Abstract
Background and Aims
Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is characterized by systemic necrotizing vasculitis in small vessels. The necrotic lesions consist of ANCA-mediated neutrophil extracellular traps (NETs) which represent a form of lytic cell death. The persistent NETs serve as autoantigens against ANCAs and cause organ damage in a vicious cycle. Considering dead cells are essentially cleared by phagocytic cells as a process of efferocytosis, why the NETs persist in tissue remains unclear. During efferocytosis, macrophages engulf apoptotic cells to prevent the leakage of intracellular components including toxic enzyme into the surrounding cells and these processes are regulated by the expression of CD47 as a “don’t eat me” signal. In this study, we hypothesized that ANCA-mediated NETs in AAV escape from efferocytosis via the up-regulation of CD47 and the persistent NETs amplify the disease.
Method
Human data: Human kidney biopsy specimens from patients with AAV and minor glomerular abnormality (MGA, as a case control) were subjected to immunohistochemistry (IHC) staining for CD47. In vitro: The expression of CD47 on neutrophils was evaluated by flow cytometry (FCM). Human neutrophils from healthy donor were treated with ANCA-IgGs from MPO-AAV patients or control IgGs. For the efferocytosis assay, macrophages were co-incubated with unstimulated, apoptotic, and ANCA-IgGs treated neutrophils in the presence of anti-CD47 monoclonal antibody (mAb) or a control antibody. The neutrophils were labeled with CFMDA cell tracker (fluorescent probe) and the efferocytosis was evaluated as neutrophil engulfed (CFMDA positive) macrophages using fluorescent microscopy. In vivo: Spontaneous crescentic glomerulonephritis-forming/Kinjoh (SCG/Kj) mice (8-week-old age) were treated with intraperitoneal injection of anti-CD47 mAb or a control antibody every 5 days for two weeks. The severity of glomerulonephritis was assessed by the levels of serum creatinine, haematuria, mRNA expression of pro-inflammatory genes, and histopathological findings. To address the immune response against the CD47 blockade, the titre of MPO-ANCA and the number of splenic cell subset was assessed by ELISA and FCM analysis, respectively.
Results
Human data: The IHC analysis of human renal specimens revealed that the positive area of CD47 of AAV was greater than that of MGA. In particular, the CD47-overexpressed cells were seen in glomeruli with necrotic crescent formation. In vitro: Mean fluorescence intensity (MFI) of CD47 in ANCA-IgGs treated neutrophils (NETs) was significantly higher than that in control IgGs treated neutrophils (ANCA-IgG; 442±21.4 a.u. vs control IgG; 402±10 a.u., p<0.05). In efferocytosis assay, apoptotic neutrophils were engulfed by macrophages (efferocytosis rate/ apoptotic neutrophil; 20.5±3.8%, live neutrophils; 0.9±0.5%). The efferocytosis rate of ANCA-induced NETs significantly decreased compared to apoptotic neutrophil, but anti-CD47 mAb improved the efferocytosis of ANCA-NETs (efferocytosis rate/ anti-CD47 mAb; 19.1±4.2%, control antibody; 7.7±2.2%, p<0.05). In vivo: the renal histopathological severity score, serum creatinine level of AAV mice treated with anti-CD47 mAb decreased compared to that of AAV mice treated with a control antibody (anti-CD47 mAb; 0.96±0.30 vs control antibody; 0.61±0.32 mg/dL). Although there was no significant difference in the number of splenic cells between anti-CD47 and control antibody treated mice, CD47 blockade therapy significantly reduced serum MPO-ANCA titre (28.5±10.4 vs 45.2±14.5 μg/mL) and renal mRNA expression (IFNα, IFNγ, MCP-1 and perforin) of AAV mice.
Conclusion
ANCA-mediated NETs might escape from efferocytosis through up-regulation of CD47 and provoke necrotizing vasculitis. CD47 blockade could be a potential novel therapeutic strategy for AAV.
In vitro activation of coagulation by human neutrophil DNA and histone proteins but not neutrophil extracellular traps
