Recombinant pseudorabies virus carrying a Plasmodium gene: herpesvirus as a new live viral vector for inducing T- and B-cell immunity

Abstract In Balb/c mice, the sterile protective immunity induced by immunization with radiation-attenuated Plasmodium yoelii sporozoites is eliminated by in vivo depletion of CD8+ Tlymphocytes, suggesting that cytotoxic T lymphocytes (CTL) against malaria antigens expressed on infected hepatocytes are required for mediating this protective immunity. To produce a vaccine that would induce CTL against the P. yoelii circumsporozoite protein (CS), we constructed an attenuated pseudorabies virus (PRV) containing a gene encoding this protein. Balb/c mice that received three doses of 10 7 plaque-forming units (p.f.u.) of this vaccine intravenously at 3 week intervals developed high levels of antibodies to sporozoites (indirect fluorescent antibody titre = 4096) and CTL against a 16 amino acid epitope (SYVPSAEQILEFVKQI, amino acids 281–296) from the P. yoelii CS protein designated PYCTLI. The cytotoxic activity of the CTL was antigen-specific, MHC-restricted, and dependent on CD8 + T cells. Furthermore, these CTL eliminated P. yoelii -infected hepatocytes from in vitro culture, indicating that they recognize this peptide on the surface of infected hepatocytes. However, all nine mice that were challenged with 200 sporozoites developed a blood-stage malaria infection. We attribute this lack of protection to the great difficulty of inducing sterile immunity against this highly infectious parasite P. yoelii . We conclude that recombinant pseudorabies virus (PRV) worked successfully as a live vaccine vector to induce both antibodies and CTL, albeit non-protective in vivo , and the herpesviruses should be considered as carriers for subunit vaccines where T- and B-cell immunity is required.