Expression of the human MxA protein is associated with hyperphosphorylation of VSV P protein in human neural Cells

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.

Interplay between innate immunity and negative-strand RNA viruses: towards a rational model

The discovery of a new class of cytosolic receptors recognizing viral RNA, called the RIG-like receptors (RLRs), has revolutionized our understanding of the interplay between viruses and host cells. A tremendous amount of work has been accumulating to decipher the RNA moieties required for an RLR agonist, the signal transduction pathway leading to activation of the innate immunity orchestrated by type I interferon (IFN), the cellular and viral regulators of this pathway, and the viral inhibitors of the innate immune response. Previous reviews have focused on the RLR signaling pathway and on the negative regulation of the interferon response by viral proteins. The focus of this review is to put this knowledge in the context of the virus replication cycle within a cell. Likewise, there has been an expansion of knowledge about the role of innate immunity in the pathophysiology of viral infection. As a consequence, some discrepancies have arisen between the current models of cell-intrinsic innate immunity and current knowledge of virus biology. This holds particularly true for the nonsegmented negative-strand viruses (Mononegavirales), which paradoxically have been largely used to build presently available models. The aim of this review is to bridge the gap between the virology and innate immunity to favor the rational building of a relevant model(s) describing the interplay between Mononegavirales and the innate immune system.

Expression of the interferon-alpha/beta-inducible MxA protein in brain lesions of subacute sclerosing panencephalitis

The type-I interferon (IFN) inducible human MxA protein exhibits antiviral activity against a variety of RNA viruses including the measles virus (MV). In this study, we investigated the association between the expression of MV antigens and MxA in subacute sclerosing panencephalitis (SSPE) brains. We analyzed the MxA expression in and around lesions in brains of three SSPE patients and compared it with normal brains. Double staining with antibodies against MxA and the MV nucleocapsid revealed that MxA was highly expressed in a belt surrounding MV-antigen-positive lesions in SSPE brains. In normal appearing regions distant from a lesion in SSPE brains and in normal brains, MxA was not detected. Furthermore, MxA was often less or not expressed in the center of lesions expressing high amounts of MV antigens. Such a pattern of MxA expression in SSPE brains clearly indicates that newly infected cells release type I IFN and will become demarcated by a protecting barrier of MxA expressing cells. Double staining with antibodies against MxA and glial fibrillary acidic protein (GFAP) showed that the MxA protein was expressed mainly in the cytoplasm of astrocytes. MxA expression did not correlate with the presence of cellular infiltrates of inflammatory cells, although some lymphoid cells were also positive for MxA. Since MxA inhibits the replication of MV, these findings suggest that the IFN-induced MxA protein plays an important role in slowing down the viral spread in SSPE brains and by doing so may contribute to the persistence of the MV-infection.

Nuclear MxA proteins form a complex with influenza virus NP and inhibit the transcription of the engineered influenza virus genome

Mx proteins belong to the dynamin superfamily of high molecular weight GTPases and interfere with multiplication of a wide variety of viruses. Earlier studies show that nuclear mouse Mx1 and human MxA designed to be localized in the nucleus inhibit the transcription step of the influenza virus genome. Here we set a transient influenza virus transcription system using luciferase as a reporter gene and cells expressing the three RNA polymerase subunits, PB1, PB2 and PA, and NP. We used this reporter assay system and nuclear-localized MxA proteins to get clues for elucidating the anti-influenza virus activity of MxA. Nuclear-localized VP16-MxA and MxA-TAg NLS strongly interfered with the influenza virus transcription. Over-expression of PB2 led to a slight resumption of the transcription inhibition by nuclear MxA, whereas over-expression of PB1 and PA did not affect the MxA activity. Of interest is that the inhibitory activity of the nuclear MxA was markedly neutralized by over-expression of NP. An NP devoid of its C-terminal region, but containing the N-terminal RNA binding domain, also neutralized the VP16-MxA activity in a dose-dependent manner, whereas an NP lacking the N-terminal region did not affect the VP16-MxA activity. Further, not only VP16-MxA but also the wild-type MxA was found to interact with NP in influenza virus-infected cells. This indicates that the nuclear MxA suppresses the influenza virus transcription by interacting with not only PB2 but also NP.

Modulation of interferon expression by hepatitis C virus NS5A protein and human homeodomain protein PTX1

Hepatitis C virus (HCV) NS5A protein transcriptionally modulates a number of cellular genes. Since there is no evidence of binding of NS5A protein to DNA, it is likely to exert its activity in concert with cellular factor(s). In this study, we have identified a specific interaction of HCV NS5A with homeodomain protein PTX1 of human origin by a yeast two-hybrid interacting cloning system. The authenticity of this interaction was verified by mammalian two-hybrid assay, in vivo co-immunoprecipitation analysis, and from a colocalization study. Recently, murine PTX1 (mPTX1) has been shown to repress virus-induced murine interferonA4 promoter activity. Interferon-à alone or together with ribavirin is the only available therapy for HCV-infected patients. Therefore, we examined whether coexpression of NS5A and human PTX1 (hPTX1) proteins modulate human IFN-à promoter activity. An in vitro reporter assay by transfection of HepG2 cells with NS5A suggested an activation of IFN-à promoter to approximately 20-fold upon Newcastle disease virus (NDV) infection. Under similar experimental conditions, hPTX1-activated IFN-à prompter to approximately sevenfold, unlike mPTX1. However, cotransfection of NS5A and hPTX1 displayed a lower interferon promoter activity, probably for physical association between these two proteins. Subsequent study demonstrated that activation of IFN promoter by NS5A is associated with an increased expression of IRF-3. Further analysis revealed that ectopic expression of NS5A in HepG2 cells enhances endogenous IFN-à secretion and MxA expression upon induction with NDV. However, exogenous expression of hPTX1 did not significantly alter NS5A-mediated function in the stable transfectants. Taken together, these results suggested that the level of endogenous hPTX1 is not sufficient to block the function of NS5A for augmentation of virus-mediated IFN activity in HepG2 cells.

Inhibition of influenza C viruses by human MxA protein

Human MxA protein was analyzed for its ability to inhibit the replication of different influenza C viruses. Three laboratory derivatives of viral strain C/Ann Arbor/1/50 were investigated, namely the parental wild-type virus C/AA-wt, the persistent variant C/AA-pi and the highly cytopathogenic variant C/AA-cyt. In addition, strain C/Paris/214/91 isolated from an influenza patient was used. Multiplication of all four viruses was suppressed in MxA-expressing Vero cells, as indicated by a decrease in viral RNA synthesis, viral protein synthesis, virion production and induction of a cytopathic effect. Inhibition correlated with the level of MxA expression. Furthermore, inhibition was independent of cell clone-specific differences in expression of virus receptors, as demonstrated by receptor reconstitution experiments. Thus, human MxA protein has antiviral activity against influenza C viruses.

MxA GTPase blocks reporter gene expression of reconstituted Thogoto virus ribonucleoprotein complexes

Human MxA protein accumulates in the cytoplasm of interferon-treated cells and inhibits the multiplication of several RNA viruses, including Thogoto virus (THOV), a tick-borne orthomyxovirus that transcribes and replicates its genome in the cell nucleus. The antiviral mechanism of MxA was investigated by using two alternative minireplicon systems in which recombinant viral ribonucleoprotein complexes (vRNPs) of THOV were reconstituted from cloned cDNAs. A chloramphenicol acetyltransferase reporter minigenome RNA was expressed either by T7 RNA polymerase in the cytoplasm of transfected cells or, alternatively, by RNA polymerase I in the nucleus. The inhibitory effect of MxA was studied in both cellular compartments by coexpressing wild-type MxA or TMxA, an artificial nuclear form of MxA. Our results indicate that both MxA proteins recognize the assembled vRNP rather than the newly synthesized unassembled components. The present findings are consistent with previous data which indicated that cytoplasmic MxA prevents transport of vRNPs into the nucleus, whereas nuclear MxA directly inhibits the viral polymerase activity in the nucleus.

Pathogenic aspects of measles virus infections

Measles virus (MV) infections normally cause an acute self limiting disease which is resumed by a virus-specific immune response and leads to the establishment of a lifelong immunity. Complications associated with acute measles can, on rare occasions, involve the central nervous system (CNS). These are postinfectious measles encephalitis which develops soon after infection, and, months to years after the acute disease, measles inclusion body encephalitis (MIBE) and subacute sclerosing panencephalitis (SSPE) which are based on a persistent MV infection of brain cells. Before the advent of HIV, SSPE was the best studied slow viral infection of the CNS, and particular restrictions of MV gene expression as well as MV interactions with neural cells have revealed important insights into the pathogenesis of persistent viral CNS infections. MV CNS complication do, however, not large contribute to the high rate of mortality seen in association with acute measles worldwide. The latter is due to a virus-induced suppression of immune functions which favors the establishment of opportunistic infections. Mechanisms underlying MV-mediated immunosuppression are not well understood. Recent studies have indicated that MV-induced disruption of immune functions may be multifactorial including the interference with cytokine synthesis, the induction of soluble inhibitory factors or apoptosis and negative signalling to T cells by the viral glycoproteins expressed on the surface of infected cells, particularly dendritic cells.

Host cell protein kinases in nonsegmented negative-strand virus (mononegavirales) infection

Phosphorylation of one or more viral proteins is probably an essential step in the life cycle of every member of the nonsegmented negative-strand RNA virus (mononegavirales [MNV]) group. Since no virally encoded protein kinases have been discovered in this group, phosphorylation is effected entirely by host cell kinases. The virally encoded P proteins of the MNV are the only ones consistently phosphorylated with a stoichiometry > or =1. The P protein of vesicular stomatitis virus (VSV), and perhaps also of respiratory syncytial virus, are the only ones for which a function of phosphorylation has been established. Phosphorylation by casein kinase 2 at one or more identified sites in the VSV P protein activates transcriptional activity by promoting formation of a homotrimer, which is then capable of binding the RNA polymerase and attaching it to the N protein-RNA template. A second phosphorylation of VSV P protein by a different kinase also occurs, dependent upon prior modification by casein kinase 2, but its function is not definitely known. Phosphorylation of the other MNV P proteins may serve a different purpose. No evidence has been obtained yet for any function for phosphorylation of any other MNV protein.

Bile acids modulate the interferon signalling pathway

We have previously shown that cholestasis and bile acids inhibit 2', 5' oligoadenylate synthetase (OAS) activity in the liver and in primary hepatocyte cultures. Here, we assessed the influence of bile acids on interferon (IFN) pathway activation in three hepatoma cell lines. In HepG2 cells, bile acids (100-200 micromol/L) inhibited IFN-induced 2',5' OAS activity to an extent depending on their surface activity index. In Western blot analysis, IFN-induced expression of two major antiviral proteins, MxA and OAS p100, was reduced by 54% +/- 8% and 44% +/- 12%, respectively, when cells were preincubated for 4 hours with 100 micromol/L chenodeoxycholic acid (CDCA). In the same conditions, CDCA did not modify the IFN-induced signal transducers and activators of transcription (STAT)s tyrosine phosphorylation. In contrast, it reduced IFN-induced MxA promoter activity by 60%. The inhibitory effect of CDCA was not mediated by a 4beta-phorbol 12beta-myristate 13alpha-acetate (PMA)-sensitive protein kinase C (PKC)-dependent pathway. Finally, using CHO cells stably expressing a functional human bile acid carrier (Na+-dependent taurocholate cotransporting polypeptide [NTCP]), we found that bile acid inhibition of the IFN pathway occurred in the range of more physiological concentrations (12-50 micromol/L). In summary, our results provide strong evidence that bile acids inhibit the induction of proteins involved in the antiviral activity of IFN. This might partly explain the lack of responsiveness to IFN therapy in some patients with advanced chronic viral liver diseases.

Dominant-negative mutants of human MxA protein: domains in the carboxy-terminal moiety are important for oligomerization and antiviral activity

Human MxA protein is an interferon-induced 76-kDa GTPase that exhibits antiviral activity against several RNA viruses. Wild-type MxA accumulates in the cytoplasm of cells. TMxA, a modified form of wild-type MxA carrying a foreign nuclear localization signal, accumulates in the cell nucleus. Here we show that MxA protein is translocated into the nucleus together with TMxA when both proteins are expressed simultaneously in the same cell, demonstrating that MxA molecules form tight complexes in living cells. To define domains important for MxA-MxA interaction and antiviral function in vivo, we expressed mutant forms of MxA together with wild-type MxA or TMxA in appropriate cells and analyzed subcellular localization and interfering effects. An MxA deletion mutant, MxA(359-572), formed heterooligomers with TMxA and was translocated to the nucleus, indicating that the region between amino acid positions 359 and 572 contains an interaction domain which is critical for oligomerization of MxA proteins. Mutant T103A with threonine at position 103 replaced by alanine had lost both GTPase and antiviral activities. T103A exhibited a dominant-interfering effect on the antiviral activity of wild-type MxA rendering MxA-expressing cells susceptible to infection with influenza A virus, Thogoto virus, and vesicular stomatitis virus. To determine which sequences are critical for the dominant-negative effect of T103A, we expressed truncated forms of T103A together with wild-type protein. A C-terminal deletion mutant lacking the last 90 amino acids had lost interfering capacity, indicating that an intact C terminus was required. Surprisingly, a truncated version of MxA representing only the C-terminal half of the molecule exerted also a dominant-negative effect on wild-type function, demonstrating that sequences in the C-terminal moiety of MxA are necessary and sufficient for interference. However, all MxA mutants formed hetero-oligomers with TMxA and were translocated to the nucleus, indicating that physical interaction alone is not sufficient for disturbing wild-type function. We propose that dominant-negative mutants directly influence wild-type activity within hetero-oligomers or else compete with wild-type MxA for a cellular or viral target.