Protective efficacy of a candidate attenuated live vaccine derived from an NADC30-like strain against homologous porcine reproductive and respiratory syndrome virus challenge.

Since its emergence in 2013, the NADC30-like variant of Porcine Reproductive and Respiratory Syndrome Virus type 2 (PRRSV-2) has become the predominant genotype circulating in China. Commercially available vaccines, developed from lineage 5 (L5) and lineage 8 (L8) strains, confer only limited cross-protection against these NADC30-like (lineage 1, L1) strains, presenting a substantial threat to swine production. In this study, the NADC30-like strain GXHX20211106 was serially passaged 100 times in MARC-145 cells. Whole-genome sequencing identified 82 nucleotide mutations in the attenuated strain (GXHX20211106-P100) relative to the parental virus, 41 of which resulted in non-synonymous amino acid substitutions. Cytopathic effect (CPE) and immunofluorescence assay (IFA) results demonstrated that increasing passage number correlated with expanded cytopathic areas and a marked enhancement in the expression level of the viral nucleocapsid (N) protein. Plaque assay and multi-step growth kinetics analysis indicated that the passaged virus exhibited significantly improved replicative fitness in vitro, suggesting enhanced adaptation to the host cell line. Animal challenge experiments demonstrated that immunization with the live-attenuated vaccine candidate GXHX20211106-P100 effectively elicited a systemic immune response, significantly reducing viremia, pulmonary viral load, and histopathological lesions following homologous challenge. However, the vaccine's efficacy in suppressing viral shedding at mucosal sites was limited. This study successfully generated an attenuated strain of NADC30-like PRRSV through cell culture adaptation and confirms its potential as a homologous vaccine candidate, thereby providing a critical foundation for the development of vaccines targeting emerging L1 branch strains.