Research note: A novel double-gene deleted vaccine against hypervirulent MDV variant generated by CRISPR/Cas9-based gene editing.

Marek's disease (MD), an important avian immunosuppressive and neoplastic diseases, has resulted in huge economic losses for the poultry industry worldwide. Over the past 50 years, the long-term prevalence of this disease and the growing immune pressure of MD vaccination have triggered persistently increased virulence. The emerging hypervirulent variant of MDV (HV-MDV) overcomes the protection conferred by commercial vaccines, highlighting the urgent need to develop novel, highly efficient MD vaccines. To address this challenge, using CRISPR/Cas9-based gene editing, we generated a novel candidate vaccine, SQ01ΔmeqΔLAT, which features double deletions of both oncogene meq and LAT-clustered miRNAs. Comprehensive experiments confirmed the gene deletions, genetic stability and retention of the replication kinetics of the mutant, without revealing any adverse impact on the expression of essential viral genes. Further animal experiments showed no histopathological lesions or neoplastic changes in SQ01ΔmeqΔLAT-challenged birds, demonstrating favourable safety to chicken hosts. More importantly, the SQ01ΔmeqΔLAT vaccine achieved a high protection index (PI) of 88.9% against HV-MDV strain HNSQ01; this was an improvement compared to CVI988, which provided a PI value of only 70.6%. Our data provide an important basis for the development of highly efficient MD vaccines and shed new light on the design of herpesvirus vaccines, simultaneously targeting both viral protein-coding genes and non-coding RNAs.