DISCUSSION
Lineage 1, especially NADC30-like PRRSV, has become the most prevalent
PPRSV lineage in North America and Asia (de Avellar and Markus, 1993;
Kawabori et al., 1983; Savary and Ferron, 1982; Sun et al., 2020). In
2013, a new PRRSV strain called NADC30-like PRRSV, which has a unique
131-aa deletion within its NSP2 protein, was isolated from diseased
piglets in China (Zhao et al., 2015; Zhou et al., 2015). This PRRSV
originated in the United States and has become one of the major endemic
strains in China since 2016 (Guo et al., 2019). The pathogenicity of
NADC30-like PRRSVs ranges from moderate (Sun et al., 2016; Wang et al.,
2018; Zhang et al., 2019a) to high (Chen et al., 2021; Liu et al.,
2017b; Zhao et al., 2015), and most of them are moderately pathogenic.
However, the current commercial vaccines, Ingelvac PRRS MLV/RespPRRS
MLV(Sun et al., 2018; Wei et al., 2019), CH-1a(Li et al., 2022),
HuN4-F112(Bai et al., 2016), JXA1-P80(Sun et al., 2018), R98(Zhang et
al., 2016), TJM-F92(Bai et al., 2016) and GDr180(Bai et al., 2016; Zhang
et al., 2019a), do not provide completely effective protection against
NADC30-like PRRSVs. Here, we selected the moderately pathogenic
NADC30-like PRRSV SD strain and described a newly developed lineage 1
PRRSV vaccine candidate, SD-R, which is efficacious in the prevention of
clinical infection caused by NADC30-like PRRSVs.
Unlike CH-1a or HP-PRRSV, NADC30-like PRRSVs have lower levels of
whole-genome similarity (Guo et al., 2019; Zhang et al., 2019a) and a
wider variety of recombination patterns (Yu et al., 2020; Zhang et al.,
2019a). Almost all NADC30-like PRRSVs are recombinant viruses (Yu et
al., 2020; Zhang et al., 2019a). Although recombination breakpoints are
relatively random, statistical analysis reveals that recombination
hotspots range from nucleotide positions of approximately 7,900 to 8,100
and 12,400 to 13,500 (Yu et al., 2020). We speculated that NADC30-like
PRRSVs with two recombination regions may be more stable and important
for viral survival. Therefore, we selected an NADC30-like PRRSV SD with
the above two recombination regions (7365-7661 in the NSP9 region and
12305-12773 in the GP2a-GP3 region) for passage in Marc-145 cells.
In the present study, SD-R, a genetically stable attenuated viral
strain, was obtained by serial passaging in Marc-145 cells with the
lineage 1 PRRSV SD strain. All amino acid mutations related to SD were
observed before the 125th passage. No nucleotide and
amino acid mutations were observed between the 125thand 150th passages. This indicated that the SD strain
at the 125th passage had adapted to the Marc-145 cells
and was subsequently stably passaged. The HP-PRRSV vaccines JXA1-R, TJM,
HuN4-F112 and GDr180 were obtained through passaging in Marc-145 cells
for 80, 92, 112 and 180 passages, respectively (Leng et al., 2012; Liu
et al., 2015; Tian et al., 2009; Yu et al., 2015). Because the
pathogenicity of SD is far lower than that of HP-PRRSV, we believe that
SD-R (the 125th passage in Marc-145 cells) is safer
for piglets than commercial HP-PRRSV vaccines. Furthermore, both
high-dose and repeated-dose tests based on SD-R were safe for all
piglets (data not shown).
In this study, piglets immunized with SD-R developed a rapid and
effective humoral response and were effectively protected against
NADC30-like PRRSV challenge. Indeed, PRRSV vaccines have poor
cross-protection effects (Bai et al., 2016; Yu et al., 2021). However,
surprisingly, SD-R could provide better cross-protection, even though
the genomic nucleotide similarity of SD and HLJWK108-1711 was only
89.9%. All the piglets immunized with SD-R and then challenged with SD
or HLJWK108-1711 survived without any major clinical signs at any point
in the experimental period. Piglets in the immunized and challenge
groups were healthier than those in the challenge control group based on
clinical signs, body temperature, body weight, viremia and viral loads
in tissues. Altogether, these results suggested that the SD-R candidate
vaccine is effective against infections caused by different NADC30-like
PRRSVs. However, the detailed molecular basis of cross-protection
induced by the SD-R vaccine and its attenuation mechanism remain
unclear. Furthermore, the cross-protection against other types of
PRRSVs, such as NADC34-like PRRSV, QYYZ-like PRRSV and HP-PRRSV, should
be studied further.
In conclusion, we developed the first attenuated lineage 1 PRRSV
candidate vaccine strain, SD-R. Furthermore, SD-R was sufficiently
attenuated and antigenic enough to confer protection against homologous
and heterologous NADC30-like PRRSV challenge.