Influenza Virus RNA Synthesis and the Innate Immune Response

Defining the minimal components of the influenza A virus replication machinery via an in vitro reconstitution system

During influenza A virus infection, the viral RNA polymerase transcribes the viral negative-sense segmented RNA genome and replicates it in a two-step process via complementary RNA within viral ribonucleoprotein (vRNP) complexes. While numerous viral and host factors involved in vRNP functions have been identified, dissecting the roles of individual factors remains challenging due to the complex cellular environment in which vRNP activity has been studied. To overcome this challenge, we reconstituted viral transcription and a full cycle of replication in a test tube using vRNPs isolated from virions and recombinant factors essential for these processes. This novel system uncovers the minimal components required for influenza virus replication and also reveals new roles of regulatory factors in viral replication. Moreover, it sheds light on the molecular interplay underlying the temporal regulation of viral transcription and replication. Our highly robust in vitro system enables systematic functional analysis of factors modulating influenza virus vRNP activity and paves the way for imaging key steps of viral transcription and replication.

Respiratory Viruses and Virus-like Particle Vaccine Development: How Far Have We Advanced?

With technological advancements enabling globalization, the intercontinental transmission of pathogens has become much easier. Respiratory viruses are one such group of pathogens that require constant monitoring since their outbreak leads to massive public health crises, as exemplified by the influenza virus, respiratory syncytial virus (RSV), and the recent coronavirus disease 2019 (COVID-19) outbreak caused by the SARS-CoV-2. To prevent the transmission of these highly contagious viruses, developing prophylactic tools, such as vaccines, is of considerable interest to the scientific community. Virus-like particles (VLPs) are highly sought after as vaccine platforms for their safety and immunogenicity profiles. Although several VLP-based vaccines against hepatitis B and human papillomavirus have been approved for clinical use by the United States Food and Drug Administration, VLP vaccines against the three aforementioned respiratory viruses are lacking. Here, we summarize the most recent progress in pre-clinical and clinical VLP vaccine development. We also outline various strategies that contributed to improving the efficacy of vaccines against each virus and briefly discuss the stability aspect of VLPs that makes it a highly desired vaccine platform.

Research progress on the effect of traditional Chinese medicine on the activation of PRRs-mediated NF-κB signaling pathway to inhibit influenza pneumonia

Influenza pneumonia has challenged public health and social development. One of the hallmarks of severe influenza pneumonia is overproduction of pro-inflammatory cytokines and chemokines, which result from the continuous activation of intracellular signaling pathways, such as the NF-κB pathway, mediated by the interplay between viruses and host pattern recognition receptors (PRRs). It has been reported that traditional Chinese medicines (TCMs) can not only inhibit viral replication and inflammatory responses but also affect the expression of key components of PRRs and NF-κB signaling pathways. However, whether the antiviral and anti-inflammatory roles of TCM are related with its effects on NF-κB signaling pathway activated by PRRs remains unclear. Here, we reviewed the mechanism of PRRs-mediated activation of NF-κB signaling pathway following influenza virus infection and summarized the influence of anti-influenza TCMs on inflammatory responses and the PRRs/NF-κB signaling pathway, so as to provide better understanding of the mode of action of TCMs in the treatment of influenza pneumonia.

Defective Interfering Particles of Influenza Virus and Their Characteristics, Impacts, and Use in Vaccines and Antiviral Strategies: A Systematic Review

Defective interfering particles (DIPs) are particles containing defective viral genomes (DVGs) generated during viral replication. DIPs have been found in various RNA viruses, especially in influenza viruses. Evidence indicates that DIPs interfere with the replication and encapsulation of wild-type viruses, namely standard viruses (STVs) that contain full-length viral genomes. DIPs may also activate the innate immune response by stimulating interferon synthesis. In this review, the underlying generation mechanisms and characteristics of influenza virus DIPs are summarized. We also discuss the potential impact of DIPs on the immunogenicity of live attenuated influenza vaccines (LAIVs) and development of influenza vaccines based on NS1 gene-defective DIPs. Finally, we review the antiviral strategies based on influenza virus DIPs that have been used against both influenza virus and SARS-CoV-2. This review provides systematic insights into the theory and application of influenza virus DIPs.

The E3 ligase subunit FBXO45 binds the interferon-λ receptor and promotes its degradation during influenza virus infection

Influenza remains a major public health challenge, as the viral infection activates multiple biological networks linked to altered host innate immunity. Following infection, IFN-λ, a ligand crucial for the resolution of viral infections, is known to bind to its cognate receptor, IFNLR1, in lung epithelia. However, little is known regarding the molecular expression and regulation of IFNLR1. Here, we show that IFNLR1 is a labile protein in human airway epithelia that is rapidly degraded after influenza infection. Using an unbiased proximal ligation biotin screen, we first identified that the Skp-Cullin-F box E3 ligase subunit, FBXO45, binds to IFNLR1. We demonstrate that FBXO45, induced in response to influenza infection, mediates IFNLR1 protein polyubiquitination and degradation through the ubiquitin-proteasome system by docking with its intracellular receptor domain. Furthermore, we found ectopically expressed FBXO45 and its silencing in cells differentially regulated both IFNLR1 protein stability and interferon-stimulated gene expression. Mutagenesis studies also indicated that expression of a K319R/K320R IFNLR1 variant in cells exhibited reduced polyubiquitination, yet greater stability and proteolytic resistance to FBXO45 and influenza-mediated receptor degradation. These results indicate that the IFN-λ-IFNLR1 receptor axis is tightly regulated by the Skp-Cullin-F box ubiquitin machinery, a pathway that may be exploited by influenza infection as a means to limit antiviral responses.

Tripartite motif-containing protein 46 accelerates influenza A H7N9 virus infection by promoting K48-linked ubiquitination of TBK1

BACKGROUND

Avian influenza A H7N9 emerged in 2013, threatening public health and causing acute respiratory distress syndrome, and even death, in the human population. However, the underlying mechanism by which H7N9 virus causes human infection remains elusive.

METHODS

Herein, we infected A549 cells with H7N9 virus for different times and assessed tripartite motif-containing protein 46 (TRIM46) expression. To determine the role of TRIM46 in H7N9 infection, we applied lentivirus-based TRIM46 short hairpin RNA sequences and overexpression plasmids to explore virus replication, and changes in type I interferons and interferon regulatory factor 3 (IRF3) phosphorylation levels in response to silencing and overexpression of TRIM46. Finally, we used Co-immunoprecipitation and ubiquitination assays to examine the mechanism by which TRIM46 mediated the activity of TANK-binding kinase 1 (TBK1).

RESULTS

Type I interferons play an important role in defending virus infection. Here, we found that TRIM46 levels were significantly increased during H7N9 virus infection. Furthermore, TRIM46 knockdown inhibited H7N9 virus replication compared to that in the control group, while the production of type I interferons increased. Meanwhile, overexpression of TRIM46 promoted H7N9 virus replication and decrease the production of type I interferons. In addition, the level of phosphorylated IRF3, an important interferon regulatory factor, was increased in TRIM46-silenced cells, but decreased in TRIM46 overexpressing cells. Mechanistically, we observed that TRIM46 could interact with TBK1 to induce its K48-linked ubiquitination, which promoted H7N9 virus infection.

CONCLUSION

Our results suggest that TRIM46 negatively regulates the human innate immune response against H7N9 virus infection.

SERINC5 restricts influenza virus infectivity

SERINC5 is a multi-span transmembrane protein that is incorporated into HIV-1 particles in producing cells and inhibits HIV-1 entry. Multiple retroviruses like HIV-1, equine infectious anemia virus and murine leukemia virus are subject to SERINC5 inhibition, while HIV-1 pseudotyped with envelope glycoproteins of vesicular stomatitis virus and Ebola virus are resistant to SERINC5. The antiviral spectrum and the underlying mechanisms of SERINC5 restriction are not completely understood. Here we show that SERINC5 inhibits influenza A virus infection by targeting virus-cell membrane fusion at an early step of infection. Further results show that different influenza hemagglutinin (HA) subtypes exhibit diverse sensitivities to SERINC5 restriction. Analysis of the amino acid sequences of influenza HA1 strains indicates that HA glycosylation sites correlate with the sensitivity of influenza HA to SERINC5, and the inhibitory effect of SERINC5 was lost when certain HA glycosylation sites were mutated. Our study not only expands the antiviral spectrum of SERINC5, but also reveals the role of viral envelope glycosylation in resisting SERINC5 restriction.

Identification of Critical Genes and Pathways for Influenza A Virus Infections via Bioinformatics Analysis

Influenza A virus (IAV) requires the host cellular machinery for many aspects of its life cycle. Knowledge of these host cell requirements not only reveals molecular pathways exploited by the virus or triggered by the immune system but also provides further targets for antiviral drug development. To uncover critical pathways and potential targets of influenza infection, we assembled a large amount of data from 8 RNA sequencing studies of IAV infection for integrative network analysis. Weighted gene co-expression network analysis (WGCNA) was performed to investigate modules and genes correlated with the time course of infection and/or multiplicity of infection (MOI). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were performed to explore the biological functions and pathways of the genes in 5 significant modules. Top hub genes were identified using the cytoHubba plugin in the protein interaction network. The correlation between expression levels of 7 top hub genes and time course or MOI was displayed and validated, including BCL2L13, PLSCR1, ARID5A, LMO2, NDRG4, HAP1, and CARD10. Dysregulated expression of these genes potently impacted the development of IAV infection through modulating IAV-related biological processes and pathways. This study provides further insights into the underlying molecular mechanisms and potential targets in IAV infection.

Anti-influenza Virus Activity of Methylthio-Formycin Distinct From That of T-705

Seasonal influenza virus epidemics result in severe illness, and occasionally influenza pandemics cause significant morbidity and mortality, although vaccines and anti-influenza virus drugs are available. By screening an in-house library, we identified methylthio-formycin (SMeFM), an adenosine analog, as a potent inhibitor of influenza virus propagation. SMeFM inhibited the propagation of influenza A and B viruses (IC₅₀: 34.1 and 37.9 nM, respectively) and viruses showing reduced susceptibility to baloxavir and neuraminidase inhibitors but not T-705 (Favipiravir). However, the combination of T-705 and SMeFM inhibited the propagation of the influenza virus not in an antagonistic but in a slightly synergistic manner, suggesting that SMeFM has targets distinct from that of T-705. SMeFM induced A-to-C transversion mutations in virus genome RNA, and SMeFM triphosphate did not inhibit in vitro viral RNA synthesis. Our results show that SMeFM inhibits the propagation of the influenza virus by a mechanism different from that of T-705 and is a potential drug candidate to develop for anti-influenza drug.

Characterization of influenza A virus induced transposons reveals a subgroup of transposons likely possessing the regulatory role as eRNAs

Although many studies have observed genome-wide host transposon expression alteration during viral infection, the mechanisms of induction and the impact on the host remain unclear. Utilizing recently published influenza A virus (IAV) time series data and ENCODE functional genomics data, we characterized virus induced host differentially expressed transposons (virus-induced-TE) by investigating genome-wide spatial and functional relevance between the virus-induced-TEs and epigenomic markers (e.g. histone modification and chromatin remodelers). We found that a significant fraction of virus-induced-TEs are derived from host enhancer regions, where CHD4 binding and/or H3K27ac occupancy is high or H3K9me3 occupancy is low. By overlapping virus-induced-TEs to human enhancer RNAs (eRNAs), we discovered that a proportion of virus-induced-TEs are either eRNAs or part of enhancer RNAs. Upon further analysis of the eRNA targeted genes, we found that the virus-induced-TE related eRNA targets are overrepresented in differentially expressed host genes of IAV infected samples. Our results suggest that changing chromatin accessibility from repressive to permissive in the transposon docked enhancer regions to regulate host downstream gene expression is potentially one of the virus and host cell interaction mechanisms, where transposons are likely important regulatory genomic elements. Our study provides a new insight into the mechanisms of virus-host interaction and may lead to novel strategies for prevention and therapeutics of IAV and other virus infectious diseases.

Gene therapy: Comprehensive overview and therapeutic applications

Gene therapy is the product of man's quest to eliminate diseases. Gene therapy has three facets namely, gene silencing using siRNA, shRNA and miRNA, gene replacement where the desired gene in the form of plasmids and viral vectors, are directly administered and finally gene editing based therapy where mutations are modified using specific nucleases such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and clustered regulatory interspaced short tandem repeats (CRISPR)/CRISPR-associated protein (Cas)-associated nucleases. Transfer of gene is either through transformation where under specific conditions the gene is directly taken up by the bacterial cells, transduction where a bacteriophage is used to transfer the genetic material and lastly transfection that involves forceful delivery of gene using either viral or non-viral vectors. The non-viral transfection methods are subdivided into physical, chemical and biological. The physical methods include electroporation, biolistic, microinjection, laser, elevated temperature, ultrasound and hydrodynamic gene transfer. The chemical methods utilize calcium- phosphate, DAE-dextran, liposomes and nanoparticles for transfection. The biological methods are increasingly using viruses for gene transfer, these viruses could either integrate within the genome of the host cell conferring a stable gene expression, whereas few other non-integrating viruses are episomal and their expression is diluted proportional to the cell division. So far, gene therapy has been wielded in a plethora of diseases. However, coherent and innocuous delivery of genes is among the major hurdles in the use of this promising therapy. Hence this review aims to highlight the current options available for gene transfer along with the advantages and limitations of every method.

Mesenchymal stem/stromal cell-based therapies for severe viral pneumonia: therapeutic potential and challenges

Severe viral pneumonia is a significant cause of morbidity and mortality globally, whether due to outbreaks of endemic viruses, periodic viral epidemics, or the rarer but devastating global viral pandemics. While limited anti-viral therapies exist, there is a paucity of direct therapies to directly attenuate viral pneumonia-induced lung injury, and management therefore remains largely supportive. Mesenchymal stromal/stem cells (MSCs) are receiving considerable attention as a cytotherapeutic for viral pneumonia. Several properties of MSCs position them as a promising therapeutic strategy for viral pneumonia-induced lung injury as demonstrated in pre-clinical studies in relevant models. More recently, early phase clinical studies have demonstrated a reassuring safety profile of these cells. These investigations have taken on an added importance and urgency during the COVID-19 pandemic, with multiple trials in progress across the globe. In parallel with clinical translation, strategies are being investigated to enhance the therapeutic potential of these cells in vivo, with different MSC tissue sources, specific cellular products including cell-free options, and strategies to ‘licence’ or ‘pre-activate’ these cells, all being explored. This review will assess the therapeutic potential of MSC-based therapies for severe viral pneumonia. It will describe the aetiology and epidemiology of severe viral pneumonia, describe current therapeutic approaches, and examine the data suggesting therapeutic potential of MSCs for severe viral pneumonia in pre-clinical and clinical studies. The challenges and opportunities for MSC-based therapies will then be considered.

PU.1 is involved in the transcriptional up-regulation of RNA and DNA sensing pathway genes in buffalo fibroblasts

PU.1, CEBPA, and CEBPB are Lineage Determining Transcription Factors (LDTFs) that play roles in biological processes such as cell differentiation and the immune system regulation including the innate immune pathways. The roles of these LDTFs in the innate RNA and DNA sensing pathways have received little attention. We show that in buffalo fibroblasts, PU.1 causes the mRNA up-regulation of the RNA and DNA sensors such as RIG-I (65.1 fold), MDA5 (20.4 fold), IFI16-l (8.0 fold), and cGAS (60.5 fold) while CEBPA does the same but to a lesser extent (RIG-I-26.4 fold, MDA5-10.8 fold, IFI16-l- 3.3 fold and cGAS-8.6 fold). CEBPB does not appear to have a role in the up-regulation of these genes. PU.1 expression also primes the cells to develop a strong immune response against the dsRNA virus mimic polyinosinic:polycytidylic acid (poly I:C) by significantly up-regulating Interferon-β (14.9 fold change with p-value <0.0001). CEBPA up-regulates Interferon-β to a lower level than PU.1 (4.7 fold change with p-value 0.0024), whereas CEBPB exhibits non-significant up-regulation (2.1 fold with p-value of 0.1449). As PU.1 robustly up-regulates the nucleic acid sensing pathways, it can prove to be useful in improving the defence against viruses that can cause losses to animal husbandry.