PRRSV GP4 subunit vaccine combined with adenovirus heterologous prime-boost immunization strategy induced a significant immune response in mice.

Background: The porcine reproductive and respiratory syndrome virus (PRRSV) continues to cause widespread infections in the pig industry worldwide. Currently, multiple PRRSV vaccine candidates are in preclinical or clinical trials, and each has different advantages and limitations. Glycoprotein 4 (GP4) is rich in epitopes, which can induce the body to produce neutralizing antibodies, plays a vital role in causing the host immune response, and is a key target for PRRSV vaccine development. In this study, we developed a novel candidate vaccine immunization strategy combining a subunit vaccine with an adenovirus vector vaccine through prokaryotic and eukaryotic systems expressing GP4.

Results: In this study, predictive analysis of PRRSV GP4 antigen structures in two expressed modes, and the results showed good antigenicity. The PRRSV GP4 subunit vaccine, as well as the adenovirus vector-based vaccine, were successfully constructed. In the immunization experiment of mouse models, a heterologous primary-boost immunization strategy was implemented: primary immunization with the GP4 subunit vaccine, and boost immunization was followed by an adenovirus vector vaccine. The safety assessment revealed that all candidate vaccine groups demonstrated good safety profiles. With an indirect enzyme-linked immunosorbent assay (ELISA) and neutralizing antibodies, mice in the combined immunization group developed higher levels of PRRSV-specific antibodies with significantly higher neutralizing antibody titers than mice alone. IgG subtype analysis indicated that the proteome favors the Th2-type immune response, while the adenoviral group favors the Th1-type immune response. The secretion levels of cytokines IL-4, IFN-γ, and TNF-α were significantly higher in the serum of the combined immunization group than in the immune group alone. Moreover, the cellular immune response test results showed that the combined immune group significantly enhanced the splenic lymphocyte proliferation capacity, IFN-γ secretion level, and cytokine transcript level. These findings suggest that the heterologous primary-boost immunization strategy of the PRRSV GP4 subunit vaccine developed here, in combination with the adenovirus vaccine, successfully induced strong humoral and cellular immune responses in mice.

Conclusions: In this study, the PRRSV GP4 subunit and adenovirus vector vaccine were successfully constructed and induced high levels of PRRSV-specific neutralizing antibody and cellular immune responses in mouse models by a heterologous primary-boost immunization strategy. These results support the clinical development of the PRRSV vaccine and bring new hope for PRRSV prevention and control strategies in the swine industry.