Poxvirus virions: their surface ultrastructure and interaction with the surface membrane of host cells

Unexpected thermal stability of two enveloped megaviruses, Emiliania huxleyi virus and African swine fever virus, as measured by viability PCR

BACKGROUND

The particle structure of Emiliania huxleyi virus (EhV), an algal infecting member of nucleocytoplasmic large DNA viruses (NCLDVs), contains an outer lipid membrane envelope similar to that found in animal viruses such as African swine fever virus (ASFV). Despite both being enveloped NCLDVs, EhV and ASFV are known for their stability outside their host environment.

METHOD

Here we report for the first time, the application of a viability qPCR (V-qPCR) method to describe the unprecedented and similar virion thermal stability of both EhV and ASFV. This result contradicts the cell culture-based assay method that suggests that virus "infectivity" is lost in a matter of seconds (for EhV) and minutes (for ASFV) at temperature greater than 50 °C. Confocal microscopy and analytical flow cytometry methods was used to validate the V-qPCR data for EhV.

RESULTS

We observed that both EhV and ASFV particles has unprecedented thermal tolerances. These two NCLDVs are exceptions to the rule that having an enveloped virion anatomy is a predicted weakness, as is often observed in enveloped RNA viruses (i.e., the viruses causing Porcine Reproductive and Respiratory Syndrome (PRRS), COVID-19, Ebola, or seasonal influenza). Using the V-qPCR method, we confirm that no PRRSV particles were detectable after 20 min of exposure to temperatures up to 100 °C. We also show that the EhV particles that remain after 50 °C 20 min exposure was in fact still infectious only after the three blind passages in bioassay experiments.

CONCLUSIONS

This study raises the possibility that ASFV is not always eliminated or contained after applying time and temperature inactivation treatments in current decontamination or biosecurity protocols. This observation has practical implications for industries involved in animal health and food security. Finally, we propose that EhV could be used as a surrogate for ASFV under certain circumstances.

The OH system: A panorama view of the PPV-host interaction

Poxviruses are a family of specialized cytoplasm-parasitic DNA viruses that replicate and assembly in virus factory. In Parapoxvirus (PPV) genus, with the orf virus (ORFV) as a representative species of this genus, their behaviors are significantly different from that of Orthopoxvirus, and the plots of viral practical solutions for evading host immunity are intricate and fascinating, particularly to anti-host and host's antiviral mechanisms. In order to protect the virus factory from immune elimination caused by infection, PPVs attempt to interfere with multiple stress levels of host, mainly by modulating innate immunity response (IIR) and adaptive immunity response (AIR). Given that temporarily constructed by virus infection, ORFV-HOST (OH) system accompanied by viral strategies is carefully managed in the virus factory, thus directing many life-critical events once undergoing the IIR and AIR. Evolutionarily, to reduce the risk of system destruction, ORFV have evolved into a mild-looking mode to avoid overstimulation. Moreover, the current version of development also focus on recognizing and hijacking more than eight antiviral security mechanisms of host cells, such as the 2',5'-oligoadenylate synthetase (OAS)/RNase L and PKR systems, the ubiquitin protease system (UPS), and so on. In summary, this review assessed inescapable pathways as mentioned above, through which viruses compete with their hosts strategically. The OH system provides a panoramic view and a powerful platform for us to study the PPV-Host interaction, as well as the corresponding implications on a great application potential in anti-virus design.

Molecular genetic analysis of orf virus: a poxvirus that has adapted to skin

Orf virus is the type species of the Parapoxvirus genus of the family Poxviridae. It induces acute pustular skin lesions in sheep and goats and is transmissible to humans. The genome is G+C rich, 138 kbp and encodes 132 genes. It shares many essential genes with vaccinia virus that are required for survival but encodes a number of unique factors that allow it to replicate in the highly specific immune environment of skin. Phylogenetic analysis suggests that both viral interleukin-10 and vascular endothelial growth factor genes have been "captured" from their host during the evolution of the parapoxviruses. Genes such as a chemokine binding protein and a protein that binds granulocyte-macrophage colony-stimulating factor and interleukin-2 appear to have evolved from a common poxvirus ancestral gene while three parapoxvirus nuclear factor (NF)-κB signalling pathway inhibitors have no homology to other known NF-κB inhibitors. A homologue of an anaphase-promoting complex subunit that is believed to manipulate the cell cycle and enhance viral DNA synthesis appears to be a specific adaptation for viral-replication in keratinocytes. The review focuses on the unique genes of orf virus, discusses their evolutionary origins and their role in allowing viral-replication in the skin epidermis.

Correlation between structure, protein composition, morphogenesis and cytopathology of Glossina pallidipes salivary gland hypertrophy virus

The Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) is a dsDNA virus with rod-shaped, enveloped virions. Its 190 kb genome contains 160 putative protein-coding ORFs. Here, the structural components, protein composition and associated aspects of GpSGHV morphogenesis and cytopathology were investigated. Four morphologically distinct structures: the nucleocapsid, tegument, envelope and helical surface projections, were observed in purified GpSGHV virions by electron microscopy. Nucleocapsids were present in virogenic stroma within the nuclei of infected salivary gland cells, whereas enveloped virions were located in the cytoplasm. The cytoplasm of infected cells appeared disordered and the plasma membranes disintegrated. Treatment of virions with 1 % NP-40 efficiently partitioned the virions into envelope and nucleocapsid fractions. The fractions were separated by SDS-PAGE followed by in-gel trypsin digestion and analysis of the tryptic peptides by liquid chromatography coupled to electrospray and tandem mass spectrometry. Using the MaxQuant program with Andromeda as a database search engine, a total of 45 viral proteins were identified. Of these, ten and 15 were associated with the envelope and the nucleocapsid fractions, respectively, whilst 20 were detected in both fractions, most likely representing tegument proteins. In addition, 51 host-derived proteins were identified in the proteome of the virus particle, 13 of which were verified to be incorporated into the mature virion using a proteinase K protection assay. This study provides important information about GpSGHV biology and suggests options for the development of future anti-GpSGHV strategies by interfering with virus-host interactions.

Investigation of orf virus structure and morphogenesis using recombinants expressing FLAG-tagged envelope structural proteins: evidence for wrapped virus particles and egress from infected cells

Orf virus (ORFV) is the type species of the genus Parapoxvirus, but little is known about the structure or morphogenesis of the virus. In contrast, the structure and morphogenesis of vaccinia virus (VACV) has been extensively studied. VACV has two main infectious forms, mature virion (MV) and extracellular virion (EV). The MV is wrapped by two additional membranes derived from the trans-Golgi to produce a wrapped virion (WV), the outermost of which is lost by cellular membrane fusion during viral egress to form the EV. Genome sequencing of ORFV has revealed that it has homologues of almost all of the VACV structural genes. Notable exceptions are A36R, K2L, A56R and B5R, which are associated with WV and EV envelopes. This study investigated the morphogenesis and structure of ORFV by fusing FLAG peptide to the structural proteins 10 kDa, F1L and ORF-110 to form recombinant viruses. 10 kDa and F1L are homologues of VACV A27L and H3L MV membrane proteins, whilst ORF-110 is homologous to VACV A34R, an EV membrane protein. Immunogold labelling of FLAG proteins on virus particles isolated from lysed cells showed that FLAG-F1L and FLAG-10 kDa were displayed on the surface of infectious particles, whereas ORF-110-FLAG could not be detected. Western blot analysis of solubilized recombinant ORF-110-FLAG particles revealed that ORF-110-FLAG was abundant and undergoes post-translational modification indicative of endoplasmic reticulum trafficking. Fluorescent microscopy confirmed the prediction that ORF-110-FLAG localized to the Golgi in virus-infected cells. Finally, immunogold labelling of EVs showed that ORF-110-FLAG became exposed on the surface of EV-like particles as a result of egress from the cell.

Egress of light particles among filopodia on the surface of Varicella-Zoster virus-infected cells

Varicella-zoster virus (VZV) is renowned for its very low titer when grown in cultured cells. There remains no single explanation for the low infectivity. In this study, viral particles on the surfaces of infected cells were examined by several imaging technologies. Few surface particles were detected at 48 h postinfection (hpi), but numerous particles were observed at 72 and 96 hpi. At 72 hpi, 75% of the particles resembled light (L) particles, i.e., envelopes without capsids. By 96 hpi, 85% of all particles resembled L particles. Subsequently, the envelopes of complete virions and L particles were investigated to determine their glycoprotein constituents. Glycoproteins gE, gI, and gB were detected in the envelopes of both types of particles in similar numbers; i.e., there appeared to be no difference in the glycoprotein content of the L particles. The viral particles emerged onto the cell surface amid actin-based filopodia, which were present in abundance within viral highways. Viral particles were easily detected at the base of and along the exterior surfaces of the filopodia. VZV particles were not detected within filopodia. In short, these results demonstrate that VZV infection of cultured cells produces a larger proportion of aberrant coreless particles than has been seen with any other previously examined alphaherpesvirus. Further, these results suggested a major disassociation between capsid formation and envelopment as an explanation for the invariably low VZV titer in cultured cells.

Genus Parapoxvirus

Highly contagious pustular skin infections of sheep, goats and cattle that were unwittingly transmitted to humans from close contact with infected animals, have been the scourge of shepherds, herdsmen and dairy farmers for centuries. In more recent times we recognise that these proliferative pustular lesions are likely to be caused by a group of zoonotic viruses that are classified as parapoxviruses. In addition to infecting the above ungulates, parapoxviruses have more recently been isolated from seals, camels, red deer and reindeer and most have been shown to infect man. The parapoxviruses have one of the smallest genomes of the poxvirus family (140 kb) yet share over 70% of their genes with the most virulent members. Like other poxviruses, the central core of the genomes encode factors for virus transcription and replication, and structural proteins, whereas the terminal regions encode accessory factors that give the parapoxvirus group many of its unique features. Several genes of parapoxviruses are unique to this genus and encode factors that target inflammation, the innate immune responses and the development of acquired immunity. These factors include a homologue of mammalian interleukin (IL)-10, a chemokine binding protein and a granulocyte-macrophage colony stimulating factor /IL-2 binding protein. The ability of this group to reinfect their hosts, even though a cell-mediated memory response is induced during primary infection, may be related to their epitheliotropic niche and the immunomodulators they produce. In this highly localised environment, the secreted immunomodulators only interfere with the local immune response and thus do not compromise the host’s immune system. The discovery of a vascular endothelial growth factor-like gene may explain the highly vascular nature of parapoxvirus lesions. There are many genes of parapoxviruses which do not encode polypeptides with significant matches with protein sequences in public databases, separating this genus from most other mammalian poxviruses. These genes appear to be involved in inhibiting apoptosis, manipulating cell cycle progression and degradation of cellular proteins that may be involved in the stress response, thus allowing the virus to subvert intracellular antiviral mechanisms and enhance the availability of cellular molecules required for replication. Parapoxviruses in common with Molluscum contagiosum virus lack a number of genes that are highly conserved in other poxviruses, including factors for nucleotide metabolism, serine protease inhibitors and kelch-like proteins. It is apparent that parapoxviruses have evolved a unique repertoire of genes that have allowed adaptation to the highly specialised environment of the epidermis.

Laboratory diagnosis of contagious ecthyma: comparison of different PCR protocols with virus isolation in cell culture

A new polymerase chain reaction (PCR) assay for rapid diagnosis of contagious ecthyma was designed and applied to 21 clinical samples from Greece. This assay, which detects a highly conserved gene from the parapox genome, was evaluated for its sensitivity and specificity in order to be considered as a useful diagnostic tool. A comparative study with two published PCR protocols one using primers PPP1-PPP3, PPP1-PPP4 which targets putative virion envelope gene B2L and the other using VIR1-VIR2 primers which amplifies ORF virus interferon resistant (VIR) gene, as well as cell culture virus neutralization assay was carried out. All samples tested were amplified successfully with the PCR protocol established in the laboratory. The combination of primers PPP1-PPP3 and PPP1-PPP4 in a semi-nested PCR gave a positive result in 20 of 21 samples while primers VIR1-VIR2 failed to amplify successfully 7 of 21 samples. The diagnostic value of parapox viral DNA amplification was also compared with the results of virus isolation by cell culture and was positive in three samples that the virus isolation was obtained.