Intraluminal proteolytic activation plays an important role in replication of type 1 reovirus in the intestines of neonatal mice

Proteasome activity is required for reovirus entry into cells

IMPORTANCE

Due to their limited genetic capacity, viruses are reliant on multiple host systems to replicate successfully. Mammalian orthoreovirus (reovirus) is commonly used as a model system for understanding host-virus interactions. In this study, we identify that the proteasome system, which is critical for cellular protein turnover, affects reovirus entry. Inhibition of the proteasome using a chemical inhibitor blocks reovirus uncoating. Blocking these events reduces subsequent replication of the virus. This work identifies that additional host factors control reovirus entry.

Reovirus genomic diversity confers plasticity for protease utility during adaptation to intracellular uncoating

IMPORTANCE

Reoviruses infect many mammals and are widely studied as a model system for enteric viruses. However, most of our reovirus knowledge comes from laboratory strains maintained on immortalized L929 cells. Herein, we asked whether naturally circulating reoviruses possess the same genetic and phenotypic characteristics as laboratory strains. Naturally circulating reoviruses obtained from sewage were extremely diverse genetically. Moreover, sewage reoviruses exhibited poor fitness on L929 cells and relied heavily on gut proteases for viral uncoating and productive infection compared to laboratory strains. We then examined how naturally circulating reoviruses might adapt to cell culture conditions. Within three passages, virus isolates from the parental sewage population were selected, displaying improved fitness and intracellular uncoating in L929 cells. Remarkably, selected progeny clones were present at 0.01% of the parental population. Altogether, using reovirus as a model, our study demonstrates how the high genetic diversity of naturally circulating viruses results in rapid adaptation to new environments.

Reovirus uses temporospatial compartmentalization to orchestrate core versus outercapsid assembly

Reoviridae virus family members, such as mammalian orthoreovirus (reovirus), encounter a unique challenge during replication. To hide the dsRNA from host recognition, the genome remains encapsidated in transcriptionally active proteinaceous core capsids that transcribe and release +RNA. De novo +RNAs and core proteins must repeatedly assemble into new progeny cores in order to logarithmically amplify replication. Reoviruses also produce outercapsid (OC) proteins μ1, σ3 and σ1 that assemble onto cores to create highly stable infectious full virions. Current models of reovirus replication position amplification of transcriptionally-active cores and assembly of infectious virions in shared factories, but we hypothesized that since assembly of OC proteins would halt core amplification, OC assembly is somehow regulated. Kinetic analysis of virus +RNA production, core versus OC protein expression, and core particles versus whole virus particle accumulation, indicated that assembly of OC proteins onto core particles was temporally delayed. All viral RNAs and proteins were made simultaneously, eliminating the possibility that delayed OC RNAs or proteins account for delayed OC assembly. High resolution fluorescence and electron microscopy revealed that core amplification occurred early during infection at peripheral core-only factories, while all OC proteins associated with lipid droplets (LDs) that coalesced near the nucleus in a μ1–dependent manner. Core-only factories transitioned towards the nucleus despite cycloheximide-mediated halting of new protein expression, while new core-only factories developed in the periphery. As infection progressed, OC assembly occurred at LD-and nuclear-proximal factories. Silencing of OC μ1 expression with siRNAs led to large factories that remained further from the nucleus, implicating μ1 in the transition to perinuclear factories. Moreover, late during infection, +RNA pools largely contributed to the production of de-novo viral proteins and fully-assembled infectious viruses. Altogether the results suggest an advanced model of reovirus replication with spatiotemporal segregation of core amplification, OC complexes and fully assembled virions.

It is important to understand how viruses replicate and assemble to discover antiviral therapies and to modify viruses for applications like gene therapy or cancer therapy. Reovirus is a harmless virus being tested as a cancer therapy. Reovirus has two coats of proteins, an inner coat and an outer coat. To replicate, reovirus particles need only the inner coat, but to become infectious they require the outer coat. Strangely, inner and outer coat proteins are all made by the virus at once, so it was unknown what determines whether newly made viruses will contain just the inner coat to continue to replicate, or both coats to transmit to new hosts. Our experiments reveal that the inner coat proteins are located in a different area of an infected cell versus the outer coat proteins. The location therefore determines if the newly made viruses contain just the inner coat versus both coats. Reoviruses have evolved extravagant mechanisms to be able to efficiently take on the best composition required for replication and transmission.

The multi-functional reovirus σ3 protein is a virulence factor that suppresses stress granule formation and is associated with myocardial injury

The mammalian orthoreovirus double-stranded (ds) RNA-binding protein σ3 is a multifunctional protein that promotes viral protein synthesis and facilitates viral entry and assembly. The dsRNA-binding capacity of σ3 correlates with its capacity to prevent dsRNA-mediated activation of protein kinase R (PKR). However, the effect of σ3 binding to dsRNA during viral infection is largely unknown. To identify functions of σ3 dsRNA-binding activity during reovirus infection, we engineered a panel of thirteen σ3 mutants and screened them for the capacity to bind dsRNA. Six mutants were defective in dsRNA binding, and mutations in these constructs cluster in a putative dsRNA-binding region on the surface of σ3. Two recombinant viruses expressing these σ3 dsRNA-binding mutants, K287T and R296T, display strikingly different phenotypes. In a cell-type dependent manner, K287T, but not R296T, replicates less efficiently than wild-type (WT) virus. In cells in which K287T virus demonstrates a replication deficit, PKR activation occurs and abundant stress granules (SGs) are formed at late times post-infection. In contrast, the R296T virus retains the capacity to suppress activation of PKR and does not mediate formation of SGs at late times post-infection. These findings indicate that σ3 inhibits PKR independently of its capacity to bind dsRNA. In infected mice, K287T produces lower viral titers in the spleen, liver, lungs, and heart relative to WT or R296T. Moreover, mice inoculated with WT or R296T viruses develop myocarditis, whereas those inoculated with K287T do not. Overall, our results indicate that σ3 functions to suppress PKR activation and subsequent SG formation during viral infection and that these functions correlate with virulence in mice.

The σ3 protein of mammalian orthoreoviruses is a double-stranded RNA binding protein that has classically been thought to function by scavenging dsRNA within infected cells and thus prevents activation of cellular sensors of dsRNA such as the kinase PKR. Here we used mutagenesis to identify the region of σ3 responsible for binding dsRNA. Characterization of mutant viruses expressing σ3 proteins incapable of binding dsRNA show that contrary to expectation, dsRNA binding is not required for σ3-mediated inhibition of PKR. We show that one mutant virus (R296T) despite being deficient in dsRNA-binding can inhibit PKR and replicates similar to WT virus. In contrast, another mutant virus (K287T) that bears a σ3 protein that cannot prevent dsRNA-mediated activation of PKR induces stress granules in infected cells and replicates less efficiently than WT virus. In vivo, the K287T mutant is attenuated in its replication and unlike WT virus and the R296T mutant virus does not cause heart disease (myocarditis).

A Single Point Mutation, Asn16→Lys, Dictates the Temperature-Sensitivity of the Reovirus tsG453 Mutant

Studies of conditionally lethal mutants can help delineate the structure-function relationships of biomolecules. Temperature-sensitive (ts) mammalian reovirus (MRV) mutants were isolated and characterized many years ago. Two of the most well-defined MRV ts mutants are tsC447, which contains mutations in the S2 gene encoding viral core protein σ2, and tsG453, which contains mutations in the S4 gene encoding major outer-capsid protein σ3. Because many MRV ts mutants, including both tsC447 and tsG453, encode multiple amino acid substitutions, the specific amino acid substitutions responsible for the ts phenotype are unknown. We used reverse genetics to recover recombinant reoviruses containing the single amino acid polymorphisms present in ts mutants tsC447 and tsG453 and assessed the recombinant viruses for temperature-sensitivity by efficiency-of-plating assays. Of the three amino acid substitutions in the tsG453 S4 gene, Asn₁₆-Lys was solely responsible for the tsG453ts phenotype. Additionally, the mutant tsC447 Ala₁₈₈-Val mutation did not induce a temperature-sensitive phenotype. This study is the first to employ reverse genetics to identify the dominant amino acid substitutions responsible for the tsC447 and tsG453 mutations and relate these substitutions to respective phenotypes. Further studies of other MRV ts mutants are warranted to define the sequence polymorphisms responsible for temperature sensitivity.

Control of Capsid Transformations during Reovirus Entry

Mammalian orthoreovirus (reovirus), a dsRNA virus with a multilayered capsid, serves as a model system for studying the entry of similar viruses. The outermost layer of this capsid undergoes processing to generate a metastable intermediate. The metastable particle undergoes further remodeling to generate an entry-capable form that delivers the genome-containing inner capsid, or core, into the cytoplasm. In this review, we highlight capsid proteins and the intricacies of their interactions that control the stability of the capsid and consequently impact capsid structural changes that are prerequisites for entry. We also discuss a novel proviral role of host membranes in promoting capsid conformational transitions. Current knowledge gaps in the field that are ripe for future investigation are also outlined.

Recognition of Reovirus RNAs by the Innate Immune System

Mammalian orthoreovirus (reovirus) is a dsRNA virus, which has long been used as a model system to study host–virus interactions. One of the earliest interactions during virus infection is the detection of the viral genomic material, and the consequent induction of an interferon (IFN) based antiviral response. Similar to the replication of related dsRNA viruses, the genomic material of reovirus is thought to remain protected by viral structural proteins throughout infection. Thus, how innate immune sensor proteins gain access to the viral genomic material, is incompletely understood. This review summarizes currently known information about the innate immune recognition of the reovirus genomic material. Using this information, we propose hypotheses about host detection of reovirus.

Components of the Reovirus Capsid Differentially Contribute to Stability

The mammalian orthoreovirus (reovirus) outer capsid is composed of 200 μ1-σ3 heterohexamers and a maximum of 12 σ1 trimers. During cell entry, σ3 is degraded by luminal or intracellular proteases to generate the infectious subviral particle (ISVP). When ISVP formation is prevented, reovirus fails to establish a productive infection, suggesting proteolytic priming is required for entry. ISVPs are then converted to ISVP*s, which is accompanied by μ1 rearrangements. The μ1 and σ3 proteins confer resistance to inactivating agents; however, neither the impact on capsid properties nor the mechanism (or basis) of inactivation is fully understood. Here, we utilized T1L/T3D M2 and T3D/T1L S4 to investigate the determinants of reovirus stability. Both reassortants encode mismatched subunits. When μ1-σ3 were derived from different strains, virions resembled wild-type particles in structure and protease sensitivity. T1L/T3D M2 and T3D/T1L S4 ISVPs were less thermostable than wild-type ISVPs. In contrast, virions were equally susceptible to heating. Virion associated μ1 adopted an ISVP*-like conformation concurrent with inactivation; σ3 preserves infectivity by preventing μ1 rearrangements. Moreover, thermostability was enhanced by a hyperstable variant of μ1. Unlike the outer capsid, the inner capsid (core) was highly resistant to elevated temperatures. The dual layered architecture allowed for differential sensitivity to inactivating agents.IMPORTANCE Nonenveloped and enveloped viruses are exposed to the environment during transmission to a new host. Protein-protein and/or protein-lipid interactions stabilize the particle and protect the viral genome. Mammalian orthoreovirus (reovirus) is composed of two concentric, protein shells. The μ1 and σ3 proteins form the outer capsid; contacts between neighboring subunits are thought to confer resistance to inactivating agents. We further investigated the determinants of reovirus stability. The outer capsid was disrupted concurrent with the loss of infectivity; virion associated μ1 rearranged into an altered conformation. Heat sensitivity was controlled by σ3; however, particle integrity was enhanced by a single μ1 mutation. In contrast, the inner capsid (core) displayed superior resistance to heating. These findings reveal structural components that differentially contribute to reovirus stability.

Protein Mismatches Caused by Reassortment Influence Functions of the Reovirus Capsid

Following attachment to host receptors via σ1, reovirus particles are endocytosed and disassembled to generate infectious subvirion particles (ISVPs). ISVPs undergo conformational changes to form ISVP*, releasing σ1 and membrane-targeting peptides from the viral μ1 protein. ISVP* formation is required for delivery of the viral core into the cytoplasm for replication. We characterized the properties of T3D^F/T3D^CS1, an S1 gene monoreassortant between two laboratory isolates of prototype reovirus strain T3D: T3D^F and T3D^C T3D^F/T3D^CS1 is poorly infectious. This deficiency is a consequence of inefficient encapsidation of S1-encoded σ1 on T3D^F/T3D^CS1 virions. Additionally, compared to T3D^F, T3D^F/T3D^CS1 undergoes ISVP-to-ISVP* conversion more readily, revealing an unexpected role for σ1 in regulating ISVP* formation. The σ1 protein is held within turrets formed by the λ2 protein. To test if the altered properties of T3D^F/T3D^CS1 are due to a mismatch between σ1 and λ2 proteins from T3D^F and T3D^C, properties of T3D^F/T3D^CL2 and T3D^F/T3D^CS1L2, which express a T3D^C-derived λ2, were compared. The presence of T3D^C λ2 allowed more efficient σ1 incorporation, producing particles that exhibit T3D^F-like infectivity. Compared to T3D^F, T3D^F/T3D^CL2 prematurely converts to ISVP*, uncovering a role for λ2 in regulating ISVP* formation. Importantly, a virus with matching σ1 and λ2 displayed a more regulated conversion to ISVP* than either T3D^F/T3D^CS1 or T3D^F/T3D^CL2. In addition to identifying new regulators of ISVP* formation, our results highlight that protein mismatches produced by reassortment can alter virus assembly and thereby influence subsequent functions of the virus capsid.IMPORTANCE Cells coinfected with viruses that possess a multipartite or segmented genome reassort to produce progeny viruses that contain a combination of gene segments from each parent. Reassortment places new pairs of genes together, generating viruses in which mismatched proteins must function together. To test if such forced pairing of proteins that form the virus shell or capsid alters the function of the particle, we investigated properties of reovirus variants in which the σ1 attachment protein and the λ2 protein that anchors σ1 on the particle are mismatched. Our studies demonstrate that a σ1-λ2 mismatch produces particles with lower levels of encapsidated σ1, consequently decreasing virus attachment and infectivity. The mismatch between σ1 and λ2 also altered the capacity of the viral capsid to undergo conformational changes required for cell entry. These studies reveal new functions of reovirus capsid proteins and illuminate both predictable and novel implications of reassortment.

Going (Reo)Viral: Factors Promoting Successful Reoviral Oncolytic Infection

Oncolytic viruses show intriguing potential as cancer therapeutic agents. These viruses are capable of selectively targeting and killing cancerous cells while leaving healthy cells largely unaffected. The use of oncolytic viruses for cancer treatments in selected circumstances has recently been approved by the Food and Drug Administration (FDA) of the US and work is progressing on engineering viral vectors for enhanced selectivity, efficacy and safety. However, a better fundamental understanding of tumour and viral biology is essential for the continued advancement of the oncolytic field. This knowledge will not only help to engineer more potent and effective viruses but may also contribute to the identification of biomarkers that can determine which patients will benefit most from this treatment. A mechanistic understanding of the overlapping activity of viral and standard chemotherapeutics will enable the development of better combinational approaches to improve patient outcomes. In this review, we will examine each of the factors that contribute to productive viral infections in cancerous cells versus healthy cells. Special attention will be paid to reovirus as it is a well-studied virus and the only wild-type virus to have received orphan drug designation by the FDA. Although considerable insight into reoviral biology exists, there remain numerous deficiencies in our understanding of the factors regulating its successful oncolytic infection. Here we will discuss what is known to regulate infection as well as speculate about potential new mechanisms that may enhance successful replication. A joint appreciation of both tumour and viral biology will drive innovation for the next generation of reoviral mediated oncolytic therapy.

Structural and Functional Features of the Reovirus σ1 Tail

Mammalian orthoreovirus attachment to target cells is mediated by the outer capsid protein σ1, which projects from the virion surface. The σ1 protein is a homotrimer consisting of a filamentous tail, which is partly inserted into the virion; a body domain constructed from β-spiral repeats; and a globular head with receptor-binding properties. The σ1 tail is predicted to form an α-helical coiled coil. Although σ1 undergoes a conformational change during cell entry, the nature of this change and its contributions to viral replication are unknown. Electron micrographs of σ1 molecules released from virions identified three regions of flexibility, including one at the midpoint of the molecule, that may be involved in its structural rearrangement. To enable a detailed understanding of essential σ1 tail organization and properties, we determined high-resolution structures of the reovirus type 1 Lang (T1L) and type 3 Dearing (T3D) σ1 tail domains. Both molecules feature extended α-helical coiled coils, with T1L σ1 harboring central chloride ions. Each molecule displays a discontinuity (stutter) within the coiled coil and an unexpectedly seamless transition to the body domain. The transition region features conserved interdomain interactions and appears rigid rather than highly flexible. Functional analyses of reoviruses containing engineered σ1 mutations suggest that conserved residues predicted to stabilize the coiled-coil-to-body junction are essential for σ1 folding and encapsidation, whereas central chloride ion coordination and the stutter are dispensable for efficient replication. Together, these findings enable modeling of full-length reovirus σ1 and provide insight into the stabilization of a multidomain virus attachment protein.IMPORTANCE While it is established that different conformational states of attachment proteins of enveloped viruses mediate receptor binding and membrane fusion, less is understood about how such proteins mediate attachment and entry of nonenveloped viruses. The filamentous reovirus attachment protein σ1 binds cellular receptors; contains regions of predicted flexibility, including one at the fiber midpoint; and undergoes a conformational change during cell entry. Neither the nature of the structural change nor its contribution to viral infection is understood. We determined crystal structures of large σ1 fragments for two different reovirus serotypes. We observed an unexpectedly tight transition between two domains spanning the fiber midpoint, which allows for little flexibility. Studies of reoviruses with engineered changes near the σ1 midpoint suggest that the stabilization of this region is critical for function. Together with a previously determined structure, we now have a complete model of the full-length, elongated reovirus σ1 attachment protein.

Differential Delivery of Genomic Double-Stranded RNA Causes Reovirus Strain-Specific Differences in Interferon Regulatory Factor 3 Activation

Serotype 3 (T3) reoviruses induce substantially more type 1 interferon (IFN-I) secretion than serotype 1 (T1) strains. However, the mechanisms underlying differences in IFN-I production between T1 and T3 reoviruses remain undefined. Here, we found that differences in IFN-I production between T1 and T3 reoviruses correlate with activation of interferon regulatory factor 3 (IRF3), a key transcription factor for the production of IFN-I. T3 strain rsT3D activated IRF3 more rapidly and to a greater extent than the T1 strain rsT1L, in simian virus 40 (SV40) immortalized endothelial cells (SVECs). Differences in IRF3 activation between rsT1L and rsT3D were observed in the first hours of infection and were independent of de novo viral RNA and protein synthesis. NF-κB activation mirrored IRF3 activation, with rsT3D inducing more NF-κB activity than rsT1L. We also found that IRF3 and NF-κB are activated in a mitochondrial antiviral-signaling protein (MAVS)-dependent manner. rsT1L does not suppress IRF3 activation, as IRF3 phosphorylation could be induced in rsT1L-infected cells. Transfected rsT1L and rsT3D RNA induced IRF3 phosphorylation, indicating that genomic RNA from both strains has the capacity to activate IRF3. Finally, bypassing the normal route of reovirus entry by transfecting in vitro-generated viral cores revealed that rsT1L and rsT3D core particles induced equivalent IRF3 activation. Taken together, our findings indicate that entry-related events that occur after outer capsid disassembly, but prior to deposition of viral cores into the cytoplasm, influence the efficiency of IFN-I responses to reovirus. This work provides further insight into mechanisms by which nonenveloped viruses activate innate immune responses.IMPORTANCE Detection of viral nucleic acids by the host cell triggers type 1 interferon (IFN-I) responses, which are critical for containing and clearing viral infections. Viral RNA is sensed in the cytoplasm by cellular receptors that initiate signaling pathways, leading to the activation of interferon regulatory factor 3 (IRF3) and NF-κB, key transcription factors required for IFN-I induction. Serotype 3 (T3) reoviruses induce significantly more IFN-I than serotype 1 (T1) strains. In this work, we found that differences in IFN-I production by T1 and T3 reoviruses correlate with differential IRF3 activation. Differences in IRF3 activation are not caused by a blockade of the IRF3 activation by a T1 strain. Rather, differences in events during the late stages of viral entry determine the capacity of reovirus to activate host IFN-I responses. Together, our work provides insight into mechanisms of IFN-I induction by nonenveloped viruses.

Cleavage of the C-Terminal Fragment of Reovirus μ1 Is Required for Optimal Infectivity

The mammalian orthoreovirus (reovirus) outer capsid, which is composed of 200 μ1/σ3 heterohexamers and a maximum of 12 σ1 trimers, contains all of the proteins that are necessary for attaching to and entering host cells. Following attachment, reovirus is internalized by receptor-mediated endocytosis and acid-dependent cathepsin proteases degrade the σ3 protein. This process generates a metastable intermediate, called infectious subviral particle (ISVP), in which the μ1 membrane penetration protein is exposed. ISVPs undergo a second structural rearrangement to deposit the genome-containing core into the host cytoplasm. The conformationally altered particle is called ISVP*. ISVP-to-ISVP* conversion culminates in the release of μ1 N- and C-terminal fragments, μ1N and Φ, respectively. Released μ1N is thought to facilitate core delivery by generating size-selective pores within the endosomal membrane, whereas the precise role of Φ, particularly in the context of viral entry, is undefined. In this report, we characterize a recombinant reovirus that fails to cleave Φ from μ1 in vitro Φ cleavage, which is not required for ISVP-to-ISVP* conversion, enhances the disruption of liposomal membranes and facilitates the recruitment of ISVP*s to the site of pore formation. Moreover, the Φ cleavage-deficient strain initiates infection of host cells less efficiently than the parental strain. These results indicate that μ1N and Φ contribute to reovirus pore forming activity.IMPORTANCE Host membranes represent a physical barrier that prevents infection. To overcome this barrier, viruses utilize diverse strategies, such as membrane fusion or membrane disruption, to access internal components of the cell. These strategies are characterized by discrete protein-protein and protein-lipid interactions. The mammalian orthoreovirus (reovirus) outer capsid undergoes a series of well-defined conformational changes, which conclude with pore formation and delivery of the viral genetic material. In this report, we characterize the role of the small, reovirus-derived Φ peptide in pore formation. Φ cleavage from the outer capsid enhances membrane disruption and facilitates the recruitment of virions to membrane-associated pores. Moreover, Φ cleavage promotes the initiation of infection. Together, these results reveal an additional component of the reovirus pore forming apparatus and highlight a strategy for penetrating host membranes.

Bacteria and bacterial envelope components enhance mammalian reovirus thermostability

Enteric viruses encounter diverse environments as they migrate through the gastrointestinal tract to infect their hosts. The interaction of eukaryotic viruses with members of the host microbiota can greatly impact various aspects of virus biology, including the efficiency with which viruses can infect their hosts. Mammalian orthoreovirus, a human enteric virus that infects most humans during childhood, is negatively affected by antibiotic treatment prior to infection. However, it is not known how components of the host microbiota affect reovirus infectivity. In this study, we show that reovirus virions directly interact with Gram positive and Gram negative bacteria. Reovirus interaction with bacterial cells conveys enhanced virion thermostability that translates into enhanced attachment and infection of cells following an environmental insult. Enhanced virion thermostability was also conveyed by bacterial envelope components lipopolysaccharide (LPS) and peptidoglycan (PG). Lipoteichoic acid and N-acetylglucosamine-containing polysaccharides enhanced virion stability in a serotype-dependent manner. LPS and PG also enhanced the thermostability of an intermediate reovirus particle (ISVP) that is associated with primary infection in the gut. Although LPS and PG alter reovirus thermostability, these bacterial envelope components did not affect reovirus utilization of its proteinaceous cellular receptor junctional adhesion molecule-A or cell entry kinetics. LPS and PG also did not affect the overall number of reovirus capsid proteins σ1 and σ3, suggesting their effect on virion thermostability is not mediated through altering the overall number of major capsid proteins on the virus. Incubation of reovirus with LPS and PG did not significantly affect the neutralizing efficiency of reovirus-specific antibodies. These data suggest that bacteria enhance reovirus infection of the intestinal tract by enhancing the thermal stability of the reovirus particle at a variety of temperatures through interactions between the viral particle and bacterial envelope components.

The Loop Formed by Residues 340 to 343 of Reovirus μ1 Controls Entry-Related Conformational Changes

Reovirus particles are covered with 200 μ1/σ3 heterohexamers. Following attachment to cell surface receptors, reovirus is internalized by receptor-mediated endocytosis. Within the endosome, particles undergo a series of stepwise disassembly events. First, the σ3 protector protein is degraded by cellular proteases to generate infectious subviral particles (ISVPs). Second, the μ1 protein rearranges into a protease-sensitive conformation to generate ISVP*s and releases two virus-encoded peptides, μ1N and Φ. The released peptides promote delivery of the genome-containing core by perforating the endosomal membrane. Thus, to establish a productive infection, virions must be stable in the environment but flexible to disassemble in response to the appropriate cellular cue. The reovirus outer capsid is stabilized by μ1 intratrimer, intertrimer, and trimer-core interactions. As a consequence of ISVP-to-ISVP* conversion, neighboring μ1 trimers unwind and separate. Located within the μ1 jelly roll β barrel domain, which is a known regulator of ISVP* formation, residues 340 to 343 form a loop and have been proposed to facilitate viral entry. To test this idea, we generated recombinant reoviruses that encoded deletions within this loop (Δ341 and Δ342). Both deletions destabilized the outer capsid. Notably, Δ342 impaired the viral life cycle; however, replicative fitness was restored by an additional change (V403A) within the μ1 jelly roll β barrel domain. In the Δ341 and Δ342 backgrounds, V403A also rescued defects in ISVP-to-ISVP* conversion. Together, these findings reveal a new region that regulates reovirus disassembly and how perturbing a metastable capsid can compromise replicative fitness.IMPORTANCE Capsids of nonenveloped viruses are composed of protein complexes that encapsulate, or form a shell around, nucleic acid. The protein-protein interactions that form this shell must be stable to protect the viral genome but also sufficiently flexible to disassemble during cell entry. Thus, capsids adopt conformations that undergo rapid disassembly in response to a specific cellular cue. In this work, we identify a new region within the mammalian orthoreovirus outer capsid that regulates particle stability. Amino acid deletions that destabilize this region impair the viral replication cycle. Nonetheless, replicative fitness is restored by a compensatory mutation that restores particle stability. Together, this work demonstrates the critical balance between assembling virions that are stable and maintaining conformational flexibility. Any factor that perturbs this balance has the potential to block a productive infection.

Reverse Genetics for Mammalian Orthoreovirus

Reverse genetics allows introduction of specific alterations into a viral genome. Studies performed with mutant viruses generated using reverse genetics approaches have contributed immeasurably to our understanding of viral replication and pathogenesis, and also have led to development of novel vaccines and virus-based vectors. Here, we describe the reverse genetics system that allows for production and recovery of mammalian orthoreovirus, a double-stranded (ds) RNA virus, from plasmids that encode the viral genome.

Lipids Cooperate with the Reovirus Membrane Penetration Peptide to Facilitate Particle Uncoating

Virus-host interactions play a role in many stages of the viral lifecycle, including entry. Reovirus, a model system for studying the entry mechanisms of nonenveloped viruses, undergoes a series of regulated structural transitions that culminate in delivery of the viral genetic material. Lipids can trigger one of these conformational changes, infectious subviral particle (ISVP)-to-ISVP* conversion. ISVP* formation releases two virally encoded peptides, myristoylated μ1N (myr-μ1N) and Φ. Among these, myr-μ1N is sufficient to form pores within membranes. Released myr-μ1N can also promote ISVP* formation in trans Using thermal inactivation as a readout for ISVP-to-ISVP* conversion, we demonstrate that lipids render ISVPs less thermostable in a virus concentration-dependent manner. Under conditions in which neither lipids alone nor myr-μ1N alone promotes ISVP-to-ISVP* conversion, myr-μ1N induces particle uncoating when lipids are present. These data suggest that the pore-forming activity and the ISVP*-promoting activity of myr-μ1N are linked. Lipid-associated myr-μ1N interacts with ISVPs and triggers efficient ISVP* formation. The cooperativity between a reovirus component and lipids reveals a distinct virus-host interaction in which membranes can facilitate nonenveloped virus entry.

Lipid Membranes Facilitate Conformational Changes Required for Reovirus Cell Entry

Cellular entry of nonenveloped and enveloped viruses is often accompanied by dramatic conformational changes within viral structural proteins. These rearrangements are triggered by a variety of mechanisms, such as low pH, virus-receptor interactions, and virus-host chaperone interactions. Reoviruses, a model system for entry of nonenveloped viruses, undergo a series of disassembly steps within the host endosome. One of these steps, infectious subviral particle (ISVP)-to-ISVP* conversion, is necessary for delivering the genome-containing viral core into host cells, but the physiological trigger that mediates ISVP-to-ISVP* conversion during cell entry is unknown. Structural studies of the reovirus membrane penetration protein, μ1, predict that interactions between μ1 and negatively charged lipid head groups may promote ISVP* formation; however, experimental evidence for this idea is lacking. Here, we show that the presence of polyanions (SO4(2-) and HPO4(2-)) or lipids in the form of liposomes facilitates ISVP-to-ISVP* conversion. The requirement for charged lipids appears to be selective, since phosphatidylcholine and phosphatidylethanolamine promoted ISVP* formation, whereas other lipids, such as sphingomyelin and sulfatide, either did not affect ISVP* formation or prevented ISVP* formation. Thus, our work provides evidence that interactions with membranes can function as a trigger for a nonenveloped virus to gain entry into host cells.

IMPORTANCE

Cell entry, a critical stage in the virus life cycle, concludes with the delivery of the viral genetic material across host membranes. Regulated structural transitions within nonenveloped and enveloped viruses are necessary for accomplishing this step; these conformational changes are predominantly triggered by low pH and/or interactions with host proteins. In this work, we describe a previously unknown trigger, interactions with lipid membranes, which can induce the structural rearrangements required for cell entry. This mechanism operates during entry of mammalian orthoreoviruses. We show that interactions between reovirus entry intermediates and lipid membranes devoid of host proteins promote conformational changes within the viral outer capsid that lead to membrane penetration. Thus, this work illustrates a novel strategy that nonenveloped viruses can use to gain access into cells and how viruses usurp disparate host factors to initiate infection.

Potential for Improving Potency and Specificity of Reovirus Oncolysis with Next-Generation Reovirus Variants

Viruses that specifically replicate in tumor over normal cells offer promising cancer therapies. Oncolytic viruses (OV) not only kill the tumor cells directly; they also promote anti-tumor immunotherapeutic responses. Other major advantages of OVs are that they dose-escalate in tumors and can be genetically engineered to enhance potency and specificity. Unmodified wild type reovirus is a propitious OV currently in phase I–III clinical trials. This review summarizes modifications to reovirus that may improve potency and/or specificity during oncolysis. Classical genetics approaches have revealed reovirus variants with improved adaptation towards tumors or with enhanced ability to establish specific steps of virus replication and cell killing among transformed cells. The recent emergence of a reverse genetics system for reovirus has provided novel strategies to fine-tune reovirus proteins or introduce exogenous genes that could promote oncolytic activity. Over the next decade, these findings are likely to generate better-optimized second-generation reovirus vectors and improve the efficacy of oncolytic reotherapy.

The requirement of environmental acidification for Ibaraki virus infection to host cells

The effect of environmental acidification on Ibaraki virus (IBAV) infection was tested using endosomal inhibitory chemicals and low pH treatment. Treatment of target cells with endosomal inhibitors significantly decreased the progeny virus production. IBAV outer capsid proteins, VP5 and VP2, were removed from virion when purified IBAV was exposed to low pH environment. Further experiment showed that the exposure to low pH buffer facilitated IBAV infection when the cellular endosomal pathway was impaired by bafilomycin A1. Results obtained in this study suggest that acidic environment is essential to initiate IBAV infection.

Conformational changes required for reovirus cell entry are sensitive to pH

During cell entry, reovirus particles disassemble to generate ISVPs. ISVPs undergo conformational changes to form ISVP(*) and this conversion is required for membrane penetration. In tissues where ISVP formation occurs within endosomes, ISVP-to-ISVP(*) conversion occurs at low pH. In contrast, in tissues where ISVP formation occurs extracellularly, ISVP-to-ISVP(*) transition occurs at neutral pH. Whether these two distinct pH environments influence the efficiency of cell entry is not known. In this study, we used Ouabain to lower the endosomal pH and determined its effect on reovirus infection. We found that Ouabain treatment blocks reovirus infection. In cells treated with Ouabain, virus attachment, internalization, and ISVP formation were unaffected but the efficiency of ISVP(*)s formation was diminished. Low pH also diminished the efficiency of ISVP-to-ISVP(*) conversion in vitro. Thus, the pH of the compartment where ISVP-to-ISVP(*) conversion takes place may dictate the efficiency of reovirus infection.

Reduction of virion-associated σ1 fibers on oncolytic reovirus variants promotes adaptation toward tumorigenic cells

Wild-type mammalian orthoreovirus serotype 3 Dearing (T3wt) is nonpathogenic in humans but preferentially infects and kills cancer cells in culture and demonstrates promising antitumor activity in vivo. Using forward genetics, we previously isolated two variants of reovirus, T3v1 and T3v2, with increased infectivity toward a panel of cancer cell lines and improved in vivo oncolysis in a murine melanoma model relative to that of T3wt. Our current study explored how mutations in T3v1 and T3v2 promote infectivity. Reovirions contain trimers of σ1, the reovirus cell attachment protein, at icosahedral capsid vertices. Quantitative Western blot analysis showed that purified T3v1 and T3v2 virions had ∼ 2- and 4-fold-lower levels of σ1 fiber than did T3wt virions. Importantly, using RNA interference to reduce σ1 levels during T3wt production, we were able to generate wild-type reovirus with reduced levels of σ1 per virion. As σ1 levels were reduced, virion infectivity increased by 2- to 5-fold per cell-bound particle, demonstrating a causal relationship between virion σ1 levels and the infectivity of incoming virions. During infection of tumorigenic L929 cells, T3wt, T3v1, and T3v2 uncoated the outer capsid proteins σ3 and μ1C at similar rates. However, having started with fewer σ1 molecules, a complete loss of σ1 was achieved sooner for T3v1 and T3v2. Distinct from intracellular uncoating, chymotrypsin digestion, as a mimic of natural enteric infection, resulted in more rapid σ3 and μ1C removal, unique disassembly intermediates, and a rapid loss of infectivity for T3v1 and T3v2 compared to T3wt. Optimal infectivity toward natural versus therapeutic niches may therefore require distinct reovirus structures and σ1 levels.

IMPORTANCE

Wild-type reovirus is currently in clinical trials as a potential cancer therapy. Our molecular studies on variants of reovirus with enhanced oncolytic activity in vitro and in vivo now show that distinct reovirus structures promote adaptation toward cancer cells and away from conditions that mimic natural routes of infection. Specifically, we found that reovirus particles with fewer molecules of the cell attachment protein σ1 became more infectious toward transformed cells. Reduced σ1 levels conferred a benefit to incoming particles only, resulting in an earlier depletion of σ1 and a higher probability of establishing productive infection. Conversely, reovirus variants with fewer σ1 molecules showed reduced stability and infectivity and distinct disassembly when exposed to conditions that mimic natural intestinal proteolysis. These findings support a model where the mode of infection dictates the precise optimum of reovirus structure and provide a molecular rationale for considering alternative reovirus structures during oncolytic therapy.

Efficient norovirus and reovirus replication in the mouse intestine requires microfold (M) cells

Microfold (M) cells are specialized intestinal epithelial cells that internalize particulate antigens and aid in the establishment of immune responses to enteric pathogens. M cells have also been suggested as a portal for pathogen entry into the host. While virus particles have been observed in M cells, it is not known whether viruses use M cells to initiate a productive infection. Noroviruses (NoVs) are single-stranded RNA viruses that infect host organisms via the fecal-oral route. Murine NoV (MNV) infects intestinal macrophages and dendritic cells and provides a tractable experimental system for understanding how an enteric virus overcomes the intestinal epithelial barrier to infect underlying target cells. We found that replication of two divergent MNV strains was reduced in mice depleted of M cells. Reoviruses are double-stranded RNA viruses that infect hosts via respiratory or enteric routes. In contrast to MNV, reovirus infects enterocytes in the intestine. Despite differences in cell tropism, reovirus infection was also reduced in M cell-depleted mice. These data demonstrate that M cells are required for the pathogenesis of two unrelated enteric viruses that replicate in different cell types within the intestine.

IMPORTANCE

To successfully infect their hosts, pathogens that infect via the gastrointestinal tract must overcome the multilayered system of host defenses. Microfold (M) cells are specialized intestinal epithelial cells that internalize particulate antigens and aid in the establishment of immune responses to enteric pathogens. Virus particles have been observed within M cells. However, it is not known whether viruses use M cells to initiate a productive infection. To address this question, we use MNV and reovirus, two enteric viruses that replicate in different cell types in the intestine, intestinal epithelial cells for reovirus and intestinal mononuclear phagocytes for MNV. Interestingly, MNV- and reovirus-infected mice depleted of M cells showed reduced viral loads in the intestine. Thus, our work demonstrates the importance of M cells in the pathogenesis of enteric viruses irrespective of the target cell type in which the virus replicates.

Mechanisms of reovirus bloodstream dissemination

Many viruses cause disease within an infected host after spread from an initial portal of entry to sites of secondary replication. Viruses can disseminate via the bloodstream or through nerves. Mammalian orthoreoviruses (reoviruses) are neurotropic viruses that use both bloodborne and neural pathways to spread systemically within their hosts to cause disease. Using a robust mouse model and a dynamic reverse genetics system, we have identified a viral receptor and a viral nonstructural protein that are essential for hematogenous reovirus dissemination. Junctional adhesion molecule-A (JAM-A) is a member of the immunoglobulin superfamily expressed in tight junctions and on hematopoietic cells that serves as a receptor for all reovirus serotypes. Expression of JAM-A is required for infection of endothelial cells and development of viremia in mice, suggesting that release of virus into the bloodstream from infected endothelial cells requires JAM-A. Nonstructural protein σ1s is implicated in cell cycle arrest and apoptosis in reovirus-infected cells but is completely dispensable for reovirus replication in cultured cells. Surprisingly, a recombinant σ1s-null reovirus strain fails to spread hematogenously in infected mice, suggesting that σ1s facilitates apoptosis of reovirus-infected intestinal epithelial cells. It is possible that apoptotic bodies formed as a consequence of σ1s expression lead to reovirus uptake by dendritic cells for subsequent delivery to the mesenteric lymph node and the blood. Thus, both host and viral factors are required for efficient hematogenous dissemination of reovirus. Understanding mechanisms of reovirus bloodborne spread may shed light on how microbial pathogens invade the bloodstream to disseminate and cause disease in infected hosts.

The μ1 72-96 loop controls conformational transitions during reovirus cell entry

The reovirus outer capsid protein μ1 forms a lattice surrounding the viral core. In the native state, μ1 determines the environmental stability of the viral capsid. Additionally, during cell entry, μ1 undergoes structural rearrangements that facilitate delivery of the viral cores across the membrane. To determine how the capsid-stabilizing functions of μ1 impinge on the capacity of μ1 to undergo conformational changes required for cell entry, we characterized viruses with mutations engineered at charged residues within the μ1 loop formed by residues 72 to 96 (72-96 loop). This loop is proposed to stabilize the capsid by mediating interactions between neighboring μ1 trimers and between trimers and the core. We found that mutations at Glu89 (E89) within this loop produced viruses with compromised efficiency for completing their replication cycle. ISVPs of E89 mutants converted to ISVP*s more readily than those of wild-type viruses. The E89 mutants yielded revertants with second-site substitutions within regions that mediate interaction between μ1 trimers at a site distinct from the 72-96 loop. These viruses also contained changes in regions that control interactions within μ1 trimers. Viruses containing these second-site changes displayed restored plaque phenotypes and were capable of undergoing ISVP-to-ISVP* conversion in a regulated manner. These findings highlight regions of μ1 that stabilize the reovirus capsid and demonstrate that an enhanced propensity to form ISVP*s in an unregulated manner compromises viral fitness.

Similar uptake but different trafficking and escape routes of reovirus virions and infectious subvirion particles imaged in polarized Madin-Darby canine kidney cells

Polarized epithelial cells that line the digestive, respiratory, and genitourinary tracts form a barrier that many viruses must breach to infect their hosts. Current understanding of cell entry by mammalian reovirus (MRV) virions and infectious subvirion particles (ISVPs), generated from MRV virions by extracellular proteolysis in the digestive tract, are mostly derived from in vitro studies with nonpolarized cells. Recent live-cell imaging advances allow us for the first time to visualize events at the apical surface of polarized cells. In this study, we used spinning-disk confocal fluorescence microscopy with high temporal and spatial resolution to follow the uptake and trafficking dynamics of single MRV virions and ISVPs at the apical surface of live polarized Madin-Darby canine kidney cells. Both types of particles were internalized by clathrin-mediated endocytosis, but virions and ISVPs exhibited strikingly different trafficking after uptake. While virions reached early and late endosomes, ISVPs did not and instead escaped the endocytic pathway from an earlier location. This study highlights the broad advantages of using live-cell imaging combined with single-particle tracking for identifying key steps in cell entry by viruses.

Transient high level mammalian reovirus replication in a bat epithelial cell line occurs without cytopathic effect

Mammalian reoviruses exhibit a large host range and infected cells are generally killed; however, most studies examined only a few cell types and host species, and are probably not representative of all possible interactions between virus and host cell. Many questions thus remain concerning the nature of cellular factors that affect viral replication and cell death. In the present work, it was observed that replication of the classical mammalian reovirus serotype 3 Dearing in a bat epithelial cell line, Tb1.Lu, does not result in cell lysis and is rapidly reduced to very low levels. Prior uncoating of virions by chymotrypsin treatment, to generate infectious subviral particles, increased the initial level of infection but without any significant effect on further viral replication or cell survival. Infected cells remain resistant to virus reinfection and secrete an antiviral factor, most likely interferon, that is protective against the unrelated encephalomyocarditis virus. Although, the transformed status of a cell is believed to promote reovirus replication and viral "oncolysis", resistant Tb1.Lu cells exhibit a classical phenotype of transformed cells by forming colonies in semisolid soft agar medium. Further transduction of Tb.Lu cells with a constitutively active Ras oncogene does not seem to affect cell growth or reovirus effect on these cells. Infected Tb1.Lu cells can produce low-level of infectious virus for a long time without any apparent effect, although these cells are resistant to reinfection. The results suggest that Tb1.Lu cells can mount an unusual antiviral response. Specific properties of bat cells may thus be in part responsible for the ability of the animals to act as reservoirs for viruses in general and for novel reoviruses in particular. Their peculiar resistance to cell lysis also makes Tb1.Lu cells an attractive model to study the cellular and viral factors that determine the ability of reovirus to replicate and destroy infected cells.

Reovirus uses multiple endocytic pathways for cell entry

Entry of reovirus virions has been well studied in several tissue culture systems. After attachment to junctional adhesion molecule A (JAM-A), virions undergo clathrin-mediated endocytosis followed by proteolytic disassembly of the capsid and penetration to the cytoplasm. However, during in vivo infection of the intestinal tract, and likely in the tumor microenvironment, capsid proteolysis (uncoating) is initiated extracellularly. We used multiple approaches to determine if uncoated reovirus particles, called intermediate subviral particles (ISVPs), enter cells by directly penetrating the limiting membrane or if they take advantage of endocytic pathways to establish productive infection. We found that entry and infection by reovirus ISVPs was inhibited by dynasore, an inhibitor of dynamin-dependent endocytosis, as well as by genistein and dominant-negative caveolin-1, which block caveolar endocytosis. Inhibition of caveolar endocytosis also reduced infection by reovirus virions. Extraction of membrane cholesterol with methyl-β-cyclodextrin inhibited infection by virions but had no effect when infection was initiated with ISVPs. We found this pathway to be independent of both clathrin and caveolin. Together, these data suggest that reovirus virions can use both dynamin-dependent and dynamin-independent endocytic pathways during cell entry, and they reveal that reovirus ISVPs can take advantage of caveolar endocytosis to establish productive infection.

Cell entry-associated conformational changes in reovirus particles are controlled by host protease activity

Membrane penetration by reovirus requires successive formation of two cell entry intermediates, infectious subvirion particles (ISVPs) and ISVP*s. In vitro incubation of reovirus virions with high concentration of chymotrypsin (CHT) results in partial digestion of the viral outer capsid to form ISVPs. When virions are instead digested with low concentrations of chymotrypsin, the outer capsid is completely proteolyzed to form cores. We investigated the basis for the inverse relationship between CHT activity and protease susceptibility of the reovirus outer capsid. We report that core formation following low-concentration CHT digestion proceeds via formation of particles that contain a protease-sensitive form of the μ1C protein, a characteristic of ISVP*s. In addition, we found that both biochemical features and viral genetic requirements for ISVP* formation and core formation following low-concentration CHT digestion are identical, suggesting that core formation proceeds via a particle resembling ISVP*s. Furthermore, we determined that intermediates generated following low-concentration CHT digestion are distinct from ISVPs and convert to ISVP*-like particles much more readily than ISVPs. These results suggest that the activity of host proteases used to generate ISVPs can influence the efficiency with which the next step in reovirus cell entry, namely, ISVP-to-ISVP* conversion, occurs.

Impact of host proteases on reovirus infection in the respiratory tract

Virion uncoating is an essential early event in reovirus infection. In natural enteric infections, rapid proteolytic uncoating of virions is mediated by pancreatic serine proteases. The proteases that promote reovirus disassembly and cell entry in the respiratory tract remain unknown. In this report, we show that endogenous respiratory and inflammatory proteases can promote reovirus infection in vitro and that preexisting inflammation augments in vivo infection in the murine respiratory tract.

Gut proteases target Yersinia invasin in vivo

BACKGROUND

Yersinia enterocolitica is a common cause of food borne gastrointestinal disease. After oral uptake, yersiniae invade Peyer's patches of the distal ileum. This is accomplished by the binding of the Yersinia invasin to β1 integrins on the apical surface of M cells which overlie follicle associated lymphoid tissue. The gut represents a barrier that severely limits yersiniae from reaching deeper tissues such as Peyer's patches. We wondered if gut protease attack on invasion factors could contribute to the low number of yersiniae invading Peyer's patches.

FINDINGS

Here we show that invasin is rapidly degraded in vivo by gut proteases in the mouse infection model. In vivo proteolytic degradation is due to proteolysis by several gut proteases such as trypsin, α-chymotrypsin, pancreatic elastase, and pepsin. Protease treated yersiniae are shown to be less invasive in a cell culture model. YadA, another surface adhesin is cleaved by similar concentrations of gut proteases but Myf was not cleaved, showing that not all surface proteins are equally susceptible to degradation by gut proteases.

CONCLUSIONS

We demonstrate that gut proteases target important Yersinia virulence factors such as invasin and YadA in vivo. Since invasin is completely degraded within 2-3 h after reaching the small intestine of mice, it is no longer available to mediate invasion of Peyer's patches.

From touchdown to transcription: the reovirus cell entry pathway

Mammalian orthoreoviruses (reoviruses) are prototype members of the Reoviridae family of nonenveloped viruses. Reoviruses contain ten double-stranded RNA gene segments enclosed in two concentric protein shells, outer capsid and core. These viruses serve as a versatile experimental system for studies of virus cell entry, innate immunity, and organ-specific disease. Reoviruses engage cells by binding to cell-surface carbohydrates and the immunoglobulin superfamily member, junctional adhesion molecule-A (JAM-A). JAM-A is a homodimer formed by extensive contacts between its N-terminal immunoglobulin-like domains. Reovirus attachment protein σ1 disrupts the JAM-A dimer, engaging a single JAM-A molecule by virtually the same interface used for JAM-A homodimerization. Following attachment to JAM-A and carbohydrate, reovirus internalization is promoted by β1 integrins, most likely via clathrin-dependent endocytosis. In the endocytic compartment, reovirus outer-capsid protein σ3 is removed by cathepsin proteases, which exposes the viral membrane-penetration protein, μ1. Proteolytic processing and conformational rearrangements of μ1 mediate endosomal membrane rupture and delivery of transcriptionally active reovirus core particles into the host cell cytoplasm. These events also allow the φ cleavage fragment of μ1 to escape into the cytoplasm where it activates NF-κB and elicits apoptosis. This review will focus on mechanisms of reovirus cell entry and activation of innate immune response signaling pathways.

Innate host barriers to viral trafficking and population diversity: lessons learned from poliovirus

Poliovirus is an error-prone enteric virus spread by the fecal-oral route and rarely invades the central nervous system (CNS). However, in the rare instances when poliovirus invades the CNS, the resulting damage to motor neurons is striking and often permanent. In the prevaccine era, it is likely that most individuals within an epidemic community were infected; however, only 0.5% of infected individuals developed paralytic poliomyelitis. Paralytic poliomyelitis terrified the public and initiated a huge research effort, which was rewarded with two outstanding vaccines. During research to develop the vaccines, many questions were asked: Why did certain people develop paralysis? How does the virus move from the gut to the CNS? What limits viral trafficking to the CNS in the vast majority of infected individuals? Despite over 100 years of poliovirus research, many of these questions remain unanswered. The goal of this chapter is to review our knowledge of how poliovirus moves within and between hosts, how host barriers limit viral movement, how viral population dynamics impact viral fitness and virulence, and to offer hypotheses to explain the rare incidence of paralytic poliovirus disease.

Murine noroviruses comprising a single genogroup exhibit biological diversity despite limited sequence divergence

Viruses within the genus Norovirus of the family Caliciviridae are the major cause of acute, nonbacterial gastroenteritis worldwide. Human noroviruses are genetically diverse, with up to 57% divergence in capsid protein sequences, and comprise three genogroups. The significance of such genetic diversity is not yet understood. The discovery of murine norovirus (MNV) and its ability to productively infect cultured murine macrophages and dendritic cells has provided an opportunity to determine the functional consequences of norovirus diversity in vitro and in vivo. Therefore, we compared the full-length genomes of 21 new MNV isolates with five previously sequenced MNV genomes and demonstrated a conserved genomic organization consisting of four open reading frames (ORFs) and a previously unknown region of nucleotide conservation in ORF2. A phylogenetic analysis of all 26 MNV genomes revealed 15 distinct MNV strains, with up to 13% divergence at the nucleotide level, that comprise a single genotype and genogroup. Evidence for recombination within ORF2 in several MNV genomes was detected by multiple methods. Serological analyses comparing neutralizing antibody responses between highly divergent strains suggested that the MNV genogroup comprises a single serotype. Within this single genogroup, MNV strains exhibited considerable biological diversity in their ability to grow in culture and to infect and/or persist in wild-type mice. The isolation and characterization of multiple MNV strains illustrate how genetic analysis may underestimate the biological diversity of noroviruses and provide a molecular map for future studies of MNV biology.

Proteolytic disassembly is a critical determinant for reovirus oncolysis

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Mammalian ortheoreoviruses are currently being investigated as novel cancer therapeutics, but the cellular mechanisms that regulate susceptibility to reovirus oncolysis remain poorly understood. In this study, we present evidence that virion disassembly is a key determinant of reovirus oncolysis. To penetrate cell membranes and initiate infection, the outermost capsid proteins of reovirus must be proteolyzed to generate a disassembled particle called an infectious subviral particle (ISVP). In fibroblasts, this process is mediated by the endo/lysosomal proteases cathepsins B and L. We have analyzed the early events of infection in reovirus-susceptible and -resistant cells. We find that, in contrast to susceptible glioma cells and Ras-transformed NIH3T3 cells, reovirus-resistant cancer cells and untransformed NIH3T3 cells restrict virion uncoating and subsequent gene expression. Disassembly-restrictive cells support reovirus infection, as in vitro-generated ISVPs establish productive infection, and pretreatment with poly(I:C) does not prevent infection in cancer cells. We find that the level of active cathepsin B and L is increased in tumors and that disassembly-restrictive glioma cells support reovirus oncolysis when grown as a tumor in vivo. Together, these results provide a model in which proteolytic disassembly of reovirus is a critical determinant of susceptibility to reovirus oncolysis.

Attachment and cell entry of mammalian orthoreovirus

Mammalian orthoreoviruses (reoviruses) serve as a tractable model system for studies of viral pathogenesis. Reoviruses infect virtually all mammals, but cause disease only in the very young. Prototype strains of the three reovirus serotypes differ in pathogenesis following infection of newborn mice. Reoviruses are nonenveloped, icosahedral particles that consist of ten segments of double-stranded RNA encapsidated within two protein shells, the inner core and outer capsid. High-resolution structures of individual components of the reovirus outer capsid and a single viral receptor have been solved and provide insight into the functions of these molecules in viral attachment, entry, and pathogenesis. Attachment of reovirus to target cells is mediated by the reovirus sigma1 protein, a filamentous trimer that projects from the outer capsid. Junctional adhesion molecule-A is a serotype-independent receptor for reovirus, and sialic acid is a coreceptor for serotype 3 strains. After binding to receptors on the cell surface, reovirus is internalized via receptor-mediated endocytosis. Internalization is followed by stepwise disassembly of the viral outer capsid in the endocytic compartment. Uncoating events, which require acidic pH and endocytic proteases, lead to removal of major outer-capsid protein sigma3, resulting in exposure of membrane-penetration mediator micro1 and a conformational change in attachment protein sigma1. After penetration of endosomes by uncoated particles, the transcriptionally active viral core is released into the cytoplasm, where replication proceeds. Despite major advances in defining reovirus attachment and entry mechanisms, many questions remain. Ongoing research is aimed at understanding serotype-dependent differences in reovirus tropism, viral cell-entry pathways, the individual and corporate roles of acidic pH and proteases in viral entry, and micro1 function in membrane penetration.

The oncolytic effect in vivo of reovirus on tumour cells that have survived reovirus cell killing in vitro

The use of oncolytic viruses has received considerable attention in recent years and many viruses have proved to be effective against a variety of cancer models and a few are currently being used in clinical trials. However, the possible emergence and outcome of virus-resistant tumour cells has not been addressed. We previously reported the effective use of reovirus against lymphoid malignancies, including the Burkitt's lymphoma cell line Raji. Here we isolated in vitro persistently infected (PI) Raji cells, and cells ‘cured’ of persistent reovirus infection (‘cured’ cells). Both PI and cured Raji cells resisted reovirus infection and cell killing in vitro. In vivo, the PI cells were non-tumorigenic in SCID mice, but cured cells regained the parental cells' ability to form tumours. Tumour xenografts from the cured cells, however, were highly susceptible to reovirus oncolysis in vivo. This susceptibility was due to the proteolytic environment within tumours that facilitates reovirus infection and cell killing. Our results show that persistent infection by reovirus impedes tumour development and that although PI cells cleared of reovirus are tumorigenic, they are killed upon rechallenge with reovirus. Both the PI and cured states are therefore not likely to be significant barriers to reovirus oncolytic therapy.

Neutrophil elastase, an acid-independent serine protease, facilitates reovirus uncoating and infection in U937 promonocyte cells

Background

Mammalian reoviruses naturally infect their hosts through the enteric and respiratory tracts. During enteric infections, proteolysis of the reovirus outer capsid protein σ3 is mediated by pancreatic serine proteases. In contrast, the proteases critical for reovirus replication in the lung are unknown. Neutrophil elastase (NE) is an acid-independent, inflammatory serine protease predominantly expressed by neutrophils. In addition to its normal role in microbial defense, aberrant expression of NE has been implicated in the pathology of acute respiratory distress syndrome (ARDS). Because reovirus replication in rodent lungs causes ARDS-like symptoms and induces an infiltration of neutrophils, we investigated the capacity of NE to promote reovirus virion uncoating.

Results

The human promonocyte cell line U937 expresses NE. Treatment of U937 cells with the broad-spectrum cysteine-protease inhibitor E64 [trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane] and with agents that increase vesicular pH did not inhibit reovirus replication. Even when these inhibitors were used in combination, reovirus replicated to significant yields, indicating that an acid-independent non-cysteine protease was capable of mediating reovirus uncoating in U937 cell cultures. To identify the protease(s) responsible, U937 cells were treated with phorbol 12-myristate 13-acetate (PMA), an agent that induces cellular differentiation and results in decreased expression of acid-independent serine proteases, including NE and cathepsin (Cat) G. In the presence of E64, reovirus did not replicate efficiently in PMA-treated cells. To directly assess the role of NE in reovirus infection of U937 cells, we examined viral growth in the presence of N-Ala-Ala-Pro-Val chloromethylketone, a NE-specific inhibitor. Reovirus replication in the presence of E64 was significantly reduced by treatment of cells with the NE inhibitor. Incubation of virions with purified NE resulted in the generation of infectious subviron particles that did not require additional intracellular proteolysis.

Conclusion

Our findings reveal that NE can facilitate reovirus infection. The fact that it does so in the presence of agents that raise vesicular pH supports a model in which the requirement for acidic pH during infection reflects the conditions required for optimal protease activity. The capacity of reovirus to exploit NE may impact viral replication in the lung and other tissues during natural infections.

Involvement of Dendritic Cell Subsets in the Induction of Oral Tolerance and Immunity

Dendritic cells (DCs) play a central role in the generation of immune responses in the intestine. DCs induce differentiation and tolerance of T cells, and may have a direct role in B cell switching to IgA. Four distinct subsets of CD11c(+) DCs are present in murine Peyer's patches, which represent primary sites for the induction of mucosal T and B cell responses. Studies suggest that CD11b(+) DCs or plasmacytoid DCs may be specialized for the induction of regulatory T cells, and CD8alpha(+) DCs for the induction of clonal deletion in response to soluble oral antigen, while all DC subsets (including CD8alpha(-)/CD11b(-) DCs) may be involved in responses to pathogens. We are currently using reovirus type-1 Lang (TIL) to explore the role of DC populations in mucosal immunity in vivo, as oral administration of live T1L to mice induces strong mucosal and systemic antiviral immune responses, whereas oral administration of inactivated T1L results in tolerance to viral proteins. We found that primary infection with T1L occurs in epithelial cells of the PP follicle-associated epithelium, but that CD8alpha(-)/CD11b(-) DCs in the subepithelial dome region (SED) are loaded with T1L antigens in the absence of active DC infection. At least a portion of this antigen is associated with cell fragments from apoptotic epithelial cells, demonstrating that SED DCs cross-present antigens from apoptotic epithelial cells. In vitro, in contrast to exposure to several TLR-ligands or anti-CD40, exposure to T1L does not activate DCs to mature or to produce cytokines, despite clear loading of the DCs with viral antigens. These data suggest that T1L is taken up by a "silent" receptor on DCs, and that the induction of immunity to T1L is dependent on signals from non-DCs following active viral infection that induce DC maturation. Thus, the decision between tolerance and immunity to inactive and live virus, respectively, likely depends on whether there is active infection of epithelial cells by T1L, which results in the elaboration of molecules, such as cytokines, that induce DC maturation.

Peyer's patch dendritic cells process viral antigen from apoptotic epithelial cells in the intestine of reovirus-infected mice

We explored the role of Peyer's patch (PP) dendritic cell (DC) populations in the induction of immune responses to reovirus strain type 1 Lang (T1L). Immunofluorescence staining revealed the presence of T1L structural (σ1) and nonstructural (σNS) proteins in PPs of T1L-infected mice. Cells in the follicle-associated epithelium contained both σ1 and σNS, indicating productive viral replication. In contrast, σ1, but not σNS, was detected in the subepithelial dome (SED) in association with CD11c⁺/CD8α⁻/CD11b^lo DCs, suggesting antigen uptake by these DCs in the absence of infection. Consistent with this possibility, PP DCs purified from infected mice contained σ1, but not σNS, and PP DCs from uninfected mice could not be productively infected in vitro. Furthermore, σ1 protein in the SED was associated with fragmented DNA by terminal deoxy-UTP nick-end labeling staining, activated caspase-3, and the epithelial cell protein cytokeratin, suggesting that DCs capture T1L antigen from infected apoptotic epithelial cells. Finally, PP DCs from infected mice activated T1L-primed CD4⁺ T cells in vitro. These studies show that CD8α⁻/CD11b^lo DCs in the PP SED process T1L antigen from infected apoptotic epithelial cells for presentation to CD4⁺ T cells, and therefore demonstrate the cross-presentation of virally infected cells by DCs in vivo during a natural viral infection.

Norovirus capture with histo-blood group antigens reveals novel virus-ligand interactions

Noroviruses are genetically diverse, uncultivable, positive-sense RNA viruses and are the most common cause of epidemic acute gastroenteritis in humans in the United States. Recent studies of norovirus attachment in vitro by using recombinant virus-like particles (VLPs) suggest that various norovirus strains exhibit different patterns of attachment to ABH histo-blood group antigens, which are carbohydrate epitopes present in high concentrations on mucosal cell surfaces of the gut. However, attachment of live norovirus strains to histo-blood group antigens has not been investigated to date. Utilizing a newly designed magnetic bead-virus capture method, we characterized histo-blood group antigen attachment properties of various norovirus strains obtained from clinical stool specimens to compare the attachment properties of wild-type virus and VLPs and to further map norovirus attachment. Consistent with previous reports using VLPs, various strains of noroviruses exhibited different patterns of attachment to histo- blood group antigens. Norwalk virus bound specifically to H type 1, H type 3, and Le(b). Two genogroup II noroviruses, one representing the Toronto genotype and the other from a novel genotype, bound specifically to Le(b). A Desert Shield-like strain did not attach to H types 1, 2, or 3, H type 1 and 3 precursors, Le(a), or Le(b). Surprisingly, wild-type Snow Mountain virus (SMV) attached specifically to H type 3, which contradicted previous findings with SMV VLPs. On further investigation, we found that stool components promote this attachment, providing the first known observation that one or more components of human feces could promote and enhance norovirus attachment to histo-blood group antigens.

Molecular mechanisms of virus spread and virion components as tools of virulence. A review

Despite of differences in replication strategy among virus families, some basic principles have remained similar. Analogous mechanisms govern virus entry into cells and the use of enzymes which direct the replication of the virus genome. The function of many cell surface receptors (such as glycosoaminoglycans, glycoproteins, proteins) which interact with viral capsid proteins or envelope glycoproteins has recently been elucidated. The list of cellular receptors (Table I) is still far from being final. The capsid components, similarly as the envelope glycoproteins, may form specific pocket like sites, which interact with the cell surface receptors. Neutralizing antibodies usually react with antigenic domains adjacent to the receptor binding site(s) and hamper the close contact inevitable for virion attachment. In the case of more complex viruses, such as herpes simplex virus, different viral glycoproteins interact with several cellular receptors. At progressed phase of adsorption the virions are engulfed into endocytic vesicles and the virion fusion domain(s) become(s) activated. The outer capsid components of reoviruses which participate in adsorption and fusion may get activated already in the lumen of digestive tract, i.e. before their engulfment by resorptive epithelium cells. Activation of the hydrophobic fusion domain(s) is a further important step allowing to pass through the lipid bilayer when penetrating the cell membrane in order to reach the cytosol. Activation of the virion fusion domain is accomplished by a conformation change, which occurs at acid pH (influenza virus hemagglutinin, sigma 1 protein of the reovirus particle) and/or after protease treatment. The herpes simplex virus fusion factors (gD and gH) undergo conformation changes by a pH-independent mechanism triggered due to interaction with the cell surface receptor(s) and mediated by mutual interactions with the viral envelope glycoproteins. The virion capsid or envelope components participating in the entry and membrane fusion are not the only tools of virulence. The correct function of virus coded proteins, which participate in replication of the viral genome, and/or in the supply of necessary nucleotides, may be very essential. In the case of enteroviruses, which RNA interacts with ribosomes directly, the correct configuration of the non-coding viral RNA sequence is crucial for initiation of translation occurring in the absence of the classical "cap" structure.

The delta region of outer-capsid protein micro 1 undergoes conformational change and release from reovirus particles during cell entry

Cell entry by reoviruses requires a large, transcriptionally active subvirion particle to gain access to the cytoplasm. The features of this particle have been the subject of debate, but three primary candidates-the infectious subvirion particle (ISVP), ISVP*, and core particle forms-that differ in whether putative membrane penetration protein micro 1 and adhesin sigma1 remain particle bound have been identified. Experiments with antibody reagents in this study yielded new information about the steps in particle disassembly during cell entry. Monoclonal antibodies specific for the delta region of micro 1 provided evidence for a conformational change in micro 1 and for release of the delta proteolytic fragment from entering particles. Antiserum raised against cores provided evidence for entry-related changes in particle structure and identified entering particles that largely lack the delta fragment inside cells. Antibodies specific for sigma1 showed that it is also largely shed from entering particles. Limited coimmunostaining with markers for late endosomes and lysosomes indicated the particles lacking delta and sigma1 did not localize to those subcellular compartments, and other observations suggested that both the particles and free delta were released into the cytoplasm. Essentially equivalent findings were obtained with native ISVPs and highly infectious recoated particles containing wild-type proteins. Poorly infectious recoated particles containing a hyperstable mutant form of micro 1, however, showed no evidence for the in vitro and intracellular changes in particle structure normally detected by antibodies, and these particles instead accumulated in late endosomes or lysosomes. Recoated particles with hyperstable micro 1 were also ineffective at mediating erythrocyte lysis in vitro and promoting alpha-sarcin coentry and intoxication of cells in cultures. Based on these and other findings, we propose that ISVP* is a transient intermediate in cell entry which mediates membrane penetration and is then further uncoated in the cytoplasm to yield particles, resembling cores, that largely lack the delta fragment of micro 1.

Secretory immunoglobulin A antibodies against the sigma1 outer capsid protein of reovirus type 1 Lang prevent infection of mouse Peyer's patches

Reovirus type 1 Lang (T1L) adheres to M cells in the follicle-associated epithelium of mouse intestine and exploits the transport activity of M cells to enter and infect the Peyer's patch mucosa. Adult mice that have previously cleared a reovirus T1L infection have virus-specific immunoglobulin G (IgG) in serum and IgA in secretions and are protected against reinfection. Our aim in this study was to determine whether secretory IgA is sufficient for protection of Peyer's patches against oral reovirus challenge and, if so, against which reovirus antigen(s) the IgA may be directed. Monoclonal antibodies (MAbs) of the IgA isotype, directed against the sigma1 protein of reovirus T1L, the viral adhesin, were produced and tested along with other, existing IgA and IgG MAbs against reovirus T1L outer capsid proteins. Anti-sigma1 IgA and IgG MAbs neutralized reovirus T1L in L cell plaque reduction assays and inhibited T1L adherence to L cells and Caco-2(BBe) intestinal epithelial cells in vitro, but MAbs against other proteins did not. Passive oral administration of anti-sigma1 IgA and IgG MAbs prevented Peyer's patch infection in adult mice, but other MAbs did not. When anti-sigma1 IgA and IgG MAbs were produced in mice from hybridoma backpack tumors, however, the IgA prevented Peyer's patch infection, but the IgG did not. The results provide evidence that neutralizing IgA antibodies specific for the sigma1 protein are protective in vitro and in vivo and that the presence of these antibodies in intestinal secretions is sufficient for protection against entry of reovirus T1L into Peyer's patches.

Cathepsin S supports acid-independent infection by some reoviruses

In murine fibroblasts, efficient proteolysis of reovirus outer capsid protein sigma3 during cell entry by virions requires the acid-dependent lysosomal cysteine protease cathepsin L. The importance of cathepsin L for infection of other cell types is unknown. Here we report that the acid-independent lysosomal cysteine protease cathepsin S mediates outer capsid processing in macrophage-like P388D cells. P388D cells supported infection by virions of strain Lang, but not strain c43. Genetic studies revealed that this difference is determined by S4, the viral gene segment that encodes sigma3. c43-derived subvirion particles that lack sigma3 replicated normally in P388D cells, suggesting that the difference in infectivity of Lang and c43 virions is at the level of sigma3 processing. Infection of P388D cells with Lang virions was inhibited by the broad spectrum cysteine protease inhibitor trans-epoxysuccinyl-l-leucylamido-(4-guanidino)butane but not by NH(4)Cl, which raises the endocytic pH and thereby inhibits acid-dependent proteases such as cathepsins L and B. Outer capsid processing and infection of P388D cells with Lang virions were also inhibited by a cathepsin S-specific inhibitor. Furthermore, in the presence of NH(4)Cl, cell lines engineered to express cathepsin S supported infection by Lang, but not c43, virions. Our results thus indicate that differences in susceptibility to cathepsin S-mediated sigma3 processing are responsible for strain differences in reovirus infection of macrophage-like P388D cells and other cathepsin S-expressing cells. Additionally, our data suggest that the acid dependence of reovirus infections of most other cell types may reflect the low pH requirement for the activities of most other lysosomal proteases rather, than some other acid-dependent aspect of cell entry.

The viral sigma1 protein and glycoconjugates containing alpha2-3-linked sialic acid are involved in type 1 reovirus adherence to M cell apical surfaces

Type 1 reoviruses invade the intestinal mucosa of mice by adhering selectively to M cells in the follicle-associated epithelium and then exploiting M cell transport activity. The purpose of this study was to identify the apical cell membrane component and viral protein that mediate the M cell adherence of these viruses. Virions and infectious subviral particles of reovirus type 1 Lang (T1L) adhered to rabbit M cells in Peyer's patch mucosal explants and to tissue sections in an overlay assay. Viral adherence was abolished by pretreatment of sections with periodate and in the presence of excess sialic acid or lectins MAL-I and MAL-II (which recognize complex oligosaccharides containing sialic acid linked alpha2-3 to galactose). The binding of T1L particles to polarized human intestinal (Caco-2(BBe)) cell monolayers was correlated with the presence of MAL-I and MAL-II binding sites, blocked by excess MAL-I and -II, and abolished by neuraminidase treatment. Other type 1 reovirus isolates exhibited MAL-II-sensitive binding to rabbit M cells and polarized Caco-2(BBe) cells, but type 2 or type 3 isolates including type 3 Dearing (T3D) did not. In assays using T1L-T3D reassortants and recoated viral cores containing T1L, T3D, or no sigma1 protein, MAL-II-sensitive binding to rabbit M cells and polarized Caco-2(BBe) cells was consistently associated with the T1L sigma1. MAL-II-recognized oligosaccharide epitopes are not restricted to M cells in vivo, but MAL-II immobilized on virus-sized microparticles bound only to the follicle-associated epithelium and M cells. The results suggest that selective binding of type 1 reoviruses to M cells in vivo involves interaction of the type 1 sigma1 protein with glycoconjugates containing alpha2-3-linked sialic acid that are accessible to viral particles only on M cell apical surfaces.

Addition of exogenous protease facilitates reovirus infection in many restrictive cells

Virion uncoating is a critical step in the life cycle of mammalian orthoreoviruses. In cell culture, and probably in extraintestinal tissues in vivo, reovirus virions undergo partial proteolysis within endosomal or/or lysosomal compartments. This process converts the virion into a form referred to as an intermediate subvirion particle (ISVP). In natural enteric reovirus infections, proteolytic uncoating takes place extracellularly within the intestinal lumen. The resultant proteolyzed particles, unlike intact virions, have the capacity to penetrate cell membranes and thereby gain access to cytoplasmic components required for viral gene expression. We hypothesized that the capacity of reovirus outer capsid proteins to be proteolyzed is a determinant of cellular host range. To investigate this hypothesis, we asked if the addition of protease to cell culture medium would expand the range of cultured mammalian cell lines that can be productively infected by reoviruses. We identified many transformed and nontransformed cell lines, as well as primary cells, that restrict viral infection. In several of these restrictive cells, virion uncoating is inefficient or blocked. Addition of proteases to the cell culture medium generates ISVP-like particles and promotes viral growth in nearly all cell lines tested. Interestingly, we found that some cell lines that restrict reovirus uncoating still express mature cathepsin L, a lysosomal protease required for virion disassembly in murine L929 cells. This finding suggests that factors in addition to cathepsin L are required for efficient intracellular proteolysis of reovirus virions. Our results demonstrate that virion uncoating is a critical determinant of reovirus cellular host range and that many cells which otherwise support productive reovirus infection cannot efficiently mediate this essential early step in the virus life cycle.