Insight to Pyroptosis in Viral Infectious Diseases

Background: Pyroptosis is defined as programmed necrosis executed by gasdermin D or E (GSDMD or GSDME), which punches cellular membrane. Morphologically, pyroptosis is characterized by cell swelling and cell membrane rupture, leading to the release of cellular contents that triggers intense inflammatory response. More and more studies have found that pyroptosis may be involved in the pathogenesis of viral infection, which may be a determinant for inflammation observed in most viral diseases. Objective: This paper aims to summarize the roles of pyroptosis in the pathogenesis of viral infectious diseases and to provide potential drug targets for the treatment of viral diseases, which will contribute to medical research and public health. Measures: This paper mainly summarizes pyroptosis occurring in diseases caused by different viruses, including human immunodeficiency virus, hepatitis virus, enterovirus, influenza virus and dengue fever virus. Meanwhile, the reported mechanism underlying pyroptosis mediating pathogenesis of these viral diseases will also be described. Conclusion: Current studies have shown that pyroptosis is a double-edged sword in viral infectious diseases. On one hand, pyroptosis leads to pathogenic inflammation of many viral infectious diseases which aggravate tissue damage initiated by viral infection, and blocking proptosis usually relieves the inflammation, which exerts therapeutic effects on viral diseases. On the other hand, moderating pyroptosis can contribute to defense against pathogen infection by releasing immune epitopes and inducing antiviral immune response.

• Pyroptosis
• Viral Infectious Diseases
• Gasdermin D
• Gasdermin E
• Caspase
• Inflammation

1. Background and Purpose

Cell death modes such as necrosis, autophagy and apoptosis have been gradually discovered. In recent years, a new type of cell death, pyroptosis, has been described. Pyroptosis is a kind of programmed and inflammatory necrosis, depending on caspase (cysteinyl aspartate specific proteinase) activation and the cell-membrane-pore forming executed by the protein family of gasdermins. Pyroptosis results in lytic cell death accompanied by the release of inflammatory factors, inducing cascade and amplification of inflammatory response. Pyroptosis is an important immune defense mechanism in the body, which plays an important role in resisting invasion of external pathogens and sensing internal pathogenic signals within cells.

Along with the whole human history, from epidemics recorded on murals of ancient Egypt to the recent global COVID-19 pandemic, viral infectious diseases have been imposing great threat to human health. Inflammation is a defensive response against viral infection in the body, but exaggerated inflammation will lead to tissue damage and exacerbated the disease. As a process of inflammatory cell death, pyroptosis has been reported to be closely related to inflammatory modulation in various viral infections. Thus, better understanding of pyroptosis will provide new ideas for the prevention and treatment of viral infectious diseases. In this paper, we summarize some studies about the relationship between pyroptosis and viral infectious diseases that have been discovered at present, focusing on a small but in-depth scope, which can provide a practical reference value for researchers studying viral infectious diseases and pyroptosis.

2. Morphological Features of Pyroptosis

Pyroptosis was first described by Zychlinsky and his colleagues in 1992 when they observed that Shigella fredori infected macrophages and underwent lytic cell death which shares some characteristics with apoptosis, such as DNA fragmentation, nuclear pyknosis and caspase-dependence, and thus was thought to be apoptosis originally [1]. In 2001, Cookson et al. found that though such lytic cell death mode depends on caspases, it shows some features different from caspase-3-mediated apoptosis, including the formation of pores on the cell membrane, cell lysis and release of contents, and thus renamed it as pyroptosis. “Pyro” means fire, indicating that this programmed cell death can trigger an inflammatory response, and “ptosis” means falling, representing the nature of programmed cell death [2]. The original pattern of pyroptosis is caspase-1-dependent in which gasdermin D, a perforating protein, is cleaved and activated by activated caspase 1, and the N-terminal cleavage band can recognize and bind to the cell membrane, assembling into hollow annular pores with a diameter of 10 - 15 nm [3]. Because the balance of the membrane permeation barrier is destroyed by the perforating, K⁺ outflows and Na⁺ inflows, accompanied by the inflow of other extracellular solution, leading to the expansion of cells [4]. Morphologically, pyroptotic cells blow out many “bubbles” on their surface at first. After a certain time, the bubbles burst, and the cell contents including interleukin 1β (IL-1β), IL-18 and lactate dehydrogenase are released to the extracellular fluid through the pore channels, which will recruit immune cells and trigger a wide range of inflammatory reactions [5] - [11].

3. The Molecular Mechanism of Pyroptosis

In the process of pyroptosis, intracellular inflammatory bodies form specific protein complexes firstly in response to the stimulation of different signals, leading to the activation of different caspases which cleave gasdermin D or E to release N-terminal fragments that form pores on the cell membrane, resulting in cell membrane rupture and release of cell contents. Pyroptosis causes a strong inflammatory response. According to the dependence on the inflammatory caspase, pyroptosis can be classified into canonical and non-canonical pathways. The mechanism of pyroptosis is summarized in Figure 1.

3.1. Overview of Caspase Family Proteins

Caspases are a group of proteases with similar structure existing in the cytoplasm. In 1993, Yuan et al. found that the CED-3 gene of C. elegans is highly homologous in function and sequence to the ICE (Interleukin-18 converting enzyme) gene of mammalian cells, which is similar to the CED-3 gene. The high expression of ICE gene can induce the apoptosis of rodent fibroblasts, attracting much attention. More and more researchers have carried out a series of studies on the relationship between caspase family proteins and apoptosis. All caspases

are characterized by the conserved pentapeptide sequence of OACXG (X being any amino acid) and their similar functions. At least 14 members of the caspase family have been found in mammals, named caspase 1 to caspase 14 in sequence according to the time of discovery [12]. The inflammatory factors related to pyroptosis include caspase 1, caspase 4, caspase 5, caspase 11, caspase 12, caspase 13 and caspase 14 [13]. The expression of caspases is species dependent. For instance, mouse mainly expresses caspase 1 and caspase 11, but human mainly expresses caspase 1, caspase 4 and caspase 5 [14].

Caspase family proteins share similar amino acid sequence, structure and substrate specificity. In live cells, they generally exist in the form of inactive proteinogen which can be activated through proteolysis. The prozyme molecule is composed of three parts, including an N-terminal primordial domain, a large subumit and a small subunit. Upon activated, the large subunit and the small subunit form a heterodimer, and the two heterodimers will assemble into an enzymatic active heterotetramer. Previous studies have confirmed that the caspase family proteins are related to a variety of physiological and pathological processes. For example, caspase 12 can inhibit the activation of caspase 1 and further inhibit the occurrence of inflammatory response. Cancer rates are higher after loss of caspase 2; Caspase 3 is associated with GSDME-mediated pyroptosis and apoptosis signaling pathways. Caspase 6, 7, 8, 9 and 10 are all involved in apoptosis, among which caspase 9 is closely related to the generation of degenerative diseases and cancer [13] [15] [16] [17] [18].

3.2. Overview of Gasdermin Family Proteins

The gasdermin protein family consists of six members, they are gasdermin A (GSDMA), gasdermin B (GSDMB), gasdermin C (GSDMC), gasdermin D (GSDMD) and gasdermin E (GSDME/DFNA5) and an autosomal recessive genetic deafness protein No. 59 (Deafness, Autosomal Recessive 59, DFNB59) [19] [20]. Each member of the gasdermin family has a different distribution and function in human tissues and organs.

GSDMA is mainly found in the epithelial cells of the skin, esophagus, tongue and umbilical cord, and is associated with the development of diabetes and asthma, as well as a number of other immunological diseases.

GSDMB is highly expressed in liver, stomach, skin and immune cells, and is associated with diabetes, inflammatory bowel disease, colitis and asthma [21] [22].

GSDMC is mainly localized in tissues such as skin, spleen, esophagia and bladder, and was initially thought to be the exonuclear factor of the melanoma-derived leucine zipper due to its dominant expression in melanoma cells [9].

The GSDMD gene is located at 8q24 and is mainly expressed in skin, immune cells and gastrointestinal tissues. It is composed of 242 amino acids and can be divided into three parts: The N-terminal region, the intermediate junction region and the C-terminal region [23] [24]. By far, GSDMD is the most common and well-studied pyroptosis associated protein. In GSDMD mediated pyroptosis, GSDMD can be cleaved by caspase1/4/5/11 to release C-terminal fragment and N-terminal fragment with pyroptosis effector. The N-terminal fragment is lipophilic and can interact with PI (Phosphatidylinositol), PS (Phosphatidylserine) on the inner side of the cell membrane, and Cardiolipin on both sides of the bacterial membrane specifically bind and assemble to form pores, which disrupt the balance of the membrane permeability barrier, resulting in K+ outflow, Na+ inflow, and water molecules from the outside. The cells swell, their membranes rupture, and their contents are released, leading to pyroptosis and an intense inflammatory response. Studies have shown that when the N-terminal domain forms fewer channels, cells can initiate compensatory repair mechanism to repair the damage of cell membrane. However, when the N terminal of GSDMD makes too many holes in the cell, it will exceed the repair ability of the cell, which will lead to cell lysis and release of cell contents, causing pyroptosis [5] [7] [9] [10] [11].

GSDMD can combine with the cardiolipin on the bacterial cell membrane and punch holes to play a bactericidal role, which can effectively kill Staphylococcus aureus, Escherichia coli and Bacillus etc. [25]. In addition, it has been found that the C-terminal fragment of GSDMD inhibits pyroptosis induced by N-terminal fragment. Under normal physiological conditions, the GSDMD-C-terminal domain inhibits the drilling activity by connecting with the GSDMD-N-terminal domain, so as to maintain the stability of the internal environment [26] [27].

GSDME is the earliest gasdermin family protein discovered, and it is the second protein capable of mediating pyroptosis following GSDMD. Because its mutation is related to the occurrence of hereditary deafness, it was originally named DFNA5. It is mainly expressed in cochlea, intestinal tract, placenta and brain. In recent years, GSDME-related pyroptosis has drawn more and more attention [28]. Studies have shown that in the treatment of chemotherapy drugs, GSDME can be cleaved by caspase 3 between the 275th and 276th amino acid residues, releasing about 33kDa GSDME-N fragment which has a function of perforating the cell membrane. This molecular mechanism is similar to that of GSDMD mediated pyroptosis [29] [30].

GSDME-mediated pyroptosis is one of the reasons for the side effects of chemotherapeutic drugs that damages normal tissues, since GSDME is expressed at a high level in many healthy tissues. Recent studies have shown that when caspase 3 is activated, the expression level of intracellular GSDME determines the mode of cell death. When the expression level of intracellular GSDME is high, activated caspase 3 cleaves GSDME and induces pyroptosis. When the expression level of GSDME is low or there is no GSDME expression, the cells undergo apoptosis [30]. Therefore, GSDME is a key molecule switching capase-3-mediated apoptosis and pyroptosis.

3.3. The Canonical Pyroptosis Pathway

When the body encounters a variety of infectious and immunological stimuli, caspase 1 can be activated through different inflammatory bodies, which initiates the canonical pyroptosis pathway. When pathogens invade, inflammasomes can recruit caspase 1 precursors directly or through ASC (apoptosis-associated speck-like protein containing C-terminal caspase recruitment domain), resulting in hydrolysis of caspase 1 precursors and production of active caspase 1. The inflammasomes containing ASC can interact with the caspase 1 homologous protein PYD (pyrin domain) to recruit the upstream and downstream PRRs (pattern recognition receptors) and the active form of caspase 1, assembling inflammatory complex [31] [32]. As for some inflammasomes lacking ASC connectors, such as NAIP-NLRC4 and NLRP1b inflammasomes, they directly recruit pro-caspase 1 and activate it, but do not produce inflammatory complex [33]. Further studies have revealed the specific molecular mechanism of GSDMD-induced pyroptosis. Accurate structural analysis results showed that when the signal was transmitted to GSDMD by the mature caspase 1 in the cell, GSDMD will be cleaved into the lipophilic fragment of the GSDMD-N terminus of 31 kDa and the hydrophilic fragment of the GSDMD-C terminus of 22 kDa [34]. GSDME-N can specifically bind to phosphatidylinositol (PI) and phosphatidylserine (PS) on the inner side of cell membrane and assemble pores. Under electron microscopy, there are about 16 N-terminal segments assembling hollow annular pores with a diameter of 10 - 15 nm. On the other hand, activated caspase 1 can also cleavage the inflammatory cytokine IL-1β and IL-18 precursors to generate mature IL-1β and IL-18. As pore-forming destroys membrane-permeability barrier, K⁺ outflows and Na⁺ inflows together with other extracellular solutes, leading to cell expansion and cell membrane rupture. Mature inflammatory cytokines IL-1β and IL-18 can also be released to extracellular fluid through the pore, which in turn stimulates the recruitment of immune cells and triggers a cascade of inflammatory responses [5] [7] [9] [10] [11].

3.4. Non-Canonical Pyroptosis Pathway

Human caspase 4/5 and murine caspase 11 can be activated by Toll-like receptor combined with lipopolysaccharide (LPS) of gram-negative bacteria. Caspase 4/5/11 directly binds to lipid A, the conserved domain of LPS, for its activation. The activated caspase 4/5/11 then cleaves GSDMD, and the GSDMD-N fragment is generated to perform the function of cell perforation, thus leading to inflammatory necrosis. This pathway, which is dependent on caspase 4/5/11, is known as the non-canonical pathway. Caspase 11 is also an upstream activator of caspase 1 that activates caspase 1 together with NLRP3 and ASC, further triggering pyroptosis and releasing mature IL-1β and IL-18 [35].

3.5. GSDME-Mediated Pyroptosis Pathway

Activation of caspase 3 was generally believed as a marker of apoptosis. For example, in the treatment of cancer with chemotherapy drugs such as doxorubicin and cisplatin, caspase 8 and caspase 9 can be activated, and then activate downstream caspase 3 to induce exogenous and endogenous apoptosis, respectively [36] [37]. But recent studies have shown that activated caspase 3 also cleaves GSDME to release active GSDME-N terminal fragment which can also drill into the cell membrane. Similar to GSDMD, GSDME-mediated pyroptosis also causes cell swell and rupture, releasing some inflammatory cytokines and other cellular contents, recruiting extracellular immune cells, and triggering a strong inflammatory cascade reaction [29] [30].

GSDME has been found to be expressed at low levels in many cancer cells, but at high levels in normal tissues, such as the placenta, kidney, lung, uterus and heart. When Shao Feng et al. studied the association between chemotherapy drugs treatment and GSDME mediated cell apoptosis, they found that damage of tissue and organ in wild-type mice treated with chemotherapy drugs was significantly more serious than that in gsdme^−/− mice, indicating that in the process of cancer treatment, the usage of chemotherapy drugs can cause serious side effects on the patient’s normal tissues and organs. Therefore, the discovery of caspase 3-GSDME mediated pyroptosis pathway can provide new ideas and new targets for the treatment of cancer and the prevention of side effects caused by chemotherapy drug [30].

4. Role of Pyroptosis in Viral Infectious Diseases

Table 1

Role of pyroptosis in viral infectious diseases

Virus	Disease	Role of pyroptosis in virus-induced disease	References
SARS-CoV-2	COVID-19, Pneumonia, Severe acute Respiratory syndrome, Kidney failure.	Hematopoietic stem cell pyroptosis. Levels of IL-1β and LDH were significantly increased in COVID-19 patients.	[38] - [49]
Hepatitis virus	Viral hepatitis, Liver fibrosis, Cirrhosis of the liver, Hepatic carcinoma.	HBeAg inhibits pyroptosis. HCV causes pyroptosis. Inhibition of NLRP3 relieves acute liver failure induced by viral hepatitis and inhibit pyroptosis.	[57] [58] [59] [60] [61]
Influenza virus	Influenza	H5N1 acts on galectin-3 and activates NLRP3, promoting pyroptosis in the lungs and triggering inflammation.	[50] [51] [52]
HIV	AIDS	HIV destroys immune system by causing pyroptosis of CD4+ T cells.	[53] [54] [55] [56]
DENV	Dengue haemorrhagic fever, Dengue shock syndrome.	Dengue virus causes pyroptosis of monocytes and macrophage.	[62] [63]
Enterovirus	HFMD, Myocarditis, Pancreatitis, Meningitis	EV71 and CVB3 induce pyroptosis via caspase 1-GSDMD pathway.	[64] [65]
Other viruses	Hydrophobia, Enterogastritis	Rabies virus activates pyroptosis via caspase 1-GSDMD pathway.	[66] [67]

4.1. Role of Pyroptosis in Novel Coronavirus Infection

Novel coronavirus (SARS-CoV-2) is a novel coronavirus discovered in December 2019 which causes global pandemic of COVID-19 and greatly threats public health. Currently, its spread is still raging around the world [38] [39]. Studies have reported that SARS-CoV-2 infection can induce a strong inflammatory cytokine storm [40] [41], resulting in loss of hematopoietic function and lymphocytopenia in the body [42] [43]. In recent years, it has been found that pyroptosis can also lead to a strong inflammatory response, and it can clear or reduce the number of cells by cleavage, which is exactly in line with the symptoms after SARS-CoV-2 infection. There is growing evidence indicating that pyroptosis may be involved in the infection and pathogenesis of SARS-CoV-2. NLRP3 inflammasome is likely to be the culprit for certain complications of SARS-CoV-2 infection that may affect multiple tissues and organs as well as potential hematopoietic function [41] [44].

During infection, SARS-CoV-2 spines can recognize ACE2, its receptor, on the cell surface and directly activate NLRP3 inflammasome. NLRP3 inflammasome can subsequently activate caspase 1 to trigger inflammatory immune response via the release of mature pro-inflammatory factors such as IL-1β and IL-18, triggering a strong inflammatory response. Caspase 1 can also directly lyse GSDMD to produce the active fragment of GSDMD-N, leading to pyroptosis [45] [46] [47]. Many evidences have been demonstrated to support the occurrence of pyroptosis in SARS-CoV-2 infection. Yhan et al. found that the level of lactate dehydrogenase (LDH) in samples from COVID-19 patients was significantly increased [48]. LDH is a cytosolutase residing in the cytoplasm. LDH can only be released into the extracellular environment when the cell membrane is ruptured. Therefore, LDH can be used as one of the indicators to monitor pyroptosis. In addition, the inflammatory cytokine IL-1β, one of the downstream indicators of pyroptosis, is significantly increased in serum of COVID-19 patients [49]. However, the specific pattern of pyroptosis involved in the infection and pathogenicity of SARS-CoV-2 remains to be clarified. Therefore, it is necessary to conduct more in-depth studies on the role of pyroptosis in SARS-CoV-2 infection, in order to provide new ideas for the development of drugs and treatment regimens for COVID-19 (Table 1, Line 1).

4.2. Role of Pyroptosis in Influenza Virus Infection

Influenza viruses mutate quickly and have a high rate of gene recombination, so they can cause a massive epidemic and spread after infection, which is one of the reasons for the limited development of influenza vaccine [50]. Influenza virus infection causes an inflammatory response that can lead to severe sequelae, other complications and even death, the study reported. NLRP3 inflammasome is the main antiviral defense mechanism in the host during influenza virus infection. Previous studies have shown that NLRP3 inflammasome activates caspase 1, which leads to the release of mature IL-1β and IL-18 as well as pyroptosis.

Galectin-3 is a β-galactose lateral binding protein that is widely distributed in immune cells and epithelial cells and can regulate the occurrence of a variety of immune processes and microbial infection. Chen et al. found that galectin-3 could promote the activation of NLRP3 inflammasome and enhance the lung inflammation induced by H5N1 avian influenza virus in the lung tissue of galectin-3 knockout mice and wild-type mice infected with H5N1 influenza virus [51]. In addition, Kuriakose et al. found that during the infection of influenza A virus (IAV), Z-DNA-binding protein 1 can activate NLRP3 inflammasome, and induce necrosis, apoptosis and pyroptosis [52]. Therefore, the inhibition of pyroptosis may provide a new preventive measure for the containment of influenza virus pandemic (Table 1, Line 3).

4.3. Role of Pyroptosis in Human Immunodeficiency Virus Infection

Human immunodeficiency virus (HIV) infection will cause acquired immune deficiency syndrome (AIDS). HIV can be divided into two related but different groups: The human immunodeficiency virus type-1 (HIV-1) and the human immunodeficiency virus type-2 (HIV-2) [53]. Studies have reported that the main pathogen of AIDS is HIV-1 which mainly attacked CD4⁺ cells including T4 lymphocytes, monocytes macrophages and dendritic cells, resulting in a gradual decline in the number of CD4⁺ T cells, lymphocyte depletion and paralysis of the host immune system [54]. Due to the correlation between apoptosis and the occurrence and progression of AIDS, previous studies believed that the death and sharp decrease of CD4⁺ T cells were mainly caused by apoptosis [54].

Doitsh et al. examined the spleen and tonsil tissues of HIV-1 infected people and found that only 5% of CD4⁺ T cells were infected with HIV-1 and underwent apoptosis [55] [56]. About 95% of CD4⁺ T cells were not infected with HIV-1 but died. Further investigation found that caspase 1 was activated in the 95% cells, and IL-1β and IL-18 secreted by the cells were significantly increased, suggesting that most of the death of CD4⁺ T cells was caused by pyroptosis mediated by caspase 1. Subsequent studies using VX-765, a caspase 1 inhibitor, showed that inhibition of caspase 1 cleavage reduces the secretion of IL-1β, as well as the death of HIV-infected CD4⁺ T cells. These results suggest that pyroptosis is involved in the molecular mechanism of the depletion of human immune cells after HIV infection [55] (Table 1, Line 4).

4.4. Role of Pyroptosis in Hepatitis Virus Infection

Human hepatitis viruses can be classified into type A, B, C, D, E and G. Hepatitis virus infection can lead to a series of liver damages such as liver fibrosis, cirrhosis of the liver and even hepatic carcinoma. However, the molecular mechanism of liver damage caused by hepatitis virus has not yet been fully confirmed.

Kupffer cells (KCs) are common non-parenchymal cells in the liver, which have a momentous position in maintaining the homeostasis and are main source of inflammasome in the liver [57]. Studies have shown that after HBV invades the host, it can infect the KCS of the host liver, which then responds by releasing inflammatory cytokines (such as IL-18) and stimulating natural killer cells (NK) [58]. In the infection of hepatitis B virus (HBV), HBeAg, a component of the core antigen of HBV, can inhibit the activation of caspase 1 and the maturation and releasing of IL-1β by suppressing the production of reactive oxygen species, which is beneficial to the persistent reproduction and immune tolerance of HBV [59]. Kofahi et al. tested the effect of HCV infection on programmed cell death of Huh-7.5 and found that the proliferation rate of HCV infected cells was significantly reduced, accompanied by caspase-3-mediated pyroptosis, and HCV resulted in a significant increase in the proportion of activated caspase 1 [60]. In addition, Wang et al. used a mouse model of hepatic failure that was pathologically similar to viral hepatitis and found that the levels of detected inflammatory cytokines were remarkably reduced after NLRP3 inflammasomes were treated with pyroptosis inhibitors [61]. These results suggest that pyroptosis is a vital mode of cell death in cells infected by hepatitis viruses. Therefore, an in-depth study on the role of pyroptosis in the pathogenesis of hepatitis virus and the way that hepatitis virus uses pyroptosis to evade the immune defense mechanism of the body may provide a new idea for the research and treatment of viral hepatitis (Table 1, Line 2).

4.5. Role of Pyroptosis in Dengue Virus Infection

Dengue is a mild and self-limited disease caused by Dengue virus (DENV) infection. A very rare group of patients may develop a worsening condition leading to a more serious and lethal Dengue haemorrhagic fever or Dengue shock syndrome. However, the pathogenic mechanism of Dengue related illness is not clear yet. Tan et al. examined the activation of caspase 1 precursors in primary monocytes infected with DENV and found that pyroptosis is involved in the activation of caspase 1 in DENV-infected monocytes and may play a pro-inflammatory role in the immune pathogenesis of dengue fever [62]. Wu et al. detected that the IL-1β precursor, IL-18 precursor and NLRP3 related to caspase 1 activation were up-regulated in DENV-infected human monocyte derived macrophages, and then they inhibited the NLRP3 inflammasome in macrophages, and found that the pyroptosis phenomenon was significantly reduced [63] (Table 1, Line 5).

4.6. Role of Pyroptosis in Enterovirus Infection

Enterovirus 71 (EV71), one of the main causes of hand, foot and mouth disease (HFMD), also affects the nervous system and causes inflammation in patients. Pyroptosis is a newly discovered way of programmed cell lysis and inflammatory necrosis. Since EV71 can also trigger inflammatory response, we speculate that pyroptosis has some connection to the pathogenesis of EV71. In 2017, Zhong et al. found that EV71 infection could induce the activation of caspase 1 and increased the secretion of IL-1β and IL-18 in infected cells [64]. However, when caspase 1 was treated with specific inhibitors, it was discovered that the degree of body damage and inflammation caused by EV71 infection in mice was remarkably decreased. In addition, they found that caspase 1 inhibitors inhibited EV71 replication in the mouse brain. Similarly, they detected that infection with coxsackie B3 (CVB3), an important member of the enterovirus family, also activated pyroptosis. In HeLa cells infected with CVB3, caspase 1 is activated, and the expression of IL-18 and NLRP3 is also increased. Caspase 1 inhibitors can also reduce the body damage of mice by inhibiting the replication of CVB3 and activation of caspase 1. These results indicated that EV71 and CVB3 infections were associated with pyroptosis. In addition, Lei et al. found that [65], EV71 infection decreased the expression of GSDMD, the specific mechanism is that EV71 virus protease 3C cleaves GSDMD at Q193-G194 and inhibits the occurrence of pyroptosis, generating a non-functional GSDMD fragment composed of GSDMD1-193. The GSDMD-N fragment composed of GSDMD1-275 generated by caspase 1 cleavage can cause pyroptosis and thus inhibit the replication of EV71. These results suggest that EV71 virus can use its own 3C protease to escape the antiviral immune mechanism of the host (Table 1, Line 6).

4.7. Role of Pyroptosis in Other Viral Infections

Koraka et al. infected mice with rabies virus and found that the expressions of caspase 1, IL-1β and IL-18, which are closely related to pyroptosis, were significantly upregulated, indicating that rabies virus infection can activate pyroptosis signaling pathway [66].

A study of mouse rotavirus showed that GSDMD knockout mice were more susceptible to rotavirus and that caspase 1/11, GSDMD, and NLRP9b knockout tissues had greater rotavirus replication. In addition, they found that NLRP9b and the adaptor proteins ASC and caspase 1 assemble the inflammasome complex, which in turn mediates the pyroptosis of intestinal epithelial cells to inhibit rotavirus replication [67] (Table 1, Line 7).

In addition, studies have found that pyroptosis is also involved in the occurrence of many bacterial infectious diseases, such as Shigella frederi, Salmonella, Listeria, Yersinia and other bacteria can induce macrophage pyroptosis through the caspase 4/5/11-GSDMD pathway [35] [68].

5. Conclusions and Perspectives

In summary, pyroptosis plays important roles in the pathogenesis of many viral infectious diseases. Current studies on viral infectious diseases associated with pyroptosis are all found to occur through the caspase 1-GSDMD pathway, so we can directly inhibit the occurrence of pyroptosis by using the specific inhibitor of caspase 1 or inhibiting GSDMD to block the process of pyroptosis and reduce the expression levels of inflammatory cytokines IL-1β and IL-18, thus achieving the therapeutic effect of viral diseases, but the inhibitor should have side effects on the body. So it is still clear how to measure the use of drugs or inhibitors clinically. In addition, different viruses activate pyroptosis in different ways, such as receptors, inflammasomes and other related factors that regulate pyroptosis, so we can also deal with different targets.

GSDMD and GSDME are the key molecules in pyroptosis. It has been only found that GSDMD plays an important role in viral diseases, but whether GSDME is also involved in the vocalization and development of viral diseases remains to be explored. In addition, in addition to GSDMD and GSDME, whether there are other key molecules of pyroptosis and the specific mechanism of pyroptosis still need to be further studied. It’s worth noting that cell pyroptosis can be used as a “double-edged sword” in viral infectious diseases. On the one hand, it helps to remove pathogens and prevent infection, which plays an indispensable role in maintaining the normal operation of the body. On the other hand, excessive pyroptosis can lead to intense inflammatory response, which aggravates the occurrence and development of the disease. As a newly discovered proinflammatory programmed cell death in recent years, pyroptosis will be proven to be involved in more and more viral diseases beyond the diseases summarized in this review. And due to the different stimulators of different viral infectious diseases, there may be a more complex mechanism to regulate the death process, which still needs to be further explored. Therefore, making an intensive study of the characteristics and mechanism of pyroptosis as well as its relationship with disease can provide a new way of thinking and effective drug targets for viral disease prevention and treatment.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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