The HSV-1 Latency-Associated Transcript Functions to Repress Latent Phase Lytic Gene Expression and Suppress Virus Reactivation from Latently Infected Neurons.

Herpes simplex virus 1 (HSV-1) establishes life-long latent infection within sensory neurons, during which viral lytic gene expression is silenced. The only highly expressed viral gene product during latent infection is the latency-associated transcript (LAT), a non-protein coding RNA that has been strongly implicated in the epigenetic regulation of HSV-1 gene expression. We have investigated LAT-mediated control of latent gene expression using chromatin immunoprecipitation analyses and LAT-negative viruses engineered to express firefly luciferase or ß-galactosidase from a heterologous lytic promoter. Whilst we were unable to determine a significant effect of LAT expression upon heterochromatin enrichment on latent HSV-1 genomes, we show that reporter gene expression from latent HSV-1 genomes occurs at a greater frequency in the absence of LAT. Furthermore, using luciferase reporter viruses we have observed that HSV-1 gene expression decreases during long-term latent infection, with a most marked effect during LAT-negative virus infection. Finally, using a fluorescent mouse model of infection to isolate and culture single latently infected neurons, we also show that reactivation occurs at a greater frequency from cultures harbouring LAT-negative HSV-1. Together, our data suggest that the HSV-1 LAT RNA represses HSV-1 gene expression in small populations of neurons within the mouse TG, a phenomenon that directly impacts upon the frequency of reactivation and the maintenance of the transcriptionally active latent reservoir.

Enhancement of CD8(+) T-cell memory by removal of a vaccinia virus nuclear factor-κB inhibitor.

Factors influencing T-cell responses are important for vaccine development but are incompletely understood. Here, vaccinia virus (VACV) protein N1 is shown to impair the development of both effector and memory CD8(+) T cells and this correlates with its inhibition of nuclear factor-κB (NF-κB) activation. Infection with VACVs that either have the N1L gene deleted (vΔN1) or contain a I6E mutation (vN1.I6E) that abrogates its inhibition of NF-κB resulted in increased central and memory CD8(+) T-cell populations, increased CD8(+) T-cell cytotoxicity and lower virus titres after challenge. Furthermore, CD8(+) memory T-cell function was increased following infection with vN1.I6E, with more interferon-γ production and greater protection against VACV infection following passive transfer to naive mice, compared with CD8(+) T cells from mice infected with wild-type virus (vN1.WT). This demonstrates the importance of NF-κB activation within infected cells for long-term CD8(+) T-cell memory and vaccine efficacy. Further, it provides a rationale for deleting N1 from VACV vectors to enhance CD8(+) T-cell immunogenicity, while simultaneously reducing virulence to improve vaccine safety.