A lineage 1 branch porcine reproductive and respiratory syndrome virus live vaccine candidate provides broad cross-protection against HP-like PRRSV in piglets

Abstract

Multiple porcine reproductive and respiratory syndrome virus (PRRSV) subtypes coinfect numerous pig farms in China, and commercial PRRSV vaccines offer limited cross-protection against heterologous strains. Our previous research confirmed that a PRRSV lineage 1 branch attenuated live vaccine (SD-R) provides cross-protection against HP-PRRSV, NADC30-like PRRSV and NADC34-like PRRSV. HP-PRRSV has undergone significant genetic variation following nearly two decades of evolution and has transformed into a subtype referred to as HP-like PRRSV, which also exhibits high pathogenicity. The effectiveness of immunising piglets with the SD-R strain to provide protection against infection with HP-like PRRSV remains uncertain. In the present study, we evaluated the protective effects of SD-R vaccine strains on DLF-challenged piglets. The results revealed that piglets challenged with DLF presented clinical symptoms such as continuous high fever and an obvious decrease in daily weight gain. Importantly, the piglets immunised with SD-R exhibited notable reductions in pathological damage, especially of decreases in DLF-induced thymic atrophy. Moreover, the serum of SD-R-immunised piglets strongly neutralised DLF, and the number of SD-R-vaccinated piglets demonstrating viraemia was greatly reduced. These results suggest that the PRRSV lineage 1 branch live vaccine candidate provides broad cross-protection against HP-like PRRSV in piglets.

Keywords: HP-like PRRSV; Lineage 1 branch; cross-protective efficacy; live vaccine.

Figure 1.

Phylogenetic analyses and inferred NSP2-encoded amino acid alignment of the SD-R and DLF strains. (a) Phylogenetic tree constructed on the basis of the PRRSV ORF5 sequences. The strains that were tested for SD-R immunoprotection are labelled with black stars, and the SD-R and DLF strains used in this study are labelled with yellow stars. (b) NSP2 deletion patterns in SD-R and DLF. The 131-aa discontinuous deletions are highlighted with a light yellow background, whereas the 30-aa discontinuous deletions are marked with a light blue background.

Figure 2.

Study design, rectal temperatures, average daily body weights, anti-prrsv antibody levels and serum neutralisation titres. (a) Study design (as described in the materials and methods section). (b) Rectal temperatures after SD-R immunisations and challenge with the DLF strain. (c) Average daily body weights after SD-R immunisations and challenge with the DLF strain. The means ± SDs (error bars) of body weight gains are shown. *, p < 0.05; ns, p > 0.05, no significant difference. (d) Serum neutralisation titres of the SD-R strain in six different groups. (e) Serum neutralisation titres of the DLF strain in six different groups. A serum neutralisation test was performed after serum collection from the three groups at 28 dpv and 21 dpi. The serum neutralisation titres were observed and statistically analysed at 3 d and 7 d after the cells were inoculated. A titre of ≥ 1:4 was considered positive. (f) Anti-prrsv antibody levels after SD-R immunisation and challenge with the DLF strain. The threshold for seroconversion was set at a sample-to-positive (S/P) ratio of 0.4. The bars represent the average S/Ps from one group of piglets. The means ± SDs (error bars) of the specific antibodies are shown.

Figure 3.

Gross and histological lesions of lungs and lymph nodes in piglets from each group after SD-R immunisation and challenge with the HP-like PRRSV DLF strain. Compared with the control piglets, the piglets infected with DLF presented varying degrees of thymic atrophy, whereas the piglets in the SD-R vaccine-treated and DLF-challenged groups did not exhibit thymic atrophy (a-c). Compared with the control group and the SD-R vaccine-treated and DLF-challenged groups(d, f, j, l), the DLF infection group presented severe interstitial pneumonia with lung consolidation (e) and lymph node haemorrhage (k) . Compared with those in the control group, different severities of pulmonary interstitial pneumonia were observed in the challenge group piglets, who presented extensive inflammatory cell infiltration, alveolar epithelial hyperplasia, and widening of the alveolar diaphragm, whereas piglets in the SD-R vaccine-treated and DLF-challenged groups presented only small amounts of inflammatory cell infiltration (g-i). Compared with the control group and the SD-R vaccine-treated and DLF-challenged group, the challenged group presented greater infiltration of eosinophils in the mandibular lymph nodes, along with scattered minor haemorrhage (m-o).

Figure 4.

Viral loads and distribution in tissues. (a) Serum viral loads on different collection dates. (b) Viral loads in different tissues. The quantification of PRRSV viral RNA in sera and tissues was performed via qPCR. The asterisk (*) indicates significant differences between the immunised and challenged groups compared with the dlf-challenged group (ns, p > 0.05; * p < 0.05; ** p < 0.01; *** p < 0.001). The numbers represent the number of piglets carrying the virus and the total number of piglets in each group. The three groups are labelled with green, purple, and orange colours, respectively.