Attenuation of a contemporary Chinese PRRSV-1 strain via serial cell passage and evaluation of its protective efficacy as a modified live virus vaccine candidate.

Porcine Reproductive and Respiratory Syndrome Virus type 1 (PRRSV-1) imposes substantial economic burdens on global swine production. In China, where only subtype 1 strains circulate across 23 provinces, evolving subpopulations challenge existing control measures. The absence of licensed PRRSV-1 vaccines in China underscores the urgency of developing tailored vaccine candidates against circulating strains. This study attenuated a contemporary Chinese PRRSV-1 isolate (GXFS20220129) through 100 serial passages (P100) on MARC-145 cells and evaluated its efficacy as a modified live virus (MLV) candidate. In vitro characterization revealed enhanced growth kinetics and increased plaque morphology in P100. Whole-genome sequencing identified 37 nucleotide mutations and 19 amino acid substitutions in P100, concentrated in the nsp1-3 and GP2 regions, potentially associated with attenuation. Immunogenicity and protection were assessed in piglets. The P100-immunized group exhibited no clinical signs post-challenge, maintained weight gain equivalent to blank controls, and demonstrated robust seroconversion by 14 dpi. The area under the curve (AUC) analyses (28-42 dpi) showed significantly lower nasal-shedding and higher antibody responses in vaccinated pigs, whereas cumulative viremia and rectal-shedding AUCs were not significantly different between challenged groups. Vaccination attenuated viremia, accelerated viral clearance, significantly reduced viral loads in systemic tissues (heart, liver, lung, kidney) and intestinal segments (duodenum, ileum, rectum), and altered shedding kinetics. Gross and histopathological lesions in lungs and intestines were substantially mitigated in immunized animals. Collectively, serial passage generated an attenuated PRRSV-1 strain conferring comprehensive protection against homologous challenge, supporting its potential as an MLV candidate for PRRSV-1 control in China.