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

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+ T lymphocytes, 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 PYCTL1. 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 subunit vaccines where T- and B-cell immunity is required.