Porcine reproductive and respiratory syndrome virus (PRRSV) infection is one of the most important swine pathogens causing significant economic impact on swine industry worldwide. Infection with PRRSV causes reproductive failure in breeding herds, respiratory disorders and predisposes to secondary infections, contributing to porcine respiratory disease complex (PRDC) in fattening pigs (Lunney et al., 2016). Several reports suggested that PRRSV-induced negative immunomodulatory effects resulted in poor anti-viral immune responses, and immunosuppression in infected pigs (Cecere et al., 2012; Kimman et al., 2009; Lopez and Osorio, 2004; Mateu and Diaz, 2008; Thanawongnuwech et al., 2000; Van Reeth and Nauwynck, 2000). Thus, controlling PRRSV infection is essential for enhancing pig growth performance and economic profit. Currently, commercially available PRRSV vaccines, both modified live PRRS vaccines (MLV) and killed vaccines (KV), have been licensed (Charerntantanakul, 2012; Kimman et al., 2009). However, these vaccines do not provide complete protection against heterologous infections. In addition, MLV also induce negative immunomodulatory effects similar to the natural infection (LeRoith et al., 2011; Suradhat et al., 2016; Thanawongnuwech and Suradhat, 2010).
Recently, a new variant type 2 PRRSV, known as highly pathogenic PRRSV (HP-PRRSV), emerged in China, causing severe clinical outcomes and high mortality in infected pigs (Tian et al., 2007). Since the initial outbreak, HP-PRRSV rapidly spread to other countries and became the ap4 virus circulating in the region, including Thailand (Jantafong et al., 2015; Nilubol et al., 2012). Previous studies indicated that HP-PRRSV derived MLV provided full protection against the HP-PRRSV infection (Leng et al., 2012; Yu et al., 2015). However, the vaccines were not licensed in other countries. The current commercially available MLV only provided partial protection against HP-PRRSV challenges (Do et al., 2015; Lager et al., 2014). Recently, a novel DNA vaccine (pORF7t) has been developed. The vaccine was designed to modulate PRRSV-specific immune responses by reducing PRRSV-induced immunomodulatory activities (Suradhat et al., 2015). Priming with the DNA vaccine could reduce MLV-induced negative immunomodulatory effects, both IL-10 and Treg, and enhance PRRSV-specific cell-mediated immunity in the immunized pigs (Suradhat et al., 2016). Thus, the heterologous DNA-MLV, prime-boost immunization may be useful for controlling PRRSV infection in heavily infected areas or areas with PRRSV circulation. We hypothesized that the DNA-MLV prime-boost immunization would enhance MLV-induced, PRRSV-specific immunity against the HP-PRRSV infection, leading to better disease protection.
Materials and methods
In this study, we hypothesized that the heterologous DNA-MLV prime-boost immunization would enhance the efficacy of the MLV against the HP-PRRSV strain. Our results demonstrated that the DNA vaccine could improve the quality of PRRSV-specific immunity in the pigs receiving DNA-MLV prime-boost immunization. However, priming with the DNA vaccine did not provide significant advantage over the MLV in clinical protection. As the DNA vaccine aimed to reduce PRRSV-induced negative immunomodulatory effects, the efficacy of the DNA-MLV immunization still mainly relied on the protective mechanisms induced by MLV. The findings that the current commercially available MLV provided only partial protection against the HP-PRRSV challenges are consistent with the previous reports (Do et al., 2015; Lager et al., 2014), suggesting the need for better vaccines against the HP-PRRSV.
PRRSV-specific IFN-γ secreting cells are believed to be the main protective mechanism against the conventional PRRSV infection (Meier et al., 2003; Park et al., 2014; Zuckermann et al., 2007). In this study, increased numbers of CD3+IFN-γ+ cells in the DNA-MLV group were observed both following MLV immunization and HP-PRRSV challenge, indicating that the broad immunomodulatory effects of the DNA vaccine on cellular immunity. Interestingly, priming with a DNA vaccine also enhanced the anti-viral cytokine gene, IFNA, expression following HP-PRRSV infection (Fig. 4a). The N protein of PRRSV has been reported as an IFN-antagonist (Huang et al., 2015, 2014; Lunney et al., 2016). It was possible that priming with the DNA vaccine resulted in enhanced immunity to N protein, which resulted in reduced IFN-antagonist activity following HP-PRRSV infection. Nonetheless, our results indicated that the well-primed cellular immunity was not sufficient to completely protect against HP-PRRSV infection.