Efficacy of a porcine reproductive and respiratory syndrome virus 1 (PRRSV-1) natural recombinant against a heterologous PRRSV-1 isolate both clustered within the subgroup of BJEU06-1-like isolates

Porcine reproductive and respiratory syndrome virus 1 (PRRSV-1) has been prevalent in more than 20 provinces of China. However, no PRRSV-1-specific vaccine is commercially available in China. To evaluate the feasibility of using a low virulent PRRSV-1 isolate against potential outbreaks caused by virulent Chinese PRRSV-1 isolates, here we evaluated the efficacy of a low virulent PRRSV-1 HLJB1 strain isolated in 2014 as live vaccine against a virulent PRRSV-1 SD1291 strain isolated in 2022. Genome-based phylogenetic analysis showed that both HLJB1 and SD1291 were grouped within BJEU06-1-like isolates. However, they shared only 85.27% genomic similarity. Piglet inoculation and challenge study showed that HLJB1 inoculation could reduce viremia but did not significantly alleviate clinical signs and tissue lesions. Virus neutralization test indicated that HLJB1 inoculation could induce homologous neutralizing antibodies (NAbs) but no heterologous NAbs at 42 dpi. In addition, flow cytometric analyses showed that no memory T follicular helper (Tfh) cells against SD1291 and SD1291-specific IFN-? secreting cells were induced by HLJB1 pre-inoculation. These results supported that HLJB1 inoculation only provides partial cross-protection against SD1291 infection even though they are clustered within the same PRRSV-1 subgroup, which is closely related to the failure in conferring cross-protective adaptive immune responses.