Interferon Inducing Porcine Reproductive and Respiratory Syndrome Virus Vaccine Candidate Protected Piglets from HP-PRRSV Challenge and Evoke a Higher Level of Neutralizing Antibodies Response

Abstract

Although widespread administration of attenuated porcine reproductive and respiratory syndrome virus (PRRSV) vaccines has been implemented since they first became commercially available two decades ago, PRRSV infection prevalence in swine herds remains high. The limited success of PRRSV vaccines is partly due to the well-established fact that a given vaccine strain confers only partial or no protection against heterologous strains. In our past work, A2MC2-P90, a novel PRRSV vaccine candidate that induced a type I IFNs response in vitro, conferred complete protection against challenge with genetically heterologous PRRSV strains. Here we assessed the ability of the PRRSV vaccine candidate A2MC2-P90 to protect piglets against the HP-PRRSV challenge and compared its efficacy to that of a licensed HP-PRRSV-specific vaccine (TJM-F92) assessed in parallel. A2MC2-P90 provided vaccinated piglets with 100% protection from a lethal challenge with extremely virulent HP-PRRSV-XJA1, while 100% mortality was observed for unvaccinated piglets by day 21 post-challenge. Notably, comparison of partial sequence (GP5) of XJA1 to A2MC2-P90 suggested there was only 88.7% homology. When comparing post-HP-PRRSV challenge responses between piglets administered A2AMC2-P90 versus those immunized with licensed vaccine TJM-F92, A2MC2-P90-vaccinated piglets rapidly developed a stronger protective humoral immune response, as evidenced by much higher titers of neutralizing antibodies, more rapid clearance of viremia and less nasal virus shedding. In conclusion, our data suggest that this novel vaccine candidate A2MC2-P90 has improved protection spectrum against heterologous HP-PRRSV strains.

Keywords: HP-PRRSV; IFN induction; PRRSV; modified live vaccines; neutralizing antibodies; protection.

Figure 1

Schematic illustration of experimental protocol and ELISA examination of seroconversion prior to the porcine reproductive and respiratory syndrome virus (PRRSV) challenge. (A). After piglets were housed, A2MC2-P90 and TJM-F92 were used to vaccinate animals via the intramuscular route using 1 mL of virus stock (1.0 × 10⁶ TCID₅₀/mL). At 21 days post vaccination, serum samples were collected then at day 0 piglets were challenged with HP-PRRSV-XJ1. Blood and nasal swab samples were collected at indicated time points and all surviving animals were necropsied at 21 dpc. (B). After immunization of piglets with A2MC2-P90 and TJM-F92, serum samples were collected 21 days later and examined to detect seroconversion using an IDEXX HerdChek PRRS X3 ELISA kit. All experiments were repeated at least three times for each serum sample. Data are expressed as the mean ± SD and were subjected to Student’s t-test. No significant differences (NS) were observed between groups immunized with A2MC2-P90 or TJM-F92.

Figure 2

PRRSV-A2MC2-P90 protection of piglets against a lethal challenge with highly pathogenetic PRRSV (HP-PRRSV). (A). All vaccinated animals or non-vaccinated controls were each inoculated with 1.0 × 10⁵ TCID₅₀ of HP-PRRSV-XJ1 via both intramuscular and intranasal routes. Clinical signs and survival rates were monitored and calculated daily for a total of 21 days. (B). After inoculation of animals, rectal temperature was recorded daily for surviving animals of all groups. Data are expressed as mean ± SD and were subjected to a Student’s t-test. Significant differences between the A2MC2-P90-vaccinated group (A2P90/HP-PRRSV), TJM F92-vaccinated group (TJM-92/HP-PRRSV) and the non-vaccinated but the HP-PRRSV challenged group (MOCK/HP-PRRSV) are marked with * (p < 0.05), or ** (p < 0.01) or *** (p < 0.001).

Figure 3

PRRSV-A2MC2-P90 vaccination prevented development of pathological lung lesions after HP-PRRSV challenge in vivo. (A). A total of 16 piglets were randomly divided into 4 groups (n = 4) that included the negative control group, A2MC2-P90-vaccinated group (A2P90/HP-PRRSV), TJM F92-vaccinated group (TJM-92/HP-PRRSV) and non-vaccinated HP-PRRSV-challenged group (MOCK/HP-PRRSV). Representative ventral and dorsal lung images from each group were captured immediately after piglets were autopsied at 21 dpc or at time of death if before 21 dpc. (B). Gross pathological changes of all animals in each group were quantified using a scoring system based on a 100-point scale. Data are expressed as the mean ± SD and were subjected to a Student’s t-test. Significant differences between indicated groups were marked with *** (p < 0.001) or NS (not significant).

Figure 4

PRRSV-A2MC2-P90 vaccination reduced nasal shedding of virus and viremia after HP-PRRSV challenge of vaccinated piglets. (A). Nasal swab samples collected at indicated time points and harvested using TRIzol reagent were subjected to PRRSV RNA detection using IDEXX RealPCR PRRSV-2 RNA Mix. The Ct value of each sample is presented for comparison and was based on the manufacturer cut-off Ct value of 38. (B). Serum samples collected from each piglet at indicated time points and processed using TRIzol reagent were subjected to PRRSV RNA detection using IDEXX RealPCR PRRSV-2 RNA Mix. The Ct value of each sample is presented for comparison and was determined based on the manufacturer cut-off the Ct value of 38.

Figure 5

PRRSV-A2MC2-P90 vaccination evoked a higher titer of neutralizing antibodies in vaccinated piglets that correlated with reduced virus shedding and viremia. (A). Serum samples from piglets in the PRRSV-A2MC2-P90-vaccinated group (A2P90/HP-PRRSV) or TJM F92-vaccinated group (TJM-92/HP-PRRSV) were collected at indicated dpc after HP-PRRSV challenge. Sera were further tested via neutralizing assays using 2-fold serial dilutions to evaluate virus neutralizing activity against HP-PRRSV infection of MARC-145 cells. Data are expressed as the mean ± SD and were subjected to a Student’s t-test. Significant differences of NAbs titers between A2P90/HP-PRRSV and TJM-92/HP-PRRSV at indicated dpc (except 3 dpc) were marked with * (p < 0.05), or ** (p < 0.01) or NS (not significant). (B). Nasal swabs and serum samples from both vaccination groups collected at 21 dpc were processed using TRIzol reagent and subjected to PRRSV RNA detection using IDEXX RealPCR PRRSV-2 RNA Mix. The Ct value of each sample is presented as the mean ± SD and all data were subjected to a Student’s t-test. Significant differences of Ct values between the A2MC2-P90-vaccinated group (A2P90/HP-PRRSV) and TJM F92-vaccinated group (TJM-92/HP-PRRSV) are marked with * (p < 0.05).