Pathophysiology of Inflammatory Bowel Disease: Innate Immune System

Inflammatory bowel disease (IBD), comprising Crohn's disease (CD) and ulcerative colitis (UC), is a heterogeneous state of chronic intestinal inflammation with no exact known cause. Intestinal innate immunity is enacted by neutrophils, monocytes, macrophages, and dendritic cells (DCs), and innate lymphoid cells and NK cells, characterized by their capacity to produce a rapid and nonspecific reaction as a first-line response. Innate immune cells (IIC) defend against pathogens and excessive entry of intestinal microorganisms, while preserving immune tolerance to resident intestinal microbiota. Changes to this equilibrium are linked to intestinal inflammation in the gut and IBD. IICs mediate host defense responses, inflammation, and tissue healing by producing cytokines and chemokines, activating the complement cascade and phagocytosis, or presenting antigens to activate the adaptive immune response. IICs exert important functions that promote or ameliorate the cellular and molecular mechanisms that underlie and sustain IBD. A comprehensive understanding of the mechanisms underlying these clinical manifestations will be important for developing therapies targeting the innate immune system in IBD patients. This review examines the complex roles of and interactions among IICs, and their interactions with other immune and non-immune cells in homeostasis and pathological conditions.

PADs and NETs in digestive system: From physiology to pathology

Peptidylarginine deiminases (PADs) are the only enzyme class known to deiminate arginine residues into citrulline in proteins, a process known as citrullination. This is an important post-translational modification that functions in several physiological and pathological processes. Neutrophil extracellular traps (NETs) are generated by NETosis, a novel cell death in neutrophils and a double-edged sword in inflammation. Excessive activation of PADs and NETs is critically implicated in their transformation from a physiological to a pathological state. Herein, we review the physiological and pathological functions of PADs and NETs, in particular, the involvement of PAD2 and PAD4 in the digestive system, from inflammatory to oncological diseases, along with related therapeutic prospects.

Neutrophils prevent rectal bleeding in ulcerative colitis by peptidyl-arginine deiminase-4-dependent immunothrombosis

OBJECTIVE

Bleeding ulcers and erosions are hallmarks of active ulcerative colitis (UC). However, the mechanisms controlling bleeding and mucosal haemostasis remain elusive.

DESIGN

We used high-resolution endoscopy and colon tissue samples of active UC (n = 36) as well as experimental models of physical and chemical mucosal damage in mice deficient for peptidyl-arginine deiminase-4 (PAD4), gnotobiotic mice and controls. We employed endoscopy, histochemistry, live-cell microscopy and flow cytometry to study eroded mucosal surfaces during mucosal haemostasis.

RESULTS

Erosions and ulcerations in UC were covered by fresh blood, haematin or fibrin visible by endoscopy. Fibrin layers rather than fresh blood or haematin on erosions were inversely correlated with rectal bleeding in UC. Fibrin layers contained ample amounts of neutrophils coaggregated with neutrophil extracellular traps (NETs) with detectable activity of PAD. Transcriptome analyses showed significantly elevated PAD4 expression in active UC. In experimentally inflicted wounds, we found that neutrophils underwent NET formation in a PAD4-dependent manner hours after formation of primary blood clots, and remodelled clots to immunothrombi containing citrullinated histones, even in the absence of microbiota. PAD4-deficient mice experienced an exacerbated course of dextrane sodium sulfate-induced colitis with markedly increased rectal bleeding (96 % vs 10 %) as compared with controls. PAD4-deficient mice failed to remodel blood clots on mucosal wounds eliciting impaired healing. Thus, NET-associated immunothrombi are protective in acute colitis, while insufficient immunothrombosis is associated with rectal bleeding.

CONCLUSION

Our findings uncover that neutrophils induce secondary immunothrombosis by PAD4-dependent mechanisms. Insufficient immunothrombosis may favour rectal bleeding in UC.

Association of neutrophil extracellular traps with fistula healing in patients with complex perianal fistulising Crohn's disease

BACKGROUND AND AIMS

Perianal fistulising Crohn's disease (pfCD) is a disabling phenotype of Crohn's disease (CD) with suboptimal outcomes. We assessed neutrophil extracellular traps (NETs) in perianal fistulas and implicated their roles in pfCD healing.

METHODS

Patients with complex pfCD who developed preplaced seton drainage were recruited during the verified maintenance of remission in CD. Fistula tracts were sampled during definitive surgery plus seton removal. Patient demographics, CD classification, medication strategy, and healing of pfCD were recorded. RNA sequencing was applied for transcriptomic profile analysis. NETs components, including myeloperoxidase (MPO), neutrophil elastase (NE), and citrullinated histone H3 (CitH3), were identified using immunofluorescence. Serum infliximab (IFX), anti-IFX antibodies, and tissue levels of IFX, adalimumab (ADA), MPO and CitH3 were determined using enzyme-linked immunosorbent assays. Peptidyl arginine deiminase IV (PAD4), tumour necrosis factor (TNF)-α, and NE were detected using immunohistochemistry. Gene expression levels of PAD family members were assessed with qPCR.

RESULTS

Twenty-one patients were included, 15 of whom adopted IFX as maintenance treatment. RNA-seq revealed difference in neutrophil associated pathways between unhealed and healed fistulas. NETs components (MPO/NE/CitH3) were detectable in the fistulas and were parallel with the PAD4 levels. Eleven of 21 (52%) patients experienced complete healing of the pfCD 108 weeks post-operatively. Fistula NETs were significantly increased in patients with unhealed pfCD. Increased NETs were associated with abundant TNF-α production and the absence of IFX in fistulas.

CONCLUSIONS

NETs exist in pfCD fistulas, which are associated with unhealed post-operative fistulas in pfCD, suggesting their prognostic roles in pfCD.

Huang Qin Decoction inhibits the initiation of experimental colitis associated carcinogenesis by controlling the PAD4 dependent NETs

BACKGROUND

Colorectal cancer is associated with ulcerative colitis (UC). The infiltration of neutrophils is the main cause of DNA damage produced by inflammation in the intestinal epithelium. Under the action of peptidyl arginine deaminase 4 (PAD4), neutrophils dissociate chromatin and form neutrophil extracellular traps (NETs), which can aggravate tissue inflammation and encourage tumor development. Although Huang Qin Decoction (HQD) was found to be useful in treating UC and was used to gradually prevent and treat digestive tract cancers, the underlying reasons were unclear.

METHODS

To demonstrate HQD could inhibits the initiation of colitis associated carcinogenesis by controlling NETs related inflammation, we first performed an AOM/DSS-generated colitis-associated carcinogenesis model to assess the efficacy of HQD in reducing neutrophil infiltration and anti-tumor activity. Then, using network pharmacology research, we investigated the potential mechanisms underlying those medicinal effects, as demonstrated by the detection of NETs aggregation and PAD4 expression changes in the colon.

RESULTS

HQD substantially reduced the number of colon cancers and the expression of Ki67, restored the level of intestinal tight junction protein occludin and ZO-1, and relieved the intestinal inflammation caused by TNF-α, IL-1β. At the same time, it inhibited neutrophil infiltration in the colon and improved the immunosurveillance of CD8⁺T cells. The potential mechanisms of HQD intervention against UC and UC with neoplasia (UCN) were studied using network pharmacology, and 156 conjunct genes as well as numerous inflammation-related pathways were identified. Protein-protein interaction (PPI) analysis indicated that HQD inhibition of intestinal tumors might be related to the deactivation of PAD4, which was verified by the down-regulation of NETs, MPO-DNA complex levels, and PAD4 expression after HQD treatment.

CONCLUSION

Huang Qin Decoction inhibits the initiation of colitis associated carcinogenesis by controlling PAD4-dependent neutrophil extracellular traps.

The emerging role of neutrophilic extracellular traps in intestinal disease

Neutrophil extracellular traps (NETs) are extracellular reticular fibrillar structures composed of DNA, histones, granulins and cytoplasmic proteins that are delivered externally by neutrophils in response to stimulation with various types of microorganisms, cytokines and host molecules, etc. NET formation has been extensively demonstrated to trap, immobilize, inactivate and kill invading microorganisms and acts as a form of innate response against pathogenic invasion. However, NETs are a double-edged sword. In the event of imbalance between NET formation and clearance, excessive NETs not only directly inflict tissue lesions, but also recruit pro-inflammatory cells or proteins that promote the release of inflammatory factors and magnify the inflammatory response further, driving the progression of many human diseases. The deleterious effects of excessive release of NETs on gut diseases are particularly crucial as NETs are more likely to be disrupted by neutrophils infiltrating the intestinal epithelium during intestinal disorders, leading to intestinal injury, and in addition, NETs and their relevant molecules are capable of directly triggering the death of intestinal epithelial cells. Within this context, a large number of NETs have been reported in several intestinal diseases, including intestinal infections, inflammatory bowel disease, intestinal ischemia–reperfusion injury, sepsis, necrotizing enterocolitis, and colorectal cancer. Therefore, the formation of NET would have to be strictly monitored to prevent their mediated tissue damage. In this review, we summarize the latest knowledge on the formation mechanisms of NETs and their pathophysiological roles in a variety of intestinal diseases, with the aim of providing an essential directional guidance and theoretical basis for clinical interventions in the exploration of mechanisms underlying NETs and targeted therapies.

Neutrophil extracellular traps in inflammatory bowel diseases: Implications in pathogenesis and therapeutic targets

Crohn's disease (CD) and ulcerative colitis (UC) are the two main forms of inflammatory bowel disease (IBD). Among the various immune cells involved in IBD, neutrophils are the first to infiltrate and appear to contribute to the impairment of the epithelial barrier, destruction of tissues by oxidative and proteolytic damage, as well as to the perpetuation of inflammation by the release of cytokines and chemokines associated with pro-inflammatory effects. In addition to basic effector mechanisms, such as phagocytosis and chemotaxis, neutrophils can also form extracellular traps (NETs), which is made up of a mesh-like structure - which contains its chromatin (DNA + histones) together with granules and enzymes, such as myeloperoxidase (MPO) and neutrophilic elastase (NE) - and that acts as a trap that can result in the death of extracellular pathogens and/or can promote tissue damage. Recent evidence indicates that NETs also play an important and significant role in the pathogenesis of IBD. Previous studies have reported increased levels of NETs in tissue and serum samples from patients with IBD, as well as in experimental colitis. In this review, we discuss current knowledge about the formation of NETs and their role in the pathophysiology of IBD, pointing out potential mechanisms by which NETs promote tissue damage, as well as their involvement in complications associated with IBD. In addition, we propose potential targets for therapy to regulate the production of NETs, making it possible to expand the current spectrum of therapies for IBD.

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.

Studies of mitochondrial and nuclear DNA released from food allergen-activated neutrophils. Implications for non-IgE food allergy

Background: Although adverse food reactions are commonly divided into immunoglobulin E (IgE) mediated food allergy (FA), and non-IgE FA, the current literature is providing support for the role of innate immune responses as an important component of non-IgE FA. Using a commercially available leukocyte activation (LA) assay, a recent quantitative study of total extracellular DNA released in cellular supernatants of human peripheral blood mononuclear cells exposed either to positive or negative tested foods demonstrated that leukocytes exposed to foods with positive LA test results showed higher DNA content than those exposed to foods with negative LA test results. In humans, the origin of DNA might be either the nucleus or the mitochondria. Analysis of emerging data from several laboratories, including our own, suggests that mitochondrial DNA induces inflammatory responses through induction of proinflammatory cytokines. Objective: This pilot study was designed primarily to convey the finding, and relevance of, mitochondrial DNA in the form of neutrophil extracellular traps (NET) as a new pathogenetic mechanism for innate immune-mediated non-IgE FA. Methods: The study population consisted of a total of six subjects, four in a major FA study group and two in a subgroup. Neutrophils were isolated and treated with food antigens that elicited positive and negative LA responses, and the released free DNA was analyzed for the cellular site of origin by using real-time polymerase chain reaction and for leukocyte calprotectin and S100 calcium-binding protein A12 (S100A12) proteins as markers of NETs. Results: We showed that cellular supernatants from neutrophils treated with foods that elicit positive LA responses can contain increased DNA levels of nuclear as well as mitochondrial origin. Supernatants from neutrophils treated with negative tested food (LA) responses did not induce the release of nuclear or mitochondrial DNA. Conclusion: Analysis of our data suggested that the induction of NETs that contain proinflammatory mitochondrial DNA may provide the critical link necessary for a better understanding of the pathogenesis of non-IgE-mediated FA. These discoveries may not only facilitate better diagnostic tests of FA but should also improve clinical management of allergic and other inflammatory diseases.