UL56 Is Essential for Herpes Simplex Virus-1 Virulence In Vivo but Is Dispensable for Induction of Host-Protective Immunity.

Herpes simplex virus-1 (HSV-1) is common and can cause significant disease in humans. Unfortunately, efforts to develop effective vaccines against HSV-1 have so far failed. A detailed understanding of how the virus infects its host and how the host mounts potent immune responses against the virus may inform new vaccine approaches. Here, using a zosteriform mouse model, we examined how the HSV-1 gene UL56 affects the ability of the virus to cause morbidity and generate protective immunity. A UL56 deletion mutant, ΔUL56, was derived from the wild-type HSV-1 strain SC16, alongside a revertant strain in which UL56 was reintroduced in ΔUL56. In vitro, the three virus strains replicated in a similar manner; however, in vivo, only the wild type and the revertant strains caused shingles-like skin lesions and death. Mice previously infected with ΔUL56 became resistant to a lethal challenge with the wild-type SC16. The protective immunity induced by ΔUL56 was independent of IL-1, IL-33, and IL-36 signaling through IL-1RAP. Both skin and intramuscular ΔUL56 inoculation generated protective immunity against a lethal SC16 challenge. After 6 months, female mice remained resistant to infection, while male mice exhibited signs of declining protection. Our data demonstrate that UL56 is important for the ability of HSV-1 to spread within the infected host and that a ∆UL56 strain elicits an effective immune response against HSV-1 despite this loss of virulence. These findings may guide further HSV-1 vaccine development.