Pseudorabies virus infections in explants of porcine nasal mucosa

Innate antiviral responses in porcine nasal mucosal explants inoculated with influenza A virus are comparable with responses in respiratory tissues after viral infection

Nasal mucosal explant (NEs) cultured at an air–liquid interface mimics in vivo conditions more accurately than monolayer cultures of respiratory cell lines or primary cells cultured in flat-bottom microtiter wells. NEs might be relevant for studies of host-pathogen interactions and antiviral immune responses after infection with respiratory viruses, including influenza and corona viruses.

Pigs are natural hosts for swine influenza A virus (IAV) but are also susceptible to IAV from humans, emphasizing the relevance of porcine NEs in the study of IAV infection. Therefore, we performed fundamental characterization and study of innate antiviral responses in porcine NEs using microfluidic high-throughput quantitative real-time PCR (qPCR) to generate expression profiles of host genes involved in inflammation, apoptosis, and antiviral immune responses in mock inoculated and IAV infected porcine NEs.

Handling and culturing of the explants ex vivo had a significant impact on gene expression compared to freshly harvested tissue. Upregulation (2–43 fold) of genes involved in inflammation, including IL1A and IL6, and apoptosis, including FAS and CASP3, and downregulation of genes involved in viral recognition (MDA5 (IFIH1)), interferon response (IFNA), and response to virus (OAS1, IFIT1, MX1) was observed. However, by comparing time-matched mock and virus infected NEs, transcription of viral pattern recognition receptors (RIG-I (DDX58), MDA5 (IFIH1), TLR3) and type I and III interferons (IFNB1, IL28B (IFNL3)) were upregulated 2–16 fold in IAV-infected NEs. Furthermore, several interferon-stimulated genes including MX1, MX2, OAS, OASL, CXCL10, and ISG15 was observed to increase 2–26 fold in response to IAV inoculation. NE expression levels of key genes involved in antiviral responses including IL28B (IFNL3), CXCL10, and OASL was highly comparable to expression levels found in respiratory tissues including nasal mucosa and lung after infection of pigs with the same influenza virus isolate.

Different replication characteristics of historical pseudorabies virus strains in porcine respiratory nasal mucosa explants

Different alphaherpesviruses, including pseudorabies virus (PRV), are able to cross the basement membrane barrier in nasal respiratory epithelium. As a first step in investigating this invasion process, a detailed quantitative analysis system was set up to determine the kinetics of horizontal and vertical virus spread in nasal explants, using the virulent PRV strain 89V87. Plaque latitudes, total depths, depths measured from the basement membrane and volumes were determined at 0, 12, 24 and 36h post inoculation (pi). PRV 89V87 was found to spread in a plaquewise manner and to cross the basement membrane between 12 and 24hpi. During the 1960s-1970s, an increase in PRV virulence has been reported. To analyse potential differences in efficiency of infection and spread for different historical PRV strains, single infected cells and plaques of infected cells were quantified at 12 and 36hpi in nasal mucosa explants for seven European PRV strains, isolated in the 1960s (Becker, NIA1), the 1970s (NS374, NIA3, 75V19) and later (89V87, 00V72). All viruses were used at second passage in cell culture, except for the Becker strain, which had an unknown passage history. Older strains, Becker, NIA1 and/or NS374, showed lower numbers of primary infectious centers, lower capacity to form plaques and/or lower capacity to cross the basement membrane. The observed differences in virus-mucosa interactions may aid in understanding the virulence increase of PRV. The quantitative assay established here will be of use in unravelling the mechanism of alphaherpesvirus-mediated invasion through the basement membrane.

Role of pseudorabies virus Us9, a type II membrane protein, in infection of tissue culture cells and the rat nervous system

The protein product of the pseudorabies virus (PRV) Us9 gene is a phosphorylated, type II membrane protein that is inserted into virion envelopes and accumulates in the trans-Golgi network. It is among a linked group of three envelope protein genes in the unique short region of the PRV genome which are absent from the attenuated Bartha strain. We found that two different Us9 null mutants exhibited no obvious phenotype after infection of PK15 cells in culture. Unlike those of gE and gI null mutants, the plaque size of Us9 null mutants on Madin-Darby bovine kidney cells was indistinguishable from that of wild-type virus. However, both of the Us9 null mutants exhibited a defect in anterograde spread in the visual and cortical circuitry of the rat. The visual system defect was characterized by restricted infection of a functionally distinct subset of visual projections involved in the temporal organization of behavior, whereas decreased anterograde spread of virus to the cortical projection targets was characteristic of animals receiving direct injections of virus into the cortex. Spread of virus through retrograde pathways in the brain was not compromised by a Us9 deletion. The virulence of the Us9 null mutants, as measured by time to death and appearance of symptoms of infection, also was reduced after their injection into the eye, but not after cortical injection. Through sequence analysis, construction of revertants, measurement of gE and gI protein synthesis in the Us9 null mutants, and mixed-infection studies of rats, we conclude that the restricted-spread phenotype after infection of the rat nervous system reflects the loss of Us9 and is not an indirect effect of the Us9 mutations on expression of glycoproteins gE and gI. Therefore, at least three viral envelope proteins, Us9, gE, and gI, function together to promote efficient anterograde transneuronal infection by PRV in the rat central nervous system.

The Us9 gene product of pseudorabies virus, an alphaherpesvirus, is a phosphorylated, tail-anchored type II membrane protein

The Us9 gene is highly conserved among the alphaherpesviruses sequenced to date, yet its function remains unknown. In this report, we demonstrate that the pseudorabies virus (PRV) Us9 protein is present in infected cell lysates as several phosphorylated polypeptides ranging from 17 to 20 kDa. Synthesis is first detected at 6 h postinfection and is sensitive to the DNA synthesis inhibitor phosphonoacetic acid. Unlike the herpes simplex virus type 1 Us9 homolog, which was reported to be associated with nucleocapsids in the nuclei of infected cells (M. C. Frame, D. J. McGeoch, F. J. Rixon, A. C. Orr, and H. S. Marsden, Virology 150:321-332, 1986), PRV Us9 localizes to the secretory pathway (predominately to the Golgi apparatus) and not to the nucleus. By fusing the enhanced green fluorescent protein (EGFP) reporter molecule to the carboxy terminus of Us9, we demonstrated that Us9 not only is capable of targeting a Us9-EGFP fusion protein to the Golgi compartment but also is able to direct efficient incorporation of such chimeric molecules into infectious viral particles. Moreover, through protease digestion experiments with Us9-EGFP-containing viral particles, we demonstrated that the Us9 protein is inserted into the viral envelope as a type II, tail-anchored membrane protein.

Construction and properties of pseudorabies virus recombinants with altered control of immediate-early gene expression

To investigate how altered control of expression of the essential immediate-early (IE) gene of pseudorabies virus influences virus replication and virulence, we replaced the IE promoter with the tissue-specific promoters of the bovine cytokeratin IV gene (CKIV), the bovine cytokeratin VIb gene (CKVIb), or the inducible promoter of Drosophila heat shock gene HSP70. We compared expression of the IE gene of the wild-type virus and recombinant viruses in different cell types and at different temperatures and found that IE expression had become cell type or temperature dependent. When a recombinant virus was titrated on nonpermissive cells or was titrated at nonpermissive temperatures in vitro, the plating efficiency was reduced by more than 99%. Mice were inoculated subcutaneously (s.c.), intraperitoneally (i.p.), or intranasally (i.n.) with a dose equal to 100 times the 50% lethal dose of the wild-type virus. After inoculation with temperature-sensitive recombinant N-HSP, two (s.c.), two (i.p.), and four (i.n.) of five mice died. However, at this dose, recombinant N-CKIV, which contains a promoter specific for stratified epithelial tissue of the tongue mucosa, was not lethal when inoculated s.c. or i.p. but killed four mice when inoculated i.n. Recombinant N-CKVIb, which contains a promoter specific for the suprabasal layers of the epidermis, was not lethal after inoculation by any of the three routes. In explant cultures of nasal mucosa of pigs, replication of N-CKIV and N-CKVIb was not markedly reduced in the epithelium. However, in contrast to results obtained with wild-type virus, infection of the stroma was not observed. We conclude that the replicative ability and virulence of pseudorabies virus can be influenced by altering control of expression of the IE gene.

Pseudorabies virus infectivity for swine skin characterized in vitro

The infectivity of pseudorabies virus (PrV) was demonstrated in a cell substrate derived from swine skin explant cultures designated primary porcine skin cells (c/cSLA PPSC). c/cSLA PPSC infected with either wild type or TK- PrV strain Kaplan (Ka) developed typical cytopathologic changes (CPE) as early as 4 h post inoculation (p.i.). The CPE caused by PrV on c/cSLA PPSC was specifically neutralized by covalescent swine sera. Synthesis of late viral proteins was demonstrated in PrV-infected c/cSLA PPSC by indirect fluorescent antibody staining using monoclonal antibodies (mAbs) specific for PrV gIII. PrV induced protein synthesis was further confirmed by specific immunoprecipitation of 35S-methionine labeled viral polypeptides from PrV-infected c/cSLA PPSC with PrV convalescent swine serum, PrV immune mouse serum or mAb to PrV gIII. Moreover, the virus progeny derived from c/cSLA PPSC was shown to be infectious for MDBK cells and this infection was specifically neutralized by PrV convalescent swine serum. The capacity c/cSLA PPSC to support a complete growth cycle of PrV and the relative ease of deriving these cells from pigs can be applied in an autologous fashion in studies of cellular immunity where the MHC needs to be matched.

Envelope glycoprotein gp50 of pseudorabies virus is essential for virus entry but is not required for viral spread in mice

Phenotypically complemented pseudorabies virus gp50 null mutants are able to produce plaques on noncomplementing cell lines despite the fact that progeny virions are noninfectious. To determine whether gp50 null mutants and gp50+gp63 null mutants are also able to replicate and spread in animals, mice were infected subcutaneously or intraperitoneally. Surprisingly, both gp50 mutants and gp50+gp63 double mutants proved to be lethal for mice. In comparison with the wild-type virus, gp50 mutants were still highly virulent, whereas the virulence of gp50+gp63 mutants was significantly reduced. Severe signs of neurological disorders, notably pruritus, were apparent in animals infected with the wild-type virus or a gp50 mutant but were much less pronounced in animals infected with a gp50+gp63 or gp63 mutant. Immunohistochemical examination of infected animals showed that all viruses were able to reach, and replicate in, the brain. Examination of visceral organs of intraperitoneally infected animals showed that viral antigen was predominantly present in peripheral nerves, suggesting that the viruses reached the central nervous system by means of retrograde axonal transport. Infectious virus could not be recovered from the brains and organs of animals infected with gp50 or gp50+gp63 mutants, indicating that progeny virions produced in vivo are noninfectious. Virions that lacked gp50 in their envelopes, and a phenotypically complemented pseudorabies virus gII mutant (which is unable to produce plaques in tissue culture cells), proved to be nonvirulent for mice. Together, these results show that gp50 is required for the primary infection but not for subsequent replication and viral spread in vivo. These results furthermore indicate that transsynaptic transport of the virus is independent of gp50. Since progeny virions produced by gp50 mutants are noninfectious, they are unable to spread from one animal to another. Therefore, such mutants may be used for the development of a new generation of safer (carrier) vaccines.

Role of different genes in the virulence and pathogenesis of Aujeszky's disease virus

In this study the role of different genes located in the unique short region of the genome of Aujeszky's disease virus was examined. Inactivation of the genes encoding the protein kinase (PK), gp63, and gI reduced virulence of the virus for pigs, in contrast to inactivation of the genes encoding the 28 kDa protein, and gX. There was no correlation between virulence and virus multiplication in vitro or in the oropharynx in vivo. The morphogenesis of the PK mutant was altered. The gI mutant replicated to normal titres in the oropharynx and could be recovered from the trigeminal ganglia but not from other parts of the central nervous system, suggesting that gI facilitates the spread of the virus from neuron to neuron. All mutants induced neutralizing antibody and complete or partial protection against a challenge infection. PK and gp63 were required for the induction of complete protection, although these proteins are reportedly not targets for neutralizing antibody or cytotoxic T cells.

Pathological, ultrastructural, and immunohistochemical changes caused by Lelystad virus in experimentally induced infections of mystery swine disease (synonym: porcine epidemic abortion and respiratory syndrome (PEARS))

The pathogenicity and pathogenesis of Lelystad virus was studied in six 6-day-old SPF piglets. A third passage of the agent was propagated on porcine alveolar macrophages and intranasally inoculated into pigs. Pigs were killed at hours 24, 48, 60, and 72, and on days 6 and 8 after inoculation. From day 2 on pigs developed diffuse interstitial pneumonia with focal areas of catarrhal pneumonia, and from this day on splenic red pulp macrophages were enlarged and vacuolated. Lelystad virus was re-isolated from the lungs of infected pigs from day 2 after inoculation. Lelystad virus antigens were detected by immunohistochemical techniques in bronchiolar epithelium and alveolar cells, and in spleen cells of infected pigs from day 2 after inoculation. Ultrastructural examination of tissues by electron microscopy revealed degenerating alveolar macrophages and epithelial cells in lungs and nasal mucosa, with excessive vacuolation of the endoplasmic reticulum. Although the respiratory tract seems to be the target organ for this virus, macrophages in other organs, such as the spleen, can also be infected. This preference for macrophages may impair immunological defences.