Deletion of gene 52 encoding glycoprotein M of equine herpesvirus type 1 strain RacH results in increased immunogenicity

The immunogenicity of equine herpesvirus type 1 (EHV-1) strain RacH was compared to a RacH virus in which gene 52 encoding glycoprotein M (gM) was interrupted by insertion of LacZ (HΔgM-Ins) and a RacH with 75% of gene 52 was deleted and replaced by LacZ (HΔgM-HS). HΔgM-Ins failed to produce full-length gM, but the carboxy-terminal portion was still expressed. No gM expression was detected in HΔgM-HS-infected cells. Mice were immunised once with 1×103 to 1×105 plaque-forming units (PFU) of RacH or mutant viruses and challenged with virulent RacL11 virus 29 days later. A dose-dependence of protection was observed in RacH-immunised mice, and following immunisation with 1×104 or 1×103 PFU body weight losses and increased virus titres in lungs were observed after challenge infection. HΔgM-HS-immunised mice were completely protected even after immunisation with 1×103 PFU. Mice immunised with 1×103 PFU of HΔgM-Ins but not the higher doses showed signs of disease after challenge infection.