kolmogorov–Smirnov test. Data with normal distribution are expressed as mean ± SD, and median and interquartile ranges are applied for data without normal distribution. Multiple sets of continuous variables which were normally distributed were evaluated using one-way ANOVA analysis followed by Tukey’s test for multiple comparisons. For multiple sets of quantitative data that do not conform to the normal distribution, the Kruskal–Wallis test was used. The Mann–Whitney U test was applied to two-independent groups that were not normally distributed. Categorical variables are presented as frequencies and performed with the χ2 test. Correlation analysis was performed by a spearman rank correlation. Statistical significance was set at a p value < 0.05. GraphPad Prism 8.0.1 software for Windows (GraphPad Software, California, USA) was used for data analysis.
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