Alphaherpesvirus-induced activation of plasmacytoid dendritic cells depends on the viral glycoprotein gD and is inhibited by non-infectious light particles

O-glycosylation in viruses: A sweet tango

O-glycosylation is an ancient yet underappreciated protein posttranslational modification, on which many bacteria and viruses heavily rely to perform critical biological functions involved in numerous infectious diseases or even cancer. But due to the innate complexity of O-glycosylation, research techniques have been limited to study its exact role in viral attachment and entry, assembly and exit, spreading in the host cells, and the innate and adaptive immunity of the host. Recently, the advent of many newly developed methodologies (e.g., mass spectrometry, chemical biology tools, and molecular dynamics simulations) has renewed and rekindled the interest in viral-related O-glycosylation in both viral proteins and host cells, which is further fueled by the COVID-19 pandemic. In this review, we summarize recent advances in viral-related O-glycosylation, with a particular emphasis on the mucin-type O-linked α-N-acetylgalactosamine (O-GalNAc) on viral proteins and the intracellular O-linked β-N-acetylglucosamine (O-GlcNAc) modifications on host proteins. We hope to provide valuable insights into the development of antiviral reagents or vaccines for better prevention or treatment of infectious diseases.

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.