NLRP3 Inflammasomes: Dual Function in Infectious Diseases

One of the most important defense mechanisms for the body is the innate immune system. To preserve tissue homeostasis, immune cells respond adaptively to unpleasant stimuli such as infection, damage, and autoimmune reactions by causing inflammation. An adequate inflammatory response may help the body rid itself of unhealthy or dead cells and speed tissue healing. However, inflammatory cytokines are generated when the endogenous danger signals are overactivated. The host may suffer negative effects from this process and become more vulnerable to pathological harm. In recent years, the relationship between the innate immune system and infectious diseases has received increasing attention (1, 2).

As one of the most researched inflammasomes, the Nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is an intracellular mediator that can react by starting an inflammatory cascade, inducing gasdermin D–dependent pyroptosis, and producing and releasing proinflammatory cytokines, including IL-1β and IL-18 (3). Various clinical and basic studies have demonstrated that the NLRP3 inflammasome contributes to immune-driven pathology in infectious diseases. However, with the deepening of research, the complex mechanism of the NLRP3 inflammasome at different stages of the disease has attracted more and more attention. Whether NLRP3 is a hero or an enemy deserves further study. This review aims to provide a comprehensive overview of the current understanding of the interplay between NLRP3 inflammasome activation and infectious diseases. We summarize the scientific data of biologics, phytochemicals, and small-molecule chemical medicines acting on the NLRP3 inflammasome signaling pathway. More information on NLRP3 can aid in a deeper understanding of the condition and the development of novel therapeutic approaches.

The NLRP3 inflammasome consists of the nucleotide-binding domain, leucine-rich repeat, pyrin domain-containing protein 3, the apoptosis-associated speck-like protein containing a CARD (ASC), and the pro-cysteinyl aspartate–specific proteinase-1 (procaspase-1) (4). Procaspase-1 is recruited through CARDs of ASC, and it triggers proximity-induced dimerization once clustered (5). A C-terminal leucine rich repeat domain, a nucleotide-binding NACHT domain, and an N-terminal pyrin domain are all present in NLRP3 (6). Typically, the NLRP3 inflammasome activation mechanism entails two steps: priming (step 1) and activation (step 2). Step 1 is carried out through PRR signals, including TLR and muramyl dipeptide (MDP)/NOD2, as well as activation and cytokines such as TNF-α/TNFR. These signals function through activating the NF-κB–dependent pathway (7), thus upregulating the expression of NLRP3, pro–IL-1β, and pro–IL-18, which empower NLRP3 and promote inflammasome assembly. It is noteworthy that pro–IL-18 and pro–IL-1β have different degrees of dependence on priming (8). For this, pro–IL-18 is constitutively expressed in many cell types, whereas the expression of pro–IL-1β needs to be induced by the above-noted inflammatory mediators (9). Meanwhile, NLRP3’s posttranslational modifications, including ubiquitination, phosphorylation, and SUMOylation, are mediated (10). A series of complex posttranslational modifications alters the protein function, activity, and/or intracellular location of NLRP3, regulating NLRP3 inflammasome activity from multiple perspectives.

Step 2 is induced by a variety of pathogen-associated molecular patterns and damage-associated molecular patterns, including extracellular ATP, viral RNA, and pore-forming toxins, as well as particulate matter, causing NLRP3, ASC, and procaspase-1 to come together to form a complex (11). For example, there is evidence that ion flux (K⁺/Cl⁻ efflux and Ca²⁺ influx) (12–15), mitochondrial impairment (16), the production of reactive oxygen species (ROS) (17), and lysosomal leakage (18) are the triggers for NLRP3 inflammasome activation. ATPase activity found in the NACHT domain is necessary for NLRP3 conformational shift and oligomerization, leading to pyrin domain clustering and ASC clustering (11). ASC polymerizes to form a stable long filamentous structure that increases the density of caspase-1 to facilitate contact with the substrate, allowing NLRP3 to interact with caspase-1 (19). These processes facilitate the assembly of inflammasome.

The formation of the NLRP3 inflammasome initiates the self-proteolytic process of caspase-1, thus cleaving cell substrates including pro–IL-1β, pro–IL-18, and gasdermin D (GSDMD) and converting them into active forms (20, 21). Subsequently, the oligomerization of GSDMD N-terminal forms pores on the plasma membrane, driving the extracellular release of IL-1β and IL-18 and mediating pyroptotic cell death (22, 23). At this time, water also enters the dying cells, causing cell swelling and nerve injury–induced protein 1 (NINJ1)–dependent plasma membrane rupture, resulting in the release of macromolecules in the cells (24, 25) (Fig. 1).

Clinical and preclinical research studies have demonstrated the involvement of the NLRP3 inflammasome in various infectious diseases. However, it is unclear whether activation of the NLRP3 inflammasome is beneficial or harmful. To better focus on this issue, we discuss common clinical situations of infectious diseases in which NLRP3 exhibits a protective effect early in the infection but becomes deleterious later in the disease. The following sections offer a concise summary of recent and significant evidence for further discussion (Fig. 2).

S. aureus is a common foodborne pathogenic microorganism that is responsible for a variety of infection diseases, including peritonitis and enteritis (26). The principal virulence factors of S. aureus consist of a range of toxins, such as pore-forming toxins including α toxin (27), toxic shock syndrome toxin 1 (28), staphylococcal enterotoxin A (29), leukocyte toxin A/B (30), and lipoprotein (31). These virulence factors engage in intricate host–pathogen interactions, triggering the activation of the NLRP3 inflammasome in macrophages, monocytes, and neutrophils through diverse signaling pathways (32). The specific activation mechanisms include affecting NF-κB/MAPK signaling pathways (29), the TLRs 1, 2, and 4/NLRP3 pathway, and ion flux (K⁺ efflux and Ca²⁺ influx) (28, 33).

The NLRP3 inflammasome is a key immune response against bacterial infections. NLRP3 inflammatory activation is usually expressed for pathogen/hazard monitoring, and activation of NLRP3-dependent caspase-1 and IL-1β is required for the process of pathogen clearance of S. aureus (34, 35). In bacterial infections of the conjunctiva, the NLRP3 inflammasome is activated in infected goblet cells to stimulate an inflammatory response to protect host tissues and eradicate pathogens (34). A similar phenomenon was found in a model of mastitis caused by S. aureus infection. To be specific, S. aureus survival increased when NLRP3 inflammasome activation was inhibited by the inhibitory effect of PINK1/parkin-mediated mitochondrial autophagy (36). In addition, NLRP3 inflammatory activation and downstream caspase-1 help to promote the bactericidal activity of S. aureus–containing phagosomes (37).

However, as the disease advances, the NLRP3 inflammasome-mediated excessive inflammatory response may aggravate various tissue damage. In the staphylococcal parasite diseases model, when the NF-κB/MAPK/NLRP3 signaling pathway is excessively activated by staphylococcal enterotoxin A (29), it exacerbates intestinal barrier dysfunction and injury in mice. The NLRP3 inflammasome, as one of the signal components of the necroptosis cascade, aggravates the lung injury caused by S. aureus (38). Furthermore, in osteomyelitis disease, S. aureus induces osteoclast differentiation and promotes bone resorption by inducing the NF-κB/NLRP3 signaling pathway (39). Interestingly, the impact of the NLRP3 inflammasome on cytokine regulation is closely linked to the duration of bacterial infection. As the infection time extends beyond 24 h, there is a gradual increase in the production of inflammatory factors, including TNF-α, chemokine (RANTES), and IL-10 (40). This phenomenon may provide an explanation for the chronic tissue damage caused by the NLRP3 inflammasome. Moreover, the NLRP3 inflammasome may participate in the elimination of human monocytes and bacterial resistance. Components of the inflammasome, that is, NLRP3 and ASC, are essential for leukocyte toxin A/B–mediated IL-1β secretion and induction of human monocyte necrosis (30). α Toxin–induced activation of NLRP3 inflammasome vesicles can enhance host cell glycolysis, leading to ATP depletion and consequent antibiotic resistance (41).

Salmonella enters the human body through the oral route, triggering systemic reactions such as intestinal injury, fever, and septicemia (42, 43). These infectious diseases are collectively referred to as salmonellosis (44). Salmonella spp. express a range of pathogen-associated molecular patterns, including but not limited to type 3 secretion systems, flagella, fimbriae, LPS, and bacterial DNA (45). These components can activate diverse innate immune signaling pathways, ultimately leading to inflammasome activation.

Inflammasomes play a protective role in the early stage of salmonellosis and contribute to the clearance of bacteria. Mice lacking the NLRP3 gene were significantly more susceptible to Salmonella infection (46). In addition, intestinal infection model mice with caspase-1^−/− and caspase-11^−/− showed a greater burden of Salmonella (47). Furthermore, inhibition of NLRP3 inflammasome recognition and decline of inflammasome signaling in the chronic phase of infection result in pathogen survival (48). In response to NLRP3 inflammatory vesicle-mediated pathogen clearance, Salmonella enteritidis T1SS protein SiiD can inhibit the production of mitochondrial ROS, thereby affecting ASC oligomerization (49). For Salmonella typhimurium, S. typhimurium TAcnB was also found to inhibit NLPR3 inflammasome activation through the same mechanism (50). A recent study has shown that Salmonella enteritidis antitoxin DinJ can specifically inhibit the activation of the NLRP3 inflammasome and downstream IL-1β and IL-18 (51).

In contrast, however, inflammasome activation and its mediated pyroptosis are associated with systemic inflammatory response and death caused by Salmonella. Downstream of NLRP3, caspase-1, for example, is essential in Salmonella-induced coagulopathy. Caspase-1– or GSDMD-deficient mice showed relatively shorter prothrombin time and lower plasma thrombin-antithrombin concentration after Salmonella infection (52). Interestingly, this phenomenon occurs in the use of higher doses to simulate severe sepsis models. In the low-dose model of Salmonella, the mortality of mice was not affected by inflammasome factors. Additionally, inflammasomes are also involved in the effects of Salmonella infection on intestinal mucosal physiological responses. IL-1β, downstream of NLRP3, showed a significant inhibitory effect on ascorbic acid uptake. The mechanism may be related to the decrease of SLC23A1 expression caused by IL-1β (53). For the dual role of the NLRP3 inflammasome in Salmonella infection, as well as its concurrent beneficial and harmful effects, we hypothesize that this duality could be influenced by the severity of the Salmonella infection and various stages of the disease progression.

Hepatitis B virus (HBV) is a noncytopathic double-stranded, hepatotropic DNA virus that can cause chronic hepatitis B (CHB), cirrhosis, and even liver cancer when the inflammatory response persists. Although the specific mechanism of NLRP3 inflammasome activation in HBV-infected individuals remains unknown, it may be indirectly induced by intrahepatic danger signals (e.g., ATP) released during infection (54). Hepatitis B core Ag and HBV X protein can promote NLRP3 inflammasome, IL-1β, and IL-18 expression (55, 56), whereas hepatitis B e Ag (HBeAg) suppresses NLRP3 activation and can partially offset IL-18 overexpression (57). Elevated levels of NLRP3, ASC, and IL-1β were detected in the adjacent tissues of patients with HBV-related liver cancer and were positively correlated with HBV-DNA (56). However, there is also evidence indicating that the mRNA level of NLRP3 in monocytes of CHB patients is unrelated to the HBV-DNA copy number/ml and HBeAg status (58). The disparate outcomes could be linked to the decrease in NLRP3 inflammasome levels as the chronic infection progresses. Analysis of inflammasome levels in PBMCs from patients with acute hepatitis B and CHB revealed elevated serum IL-1β and IL-18 levels solely in patients with acute hepatitis B, not in those with CHB (59).

On one hand, the innate immune mechanism plays an important role in clearing HBV. As an important component of innate immunity, the NLRP3 inflammasome and IL-1β are critical in the recognition and clearance of HBV. Yu et al. (57) have reported that suppressed NLRP3 inflammasome activation contributes to HBV persistence and immune tolerance. It has also been shown that the NLRP3 inflammasome, along with other NLRs such as NLRP12 and NLRP1, may trigger humoral immunity against HBV because they trigger an immune response against hepatitis B surface Ag (60). Moreover, increasing evidence indicates that the HBV has evolved numerous mechanisms to disrupt the NLRP3 inflammasome, thereby sustaining viral persistence. HBeAg-mediated inhibition of the NF-κB pathway and ROS as well as downregulation of NLRP3 and IL-1β secretion are examples (57).

On the other hand, the NLRP3 inflammasome mediates cytokines such as IL-1β and IL-18, which are implicated in the inflammatory response and damage of liver tissues. In the early stages of HBV infection, the changes in cytokine levels in the peripheral blood of patients were relatively minor (61). As the disease progresses, the levels of NLRP3, GSDMD, caspase-1, IL-1β, and IL-18 have been found to positively correlate with the extent of liver pathological inflammation in CHB patients (62). The NLRP3 inflammasome regulates pyroptosis by affecting its downstream production of pro–caspase-1 and pro–IL-1β, driving inflammation toward a progressive disease state and hastening the development of chronic liver conditions such cirrhosis and HBV-related acute-on-chronic liver failure (63, 64). In addition, NLRP3 knockout improved pathological changes in liver tissue, including lipid accumulation, dilated hepatocytes, focal inflammation, and collagen formation (65). Interestingly, NLRP3 was predominantly expressed in CD14⁺CD16⁺ intermediate monocytes (63). These findings contribute to a deeper understanding of the involvement of inflammasomes and immune cells in chronic diseases. In conclusion, in studying the interaction between innate and adaptive immunity in chronic HBV, the role of the NLRP3 inflammasome has not been fully elucidated and needs to be further explored.

COVID-19 is caused by infection with SARS-CoV-2 (66). Excessively elevated NLRP3 has been found in macrophages and monocytes infected in vitro, mouse infection models, and lung tissues of severe COVID-19 cases (67, 68). The NLRP3 inflammasome is activated by SARS-CoV-2 infection both directly and indirectly (69). More specifically, viral N and S proteins can facilitate NLRP3 binding to ASC, promote NLRP3 inflammasome assembly, and trigger the NLRP3 inflammasome directly (70). The NLRP3 inflammasome can also be activated by SARS-CoV E protein, spike 1 protein, and ORF3a through modifications to the K⁺ ion permeability, inhibition of mitophagy, and an increase of mitochondrial ROS generation (71–73).

The innate immune response helps inhibit pathogen spread. NLRP3 inflammasomes propagate inflammatory signals that alert the immune system to infection and play a vital role in viral clearance. Inflammasome activation prevents viruses from replicating efficiently in these sentinel immune cells. Inhibition of the NLRP3 inflammasome pathway, in contrast, leads to a significant increase in infectious viruses produced by infected macrophages (74).

Excessive activation of the NLRP3 inflammasome has been identified as a key contributor to the creation of the inflammatory cytokine storm (75), resulting in acute respiratory distress syndrome and serious injury to multiple organs, finally leading to death (67). After viral infection, infected, apoptotic pneumocytes activate macrophages and produce a substantial quantity of TNF-α and IL-1β, leading in immune cell recruitment and widespread NLRP3 activation, generating a proinflammatory positive cascade feedback loop (76, 77). Abnormal elevations of NLRP3, IL-18, and IL-1β were linked to lung damage and acute respiratory distress syndrome (77–79). IL-1β and IL-18 are two potent proinflammatory cytokines with prothrombotic action, which may be involved in the formation of immune thrombosis. Moreover, prothrombotic endothelial and platelet reactions were involved (80). Upregulated GSDMD levels are associated with the release of neutrophil extracellular traps, which together contribute to organ damage and microvascular thrombosis in COVID-19 patients (81). Interestingly, NLRP3 may also be involved in COVID-19–related adverse reactions or diseases, including follicular conjunctivitis and bulbar congestion (82), adverse pregnancy outcomes (82), and periodontitis (83), among others.

Influenza A virus (IAV) infection can cause acute respiratory illness. The symptoms and severity of IAV infections differ among individuals, and one of the causes for severe infection is abnormal cytokine and cellular inflammatory responses (84). The processes behind NLRP3 activation in IAV-infected cells are unknown, and lysosomal rupture, ionic imbalances, mitochondrial disruption, trans-Golgi network dispersion, and ROS generation may be involved (85–88).

NLRP3 protects against infection in the early stages but contributes to disease pathogenesis later in infection. The NLRP3 inflammasome is critical in the body’s response to influenza virus interference. Specifically, many studies have shown that NLRP3 deficiency leads to more serious diseases, and enhanced NLRP3 activity can improve the body’s resistance to IAV infection (89, 90), including hyperactivation of the NLRP3 inflammasome and subsequent neutrophil recruitment caused by IL-1β. The protective effect of NLRP3 is also manifested in helping virus clearance during influenza virus interference (90). NLRP3 is also a key factor in regulating the cytodifferentiation of Th2, Th17, and regulatory T cells, suggesting that it may play a facilitating role in the adaptive immune response during influenza virus infection (91–93). Meanwhile, NLRP3 plays a crucial role in the repair of lung injury caused by influenza virus infection. Animal experiments have shown that NLRP3 gene–deficient mice showed bronchiolar obstruction caused by neutrophils, macrophages, and necrotic cells, as well as a large amount of collagen deposition in alveoli and pulmonary lobules after IAV infection, whereas normal mice showed no relevant pathological manifestations in bronchioles, and only a small amount of collagen deposition in alveoli (94).

The protective effect of the NLRP3 inflammasome appeared to be IAV dose related, because when a modest dosage of IAV was given, NLRP3 deficiency had no effect on mortality. However, delayed administration of MCC950 until day 3 or 7 of infection alleviated disease and reduced lung inflammation, indicating that overexpression of the NLRP3 inflammasome contributes to disease pathogenesis later in infection (95). Moreover, NLRP3-dependent IL-1β has been observed to rapidly rise during the course of a severe infection, exacerbating hyperinflammation and illness (96). Additionally, IAV also triggers acute exacerbation of chronic obstructive pulmonary disease by increasing the secretion of proinflammatory cytokines such as caspase-1, IL-1β, and IL-18 through NLRP3 inflammasome activation (97). The possible mechanism is that IAV nucleoprotein induces IL-1β production by activating the NLRP3 inflammasome, which subsequently leads to an increase in tryptase and viral infectivity by enhancing proteolytic cleavage of envelope protein hemagglutinin (98). Current IAV infection therapies are limited, and a better knowledge of the processes of NLRP3-dependent inflammation during severe IAV infections may lead to novel therapeutic options.

AIDS is a chronic infectious disease without a known cure, which is caused by HIV-1 attacking the human immune system and forming immune deficiency (99). Previous studies have demonstrated that HIV infection activates the NLRP3 inflammasome, and the mechanism remained poorly understood, as HIV-1 transactivator of transcription protein (38), LPS, and IFN-γ–inducible protein 16 may be involved (100).

The role of the NLRP3 inflammasome in HIV-1 infection is double-faced. On one hand, activation of the NLRP3 inflammasome is associated with a better control of HIV-1 infection. In the early stage of HIV infection, the NLRP3 inflammasome pathway plays a protective role in inhibiting viral entry and intracellular accumulation of viral nucleocapsid (101). NLRP3 protein can inhibit HIV-1 infection through the functional crosstalk with purinergic receptor P2Y2 by repressing the F-actin remodeling required for HIV-1 entry (102). Moreover, NLRP3 inflammasomes are involved in the activation of dendritic cells by vaccine adjuvants, and the loss of NLRP3 inflammasomes will lead to the loss of adjuvant response (103). Therefore, defects in NLRP3 inflammasomes may be one of the reasons for the lower range of immune responses in HIV–infected patients (104). In addition, gain-of-function variants of the NLRP3 inflammasome and IL-18 gene help chronic HIV-1–infected individuals be more protected than noncarriers against gut damage and microbial translocation, as well as the consequent systemic activation and exhaustion of immune cells (105).

On the other hand, activity of the NLRP3 inflammasome contributes to CD4⁺ T cell loss, cardiovascular disease, neurocognitive impairment, and HIV-1 persistent infection. The NLRP3 inflammasome participated in mediating caspase-1 activation and pyroptosis, which triggered abortive infection induced by cell-to-cell viral spread, killed 95% of quiescent lymphoid CD4⁺ T cells, and was recognized as the dominant pathway in CD4⁺ T cell loss (106). The decreased and even exhausted level of CD4⁺ T cells in lymphoid tissue exposes the host to opportunistic infections and cancer and has long been considered as the core of the pathogenesis of HIV-1 (107). In addition, overexpression of NLRP3 inflammasome components and downstream cytokines (caspase-1, IL-1β, and IL-18) increases foam cell formation, resulting in a higher risk of cardiovascular disease in HIV patients (108). However, inhibition of NLRP3 activity reduced foam cell formation and expression (109). NLRP3 has also been recognized as a possible biomarker related to neurocognitive impairment and increases with increasing illness severity (110), which continues to afflict almost 50% of HIV-1–infected individuals. Specifically, NLRP3 inflammatory activation in microglia leads to increased expression and extracellular secretion of the proinflammatory factors IL-1β and IL-18, as well as the neurotoxic factors TNF-α, IL-1α, and C1q. NLRP3 inflammatory activation leads to neurotoxicity and neurodegeneration by mediating microglial cell pyroptosis (111). Bandera et al. (112) found that the NLRP3 inflammasome and caspase-1 may be involved and play an important role in the lack of immunological response and higher immune activation status of immunological nonresponders during antiretroviral therapy and affect the effectiveness of treatment.

The understanding of the pathophysiology of infectious illnesses has been improved by the discovery of inflammasomes. The full release of cytokines represented by NLRP3 is crucial for the early defense of pathogen in humans, and keeping an anti-inflammatory cytokine response at this point aids in resolution of disease. However, prolonged abnormal NLRP3 inflammatory activation has been found to be associated with severe infections, persistent infections, immune cell depletion, tissue damage, drug tolerance, coagulation disorders, and neurologic injury. This may be the reason for the dual function in infectious disorders of the NLRP3 inflammasome.

However, certain NLRP3 inflammasome processes remain unclear, and some are even debatable and inconsistent, including the specific mechanism by which NLRP3 is activated, the consumption of immune cells by long-term elevated NLRP3, and the relationship between other inflammatory pathways and the NLRP3 inflammasome. In addition, studies on the function of the NLRP3 inflammasome in two or more infectious illnesses are few. Coinfection frequently comes with a bad prognosis, a high risk of severe illness, and fatality (113). The intricate interplay between pathogen and the inflammasome encompasses numerous complex pathological mechanisms, with different research perspectives potentially illuminating only a fraction of this complexity. Further investigation is warranted to comprehensively unveil the multifaceted role of NLRP3 in infectious diseases. Understanding the host response to pathogen infection and NLRP3 inflammatory hyperactivation can help develop more effective treatment regimens and improve clinical outcomes.

At present, many infectious diseases are still a global health burden, and the existing treatment strategies need to be supplemented. Due to its crucial function in infectious illnesses, NLRP3 inflammasome inhibition has the potential to be a more effective, economical, and precise technique on a variety of levels (114). In this section, we review recently discovered biological small compounds as well as pharmacological NLRP3 inflammasome inhibitors. The mechanism of action of these compounds was demonstrated based on data from tests both in vivo and in vitro.

Small molecules directly targeting NLRP3 are developing rapidly. Some small molecule compounds are able to block the ATPase activity of NLRP3. Representative small molecules include MCC950 and BAY 11-7082. MCC950 is a potent and well-studied inflammasome lead compound, which forms interactions with multiple substructural domains in NACHT, inhibits NLRP3 ATPase activity, and maintains NLRP3 in an inactive state (115, 116). MCC950 also inhibits the development and release of IL-1β and IL-18 as well as the activation of caspase-1 (90). MCC950 has shown a therapeutic effect in HBV-related liver damage (65), a juvenile mouse model of IAV infection (95), and IAV-induced COPD rat models (97). The therapeutic effect is mainly manifested in improving hepatocellular ballooning, inflammation, and collagen formation. Notably, although MCC950 treatment improved pathology and fibrosis in the early stage of the disease, it aggravated the pathological progression at day 22 postinfection. The reason may be related to the dual role of NLRP3. During the SARS-CoV-2 pandemic, the potential therapeutic effect of MCC950 in alleviating immune overactivation was considered, which mainly focuses on reducing inflammation, alleviating the pathology in lung tissues, and reducing the mortality of infected mice (117, 118). It has been found that the therapeutic effect of NLRP3 may be related to the combined inhibition of virus-encoded open reading frame 8 (ORF8) protein (119). However, MCC950 did not affect virus clearance (95). Considering the complex issues such as the pharmacokinetic and toxic properties, a large number of studies remain preclinical (120). BAY 11-7082 can also inhibit the ATPase activity of NLRP3 (121). BAY 11-7082 limits SARS-CoV-2 infections. Treatment with BAY 11-7082 significantly reduced the single-guide RNA level of SARS-CoV-2 N protein in vitro (121).

Direct inhibitors of NLRP3 inflammatory that work through binding to NLRP3 and affecting protein–protein interactions are ADS032 and tranilast. ADS032, a novel dual inhibitor of NLRP1 and NLRP3, can directly interact proximal to the Walker B motif within the NACHT domain of NLRP3, thus reducing the secretion and maturation of IL-1β and blocking the formation of spots. ADS032 protects mice from lethal IAV attack, increases survival, and reduces lung inflammation (122). Tranilast can also directly bind to NACHT, disrupt NLRP3–NLRP3 interactions, and block oligomerization (123). Tranilast has been found to inhibit HBV replication and dose-dependently reduce HB Ag levels without affecting the cell viability of hepatocytes (124). Tranilast also reduces TGF-β activation induced by IAV infection (125).

In addition to directly inhibiting the NLRP3 inflammasome, some pharmacological strategies can also indirectly inhibit the inflammasome by acting on its upstream and downstream complex signaling pathways, including NF-κB/NLRP3 signaling, IL-1 receptor and GSDMD cleavage inhibition, and others.

The representative drugs acting on the upstream pathway of the NLRP3 inflammasome are colchicine, statins, melatonin, sulfasalazine, and baricitinib. The potential therapeutic effect of colchicine, a bioactive constituent from an ancient medicinal herb, as an anti-inflammatory drug in the novel coronavirus epidemic has attracted wide attention but has not yet been approved for treatment (80). It can inhibit the NLRP3 inflammasome by preventing caspase activation, inhibiting P2X7 receptors, interfering with inflammasome oligomerization, decreasing pyrin gene expression, and repressing NLRP3 promoter acetylation by inhibiting STAT3 phosphorylation (126–128). Colchicine has previously shown therapeutic potential in many infectious diseases, including CHB and AIDS (129). During the COVID-19 outbreak, a number of clinical studies have fully proved that it can prolong the time of clinical deterioration (130), improve survival rate, reduce the risk of death (131), and reduce the length of supplemental oxygen therapy and hospitalization (132). However, it is noteworthy that there are also reports of serious adverse events after taking colchicine in patients with novel coronavirus (133) and that diarrhea and nausea were the main symptoms (134). Therefore, the balance between patient benefit and safety should be considered in clinical use. Similarly, statins have received attention because of their strong anti-inflammatory properties and induction of autophagy. Statins can inhibit the NLRP3 inflammasome through regulating the TLR4/MyD88/NF-κB pathway and downregulate expression of its downstream cytokines IL-18 and IL-1β. However, clinical studies on statins in the treatment of CIVID-19 are ongoing (NCT04372589, NCT04345848) (135). Of note, the combination of colchicine and rosuvastatin has been found to reduce mortality and the need for mechanical ventilation (136). Melatonin is a potent antioxidant and anti-inflammatory molecule, which specifically inhibits the NLRP3 inflammasome. It also downregulates NF-κB expression and releases the anti-inflammatory cytokines IL-4 and IL-10 (137). A recent clinical trial evaluated the therapeutic effects of melatonin in inhibiting the NLRP3 inflammasome in COVID-19 patients and found that melatonin could effectively reduce the inflammasome polyprotein complex (TNF-α and IL-1β) and oxidative stress (MDA and NO levels), thereby alleviating the cytokine storm in lung tissue (138). Sulfasalazine could downregulate the expression levels of NLRs, ASC, IL-1β, and IL-18 by interfering with the NF-κB pathway, so as to play a role in immune regulation in the environment of HIV infection (139). Baricitinib, which can regulate the NF-κB/NLRP3 pathway (140), has been found to reduce mortality in patients with COVID-19 as a complementary therapy (141).

Caspase-1, downstream of NLRP3, is involved in the induction of pyroptosis and plays an important role in various infectious diseases. Therefore, there are many studies focused on caspase-1 inhibitors. Salmeterol, a β₂-adrenergic receptor agonist, can inhibit the cleavage of caspase-1, reduce the release of TNF-α and IL-1β, and the mechanism may be related to the classic cAMP/PKA signaling pathway (142). Most recently, it has been found that salmeterol has the characteristics of anti-IAV, which can improve the spleen morphology of IAV-infected mice, significantly increase the CD4⁺/CD8⁺ ratio of lymphocytes, and improve immune function (143). VX-765, a caspase-1 inhibitor, was found to improve CD4⁺ T cell homeostasis and reduce viral load and immune activation early after HIV-1 infection (144).

Studies on biological agents that inhibit NLRP3 inflammasome have also been carried out, which may inhibit the inflammasome’s activation, or upstream and downstream products, to prevent the inflammatory cascade.

The effect of direct inhibitors of the NLRP3 inflammasome is clear, including neuronal precursor cell–expressed developmentally downregulated 4 (NEDD4) and vitamin D3. NEDD4 inhibits NLRP3 by promoting proteasome degradation through direct binding to NLRP3 and mediates NLRP3 degradation through ubiquitination of SF3A2 (145). A recent study found that NEDD4 can inhibit NLRP3 and regulate GSDMD-mediated pyroptosis, inhibiting the synergistic inflammation caused by coinfection of IAV and Streptococcus pneumoniae (146). Vitamin D3 has shown beneficial therapeutic effects in clinical studies of novel coronavirus and influenza (147, 148). Recently, vitamin D3 has been found to reduce excessive inflammation by inactivating the NLRP3 inflammasome, and its mechanism may be to bind to NLRP3, block NLRP3 binding to BRCC3, and influence NLRP3 deubiquitination (149).

Other indirect inhibitors act on the upstream and downstream signaling pathways of the NLRP3 inflammasome. Biologics that act upstream are Lactiplantibacillus plantarum–derived postbiotics and epigallocatechin-3-gallate. Postbiotics, which are metabolites and bacterial components of probiotics, possess similar or even better therapeutic effects with probiotics. Lactiplantibacillus plantarum–derived postbiotics can downregulate the gene expression of NLRP3, caspase-1, IL-1β, and IL-18 and protect against Salmonella infection, which may be related to the induction of autophagy by triggering the AMP-activated protein kinase (AMPK) signaling pathway (150). Epigallocatechin-3-gallate, a polyphenolic catechin extracted from green tea, has been shown to inhibit activation of the NLRP3 inflammasome through the HMGB1/NLRP3 signaling pathway and reduce liver injury and fibrosis caused by HBV (151).

Therapeutic strategies targeting IL-1β, the downstream of NLRP3 inflammasome, have also been extensively studied. Canakinumab and anakinra are IL-1 receptor inhibitors that have been shown to be a therapy for an excessive inflammatory response to COVID-19 (152). Canakinumab, a neutralizing mAb of IL-1β, is associated with improved overall mortality, decreased serum C-reactive protein levels, and reduced need for mechanical ventilation (153, 154). Anakinra, a human IL-1 receptor antagonist, has been found to shorten hospital stay, improve clinical status, and reduce 28-d mortality (155). However, there is also systematic analysis of randomized controlled studies showing that the therapeutic effects are limited, although safe (156). Different conclusions may be related to different research protocols, different basic conditions of the patients included, and different severity of the disease. We expect more studies to perform subgroup analysis on baseline characteristics to further evaluate the therapeutic effect. To elucidate the mechanism of indirect inhibitors, more research is necessary because of the intricate processes behind the NLRP3 inflammasome.

The ability of phytochemicals to prevent and treat infectious illnesses by inhibiting the activation of the NLRP3 inflammasome has sparked intense attention among scientists. Due to its diverse therapeutic targets and few side effects, traditional Chinese medicine is frequently used to treat infectious disorders. Traditional Chinese medicine extracts may currently be proven to block activation of the NLRP3 inflammasome and have therapeutic benefits on a number of infectious disorders by several in vivo and in vitro investigations.

Some phytochemicals can directly act on NLRP3 inflammasome activation, including Korean red ginseng and celastrol. Ginsenosides from Korean red ginseng inhibit the initiation and activation of NLRP3 inflammasome activation and attenuate ASC pyroptosome formation (157). Korean red ginseng treatment of HIV-1–infected patients slows the depletion of CD4⁺ T cells, prolongs the lifespan of AIDS patients, and acts as an antiviral agent by delaying coreceptor switching (158). Celastrol, a substance derived from Celastraceae, can stop NLRP3 from combination and from being deubiquitinated at K63, which prevents the generation and secretion of IL-18 and IL-1β (159). In vitro, celastrol is found toxic to planktonic S. aureus and also active against clinical methicillin-resistant S. aureus (160).

Additionally, numerous phytochemicals can indirectly act on NLRP3 inflammatory activation. Triptolide, Scutellaria baicalensis, isoforskolin, phytosphingosine, notoamides, wampee leaf volatile oil emulsions, and berberine can affect the upstream pathway of NLRP3. Triptolide (TP), an ingredient in the traditional Chinese medication Tripterygium wilfordii, is a diterpene lactone that has been epoxidized. It has been revealed recently that TP possesses antiviral properties that include IAV, HIV, and human papillomavirus (161). The primary function of TP is to control IL-6, TNF-α, and IL-1β levels by blocking the NF-κB/NLRP3 signaling pathway, thus reducing the cytokine storm caused by influenza virus. The anti-influenza virus and anti-inflammatory properties of S. baicalensis may be connected to the suppression of the NF-κB/NLRP3 pathway, which lowers caspase-1 activation and IL-1β production (162). It was further verified in the IAV-infected macrophage model that S. baicalensis may prevent NF-κB p65 from translocating into the nucleus (163). Isoforskolin is a plant medicine produced in Yunnan, China. Isoforskolin can regulate NF-κB/NLRP3 pathways, improve lung function by reducing inflammatory mediator levels (e.g., TNF-α, IL-1β, IL-6), and reduce inflammation, thus alleviating acute exacerbation of chronic obstructive pulmonary disease caused by influenza virus infection (164). Phytosphingosine, a product from rumen fluid and milk of cows, has recently been found to improve S. aureus–induced mastitis by inhibiting the activation of the NF-κB/NLRP3 signaling pathway (165). Notoamides, a group of prenylated indole alkaloids, have potential biological activities such as antibacterial, antiparasitic, and insecticidal activities (166). Notoamides can improve mitochondrial damage to block NLRP3 inflammasome-induced pyroptosis, thereby inhibiting pathogenic S. aureus ATCC 29213 (167). Wampee leaf volatile oil emulsions have recently been found to induce the formation of voids and abnormal binary fission in S. aureus and significantly inhibit the formation of cell biofilms, which may be related to its inhibitory effect on the NF-κB/NLRP3 pathway (168). By inhibiting NLRP3 inflammasome activation and downregulating GSDMD-mediated pyroptosis (169), berberine can enhance survival rates, lessen lung inflammation, and lessen weight loss in mice with influenza virus pneumonia and induce regular mitophagy (170). The possible mechanisms may be induction of mitochondrial autophagy and inhibition of mitochondrial ROS production (171).

At present, the development and application of antibiotics, antiviral drugs, and vaccines have controlled the harm of infectious diseases to a certain extent, but there are still some infectious diseases such as viral hepatitis, influenza, and others that are still widespread. Coupled with the repeated epidemic of new infectious diseases such as COVID-19, it has caused a heavy medical and social economic burden. Due to its effective immune regulation and antiviral effects, the NLRP3 inflammasome and related upstream and downstream pathway inhibitors have become a research hotspot in infectious diseases. In this review, we summarize the research progress of small molecules, biologics, and phytochemicals targeting NLRP3 inflammasomes and upstream and downstream pathways, which are expected to achieve significant results in various disease models. Most inhibitors target the excessive damage to the immune system, organs, and tissues caused by NLRP3 inflammatory overactivation. This may be due to the fact that the mechanisms investigated for the protective effects of NLRP3 are unclear and the short time window for its protective effects. However, from the information we have so far, appropriate modulation of NLRP3-mediated intrinsic immunity in the early stages of pathogen infection is beneficial. It helps to cut off the course of the disease early and protects the organism from excessive inflammatory damage and chronic damage.

Even with the aforementioned successes, a few issues remain. Enhancing our comprehension of the pathogenic mechanism and life cycle of infectious pathogens is vital, as is conducting more research on the critical connections underlying their pathological processes. To get a better understanding of the process behind host immune system diseases, it is imperative to elucidate the specific method by which NLRP3 interacts with host cells. These insights will ultimately aid in the advancement of treatment plans centered on the NLRP3 inflammasome. In addition, there are more questions to be answered about the safety, potential side effects, economic value, and other factors related to NLRP3 inflammasome inhibitors, as well as their possible synergies with other inflammatory pathways. In the future, it is urgent to carry out large-scale, multicenter clinical studies to evaluate its efficacy alone and the superiority of combined application.

The activation of the NLRP3 inflammasome together with the production of IL-1β and IL-18 are a key step in the innate immune response to invasive infections and show a complex duality. It has been revealed that NLRP3 plays a beneficial role in the early stage of infection by promoting immune recognition and pathogen clearance. As the disease progresses, NLRP3 plays a role in fibrosis and the inflammatory response in different tissues, the immune tolerance mechanism of virus, the consumption of lymphocytes, and other factors, showing a negative effect. Further study on the mechanism of the dual role of NLRP3 is required to more clearly understand the pathophysiological involvement of the NLRP3 inflammasome in infectious disorders. Given the complexity of the NLRP3 inflammasome, it is necessary to examine its role in different tissues, different types of cells, and different stages in more detail, and to explore the mechanism behind the different therapeutic effects of NLRP3 inhibitors. The multilevel regulation of the NLRP3 inflammasome has shown effectiveness in clinical studies and basic models of various infectious diseases. Notably, considering the complex mechanism of the NLRP3 inflammasome in different stages of infectious disease, the timing of intervention for NLRP3 inhibitors seems to be crucial. We noticed that some drugs, such as MCC950, showed opposite effects at different stages of the disease. In some instances, excessive immune regulation may be beneficial to virus transmission, and delayed immune regulation may be ineffective for chronic tissue damage. Therefore, future research should focus on the optimal timing of drug application. Undoubtedly, future research will uncover fresh approaches to treating infectious disorders. We should further define the drug’s target range in the inhibitor screening procedure going forward and work to prevent any negative effects.

The authors have no financial conflicts of interest.

We are grateful to the Yingke Qianxin team for helping us to search the literature.