key: cord-262863-f07v5uk8
authors: Bertocchi, Ilaria; Foglietta, Federica; Collotta, Debora; Eva, Carola; Brancaleone, Vincenzo; Thiemermann, Christoph; Collino, Massimo
title: The hidden role of NLRP3 inflammasome in obesity‐related COVID‐19 exacerbations: lessons for drug repurposing
date: 2020-08-09
journal: Br J Pharmacol
DOI: 10.1111/bph.15229
sha: 
doc_id: 262863
cord_uid: f07v5uk8

COVID‐19, the illness caused by SARS‐CoV‐2, has a wide‐ranging clinical spectrum that, in the worst‐case scenario, involves a rapid progression to severe acute respiratory syndrome and even death. Epidemiological data show that obesity and diabetes are among the main risk factors associated with high morbidity and mortality. The increased susceptibility to SARS‐CoV‐2 infection documented in obesity‐related metabolic derangements argues for initial defects in defense mechanisms, most likely due to an elevated systemic metabolic inflammation (“metaflammation”). The NLRP3 inflammasome is a master regulator of metaflammation and has a pivotal role in the pathophysiology of either obesity and diabetes. Here, we discuss the most recent findings suggesting contribution of NLRP3 inflammasome to the increase in complications in COVID‐19 patients with diabesity. We also review current pharmacological strategies for COVID‐19, focusing on treatments whose efficacy could be due, at least in part, to interference with the activation of the NLRP3 inflammasome.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel member of human betacoronavirus (mutational ssRNA viruses) causing the disease named (for coronavirus disease 2019), which was recognized as a pandemic on March 11, 2020 by the World Health Organization (WHO). COVID-19 is a syndrome with a wide clinical spectrum: it can be asymptomatic or, in the majority of cases, cause mild symptoms that are indistinguishable from other respiratory infections, but it may also lead to a rapid progression to severe acute respiratory syndrome (SARS), with respiratory failure and even death (Zhou et al., 2020) .

The activation of signalling receptors of the innate immune system is the first step of the physiological response to virus infection. However, when excessive, this innate immune system activation may evoke hyper-inflammation and tissue damage in patients with severe COVID-19, making it an important aspect of the pathophysiology of this syndrome (Netea et al., 2020) .

Older people are more likely to suffer from the most serious complications of SARS-CoV-2 infection: this is likely due to the physiological decrease in the efficacy of the immune system and the simultaneous increase in age-related inflammatory response (Latz & Duewell, 2018) .

Moreover, aging and the modern lifestyle typical of Western societies are both associated with an increase in comorbidities, such as "diabesity", the association of obesity and diabetes, which defines a combination of primarily metabolic disorders evoked by impairment of both lipids and sugars metabolisms (Potenza, Nacci, De Salvia, Sgarra, Collino & Montagnani, 2017; Tschop & DiMarchi, 2012) . The close relationship existing between impaired metabolism and immunity in obese-related metabolic derangements is well documented, and the term "metaflammation" has been coined to indicate a pathophysiological inflammatory response resulting from metabolic alterations and dysfunctions (Hotamisligil, 2017; Mastrocola, Aragno, Alloatti, Collino, Penna & Pagliaro, 2018 ). An increasing number of studies report that patients with severe obesity or type 2 diabetes mellitus (T2DM) exhibit an increased circulating concentration of proinflammatory cytokines, such as IL-1β, IL-6 and TNF-α . The metaflammation induced in response to excess calories intake in the adipose tissue may subsequently involve other organs, such as the skeletal muscle, liver, heart and lung, leading to metabolic and cardiovascular alterations.

Compared with people of a normal weight, obese individuals have an increased susceptibility to develop chronic diseases and infections (Huttunen & Syrjanen, 2010; Milner & Beck, 2012; Wolowczuk et al., 2008) . Patients with T2DM developing COVID-19 are twice as likely to require ventilation and intensive care unit (ICU) supports, and their mortality is 3fold higher than in non-diabetic COVID-19 patients Yang et al., 2020) .

Obesity carries a 5-fold increased risk of developing severe pneumonia from COVID-19 (Cai et al., 2020) and diabesity is a well-recognised risk factor for severe infections (Almond, Edwards, Barclay & Johnston, 2013; Huttunen & Syrjanen, 2013) , sleep apnoea (Dixon & Peters, 2018) , poor immune response and scant outcomes in patients with respiratory disease (Green & Beck, 2017) . According to United States data, 37% of 3,615 hospitalized COVID-19 cases were obese, and the obesity increased the odds of ICU admission (Lighter et al., 2020) . In a French study enrolling COVID-19 patients admitted to the ICU, the requirement for ventilation was increased by 7-fold in patients with a body mass index (BMI) > 35 kg/m 2 , and this was independent of age and presence of diabetes or hypertension (Simonnet et al., 2020) . Direct comparison of odds ratios for the risk of SARS in obese and/or diabetic subjects in comparison to other lung disorders such as asthma, pulmonary hypertension and pneumonia in the same populations, would be useful to understand the specific contribution of SARS-CoV-2 for both severity and prevalence of a specific organ injury. Adipocytes express high levels of angiotensin-converting enzyme-2 (ACE2), which is the receptor for SARS-CoV-2 (Gembardt et al., 2005; Gupte et al., 2008) . Similar to infections with either human or simian immunodeficiency virus (HIV/SIV), high virus concentrations in adipose tissue may be secondary to high levels of binding cells (CD4+). Indeed, it is possible that SARS-CoV-2 accesses adipose tissue more easily due to the high ACE2 expression, possibly generating a viral "reservoir" in adipose tissue (Wanjalla et al., 2019) . So far, most of the experimental findings have evaluated ACE2 gene expression only, whereas its parallelism with protein expression as well as enzyme functionality still needs to be confirmed to better clarify the effective role of ACE2 in facilitating SARS-CoV-2 localization in adipose tissue.

As widely documented (Huttunen & Syrjanen, 2013; Singh, Gupta, Ghosh & Misra, 2020) , obese people and individuals with T2DM are more likely to develop a dysfunctional immune response that provokes viral infection and makes failed the elimination of the pathogen.

Specific reasons for this are not still understood, even if one reasonable explanation could be an excessive local inflammatory response, often due to an over-activation of specific inflammatory pathways, including, but not limited to, the NLRP3 inflammasome.

NLRP3 inflammasome is a large, multimeric protein complex assembled within the cell upon the recognition of unique molecular patterns by a germline-encoded pattern recognition receptor (Yang, Wang, Kouadir, Song & Shi, 2019) . Among the pattern recognition receptors known to form inflammasomes, NLRP3 belongs to the family containing both a nucleotide-binding oligomerization domain (NOD) and a leucine-rich repeat-containing proteins. NLRP3 is expressed in several cell types, as innate immunity, endothelial, hematopoietic, lung epithelial, kidney and cardiac cells (Ratajczak & Kucia, 2020) . We and others have previously demonstrated that NLRP3 inflammasome over-activation contributes to the pathogenesis of cardio-metabolic disorders (Mastrocola et al., 2016a; O'Riordan et al., 2019; Wang et al., 2020; Zuurbier et al., 2019) , whereas NLRP3 deficiency results in reduced systemic inflammation, along with a decreased immune cell activation and improved insulin resistance (Grant & Dixit, 2013; Vandanmagsar et al., 2011) . NLRP3 inflammasome is also a crucial player in immune defence of the host against many pathogens, including viruses . Once activated, its amino-terminal pyrin domain is able to recruit the downstream adaptor protein apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC). In addition, the amino-terminal pyrin domain triggers the assembly of the scaffold complex important to recruit the inflammasome effector, procaspase-1, which becomes activated. Active form of caspase-1 cleaves both pro-IL-1β and pro-IL-18 into their biologically active forms (Dinarello, 2009 ). Caspase-1 can also activate an additional substrate, the gasdermin D, which mediates pyroptosis by creating pore channels in cell membranes. Pyroptosis is a lytic form of necrosis used by the innate immune system to disrupt pathogen replication and intracellular accumulation, due to the formation of pore-induced intracellular traps (Jorgensen, Zhang, Krantz & Miao, 2016) . Despite its role in facilitating pathogen clearance, inflammasome activation can also be detrimental to the host by enhancing viral dissemination (Lupfer, Malik & Kanneganti, 2015) . To date, it is still unclear whether SARS-CoV-2 activates the NLRP3 inflammasome (Yap, Moriyama & Iwasaki, 2020) , but this is likely, as SARS-CoV also activates the NLRP3 inflammasome by modulating either ion channel activity or ASC ubiquitination. Moreover, after binding to SARS-CoV, ACE2 is internalized, leading to high cytosolic levels of angiotensin II, which is known to act as an activator of the NLRP3 inflammasome in lung, kidney cells and cardiomyocytes (Deshotels, Xia, Sriramula, Lazartigues & Filipeanu, 2014; Wang et al., 2008) . In particular, activation of the ACE2/Ang(1-7) axis leads to inhibition of NLRP3 inflammasome associated with the downregulation of angiotensin II-induced mir-21 (Sun et al., 2017) . It is possible that a similar pathway may be activated by SARS-CoV-2. In addition, given that caspase-1 affects expression of several innate immunity genes and viral replication (Bauer et al., 2012) , the NLRP3 inflammasome may alter cellular physiology and homeostasis also modulating gene expression, leading to altered innate antiviral defence pathways and impacting therefore on host responsiveness to pathogens like SARS-CoV-2.

Diabesity is a high-risk factor for both influenza infections and hospitalization for respiratory illness during seasonal influenza (Karlsson, Sheridan & Beck, 2010; Kwong, Campitelli & Rosella, 2011) . Thus, we may speculate that the chronic over-activation of the NLRP3 inflammasome, due to metabolic impairments, may contribute to alterations in the innate immune response to viral infection, making it more permissive and severe ( Figure 1 ).

The activation of both innate and adaptive immunity after recognition of viral antigens produces a large amount of pro-inflammatory cytokines and chemokines even in moderate cases of COVID-19. In some patients, this activation exacerbates, leading to a 'cytokine storm', which in turn causes severe lung injury, multiple organ failure and, eventually, death (Sarzi-Puttini et al., 2020; Xu et al., 2020) . The mechanisms by which SARS-CoV-2 subverts the body's innate antiviral cytokine responses are yet to be elucidated. Nevertheless, most recent research on SARS-CoV suggests that pyroptosis may have a pivotal role. Pyroptosis is a form of programmed cell death with an high inflammatory scenario that is commonly observed with cytopathic viruses (Fink & Cookson, 2005) . As mentioned before, NLRP3 inflammasome regulates pyroptosis by gasdermin D-mediated membrane rupture along with spontaneous release of cytosolic contents into the extracellular spaces. The activation of pyroptosis in alveolar macrophages and in recruited monocyte-derived macrophages by SARS-CoV-2 aggravates pneumonia and acute respiratory distress syndrome (ARDS). The release of alarmins, including viral particles, ATP, reactive oxygen species (ROS) as well as cytokines, chemokines and lactate dehydrogenase (LDH), elicits an immediate reaction from surrounding immune cells, inducing a pyroptotic chain reaction. Different studies have reported elevated levels of LDH, a cytosolic enzyme that is measured for monitoring pyroptosis (Rayamajhi, Zhang & Miao, 2013) , in patients with the severe form of the disease and worse outcomes (Lippi, Mattiuzzi, Bovo & Plebani, 2020) . In keeping with this and other observations, we can suggest the NLRP3 inflammasome-mediated pyroptosis as one of the key mechanisms involved in COVID-19 ( Figure 1 ). In accordance, a further exacerbation of symptoms may derive from the pyroptosis induced release of viral RNA and antigens that, by reaching the circulation, may generate immune complex and depositions in other target organs, initiating severe inflammatory cascades. Hence, in addition to local damage, cytokine storm also has ripple effects across the body, evoking organ damage and multi-organ failure (Ruan, Yang, Wang, Jiang & Song, 2020) .

Resolution of inflammation is an active process triggered by the onset of inflammation itself (Perretti & D'Acquisto, 2009) . The main actors of this process consist of specialized proresolving mediators (SPMs), whose list is constantly updated and includes annexin A1 (AnxA1), formyl-peptide receptor 2 (FPR2) and lipoxin A4 (Brancaleone et al., 2013; Dufton et al., 2010; Norling, Dalli, Flower, Serhan & Perretti, 2012; Serhan, Chiang & Dalli, 2015) .

Very recently, AnxA1 has been reported to be involved in the regulation of NLRP3 inflammasome function (Galvao et al., 2020) , independently from the binding to its cognate receptor FPR2 (Galvao et al., 2020) . Indeed, lower levels of AnxA1 are associated to an increased activation of NLRP3, thus raising IL-1β release (Kelley, Jeltema, Duan & He, 2019 ). In addition, AnxA1 is also known to trigger resolution pathway by engaging FPR2 (Brancaleone, Dalli, Bena, Flower, Cirino & Perretti, 2011; Perretti & D'Acquisto, 2009 ) and (Machado et al., 2020) AnxA1/FPR2 signaling axis has been identified to control and limit viral infection (Alessi, Cenac, Si-Tahar & Riteau, 2017; Lopategi et al., 2019) . Therefore, impairment of this axis could unbalance the immune response to infections, thus generating an ineffective response, which in turn could lead to increased cytokine storm and exacerbation of COVID-19 symptoms.

The specific reasons for the higher susceptibility of obese and diabetic patients to SARS-CoV2 are unclear, but chronic exposure to a low-grade NLRP3 inflammasome-dependent inflammatory response may well be a key driver. Diet-induced alterations in the gut microbiome and related increased gut leakiness of bacterial wall lipopolysaccharides (endotoxins) are known to promote activation of NLRP3 inflammasome by Toll-like receptor (TLR) pathways. This event is followed by the accumulation of IL-1 family cytokines, which modulate the function of insulin producing pancreatic -cell (Tack, Stienstra, Joosten & Netea, 2012) . We and others have demonstrated that NLRP3 inflammasome over-activation is involved not only in the pathogenesis of diabesity, but also in the exacerbation of related cardiovascular injuries, including myocardial infarction, and this process is associated to an increase in the local inflammatory response. At the same time, a decrease in the efficiency of endogenous protective responses occurs (Mastrocola et al., 2016a; Xu et al., 2013) . NLRP3 inflammasome activation is involved in endothelial lysosome membrane permeabilization, cathepsin B release, impaired glycocalyx thickness (Ikonomidis et al., 2019) , thus further contributing to the enhanced susceptibility to cardiovascular injury. Similarly, the diabesityrelated basal activation of the NLRP3 inflammasome cascade, leading to increase in either gastrointestinal or vascular permeability, may contribute to exacerbate SARS-CoV-2 systemic diffusion and enhance the intricate mechanisms of intracellular cross talk operational in the pathogenesis of COVID-19.

To date, there are no specific medications available to treat COVID-19. Clinical trials are in process on several drugs, mainly based on the drug repurposing approach to redevelop a compound/drug for the use in a different disease other than that of its original use. This review summarizes recent documentations on clinically approved drugs, repurposed to counteract COVID-19 infection, whose potential efficacy can be due, at least in part, to interference with the NLRP3 inflammasome cascade, at different levels ( Figure 2 ).

We systematically searched the PubMed and Google Scholar databases, clinical trials.gov, chictr.org.cn/searchprojen.aspx and https://www.clinicaltrialsregister.eu until June 25th, 2020

to prepare this section of the narrative review on the role of NLRP3 inflammasome in drugs repurposed in COVID-19. We also accessed the full text of the relevant cross references from the search results.

To date, no specific and selective compounds which inhibit the activation of the NLRP3 inflammasome are clinically available. Nonetheless, several drugs have been proposed to exert their beneficial effects, at least in part, by interfering with the activity of the NLRP3 inflammasome, although detailed mechanistic information on the mode of inhibition is lacking for many of these molecules (Mangan, Olhava, Roush, Seidel, Glick & Latz, 2018) .

One of the most well-characterized compounds used for exploring NLRP3 biology and drugability is MCC950, which binds to NLRP3 and affects a key step in its activation (Coll et al., 2015) . We recently also characterized the pharmacological profile of a newly synthesized NLRP3 inflammasome inhibitor, named INF4E, which directly targets the NLRP3 inflammasome and inhibits the ATPase activity of NLRP3 required for its activation (Mastrocola et al., 2016b) .

Among the drugs under investigation for the treatment of COVID-19, interferons (IFNs) are those whose effects observed may be, at least in part, the result of altering activation of the NLRP3 inflammasome pathway. Clinical trials are investigating the effects of IFNs either in combination with lopinavir/ritonavir (Chinese Clinical Trial Registry identifier: ChiCTR2000029387) or lopinavir/ritonavir/ribavirin (NCT04276688 and EudraCT database identifier: 2020-001023-14).

Selective interactions between type I-IFNs and NLRP3 inflammasome have been documented in the literature. For instance, both type-I IFNs and IFN-γ evoke higher expression of inducible nitric oxide (NO) synthase, increasing levels of endogenous NO, which inhibits NLRP3 oligomerization by means of direct S-nitrosylation of the NLRP3 protein, avoiding full inflammasome organization (Mishra et al., 2013) . Another mechanism resides in the ability of type-I IFN to decrease the NLRP3 inflammasome activation via the signal transducer and activator of transcription 1 that induces caspase-1 to process the IL-1β precursor (Kopitar-Jerala, 2017).

The administration of the protease inhibitor ritonavir in association with lopinavir, has been tested for the treatment of SARS-CoV-2 infection and COVID-19 pneumonia (Cao et al., 2020) and there are several on-going clinical trials in China (NCT04255017, NCT04261907, NCT04286503, NCT04295551), Hong Kong (NCT04276688), Republic of Korea (NCT04307693) and in Europe (NCT04315948, NCT04328285). The protease inhibitor darunavir is under investigation as an alternative drug to lopinavir/ritonavir treatment in COVID-19 (NCT04252274 and ChiCTR2000029541) . Other two clinical trials (NCT04261270 and NCT04303299) have been proposed to evaluate oseltamivir, a neuraminidase inhibitor approved for the treatment of influenza, alone or in combination with ritonavir or favipiravir, darunavir and chloroquine. Interestingly, the three antiviral drugs here mentioned interfere with NLRP3 inflammasome activation. Ritonavir has been demonstrated to inhibit caspase-1 activation leading to reduced cleavage of pro-IL-18 (Weichold, Bryant, Pati, Barabitskaya, Gallo & Reitz, 1999) , whereas darunavir limits the expression of key regulators of NLRP3 inflammasome complex formation, TLR4 and NF-κB (Zhang et al., 2018) . In contrast, oseltamivir had no effect on the expression of NLRP3 and IL-1β, however, when administered in combination with sirolimus it drastically improves the ability of sirolimus to repress NLRP3 inflammasome activation and NLRP3 inflammasome-mediated secretion of IL-1β in lung epithelial cells infected with influenza virus (Jia et al., 2018) .

Few clinical trials are recruiting patients with COVID-19 to test the efficacy of the known macrolide antibiotic azithromycin alone (NCT04332107, NCT04381962, NCT04369365, NCT04371107) or in combination with hydroxychloroquine (NCT04339816, and NCT04336332).

The beneficial effects of azithromycin against lipopolysaccharides-induced pulmonary neutrophilia have been documented to involve NLRP3 inflammasome inhibition (Bosnar et al., 2011; Gualdoni, Lingscheid, Schmetterer, Hennig, Steinberger & Zlabinger, 2015) , which is also involved in mediating tetracycline beneficial effects in experimental models of diabetic retinopathy and ischemic stroke Lu, Xiao & Luo, 2016) .

Three clinical trials are recruiting COVID-19 patients to test the efficacy of the immunosuppressant sirolimus (NCT04341675, NCT04371640 and NCT04374903), whereas no clinical study on the calcineurin inhibitor cyclosporine in COVID-19 has been recorded, despite it has been hypothesized as potential candidate for SARS-CoV-2 infection (Cure, Kucuk & Cumhur Cure, 2020) . Both the immunosuppressants can interfere with NLRP3 inflammasome. Sirolimus inhibits NLRP3 inflammasome activation in macrophages through autophagy induction (Ko, Yoon, Lee & Oh, 2017) , whereas cyclosporine targets cyclophilin D to avoid the opening of mitochondria permeability transition pore, which is strictly involved in NLRP3 inflammasome activation (Iyer et al., 2013) .

patients is the classical anti-mitotic drug colchicine, with at least ten ongoing clinical trials (NCT04328480, NCT04322682, 2020 -001841-38, NCT04392141, 2020 -001511-25, NCT04322565, NCT04375202, NCT04360980, 2020 ) and 3 others ready to start (NCT04367168, NCT04355143, NCT04326790). Several mechanisms have been proposed to explain how colchicine can suppress NLRP3 inflammasome activation. First, colchicine can inhibit the expression of pyrin gene, avoiding the assembly of the NLRP3 inflammasome complex (Nidorf, Eikelboom & Thompson, 2014) . Second, colchicine inhibits activation of caspase-1 and mature IL-1β (Otani et al., 2016) . Third, colchicine inhibits P2X7-induced pore formation, a key step in NLRP3 inflammasome activation following ATP exposure (Marques-da-Silva, Chaves, Castro, Coutinho-Silva & Guimaraes, 2011).

Tranilast, an analog of a tryptophan metabolite, which has been used in allergic disorders such as bronchial asthma, has been recently suggested for repurposing in the treatment of novel coronavirus pneumonia (ChiCTR2000030002). As convincingly demonstrated (Huang et al., 2018) , tranilast targets NLRP3 or NLRP3-ASC interaction to block the assembly of NLRP3 inflammasome, thus exerting significant beneficial effects in experimental models of inflammation-associated diseases.

Inhibition of NLRP3 inflammasome activation can be achieved by limiting the expression of the main component of the inflammasome complex, although this approach is not specific and is likely to produce many off-target effects.

Small molecules with immunomodulatory properties, already proposed for COVID-19 infection and with documented modulatory activity on expression of NLRP3 inflammasome components, are the JAK inhibitor baricitinib and the BTK inhibitor acalabrutinib. A study on baricitinib has already been concluded with interesting results (NCT04358614) and others are recruiting patients for both the treatments (Baricitinib: NCT04320277 and NCT04390464; Acalabrutinib: NCT04346199 and NCT04380688). Experimental evidence for key roles of the JAK and BTK cascades in either modulating either expression of the NLRP3 inflammasome and diabesity pathogenesis have been reported in the literature (Collotta et al., 2020; Furuya et al., 2018; Purvis et al., 2020) .

Up to nowadays six clinical trials (NCT04347980, NCT04325061, NCT04395105, NCT04344730, NCT04360876, NCT04327401), reported on clinicaltrials.gov are recruiting patients to test the efficacy of the corticosteroid dexamethasone, whose beneficial effects in airway inflammation has been recently demonstrated to involve lung inhibition of the activity of NLRP3 inflammasome and the release of IL-1β and IL-18 (Guan, Ma, Fan, Chen, Miao & Wu, 2020) .

Chloroquine and its analogue hydroxychloroquine, two well-known anti-malarial drugs, have been proposed for severe COVID-19 treatment, with more than 70 clinical trials registered on clinicaltrials.gov. Several studies have demonstrated the anti-inflammatory activity of chloroquine mainly due to significant inhibition of cytokines release, including IL-1β, from monocytes and macrophages (Hong et al., 2004) . Recently, a direct role for chloroquine in inhibiting the expression of NLRP3 inflammasome components has been demonstrated (Chen, Wang, Zhu, Lu, Liu & Zheng, 2017) . Besides, hydroxychloroquine exerts its antiinflammatory effect by inhibiting cathepsin-mediated NLRP3 inflammasome activation (Tang et al., 2018) . As recently reported (Lucchesi et al., 2020) , the pleiotropic effects of chloroquine and hydroxychloroquine on NLRP3 inflammasome expression and activation may significantly contribute to the ability of these anti-malarial drugs to accelerate the recovery of patients with SARS-CoV-2-induced ARDS.

Blocking downstream signalling molecules, such as IL-1β, may provide a means to regulate NLRP3 inflammasome activation. However, the efficacy of this approach is limited because NLRP3 induces a variety of pro-inflammatory mediators in addition to IL-1β, and conversely, the risk of immune suppression would be higher because IL-1β is produced by a variety of non-NLRP3 inflammasome dependent pathways in response to a variety of stimuli.

There are at least 8 clinical trials on pharmacological strategies specifically aimed to target the main product of NLRP3 inflammasome activation, the cytokine IL-1β. They include studies on the human anti-IL-1β monoclonal antibody Canakinumab (ClinicalTrials.gov identifier: NCT04362813, NCT04365153, NCT04348448) and the recombinant nonglycosylated human IL-1 receptor antagonist Anakinra (NCT04324021, NCT04330638, NCT0432402, NCT04357366 and NCT04339712). Besides, several other clinical trials are evaluating the effects of drugs targeting IL-6, whose serum concentration are drastically increased in patients with NLRP3 inflammasome-mediated condition (Abbate, Toldo, Marchetti, Kron, Van Tassell & Dinarello, 2020) . A cross-talk mechanism linking IL-1β to IL-6 has been documented. IL-1β induces IL-6 gene transcription via a PI-3-kinase/AKT-dependent pathways (Cahill & Rogers, 2008 ) and, at the same time, blockade of IL-6 signalling blunts the activation of NLRP3 inflammasome (Powell et al., 2015; Wu et al., 2018) . Thus, drugs able to interfere with IL-6 receptor activation may inhibit NLRP3dependent IL-1β release and, in a similar way, anti-IL-1β strategies significantly reduce IL-6 release, with an effective control of the cytokine-storm. Several clinical trials have been approved to test the efficacy of Sarilumab (NCT04315298 and NCT04327388, EudraCT identifier: 2020-001162-12 and 2020-001390-76) and Tocilizumab (ChinaXiv identifier:

202003.00026 and 20000.30894), two humanized monoclonal antibodies against the IL-6 receptor, thus confirming that targeting cellular machineries leading to cytokine overproduction, such as the NLRP3 inflammasome, holds promise for COVID-19 therapy.

Both infection and individual dysfunctional immune response elicited by SARS-CoV-2 have been reported to contribute to COVID-19 pathogenesis. Thus, it is more than a speculation that controlling the inflammatory response may be as important as targeting the virus. The evidences here reported suggest a potential role for the NLRP3 inflammasome pathway as cross-talk mechanism involved in both cellular events. Indeed, pharmacological targeting of NLRP3 inflammasome could be a feasible approach to counteract the pathology at multiple levels, ranging from the interference with viral infection to the reassessment of unbalanced immune responses. In addition, patients with exacerbated disease might reveal an impaired resolution process, which could again reflect on NLRP3 hyper-activation contributing to the "cytokine storm". Interestingly, some of the pharmacological strategies under investigation for COVID-19 therapy include drugs able to directly disrupt the NLRP3 inflammasome cascade or to affect the transcriptional activity of factors involved in the synthesis of the main components of the NLRP3 inflammasome complex. Further studies are needed to better elucidate the pathophysiological involvement of NLRP3 inflammasome in this specific context and to prove the efficacy and safety of related pharmacological strategies. Overall, the findings here reviewed underlie intriguing perspectives within the intricate mechanisms involved in the COVID-19 pathogenesis, and, thus, they may contribute to raise hope for discovering new strategies to battle one of the main pandemic of our modern times. 

Interleukin-1 and the Inflammasome as Therapeutic Targets in Cardiovascular Disease

FPR2: A Novel Promising Target for the Treatment of Influenza

Obesity and susceptibility to severe outcomes following respiratory viral infection

Influenza enhances caspase-1 in bronchial epithelial cells from asthmatic volunteers and is associated with pathogenesis

Azithromycin inhibits macrophage interleukin-1beta production through inhibition of activator protein-1 in lipopolysaccharide-induced murine pulmonary neutrophilia

Evidence for an anti-inflammatory loop centered on polymorphonuclear leukocyte formyl peptide receptor 2/lipoxin A4 receptor and operative in the inflamed microvasculature

A vasculo-protective circuit centered on lipoxin A4 and aspirin-triggered 15-epi-lipoxin A4 operative in murine microcirculation

Interleukin (IL) 1beta induction of IL-6 is mediated by a novel phosphatidylinositol 3-kinase-dependent AKT/IkappaB kinase alpha pathway targeting activator protein-1

Obesity and COVID-19 Severity in a Designated Hospital in Shenzhen

A Trial of Lopinavir-Ritonavir in Adults Hospitalized with Severe Covid-19

Activation of the TXNIP/NLRP3 inflammasome pathway contributes to inflammation in diabetic retinopathy: a novel inhibitory effect of minocycline

The Antimalarial Chloroquine Suppresses LPS-Induced NLRP3 Inflammasome Activation and Confers Protection against Murine Endotoxic Shock

A small-molecule inhibitor of the NLRP3 inflammasome for the treatment of inflammatory diseases

Baricitinib counteracts metaflammation, thus protecting against diet-induced metabolic abnormalities in mice

Cyclosporine therapy in cytokine storm due to coronavirus disease 2019 (COVID-19)

Angiotensin II mediates angiotensin converting enzyme type 2 internalization and degradation through an angiotensin II type I receptor-dependent mechanism

Immunological and inflammatory functions of the interleukin-1 family

The effect of obesity on lung function

Antiinflammatory role of the murine formyl-peptide receptor 2: ligand-specific effects on leukocyte responses and experimental inflammation

Apoptosis, pyroptosis, and necrosis: mechanistic description of dead and dying eukaryotic cells

Tofacitinib inhibits granulocyte-macrophage colony-stimulating factor-induced NLRP3 inflammasome activation in human neutrophils

The role of annexin A1 in the modulation of the NLRP3 inflammasome

Organ-specific distribution of ACE2 mRNA and correlating peptidase activity in rodents

Mechanisms of disease: inflammasome activation and the development of type 2 diabetes

Obesity Impairs the Adaptive Immune Response to Influenza Virus

Azithromycin inhibits IL-1 secretion and non-canonical inflammasome activation

Dexamethasone alleviate allergic airway inflammation in mice by inhibiting the activation of NLRP3 inflammasome

Clinical Characteristics of Coronavirus Disease 2019 in China

ACE2 is expressed in mouse adipocytes and regulated by a high-fat diet

Chloroquine protects mice from challenge with CpG ODN and LPS by decreasing proinflammatory cytokine release

Inflammation, metaflammation and immunometabolic disorders

Clinical features of patients infected with 2019 novel coronavirus in Wuhan

Tranilast directly targets NLRP3 to treat inflammasome-driven diseases

Obesity and the outcome of infection

Obesity and the risk and outcome of infection

Differential effects of inhibition of interleukin 1 and 6 on myocardial, coronary and vascular function

Mitochondrial cardiolipin is required for Nlrp3 inflammasome activation

Delayed oseltamivir plus sirolimus treatment attenuates H1N1 virus-induced severe lung injury correlated with repressed NLRP3 inflammasome activation and inflammatory cell infiltration

Pyroptosis triggers pore-induced intracellular traps (PITs) that capture bacteria and lead to their clearance by efferocytosis

Diet-induced obesity impairs the T cell memory response to influenza virus infection

The NLRP3 Inflammasome: An Overview of Mechanisms of Activation and Regulation

Rapamycin regulates macrophage activation by inhibiting NLRP3 inflammasome-p38 MAPK-NFkappaB pathways in autophagy-and p62-dependent manners

The Role of Interferons in Inflammation and Inflammasome Activation

Obesity and respiratory hospitalizations during influenza seasons in Ontario, Canada: a cohort study

NLRP3 inflammasome activation in inflammaging

Obesity in patients younger than 60 years is a risk factor for Covid-19 hospital admission

Current laboratory diagnostics of coronavirus disease 2019 (COVID-19)

Frontline Science: Specialized proresolving lipid mediators inhibit the priming and activation of the macrophage NLRP3 inflammasome

Minocycline Suppresses NLRP3 Inflammasome Activation in Experimental Ischemic Stroke

Clinical and biological data on the use of hydroxychloroquine against SARS-CoV-2 could support the role of the NLRP3 inflammasome in the pathogenesis of respiratory disease

Inflammasome control of viral infection

The Annexin A1/FPR2 pathway controls the inflammatory response and bacterial dissemination in experimental pneumococcal pneumonia

Targeting the NLRP3 inflammasome in inflammatory diseases

Colchicine inhibits cationic dye uptake induced by ATP in P2X2 and P2X7 receptorexpressing cells: implications for its therapeutic action

Metaflammation: Tissue-Specific Alterations of the NLRP3 Inflammasome Platform in Metabolic Syndrome

Maladaptive Modulations of NLRP3 Inflammasome and Cardioprotective Pathways Are Involved in Diet-Induced Exacerbation of Myocardial Ischemia/Reperfusion Injury in Mice

Pharmacological Inhibition of NLRP3 Inflammasome Attenuates Myocardial Ischemia/Reperfusion Injury by Activation of RISK and Mitochondrial Pathways

The impact of obesity on the immune response to infection

Nitric oxide controls the immunopathology of tuberculosis by inhibiting NLRP3 inflammasome-dependent processing of IL-1beta

Trained Immunity: a Tool for Reducing Susceptibility to and the Severity of SARS-CoV-2 Infection

Targeting cholesterol crystal-induced inflammation for the secondary prevention of cardiovascular disease

Resolvin D1 limits polymorphonuclear leukocyte recruitment to inflammatory loci: receptor-dependent actions

Bruton's Tyrosine Kinase Inhibition Attenuates the Cardiac Dysfunction Caused by Cecal Ligation and Puncture in Mice

Colchicine prevents NSAID-induced small intestinal injury by inhibiting activation of the NLRP3 inflammasome

Annexin A1 and glucocorticoids as effectors of the resolution of inflammation

Targeting endothelial metaflammation to counteract diabesity cardiovascular risk: Current and perspective therapeutic options

Interleukin 6 Increases Production of Cytokines by Colonic Innate Lymphoid Cells in Mice and Patients With Chronic Intestinal Inflammation

Inhibition of Bruton's tyrosine kinase regulates macrophage NF-κB and NLRP3 inflammasome activation in metabolic inflammation

SARS-CoV-2 infection and overactivation of Nlrp3 inflammasome as a trigger of cytokine "storm" and risk factor for damage of hematopoietic stem cells

Detection of pyroptosis by measuring released lactate dehydrogenase activity

Clinical predictors of mortality due to COVID-19 based on an analysis of data of 150 patients from Wuhan, China

COVID-19, cytokines and immunosuppression: what can we learn from severe acute respiratory syndrome?

The resolution code of acute inflammation: Novel pro-resolving lipid mediators in resolution

Lille Intensive Care COVID-19 and Obesity study group. High prevalence of obesity in severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) requiring invasive mechanical ventilation

Diabetes in COVID-19: Prevalence, pathophysiology, prognosis and practical considerations

Mir-21 Mediates the Inhibitory Effect of Ang (1-7) on AngII-induced NLRP3 Inflammasome Activation by Targeting Spry1 in lung fibroblasts

Inflammation links excess fat to insulin resistance: the role of the interleukin-1 family

Hydroxychloroquine attenuates renal ischemia/reperfusion injury by inhibiting cathepsin mediated NLRP3 inflammasome activation

Outstanding Scientific Achievement Award Lecture 2011: defeating diabesity: the case for personalized combinatorial therapies

The NLRP3 inflammasome instigates obesity-induced inflammation and insulin resistance

Endocytosis of the receptorbinding domain of SARS-CoV spike protein together with virus receptor ACE2

NLRP3 inflammasome, an immuneinflammatory target in pathogenesis and treatment of cardiovascular diseases

Adipose Tissue in Persons With HIV Is Enriched for CD4(+) T Effector Memory and T Effector Memory RA(+) Cells, Which Show Higher CD69 Expression and CD57, CX3CR1, GPR56 Co-expression With Increasing Glucose Intolerance

HIV-1 protease inhibitor ritonavir modulates susceptibility to apoptosis of uninfected T cells

Feeding our immune system: impact on metabolism

IL-6 receptor blockade ameliorates diabetic nephropathy via inhibiting inflammasome in mice

Hypercholesterolemic myocardium is vulnerable to ischemia-reperfusion injury and refractory to sevofluraneinduced protection

Pathological findings of COVID-19 associated with acute respiratory distress syndrome

Clinical course and outcomes of critically ill patients with SARS-CoV-2 pneumonia in Wuhan, China: a single-centered, retrospective, observational study

Recent advances in the mechanisms of NLRP3 inflammasome activation and its inhibitors

Inflammasomes and Pyroptosis as Therapeutic Targets for COVID-19

Darunavir alleviates irinotecaninduced intestinal toxicity in Vivo

NLRP3 Inflammasome-A Key Player in Antiviral Responses

Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study

Innate immunity as a target for acute cardioprotection