Mx GTPases: dynamin-like antiviral machines of innate immunity

Expression of Myxovirus Resistance A (MxA) Is Associated with Tumor-Infiltrating Lymphocytes in Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Breast Cancers

Purpose

The prognostic significance of tumor-infiltrating lymphocytes (TILs) has been determined in breast cancers. Interferons can affect T-cell activity through direct and indirect mechanisms. Myxovirus resistance A (MxA) is an excellent marker of interferon activity. Here,we evaluated TILs and MxA expression in human epidermal growth factor receptor 2 (HER2)–positive breast cancers.

Materials and Methods

Ninety cases of hormone receptor (HR)+/HER2+ tumors and 78 cases of HR–/HER2+ tumors were included. The TILs level was assessed using hematoxylin and eosin–stained full face sections, and MxA expressionwas evaluated by immunohistochemistrywith a tissue microarray.

Results

MxA protein expression was significantly higher in tumors with high histologic grade (p=0.023) and high levels of TILs (p=0.002). High levels of TILs were correlated with high histological grade (p=0.001), negative lymphovascular invasion (p=0.007), negative lymph node metastasis (p=0.007), absence of HR expression (p < 0.001), abundant tertiary lymphoid structures (TLSs) around ductal carcinoma in situ (p=0.018), and abundant TLSs around the invasive component (p < 0.001). High levels of TILs were also associated with improved disease-free survival, particularly in HR–/HER2+ breast cancers. However, MxA was not a prognostic factor.

Conclusion

High expression of MxA in tumor cells was associated with high levels of TILs in HER2-positive breast cancers. Additionally, a high level of TILs was a prognostic factor for breast cancer, whereas the level of MxA expression had no prognostic value.

JC Polyomavirus Infection of Primary Human Renal Epithelial Cells Is Controlled by a Type I IFN-Induced Response

The JC and BK human polyomaviruses (JCPyV and BKPyV, respectively) establish lifelong persistent infections in the kidney. In immunosuppressed individuals, JCPyV causes progressive multifocal leukoencephalopathy (PML), a fatal neurodegenerative disease, and BKPyV causes polyomavirus-associated nephropathy (PVN). In this study, we compared JCPyV and BKPyV infections in primary human renal proximal tubule epithelial (HRPTE) cells. JCPyV established a persistent infection, but BKPyV killed the cells in 15 days. To identify the cellular factors responsible for controlling JCPyV infection and promoting viral persistence, we profiled the transcriptomes of JCPyV- and BKPyV-infected cells at several time points postinfection. We found that infection with both viruses induced interferon production but that interferon-stimulated genes (ISGs) were only activated in the JCPyV-infected cells. Phosphorylated STAT1 and IRF9, which are responsible for inducing ISGs, translocated to the nucleus of JCPyV-infected cells but did not in BKPyV-infected cells. In BKPyV-infected cells, two critical suppressors of cytokine signaling, SOCS3 and SOCS1, were induced. Infection with BKPyV but not JCPyV caused reorganization of PML bodies that are associated with inactivating antiviral responses. Blockade of the interferon receptor and neutralization of soluble interferon alpha (IFN-α) and IFN-β partially alleviated the block to JCPyV infection, leading to enhanced infectivity. Our results show that a type I IFN response contributes to the establishment of persistent infection by JCPyV in HRPTE cells.

The human polyomaviruses JCPyV and BKPyV both establish lifelong persistent infection in the kidneys. In immunosuppressed patients, BKPyV causes significant pathology in the kidney, but JCPyV is only rarely associated with disease in this organ. The reasons behind this striking difference in kidney pathology are unknown. In this study, we show that infection of primary human renal tubule epithelial cells with JCPyV and BKPyV results in divergent innate immune responses that control JCPyV but fail to control BKPyV. This is the first study that directly compares JCPyV and BKPyV infection in vitro in the same cell type they naturally infect, and the significant differences that have been uncovered could in part explain the distinct disease outcomes.

Phleboviruses and the Type I Interferon Response

The genus Phlebovirus of the family Bunyaviridae contains a number of emerging virus species which pose a threat to both human and animal health. Most prominent members include Rift Valley fever virus (RVFV), sandfly fever Naples virus (SFNV), sandfly fever Sicilian virus (SFSV), Toscana virus (TOSV), Punta Toro virus (PTV), and the two new members severe fever with thrombocytopenia syndrome virus (SFTSV) and Heartland virus (HRTV). The nonstructural protein NSs is well established as the main phleboviral virulence factor in the mammalian host. NSs acts as antagonist of the antiviral type I interferon (IFN) system. Recent progress in the elucidation of the molecular functions of a growing list of NSs proteins highlights the astonishing variety of strategies employed by phleboviruses to evade the IFN system.

Molecular identification and comparative transcriptional analysis of myxovirus resistance GTPase (Mx) gene in goose (Anser cygnoide) after H9N2 AIV infection

Interferon (IFN)-induced myxovirus resistance (Mx) GTPases belong to the family of dynamin-like GTPases and control a diverse range of viruses. In this study, the identified goose Mx (goMx) mRNA is 2009bp long, shares partially conserved exons with other homologues, and shares highly conserved domains in its primary structure. The amino acid position 629 (629aa) of the goMx protein was identified as serine (Ser), in contrast to the Ser located at 631aa in chicken Mx, which is considered to be responsible for the lack of chicken Mx antiviral activity. In addition, the goMx 142aa residue in the dynamin family signature differs from that of other functional Mx proteins. Transcriptional analysis revealed that goMx was mainly expressed in the digestive, respiratory and immune systems in an age-specific manner. GoMx transcript levels in goose peripheral blood mononuclear cells (PBMCs) were found to be significantly up-regulated by various agonists and avian viruses. Furthermore, a time course study of the effects of H9N2 avian influenza virus (AIV) on goMx expression in infected goslings suggested that H9N2 AIV affected goMx expression. However, significant changes in goMx expression were observed in the trachea, lung and small intestine of infected birds. Altogether, these results indicate that goMx protein may have acquired its broad antiviral activity by changing only a few amino acids at select sites, even as it shares a conserved architectures with species.

Interferon-Inducible GTPases in Host Resistance, Inflammation and Disease

Cell-autonomous immunity is essential for host organisms to defend themselves against invasive microbes. In vertebrates, both the adaptive and the innate branches of the immune system operate cell-autonomous defenses as key effector mechanisms that are induced by pro-inflammatory interferons (IFNs). IFNs can activate cell-intrinsic host defenses in virtually any cell type ranging from professional phagocytes to mucosal epithelial cells. Much of this IFN-induced host resistance program is dependent on four families of IFN-inducible GTPases: the myxovirus resistance proteins, the immunity-related GTPases, the guanylate-binding proteins (GBPs), and the very large IFN-inducible GTPases. These GTPase families provide host resistance to a variety of viral, bacterial, and protozoan pathogens through the sequestration of microbial proteins, manipulation of vesicle trafficking, regulation of antimicrobial autophagy (xenophagy), execution of intracellular membranolytic pathways, and the activation of inflammasomes. This review discusses our current knowledge of the molecular function of IFN-inducible GTPases in providing host resistance, as well as their role in the pathogenesis of autoinflammatory Crohn's disease. While substantial advances were made in the recent past, few of the known functions of IFN-inducible GTPases have been explored in any depth, and new functions await discovery. This review will therefore highlight key areas of future exploration that promise to advance our understanding of the role of IFN-inducible GTPases in human diseases.

The implication of SUMO in intrinsic and innate immunity

Since its discovery, SUMOylation has emerged as a key post-translational modification involved in the regulation of host-virus interactions. SUMOylation has been associated with the replication of a large number of viruses, either through the direct modification of viral proteins or through the modulation of cellular proteins implicated in antiviral defense. SUMO can affect protein function via covalent or non-covalent binding. There is growing evidence that SUMO regulates several host proteins involved in intrinsic and innate immunity, thereby contributing to the process governing interferon production during viral infection; as well as the interferon-activated Jak/STAT pathway. Unlike the interferon-mediated innate immune response, intrinsic antiviral resistance is mediated by constitutively expressed antiviral proteins (defined as restriction factors), which confer direct viral resistance through a variety of mechanisms. The aim of this review is to evaluate the role of SUMO in intrinsic and innate immunity; highlighting the involvement of the TRIM family proteins, with a specific focus on the mechanism through which SUMO affects i- interferon production upon viral infection, ii-interferon Jak/STAT signaling and biological responses, iii-the relationship between restriction factors and RNA viruses.

Mx1 reveals innate pathways to antiviral resistance and lethal influenza disease

Influenza A virus (IAV) causes up to half a million deaths worldwide annually, 90% of which occur in older adults. We show that IAV-infected monocytes from older humans have impaired antiviral interferon production but retain intact inflammasome responses. To understand the in vivo consequence, we used mice expressing a functional Mx gene encoding a major interferon-induced effector against IAV in humans. In Mx1-intact mice with weakened resistance due to deficiencies in Mavs and Tlr7, we found an elevated respiratory bacterial burden. Notably, mortality in the absence of Mavs and Tlr7 was independent of viral load or MyD88-dependent signaling but dependent on bacterial burden, caspase-1/11, and neutrophil-dependent tissue damage. Therefore, in the context of weakened antiviral resistance, vulnerability to IAV disease is a function of caspase-dependent pathology.

Influenza A viruses escape from MxA restriction at the expense of efficient nuclear vRNP import

To establish a new lineage in the human population, avian influenza A viruses (AIV) must overcome the intracellular restriction factor MxA. Partial escape from MxA restriction can be achieved when the viral nucleoprotein (NP) acquires the critical human-adaptive amino acid residues 100I/V, 283P, and 313Y. Here, we show that introduction of these three residues into the NP of an avian H5N1 virus renders it genetically unstable, resulting in viruses harboring additional single mutations, including G16D. These substitutions restored genetic stability yet again yielded viruses with varying degrees of attenuation in mammalian and avian cells. Additionally, most of the mutant viruses lost the capacity to escape MxA restriction, with the exception of the G16D virus. We show that MxA escape is linked to attenuation by demonstrating that the three substitutions promoting MxA escape disturbed intracellular trafficking of incoming viral ribonucleoprotein complexes (vRNPs), thereby resulting in impaired nuclear import, and that the additional acquired mutations only partially compensate for this import block. We conclude that for adaptation to the human host, AIV must not only overcome MxA restriction but also an associated block in nuclear vRNP import. This inherent difficulty may partially explain the frequent failure of AIV to become pandemic.

Host genetics of severe influenza: from mouse Mx1 to human IRF7

Influenza viruses cause mild to moderate respiratory illness in most people, and only rarely devastating or fatal infections. The virulence factors encoded by viral genes can explain seasonal or geographic differences at the population level but are unlikely to account for inter-individual clinical variability. Inherited or acquired immunodeficiencies may thus underlie severe cases of influenza. The crucial role of host genes was first demonstrated by forward genetics in inbred mice, with the identification of interferon (IFN)-α/β-inducible Mx1 as a canonical influenza susceptibility gene. Reverse genetics has subsequently characterized the in vivo role of other mouse genes involved in IFN-α/β and -λ immunity. A series of in vitro studies with mouse and human cells have also refined the cell-intrinsic mechanisms of protection against influenza viruses. Population-based human genetic studies have not yet uncovered variants with a significant impact. Interestingly, human primary immunodeficiencies affecting T and B cells were also not found to predispose to severe influenza. Recently however, human IRF7 was shown to be essential for IFN-α/β- and IFN-λ-dependent protective immunity against primary influenza in vivo, as inferred from a patient with life-threatening influenza revealed to be IRF7-deficient by whole exome sequencing. Next generation sequencing of human exomes and genomes will facilitate the analysis of the human genetic determinism of severe influenza.

Gene expression profiling in necrotizing enterocolitis reveals pathways common to those reported in Crohn's disease

BACKGROUND

Necrotizing enterocolitis (NEC) is the most frequent life-threatening gastrointestinal disease experienced by premature infants in neonatal intensive care units. The challenge for neonatologists is to detect early clinical manifestations of NEC. One strategy would be to identify specific markers that could be used as early diagnostic tools to identify preterm infants most at risk of developing NEC or in the event of a diagnostic dilemma of suspected disease. As a first step in this direction, we sought to determine the specific gene expression profile of NEC.

METHODS

Deep sequencing (RNA-Seq) was used to establish the gene expression profiles in ileal samples obtained from preterm infants diagnosed with NEC and non-NEC conditions. Data were analyzed with Ingenuity Pathway Analysis and ToppCluster softwares.

RESULTS

Data analysis indicated that the most significant functional pathways over-represented in NEC neonates were associated with immune functions, such as altered T and B cell signaling, B cell development, and the role of pattern recognition receptors for bacteria and viruses. Among the genes that were strongly modulated in neonates with NEC, we observed a significant degree of similarity when compared with those reported in Crohn's disease, a chronic inflammatory bowel disease.

CONCLUSIONS

Gene expression profile analysis revealed a predominantly altered immune response in the intestine of NEC neonates. Moreover, comparative analysis between NEC and Crohn's disease gene expression repertoires revealed a surprisingly high degree of similarity between these two conditions suggesting a new avenue for identifying NEC biomarkers.

MxB Is Not Responsible for the Blocking of HIV-1 Infection Observed in Alpha Interferon-Treated Cells

MxB restricts HIV-1 infection by directly interacting with the HIV-1 core, which is made of viral capsid; however, the contribution of MxB to the HIV-1 restriction observed in alpha interferon (IFN-α)-treated human cells is unknown. To understand this contribution, we used HIV-1 bearing the G208R capsid mutant (HIV-1-G208R), which overcomes the restriction imposed by cells expressing MxB. Here we showed that the reason why MxB does not block HIV-1-G208R is that MxB does not interact with HIV-1 cores bearing the mutation G208R. To understand whether MxB contributes to the HIV-1 restriction imposed by IFN-α-treated human cells, we challenged IFN-α-treated cells with HIV-G208R and found that MxB does not contribute to the restriction imposed by IFN-α-treated cells. To more directly test the contribution of MxB, we challenged IFN-α-treated human cells that are knocked out for the expression of MxB with HIV-1. These experiments suggested that MxB does not contribute to the HIV-1 restriction observed in IFN-α-treated human cells.

IMPORTANCE

MxB is a restriction factor that blocks HIV-1 infection in human cells. Although it has been postulated that MxB is the factor that blocks HIV-1 infection in IFN-α-treated cells, this is a hard concept to grasp due to the great number of genes that are induced by IFN-α in cells from the immune system. The work presented here elegantly demonstrates that MxB has minimal or no contribution to the ability of IFN-α-treated human cells to block HIV-1 infection. Furthermore, this work suggests the presence of novel restriction factors in IFN-α-treated human cells that block HIV-1 infection.

Interferon-stimulated genes-essential antiviral effectors implicated in resistance to Theiler's virus-induced demyelinating disease

Background

Experimental infection of mice with Theiler’s murine encephalomyelitis virus (TMEV) is used as an animal model of human multiple sclerosis. TMEV persists in susceptible mouse strains and causes a biphasic disease consisting of acute polioencephalomyelitis and chronic demyelinating leukomyelitis. In contrast, resistant mice eliminate the virus within 2 to 4 weeks, which seems to be based on a strong antiviral innate immune response including the activation of the type I interferon (IFN) pathway. Several interferon-stimulated genes (ISGs) such as IFN-stimulated protein of 15 kDa (ISG15), protein kinase R (PKR), and 2′5′-oligoadenylate synthetase (OAS) function as antiviral effectors and might contribute to virus elimination. Nevertheless, detailed investigations of the type I IFN pathway during TMEV-induced demyelinating disease (TMEV-IDD) are lacking.

Methods

The present study evaluated microarray data of the spinal cord obtained from susceptible SJL/J mice after TMEV infection focusing on IFN-related genes. Moreover, ISG gene and protein expression was determined in mock- and TMEV-infected SJL/J mice and compared to its expression in resistant C57BL/6 mice using real- time PCR, immunohistochemistry, and immunofluorescence.

Results

Interestingly, despite of increased ISG gene expression during TMEV-IDD, ISG protein expression was impaired in SJL/J mice and mainly restricted to demyelinated lesions. In contrast, high ISG protein levels were found in spinal cord gray and white matter of C57BL/6 compared to SJL/J mice in the acute and chronic phase of TMEV-IDD. In both mouse strains, ISG15 was mainly found in astrocytes and endothelial cells, whereas PKR was predominantly expressed by microglia/macrophages, oligodendrocytes, and neurons. Only few cells were immunopositive for OAS proteins.

Conclusions

High levels of antiviral ISG15 and PKR proteins in the spinal cord of C57BL/6 mice might block virus replication and play an important role in the resistance to TMEV-IDD.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0462-x) contains supplementary material, which is available to authorized users.

A dynamin-like protein involved in bacterial cell membrane surveillance under environmental stress

In ever-changing natural environments, bacteria are continuously challenged with numerous biotic and abiotic stresses. Accordingly, they have evolved both specific and more general mechanisms to counteract stress-induced damage and ensure survival. In the soil habitat of Bacillus subtilis, peptide antibiotics and bacteriophages are among the primary stressors that affect the integrity of the cytoplasmic membrane. Dynamin-like proteins (DLPs) play a major role in eukaryotic membrane re-modelling processes, including antiviral activities, but the function of the corresponding bacterial homologues was so far poorly understood. Here, we report on the protective function of a bacterial DLP, DynA from B. subtilis. We provide evidence that DynA plays an important role in a membrane surveillance system that counteracts membrane pore formation provoked by antibiotics and phages. In unstressed cells, DynA is a highly dynamic membrane-associated protein. Upon membrane damage, DynA localizes into large and static assemblies, where DynA acts locally to counteract stress-induced pores, presumably by inducing lipid bilayer fusion and sealing membrane gaps. Thus, lack of DynA increases the sensitivity to antibiotic exposure and phage infection. Taken together, our work suggests that DynA, and potentially other bacterial DLPs, contribute to the innate immunity of bacteria against membrane stress.

Phylogenomic analyses reveal a molecular signature linked to subterranean adaptation in rodents

Background

Genome-wide signatures of convergent evolution are widely expected but rarely revealed in animals. Subterranean rodent genome and transcriptome data produced by next-generation sequencing facilitate the use of phylogenetic methods to infer non-synonymous and synonymous substitution rates within coding regions, which can reveal changes at the molecular level that are correlated with the dramatic shift from a terrestrial to subterranean habitat.

Results

Our study used previously sequenced genome or transcriptome data of two subterranean rodents, the blind mole rat and naked mole rat, and their terrestrial relatives, the mouse and guinea pig, to investigate the genetic basis of rodent subterranean adaptation. An analysis of 4996 orthologous genes revealed that the substitution pace of coding sequences was significantly slower in the blind mole rat than in the mouse, and slower in the naked mole rat than in the guinea pig. The dN/dS ratio was significantly higher in the blind mole rat than in the mouse and in the naked mole rat than in the guinea pig. These patterns are most likely related to the longer generation time and lower effective population size of subterranean rodents caused by subterranean ecological constraints. We also identified some genes and gene ontology (GO) categories that might be candidates for adaptation to subterranean life.

Conclusions

Our study reveals a case of subterranean convergent evolution in rodents that is correlated with change in the pace and mode of molecular evolution observed at the genome scale. We believe that this genomic signature could have also evolved in other cases of subterranean convergence. Additionally, the genes that displayed the most radical changes in their patterns of evolution and their associated GO categories provide a strong basis for further comparative and functional studies, and potentially reveal molecular signatures of adaptation to subterranean life.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0564-1) contains supplementary material, which is available to authorized users.

Targeting Innate Immunity for Antiviral Therapy through Small Molecule Agonists of the RLR Pathway

The cellular response to virus infection is initiated when pathogen recognition receptors (PRR) engage viral pathogen-associated molecular patterns (PAMPs). This process results in induction of downstream signaling pathways that activate the transcription factor interferon regulatory factor 3 (IRF3). IRF3 plays a critical role in antiviral immunity to drive the expression of innate immune response genes, including those encoding antiviral factors, type 1 interferon, and immune modulatory cytokines, that act in concert to restrict virus replication. Thus, small molecule agonists that can promote IRF3 activation and induce innate immune gene expression could serve as antivirals to induce tissue-wide innate immunity for effective control of virus infection. We identified small molecule compounds that activate IRF3 to differentially induce discrete subsets of antiviral genes. We tested a lead compound and derivatives for the ability to suppress infections caused by a broad range of RNA viruses. Compound administration significantly decreased the viral RNA load in cultured cells that were infected with viruses of the family Flaviviridae, including West Nile virus, dengue virus, and hepatitis C virus, as well as viruses of the families Filoviridae (Ebola virus), Orthomyxoviridae (influenza A virus), Arenaviridae (Lassa virus), and Paramyxoviridae (respiratory syncytial virus, Nipah virus) to suppress infectious virus production. Knockdown studies mapped this response to the RIG-I-like receptor pathway. This work identifies a novel class of host-directed immune modulatory molecules that activate IRF3 to promote host antiviral responses to broadly suppress infections caused by RNA viruses of distinct genera.

IMPORTANCE

Incidences of emerging and reemerging RNA viruses highlight a desperate need for broad-spectrum antiviral agents that can effectively control infections caused by viruses of distinct genera. We identified small molecule compounds that can selectively activate IRF3 for the purpose of identifying drug-like molecules that can be developed for the treatment of viral infections. Here, we report the discovery of a hydroxyquinoline family of small molecules that can activate IRF3 to promote cellular antiviral responses. These molecules can prophylactically or therapeutically control infection in cell culture by pathogenic RNA viruses, including West Nile virus, dengue virus, hepatitis C virus, influenza A virus, respiratory syncytial virus, Nipah virus, Lassa virus, and Ebola virus. Our study thus identifies a class of small molecules with a novel mechanism to enhance host immune responses for antiviral activity against a variety of RNA viruses that pose a significant health care burden and/or that are known to cause infections with high case fatality rates.

Increased expression of Toll-like receptors 7 and 9 in myasthenia gravis thymus characterized by active Epstein-Barr virus infection

Considerable data implicate the thymus as the main site of autosensitization to the acetylcholine receptor in myasthenia gravis (MG), a B-cell-mediated autoimmune disease affecting the neuromuscular junction. We recently demonstrated an active Epstein-Barr virus (EBV) infection in the thymus of MG patients, suggesting that EBV might contribute to the onset or maintenance of the autoimmune response within MG thymus, because of its ability to activate and immortalize autoreactive B cells. EBV has been reported to elicit and modulate Toll-like receptor (TLR) 7- and TLR9-mediated innate immune responses, which are known to favor B-cell dysfunction and autoimmunity. Aim of this study was to investigate whether EBV infection is associated with altered expression of TLR7 and TLR9 in MG thymus. By real-time PCR, we found that TLR7 and TLR9 mRNA levels were significantly higher in EBV-positive MG compared to EBV-negative normal thymuses. By confocal microscopy, high expression levels of TLR7 and TLR9 proteins were observed in B cells and plasma cells of MG thymic germinal centers (GCs) and lymphoid infiltrates, where the two receptors co-localized with EBV antigens. An increased frequency of Ki67-positive proliferating B cells was found in MG thymuses, where we also detected proliferating cells expressing TLR7, TLR9 and EBV antigens, thus supporting the idea that EBV-associated TLR7/9 signaling may promote abnormal B-cell activation and proliferation. Along with B cells and plasma cells, thymic epithelium, plasmacytoid dendritic cells and macrophages exhibited enhanced TLR7 and TLR9 expression in MG thymus; TLR7 was also increased in thymic myeloid dendritic cells and its transcriptional levels positively correlated with those of interferon (IFN)-β. We suggested that TLR7/9 signaling may be involved in antiviral type I IFN production and long-term inflammation in EBV-infected MG thymuses. Our overall findings indicate that EBV-driven TLR7- and TLR9-mediated innate immune responses may participate in the intra-thymic pathogenesis of MG.

Evolutionary Analyses Suggest a Function of MxB Immunity Proteins Beyond Lentivirus Restriction

Viruses impose diverse and dynamic challenges on host defenses. Diversifying selection of codons and gene copy number variation are two hallmarks of genetic innovation in antiviral genes engaged in host-virus genetic conflicts. The myxovirus resistance (Mx) genes encode interferon-inducible GTPases that constitute a major arm of the cell-autonomous defense against viral infection. Unlike the broad antiviral activity of MxA, primate MxB was recently shown to specifically inhibit lentiviruses including HIV-1. We carried out detailed evolutionary analyses to investigate whether genetic conflict with lentiviruses has shaped MxB evolution in primates. We found strong evidence for diversifying selection in the MxB N-terminal tail, which contains molecular determinants of MxB anti-lentivirus specificity. However, we found no overlap between previously-mapped residues that dictate lentiviral restriction and those that have evolved under diversifying selection. Instead, our findings are consistent with MxB having a long-standing and important role in the interferon response to viral infection against a broader range of pathogens than is currently appreciated. Despite its critical role in host innate immunity, we also uncovered multiple functional losses of MxB during mammalian evolution, either by pseudogenization or by gene conversion from MxA genes. Thus, although the majority of mammalian genomes encode two Mx genes, this apparent stasis masks the dramatic effects that recombination and diversifying selection have played in shaping the evolutionary history of Mx genes. Discrepancies between our study and previous publications highlight the need to account for recombination in analyses of positive selection, as well as the importance of using sequence datasets with appropriate depth of divergence. Our study also illustrates that evolutionary analyses of antiviral gene families are critical towards understanding molecular principles that govern host-virus interactions and species-specific susceptibility to viral infection.

Evolutionary analyses have the potential to reveal not only biochemical details about host-virus arms-races but also the nature of the pathogens that drove them. Primate MxB was recently shown to restrict the replication of primate lentiviruses, including HIV-1. However, we find that positive selection in primate MxB is incongruent with known molecular determinants of lentiviral restriction. This suggests that MxB has antiviral activity against a broader range of viruses than is currently appreciated. We also identified multiple losses of MxB in mammals, as well as rampant recombination between Mx paralogs, which has distorted gene orthology. Our study illustrates the importance of evolution-guided functional analyses of antiviral gene families.

The Interferon-Induced Mx2 Inhibits Porcine Reproductive and Respiratory Syndrome Virus Replication

Porcine reproductive and respiratory syndrome virus (PRRSV) causes one of the most economically important diseases of swine in the world. Current vaccination strategies provide only limited protection against PRRSV infection. Recently, myxovirus resistance 2 (Mx2) has been identified as a novel interferon (IFN)-induced, innate immunity restriction factor that inhibits some viral infections. However, the role of Mx2 in PRRSV infection is not well understood. In this study, we cloned the full-length monkey Mx2 (mMx2) complementary DNA (cDNA) from IFN-β-treated African green monkey Marc-145 cells, and found that overexpression of mMx2 inhibited PRRSV replication in Marc-145 cells. IFN-β induced expression of mMx2 in Marc-145 cells and suppressed PRRSV replication in a dose-dependent manner. Knockdown of mMx2 impaired the antiviral activity mediated by IFN-β. Confocal imaging and immunoprecipitation assays indicated that mMx2 interacted with PRRSV N protein in virus-infected cells. Furthermore, we showed that GTPase activity of mMx2 is necessary, but that the first N-terminal 51 amino acids are dispensable for antiviral activity. Finally, porcine Mx2 was also found to have the antiviral activity against PRRSV in Marc-145 cells. We conclude that mMx2 protein inhibits PRRSV replication by interaction with the viral N protein.

Oligomerization Requirements for MX2-Mediated Suppression of HIV-1 Infection

Human myxovirus resistance 2 (MX2/MXB) is an interferon-stimulated gene (ISG) and was recently identified as a late postentry suppressor of human immunodeficiency virus type 1 (HIV-1) infection, inhibiting the nuclear accumulation of viral cDNAs. Although the HIV-1 capsid (CA) protein is believed to be the viral determinant of MX2-mediated inhibition, the precise mechanism of antiviral action remains unclear. The MX family of dynamin-like GTPases also includes MX1/MXA, a well-studied inhibitor of a range of RNA and DNA viruses, including influenza A virus (FLUAV) and hepatitis B virus but not retroviruses. MX1 and MX2 are closely related and share similar domain architectures and structures. However, MX2 possesses an extended N terminus that is essential for antiviral function and confers anti-HIV-1 activity on MX1 [MX1(N_MX2)]. Higher-order oligomerization is required for the antiviral activity of MX1 against FLUAV, with current models proposing that MX1 forms ring structures that constrict around viral nucleoprotein complexes. Here, we performed structure-function studies to investigate the requirements for oligomerization of both MX2 and chimeric MX1(N_MX2) for the inhibition of HIV-1 infection. The oligomerization state of mutated proteins with amino acid substitutions at multiple putative oligomerization interfaces was assessed using a combination of covalent cross-linking and coimmunoprecipitation. We show that while monomeric MX2 and MX1(N_MX2) mutants are not antiviral, higher-order oligomerization does not appear to be required for full antiviral activity of either protein. We propose that lower-order oligomerization of MX2 is sufficient for the effective inhibition of HIV-1.

IMPORTANCE Interferon plays an important role in the control of virus replication during acute infection in vivo. Recently, cultured cell experiments identified human MX2 as a key effector in the interferon-mediated postentry block to HIV-1 infection. MX2 is a member of a family of large dynamin-like GTPases that includes MX1/MXA, a closely related interferon-inducible inhibitor of several viruses, including FLUAV, but not HIV-1. MX GTPases form higher-order oligomeric structures, and the oligomerization of MX1 is required for inhibitory activity against many of its viral targets. Through structure-function studies, we report that monomeric mutants of MX2 do not inhibit HIV-1. However, in contrast to MX1, oligomerization beyond dimer assembly does not seem to be required for the antiviral activity of MX2, implying that fundamental differences exist between the antiviral mechanisms employed by these closely related proteins.

No Love Lost Between Viruses and Interferons

The interferon system protects mammals against virus infections. There are several types of interferons, which are characterized by their ability to inhibit virus replication and resultant pathogenesis by triggering both innate and cell-mediated immune responses. Virus infection is sensed by a variety of cellular pattern-recognition receptors and triggers the synthesis of interferons, which are secreted by the infected cells. In uninfected cells, cell surface receptors recognize the secreted interferons and activate intracellular signaling pathways that induce the expression of interferon-stimulated genes; the proteins encoded by these genes inhibit different stages of virus replication. To avoid extinction, almost all viruses have evolved mechanisms to defend themselves against the interferon system. Consequently, a dynamic equilibrium of survival is established between the virus and its host, an equilibrium that can be shifted to the host's favor by the use of exogenous interferon as a therapeutic antiviral agent.

Interferon-inducible cholesterol-25-hydroxylase restricts hepatitis C virus replication through blockage of membranous web formation

Hepatitis C virus (HCV) is a positive-strand RNA virus that primarily infects human hepatocytes. Infections with HCV constitute a global health problem, with 180 million people currently chronically infected. Recent studies have reported that cholesterol 25-hydroxylase (CH25H) is expressed as an interferon-stimulated gene and mediates antiviral activities against different enveloped viruses through the production of 25-hydroxycholesterol (25HC). However, the intrinsic regulation of human CH25H (hCH25H) expression within the liver as well as its mechanistic effects on HCV infectivity remain elusive. In this study, we characterized the expression of hCH25H using liver biopsies and primary human hepatocytes. In addition, the antiviral properties of this protein and its enzymatic product, 25HC, were further characterized against HCV in tissue culture. Levels of hCH25H messenger RNA were significantly up-regulated both in HCV-positive liver biopsies and in HCV-infected primary human hepatocytes. The expression of hCH25H in primary human hepatocytes was primarily and transiently induced by type I interferon. Transient expression of hCH25H in human hepatoma cells restricted HCV infection in a genotype-independent manner. This inhibition required the enzymatic activity of CH25H. We observed an inhibition of viral membrane fusion during the entry process by 25HC, which was not due to a virucidal effect. Yet the primary effect by 25HC on HCV was at the level of RNA replication, which was observed using subgenomic replicons of two different genotypes. Further analysis using electron microscopy revealed that 25HC inhibited formation of the membranous web, the HCV replication factory, independent of RNA replication.

CONCLUSION

Infection with HCV causes up-regulation of interferon-inducible CH25H in vivo, and its product, 25HC, restricts HCV primarily at the level of RNA replication by preventing formation of the viral replication factory.

Functional Comparison of Mx1 from Two Different Mouse Species Reveals the Involvement of Loop L4 in the Antiviral Activity against Influenza A Viruses

The interferon-induced Mx1 gene is an important part of the mammalian defense against influenza viruses. Mus musculus Mx1 inhibits influenza A virus replication and transcription by suppressing the polymerase activity of viral ribonucleoproteins (vRNPs). Here, we compared the anti-influenza virus activity of Mx1 from Mus musculus A2G with that of its ortholog from Mus spretus. We found that the antiviral activity of M. spretus Mx1 was less potent than that of M. musculus Mx1. Comparison of the M. musculus Mx1 sequence with the M. spretus Mx1 sequence revealed 25 amino acid differences, over half of which were present in the GTPase domain and 2 of which were present in loop L4. However, the in vitro GTPase activity of Mx1 from the two mouse species was similar. Replacement of one of the residues in loop L4 in M. spretus Mx1 by the corresponding residue of A2G Mx1 increased its antiviral activity. We also show that deletion of loop L4 prevented the binding of Mx1 to influenza A virus nucleoprotein and, hence, abolished the antiviral activity of mouse Mx1. These results indicate that loop L4 of mouse Mx1 is a determinant of antiviral activity. Our findings suggest that Mx proteins from different mammals use a common mechanism to inhibit influenza A viruses.

IMPORTANCE

Mx proteins are evolutionarily conserved in vertebrates and inhibit a wide range of viruses. Still, the exact details of their antiviral mechanisms remain largely unknown. Functional comparison of the Mx genes from two species that diverged relatively recently in evolution can provide novel insights into these mechanisms. We show that both Mus musculus A2G Mx1 and Mus spretus Mx1 target the influenza virus nucleoprotein. We also found that loop L4 in mouse Mx1 is crucial for its antiviral activity, as was recently reported for primate MxA. This indicates that human and mouse Mx proteins, which have diverged by 75 million years of evolution, recognize and inhibit influenza A viruses by a common mechanism.

Resistance to Rhabdoviridae Infection and Subversion of Antiviral Responses

Interferon (IFN) treatment induces the expression of hundreds of IFN-stimulated genes (ISGs). However, only a selection of their products have been demonstrated to be responsible for the inhibition of rhabdovirus replication in cultured cells; and only a few have been shown to play a role in mediating the antiviral response in vivo using gene knockout mouse models. IFNs inhibit rhabdovirus replication at different stages via the induction of a variety of ISGs. This review will discuss how individual ISG products confer resistance to rhabdoviruses by blocking viral entry, degrading single stranded viral RNA, inhibiting viral translation or preventing release of virions from the cell. Furthermore, this review will highlight how these viruses counteract the host IFN system.

Orthobunyaviruses and innate immunity induction: alieNSs vs. PredatoRRs

The taxonomic group of Orthobunyaviruses is gaining increased attention, as several emerging members are causing devastating illnesses among humans and livestock. These viruses are transmitted to mammals by arthropods (mostly mosquitoes) during the blood meal. The nature of their genomic RNA predisposes orthobunyaviruses for eliciting a strong innate immune response mediated by pathogen recognition receptors (PRRs), especially the cytoplasmic RIG-I. However, the PRR responses are in fact disabled by the viral non-structural protein NSs. NSs imposes a strong block of cellular gene expression by inhibiting elongating RNA polymerase II. In this review, we will give an overview on the current state of knowledge regarding the interactions between orthobunyaviruses, the PRR axis, and NSs.

Restriction of HIV-1 Requires the N-Terminal Region of MxB as a Capsid-Binding Motif but Not as a Nuclear Localization Signal

The interferon alpha (IFN-α)-inducible restriction factor MxB blocks HIV-1 infection after reverse transcription but prior to integration. Fate-of-capsid experiments have correlated the ability of MxB to block HIV-1 infection with stabilization of viral cores during infection. We previously demonstrated that HIV-1 restriction by MxB requires capsid binding and oligomerization. Deletion and gain-of-function experiments have mapped the HIV-1 restriction ability of MxB to its N-terminal 25 amino acids. This report reveals that the N-terminal 25 amino acids of MxB exhibit two separate functions: (i) the ability of MxB to bind to HIV-1 capsid and (ii) the nuclear localization signal of MxB, which is important for the ability of MxB to shuttle into the nucleus. To understand whether MxB restriction of HIV-1 requires capsid binding and/or nuclear localization, we genetically separated these two functions and evaluated their contributions to restriction. Our experiments demonstrated that the (11)RRR(13) motif is important for the ability of MxB to bind capsid and to restrict HIV-1 infection. These experiments suggested that capsid binding is necessary for the ability of MxB to block HIV-1 infection. Separately from the capsid binding function of MxB, we found that residues (20)KY(21) regulate the ability of the N-terminal 25 amino acids of MxB to function as a nuclear localization signal; however, the ability of the N-terminal 25 amino acids to function as a nuclear localization signal was not required for restriction.

IMPORTANCE

MxB/Mx2 blocks HIV-1 infection in cells from the immune system. MxB blocks infection by preventing the uncoating process of HIV-1. The ability of MxB to block HIV-1 infection requires that MxB binds to the HIV-1 core by using its N-terminal domain. The present study shows that MxB uses residues (11)RRR(13) to bind to the HIV-1 core during infection and that these residues are required for the ability of MxB to block HIV-1 infection. We also found that residues (20)KY(21) constitute a nuclear localization signal that is not required for the ability of MxB to block HIV-1 infection.

Role of nucleotide binding and GTPase domain dimerization in dynamin-like myxovirus resistance protein A for GTPase activation and antiviral activity

Myxovirus resistance (Mx) GTPases are induced by interferon and inhibit multiple viruses, including influenza and human immunodeficiency viruses. They have the characteristic domain architecture of dynamin-related proteins with an N-terminal GTPase (G) domain, a bundle signaling element, and a C-terminal stalk responsible for self-assembly and effector functions. Human MxA (also called MX1) is expressed in the cytoplasm and is partly associated with membranes of the smooth endoplasmic reticulum. It shows a protein concentration-dependent increase in GTPase activity, indicating regulation of GTP hydrolysis via G domain dimerization. Here, we characterized a panel of G domain mutants in MxA to clarify the role of GTP binding and the importance of the G domain interface for the catalytic and antiviral function of MxA. Residues in the catalytic center of MxA and the nucleotide itself were essential for G domain dimerization and catalytic activation. In pulldown experiments, MxA recognized Thogoto virus nucleocapsid proteins independently of nucleotide binding. However, both nucleotide binding and hydrolysis were required for the antiviral activity against Thogoto, influenza, and La Crosse viruses. We further demonstrate that GTP binding facilitates formation of stable MxA assemblies associated with endoplasmic reticulum membranes, whereas nucleotide hydrolysis promotes dynamic redistribution of MxA from cellular membranes to viral targets. Our study highlights the role of nucleotide binding and hydrolysis for the intracellular dynamics of MxA during its antiviral action.

A triple-arginine motif in the amino-terminal domain and oligomerization are required for HIV-1 inhibition by human MX2

We have employed molecular genetic approaches to understand the domain organization of the HIV-1 resistance factor myxovirus resistance 2 (MX2). First, we describe an essential triple-arginine motif in the amino-terminal domain. Second, we demonstrate that this 91-residue domain mediates antiviral activity when appended to heterologous proteins, and we provide genetic evidence that protein oligomerization is required for MX2 function. These insights will facilitate future work aiming to elucidate MX2's mechanism of action.