In recent years, the L1 PRRSV has emerged as the predominant prevalent strain in numerous pig-raising countries. In 2017, the L1.5 PRRSV (NADC34-like) emerged in China and has become one of the dominant strains in China since 2022. In this study, the subclinical NADC34-like PRRSV HLJ strain was passaged in vitro, and the passaged viruses adapted to Marc-145 cells were obtained. The results indicated that the HLJ strain exhibited a high degree of adaptation to Marc-145 cells after 60 passages, and the virus titer remained stable after 80 passages. Subsequently, 32 four-week-old PRRSV-negative pigs were selected and divided into 7 groups. Groups A/E, B, and C were intramuscularly administered 2 ml of virus solution of HLJ-80, HLJ-100, and HLJ-150, respectively, each with a concentration of 1 × 105.0 TCID50/ml. After a 28-day period, Groups A/B/C/D and Groups E/F were challenged intramuscularly (2 ml) and intranasally (2 ml) with the NADC30-like PRRSV SD strain and HP-PRRSV HuN4 strain (1 × 105.0 TCID50/ml), respectively. The findings of the immunization and challenge experiment indicated that following immunization with different passages of the HLJ strain, no evident clinical symptoms were observed, and the viremia levels remained at a low level. As the number of passages increased, the immunogenicity decreased significantly. The HLJ-R strain (HLJ-80) offered effective clinical protection against both the L1 SD strain and L8 HuN4 strain. In conclusion, the NADC34-like PRRSV HLJ strain holds the potential to serve as a promising vaccine candidate against PRRS following serial passage and adaptation to Marc-145 cells.

Keywords: Broad-spectrum cross-protection effects; HLJ-R; Low viremia; NADC34-like PRRSV; Subclinical strain.