Reduced virulence of a pseudorabies virus isolate from wild boar origin in domestic pigs correlates with hampered visceral spread and age-dependent reduced neuroinvasive capacity

MicroRNA-194-5p/Heparin-binding EGF-like growth factor signaling mediates dexamethasone-induced activation of pseudorabies virus in rat pheochromocytoma cells

Pseudorabies virus (PRV) is a neurotropic virus, which infects a wide range of mammals. The activity of PRV is gradually suppressed in hosts that have tolerated the primary infection. Increased glucocorticoid levels resulting from stressful stimuli overcome repression of PRV activity. However, the host cell mechanism involved in the activation processes under stressful conditions remains unclear. In this study, infection of rat PC-12 pheochromocytoma cells with neuronal properties using PRV at a multiplicity of infection (MOI) = 1 for 24 h made the activity of PRV be the relatively repressed state, and then incubation with 0.5 μM of the corticosteroid dexamethasone (DEX) for 4 h overcomes the relative repression of PRV activity. RNA-seq deep sequencing and bioinformatics analyses revealed different microRNA and mRNA profiles of PC-12 cells with/without PRV and/or DEX treatment. qRT-PCR and western blot analyses confirmed the negative regulatory relationship of miRNA-194-5p and its target heparin-binding EGF-like growth factor (Hbegf); a dual-luciferase reporter assay revealed that Hbegf is directly targeted by miRNA-194-5p. Further, miRNA-194-5p mock transfection contributed to PRV activation, Hbegf was downregulated in DEX-treated PRV infection cells, and Hbegf overexpression contributed to returning activated PRV to the repression state. Moreover, miRNA-194-5p overexpression resulted in reduced levels of HBEGF, c-JUN, and p-EGFR, whereas Hbegf overexpression suppressed the reduction caused by miRNA-194-5p overexpression. Overall, this study is the first to report that changes in the miR-194-5p-HBEGF/EGFR pathway in neurons are involved in DEX-induced activation of PRV, laying a foundation for the clinical prevention of stress-induced PRV activation.

Microglia play an important role in PRV infection-induced immune responses of the central nervous system

Pseudorabies virus (PRV) can infect multiple hosts and lead to fatal encephalitis. There is a significant increase in the number of microglia in the brain of animals infected with PRV. However, whether and how microglia contribute to central nervous system damage in PRV infection remain unknown. In the present study, we elucidated that PRV infection can cause more severe inflammatory cell infiltration, thicker and more numerous vessel sleeve walls, and more severe inflammatory responses in the brains of natural hosts (pigs) than in those of nonnatural hosts (mice). In a mice infection model, activated microglia restricted viral replication in the early stage of infection. Acute neuroinflammation caused by microglia hyperactivation at late-stage of infection. Furthermore, in vitro experiments revealed that microglia restricted viral replication and decreased viral infectivity. This may be associated with the phagocytic ability of microglia because we observed a significant increase in the expression of the membrane receptor TREM2 in microglia, which is closely related to phagocytosis, we observed that depletion of microglia exacerbated neurological symptoms, blood-brain barrier breakdown, and peripheral lymphocyte infiltration. Taken together, we revealed the dual role of microglia in protecting the host and neurons from PRV infection.

Japanese Encephalitis Virus Persistence in Porcine Tonsils Is Associated With a Weak Induction of the Innate Immune Response, an Absence of IFNγ mRNA Expression, and a Decreased Frequency of CD4+CD8+ Double-Positive T Cells

In humans, Japanese encephalitis virus (JEV) causes a devastating neurotropic disease with high mortality, whereas in pigs, the virus only causes mild symptoms. Besides tropism to the central nervous system, JEV seems to harbor a particular tropism for the tonsils in pigs. This secondary lymphoid organ appears to act as a reservoir for the virus, and we show that it is found up to 21 days post infection at high viral titers. The immune response in the tonsils was studied over time upon intradermal inoculation of pigs. Entry of the virus in the tonsils was accompanied by a significant increase in anti-viral OAS1 and IFNβ mRNA expression. This limited antiviral response was, however, not sufficient to stop JEV replication, and importantly, no IFNγ or innate inflammatory cytokine mRNA expression could be observed. Strikingly, the persistence of JEV in tonsils was also associated with a significant decreased frequency of CD4⁺CD8⁺ double-positive T lymphocytes. Furthermore, it is important to note that JEV persistence in tonsils occurred despite a strong induction of the adaptive immune response. JEV-specific antibodies were found after 6 days post infection in serum, and cell-mediated immune responses upon NS3 restimulation of PBMCs from experimentally infected pigs showed that CD4⁺CD8⁺ double-positive T cells were found to display the most prominent proliferation and IFNγ production among lymphocyte subtypes. Taken together, these results suggest that an inadequate induction of the innate immune response and the absence of an IFNγ antiviral response contribute to the persistence of JEV in the tonsils and is associated with a decrease in the frequency of CD4⁺CD8⁺ double-positive T cells.

Latent pseudorabies virus infection in medulla oblongata from quarantined pigs

Pseudorabies virus (PRV) is a major pathogen in pig husbandry and is also a risk to human well-being. Pigs with latent PRV infection carry the virus lifelong, and it can be activated under conducive conditions. This poses a very important challenge to the control of the virus and may even prevent its elimination. To investigate latent infection with wild-type (wt) PRV, and also infection due to the use of live attenuated vaccines on farms, 80 pigs from two large-scale swine operations were traced. At 6 months old, the quarantined pigs were slaughtered and brain samples were collected. A PCR assay targeting the gB and gE genes was developed to detect PRV DNA fragments in medulla oblongata. Five of the samples (6.3%) were gB and gE gene fragment double-positive, 60 of the samples (75%) were gB single-positive, and 15 samples (18.7%) showed double-negative. A portion of latency-associated transcripts (LATs), EP0 mRNA, were found to be present in the gB gene fragment positive samples. Furthermore, the five double-positive samples were transmitted blindly, and apparent cytopathic effects were found in three of the five samples in the fourth generation. By means of Western blotting, PCR and sequencing, two of the isolated viruses were found to be related to vaccine strain Bartha-K61. Another was closely related to domestic epidemic strains HN1201 and LA and relatively unrelated to other Asian isolates. These results suggest that the live vaccines are latently present in brains, in a manner similar to wt PRV, and this poses potential safety issues in the pig husbandry industry. Wt PRV and live vaccine viruses were found to co-exist in pigs, demonstrating that the live vaccines were unable to confer complete sterilizing immunity, which may explain outbreaks of pseudorabies on vaccinated farms.

Effect of porcine circovirus type 2 on the severity of lung and brain damage in piglets infected with porcine pseudorabies virus

Porcine circovirus type 2 (PCV2) is widespread throughout Chinese farms, and the infection rate of porcine pseudorabies virus (PRV) is very high. The emergence of mixed infection involving PCV2 and PRV has been difficult to prevent and control and has caused considerable economic loss. The present study investigated lung and brain damage caused by PRV in piglets with PCV2 infection. Twenty piglets were divided randomly into two experiment groups (PRV group and PRV + PCV2 group; n = 10 per group). The pigs were observed for clinical signs at specified times. At necropsy, lung and brain tissue samples were collected for histopathological examination, and tissue virus load was determined using quantitative polymerase chain reaction. Severe pathogenicity due to PRV was evident in two-month-old piglets. PCV2 and PRV co-infection led to more severe neurological and respiratory symptoms and a higher mortality rate in the piglets. In addition, the pathological damage to the lung and brain was also aggravated. The co-infection was associated with a significant increase in the content of PRV in the brain and lung tissue. In conclusion, PCV2 and PRV co-infection could cause severe and irreversible damage to piglets.

Effects of Intranasal Pseudorabies Virus AH02LA Infection on Microbial Community and Immune Status in the Ileum and Colon of Piglets

Pseudorabies virus (PRV) variants broke out in china since 2011, causing high fever, respiratory distress, systemic neurological symptoms, and diarrhea in piglets. This study investigated the effect of intranasal PRV variant (AH02LA) infection on ileal and colonic bacterial communities and immune status in piglets. Ten piglets (free of PRV) were assigned to PRV variant and control groups (uninfected). At day 5 after inoculation, all piglets were euthanized. No PRV was detected in the ileal and colonic mucosa. In the PRV group, we observed up-regulation of specific cytokines gene expression, down-regulation of intestinal barrier-related gene expression, and reduction of secretory immunoglobulin A (sIgA) concentration in the ileum and colon. PRV infection increased the diversity of ileal bacterial community composition. PRV infection reduced the abundance of some beneficial bacteria (Lactobacillus species in the ileum and colon; butyrate-producing bacteria species in the colon) and increased the abundance of potentially pathogenic Fusobacterium nucleatum in the ileum and Sphingomonas paucimobilis in the colon. Moreover, PRV infection decreased concentrations of the beneficial lactate in the ileum and butyrate in the colon. However, this study does not allow to evaluate whether the observed changes are directly due to the PRV infection or rather to indirect effects (fever, clinical signs and changes in diet), and will be our next research content. In summary, our findings provide evidence that intranasal PRV infection directly or indirectly brings gut health risks and implications, although no PRV was detected in the ileum and colon.