The canonical pathway for NLRP3 inflammasome activation generally requires two distinct signals: a priming signal and an activation signal. The priming signal (signal I), involves the upregulation of NLRP3 and pro-IL-1$\beta$, typically triggered by NF-$\kappa$B activation via PRRs like TLRs or cytokine receptors. The activation signal (signal II) triggers the assembly of the inflammasome complex, often involving substances like double-stranded deoxyribonucleic acid (dsDNA), double-stranded ribonucleic acid (dsRNA), extracellular adenosine triphosphate (eATP), and potassium ions. Thus, the NLRP3 inflammasome is activated through a series of events rather than a direct interaction [50].

There is evidence supporting the idea that HSP65 is highly expressed in vascular tissues and may act as a priming signal as a TLR agonist [51]. It is a member of the HSP60 family, mainly presenting in M. tuberculosis and originally isolated from My. bovis. HSP65 shares a high degree of similarity with human HSP60 [52]. Additionally, HSP60 can activate macrophages by interacting with TLRs, initiating the synthesis of NLRP3 and triggering inflammatory cascades [53]. Huang et al. [54] found that in patients with gouty arthritis, HSP60 expression was increased in peripheral blood mononuclear cells (PBMCs) and HSP60 activated the TLR4/myeloid differentiation primary response protein 88 (MyD88)/NF-$\kappa$B signaling pathway upon binding to TLR4, resulting in the synthesis of the NLRP3 inflammasome and contributing to mitochondrial dysfunction. Naturally, there is a question of whether HSP65 also possesses the same function, and there are some clues supporting this idea. First, scholars replicated animal models of gouty arthritis by injecting inactivated bacillus Calmette-Guerin vaccine because HSP65 can serve as an antigen in synovial tissue and shares a conserved sequence with HSP60 [55]. Moreover, Bulut et al. [56] summarized the mechanisms by which HSP65 and HSP70 induce inflammation through studies of immune processes in M. tuberculosis infection. The authors found that HSP65 can activate NF-$\kappa$B by stimulating TLR4 with or without MyD88 protein and promote inflammation [56]. However, an earlier study showed that HSP65 has weak binding affinity to TLR4 in the presence of cluster of differentiation 14 and myeloid differentiation factor 2, and that HSP65 cannot activate NF-$\kappa$B through the TLR4-mediated pathway [57]. In addition, Wu et al. [58] found that HSP65 upregulates the expression of IL-6 and IL-1$\beta$ in aorta adventitial fibroblasts through the TLR4/JAK2/AKT/STAT3 pathway in vitro. Based on the information available, there is no direct evidence that HSP65 directly initiates the synthesis of the NLRP3 inflammasomes. However, there is evidence that HSP65 can trigger the production of pro-inflammatory cytokines, suggesting a role in these immune responses.

TNF-$\alpha$ plays a key role in the pathogenesis of TAK. The levels of TNF-$\alpha$ in the peripheral blood and lesion tissues of patients with TAK are significantly higher than those of healthy individuals. TNF-$\alpha$ can bind to the TNF receptor and send a priming signal to promote the upregulation of NLRP3 [59]. Studies have shown that the use of TNF-$\alpha$ inhibitors can alleviate the activity of TAK, which we will describe in detail later.

According to current understanding, activation signals may involve multiple factors. After cell perforation, DNA fragments, specifically extracellular dsDNA (eDNA), are released from cells undergoing death due to the activity of $\gamma$$\delta$ T cells or NK cells and are engulfed by macrophages. Within these macrophages, the engulfed DNA fragments can activate the NLRP3 receptor, which in turn leads to the activation of caspase-1, ultimately resulting in the formation of the NLRP3 inflammasome. Inflammasomes activation leads to the production of inflammatory factors such as IL-1 and IL-18, as well as the activation of GSDMD, resulting in pyroptosis. Inflammatory factors and eDNA released after pyroptosis then initiate the next cycle [60].

Also, an alternative activation pathway of NLRP3 inflammasomes exists in human monocytes. The binding between LPS and TLR4 can stimulate the assembly of NLRP3 inflammasomes, and this can occur through a pathway involving receptor-interacting serine/threonine-protein kinase 1, Fas-mediated death domain protein, and caspase-8, bypassing the priming step [61]. It has also been found that TLR4 recognizes LPS in the PBMCs of patients with TAK, which increases the expression of IL-1$\beta$ and IL-1R2 [62]. LPS can independently trigger assembly of the NLRP3 inflammasome and increase the level of IL-1$\beta$ in the absence of a priming signal in macrophages [63].

In 2014, Maejima et al. [9] found a correlation between the severity of symptoms in patients with TAK and polymorphism of the MAX Dimerization Protein MLX (MLX) gene. Specifically, patients with TAK with severe symptoms had a higher frequency of the missense mutation rs665268 in the MLX gene, indicating a potential association between MLX protein and TAK pathogenesis [9]. Previous studies have established a correlation among MLX protein, TXNIP, and NLRP3 inflammasome activation, particularly under conditions of oxidative stress and involving the mitochondria [64, 65]. Research suggests a link between MLX allelic variants and the NLRP3 inflammasome, which plays a role in the pathogenesis of TAK. In 2014, a study by Maejima et al. [9] confirmed that the MLX risk genotype upregulates MLX protein transcription, which in turn promotes the expression of TXNIP through the MLX/Mondo-A complex. The authors also observed an increase in NLRP3 inflammasome and caspase-1 levels in the cultured macrophages of patients with TAK harboring this allele compared to the control group [9].

Additionally, this genotype was associated with decreased autophagy levels, possibly due to reduced free Mondo-A resulting from its binding to the MLX protein. Mondo A promotes autophagy by inhibiting the expression of Rubicon, which in turn acts as a brake on autophagy, particularly by inhibiting the formation of auto-phagolysosomes (autophagosomes fused with lysosomes for degradation). Rubicon accomplishes this by binding to the phosphoinositide 3-kinase complex and inhibiting its activity [66].

Mitophagy plays a crucial role in the negative regulation of the NLRP3 inflammasome by clearing damaged mitochondria, one of the sites that generate upstream activation signals for NLRP3. Damaged mitochondria release ROS and oxidized mitochondrial DNA into the cytosol, which induce activation of the NLRP3 inflammasome through mechanisms that are not fully understood [67]. Moreover, the NLRP3 inflammasome can be engulfed by autophagosomes through ubiquitination and subsequent degradation. Ubiquitination can activate autophagy, resulting in the inactivation of NLRP3 inflammasomes [68].

Macro-autophagy and ROS act as intermediaries in the correlation between STAT3 and NLRP3. Phosphorylated STAT3 acts as a transcriptional activator, binding to target genes and promoting their expression. In mice, activation of STAT3 increases the acetylation of histones H3 and H4 on the NLRP3 promoter, thereby promoting the expression of effector proteins. Applying JAK inhibitors to reduce STAT3 phosphorylation will continuously inhibit NLRP3 transcription, suppressing inflammasome expression [69]. In addition, stimulating the marrow-derived macrophages of mice with LPS showed that STAT3 inactivation blocked NLRP3 inflammasome-induced caspase-1 activation and IL-1$\beta$ secretion and STAT3 downregulated mitochondrial autophagy mediated by PTEN-induced kinase 1 (PINK1), positively regulating macrophage NLRP3 inflammasome activation [70].

One of the activation pathways of NLRP3 inflammasome is mediated by the P2X7R starting with signal II [47, 71]. P2X7R is a transmembrane receptor, and ATP is its only physiological agonist [72]. It is widely expressed in various cells of the human body including endothelial cells, fibroblasts, neutrophils, monocytes/macrophages, dendritic cells, and lymphocytes [73]. Binding of the P2X7R with its agonist causes potassium efflux, sodium and calcium influx, and the formation of non-selective pores that allow the passage of macromolecules [74]. Cell damage or death leads to the release of ATP, which activates the P2X7R, resulting in the variation of these electrolytes and the entry of PAMPs through the pores [75, 76]. These actions disrupt the cellular homeostasis, leading to organelle damage, ROS release, initiation of the ROS-NLRP3 axis, and assembly of the NLRP3 inflammasome [77]. P2X7R can also mediate the release of IL-1$\beta$ and IL-18, inhibit the differentiation of regulatory T cells, induce the conversion of T helper 17 cells (Th17 cells), and promote inflammatory responses [78, 79].

Monocytes/macrophages with high expression of P2X7R are found in the vascular wall [80]. P2X7 polymorphisms are associated with both TAK and tuberculosis infection. The P2X7-1513C is a mutation that leads to non-functional P2X7R expression in macrophages. The frequency of this polymorphism is higher in patients with TAK than those with only pulmonary tuberculosis, and tuberculosis genetic material can also be detected in the aortic arch and other arterial tissues of patients with TAK [81]. Theoretically, the extent of pyroptosis mediated by the eATP-P2X7R pathway in the macrophages of patients with TAK would be weakened according to this finding, which would partially compromise the innate immunity against M. tuberculosis. It has been hypothesized that the proliferation of M. tuberculosis through the aforementioned mechanism can lead to the increased synthesis of HSP65 protein and exacerbate TAK.

Cells release two main inflammatory mediators, IL-1 and IL-18, after NLRP3 inflammasome activation. IL-1 has strong inflammatory-inducing effects that can lead to prostaglandin secretion and activate and differentiate neutrophils, monocytes, and lymphocytes [82]. IL-18 is a key player in inducing IFN-$\gamma$ production in macrophages and NK cells, as well as increasing the synthesis of TNF-$\alpha$ and IL-2 [79]. The high expression of the IL-1$\beta$ gene has been observed in patients with TAK, and a polymorphism in the IL-1$\beta$ gene is associated with TAK susceptibility [62, 83]. In addition, plasma levels of IL-18 are significantly elevated in patients with TAK, and a polymorphism in the promoter region of the IL-18 gene is correlated with TAK susceptibility. There are three tag single nucleotide polymorphisms, namely, IL18-656G/T (rs1946518), IL18-607C/A (rs1946519), and IL18-137G/C (rs187238) [84]. Among them, IL18-607C/A is a protective factor that may reduce the risk, whereas IL18-137G/C and IL18-656G/T are considered risk factors that may increase the risk of TAK. The IL18-137G/C and IL18-656G/T polymorphisms are associated with higher IL-18 levels [44, 46].

As aforementioned, there is a close relationship between IL-6 and TAK. The levels of IL-6 in the peripheral blood and lesion tissues of patients with TAK are significantly elevated compared to healthy individuals, and IL-6 is significantly involved in the pathogenesis of TAK. First, IL-6 is closely associated with changes in inflammatory markers such as ESR and CRP, which can reflect the severity of inflammation during the acute phase of the disease. Second, in the early stage of TAK, monocytes and fibroblasts are activated through the TLR/NF-$\kappa$B pathway, and the recognition of PAMPs induces the expression of IL-6. In other words, the level of IL-6 can reflect the expression level of TLRs. Therefore, IL-6 may be a sensitive indicator in the early stage of the disease. Lastly, the high level of IL-6 is also prominent among cytokines during the fibroproliferative phase. IL-6 promotes the proliferation of lymphocytes and is secreted by lymphocytes, as well as macrophages, epithelial cells, and fibroblasts [85]. In conclusion, IL-6 is closely related to the inflammation, fibroproliferation, and granuloma formation observed in TAK [86, 87].

Researchers have observed in a mouse model of rheumatoid arthritis (RA) that IL-6 can activate the NLRP3 inflammasome in the presence of ATP. Conversely, blocking the IL-6 pathway leads to reduced NLRP3 inflammasome [62]. The two major downstream signaling pathways activated by IL-6/IL-6R are JAK/STAT3 and JAK/mitogen-activated protein kinase. Studies have shown that IL-6 may also influence the initiation of NLRP3 inflammasome mediated by NF-$\kappa$B. Dai et al. [88] knocked in human IL-6R genes into mice and induced them to express, and then synthesized antibodies against these receptors to block their effects. The results showed that the NF-$\kappa$B signaling pathway was also affected in specific cells, and the initiation and activation of the NLRP3 inflammasome, IL-1$\beta$ secretion, pro-caspase-1 cleavage, and GSDMD activation were all inhibited [88]. Researchers showed that certain herbal extracts used to treat ulcerative colitis demonstrated dual effects by blocking the IL-6/JAK2/STAT pathway and also reducing ASC protein expression and NLRP3 inflammasome formation. They interpreted this phenomenon as double inhibition of two distinct pathways involved in inflammation. However, combined with previous findings, this may be due to blocking the upregulation of ASC protein expression mediated by IL-6, thereby affecting the NLRP3 inflammasome [89]. Furthermore, IL-6/JAK/STAT3 pathway activation can lead to inhibition of mitophagy, increased ROS release, and promotion of inflammasome activation, a process that is dependent on PINK1 [90].

Research on lung tissues infected with SAR-CoV-2 has revealed a cascade reaction of NLRP3/IL-1$\beta$/IL-6 in patients with coronavirus disease 2019 (COVID-19). The secretion of IL-1$\beta$ related to NLRP3 inflammasome-mediated pyroptosis can promote the release of IL-6 by leukocytes, exacerbating the condition of COVID-19 [91]. Sonowal et al. [92] showed that luxeptinib can inhibit the hydrolysis of caspase-1 and NLRP3-mediated IL-1$\beta$ release. It also inhibits the release of TNF-$\alpha$ and IL-6 by influencing other inflammatory signaling pathways, including those activated independently of the NLRP3 inflammasome. Luxeptinib directly affects IL-6 and TNF-$\alpha$ release through these distinct pathways, not solely via its impact on IL-1$\beta$ [92]. This suggests that IL-6 is a downstream factor of IL-1$\beta$ regulated by the NLRP3 inflammasome, and seems to affect the levels of IL-6 through IL-1$\beta$, suggesting the existence of a positive feedback regulation [58]. In conclusion, NLRP3 and ILs are intricately linked and play a crucial role in inflammation, with complex signaling pathways like those involving TAK.

Infliximab is a TNF-$\alpha$ inhibitor, and TNF-$\alpha$ is produced by macrophages, T cells, and NK cells, promoting the proliferation of Th1 cells and playing a crucial role in granuloma formation. TNF-$\alpha$ can bind to TNF receptors and activate NF-$\kappa$B, priming the synthesis of components of the inflammasome. NF-$\kappa$B and NLRP3 also promote the upregulation of TNF-$\alpha$ [59, 108]. This feedback loop enhances the activity of NLRP3. The specific inhibition of TNF-$\alpha$ by infliximab blocks the aforementioned pathway and alleviates symptoms of TAK. Case reports and clinical trials have shown that infliximab may be effective in treating patients with refractory TAK and potentially reduces their dependence on GCs. Adalimumab is another type of TNF-$\alpha$ inhibitor that may be more efficacious than tocilizumab when combined with GCs and MTX for patients with active and severe TAK [109].

The formation of NLRP3 inflammasomes in TAK involves the canonical pathway mediated by a priming signal and activation signal. TNF-$\alpha$ and IL-6 released by tissue cells, or LPS and HSP65 released by M. tuberculosis, which after binding to their respective receptors, activate NF-$\kappa$B, thereby enhancing the expression of NLRP3, ASC, pro-IL1$\beta$, and IL-6. LPS, HSP65, and IL-6 may also inhibit damaged mitochondrial clearance via the TLR4/JAK/STAT3 or IL-6/JAK/STAT3 pathway, leading to increased generation of ROS and mitochondrial DNA. TXNIP plays a crucial role in activation of the ROS pathway and its expression is facilitated by the MLX protein. There is a link among cell death, inflammasome activation, and the P2X7R in TAK. IL-1$\beta$ activated by the NLRP3 inflammasome increases IL-6 expression, further enhancing the activation of the NLRP3 inflammasome itself in a positive feedback loop. GCs reduce the production of inflammasomes within the cells of patients with TAK by inhibiting NF-$\kappa$B activity and ROS generation, whereas tocilizumab, TNF-$\alpha$ inhibitors, and tofacitinib block target proteins, reducing inflammasome production. Luxeptinib inhibits GSDMD cleavage and thus halts pyroptosis.

Etanercept is another type of TNF inhibitor used for the treatment of TAK; it blocks the effects of TNF-$\alpha$ and TNF-$\beta$ by competitive inhibition. A prospective cohort study showed that anti-TNF-$\alpha$ therapy, such as Etanercept, as a succedaneum of infliximab in the trial, in combination with GCs, can be beneficial in treating patients with recurrent TAK, with 10 of 15 patients able to gradually reduce the dose of GCs or discontinue GC and achieve sustained remission [110]. Further research is needed to compare the respective advantages and disadvantages of Etanercept and infliximab in treating TAK.

Tocilizumab is an IL-6R monoclonal antibody, and the NLRP3 inflammasome may be a crucial component in TAK. Targeting this pathway with drugs like tocilizumab may be a relevant treatment strategy that could affect ASC synthesis, the clearance of damaged mitochondria, or even directly inhibit the function of NLRP3 by blocking the IL-6R, as discussed previously. Tocilizumab not only blocks the IL-6R but also inhibits activation of the NLRP3 inflammasome through the IL-17a pathway, leading to a significant decrease in IL-17a expression [111]. IL-6 promotes the differentiation and formation of Th17 cells, and the secretion of cytokines such as IL-17 and TNF-$\alpha$ [112]. Tocilizumab blocks the binding of IL-6R to IL-6, thereby weakening the secretion of IL-17 and its positive promotion of NLRP3 inflammasome. Numerous studies have observed the differentiation of CD4+ cells in the Th17 cells of patients with TAK, as well as increased levels of IL-17 in the arterial tissues and peripheral blood [113, 114]. Tocilizumab shows better efficacy in treating refractory TAK compared to traditional therapies. It also has a good safety profile but has raised concerns about rapid symptom relapse after discontinuation [115, 116]. The addition of tocilizumab in patients with poor GC response is recommended in ACR guideline [1]. There are also some targeted drugs that have not been sufficiently studied in clinical research, which may alleviate TAK and reduce recurrence.

Ziltivekimab is a novel IL-6 ligand human monoclonal antibody that alleviates inflammation and reduces serum CRP levels. It was developed to provide a new option for the anti-inflammatory treatment of atherosclerosis in patients with chronic kidney disease, as increasing data suggest that the pro-inflammatory effects of IL-6 can exacerbate atherosclerosis and thrombosis. Studies have found that regular use of this drug, with subcutaneous injections of 7.5, 15, or 30 mg every 4 weeks for 24 weeks, can effectively lower CRP levels in patients with significant inflammatory responses [117]. It is currently unknown whether ziltivekimab has a positive effect on cardiovascular outcomes, as clinical trial results are not yet available. To date, ziltivekimab has not been used for the treatment of TAK and its therapeutic efficacy remains to be substantiated.

TAK-242 (also known as Resatorvid) is a specific inhibitor of TLR4 signaling that shows promise for alleviating diseases such as ulcerative colitis [118, 119]. Wu et al. [58] used TAK-242 to block HSP65 activation of the TLR4-JAK2/AKT/STAT3 pathway and reduce the phosphorylation levels of JAK2, AKT, and STAT3 in aortic adventitial fibroblasts in vitro, showing a possibility as a targeted therapy. However, for treating other indications like sepsis, TAK-242 is either undergoing Phase 3 clinical trials or the trial results are unsatisfactory, limiting its clinical application.

JAK inhibitors work by blocking the JAK/STAT3 signaling pathway. Traditionally, it is believed that JAK inhibitors achieve therapeutic effects by reducing Th1- and Th17-derived cytokines and activating CD4+ cells. Therefore, JAK inhibitors may also influence the initiation and activation signals of the NLRP3 inflammasome through two mechanisms: blocking STAT3 upregulation of NLRP3 protein expression and enhancing the clearance of damaged mitochondria. Clinical trials have shown that the combined treatment with drugs such as tofacitinib and ruxolitinib for the treatment of TAK yields better results than using GCs alone [120, 121]. Currently, several clinical trials are underway (e.g., NCT04161898 and NCT04299971) (Table 3, Ref. [58, 109, 115, 120, 121, 122, 123, 124, 125, 126, 127]).

IL-1$\beta$ is closely related to the pathogenesis of TAK and is involved in regulating the activation of IL-6. Canakinumab is a humanized monoclonal antibody and inhibitor targeting the IL-1 signaling pathway to exert anti-inflammatory effects and significantly reduces plasma levels of IL-6. The Canakinumab Anti-inflammatory Thrombosis Outcomes Study (CANTOS) demonstrated that administering 150 mg canakinumab every 3 months, a drug that targets the IL-1$\beta$ pathway to reduce inflammation, significantly decreased the incidence of recurrent cardiovascular events compared to the placebo group. The mechanism by which canakinumab decreased the risk may involve downregulation of IL-1$\beta$ associated with activation by cholesterol. Currently, canakinumab is being tested in clinical trials for atherosclerosis, diabetes, chronic kidney disease, gout, and some autoimmune diseases. Despite the lack of data on the use of canakinumab for TAK treatment, it has value as a potential anti-inflammatory therapeutic target.

In conclusion, TAK, is a rare but serious and complex inflammatory disease with a still largely unknown pathogenesis, posing a significant challenge to human health. While progress has been made in understanding the risk factors, pathogenesis mechanisms, pathological changes, and clinical features in TAK in recent years, the specific role of the NLRP3 inflammasome in the development of TAK needs further in-depth investigation to improve our knowledge and promote a novel targeted therapeutic strategy.