Influenza virus transcription occurs in the nuclei of infected cells, where the viral genomic RNAs are complexed with a nucleoprotein (NP) to form ribonucleoprotein (RNP) structures. Prior to assembly into progeny virions, these RNPs exit the nucleus and accumulate in the cytoplasm. The mechanisms responsible for RNP export are only partially understood but have been proposed to involve the viral M1 and NS2 polypeptides. We found that the drug leptomycin B (LMB), which specifically inactivates the cellular CRM1 polypeptide, caused nuclear retention of NP in virus-infected cells, indicating a role for the CRM1 nuclear export pathway in RNP egress. However, no alteration was seen in the cellular distribution of M1 or NS2, even in the case of a mutant virus which synthesizes greatly reduced amounts of NS2. Furthermore, NP was distributed throughout the nuclei of infected cells at early times postinfection but, when retained in the nucleus at late times by LMB treatment, was redistributed to the periphery of the nucleoplasm. No such change was seen in the nuclear distribution of M1 or NS2 after drug treatment. Similar to the behavior of NP, M1 and NS2 in infected cells, LMB treatment of cells expressing each polypeptide in isolation caused nuclear retention of NP but not M1 or NS2. Conversely, overexpression of CRM1 caused increased cytoplasmic accumulation of NP but had little effect on M1 or NS2 distribution. Consistent with this, NP bound CRM1 in vitro. Overall, these data raise the possibility that RNP export is mediated by a direct interaction between NP and the cellular CRM1 export pathway.The influenza virus genome consists of eight segments of single-stranded RNA that encode a total of 10 identified polypeptides. The genomic RNA segments are of negative sense and are always found in association with viral polypeptides: the three subunits of an RNA-dependent RNA polymerase (PB1, PB2, and PA) and, in stoichiometric quantities, a single-strand RNA-binding nucleoprotein (NP) (28). In virions, these ribonucleoprotein (RNP) structures are packaged within a shell of the viral M1 polypeptide underlying the lipid bilayer, along with the hemagglutinin (HA) and neuraminidase integral membrane glycoproteins. Minor virion components include M2, a small transmembrane ion channel, and the NS2 polypeptide (28). Influenza virus particles enter the cell by receptor-mediated endocytosis. Following acidification of the endosome, the M1 polypeptide dissociates from the RNP segments and virion RNPs (vRNPs) are released into the cytoplasm (30, 31). Unusually for a virus with no DNA coding stage, influenza virus transcription occurs in the nucleus (20,22). Accordingly, after release of the RNPs into the cytoplasm, they migrate into the nucleus, in an active process that is thought to be mediated by the cellular importin ␣/␤ pathway (39). Once in the nucleus, vRNPs act as the template for synthesis of mRNAs, which are exported into the cytoplasm for translation. The vRNPs also act as the template for synthesis of full-length cRNA co...

HPV late gene expression is initiated as an infected basal cell migrates through the differentiating layers of the epidermis, resulting in the onset of vegetative viral DNA replication and the expression of viral late proteins. We have used a large synthetic immunoglobulin library displayed on phage (diversity 6.5 x 10(10) phage) to isolate three Fabs (TVG405, 406, and 407) which recognize distinct epitopes on the E4 late protein of HPV16. A C-terminal monoclonal (TVG404) was generated by hybridoma technology, and N-terminal polyclonal antiserum was prepared by peptide immunization (alpha N-term). The most potent antibody (TVG405) had an affinity for E4 of approximately 1.0 nM. All antibodies recognized the protein in paraffin-embedded archival material, allowing us to map events in the late stages of virus infection. Expression of E4 in vivo does not coincide with synthesis of the major virus coat protein L1, but precedes it by 1 or 2 cell layers in premalignant lesions caused by HPV16 and by up to 20 cell layers in HPV63-induced warts. In higher grade lesions associated with HPV16, E4 is produced in the absence of L1. By contrast, vegetative viral DNA replication and E4 expression correlate exactly and in some lesions begin as the infected epithelial cell leaves the basal layer. Differentiation markers such as filaggrin, loricrin, and certain keratins are not detectable in E4-positive cells, and nuclear degeneration is delayed. HPV16 E4 has a filamentous distribution in the lower epithelial layers, but associates with solitary perinuclear structures in more differentiated cells. Antibodies to the N-terminus of the protein stained these structures poorly. Our findings are compatible with a role for the HPV16 E4 protein in vegetative DNA replication or in modifying the phenotype of the infected cell to favor virus synthesis or virus release. The Fabs will be of value in the evaluation of model systems for mimicking HPV infection in vitro.

Influenza virus genomic RNA segments are packaged into ribonucleoprotein (RNP) structures by the PB1, PB2, and PA subunits of an RNA polymerase and a single-strand RNA-binding nucleoprotein (NP). Assembly and function of these ribonucleoproteins depend on a complex set of protein-protein and protein-RNA interactions. Here, we identify new functional domains of PB2. We show that PB2 contains two regions that bind NP and also identify a novel PB1 binding site. The regions of PB2 responsible for binding NP and PB1 show considerable overlap, and binding of NP to the PB2 fragments could be outcompeted by PB1. The binding domains of PB2 acted as trans-dominant inhibitors of viral gene expression, and consistent with the in vitro binding data, their inhibitory activity depended on the concentration of wild-type PB2, NP, and PB1. This provides evidence for functionally significant and potentially regulatory interactions between PB2 and NP.

Influenza A virus transcribes its segmented negative sense RNA genome in the nuclei of infected cells in a process long known to require host RNA polymerase II (RNAP-II). RNA polymerase II synthesizes pre-mRNAs whose 5 0 -cap structures are scavenged by the viral RNAdependent RNA polymerase during synthesis of viral mRNAs. Drugs that inhibit RNAP-II therefore block viral replication, but not necessarily solely by denying the viral polymerase a source of cap-donor molecules. We show here that 5,6-dichloro-1-b-D-ribofuranosyl-benzimidazole (DRB), a compound that prevents processive transcription by RNAP-II, inhibits expression of the viral HA, M1 and NS1 genes at the post-transcriptional level. Abundant quantities of functionally and structurally intact viral mRNAs are made in the presence of DRB but with the exception of NP and NS2 mRNAs, are not efficiently translated. Taking M1 and NP mRNAs as representatives of DRB-sensitive and insensitive mRNAs, respectively, we found that the block to translation operates at the level of nuclear export. Furthermore, removal of DRB reversed this block unless a variety of chemically and mechanistically distinct RNAP-II inhibitors were added instead. We conclude that influenza A virus replication requires RNAP-II activity not just to provide capped mRNA substrates but also to facilitate nuclear export of selected viral mRNAs.

Equine influenza viruses are a major cause of respiratory disease in horses worldwide and undergo antigenic drift. Several outbreaks of equine influenza occurred worldwide during 2010-2012, including in vaccinated animals, highlighting the importance of surveillance and virus characterisation. Virus isolates were characterised from more than 20 outbreaks over a 3-year period, including strains from the UK, Dubai, Germany and the USA. The haemagglutinin-1 (HA1) sequence of all isolates was determined and compared with OIE-recommended vaccine strains. Viruses from Florida clades 1 and 2 showed continued divergence from each other compared with 2009 isolates. The antigenic interrelationships among viruses were determined using a haemagglutination-inhibition (HI) assay with ferret antisera and visualised using antigenic cartography. All European isolates belonged to Florida Clade 2, all those from the USA belonged to Florida Clade 1. Two subpopulations of Clade 2 viruses were isolated, with either substitution A144V or I179V. Isolates from Dubai, obtained from horses shipped from Uruguay, belonged to Florida Clade 1 and were similar to viruses isolated in the USA the previous year. The neuraminidase (NA) sequence of representative strains from 2007 and 2009 to 2012 was also determined and compared with that of earlier isolates dating back to 1963. Multiple changes were observed at the amino acid level and clear distinctions could be made between viruses belonging to Florida Clade 1 and Clade 2. Revision note VETMIC-D-13-8580Authors' response to reviewer comments: We agree with the reviewer that there are two topics here. However, we also believe that they are closely linked and that the increase in diagnostic sample submission should be sufficient justification to include the strategies used to encourage practitioners to submit nasal swabs. In response, we have therefore deleted surveillance data from the results & discussion, but retained the approaches used in the method section, for reference purposes.The scheme only relates to equine influenza as the financial support we have from the HBLB only covers 'flu. This is considered the major threat to horse racing and breeding in the UK, due to its rapid spread in an unprotected population.The 'free' surveillance scheme allows us to characterise samples sent from other countries as well as the UK. We believe that for countries where funding could potentially be sought, this scheme provides examples of low-cost methods that can be used to improve surveillance & sample submission, such as Twitter. We fully accept that this isn't possible in all countries. The isolate characterization data is certainly of value in monitoring the changes in EIV. What is disappointing is that all of the work done so far does not seem able to predict when a new vaccine will be needed. The antigenic mapping and serology tests tell us that changes are occurring and where in the HA and NA, but little about the efficacy of the current vaccines. Field data still seems most reliable. On...

SummaryEquine herpesvirus 1 (EHV‐1) causes respiratory disease, abortion, neonatal death and neurological disease in equines and is endemic in most countries. The viral factors that influence EHV‐1 disease severity are poorly understood, and this has hampered vaccine development. However, the N752D substitution in the viral DNA polymerase catalytic subunit has been shown statistically to be associated with neurological disease. This has given rise to the term “neuropathic strain,” even though strains lacking the polymorphism have been recovered from cases of neurological disease. To broaden understanding of EHV‐1 diversity in the field, 78 EHV‐1 strains isolated over a period of 35 years were sequenced. The great majority of isolates originated from the United Kingdom and included in the collection were low passage isolates from respiratory, abortigenic and neurological outbreaks. Phylogenetic analysis of regions spanning 80% of the genome showed that up to 13 viral clades have been circulating in the United Kingdom and that most of these are continuing to circulate. Abortion isolates grouped into nine clades, and neurological isolates grouped into five. Most neurological isolates had the N752D substitution, whereas most abortion isolates did not, although three of the neurological isolates from linked outbreaks had a different polymorphism. Finally, bioinformatic analysis suggested that recombination has occurred between EHV‐1 clades, between EHV‐1 and equine herpesvirus 4, and between EHV‐1 and equine herpesvirus 8.

The first 11 nt at the 5' end of influenza virus genomic RNA were shown to be both necessary and sufficient for specific binding by the influenza virus polymerase. A novel in vitro transcription assay, in which the polymerase was bound to paramagnetic beads via a biotinylated 5'-vRNA oligonucleotide, was used to study the activities of different forms of the polymerase. Complexes composed of co-expressed PB1/PB2/PA proteins and a sub-complex composed of PB1/PA bound to the 5'-vRNA oligonucleotide, whereas PB1 expressed alone did not. The enriched 5'-vRNA/PB1/PB2/PA complex was highly active for ApG and globin mRNA primed transcription on a model 3'-vRNA template. RNA synthesis in the absence of added primers produced products with 5'-terminal tri- or diphosphate groups, indicating that genuine unprimed initiation of transcription also occurred. No transcriptase activity was detected for the PB1/PA complex. These results demonstrate a role for PA in the enhancement of 5' end binding activity of PB1, a role for PB2 in the assembly of a polymerase complex able to perform both cap-dependent and -independent synthesis and that NP is not required for the initiation of replicative transcription.

The mechanism of membrane scission during influenza A virus budding has been the subject of controversy. We confirm that influenza M1 binds VPS28, a subunit of the ESCRT-1 complex. However, confocal microscopy of infected cells showed no marked colocalisation between M1 and VPS28 or VPS4 ESCRT proteins, or relocalisation of the cellular proteins. Trafficking of HA and M1 appeared normal when endosomal sorting was impaired by expression of inactive VPS4. Overexpression of either isoform of VPS28 or wildtype or dominant negative VPS4 proteins did not alter production of filamentous virions. SiRNA depletion of endogenous VPS28 had no significant effect on influenza virus replication. Furthermore, cells expressing wildtype or dominant-negative VPS4 replicated filamentous and non-filamentous strains of influenza to similar titres, indicating that influenza release is VPS4-independent. Overall, we see no role for the ESCRT pathway in influenza virus budding and the significance of the M1-VPS28 interaction remains to be determined.