Vaccine Efficacy of a Replication-Competent Interferon-Expressing Porcine Reproductive and Respiratory Syndrome (PRRS) Virus against NADC-34 Challenge

Porcine reproductive and respiratory syndrome virus (PRRSV) poses significant challenges to swine production due to its rapid generation of genetic variants and its ability to suppress antiviral interferon (IFN) responses, leading to ineffective immunity. To address this, we developed a replication-competent PRRSV modified live vaccine (MLV) candidate, IFNmix, engineered to co-express three type I IFN subclasses (IFNα, IFNβ, IFNδ) to enhance antiviral immunity. In two independent experiments, we compared IFNmix to two commercial PRRSV MLV vaccines. Pigs vaccinated with IFNmix exhibited similar anti-PRRSV antibody development, serum viral loads, lung lesions, and cytokine responses post-challenge with the virulent NADC34 strain. IFNmix induced comparable or lower body temperatures and weight gain patterns relative to commercial vaccines. While IFNmix showed viral load reduction compared to a commercial vaccine especially during the early phase (Day 7-14 post challenges), it demonstrated similar efficacy in controlling PRRSV replication and lung pathology. These findings suggest that IFNmix, by expressing multiple IFNs, can potentially enhance innate and adaptive immune responses, offering a promising approach to improving PRRSV vaccine efficacy. Further studies are needed to evaluate IFNmix against a broader range of PRRSV strains and to optimize its attenuation and immunogenicity.