Analysis of protective immunity against PRV infection in pigs using attenuated and inactivated PRV vaccines

Knowledge on the immune mechanisms of protective immunity against Pseudorabies virus (PRV) infections in pigs is essential for the development of new, biologically safe vaccines. To evaluate the role of different immune mechanisms in relation to protection, we studied the humoral and cell-mediated immune responses in pigs after vaccination with attenuated or inactivated PRV vaccines and after challenge infection with wild type PRV. In addition, we also studied the role of the adjuvant component in the induction of protective immunity. Groups of 10-12 week old SPF pigs were intramuscularly vaccinated twice, with an interval of 4 weeks. All vaccines were based on the Bartha strain. Group 1 received the modified live vaccine in o/w adjuvant (MLV +A), group 2 the modified live vaccine without adjuvant (MLV), group 3 the inactivated vaccine with o/w adjuvant (IV +A) and group 4 the inactivated vaccine without adjuvant (IV). Group 5 was sham-vaccinated with PBS. Six weeks after the second vaccination, all pigs were intranasally challenged with $10^5$ plaque forming units of the virulent wild type PRV strain NIA3 per animal to assess the protection. The animals were screened for humoral and cellular immune responses, including Facscan analysis of the proliferating cell compartment at regular intervals after vaccination and challenge infection. Virus excretion, body weight, fever and clinical signs were assessed after challenge infection. Pigs vaccinated with MLV +A, MLV or IV +A induced high levels of neutralising antibodies in pigs whereas pigs vaccinated with IV developed significantly lower levels of VN antibody titers. Pigs vaccinated with MLV +A developed the strongest lymphoproliferation (LPT) responses followed by pigs vaccinated with MLV. In contrast, pigs vaccinated with inactivated vaccines developed weak LPT (IV +A) or very weak (IV) LPT responses. The analysis of the proliferating cell compartment showed that MLV +A vaccinated pigs had significantly more CD4$^+$/CD8$^{\rm +low}$/CD6$^+$ blast cells than the other groups of pigs, and significantly more CD4$^-$/CD8$^{\rm +high}$/CD6$^+$ blast cells than the IV +A, IV and sham vaccinated pigs. After challenge infection, MLV +A vaccinated pigs were protected the best followed by MLV vaccinated pigs, as illustrated by the reduction of virus excretion. Although pre-challenge VN antibody levels of IV +A vaccinated pigs were comparable with antibody levels of MLV (+ or -A) vaccinated pigs, IV +A pigs were less protected during the first week after challenge infection. Moreover, although antibody levels between pigs vaccinated with IV +A and IV differed significantly, both groups of pigs were equally poorly protected, confirming the lack of correlation between VN antibody titres and early reduction in virus excretion. After challenge infection, pigs vaccinated with MLV+A showed strongest LPT responses with high numbers of both CD4$^+$/CD8$^{\rm +low}$/CD6$^+$ and CD4$^-$/CD8$^{\rm +high}$/CD6$^+$ blast cells at day 3 post-challenge. Pigs vaccinated with MLV showed intermediate LPT responses with high numbers of CD4$^-$/CD8$^{\rm +high}$/CD6$^+$ blast cells whereas pigs vaccinated with IV+A showed weak LPT responses with high numbers of CD4$^+$/CD8$^{\rm +low}$/CD6$^+$ blast cells. These data indicate that protection and early reduction in virus excretion is not related to VN antibody titers but to the strength of LPT responses, further characterised by high numbers of both CD4$^+$/CD8$^{\rm +low}$/CD6$^+$ and CD4$^-$/CD8$^{\rm +high}$/CD6$^+$ cell subsets. The use of the o/w adjuvant led to stronger antibody and stronger LPT responses after vaccination with MLV and IV with an increase of the CD4$^+$/CD8$^{\rm +low}$/CD6$^+$ blast cell population