A Subunit Vaccine Based on the VP2 Protein of Porcine Parvovirus 1 Induces a Strong Protective Effect in Pregnant Gilts

Abstract

Porcine parvovirus 1 (PPV1) is one of the most prevalent pathogens that can cause reproductive disorder in sows. The VP2 protein of PPV1 is the most important immunogenic protein that induces neutralizing antibodies and protective immunity. Thus, VP2 is considered an ideal target antigen for the development of a genetically engineered PPV1 vaccine. In this study, the baculovirus transfer vector carrying the HR5-P10-VP2 expression cassette was successfully constructed with the aim of increasing the expression levels of the VP2 protein. The VP2 protein was confirmed using SDS‒PAGE and Western blot analyses. Electronic microscope analysis showed that the recombinant VP2 proteins were capable of self-assembling into VLPs with a diameter of approximately 25 nm. The immunogenicity of the VP2 subunit vaccine was evaluated in pigs. The results showed that VP2 protein emulsified with ISA 201VG adjuvant induced higher levels of HI antibodies and neutralizing antibodies than VP2 protein emulsified with IMS 1313VG adjuvant. Furthermore, the gilts immunized with the ISA 201VG 20 μg subunit vaccine acquired complete protection against PPV1 HN2019 infection. In contrast, the commercial inactivated vaccine provided incomplete protection in gilts. Therefore, the VP2 subunit vaccine is a promising genetically engineered vaccine for the prevention and control of PPV1.

Keywords: VP2 protein; porcine parvovirus 1; protective immunity; subunit vaccine.

Figure 1

Schematic diagrams of the recombinant transfer vector and identification of recombinant VP2 protein. (A) Scheme of pFastBacHT-HR5-P10-VP2. (B) Scheme of pFastBacHT-P10-VP2. (C) SDS‒PAGE analysis of the recombinant VP2 protein. Lane 1: the culture supernatant of AC-P10-VP2; Lane 2: the culture supernatant of AC-HR5-P10-VP2. (D) Western blot analysis of the recombinant VP2 protein. Lane 1: the culture supernatant of AC-P10-VP2; Lane 2: the culture supernatant of AC-HR5-P10-VP2.

Figure 2

Analysis of the recombinant VP2 proteins through electron microscopy. The recombinant VP2 proteins were prepared by ultracentrifugation and negatively stained with 2% (wt/vol) aqueous uranyl acetate. (A) Scale bar indicates 100 nm. (B) Scale bar indicates 200 nm.

Figure 3

Detection of PPV1-specific antibody in the sera of the immunized pigs. (A) Serum HI antibody titers and (B) neutralization antibody titers in different groups. The sample size was five piglets. All data are expressed as the mean ± SEM. The data were analyzed by using one-way ANOVA to compare the difference among groups immunized with different vaccine at the same time. Different letters (a and b, c) indicate a statistically significant difference between different groups (p < 0.05).

Figure 4

Detection of HI antibody titers in gilts after vaccination and PPV1 challenge. The sample size was four gilts. All data are expressed as the mean ± SEM. The data were analyzed by using one-way ANOVA to compare the difference among groups immunized with different vaccine at the same time. Different letters (a, and b) indicate a statistically significant difference between different groups (p < 0.05).

Figure 5

Observation of clinical symptoms in gilts after PPV1 challenge. (A) ISA 201VG 20 μg VP2 subunit vaccine group. (B) Commercial inactivated vaccine. (C) Negative control group.

Figure 6

Detection of PPV1 DNA in the nasal swab (A) and blood (B) samples after challenge by real-time RT‒PCR. The samples were collected at 0, 3, 7, 14, 21, 35, and 60 dpc, and DNA was extracted using a DNA isolation kit (TIANGEN, Beijing, China). The virus content was determined using real-time RT‒PCR as described previously; the samples with Ct values of ≥37 were considered negative or undetectable; the samples with Ct values < 37 were considered positive. The missing panels indicated that the Ct number was not detected.