Nature Immunology Cao Xuetao was invited to write a review on the inflammatory response prone to new coronavirus infections

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iNature On October 21, 2021, the journal Nature Immunology (Nature Immunology) published a review written by Cao Xuetao, an academician of the Chinese Academy of Engineering and Nankai University
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In response to the research paper published by Oxford University scientists in the same period that the new coronavirus infection triggers the secretion of inflammatory cytokines in macrophages, combined with the international research progress in the field of immune inflammation regulation, Cao Xuetao's review of why the new coronavirus infection is better than the general virus The cellular and molecular mechanisms that infection is more likely to induce inflammatory damage in the body have been explained, and new potential targets and suggestions have been proposed for preventive intervention or therapeutic blocking of the development of patients with new coronavirus pneumonia
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As we all know, once the body is infected by a virus, it can induce the production of interferon to exert its antiviral effect
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Interferon can exert its antiviral effect because it can stimulate cells to express hundreds of interferon-induced genes (ISG), and the ISG protein molecules produced have various antiviral mechanisms
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Among them, interferon-stimulating gene 15 (ISG15) has a unique role.
As an ubiquitin-like protein, the ISG15 protein molecule can covalently bind its target molecule.
This ISG (ISGylation) is beneficial to the body's antiviral defense
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In the new issue of Nature Immunology, Professor Sanyal from the University of Oxford in the United Kingdom, together with scholars from the School of Public Health of the University of Hong Kong Li Ka-shing School of Medicine and the VIB-UGent Center for Medical Biotechnology in Belgium, jointly reported that the SARS-CoV-2 virus can be encoded by the papain-like protease (Papain-like protease, PLpro) mediates the deISGization of antiviral proteins, thereby inducing human macrophages to secrete ISG15, and the uncoupled ISG15 outside the cell can play a cytokine-like effect, which stimulates macrophages in feedback and aggravates SARS-CoV-2 triggers macrophages to release large amounts of inflammatory cytokines and induce inflammatory damage
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Nature Immunology invited Academician Cao Xuetao to comment on the characteristics and significance of the paper in the same issue
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The severity and mortality of COVID-19 are related to viral immune evasion and triggering excessive inflammation and cytokine storms
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Some immunotherapy strategies that can block the production and function of pro-inflammatory cytokines have been clinically proven to control the course of COVID-19 to a certain extent
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Activation of the natural immune response is necessary to effectively eliminate the invading virus, but its abnormal activation and excessive production of pro-inflammatory cytokines may cause the host's own tissue damage
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One of the immunopathological characteristics of COVID-19 is the imbalance of natural immune response, which is characterized by weak response to type I interferon but significant release of pro-inflammatory cytokines (including a mixture of IL-1β, IL-6 and TNFα and chemokines) , Including CCL2, CCL3 and CXCL8)
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Based on the study of the escape mechanism of the SARS-CoV-2 encoded protein, there are currently some explanations for the production of type I interferon and the abnormal antiviral effect induced by type I interferon.
However, for SARS-CoV-2 infection The reason why the production of pro-inflammatory cytokines is significantly increased to induce more severe inflammatory damage is still poorly understood
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The paper found that after SARS-CoV-2 infects human macrophages, the papain-like protease PLpro encoded by SARS-CoV-2 can induce the release of intracellular ISG15 to the outside of the cell, and the free ISG15 outside the cell continues to be amplified in a cytokine-like manner The expression of a variety of pro-inflammatory cytokines and chemokines is one of the possible causes of excessive inflammation in patients with COVID-19
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The findings of Sanyal’s team not only provide relevant immunopathological mechanisms for the excessive inflammation caused by SARS-CoV-2, but also suggest that blocking the activity of the virus PLpro or neutralizing extracellular ISG15 can be used as a countermeasure for the critical illness and "inflammatory storm" of COVID-19.
Potential treatment strategies
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Cao Xuetao drew Figure 1 to summarize this work
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Figure 1: Secreted ISG15 feedback stimulates macrophages to amplify the inflammatory response caused by SARS-CoV-2 infection
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In the immune response to viral infection, the host cell activates the natural immune signal pathway to induce the expression of IFN-I and pro-inflammatory cytokines
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The IFN-I signal induces hundreds of ISGs, including ISG15, to activate the antiviral natural immune response
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ISG15 covalently binds to the target protein in the cell.
This process is called ISGization, which helps protect against viruses
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SARS-CoV-2 infection induced a weak IFN-I response and severe pro-inflammatory cytokines (including IL-6 and IL-1β) and chemokines including CCL2
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SARS-CoV-2 can induce macrophages to preferentially secrete ISG15 through its PLpro (as a de-ISG enzyme), and in turn, secrete ISG15 freely outside the cell as a cytokine to further promote macrophage pro-inflammatory cytokines and chemotaxis The production of cytokines, which intensifies the inflammation caused by SARS-CoV-2, and may cause a cytokine storm
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After viral RNA is recognized, RNA receptors such as RIG-I and MDA-5 can activate kinases TBK1 and IKKε to initiate downstream innate immune signal transduction, activate transcription factors IRF3 and IRF7, and express IFN-I and pro-inflammatory cytokines
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IFN-I molecules activate the JAK-STAT pathway and induce the expression of hundreds of ISG genes, including ISG15, which directly or indirectly exerts an antiviral natural immune effect
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RIG-1 and/or MDA5 are considered as potential SARS-CoV-2 receptors, and SARS-CoV-2 can escape the natural antiviral response by disrupting ISG15-dependent MDA5 activation
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In combination with previous reports that free extracellular ISG15 can be used as a cytokine to induce inflammation, the paper found that a "second wave" (Second wave) pro-inflammatory mode was proposed, that is, SARS-CoV-2 mediated extracellular ISG15 release once again promoted And amplify the inflammatory response
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ISG15 is a 15kDa ubiquitin-like protein that can be covalently bound to other protein molecules and participates in the defense against infections by a variety of viral pathogens
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The covalent binding of ISG15, commonly referred to as ISG, is a three-step enzyme cascade reaction: E1 enzyme UBE1L (also known as UBA7), E2 enzyme UBCH8 (also known as UBE2L6) and the main E3 enzyme HERC5, and USP18 (also known as UBP43) encoded by the host itself mediates the deISG process
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The antiviral effect of ISG15 mainly depends on the direct ISGization of various viral proteins by ISG15, thereby inhibiting virus replication and interrupting the virus life cycle
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The role of ISG15 in the antiviral response is controversial.
For example, there are reports showing that ISG15 is an important effector molecule in the antiviral response of mice but not humans.
Patients with ISG15 deficiency even show enhanced type I IFN response characteristics, which may be due to USP18 (Negative regulator of IFNAR signal) decreased stability
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In host cells, ISG15 exists in three forms: ISG15 connected in the ISG protein, ISG15 free in the cell, and ISG15 secreted outside the cell
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ISGylation of MDA5 triggers the activation of innate immunity, ISGylation of IRF3 maintains its stability, and ISGylation of STAT1 maintains its phosphorylation and activation
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ISG15 non-covalently binds to USP18 in human cells and inhibits its degradation
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Intracellular ISG15 binds ubiquitin-binding protein p62, and histone deacetylases HDAC6 and RIG-I to regulate their autophagy clearance
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As an extracellular free factor, ISG15 has been confirmed to have a cytokine-like function, which mediates signal transduction by binding to the cell surface integrin receptor CD11a-CD18
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These findings prove that the multiple functions of ISG15 in innate immunity and inflammatory response depend on its specific location and the target protein it binds
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Further research found that SARS-CoV-2 infection induced macrophages derived from human peripheral blood monocytes to express ISG-related enzymes, including ISG15, UBE1L, UBCH8 and HERC5, and USP18
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However, SARS-CoV-2 infection does not enhance the ISGization of intracellular proteins, but promotes the extracellular release of ISG15
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This phenomenon is not caused by IFN-I signal, but by live virus infection
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The massive release of ISG15 by macrophages is selectively triggered by SARS-CoV-2, but Zika virus and influenza virus infections cannot trigger this phenomenon
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The release of ISG15 is mediated in an autophagy-dependent manner and a non-classical secretion mechanism
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These findings are consistent with previous reports that the ISG modified enzyme system is induced by IFN-I, and pathogen infection induces the release of ISG15
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Intracellular ISG may be an inhibitor of the secretion of inflammatory factors (such as IL-1β, CCL2 and IL-6), because Sanyal et al.
found that macrophages infected with SARS-CoV after knocking out ISG15, UBE1L and HERC5 genes -2 The production of inflammatory cytokines in cells was significantly reduced
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Therefore, the authors proposed that the high ratio of free ISG15 to ISG15 in macrophages may be related to the polarization of M1 pro-inflammatory macrophages, and can be used as an indicator of the severity of SARS-CoV-2 mediated inflammation
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The author further performed quantitative proteomic analysis of the intracellular ISGylated protein and secretome of ISG15 knockout or knockdown cells and found that in the ISGylome protein profile, some previously reported ISG15 target proteins, such as RIG-I and STAT1 -3 The expression of UBE1L, UBCH8 and HERC5 was significantly down-regulated in ISG15 knockout cells
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The authors used recombinant ISG15 to stimulate ISG15 knockdown cells to release inflammatory cytokines, confirming the inflammation amplification effect of secreted ISG15
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Although the authors observed an increase in serum ISG15 in patients one week after the onset of COVID-19, the clinical significance of SARS-CoV-2 infection of macrophages to secrete ISG15 is still uncertain
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It is necessary to further analyze the correlation between serum ISG15 levels and other inflammatory cytokines, clinical manifestations and pathological processes at different stages of COVID-19 in a larger cohort to confirm its significance
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Further research using recombinant ISG15 may help to understand the secretion group affected by extracellular ISG15 and the mechanism of ISG15-triggered secretion in macrophages where the tissue resides and migrates
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Another exciting result of this study is the confirmation that PLpro encoded by SARS-CoV-2 acts as a de-ISGase to mediate immune escape and regulate the production of inflammatory factors
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PLpro is one of the non-structural proteins encoded by the SARS-CoV-2 genome, which acts as an enzyme that can cleave proteins or remove ubiquitin or ISG15 from modified proteins
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PLpro is indispensable for the efficient cleavage of viral polyproteins, and this process is critical to the transcription and replication of the viral genome
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The PLpro of SARS-CoV-2 can enzymatically reduce the ISG of MDA5 and IRF3, thereby inhibiting the innate antiviral response
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Studies have found that PLpro with catalytic activity promotes the production of pro-inflammatory cytokines and the release of ISG15 in SARS-CoV-2 infected cells, suggesting that PLpro can be used as a potential target to reverse the destroyed natural antiviral response and reduce the aggravation Inflammation storm
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Future work may focus on solving how PLpro or ISG15 affect autophagy-related secretion pathways, which is a way to selectively block the release of free ISG15 induced by SARS-CoV-2 infection, and explore the effects of PLpro inhibitors on SARS-CoV -2 The effect of inflammatory cytokine storm and lung injury induced
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This study revealed the important role of extracellular ISG15 in amplifying the inflammation induced by SARS-CoV-2, and proposed that PLpro can be used as a key target for therapeutic strategies to reverse virus escape and combat SARS-CoV-2 induced inflammation
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Whether the neutralization of free ISG15 can be achieved clinically and whether it is applicable to the control of inflammatory diseases induced by other viruses is a scientific issue worthy of further exploration
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