Correspondence on 'Critical role of neutrophil extracellular traps (NETs) in patients with Behcet's disease'

Plasma-derived exosomal protein SHP2 deficiency induces neutrophil hyperactivation in Behcet's uveitis

To investigate the effect of plasma-derived exosomal proteins on neutrophil hyperactivation in Behcet's uveitis (BU), we treated neutrophils from healthy controls with plasma-derived exosomes from active BU patients, and determined the level of neutrophil activation by real-time quantitative PCR (RT-qPCR) and cytokine detection assay. The results revealed that exosomes from active BU patients could activate neutrophils as shown by increasing the expression levels of pro-inflammatory cytokines (IL-17 and IL-6), chemokines (IL-8 and MCP-1), and NETs (MPO and ELANE). Label-free quantitative proteomic analysis of plasma-derived exosomes from patients and healthy controls found a remarkably distinct protein profile and identified differentially expressed proteins (DEPs) between the two groups. The results of GO, KEGG, and GSEA enrichment analysis showed that DEPs were enriched in innate immune-mediated and neutrophil hyperactivation-related signaling pathways. The protein-protein interaction (PPI) analysis determined that SHP2 was a downregulated key hub protein in the exosomes of active BU patients. Knockdown of SHP2 in human neutrophil cell lines (NB4 cells) was shown to promote the secretion of pro-inflammatory cytokines, chemokines, and NETs. The converse effects were observed following SHP2 overexpression. In conclusion, we highlighted a pathogenic role of plasma-derived exosomal SHP2 deficiency in facilitating neutrophil activation and suggested that SHP2 might be an immunoprotective factor in BU pathologic process.

Low density neutrophils are increased in patients with Behçet's disease but do not explain differences in neutrophil function

BACKGROUND

Behçet's disease (BD) is a multisystem autoinflammatory disease characterised by mucosal ulceration, ocular, neural, joint and skin inflammation. The cause of BD is not known but there is a strong genetic association with HLA-B*51, IL10 and IL23R. Neutrophils are a first line of defence against invading pathogens and have been described as activated in patients with BD. Neutrophils can now be separated into different subsets, such as low density (LDN) and normal density (NDN) that have diverse functional roles. We wished to address neutrophil heterogeneity in patients with BD.

METHODS

Peripheral blood neutrophils were obtained from 32 BD patients and 37 healthy aged-matched controls. Percoll isolation was used to isolate all neutrophils, while Ficol-Hypaque was used to obtain LDN and NDN. Phagocytic capacity and production of reactive oxygen species (ROS), and neutrophil extracellular traps (NET) stimulated with phorbol 12-myristate 13-acetate (PMA) and Escherichia coli (E.coli) were assessed in both groups.

RESULTS

We have demonstrated reduced phagocytic capacity and ROS production but greater NET production by total neutrophils stimulated with PMA or E.coli from BD patients in comparison with healthy controls. Patients with BD had elevated numbers of LDN and lower number of NDN compared with healthy controls. However, both neutrophil subsets showed the same reduced ROS production and phagocytic function as total neutrophils in both groups.

CONCLUSION

Our novel findings indicate that the neutrophil population in BD is heterogeneous and the increased number of LDN in combination with greater NET production may contribute to the inflammatory response and pathogenesis.

Pathogenesis of Behçet's Syndrome: Genetic, Environmental and Immunological Factors

Behçet's syndrome (BS) is a rare systemic vasculitis, characterized by a wide range of different clinical involvements and unpredictable phases of recurrence and remission. BS can be described as a multifactorial disease with an incompletely known etiopathogenesis; in fact, though presenting some peculiar features, such as its typical geographic distribution and the strong association with the well-known genetic predisposing factor HLA-B*51, the cause behind the onset and progression of the disease remains currently not fully understood. Besides genetic HLA and non-HLA predisposing associations and epigenetic influence, environmental factors also play an important role in the pathogenesis of the disease, and among these, infectious agents (both bacterial and viral) and specific microbiome alterations are considered of particular relevance in BS pathogenesis. BS has been included for decades among autoimmune diseases, in light of evidence showing T- and B-cell aberrant responses. However, because of recurrent mucocutaneous lesions and episodes of inflammation without antigen-specific T-cell or autoantibody responses, BS has also been classified among autoinflammatory disorders. Nevertheless, differently from autoinflammatory diseases, BS mildly responds to therapies targeting IL-1, its onset is not usually in childhood, and has high neutrophilic vasculitic involvement. Finally, given the association with HLA class I alleles, similar to spondyloarthropathies, the concept of BS as a major histocompatibility complex (MHC) I -opathy has been introduced. Understanding the complex etiopathogenesis of BS is essential to identify modifiable risk factors of BS occurrence or exacerbation and to develop targeted therapies. This review summarizes current evidence on the main genetic, environmental and immunological factors contributing to BS development.

Neutrophil-mediated mechanisms of damage and in-vitro protective effect of colchicine in non-vascular Behçet's syndrome

Behçet’s syndrome (BS) is a systemic vasculitis with several clinical manifestations. Neutrophil hyperactivation mediates vascular BS pathogenesis, via both a massive reactive oxygen species (ROS) production and neutrophil extracellular traps (NETs) release. Here, we investigated neutrophil‐mediated mechanisms of damage in non‐vascular BS manifestations and explored the in‐vitro effects of colchicine in counteracting these mechanisms. NETs and intracellular ROS production was assessed in blood samples from 80 BS patients (46 with active non‐vascular BS, 34 with inactive disease) and 80 healthy controls. Moreover, isolated neutrophils were incubated for 1 h with an oxidating agent [2,2′‐azobis (2‐amidinopropane) dihydrochloride; 250 nM] and the ability of pure colchicine pretreatment (100 ng/ml) to counteract oxidation‐induced damage was assessed. Patients with active non‐vascular BS showed remarkably increased NET levels [21.2, interquartile range (IQR) = 18.3–25.9 mU/ml] compared to patients with inactive disease (16.8, IQR = 13.3–20.2 mU/ml) and to controls (7.1, IQR = 5.1–8.7 mU/ml, p < 0.001]. Also, intracellular ROS tended to increase in active BS, although not significantly. In active non‐vascular BS, NETs correlated with neutrophil ROS production (p < 0.001) and were particularly increased in patients with active mucosal (p < 0.001), articular (p = 0.004) and gastrointestinal symptoms (p = 0.006). In isolated neutrophils, colchicine significantly reduced oxidation‐induced NET production and cell apoptosis, although not via an anti‐oxidant activity. Neutrophil‐mediated mechanisms might be directly involved in non‐vascular BS, and NETs, more than ROS, might drive the pathogenesis of mucosal, articular and intestinal manifestations. Colchicine might be effective in counteracting neutrophils‐mediated damage in BS, although further studies are needed.

To Trap a Pathogen: Neutrophil Extracellular Traps and Their Role in Mucosal Epithelial and Skin Diseases

Neutrophils are the most abundant circulating innate immune cells and comprise the first immune defense line, as they are the most rapidly recruited cells at sites of infection or inflammation. Their main microbicidal mechanisms are degranulation, phagocytosis, cytokine secretion and the formation of extracellular traps. Neutrophil extracellular traps (NETs) are a microbicidal mechanism that involves neutrophil death. Since their discovery, in vitro and in vivo neutrophils have been challenged with a range of stimuli capable of inducing or inhibiting NET formation, with the objective to understand its function and regulation in health and disease. These networks composed of DNA and granular components are capable of immobilizing and killing pathogens. They comprise enzymes such as myeloperoxidase, elastase, cathepsin G, acid hydrolases and cationic peptides, all with antimicrobial and antifungal activity. Therefore, the excessive formation of NETs can also lead to tissue damage and promote local and systemic inflammation. Based on this concept, in this review, we focus on the role of NETs in different infectious and inflammatory diseases of the mucosal epithelia and skin.