Comparison of live and inactivated tick-borne encephalitis virus vaccines for safety, immunogenicity and efficacy in rhesus monkeys

Three antigenic chimeric live attenuated tick-borne encephalitis virus (TBEV) vaccine candidates were compared for level of replication in murine and human neuroblastoma cells, for neurovirulence and neuroinvasiveness in mice, and for safety, immunogenicity and efficacy in rhesus monkeys. Two chimeric viruses were generated by replacing the membrane precursor and envelope protein genes of dengue type 4 virus (DEN4) with the corresponding genes of a Far Eastern TBEV, Sofjin strain, in the presence (TBEV/DEN4Δ30) or absence (TBEV/DEN4) of a 30 nucleotide deletion (Δ30) in the 3′ noncoding region of the DEN4 part of the chimeric genome. A third chimeric TBEV vaccine candidate was based on the antigenically distant, but naturally attenuated Langat virus (LGT). Chimerization of LGT with DEN4 resulted in decreased neurovirulence and neuroinvasiveness in mice and highly restricted viremia in rhesus monkeys. Also, the LGT/DEN4 chimera was highly restricted in replication in both murine and human neuroblastoma cells. In contrast, TBEV/DEN4 and TBEV/DEN4Δ30 were neither attenuated for neurovirulence in the mice nor restricted in replication in the neuroblastoma cells. However, both were highly attenuated for neuroinvasiveness in mice. TBEV/DEN4 replicated to moderately high titer in rhesus monkeys (mean peak viremia = 103.1 PFU/ml) indicating that the TBEV/DEN4 chimerization had only a modest, if any, attenuating effect in monkeys. However, the addition of the Δ30 mutation to TBEV/DEN4 greatly attenuated the chimeric virus for rhesus monkeys (mean peak viremia = 100.7 PFU/ml) and induced a higher level of antibody against the TBEV than did LGT/DEN4. A single dose of either highly attenuated TBEV/DEN4Δ30 or LGT/DEN4 vaccine candidate or three doses of an inactivated TBEV vaccine were efficacious in monkeys against wild-type LGT challenge. These results indicate that both TBEV/DEN4Δ30 and LGT/DEN4 are safe and efficacious in rhesus monkeys and should be further evaluated as vaccine candidates for use in humans.