2-5A and virus infection

Cytokine regulation in SARS coronavirus infection compared to other respiratory virus infections

The pathogenesis of severe acute respiratory syndrome (SARS) is poorly understood and cytokine dysregulation has been suggested as one relevant mechanism to be explored. We compared the cytokine profile in Caco2 cells after infection of SARS coronavirus (SARS‐CoV) with other respiratory viruses including respiratory syncytial virus (RSV), influenza A virus (FluAV), and human parainfluenza virus type 2 (hPIV2). Interferon (IFN) system (production and response) was not suppressed by SARS‐CoV infection. Therefore, SARS‐CoV replication was suppressed by pretreatment with IFN. SARS‐CoV and RSV induced high levels of IL‐6 and RANTES compared with FluAV and hPIV2. Induction level of suppressor of cytokine signaling‐3 (SOCS3) by SARS‐CoV was significantly lower than that by RSV in spite of the significant production of IL‐6. Toll‐like receptors 4 and 9, which correlate with the induction of inflammatory response, were upregulated by SARS‐CoV infection. Collectively, overinduction of inflammatory cytokine and dysregulation of cytokine signaling may contribute to the immunopathology associated with “severe” inflammation in SARS. J. Med. Virol. 78:417–424, 2006. © 2006 Wiley‐Liss, Inc.

Induction of suppressor of cytokine signaling-3 by herpes simplex virus type 1 contributes to inhibition of the interferon signaling pathway

We showed previously that herpes simplex virus type 1 (HSV-1) suppresses the interferon (IFN) signaling pathway during the early infection stage in the human amnion cell line FL. HSV-1 inhibits the IFN-induced phosphorylation of Janus kinases (JAK) in infected FL cells. In the present study, we showed that the suppressor of cytokine signaling-3 (SOCS3), a host negative regulator of the JAK/STAT pathway, is rapidly induced in FL cells after HSV-1 infection. Maximal levels of SOCS3 protein were detected at around 1 to 2 h after infection. This is consistent with the occurrence of HSV-1-mediated inhibition of IFN-induced JAK phosphorylation. The HSV-1 wild-type strain VR3 induced SOCS3 more efficiently than did mutants that are defective in UL41 or UL13 and that are hyperresponsive to IFN. Induction of the IRF-7 protein and transcriptional activation of IFN-alpha4, which occur in a JAK/STAT pathway-dependent manner, were poorly induced by VR3 but efficiently induced by the mutant viruses. In contrast, phosphorylation of IRF-3 and transcriptional activation of IFN-beta, which are JAK/STAT pathway-independent process, were equally well induced by the wild-type strain and the mutants. In conclusion, the SOCS3 protein appears to be mainly responsible for the suppression of IFN signaling and IFN production that occurs during HSV-1 infection.

Suppression of thermotolerance in mumps virus-infected cells is caused by lack of HSP27 induction contributed by STAT-1

Viral infection modulates the regulation of apoptosis in host cells. Here, we report a novel mechanism by which human cells infected with mumps virus become susceptible to apoptosis caused by extracellular stresses. Mumps virus stimulates proteasome-dependent degradation of STAT-1 by action of viral accessory protein V, resulting in a severe decrease in STAT-1 protein in infected cells. We exposed mumps virus-infected and uninfected cells to heat and chemical stress. The infected cells failed to acquire resistance to apoptotic stimuli (thermotolerance) after exposure to these mild stresses. The induction of HSP27 by stress exposure was dramatically suppressed in the infected cells, but HSP70 induction was not affected. STAT-1 was required for transcriptional activation of the HSP27 gene, but not for the HSP70 gene, and cDNA transfection of STAT-1 in mumps virus-infected cells restored thermotolerance. Phosphorylated heat shock factor-1 (HSF-1) and STAT-1 phosphorylated on neither tyrosine nor serine residues were co-transported to the nucleus in response to stress. Furthermore, overexpression of unphosphorylatable mutants of STAT-1 also restored thermotolerance in mumps virus-infected cells. These lines of evidence indicate that the induction of HSP27 by stress requires STAT-1 in addition to the activated HSF-1. Furthermore, STAT-1 required for the induction of HSP27 worked independent to its phosphorylation. Thus, HSP27-dependent thermotolerance is suppressed by mumps virus infection through the destruction of STAT-1. The lack of thermotolerance should allow the infected cells to be eliminated by apoptosis and might be a host defense against viral infection.

Measles virus suppresses interferon-alpha signaling pathway: suppression of Jak1 phosphorylation and association of viral accessory proteins, C and V, with interferon-alpha receptor complex

To establish infections, viruses use various strategies to suppress the host defense mechanism, such as interferon (IFN)-induced antiviral state. We found that cells infected with a wild strain of measles virus (MeV) displayed nearly complete suppression of IFN-alpha-induced antiviral state, but not IFN-gamma-induced state. This phenomenon is due to the suppression of IFN-alpha-inducible gene expression at a transcriptional level. In the IFN-alpha signal transduction pathway, Jak1 phosphorylation induced by IFN-alpha is dramatically suppressed in MeV-infected cells; however, phosphorylation induced by IFN-gamma is not. We performed immunoprecipitation experiments using antibodies against type 1 IFN receptor chain 1 (INFAR1) and antibody against RACK1, which is reported to be a scaffold protein interacting with type I IFN receptor chain 2 and STAT1. These experiments indicated that IFNAR1 forms a complex containing the MeV-accessory proteins C and V, RACK1, and STAT1 in MeV-infected cells but not in uninfected cells. Composition of this complex in the infected cells altered little by IFN-alpha treatment. These results indicate that MeV suppresses the IFN-alpha, but not IFN-gamma, signaling pathway by inhibition of Jak1 phosphorylation. Our data suggest that functional disorder of the type I IFN receptor complex is due to "freezing" of the receptor through its association with the C and/or V proteins of MeV.

Association of mumps virus V protein with RACK1 results in dissociation of STAT-1 from the alpha interferon receptor complex

It has been reported that mumps virus protein V or the C-terminal Cys-rich region of protein V (Vsp) is associated with blocking of the interferon (IFN) signal transduction pathway through a decrease in STAT-1 production. The intracellular target of the V protein was investigated by using a two-hybrid screening system with Vsp as bait. Full-length V protein and Vsp were able to bind to RACK1, and the interaction did not require two WD domains, WD1 and WD2, in RACK1. A significant interaction between V protein and RACK1 was also demonstrated in cells persistently infected with mumps virus (FLMT cells), and the formation of the complex was not affected by treatment with IFN. On the other hand, in uninfected cells, STAT-1 was associated with the long form of the beta subunit of the alpha IFN receptor, and this association was mediated by the function of RACK1 as an adaptor protein. Immunoprecipitation and glutathione S-transferase pull-down experiments revealed that the association of RACK1 or mumps virus V protein with the IFN receptor was undetectable in mumps virus-infected cells. Furthermore, RACK1 interacted with mumps virus V protein with a higher affinity than STAT-1 did. Therefore, it is suggested that mumps virus V protein has the ability to interact strongly with RACK1 and consequently to bring about the disruption of the complex formed from STAT-1, RACK1, and the IFN receptor.

Herpes simplex virus type 1 suppresses the interferon signaling pathway by inhibiting phosphorylation of STATs and janus kinases during an early infection stage

We examined the influence on the interferon (IFN) signaling pathway of infection with herpes simplex virus type 1 (HSV-1) strain VR3. Data from reporter gene assays showed that expression of both type I and type II IFN-inducible genes was dramatically suppressed during the early stage of HSV-1 infection (2 to 3 h postinfection). During these periods, phosphorylation levels of janus kinases (JAKs) and STATs did not increase after treatment of HSV-1-infected FL cells with IFN-alpha or IFN-gamma, although cellular protein levels of the JAKs and the STATs were not significantly changed. In contrast, the inhibitory effect of HSV-1 on phosphorylation of STAT1 was not observed in U937 cells, which show resistance to steady-state accumulation of RNA for HSV-1 immediate-early genes. The phosphorylation of STAT1 in FL cells was not inhibited by infection with a UV-inactivated virus. These results indicate that viral gene expression or viral protein production is necessary for the inhibition of phosphorylation by HSV-1.

Origin of the interferon-inducible (2'-5')oligoadenylate synthetases: cloning of the (2'-5')oligoadenylate synthetase from the marine sponge Geodia cydonium

In vertebrates cytokines mediate innate (natural) immunity and protect them against viral infections. The cytokine interferon causes the induction of the (2'-5')oligoadenylate synthetase [(2-5)A synthetase], whose product, (2'-5')oligoadenylate, activates the endoribonuclease L which in turn degrades (viral) RNA. Three isoforms of (2-5)A synthetases exist, form I (40-46 kDa), form II (69 kDa), and form III (100 kDa). Until now (2-5)A synthetases have only been cloned from birds and mammals. Here we describe the cloning of the first putative invertebrate (2-5)A synthetase from the marine sponge Geodia cydonium. The deduced amino acid sequence shows signatures characteristic for (2-5)A synthetases of form I. Phylogenetic analysis of the putative sponge (2-5)A synthetase indicates that it diverged first from a common ancestor of the hitherto known members of (vertebrate) (2-5)A synthetases I, (2-5)A synthetases II and III. Moreover, it is suggested that the (2-5)A synthetases II and III evolved from this common ancestor (very likely) by gene duplication. Together with earlier results on the existence of the (2'-5')oligoadenylates in G. cydonium, the data presented here demonstrate that also invertebrates, here sponges, are provided with the (2-5)A system. At present, it is assumed that this system might be involved in growth control, including control of apoptosis, and acquired its additional function in innate immune response in evolutionarily younger animals, in vertebrates.

Evolution of the innate and adaptive immune systems: relationships between potential immune molecules in the lowest metazoan phylum (Porifera) and those in vertebrates

Porifera (sponge) form the lowest metazoan phylum and share a common ancestor with other metazoan phyla. In the present study, it is reported that sponges possess molecules that are similar in structure to those molecules involved in the immune system in mammals. Experiments with the marine sponges Geodia cydonium and Suberites domuncula have been performed on tissue (auto- and allografting) as well as on a cellular level. The studies revealed that sponges are provided with elements of the mammalian innate immune system, such as molecules containing scavenger receptor cysteine-rich domains. Furthermore, macrophage-derived cytokine-like molecules have been identified that are up-regulated during the grafting process. In addition, the (2'-5')oligoadenylate synthetase system exists in sponges. "Precursors" of the second type of immune response in mammals, the adaptive immune system, have been traced in sponges. It is shown that the expression of a lymphocyte-derived cytokine from mammals is up-regulated during non-self-recognition in S. domuncula. Finally, in G. cydonium, two classes of receptors that comprise Ig-like domains have been identified: the receptor tyrosine kinases and the non-enzymic sponge adhesion molecules. They contain two polymorphic Ig-like domains that are grouped to the variable set of immunoglobulins. The expression of these molecules is also up-regulated during the grafting process. It is concluded that sponges are already provided with a series of elements used in higher vertebrates for both the innate and the adaptive immune recognition.

Augmentation of verotoxin-induced cytotoxicity/apoptosis by interferon is repressed in cells persistently infected with mumps virus

Verotoxin type 2 (VT2) produced by enterohemorrhagic Escherichia coli (EHEC) has been shown to have high cytotoxic potency toward several human B lymphoid cell lines with and without Epstein-Barr virus (EBV). Cell death, apoptosis induced by VT2, is closely correlated with the expression of receptor molecule Gb3/CD77, recognized by the toxin, but not with the infection or presence of EBV. Pretreatment of cells with interferon-alpha (IFN-alpha) for 24 h resulted in augmentation of apoptosis by VT2. Pretreatment within 8 h, however, was not effective. It has been reported that IFN-alpha-induced apoptosis is correlated with the induction of the 2',5'-OAS/RNase L system or dsRNA-activated protein kinase (PKR) or both. We have established persistent infection in both Akata and P3HR-1 cells with mumps virus. The persistently infected cell lines, P3HR-MP2 and Akata-MP2, showed poor induction of 2',5'-OAS and PKR in response to IFN-alpha. Augmentation of VT2-induced apoptosis by IFN-alpha was not found in the cell lines P3HR-MP2 and Akata-MP2. Therefore, these findings were interpreted to indicate that augmentation of VT2-induced apoptosis by IFN-alpha may be mediated by PKR and the 2',5'-OAS/RNaseL system. It is also suggested that mumps virus can suppress apoptosis and establish persistent infection.

Transgenic Expression of IFN-α in the Central Nervous System of Mice Protects Against Lethal Neurotropic Viral Infection but Induces Inflammation and Neurodegeneration

Type I IFNs, which include IFN-α, appear to have complex and broad-ranging actions in the central nervous system (CNS) that may result in protection or injury. To better understand these issues, we generated transgenic mice that produce IFN-α1 chronically from astrocytes. These glial fibrillary acidic protein-IFN-α transgenic mice developed a progressive inflammatory encephalopathy, with marked calcium mineralization, meninoencephalitis, gliosis, and neurodegeneration. Many features of this murine encephalopathy resembled those found in certain human encephalopathies of unknown etiology; these diseases, exemplified by Aicardi-Goutières syndrome and some viral encephalopathies, show increased intrathecal production of IFN-α. Our data suggest that IFN-α overproduction may be the primary factor initiating these human diseases. Following intracerebral infection with lymphocytic choriomeningitis virus, glial fibrillary acidic protein-IFN-α mice had significantly increased survival rates associated with markedly reduced virus titers and immune pathology in the brain but normal peripheral CTL responses. Therefore, the production of IFN-α in the CNS can be a two-edged sword that on the one hand confers protection against a lethal viral infection but on the other causes significant injury to the brain. These transgenic mice provide a novel animal model in which to further evaluate the mechanisms that underlie the diverse actions of type I IFNs in the intact CNS.

Specific Antiviral Activity Demonstrated by TGTP, A Member of a New Family of Interferon-Induced GTPases

The GTPase superfamily includes a diversity of molecules whose functions are regulated through the binding and hydrolysis of GTP. This superfamily can be segregated into families of functionally related molecules that typically share amino acid sequence similarity within and around the nucleotide-binding domains. A new family of putative GTPases, including IRG-47, LRG-47, IGTP, and TGTP/Mg21, has recently emerged that share significant sequence identity (25–40%). Expression of these molecules has been shown to be selectively induced by IFN-γ and in some cases by IFN-αβ or bacterial LPS. This induction pattern implicates these putative GTPases as part of the innate defense of cells to infection, but their role in such defense has not yet been defined. We have previously described the cloning of TGTP and now confirm its intrinsic activity as a GTPase. We found that TGTP is strongly induced by endogenous IFN-αβ produced in response to standard lipofection of plasmid DNA or polyinosinic polycytidilic acid. The ability of endogenously produced IFN-αβ to efficiently induce expression of TGTP under these conditions suggested that TGTP might participate in defense against viral infection. This proposal was borne out when TGTP-transfected L cells displayed relative resistance to plaque formation by vesicular stomatitis virus but not herpes simplex virus. This observation places TGTP among a small family of innate antiviral agents and has implications for the functions of other members of this family of GTPases.