Safety and immunogenicity of a TK/ gI/gE gene-deleted feline herpesvirus-1 mutant constructed via CRISPR/Cas9 in feline

Feline herpesvirus-1 (FHV-1) is the aetiological agent of feline viral rhinotracheitis, which accounts for approximately 50 % of all viral upper respiratory diseases in cats. Commercially available modified live vaccines containing FHV-1 are generally safe and effective, but these FHV-1 vaccines retain full virulence genes and can establish latency and reactivate to cause infectious rhinotracheitis in vaccine recipients, raising safety concerns. To address this shortcoming, we constructed a novel TK/gI/gE -gene-deleted recombinant FHV-1 (WH2020-ΔTK/gI/gE) through CRISPR/Cas9-mediated homologous recombination. The growth kinetics of WH2020-ΔTK/gI/gE were slightly delayed compared to those of the parent strain WH2020. Recombinant FHV-1 had severely impaired pathogenicity in cats. Felines immunized with WH2020-ΔTK/gI/gE produced high levels of gB-specific antibodies, neutralizing antibodies and IFN-β. Additionally, WH2020-ΔTK/gI/gE provided greater protection against challenge with FHV-1 field strain WH2020 than did the commercial modified live vaccine. After challenge, the cats vaccinated with WH2020-ΔTK/gI/gE showed significantly fewer clinical signs, pathological changes, viral shedding, and viral loads in the lung and trigeminal ganglia than those vaccinated with the commercial vaccine or unvaccinated. Our results suggest that WH2020-ΔTK/gI/gE is a promising candidate as a safer and more efficacious live FHV-1 vaccine, with a decreased risk of vaccine-related complications, and could inform the design of other herpesvirus vaccines.