Inhibition of the IFN-β Response in Hepatocellular Carcinoma by Alternative Spliced Isoform of IFN Regulatory Factor-3

Eukaryotic splicing machinery in the plant-virus battleground

Plant virual infections are mainly caused by plant-virus parasitism which affects ecological communities. Some viruses are highly pathogen specific that can infect only specific plants, while some can cause widespread harm, such as tobacco mosaic virus (TMV) and cucumber mosaic virus (CMV). After a virus infects the host, undergoes a series of harmful effects, including the destruction of host cell membrane receptors, changes in cell membrane components, cell fusion, and the production of neoantigens on the cell surface. Therefore, competition between the host and the virus arises. The virus starts gaining control of critical cellular functions of the host cells and ultimately affects the fate of the targeted host plants. Among these critical cellular processes, alternative splicing (AS) is an essential posttranscriptional regulation process in RNA maturation, which amplify host protein diversity and manipulates transcript abundance in response to plant pathogens. AS is widespread in nearly all human genes and critical in regulating animal-virus interactions. In particular, an animal virus can hijack the host splicing machinery to re-organize its compartments for propagation. Changes in AS are known to cause human disease, and various AS events have been reported to regulate tissue specificity, development, tumour proliferation, and multi-functionality. However, the mechanisms underlying plant-virus interactions are poorly understood. Here, we summarize the current understanding of how viruses interact with their plant hosts compared with humans, analyze currently used and putative candidate agrochemicals to treat plant-viral infections, and finally discussed the potential research hotspots in the future. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Processing > Splicing Regulation/Alternative Splicing.

Rift Valley Fever Virus Nucleoprotein Triggers Autophagy to Dampen Antiviral Innate Immune Responses

Rift Valley fever virus (RVFV) is a mosquito-borne bunyavirus that causes severe and potentially fatal hemorrhagic fever in humans. Autophagy is a self-degradative process that can restrict viral replication at multiple infection steps. In this study, we evaluated the effects of RVFV-triggered autophagy on viral replication and immune responses. Our results showed that RVFV infection triggered autophagosome formation and induced complete autophagy. Impairing autophagy flux by depleting autophagy-related gene 5 (ATG5), ATG7, or sequestosome 1 (SQSTM1) or treatment with autophagy inhibitors markedly reduced viral RNA synthesis and progeny virus production. Mechanistically, our findings demonstrated that the RVFV nucleoprotein (NP) C-terminal domain interacts with the autophagy receptor SQSTM1 and promotes the SQSTM1-microtubule-associated protein 1 light chain 3 B (LC3B) interaction and autophagy. Deletion of the NP C-terminal domain impaired the interaction between NP and SQSTM1 and its ability to trigger autophagy. Notably, RVFV-triggered autophagy promoted viral infection in macrophages but not in other tested cell types, including Huh7 hepatocytes and human umbilical vein endothelial cells, suggesting cell type specificity of this mechanism. It was further revealed that RVFV NP-triggered autophagy dampens antiviral innate immune responses in infected macrophages to promote viral replication. These results provide novel insights into the mechanisms of RVFV-triggered autophagy and indicate the potential of targeting the autophagy pathway to develop antivirals against RVFV. IMPORTANCE We showed that RVFV infection induced the complete autophagy process. Depletion of the core autophagy genes ATG5, ATG7, or SQSTM1 or pharmacologic inhibition of autophagy in macrophages strongly suppressed RVFV replication. We further revealed that the RVFV NP C-terminal domain interacted with SQSTM1 and enhanced the SQSTM1/LC3B interaction to promote autophagy. RVFV NP-triggered autophagy strongly inhibited virus-induced expression of interferon-stimulated genes in infected macrophages but not in other tested cell types. Our study provides novel insights into the mechanisms of RVFV-triggered autophagy and highlights the potential of targeting autophagy flux to develop antivirals against this virus.

Serine-arginine splicing factor 2 promotes oesophageal cancer progression by regulating alternative splicing of interferon regulatory factor 3

OBJECTIVE

Often, alternative splicing is used by cancer cells to produce or increase proteins that promote growth and survival through alternative splicing. Although RNA-binding proteins are known to regulate alternative splicing events associated with tumorigenesis, their role in oesophageal cancer (EC) has rarely been explored.

METHODS

We analysed the expression pattern of several relatively well characterized splicing regulators on 183 samples from TCGA cohort of oesophageal cancer; the effectiveness of the knockdown of SRSF2 was subsequently verified by immunoblotting; we measured the ability of cells treated with lenti-sh-SRSF2/lenti-sh2-SRSF2 to invade through an extracellular matrix coating by transwell invasion assay; using RNA-seq data to identify its potential target genes; we performed qRT-PCR to detect the changes of exon 2 usage in lenti-sh-SRSF2 transduced KYSE30 cells to determine the possible effect of SRSF2 on splicing regulation of IRF3; RNA Electrophoretic mobility shift assay (RNA-EMSA) was performed by the incubation of purified SRSF2 protein and biotinylated RNA probes; we performed luciferase assay to confirm the effect of SRSF2 on IFN1 promoter activity.

RESULTS

We found upregulation of SRSF2 is correlated with the development of EC; Knock-down of SRSF2 inhibits EC cell proliferation, migration, and invasion; SRSF2 regulates the splicing pattern of IRF3 in EC cells; SRSF2 interacts with exon 2 of IRF3 to regulate its exclusion; SRSF2 inhibits the transcription of IFN1 in EC cells.

CONCLUSION

This study identified a novel regulatory axis involved in EC from the various aspects of splicing regulation.

Alternative pre-mRNA splicing as a mechanism for terminating Toll-like Receptor signaling

While inflammation induced by Toll-like receptor (TLR) signaling is required to combat infection, persistent inflammation can damage host tissues and contribute to a myriad of acute and chronic inflammatory disorders. Thus, it is essential not only that TLR signaling be activated in the presence of pathogens but that TLR signaling is ultimately terminated. One mechanism that limits persistent TLR signaling is alternative pre-mRNA splicing. In addition to encoding the canonical mRNAs that produce proteins that promote inflammation, many genes in the TLR signaling pathway also encode alternative mRNAs that produce proteins that are dominant negative inhibitors of signaling. Many of these negative regulators are induced by immune challenge, so production of these alternative isoforms represents a negative feedback loop that limits persistent inflammation. While these alternative splicing events have been investigated on a gene by gene basis, there has been limited systemic analysis of this mechanism that terminates TLR signaling. Here we review what is known about the production of negatively acting alternative isoforms in the TLR signaling pathway including how these inhibitors function, how they are produced, and what role they may play in inflammatory disease.

Ebola virus infection induces a delayed type I IFN response in bystander cells and the shutdown of key liver genes in human iPSC-derived hepatocytes

Liver damage and an exacerbated inflammatory response are hallmarks of Ebola virus (EBOV) infection. Little is known about the intrinsic response to infection in human hepatocytes and their contribution to inflammation. Here, we present an induced pluripotent stem cell (iPSC)-derived hepatocyte-like cell (HLC) platform to define the hepato-intrinsic response to EBOV infection. We used this platform to show robust EBOV infection, with characteristic ultrastructural changes and evidence for viral replication. Transcriptomics analysis revealed a delayed response with minimal early transcriptomic changes, followed by a general downregulation of hepatic function and upregulation of interferon signaling, providing a potential mechanism by which hepatocytes participate in disease severity and liver damage. Using RNA-fluorescence in situ hybridization (FISH), we showed that IFNB1 and CXCL10 were mainly expressed in non-infected bystander cells. We did not observe an inflammatory signature during infection. In conclusion, iPSC-HLCs are an immune competent platform to study responses to EBOV infection.

Discovery Proteomics Analysis Determines That Driver Oncogenes Suppress Antiviral Defense Pathways Through Reduction in Interferon-β Autocrine Stimulation

Since the discovery of oncogenes, there has been tremendous interest to understand their mechanistic basis and to develop broadly actionable therapeutics. Some of the most frequently activated oncogenes driving diverse cancers are c-MYC, EGFR, HER2, AKT, KRAS, BRAF, and MEK. Using a reductionist approach, we explored how cellular proteomes are remodeled in isogenic cell lines engineered with or without these driver oncogenes. The most striking discovery for all oncogenic models was the systematic downregulation of scores of antiviral proteins regulated by type 1 interferon. These findings extended to cancer cell lines and patient-derived xenograft models of highly refractory pancreatic cancer and osteosarcoma driven by KRAS and MYC oncogenes. The oncogenes reduced basal expression of and autocrine stimulation by type 1 interferon causing remarkable convergence on common phenotypic and functional profiles. In particular, there was dramatically lower expression of dsRNA sensors including DDX58 (RIG-I) and OAS proteins, which resulted in attenuated functional responses when the oncogenic cells were treated with the dsRNA mimetic, polyI:C, and increased susceptibility to infection with an RNA virus shown using SARS-CoV-2. Our reductionist approach provides molecular and functional insights connected to immune evasion hallmarks in cancers and suggests therapeutic opportunities.

The Alternatively Spliced Isoforms of Key Molecules in the cGAS-STING Signaling Pathway

Alternative splicing of pre-mRNA increases transcriptome and proteome diversity by generating distinct isoforms that encode functionally diverse proteins, thus affecting many biological processes, including innate immunity. cGAS-STING signaling pathway, whose key molecules also undergo alternative splicing, plays a crucial role in regulating innate immunity. Protein isoforms of key components in the cGAS-STING-TBK1-IRF3 axis have been detected in a variety of species. A chain of evidence showed that these protein isoforms exhibit distinct functions compared to their normal counterparts. The mentioned isoforms act as positive or negative modulators in interferon response via distinct mechanisms. Particularly, we highlight that alternative splicing serves a vital function for the host to avoid the overactivation of the cGAS-STING signaling pathway and that viruses can utilize alternative splicing to resist antiviral response by the host. These findings could provide insights for potential alternative splicing-targeting therapeutic applications.

Alternative Splicing-Based Differences Between Hepatocellular Carcinoma and Intrahepatic Cholangiocarcinoma: Genes, Immune Microenvironment, and Survival Prognosis

Alternative splicing (AS) event is a novel biomarker of tumor tumorigenesis and progression. However, the comprehensive analysis of hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (ICC) is lacking. Differentially expressed analysis was used to identify the differentially expressed alternative splicing (DEAS) events between HCC or ICC tissues and their normal tissues. The correlation between DEAS events and functional analyses or immune features was evaluated. The cluster analysis based on DEAS can accurately reflect the differences in the immune microenvironment between HCC and ICC. Forty-five immune checkpoints and 23 immune features were considered statistically significant in HCC, while only seven immune checkpoints and one immune feature in ICC. Then, the prognostic value of DEAS events was studied, and two transcripts with different basic cell functions (proliferation, cell cycle, invasion, and migration) were produced by ADHFE1 through alternative splicing. Moreover, four nomograms were established in conjunction with relevant clinicopathological factors. Finally, we found two most significant splicing factors and further showed their protein crystal structure. The joint analysis of the AS events in HCC and ICC revealed novel insights into immune features and clinical prognosis, which might provide positive implications in HCC and ICC treatment.

Role of Alternative Splicing in Regulating Host Response to Viral Infection

The importance of transcriptional regulation of host genes in innate immunity against viral infection has been widely recognized. More recently, post-transcriptional regulatory mechanisms have gained appreciation as an additional and important layer of regulation to fine-tune host immune responses. Here, we review the functional significance of alternative splicing in innate immune responses to viral infection. We describe how several central components of the Type I and III interferon pathways encode spliced isoforms to regulate IFN activation and function. Additionally, the functional roles of splicing factors and modulators in antiviral immunity are discussed. Lastly, we discuss how cell death pathways are regulated by alternative splicing as well as the potential role of this regulation on host immunity and viral infection. Altogether, these studies highlight the importance of RNA splicing in regulating host–virus interactions and suggest a role in downregulating antiviral innate immunity; this may be critical to prevent pathological inflammation.

Identification of prognostic biomarkers and correlations with immune infiltrates among cGAS-STING in hepatocellular carcinoma

The cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway induces innate immunity by activating the production of inflammatory cytokines and type I interferons. Recently, studies revealed that self-DNA from by-products of chromosome instability and tumors could activate the cGAS-STING pathway, and subsequently promote or inhibit tumor development. However, the prognostic value and correlations with immune infiltrates of the cGAS-STING pathway in hepatocellular carcinoma (HCC) have not been clarified. In the present study, we used the Molecular Signatures Database, Oncomine, UALCAN, Human Protein Atlas, Kaplan–Meier plotter, LinkedOmics, and Tumor Immune Estimation Resource databases. Overexpression of XRCC5, IRF3, TRIM21, STAT6, DDX41, TBK1, XRCC6, TREX1, PRKDC, and TMEM173 was markedly correlated with clinical stages and pathological grades in HCC. Moreover, higher mRNA expression of XRCC5, XRCC6, and PRKDC was significantly related with shorter overall survival. However, higher mRNA expression of IFI16, STAT6, NLRC3, and TMEM173 was associated with favorable overall survival. Our results suggested that the kinase targets of the cGAS-STING pathway included the SRC family of tyrosine kinases (LCK and LYN), phosphoinositide 3-kinase-related protein kinase (PIKK) family kinases (ATM and ATR), and mitogen-activated protein kinase 1 (MAPK1). Furthermore, we identified significant correlations among the expression of cGAS-STING pathway and infiltration of B cells, CD4+T cells, CD8+ T cells, macrophages, neutrophils, and dendritic cells in HCC. The expression of the cGAS-STING pathway also exhibited strong relationships with diverse immune marker sets in HCC. These findings suggest that cGAS-STING pathway members may be used as prognostic biomarkers and immunotherapeutic targets HCC patients.

Characterization of a Cell Culture System of Persistent Hepatitis E Virus Infection in the Human HepaRG Hepatic Cell Line

Hepatitis E virus (HEV) is considered as an emerging global health problem. In most cases, hepatitis E is a self-limiting disease and the virus is cleared spontaneously without the need of antiviral therapy. However, immunocompromised individuals can develop chronic infection and liver fibrosis that can progress rapidly to cirrhosis and liver failure. The lack of efficient and relevant cell culture system and animal models has limited our understanding of the biology of HEV and the development of effective drugs for chronic cases. In the present study, we developed a model of persistent HEV infection in human hepatocytes in which HEV replicates efficiently. This HEV cell culture system is based on differentiated HepaRG cells infected with an isolate of HEV-3 derived from a patient suffering from acute hepatitis E. Efficient replication was maintained for several weeks to several months as well as after seven successive passages on HepaRG naïve cells. Moreover, after six passages onto HepaRG, we found that the virus was still infectious after oral inoculation into pigs. We also showed that ribavirin had an inhibitory effect on HEV replication in HepaRG. In conclusion, this system represents a relevant and efficient in vitro model of HEV replication that could be useful to study HEV biology and identify effective antiviral drugs against chronic HEV infection.

Of Keeping and Tipping the Balance: Host Regulation and Viral Modulation of IRF3-Dependent IFNB1 Expression

The type I interferon (IFN) response is a principal component of our immune system that allows to counter a viral attack immediately upon viral entry into host cells. Upon engagement of aberrantly localised nucleic acids, germline-encoded pattern recognition receptors convey their find via a signalling cascade to prompt kinase-mediated activation of a specific set of five transcription factors. Within the nucleus, the coordinated interaction of these dimeric transcription factors with coactivators and the basal RNA transcription machinery is required to access the gene encoding the type I IFN IFNβ (IFNB1). Virus-induced release of IFNβ then induces the antiviral state of the system and mediates further mechanisms for defence. Due to its key role during the induction of the initial IFN response, the activity of the transcription factor interferon regulatory factor 3 (IRF3) is tightly regulated by the host and fiercely targeted by viral proteins at all conceivable levels. In this review, we will revisit the steps enabling the trans-activating potential of IRF3 after its activation and the subsequent assembly of the multi-protein complex at the IFNβ enhancer that controls gene expression. Further, we will inspect the regulatory mechanisms of these steps imposed by the host cell and present the manifold strategies viruses have evolved to intervene with IFNβ transcription downstream of IRF3 activation in order to secure establishment of a productive infection.

Experimental Evolution Generates Novel Oncolytic Vesicular Stomatitis Viruses with Improved Replication in Virus-Resistant Pancreatic Cancer Cells

Vesicular stomatitis virus (VSV)-based oncolytic viruses are promising agents against pancreatic ductal adenocarcinoma (PDAC). However, some PDAC cell lines are resistant to VSV. Here, using a directed viral evolution approach, we generated novel oncolytic VSVs with an improved ability to replicate in virus-resistant PDAC cell lines, while remaining highly attenuated in nonmalignant cells. Two independently evolved VSVs obtained 2 identical VSV glycoprotein mutations, K174E and E238K. Additional experiments indicated that these acquired G mutations improved VSV replication, at least in part due to improved virus attachment to SUIT-2 cells. Importantly, no deletions or mutations were found in the virus-carried transgenes in any of the passaged viruses. Our findings demonstrate long-term genomic stability of complex VSV recombinants carrying large transgenes and support further clinical development of oncolytic VSV recombinants as safe therapeutics for cancer.

Vesicular stomatitis virus (VSV) based oncolytic viruses are promising agents against various cancers. We have shown that pancreatic ductal adenocarcinoma (PDAC) cell lines exhibit great diversity in susceptibility and permissibility to VSV. Here, using a directed evolution approach with our two previously described oncolytic VSV recombinants, VSV-p53wt and VSV-p53-CC, we generated novel oncolytic VSVs with an improved ability to replicate in virus-resistant PDAC cell lines. VSV-p53wt and VSV-p53-CC encode a VSV matrix protein (M) with a ΔM51 mutation (M-ΔM51) and one of two versions of a functional human tumor suppressor, p53, fused to a far-red fluorescent protein, eqFP650. Each virus was serially passaged 32 times (which accounts for more than 60 viral replication cycles) on either the SUIT-2 (moderately resistant to VSV) or MIA PaCa-2 (highly permissive to VSV) human PDAC cell lines. While no phenotypic changes were observed for MIA PaCa-2-passaged viruses, both SUIT-2-passaged VSV-p53wt and VSV-p53-CC showed improved replication in SUIT-2 and AsPC-1, another human PDAC cell line also moderately resistant to VSV, while remaining highly attenuated in nonmalignant cells. Surprisingly, two identical VSV glycoprotein (VSV-G) mutations, K174E and E238K, were identified in both SUIT-2-passaged viruses. Additional experiments indicated that the acquired G mutations improved VSV replication, at least in part due to improved virus attachment to SUIT-2 cells. Importantly, no mutations were found in the M-ΔM51 protein, and no deletions or mutations were found in the p53 or eqFP650 portions of virus-carried transgenes in any of the passaged viruses, demonstrating long-term genomic stability of complex VSV recombinants carrying large transgenes.

IMPORTANCE Vesicular stomatitis virus (VSV)-based oncolytic viruses are promising agents against pancreatic ductal adenocarcinoma (PDAC). However, some PDAC cell lines are resistant to VSV. Here, using a directed viral evolution approach, we generated novel oncolytic VSVs with an improved ability to replicate in virus-resistant PDAC cell lines, while remaining highly attenuated in nonmalignant cells. Two independently evolved VSVs obtained 2 identical VSV glycoprotein mutations, K174E and E238K. Additional experiments indicated that these acquired G mutations improved VSV replication, at least in part due to improved virus attachment to SUIT-2 cells. Importantly, no deletions or mutations were found in the virus-carried transgenes in any of the passaged viruses. Our findings demonstrate long-term genomic stability of complex VSV recombinants carrying large transgenes and support further clinical development of oncolytic VSV recombinants as safe therapeutics for cancer.

The RNA binding protein Quaking represses host interferon response by downregulating MAVS

Quaking (QKI) is an RNA-binding protein (RBP) involved in multiple aspects of RNA metabolism and many biological processes. Despite a known immune function in regulating monocyte differentiation and inflammatory responses, the degree to which QKI regulates the host interferon (IFN) response remains poorly characterized. Here we show that QKI ablation enhances poly(I:C) and viral infection-induced IFNβ transcription. Characterization of IFN-related signalling cascades reveals that QKI knockout results in higher levels of IRF3 phosphorylation. Interestingly, complementation with QKI-5 isoform alone is sufficient to rescue this phenotype and reduce IRF3 phosphorylation. Further analysis shows that MAVS, but not RIG-I or MDA5, is robustly upregulated in the absence of QKI, suggesting that QKI downregulates MAVS and thus represses the host IFN response. As expected, MAVS depletion reduces IFNβ activation and knockout of MAVS in the QKI knockout cells completely abolishes IFNβ induction. Consistently, ectopic expression of RIG-I activates stronger IFNβ induction via MAVS-IRF3 pathway in the absence of QKI. Collectively, these findings demonstrate a novel role for QKI in negatively regulating host IFN response by reducing MAVS levels.

Cell Cycle Arrest in G2/M Phase Enhances Replication of Interferon-Sensitive Cytoplasmic RNA Viruses via Inhibition of Antiviral Gene Expression

Vesicular stomatitis virus (VSV) (a rhabdovirus) and its variant VSV-ΔM51 are widely used model systems to study mechanisms of virus-host interactions. Here, we investigated how the cell cycle affects replication of these viruses using an array of cell lines with different levels of impairment of antiviral signaling and a panel of chemical compounds arresting the cell cycle at different phases. We observed that all compounds inducing cell cycle arrest in G₂/M phase strongly enhanced the replication of VSV-ΔM51 in cells with functional antiviral signaling. G₂/M arrest strongly inhibited type I and type III interferon (IFN) production as well as expression of IFN-stimulated genes in response to exogenously added IFN. Moreover, G₂/M arrest enhanced the replication of Sendai virus (a paramyxovirus), which is also highly sensitive to the type I IFN response but did not stimulate the replication of a wild-type VSV that is more effective at evading antiviral responses. In contrast, the positive effect of G₂/M arrest on virus replication was not observed in cells defective in IFN signaling. Altogether, our data show that replication of IFN-sensitive cytoplasmic viruses can be strongly stimulated during G₂/M phase as a result of inhibition of antiviral gene expression, likely due to mitotic inhibition of transcription, a global repression of cellular transcription during G₂/M phase. The G₂/M phase thus could represent an "Achilles' heel" of the infected cell, a phase when the cell is inadequately protected. This model could explain at least one of the reasons why many viruses have been shown to induce G₂/M arrest.IMPORTANCE Vesicular stomatitis virus (VSV) (a rhabdovirus) and its variant VSV-ΔM51 are widely used model systems to study mechanisms of virus-host interactions. Here, we investigated how the cell cycle affects replication of VSV and VSV-ΔM51. We show that G₂/M cell cycle arrest strongly enhances the replication of VSV-ΔM51 (but not of wild-type VSV) and Sendai virus (a paramyxovirus) via inhibition of antiviral gene expression, likely due to mitotic inhibition of transcription, a global repression of cellular transcription during G₂/M phase. Our data suggest that the G₂/M phase could represent an "Achilles' heel" of the infected cell, a phase when the cell is inadequately protected. This model could explain at least one of the reasons why many viruses have been shown to induce G₂/M arrest, and it has important implications for oncolytic virotherapy, suggesting that frequent cell cycle progression in cancer cells could make them more permissive to viruses.

Defects in interferon pathways as potential biomarkers of sensitivity to oncolytic viruses

Increased sensitivity of cancer cells to viruses is a prerequisite for the success of oncolytic virotherapy. One of the major causes of such a phenotype is the disruption of innate antiviral defenses associated with dysfunction of type 1 interferons (IFNs) that permits unlimited replication of viruses in cancer cells. Defects in IFN pathways help cancer progression by providing additional advantages to tumor cells. However, while these defects promote the survival and accelerated proliferation of malignant cells, they facilitate viral replication and thus enhance the efficiency of viral oncolysis. This review describes a broad spectrum of defects in genes that participate in IFN induction and IFN response pathways. Expression levels and/or functional activities of these genes are frequently low or absent in cancer cells, making them sensitive to virus infection. Therefore, certain specific defects in IFN signaling cascades might serve as potential biomarkers to help in identifying individual cancer patients who are likely to benefit from oncolytic virotherapy.

Plasticity of Type I Interferon-Mediated Responses in Cancer Therapy: From Anti-tumor Immunity to Resistance

The efficacy of several therapeutic strategies against cancer, including cytotoxic drugs, radiotherapy, targeted immunotherapies and oncolytic viruses, depend on intact type I interferon (IFN) signaling for the promotion of both direct (tumor cell inhibition) and indirect (anti-tumor immune responses) effects. Malfunctions of this pathway in tumor cells or in immune cells may be responsible for the lack of response or resistance. Although type I IFN signaling is required to trigger anti-tumor immunity, emerging evidence indicates that chronic activation of type I IFN pathway may be involved in mediating resistance to different cancer treatments. The plastic and dynamic features of type I IFN responses should be carefully considered to fully exploit the therapeutic potential of strategies targeting IFN signaling. Here, we review available evidence supporting the involvement of type I IFN signaling in mediating resistance to various cancer therapies and highlight the most promising modalities that are being tested to overcome resistance.

TANK-Binding Kinase 1 (TBK1) Isoforms Negatively Regulate Type I Interferon Induction by Inhibiting TBK1-IRF3 Interaction and IRF3 Phosphorylation

TANK-binding kinase 1 (TBK1) is an important serine/threonine-protein kinase that mediates phosphorylation and nuclear translocation of IRF3, which contributes to induction of type I interferons (IFNs) in the innate antiviral response. In mammals, TBK1 spliced isoform negatively regulates the virus-triggered IFN-β signaling pathway by disrupting the interaction between retinoic acid-inducible gene I (RIG-I) and mitochondria antiviral-signaling protein (MAVS). However, it is still unclear whether alternative splicing patterns and the function of TBK1 isoform(s) exist in teleost fish. In this study, we identify two alternatively spliced isoforms of TBK1 from zebrafish, termed TBK1_tv1 and TBK1_tv2. Both TBK1_tv1 and TBK1_tv2 contain an incomplete STKc_TBK1 domain. Moreover, the UBL_TBK1_like domain is also missing for TBK1_tv2. TBK1_tv1 and TBK1_tv2 are expressed in zebrafish larvae. Overexpression of TBK1_tv1 and TBK1_tv2 inhibits RIG-I-, MAVS-, TBK1-, and IRF3-mediated activation of IFN promoters in response to spring viremia of carp virus infection. Also, TBK1_tv1 and TBK1_tv2 inhibit expression of IFNs and IFN-stimulated genes induced by MAVS and TBK1. Mechanistically, TBK1_tv1 and TBK1_tv2 competitively associate with TBK1 and IRF3 to disrupt the formation of a functional TBK1-IRF3 complex, impeding the phosphorylation of IRF3 mediated by TBK1. Collectively, these results demonstrate that TBK1 spliced isoforms are dominant negative regulators in the RIG-I/MAVS/TBK1/IRF3 antiviral pathway by targeting the functional TBK1-IRF3 complex formation. Identification and functional characterization of piscine TBK1 spliced isoforms may contribute to understanding the role of TBK1 expression in innate antiviral response.

A host subtraction database for virus discovery in human cell line sequencing data

The human cell lines HepG2, HuH-7, and Jurkat are commonly used for amplification of the RNA viruses present in environmental samples. To assist with assays by RNAseq, we sequenced these cell lines and developed a subtraction database that contains sequences expected in sequence data from uninfected cells. RNAseq data from cell lines infected with Sendai virus were analyzed to test host subtraction. The process of mapping RNAseq reads to our subtraction database vastly reduced the number non-viral reads in the dataset to allow for efficient secondary analyses.

Employing RNA viruses to fight cancer: novel insights into oncolytic virotherapy

Within recent decades, viruses that specifically target tumor cells have emerged as novel therapeutic agents against cancer. These viruses do not only act via their cell-lytic properties, but also harbor immunostimulatory features to re-direct the tumor microenvironment and stimulate tumor-directed immune responses. Furthermore, oncolytic viruses are considered to be superior to classical cancer therapies due to higher selectivity towards tumor cell destruction and, consequently, less collateral damage of non-transformed healthy tissue. In particular, the field of oncolytic RNA viruses is rapidly developing since these agents possess alternative tumor-targeting strategies compared to established oncolytic DNA viruses. Thus, oncolytic RNA viruses have broadened the field of virotherapy facilitating new strategies to fight cancer. In addition to several naturally occurring oncolytic viruses, genetically modified RNA viruses that are armed to express foreign factors such as immunostimulatory molecules have been successfully tested in early clinical trials showing promising efficacy. This review aims to provide an overview of the most promising RNA viruses in clinical development, to summarize the current knowledge of clinical trials using these viral agents, and to discuss the main issues as well as future perspectives of clinical approaches using oncolytic RNA viruses.

Alternative Pre-mRNA Splicing in Mammals and Teleost Fish: A Effective Strategy for the Regulation of Immune Responses Against Pathogen Infection

Pre-mRNA splicing is the process by which introns are removed and the protein coding elements assembled into mature mRNAs. Alternative pre-mRNA splicing provides an important source of transcriptome and proteome complexity through selectively joining different coding elements to form mRNAs, which encode proteins with similar or distinct functions. In mammals, previous studies have shown the role of alternative splicing in regulating the function of the immune system, especially in the regulation of T-cell activation and function. As lower vertebrates, teleost fish mainly rely on a large family of pattern recognition receptors (PRRs) to recognize pathogen-associated molecular patterns (PAMPs) from various invading pathogens. In this review, we summarize recent advances in our understanding of alternative splicing of piscine PRRs including peptidoglycan recognition proteins (PGRPs), nucleotide binding and oligomerization domain (NOD)-like receptors (NLRs), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) and their downstream signaling molecules, compared to splicing in mammals. We also discuss what is known and unknown about the function of splicing isoforms in the innate immune responses against pathogens infection in mammals and teleost fish. Finally, we highlight the consequences of alternative splicing in the innate immune system and give our view of important directions for future studies.

Type I interferon responses are impaired in latently HIV infected cells

BACKGROUND

The latent HIV-1 reservoir represents the primary barrier to the eradication of HIV-1 infection. The design of novel reservoir-clearance strategies, however, is impeded in part by the inability to distinguish latently HIV-infected cells from uninfected cells. Significant impairment of the type I interferon (IFN-I) response is observed during productive HIV-1 infection. Although this remains poorly described in the context of latent HIV-1 infection, presence of potential defects may serve as a novel therapeutic target. Therefore, IFN-I pathways were characterized using two latently HIV-1-infected cell lines, U1 and OM10.1, in comparison to their respective uninfected parental U937 and HL60 cell lines.

FINDINGS

Constitutive expression and induction of important mediators of IFN-I signaling including IFNα/β cytokines, IFNAR1, MHC-I, ISG15, and PKR were evaluated following exogenous IFNα or poly(I:C) treatment. Differences in basal expression of IFNAR1, MHC-I, and PKR were observed between the latently HIV-1 infected and uninfected cell lines. In parallel, significant impairments in the induction of MHC-I, ISG15 and PKR, as well as secretion of IFNα/β cytokines were observed in response to appropriate exogenous stimulation within the two latently HIV-infected U1 and OM10.1 cells, relative to their HIV-uninfected parental cells.

CONCLUSIONS

In comparison to the HIV-uninfected U937 and HL60 cell lines, widespread defects in IFN-I responsiveness were observed within the latently HIV-infected U1 and OM10.1 cells. These impairments represent novel therapeutic targets, which may be amenable to strategies currently employed in cancer therapy.

An unexpected inhibition of antiviral signaling by virus-encoded tumor suppressor p53 in pancreatic cancer cells

Virus-encoded tumor suppressor p53 transgene expression has been successfully used in vesicular stomatitis virus (VSV) and other oncolytic viruses (OVs) to enhance their anticancer activities. However, p53 is also known to inhibit virus replication via enhanced type I interferon (IFN) antiviral responses. To examine whether p53 transgenes enhance antiviral signaling in human pancreatic ductal adenocarcinoma (PDAC) cells, we engineered novel VSV recombinants encoding human p53 or the previously described chimeric p53-CC, which contains the coiled-coil (CC) domain from breakpoint cluster region (BCR) protein and evades the dominant-negative activities of endogenously expressed mutant p53. Contrary to an expected enhancement of antiviral signaling by p53, our global analysis of gene expression in PDAC cells showed that both p53 and p53-CC dramatically inhibited type I IFN responses. Our data suggest that this occurs through p53-mediated inhibition of the NF-κB pathway. Importantly, VSV-encoded p53 or p53-CC did not inhibit antiviral signaling in non-malignant human pancreatic ductal cells, which retained their resistance to all tested VSV recombinants. To the best of our knowledge, this is the first report of p53-mediated inhibition of antiviral signaling, and it suggests that OV-encoded p53 can simultaneously produce anticancer activities while assisting, rather than inhibiting, virus replication in cancer cells.

STAT3 inhibition reduces toxicity of oncolytic VSV and provides a potentially synergistic combination therapy for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a refractory malignancy with a high mortality and increasing worldwide incidence rates, including the United States and central Europe. In this study, we demonstrate that a specific inhibitor of signal transducer and activator of transcription 3 (STAT3), NSC74859, efficiently reduces HCC cell proliferation and can be successfully combined with oncolytic virotherapy using vesicular stomatitis virus (VSV). The potential benefits of this combination treatment are strengthened by the ability of NSC74859 to protect primary hepatocytes and nervous system cells against virus-induced cytotoxicity, with an elevation of the VSV maximum tolerated dose in mice. Hereby we propose a strategy for improving the current regimen for HCC treatment and seek to further explore the molecular mechanisms underlying selective oncolytic specificity of VSV.

Blockade of type I interferon (IFN) production by retroviral replicating vectors and reduced tumor cell responses to IFN likely contribute to tumor selectivity

We developed a Moloney mouse leukemia virus (MLV)-based retroviral replicating vector (RRV), Toca 511, which has displayed tumor specificity in resected brain tumor material and blood in clinical trials. Here, we investigated the interaction between Toca 511 and human host cells, and we show that RRVs do not induce type I interferon (IFN) responses in cultured human tumor cells or cultured human primary cells. However, exogenous type I IFN inhibited RRV replication in tumor cells and induced IFN-regulated genes, albeit at a lower level than in primary cells. Unexpectedly, RRVs did not induce IFN-α production upon incubation in vitro with human plasmacytoid dendritic cells (pDCs), whereas lentivirus vector and heat-treated RRVs did. Coincubation of RRVs with heat-treated RRVs or with lentivirus vector suppressed IFN-α production in pDCs, suggesting that native RRV has a dominant inhibitory effect on type I IFN induction. This effect is sensitive to trypsin treatment. In addition, heat treatment inactivated that activity but exposed an immune-stimulatory activity. The immune-stimulating component is sensitive to deglycosidases, trypsin, and phospholipase C treatment. Experiments with retroviral nonreplicating vectors and virus-like particles demonstrated that the immunosuppressive activity is not associated with the amphotropic envelope or the glyco-Gag protein. In summary, our data provide evidence that RRVs do not directly trigger type I IFN responses in IFN-responsive tumor cells. Moreover, RRVs appear to carry a heat-labile component that actively suppresses activation of cellular innate immune responses in pDCs. Inhibition of IFN induction by RRVs and the reduced response to IFN should facilitate tumor-specific infection in vivo.

IMPORTANCE

RRVs have a convincing preference for replicating in tumor cells in animal models, and we observed similar preferences in the initial treatment of human glioblastoma patients. This study investigates the basis for the interaction between RRV and human host cells (tumor versus nontumor) in vitro. We found that RRVs do not trigger an IFN-α/β response in tumor cells, but the cells are capable of responding to type I IFNs and of producing them when stimulated with known agonists. Surprisingly, the data show that RRVs can actively inhibit induction of cellular innate immunity and that this inhibitory activity is heat labile and trypsin sensitive and not attributable to the envelope protein. These data partially explain the observed in vivo tumor specificity.

Attenuated and protease-profile modified sendai virus vectors as a new tool for virotherapy of solid tumors

Multiple types of oncolytic viruses are currently under investigation in clinical trials. To optimize therapeutic outcomes it is believed that the plethora of different tumor types will require a diversity of different virus types. Sendai virus (SeV), a murine parainfluenza virus, displays a broad host range, enters cells within minutes and already has been applied safely as a gene transfer vector in gene therapy patients. However, SeV spreading naturally is abrogated in human cells due to a lack of virus activating proteases. To enable oncolytic applications of SeV we here engineered a set of novel recombinant vectors by a two-step approach: (i) introduction of an ubiquitously recognized cleavage-motive into SeV fusion protein now enabling continuous spreading in human tissues, and (ii) profound attenuation of these rSeV by the knockout of viral immune modulating accessory proteins. When employing human hepatoma cell lines, newly generated SeV variants now reached high titers and induced a profound tumor cell lysis. In contrast, virus release from untransformed human fibroblasts or primary human hepatocytes was found to be reduced by about three log steps in a time course experiment which enables the cumulation of kinetic differences of the distinct phases of viral replication such as primary target cell infection, target cell replication, and progeny virus particle release. In a hepatoma xenograft animal model we found a tumor-specific spreading of our novel recombinant SeV vectors without evidence of biodistribution into non-malignant tissues. In conclusion, we successfully developed novel tumor-selective oncolytic rSeV vectors, constituting a new tool for virotherapy of solid tumors being ready for further preclinical and clinical development to address distinct tumor types.

Molecular determinants of susceptibility to oncolytic vesicular stomatitis virus in pancreatic adenocarcinoma

BACKGROUND

M protein mutant vesicular stomatitis virus (M51R-VSV) has oncolytic properties against many cancers. However, some cancer cells are resistant to M51R-VSV. Herein, we evaluate the molecular determinants of vesicular stomatitis virus (VSV) resistance in pancreatic adenocarcinoma cells.

METHODS

Cell viability and the effect of β-interferon (IFN) were analyzed using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay. Gene expression was evaluated via microarray analysis. Cell infectability was measured by flow cytometry. Xenografts were established in athymic nude mice and treated with intratumoral M51R-VSV.

RESULTS

Four of five pancreatic cancer cell lines were sensitive to M51R-VSV, whereas Panc 03.27 cells remained resistant (81 ± 3% viability 72 h after single-cycle infection). Comparing sensitive MiaPaCa2 cells with resistant Panc 03.27 cells, significant differences in gene expression were found relating to IFN signaling (P = 2 × 10(-5)), viral entry (P = 3 × 10(-4)), and endocytosis (P = 7 × 10(-4)). MiaPaCa2 cells permitted high levels of VSV infection, whereas Panc 03.27 cells were capable of resisting VSV cell entry even at high multiplicities of infection. Extrinsic β-IFN overcame apparent defects in IFN-mediated pathways in MiaPaCa2 cells conferring VSV resistance. In contrast, β-IFN decreased cell viability in Panc 3.27 cells, suggesting intact antiviral mechanisms. VSV-treated xenografts exhibited reduced tumor growth relative to controls in both MiaPaCa2 (1423 ± 345% versus 164 ± 136%; P < 0.001) and Panc 3.27 (979 ± 153% versus 50 ± 56%; P = 0.002) tumors. Significant lymphocytic infiltration was seen in M51R-VSV-treated Panc 03.27 xenografts.

CONCLUSIONS

Inhibition of VSV endocytosis and intact IFN-mediated defenses are responsible for M51R-VSV resistance in pancreatic adenocarcinoma cells. M51R-VSV treatment appears to induce antitumor cellular immunity in vivo, which may expand its clinical efficacy.

Antifibrotic properties of transarterial oncolytic VSV therapy for hepatocellular carcinoma in rats with thioacetamide-induced liver fibrosis

Recombinant vesicular stomatitis virus (VSV) shows promise for the treatment of hepatocellular carcinoma (HCC), but its safety and efficacy when administered in a setting of hepatic fibrosis, which occurs in the majority of clinical cases, is unknown. We hypothesized that VSV could provide a novel benefit to the underlying fibrosis, due to its ability to replicate and cause cell death specifically in activated hepatic stellate cells. In addition to the ability of VSV to produce a significant oncolytic response in HCC-bearing rats in the background of thioacetamide-induced hepatic fibrosis without signs of hepatotoxicity, we observed a significant downgrading of fibrosis stage, a decrease in collagen content in the liver, and modulation of gene expression in favor of fibrotic regression. Together, this work suggests that VSV is not only safe and effective for the treatment of HCC with underlying fibrosis, but it could potentially be developed for clinical application as a novel antifibrotic agent.

Understanding and altering cell tropism of vesicular stomatitis virus

Vesicular stomatitis virus (VSV) is a prototypic nonsegmented negative-strand RNA virus. VSV's broad cell tropism makes it a popular model virus for many basic research applications. In addition, a lack of preexisting human immunity against VSV, inherent oncotropism and other features make VSV a widely used platform for vaccine and oncolytic vectors. However, VSV's neurotropism that can result in viral encephalitis in experimental animals needs to be addressed for the use of the virus as a safe vector. Therefore, it is very important to understand the determinants of VSV tropism and develop strategies to alter it. VSV glycoprotein (G) and matrix (M) protein play major roles in its cell tropism. VSV G protein is responsible for VSV broad cell tropism and is often used for pseudotyping other viruses. VSV M affects cell tropism via evasion of antiviral responses, and M mutants can be used to limit cell tropism to cell types defective in interferon signaling. In addition, other VSV proteins and host proteins may function as determinants of VSV cell tropism. Various approaches have been successfully used to alter VSV tropism to benefit basic research and clinically relevant applications.

Resistance of pancreatic cancer cells to oncolytic vesicular stomatitis virus: role of type I interferon signaling

Oncolytic virus (OV) therapy takes advantage of common cancer characteristics, such as defective type I interferon (IFN) signaling, to preferentially infect and kill cancer cells with viruses. Our recent study (Murphy et al., 2012. J. Virol. 86, 3073-87) found human pancreatic ductal adenocarcinoma (PDA) cells were highly heterogeneous in their permissiveness to vesicular stomatitis virus (VSV) and suggested at least some resistant cell lines retained functional type I IFN responses. Here we examine cellular responses to infection by the oncolytic VSV recombinant VSV-ΔM51-GFP by analyzing a panel of 11 human PDA cell lines for expression of 33 genes associated with type I IFN pathways. Although all cell lines sensed infection by VSV-ΔM51-GFP and most activated IFN-α and β expression, only resistant cell lines displayed constitutive high-level expression of the IFN-stimulated antiviral genes MxA and OAS. Inhibition of JAK/STAT signaling decreased levels of MxA and OAS and increased VSV infection, replication and oncolysis, further implicating IFN responses in resistance. Unlike VSV, vaccinia and herpes simplex virus infectivity and killing of PDA cells was independent of the type I IFN signaling profile, possibly because these two viruses are better equipped to evade type I IFN responses. Our study demonstrates heterogeneity in the type I IFN signaling status of PDA cells and suggests MxA and OAS as potential biomarkers for PDA resistance to VSV and other OVs sensitive to type I IFN responses.

Vesicular stomatitis virus as a flexible platform for oncolytic virotherapy against cancer

Oncolytic virus (OV) therapy is an emerging anti-cancer approach that utilizes viruses to preferentially infect and kill cancer cells, while not harming healthy cells. Vesicular stomatitis virus (VSV) is a prototypic non-segmented, negative-strand RNA virus with inherent OV qualities. Antiviral responses induced by type I interferon pathways are believed to be impaired in most cancer cells, making them more susceptible to VSV than normal cells. Several other factors make VSV a promising OV candidate for clinical use, including its well-studied biology, a small, easily manipulated genome, relative independence of a receptor or cell cycle, cytoplasmic replication without risk of host-cell transformation, and lack of pre-existing immunity in humans. Moreover, various VSV-based recombinant viruses have been engineered via reverse genetics to improve oncoselectivity, safety, oncotoxicity and stimulation of tumour-specific immunity. Alternative delivery methods are also being studied to minimize premature immune clearance of VSV. OV treatment as a monotherapy is being explored, although many studies have employed VSV in combination with radiotherapy, chemotherapy or other OVs. Preclinical studies with various cancers have demonstrated that VSV is a promising OV; as a result, a human clinical trial using VSV is currently in progress.

Activation and regulation of interferon-β in immune responses

Interferons (IFNs) were discovered more than half a century ago, and extensive research has since identified multifarious roles for type I IFN in human immune responses. Here, we review the functions of IFN-β in innate and adaptive immunity. We also discuss the activation and influence of IFN-β on myeloid cell types, including monocytes and dendritic cells, as well as address the effects of IFN-β on T cells and B cells. Findings from our own laboratory, which explores the molecular mechanisms of IFN-β activation by LPS and viruses, as well as from other groups investigating the regulation of IFN-β by viral proteins and endogenous factors are described. The effects of post-translational modifications of the interferon regulatory factor (IRF)-3 on IFN-β induction are also highlighted. Many unanswered questions remain concerning the regulation of the type I IFN response in inflammation, especially the role of transcription factors in the modulation of inflammatory gene expression, and these questions will form the basis for exciting avenues of future research.

Adenovirus evasion of interferon-mediated innate immunity by direct antagonism of a cellular histone posttranslational modification

Overcoming the cellular type I interferon (IFN) host defense response is critical for a virus to ensure successful infection. Investigating the effects of human adenovirus (HAdV) infection on global cellular histone posttranslational modification (hPTM), we discovered that virus infection-induced activation of IFN signaling triggers a global increase in the monoubiquitination of histone 2B (H2B) at lysine 120, which is a mark for transcriptionally active chromatin. This hPTM, catalyzed by the hBre1/RNF20 complex, is necessary for activation of the cellular IFN-stimulated gene (ISG) expression program in response to viruses. To establish effective infection, the HAdV E1A protein binds to and dissociates the hBre1 complex to block IFN-induced H2B monoubiquitination and associated ISG expression. Together, these data uncover a key role for H2B monoubiquitination in the type I IFN response and a viral mechanism of antagonizing this hPTM to evade the IFN response.

Posttranslational modification of vesicular stomatitis virus glycoprotein, but not JNK inhibition, is the antiviral mechanism of SP600125

Vesicular stomatitis virus (VSV), a negative-sense single-stranded-RNA rhabdovirus, is an extremely promising oncolytic agent for cancer treatment. Since oncolytic virotherapy is moving closer to clinical application, potentially synergistic combinations of oncolytic viruses and molecularly targeted antitumor agents are becoming a meaningful strategy for cancer treatment. Mitogen-activated protein kinase (MAPK) inhibitors have been shown to impair liver cell proliferation and tumor development, suggesting their potential use as therapeutic agents for hepatocellular carcinoma (HCC). In this work, we show that the impairment of MAPK in vitro did not interfere with the oncolytic properties of VSV in HCC cell lines. Moreover, the administration of MAPK inhibitors did not restore the responsiveness of HCC cells to alpha/beta interferon (IFN-α/β). In contrast to previous reports, we show that JNK inhibition by the inhibitor SP600125 is not responsible for VSV attenuation in HCC cells and that this compound acts by causing a posttranslational modification of the viral glycoprotein.

Evaluation of innate immune signaling pathways in transformed cells

Oncolytic viruses, the use of viruses to treat cancer, is emerging as a new option for cancer therapy. Oncolytic viruses, of both DNA and RNA origin, exhibit the ability to preferentially replicate in and kill cancer cells plausibly due to defects in innate immune signaling or translation regulation that are acquired during cellular transformation. Here, we review concepts and assays that describe how to analyze signaling pathways that govern the regulation of Type I IFN production as well as the induction of interferon-stimulated antiviral genes, events that are critical for mounting an effective antiviral response. The following procedures can be used to assess whether innate immune pathways that control antiviral host defense are defective in tumor cells - mechanisms that may help to explain viral oncolysis.

Interferon regulatory factor 3-CL, an isoform of IRF3, antagonizes activity of IRF3

Interferon regulatory factor 3 (IRF3), one member of the IRF family, plays a central role in induction of type I interferon (IFN) and regulation of apoptosis. Controlled activity of IRF3 is essential for its functions. During reverse transcription (RT)-PCR to clone the full-length open reading frame (ORF) of IRF3, we cloned a full-length ORF encoding an isoform of IRF3, termed as IRF3-CL, and has a unique carboxyl-terminus of 125 amino acids. IRF3-CL is ubiquitously expressed in distinct cell lines. Overexpression of IRF3-CL inhibits Sendai virus (SeV)-triggered induction of IFN-β and SeV-induced and inhibitor of NF-κB kinase-ε (IKKε)-mediated nuclear translocation of IRF3. When IKKε is overexpressed, IRF3-CL is associated with IRF3. These results suggest that IRF3-CL, the alternative splicing isoform of IRF-3, may function as a negative regulator of IRF3.

Down-regulation of type I interferon receptor sensitizes bladder cancer cells to vesicular stomatitis virus-induced cell death

The intrinsic oncolytic specificity of vesicular stomatitis virus (VSV) is currently being exploited to develop alternative therapeutic strategies for bladder cancer and other cancers. Previously we reported that oncolytic VSV is a potent agent for intravesical treatment of high risk bladder cancer. We observed that VSV preferentially targeted bladder cancer cells resistant to type I interferon (IFN) treatment. The goal of the current study was to further elucidate the nature of the molecular defect of IFN signaling by which bladder cancer cells become susceptible to VSV infection. Using a tissue microarray composed of human bladder cancer cores, we observed that expression of type I IFN receptor (IFNAR) was decreased relative to normal bladder tissue. Advanced bladder cancers had even lower expression of IFNAR. We found that bladder cancer cells susceptible to VSV-induced lysis had low expression of IFNAR as well. We hypothesized that down-regulation of IFNAR in bladder cancer cells may be a molecular mechanism responsible for resistance to type I IFN treatment and sensitivity to VSV oncolysis. SiRNA knockdown of IFNAR indeed facilitated replication of VSV in cells previously resistant to VSV treatment. Blocking IFNAR with a neutralizing antibody showed a similar effect. Hence down-regulation of IFNAR in bladder cancer may be one of the primary molecular mechanisms for clinical IFN resistance. However, this also facilitates VSV replication and oncolysis in high risk bladder cancers and provides a basis for selecting bladder cancer patients for IFN or oncolytic VSV therapy in future clinical trials.

Cell cycle progression or translation control is not essential for vesicular stomatitis virus oncolysis of hepatocellular carcinoma

The intrinsic oncolytic specificity of vesicular stomatitis virus (VSV) is currently being exploited to develop alternative therapeutic strategies for hepatocellular carcinoma (HCC). Identifying key regulators in diverse transduction pathways that define VSV oncolysis in cancer cells represents a fundamental prerequisite to engineering more effective oncolytic viral vectors and adjusting combination therapies. After having identified defects in the signalling cascade of type I interferon induction, responsible for attenuated antiviral responses in human HCC cell lines, we have now investigated the role of cell proliferation and translation initiation. Cell cycle progression and translation initiation factors eIF4E and eIF2Bε have been recently identified as key regulators of VSV permissiveness in T-lymphocytes and immortalized mouse embryonic fibroblasts, respectively. Here, we show that in HCC, decrease of cell proliferation by cell cycle inhibitors or siRNA-mediated reduction of G(1) cyclin-dependent kinase activities (CDK4) or cyclin D1 protein expression, do not significantly alter viral growth. Additionally, we demonstrate that translation initiation factors eIF4E and eIF2Bε are negligible in sustaining VSV replication in HCC. Taken together, these results indicate that cellular proliferation and the initiation phase of cellular protein synthesis are not essential for successful VSV oncolysis of HCC. Moreover, our observations indicate the importance of cell-type specificity for VSV oncolysis, an important aspect to be considered in virotherapy applications in the future.

Engineering oncolytic viruses to exploit tumor specific defects in innate immune signaling pathways

BACKGROUND

The use of oncolytic viruses for treatment of cancer marks a significant alteration in the battle between host and virus. Viruses are confronted by cellular innate immune responses and contain an armamentarium of immunomodulatory proteins that suppress innate immunity. Tumorigenesis can result in impairment of innate immune responses. Viruses engineered to be vulnerable to normal responses may mediate tumor-specific killing with minimal off-target toxicity.

OBJECTIVE

To examine the mechanisms by which mammalian cells respond to viral infections in normal versus cancer cells and how viruses overcome these responses and to illustrate how this knowledge is used to develop physiologically targeted oncolytic viruses.

METHODS

Literature describing studies investigating innate responses to virus infections, cancer-specific molecular defects, immunosuppressive viral products and design of oncolytic viruses is extensively reviewed, and pertinent concepts are distilled and developed.

RESULTS/CONCLUSION

Innate responses to viral infections are complex involving i) viral detection; ii) induction of interferon and other cytokines; and iii) establishment of an antiviral state. Oncolytic viruses are engineered to be susceptible to antiviral responses in normal cells. Cancers can be partially vulnerable to these viruses because they have defective antiviral responses but the antitumor potency of physiologically targeted viruses may be significantly diminished.

Engineered newcastle disease virus as an improved oncolytic agent against hepatocellular carcinoma

Newcastle disease virus (NDV) is an intrinsically tumor-specific virus, which is currently under investigation as a clinical oncolytic agent. Several clinical trials have reported NDV to be a safe and effective agent for cancer therapy; however, there remains a clear need for improvement in therapeutic outcome. The endogenous NDV fusion (F) protein directs membrane fusion, which is required for virus entry and cell-cell fusion. Here, we report a novel NDV vector harboring an L289A mutation within the F gene, which resulted in enhanced fusion and cytotoxicity of hepatocellular carcinoma (HCC) cells in vitro, as compared with the rNDV/F3aa control virus. In vivo administration of the recombinant vector, termed rNDV/F3aa(L289A), via hepatic arterial infusion in immune-competent Buffalo rats bearing multifocal, orthotopic liver tumors resulted in tumor-specific syncytia formation and necrosis, with no evidence of toxicity to the neighboring hepatic parenchyma. Furthermore, the improved oncolysis conferred by the L289A mutation translated to significantly prolonged survival compared with control NDV. Taken together, rNDV/F(L289A) represents a safe, yet more effective vector than wild-type NDV for the treatment of HCC, making it an ideal candidate for clinical application in HCC patients.