The Attenuated Pseudorabies Virus Vaccine Strain Bartha Hyperactivates Plasmacytoid Dendritic Cells by Generating Large Amounts of Cell-Free Virus in Infected Epithelial Cells

Pseudorabies virus (PRV) is a porcine alphaherpesvirus and the causative agent of Aujeszky's disease. Successful eradication campaigns against PRV have largely relied on the use of potent PRV vaccines. The live attenuated Bartha strain, which was produced by serial passaging in cell culture, represents one of the hallmark PRV vaccines. Despite the robust protection elicited by Bartha vaccination, very little is known about the immunogenicity of the Bartha strain. Previously, we showed that Bartha-infected epithelial cells trigger plasmacytoid dendritic cells (pDC) to produce much higher levels of type I interferons than cells infected with wild-type PRV. Here, we show that this Bartha-induced pDC hyperactivation extends to other important cytokines, including interleukin-12/23 (IL-12/23) and tumor necrosis factor alpha (TNF-α) but not IL-6. Moreover, Bartha-induced pDC hyperactivation was found to be due to the strongly increased production of extracellular infectious virus (heavy particles [H-particles]) early in infection of epithelial cells, which correlated with a reduced production of noninfectious light particles (L-particles). The Bartha genome is marked by a large deletion in the US region affecting the genes encoding US7 (gI), US8 (gE), US9, and US2. The deletion of the US2 and gE/gI genes was found to be responsible for the observed increase in extracellular virus production by infected epithelial cells and the resulting increased pDC activation. The deletion of gE/gI also suppressed L-particle production. In conclusion, the deletion of US2 and gE/gI in the genome of the PRV vaccine strain Bartha results in the enhanced production of extracellular infectious virus in infected epithelial cells and concomitantly leads to the hyperactivation of pDC. IMPORTANCE The pseudorabies virus (PRV) vaccine strain Bartha has been and still is critical in the eradication of PRV in numerous countries. However, little is known about how this vaccine strain interacts with host cells and the host immune system. Here, we report the surprising observation that Bartha-infected epithelial porcine cells rapidly produce increased amounts of extracellular infectious virus compared to wild-type PRV-infected cells, which in turn potently stimulate porcine plasmacytoid dendritic cells (pDC). We found that this phenotype depends on the deletion of the genes encoding US2 and gE/gI. We also found that Bartha-infected cells secrete fewer pDC-inhibiting light particles (L-particles), which appears to be caused mainly by the deletion of the genes encoding gE/gI. These data generate novel insights into the interaction of the successful Bartha vaccine with epithelial cells and pDC and may therefore contribute to the development of vaccines against other (alphaherpes)viruses.

Development and characterization of ORF68 negative equine herpes virus type-1, Ab4p strain

Equine herpesvirus-1 (EHV-1) is an important pathogen, which infects horses worldwide with high morbidity but low mortality rates. The respiratory disorders and abortions are the most common indicators. Ab4p (an abortigenic and paralytic virus) is one of the most important and virulent strains. The development and functional characterization of the open reading frame-68 (ORF68) negative EHV-1 Ab4p mutants and an assessment of their roles in the infection at the cellular level were the main targets of the current study. Escherichia coli DH10β containing the Ab4p bacterial artificial chromosome (pAb4pBAC) and Red/ET expression vector were used to develop different ORF68 mutants. Multi-step growth kinetic experiments were conducted in order to evaluate the growth properties of the constructed mutant viruses. Growth of the Ab4pΔORF68 showed the lowest titer, compared to the Ab4pΔORF68R, Ab4pΔORF68R non-sense, and the parent Ab4p viruses without any significant difference (P > 0.05). The growth of the mutant viruses was almost similar across the cell types, but viruses growth was more efficient in FHK cells as judged by the number of the obtained virus particles. The plaque size of Ab4pΔORF68 was significantly (40%) smaller than those of Ab4p (P < 0.01), Ab4pΔORF68R, and Ab4pΔORF68R non-sense viruses which confirmed the importance of ORF68 protein in the cell-to-cell transmission of EHV-1. Subcellular localization of the green fluorescent protein (GFP) ORF68 gene fusion product showed late expression with intranuclear localization of the transfected cells while immunofluorescent antibody technique (IFAT) localized it at the nucleus and nuclear membranes of the infected cells. Hence, it could be concluded that ORF68 protein may not be essential for EHV-1 Ab4p growth but plays a crucial role in virus penetration and transmission at the cellular level. Therefore, the generated EHV-1 ORF68 negative mutant could be a prospective candidate for the development of a vaccine marker.

Deletion of pseudorabies virus US2 gene enhances viral titers in a porcine cerebral cortex primary culture system

Pseudorabies virus (PRV) is a neurotropic virus with the ability to infect peripheral sensory ganglia. The transport of PRV from the peripheral to the central nervous system can cause lethal encephalitis in young piglets. However, the pathogenicity of PRV in the cerebral cortex remains poorly understood. In the present study, we developed a porcine cerebral cortex primary culture system (PCCS) using cerebral cortex tissue dissected from a 3-day-old piglet to investigate the pathogenicity of wild-type (WT) and US2 deleted (ΔUS2) PRV in the CNS in vitro. Immunofluorescence assays revealed cell bodies and neurites as the cellular locations infected by PRV. Growth kinetic analysis showed a persistent increase in WT and ΔUS2 viral titers in PCCS from 4 to 24 h post infection (hpi), thus indicating that US2 deletion did not disrupt viral growth. However, the mean plaque size was significantly higher in ΔUS2 PRV than in WT PRV in infected Vero cells. The viral titers and DNA levels of ΔUS2 PRV were significantly higher at 8 hpi than at 4 hpi, whereas those of WT showed no significant difference between the two time points in PCCS. Morphological investigation revealed induction of massive amounts of bouton-like swellings (varicosities) along the axon shaft in both WT and ΔUS2 PRV-infected neurons in the PCCS. Our data suggest that PRV US2 gene deletion enhances viral titers in PCCS but does not affect the varicosities induced by the viral infection.

Multiple E2 ubiquitin-conjugating enzymes regulate human cytomegalovirus US2-mediated immunoreceptor downregulation

Misfolded endoplasmic reticulum (ER) proteins are dislocated towards the cytosol and degraded by the ubiquitin-proteasome system in a process called ER-associated protein degradation (ERAD). During infection with human cytomegalovirus (HCMV), the viral US2 protein targets HLA class I molecules (HLA-I) for degradation via ERAD to avoid elimination by the immune system. US2-mediated degradation of HLA-I serves as a paradigm of ERAD and has facilitated the identification of TRC8 (also known as RNF139) as an E3 ubiquitin ligase. No specific E2 enzymes had previously been described for cooperation with TRC8. In this study, we used a lentiviral CRISPR/Cas9 library targeting all known human E2 enzymes to assess their involvement in US2-mediated HLA-I downregulation. We identified multiple E2 enzymes involved in this process, of which UBE2G2 was crucial for the degradation of various immunoreceptors. UBE2J2, on the other hand, counteracted US2-induced ERAD by downregulating TRC8 expression. These findings indicate the complexity of cellular quality control mechanisms, which are elegantly exploited by HCMV to elude the immune system.