Recombinant Porcine IFN-γ Potentiates the Secondary IgG and IgA Responses to an Inactivated Suid Herpesvirus-1 Vaccine and Reduces Postchallenge Weight Loss and Fever in Pigs

The effect of recombinant porcine interferon-γ (IFN-γ) on the immunogenicity in vivo of inactivated suid herpesvirus-1 (SHV-1, Phylaxia strain) was studied applying two successive i.m. immunizations. The animals were injected with inactivated virus alone or inactivated virus supplemented with 10⁴ or 10⁶ U IFN-γ. After the first immunization, none of the animals responded with measurable virus-neutralizing antibody (VNAb), virus-specific IgG or IgA. Following a second immunization 4 weeks later, a significantly increased VNAb response was noted in animals that had received vaccine doses containing 10⁴ U IFN-γ (p < 0.05). These animals also had significantly augmented serum levels of IgG (p < 0.01) and IgA (p < 0.05). Inclusion of 10⁶ U IFN-γ in the vaccine preparation did not affect the antibody response. In one experiment, the pigs were challenged oronasally with 10⁵ TCID₅₀ of the 75V19 strain of SHV-1, 7 weeks after administration of the second vaccine dose. Those that had received 10⁴ U IFN-γ in the vaccination developed less fever during the postchallenge period (p < 0.004). In all challenged pigs, growth performance was compromised during the first week after challenge. However, the only animals retaining an average net increase in body mass were those covaccinated with 10⁴ U IFN-γ (p < 0.05). Nasal excretion of virus was not significantly different between groups that had been vaccinated with or without IFN-γ. Multiple linear regression analysis of variables from individual vaccinated animals revealed the VNAb response to be correlated with serum IgG levels (p < 0.025) and with postchallenge growth performance (p < 0.0001) but not with serum IgA levels (p > 0.5). On the other hand, serum IgA appeared to be inversely correlated with early nasal virus excretion after challenge (p < 0.006). Taken together, our data suggest that addition of IFN-γ to inactivated SHV-1 vaccine may be a useful tool for enhancement of both mucosal and systemic immune responses in pigs.