The whole genome analysis of the wild-type and attenuated orf virus reveals that ORF022 facilitates viral replication.

Abstract

Background: Contagious ecthyma is an acute infectious zoonosis caused by orf virus (ORFV). Live-attenuated ORFV vaccines have played a crucial role in preventing contagious ecthyma for decades. However, these vaccines often fail to induce long-lasting immunity. In recent years, numerous ORFV genome sequences have been published, yet genomic data for attenuated strains remain limited. Furthermore, no comprehensive whole-genome-based single nucleotide polymorphisms (SNPs) analysis has been conducted to compare ORFV wild-type and attenuated strains.

Results: In this study, we performed whole-genome sequencing of ORFV wild-type and attenuated strains from Shaanxi Province. We identified two ORFV strains with genomes shorter than 130 kb, which are closely related to the SC1 attenuated strain from Sichuan Province. Additionally, we noticed that 24 genes in the attenuated strain had SNPs, with the highest number of mutations occurring in the ORF022 gene. The function of the ORF022 gene has not been previously reported. Through in vitro experiments, we demonstrated that overexpression of ORF022 enhances ORFV replication in cells. The RNA-sequencing analysis revealed that ORF022 modulates host inflammation-related signaling pathways, as evidenced by the suppression of TNF, IL-17, and Toll-like receptor signaling pathways.

Conclusions: Our findings suggest that the ORF022 in ORFV wild-type strain inhibits the host inflammatory response, reduces the immune response to ORFV, and facilitates viral replication. SNP events in attenuated strains (aFX0910) are one of the reasons for its attenuation. Investigations into the genomic sequences of attenuated viruses and the functional impact of mutated genes provide valuable insights into the mechanisms underlying ORFV attenuation and offer a foundation for the development of more effective ORFV vaccines.