The most extensively studied type of NET formation is called “suicidal” or “lytic”, also referred as NETosis. This is indeed a form of cell death because once the neutrophils release their contents, they eventually perish ( Figure 1A ).

Based on the involvement of nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase, lytic NETosis was used to be divided into NADPH-dependent and NADPH-independent forms. Later it became clear that the NADPH-independent pathway is conducted by mitochondrial ROS (mtROS) production, hence, the term, “NADPH-independent” was modified to “mitochondria-dependent NETosis” (15). On the other hand, since NADPH-oxidase may be induced by mtROS - NADPH-oxidase can engage in both processes.

Furthermore, triggering stimuli have a pivotal role and the signalization of NETosis depends on them. Proteomic analysis showed diversity in terms of NET-constitution and post-translational modifications, which suggests that the stimulus determines the biological effects (16).

Conventional lytic NETosis is usually initiated by complement proteins (such as C3b, C5a), cytokines, and ligands binding to Toll-like receptors (TLR) or IgG-Fc receptors (17). Upon receptor activation, calcium storage is released from the endoplasmic reticulum. Increased cytoplasmic calcium levels activate protein kinase C (PKC) via the RAF-MEK-MAPK/ERK signaling pathways and induce the phosphorylation of gp91phox. This stimulates the membrane-bound subunits of NADPH-oxidase to assemble into the functional enzyme in the cytoplasmic or phagosome membranes. Thus, the reactive oxygen species (ROS) generation is initiated (18, 19).

Simultaneously, the disintegrated azurophilic granules, MPO, NE, and other lytic enzymes are released into the cytosol. By oxidizing it, MPO facilitates the release of NE into the cytoplasm, however, the exact role of the lytic enzymes in this process still needs to be elucidated (20, 21).

The cytosolic fibrous network also falls victim to degradation. The disassembly of actin and vimentin molecules is promoted by calcium influx. Activated by calcium level increase and citrullination, calpain - a serine protease – facilitates the decondensation of micro- and intermedier filaments and nuclear lamin as well (22). The intactness of the microtubular network, however, does not affect the NETosis (23). NE binds and degrades the actin fibers (preventing concurrent phagocytosis), thus making its way to the nucleus. Cooperating with Gasdermin-D, NE destructs the nucleus envelope and enables water influx (21). Here NE proceeds to cleave histones which eventually leads to chromatin decondensation (24–26).

By binding calcium ions or activated by ROS, protein-arginine deiminase 4 (PAD4) adopts its catalytically active form. PAD4 translocates to the nucleus which induces citrullination of the arginine amino acids, thus reducing the positive charge and loosening the histone-DNA interaction. PAD4 also has direct effects on chromatin decondensation via inhibition of linker histone-mediated compaction (23). Ultimately, these processes lead to the decondensation of the chromatin structure (27, 28). Gasdermin D-mediated pore openings induce the progressive permeabilization of the plasma membrane which leads to the externalization of the DNA, cytosolic and granular proteins (29). Of note, pore formation changes the intracellular calcium gradient, which further promotes PAD4 activation (30).

During “vital” or “non-lytic” NET formation neutrophils maintain their viability and antimicrobial functions (e.g., engaging in recruitment, chemotaxis, and phagocytosis) after they release the nuclear or mitochondrial DNA ( Figure 1B ).

Non-lytic NETosis is initiated by the detection of stimuli of complement receptors and activated platelets. Staphylococcus aureus, Escherichia coli, and Candida albicans can also cause non-lytic NET formation by activation of complement receptor (CR) -1, -3, -4 and TLR - 2, -4 ligands NADPH independently. Calcium influx starts through the small conductance potassium channel member three (SK) and activates PAD4 and NE. They translocate to the nucleus where they decondense the chromatin. Eventually, chromatin is expelled to the extracellular space by vesicular transport, in which the blebs fuse with the plasma membrane again. This mechanism maintains the integrity of the plasma membrane and the anucleated neutrophils (cytoplasts) stay alive, keeping their ability to do their antimicrobial functions: migrate and phagocytose. This process takes approximately 5-60 minutes, compared to the lytic form of NETosis, which requires 3-4 hours (20).

The third form of NET formation is conducted by caspases. Cytosolic lipopolysaccharide (LPS) induction of neutrophils activates murine caspase-11 or the human ortholog caspase-4 via an uncanonical inflammasome pathway to enable Gasdermin D cleavage into the pore-forming fragments. Upon entering the nucleus, caspase-11/4 degrades the histones and chromatin. Gasdermin D forms pores on granules as well, liberating NE/MPO. Calcium influx through the pores activates PAD4 and citrullinates histones. Although NE, MPO, and PAD4 facilitate the process, their activities are not prerequisites for NETosis – caspase-induced chromatin cleavage converges in a similar molecular pathway as lytic NETosis (31) ( Figure 1C ).

Another pathway that leads to NETosis is the Ca²⁺-ionophore-induced mitochondrial ROS (mtROS) production. Elevated cytosolic calcium level opens SK channels, mediates mtROS production, and induces apoptosis and NET formation (15, 32). Granulocyte-macrophage colony-stimulating factor (GM-CSF), LPS, complement component 5a (C5a), or ribonucleoprotein-containing immune complexes also activate neutrophils to release mitochondrial DNA and ROS (33). Oxidized mtDNA has potent proinflammatory and type I interferon stimulatory properties that seem to play a pivotal role in the pathogenesis of lupus nephritis (LN) (34). Although mtROS-induced NETosis is known as NADPH-oxidase-independent NETosis, the crosstalk with NADPH-oxidase to produce additional ROS has already been demonstrated (32) ( Figure 1D ).

In conclusion, the contribution of PAD4 and ROS are the most relevant factors to group the processes. Lytic NETosis is ROS-dependent, and the participation of PAD4 is not obligatory, while non-lytic NET formation occurs in the absence of ROS with the important role of PAD4. Table 1 summarizes the main characteristics of NET formation.

The control of NET formation and elimination is inevitable to maintain tissue homeostasis. Lytic neutrophil remnants are mainly taken up by macrophages. NET-contents function as danger-associated molecules and they are recognized by membrane-bound receptors (intracellular adhesion molecule 1, 3, liver X receptor, Mer tyrosine kinase) or soluble pattern recognition proteins (galectin-3, C3, C1q, annexin A1, factor-H related protein, C-reactive protein, clusterin, milk fat globule EGF factor 8). Scavenging macrophages identify NETotic neutrophils by “eat-me” signals or by the loss of “don’t eat-me” signals and phagocytosis takes place. Interestingly, neutrophil LL-37 interacts with extracellular anionic molecules such as dsDNA or dsRNA and aids their uptake by macrophages. Typically, the degradation of the apoptotic (and the NETotic) neutrophils creates a net anti-inflammatory microenvironment, hence this process differs from the traditional phagocytosis, so the term, “efferocytosis” was coined (35). After NETosis, interferons and other proinflammatory cytokines prime macrophages to M1 macrophages. M1 macrophages are characterized by the ability to secrete inflammatory cytokines and costimulatory molecules. Furthermore, upon interaction with NETs, M1 macrophages release DNA in a PAD4-dependent manner (36). Hence, they not only phagocyte the intercellular debris but also exacerbate inflammation. However, over time, M1 macrophages also activate their own caspase-activated DNase to degrade the surrounding extracellular DNA, and, the macrophage phenotype shifts toward an anti-inflammatory effector function, to the M2 macrophages (36). Although the molecular background of phenotype shifting is not fully unraveled, a crosstalk between the M1 and M2 polarizing pathways is proved. The increase of cell death by M1 macrophages is recognized by M2 phenotypes and they establish a predominantly anti-inflammatory milieu, therefore, the microenvironment commits to tissue remodeling and immune tolerance (37–39). Interestingly, M2 macrophages also contribute to the initial M1 polarization by releasing inflammatory cytokines upon encountering NETs (36). This highlights that macrophages could be further subdivided based on their functionally distinct roles (40).

The complement system also facilitates the clearance of cellular debris. All three complement pathways are involved in the removal of NETs; however, the classical pathway components, C3, C4, C5, and C1q have a higher affinity toward secondary necrotic cells like NETs. They either interact directly with DNA and mitochondrial DAMPs (damage-associated molecular patterns) or via immune complexes. Complement members do not only take part in opsonization but also aid the removal of NET remnants by activating serine proteases C1r and C1s (35).

Since DNA forms the backbone of NETs, DNases are central participants in degradation and digestion. The DNase complex contains three different enzymes (DNase I, DNase II, and DNase1-like 3 protein) with different functions (41). DNase I is responsible for the removal of protein-free DNA; while DNase1-like 3 protein degrades protein-associated DNA, including DNA packed in microvesicles, and DNase II digests the DNA from apoptotic cells (41, 42). Plasminogen, on the other hand, penetrates necrotic cells, accumulates in the cytoplasm and nucleus, and activates plasmin by tissue-type or urokinase-type plasminogen activator. Plasmin degrades histone H1 and facilitates internucleosomal DNA cleavage by DNase1 (43).

During NETosis, a substantial amount of serine proteases (e.g., PR3, MPO, NE, cathepsin G) are released into the extracellular space increasing the detrimental effects. To oppose this, ceruloplasmin disrupts MPO and limits MPO-dependent ROS generation, and alpha1-antitrypsin inhibits NE, PR3, and cathepsin G by forming complexes with them (44, 45).

While the beneficial effects of NETs have long been recognized, more recent studies have revealed that NETs also play a role in various pathological conditions, some of which may not be beneficial. The possibility that NETs may be involved in autoimmune diseases was initially suggested in 2004 by Brinkmann et al., leading to an increase in research on the subject (4). The role of NETosis in systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and ANCA-associated vasculitis (AAV) is well known today, but authors suggest that NETosis may also play a crucial role in other autoimmune and autoinflammatory conditions, such as type 1 diabetes mellitus, inflammatory bowel disease, gout, antiphospholipid syndrome, and in other rheumatic diseases (46–50).

Autoimmune diseases arise from the combination of genetic and environmental factors. NETs can be involved in breaking immune tolerance and triggering autoimmunity in a variety of pathways. After the initiation of an environmental factor, the propagation phase is characterized by inflammation and tissue damage. NETs contribute to propagation by epitope spreading and NET-forming cellular components (such as histones, DNA, and granular proteins) can serve as autoantigens for later antibody production.

Furthermore, NETs interplay with the adaptive immune system. LL-37-DNA complexes enhance autoantibody production on B-cells via TLR9 (51). Activated neutrophils release B cell-activating factor (BAFF) which upregulates the CD21 and CD19 co-receptor expression and prolongs the B-cell survival by decreasing proapoptotic proteins (52). The NET-derived immunoglobulins bind to the FcγR of the neutrophils and initiate further NETosis, creating a vicious circle (53). NETs prime CD4+ T-cells directly on T-cell receptors with a lower threshold, thus T-cell response is increased upon suboptimal NET component stimulation. On the other hand, T-cell response is also elicited in a dendritic cell-mediated manner. NETs induce costimulatory CD80 and CD86 on dendritic cells which interacts with T-cell CD28 for activation and survival, as well as interleukin (IL) production.

Interferon overproduction is a hallmark in the pathogenesis of several autoimmune disorders. Plasmacytoid dendritic cells are the primary interferon-producing cells, and they can be activated by NET-induced LL-37, HMGB1 (high mobility group box 1 protein), and DNA via TLR9 (54). NETs also mediate the activation of caspase-1, leading to inflammasome activation via the NLRP3 (NOD-like receptor family, pyrin domain containing) in macrophages. Inflammasome activation results in IL-18 and -1β secretion which further promotes NET formation (55).

Autoimmunity also occurs in case of failure to remove apoptotic and NETotic cells. As an example, NADPH-oxidase or PAD4 knock-out mice develop lupus-like disorders which contradict the studies that suggested improvement in the case of administration of NADPH-oxidase or PAD4 inhibitors in NET-related autoimmune diseases (56). Both enzymes are required for cellular debris removal by macrophages and the total absence of them exacerbates autoimmunity instead of ameliorating it (57). Furthermore, DNA accumulation as a result of the lack of DNase activity leads to prolonged inflammation and the presence of NET autoantigens. In susceptible individuals self-tolerance breaks, and an autoimmune response develops (41).

Based on the current classification, autoimmune and autoinflammatory diseases are positioned at opposite ends of a spectrum. In autoinflammatory diseases, immune tolerance remains intact and the pathomechanism is not driven by autoantigen-autoantibody interactions. Local and external factors, such as infections, mechanical damage, or temperature effects have a significant impact in triggering the disease, leading to innate immune responses and tissue damage (58, 59). These pathologies can be classified based on their genetic origin, distinguishing between monogenic and polygenic diseases, where the NF-κB, the NLRP3 inflammasome, and the IL-1β pathway play an important role in the signalization (60, 61).

Based on Matzinger’s “danger-theory”, in autoinflammatory diseases damage-associated molecular patterns (DAMPs) and pathogene-associated molecular patterns (PAMPs) serve as trigger factors for the innate immune system (62). Neutrophils engage in the process, however, the exact role of NETosis is still elusive. The various immunogenic proinflammatory factors released during NETosis can maintain sterile inflammation as an amplification loop, partly by stimulating the innate immune system to further recruitment and cytokine production by netting DNA, histone, and antimicrobial peptides; and partly via the activation of NLRP3-caspase 1 inflammasome system and IL-1β production (63, 64). matrix metalloproteinase (MMP)-mediated endothelial injury is a direct tissue-damaging consequence of histones and LL-37 (65). Prolonged IL-1β and IL-18 production can trigger T-cell differentiation and IL-18 can induce T-helper 1 and B cells. Accordingly, in some immunological cases, we can speak of autoinflammatory-autoimmune diseases of mixed etiology (like juvenile idiopathic arthritis, rheumatoid arthritis, Behcet’s disease, or adult-onset Still syndrome) (59, 66).

Arterial and venous thrombotic and thromboembolic events can also occur via NETosis. NETs can serve as a scaffold for platelets to aggregate (10). Local hypoxia, NET compounds stimulate the endothelium further to release procoagulant factors and promote thrombus and NET formation. Additionally, tissue factor (TF) is released along with DNA, activating the extrinsic coagulation cascade pathway and increasing the risk of blood clots in the arterial and venous systems (67, 68). Along with the immunogenic effects, NETs also significantly contribute to vascular injury. Cytokines and extracellular histones damage endothelial cells directly as well as promote endothelial-mesenchymal transition (69). The NET-compound MMP-9 activates endothelial MMP-2 leading to endothelial cell death and augmenting the activity of collagenolysis, while interferons inhibit the endothelial progenitor’s differentiation (65, 70). NET proteins also modify the high-density lipoprotein (HDL) via oxidation, stirring it into a proatherogenic direction (71). Endothelial cells have limited capacity to take up the remnants of NETs, and persistent exposure to them provokes vascular leakage by demolishing intercellular junction proteins (69). Ultimately, these changes result in compromised endothelial function and vascular damage. Macrophages are recruited to remove NETs and promote plaque formation (11). Taken together, all these alterations lead to atherosclerotic diseases and an augmented potential to rupture and thrombus formation.

The cancerogenic and tumorigenic roles of NETosis were observed as well (72). NETs can be found in large quantities around the tumorous microenvironment. NETs abolish tumor cells through their lytic enzymes and disrupt the extracellular matrix and intercellular connections, which can facilitate the migration of tumor cells (12, 73). NETs also assist the spread of cancer by capturing and helping metastatic cells to adhere to the tissue. They can also serve as a physical barrier, protecting tumor cells from cytotoxicity (73).

The effect of NETosis on renal cells is primarily conducted by the immune complexes and the consequently induced cytokines. However, the direct effect of NETosis on renal cells is not fully elucidated and only a limited number of studies are available. The releasing endogenous antigens from NETs are aggravating the inflammation since they act as DAMPs and further prime neutrophils and trigger NET production (37). Most importantly, extracellular histones convey significant cytotoxic effects with cytokines such as interferon and induce epithelial-mesenchymal and PEC (parietal epithelial cell) progenitor proliferation, thus crescent formation (74). Histones activate TLR-2,4 and NLRP3. NLRP3 mediates caspase-1 molecular complex (inflammasome) activation and promotes IL-1 and IL-18 cleavage to mature forms. Caspase-1 elicits pyroptosis in endothelial cells and it also contributes to platelet activation, aggregation, and microthrombus formation thus hemodynamic disturbances (75). NETs cause changes in the slit diaphragm-associated proteins, such as podocin and nephrin, provoking podocyte effacement and inducing consequential proteinuria. They induce podocyte cell hypertrophy, mitotic catastrophe, and eventually podocyte cell death as well (76). NETs hasten tubular epithelial cell apoptosis and further NET formation (74, 77) ( Figures 2A-E ).

SLE is a multiorgan autoimmune disease characterized by the dysregulation of both the innate and adaptive immune systems, leading to inflammation and severe tissue damage across the body. About 30-50% of SLE patients develop lupus nephritis, which is a severe complication with proteinuria, kidney function loss, and increasing risk of mortality. 10-30% of the patients will progress to end-stage renal disease in 5 years. Impaired tolerance, aberrant response to self-antigens, and type I interferons are considered crucial in SLE development and pathogenesis, in which NETosis plays a critical role (4) ( Figure 3A ).

Low-density granulocytes (LDGs) are highly granular pathogenic granulocytes displayed in a high concentration in SLE patients. These neutrophils can be characterized by their ability to produce high amounts of proinflammatory cytokines, including type I interferons, and their increased propensity to undergo spontaneous NETosis (70, 78). Moreover, LDGs have a hyperability to produce mitochondrial ROS, which - as mentioned above - is adequate to generate NETs even in the lack of NADPH-oxidase and further stimulates interferon gene transcription (79).

This process is perpetuated by nucleic-acid-containing immune complexes. They activate neutrophils via FcγR pathways and induce mitochondria-dependent NETosis (79, 80). It is widely known that ultraviolet (UV) radiation exacerbates the pre-existing lupus disease. UV radiation induces NETosis wavelength- and dose-dependently. Penetrating the epidermis, UV-A and blue light induce ROS-production and subsequent MPO- and NE-dependent NETosis (81). UV-C radiation triggers mtROS generation, mtDNA decondensation, and caspase 3 cleavage, hence, features of both apoptosis and NETosis. Nevertheless, it is important to bear in mind that UV-C does not have biological importance since it is completely absorbed by the ozone layer (82)

On the other hand, oxidized mtDNA, exposed LL-37, and HMGB1-DNA complexes activate plasmacytoid dendritic cells via TLR9/TLR7, as well as endocytosed nucleic acid-autoantibody complexes can initiate type I interferon production, contributing to the prominent interferon signature in SLE (69, 83, 84). LL-37 and other NET proteins activate the NLRP3 inflammasome in macrophages, increasing the IL-1 and IL-18 secretion. As positive feedback, IL-18 and interferon α further stimulate the process of NET formation (2).

To conclude, NET formation in SLE is mostly NADPH-oxidase independent and induced primarily by immune complexes through FcγR signaling (85).

The accumulation of NETs is a common phenomenon in SLE and increases the exposure of nucleic acids and proteins to B-cells and plasmacytoid dendritic cells to generate high levels of interferons. There are three known ways in which the clearance of NETs may be impaired: 1) mutations and polymorphisms of DNase I, which lead to inadequate enzyme function, 2) inhibition of DNase by autoantibodies, and 3) the presence of anti-NET antibodies, that hide the binding sites from DNase I (86–89). In the latter form, autoantibodies recruit complement; C1q binds to DNA and prevents DNase I from degrading NETs (90). Furthermore, oxidized DNA is more resilient to DNase degradation (91). In summary, the altered clearance results in a longer exposure time to NETs and autoantigens, therefore it correlates with the severity of the disease. In parallel with this, the amount of DNase I in the kidney and urine decreases as lupus nephritis progresses (92).

Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a necrotizing small vessel vasculitis characterized by the presence of circulating ANCAs. Based on the circulating antibodies and clinical manifestations, three main forms of AAV are distinguished: microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA), and eosinophil granulomatosis with polyangiitis (EGPA). Renal involvement with nephritis syndrome (kidney function loss, hematuria, high blood pressure) is a common manifestation of AAVs occurring in most cases of MPA and frequently in GPA, however, it is rare in EGPA (93). The diagnostic hallmark of AAV is antibodies targeted against granular proteins of neutrophils: MPO and PR3.

Neutrophil priming is presumed to be the key step in NET formation in AAV. Priming is a process in which the neutrophil response is enhanced by an activating stimulus. Priming undergoes with the help of proinflammatory cytokines (such as transforming growth factor-beta, tumor necrosis factor α, interferon-α, -γ, IL-1, -6, -8, -17) complements (C5a), and LPS. After neutrophil activation, MPO and PR3 exteriorize on the cell surface. ANCA crossbinds these antigens with neutrophil FcγRIIa and induces uncontrolled ROS and lytic enzyme bursts. After the burst, MPO and NE migrate to the nucleus and NETosis begins and run its course in an NADPH-oxidase-dependent pathway, via the lytic NETosis. This is a pivotal difference compared to SLE (94, 95).

Although ANCA stimulation is suggested to be a key to NET formation, recent studies highlight a possibility of an ANCA-independent manner of NETosis. NETosis still occurs when ANCA IgG and IgA are depleted, or when the C5a receptor is inhibited. It does not correlate with serum tumor necrosis factor α (TNF-α), IL-6, -8 (priming cytokines), or conventional inflammatory markers like CRP or erythrocyte sedimentation rate. In fact, the level of NETosis is higher in ANCA-negative AAV patients. Additionally, even though relapse in AAV is commonly triggered by a concurrent infection, NET formation in AAV patients was lower during severe infection than in patients with relapsing AAV. To sum up, the exact mechanism of NET formation in AAV patients remains unknown and the increased level of NETs is linked to autoimmunity and clinical disease activity rather than concomitant infection (96) ( Figure 3B ).

Diabetes can affect kidney function through several mechanisms. Classic diabetic nephropathy is considered a microvascular complication of diabetes, which will lead to proteinuria and glomerulosclerosis. Although diabetes is closely associated with inflammation and oxidative stress, the role of neutrophils in this process is almost neglected. Increased glucose level upregulates PKC activity and induces NADPH-oxidase overstimulation and oxidative burst regardless of the type of diabetes. As described above, oxidative burst is a pivotal step in NET formation. Inflammatory cytokines and free fatty acids inhibit insulin signaling by phosphorylation of inhibitors of nuclear factor kappa-B kinase (IKKβ) and c-Jun N-terminal kinase 1 (JNK1), which are also inflammatory pathway mediators, and induce NFκβ (nuclear factor kappa-light-chain-enhancer of activated B cells) translocation to the nucleus, resulting in various proinflammatory gene activation which is necessary for the priming process (97). Moreover, high extracellular glucose polarizes macrophages to a proinflammatory M1 phenotype. Interacting with NETs, M1-macrophages not only exacerbate the proinflammatory response but also go under apoptosis and release extracellular DNA. This mechanism is initiated specifically by NETs and contributes to an augmented load of free DNA and the progression of the disease (36). Although the underlying pathomechanisms in the development of diabetic kidney disease are complex and go beyond the scope of this review, it is clear that NETosis is noticeably involved in the process.

Acute tubular necrosis (ATN) is the most common form of acute kidney injury (AKI), and it is characterized by the destruction of the tubular epithelial cells leading to a rapid kidney function loss with oliguria or anuria. It may occur as a result of ischemic or toxic impacts. Acute tubular necrosis is accompanied by a massive inflammatory response including recruitment, activation of immune cells, and increased proinflammatory cytokine production. Renal cell necrosis releases necrotic cell debris which is introduced as DAMPs. The innate immune system and especially neutrophils are the major responder and effector cells in AKI and play a role in the crescendo-type inflammation (98). DAMPs, such as histones generate a secondary autoimmune amplification loop by further priming neutrophils, activating them to form NETs, further deteriorating the kidney injury and exposing even more endogenous epitopes (77). Uromodulin is a constitutively secreted protein by epithelial cells in the thick ascending limb and the distal tubule into the tubular lumen. Uromodulin binds and aggregates cytokines in the lumen and stays inert inside the luminal compartment. However, in case of tubular damage, uromodulin compiles in the interstitium as crystal-like structures and they can activate antigen-presenting cells and elicit the NLRP3-inflammasome-caspase-1 pathway (99). In the late phase of the injury, macrophages infiltrate the injured tissue and in the proinflammatory environment, they undergo an M1-phenotype shift contributing to necroinflammation. Contrary to necrosis, apoptosis is a balanced and regulated physiological mechanism that halts the exaggerated inflammatory process. Apoptosis of the recruited immune cells polarizes phagocytes to M2 phenotype and favors an anti-inflammatory milieu by secreting Il-10, TGF-β, and growth factors. This environment promotes not only suspending the inflammatory vicious circle but also contributes to the regeneration of epithelial and vascular healing and fibrosis (37).

Anti-glomerular basement membrane (GBM) disease is a rare autoimmune disorder in which antibodies are produced against type IV collagen and presents with rapidly progressive glomerulonephritis and alveolar hemorrhage (100). Anti-GBM IgG binds FcγRIIa in a shear-force-dependent manner. FcγRIIa binding causes F-actin polymerization via the Abl/Src mediated pathway. Their interaction and actin polymerization leads to an endothelial CD18 integrin (Mac-1) activation, and integration-mediated adhesion takes over selectin-mediated rolling. Thus, neutrophil attachment to the endothelial cells in the capillaries is sustained. Eventually, FcγR engagement triggers intravascular ROS, protease, and NET generation (101, 102). This mechanism with the extensive NET formation and the consequent exaggerating necroinflammation explains the substantial renal damage in anti-GBM glomerulonephritis which not only confines to the glomeruli but also affects the interstitium and the tubules.

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy caused by enterohemorrhagic Shiga toxin (Stx) - producing bacteria (Shigella dysenteriae or enterohemorrhagic Escherichia coli). The syndrome is characterized by thrombocytopenia, hemolytic anemia, and acute renal failure. It is one of the main causes of acute kidney injury in children and has no specific treatment.

In hemolytic uremic syndrome, neutrophils are prone to undergo NETosis. There are several mechanisms behind it: 1) Shiga toxin is believed to be a potent neutrophil activator and NET inducer. 2) Uric acid is released from the injured cells in a great quantity, crystallizes, and triggers NET formation. 3) Higher dose of Stx induces NETosis in a ROS-dependent manner. 4) Stx enhances P-selectin expression on LPS-treated platelets and promotes neutrophil activation and aggregation (102).

Furthermore, exaggerated endothelial damage and thrombosis formation can be observed in HUS. Besides the excessive release of endothelial toxic NET contents, LPS, and Stx-treated platelets also contribute to endothelial damage (102). The role of neutrophil extracellular traps in atypical hemolytic uremic syndrome and thrombotic microangiopathies, however, is unclear.

Gout is an autoinflammatory condition characterized by monosodium urate (MSU) crystal deposition in the joints and the kidneys. High uric acid concentration is also a risk for nephrolithiasis.

MSUs are taken up by phagocytes and either directly damage the cell membrane causing necrosis and necrotic debris release leading to inflammatory cell recruitment, cytokine and chemokine production, or activating the NLRP3-caspase-1 inflammasome system resulting IL-1β release (50, 103). Recruited neutrophils undergo NADPH-dependent lytic NETosis, partly via direct activation by MSUs and partly by the inflammatory mediators (104). Uniquely, NETosis has a dual role in gout. Besides perpetuating inflammation, NETosis also limits it by engulfing MSUs preventing them from further phagocyte activation. MSU-stimulated neutrophils form aggregated NETs, which can proteolytically degrade and inactivate cytokines and chemokines stopping gout attack and resolving inflammation (105, 106).

Familial Mediterranean Fever (FMF) is a monogenic autoinflammatory disease, with clinical characteristics of fever attacks, joint pain, and skin rashes. During fever attacks, neutrophils produce large amounts of NETs which perpetuate inflammation via directly derived IL-1β and by inducing polymorphonuclear cells for further IL-1β production. However, NETs also have a self-limiting, therefore anti-inflammatory effect, as they can prevent further NETosis and stop the fever attack (107, 108). The most common kidney damage caused by FMF is amyloidosis, but IgA nephropathy and mesangioproliferative glomerulonephritis were also described (109).