Search

The chimeric flavivirus LGT/DEN4 containing prM and E genes of naturally attenuated Langat virus with remaining sequence derived from low neuroinvasive Dengue 4 virus was previously produced and assessed as a candidate for live vaccine against tick-borne encephalitis (TBE) [Pletnev and Men (1998): Proc Natl Acad Sci USA 95:1746-1751; Pletnev et al. (2000): Virology 274:23-31; Pletnev et al. (2001): J Virol 75:8259-8267; Wright et al. (2008): Vaccine 26:882-890]. In this article we compared two animal species: mice and monkeys, in order to select most sensitive models for safety evaluation of new vaccine candidates against TBE. Direct neurovirulence in suckling mice, neuroinvasiveness upon peripheral inoculation, rate of virus multiplication and expansion in CNS and its ability to persist in the central nervous system (CNS) were studied in adult mice; virological and pathomorphological examination of the CNS and visceral organs after intrathalamic virus inoculation was selected as a safety neurovirulence test in monkeys. The chimera was substantially less virulent in both animal models compared to the Absettarov strain of TBE virus. LGT/DEN4 was highly attenuated in suckling and adult mice with no evidence of viral persistence in CNS. In contrast to the mouse model, the chimera was able to reproduce in the CNS of monkeys to moderate titers, caused pathomorphological lesions in two and even illness in one of four animals, and was registered in simian brain on the 30th day post-infection. The presented data show that tests in mice solely might not be a sufficient model for safety testing of chimeric viruses.

Tick-borne encephalitis virus (TBEV) is one of the most prevalent and medically important tick-borne arboviruses in Eurasia. There are overlapping foci of two flaviviruses: TBEV and Omsk hemorrhagic fever virus (OHFV) in Russia. Inactivated vaccines exist only against TBE. There are no antiviral drugs for treatment of both diseases. Optimal animal models are necessary to study efficacy of novel vaccines and treatment preparations against TBE and relative flaviviruses. The models for TBE and OHF using subcutaneous inoculation were tested in Cercopithecus aethiops and Macaca fascicularis monkeys with or without prior immunization with inactivated TBE vaccine. No visible clinical signs or severe pathomorphological lesions were observed in any monkey infected with TBEV or OHFV. C. aethiops challenged with OHFV showed massive hemolytic syndrome and thrombocytopenia. Infectious virus or viral RNA was revealed in visceral organs and CNS of C. aethiops infected with both viruses; however, viremia was low. Inactivated TBE vaccines induced high antibody titers against both viruses and expressed booster after challenge. The protective efficacy against TBE was shown by the absence of virus in spleen, lymph nodes and CNS of immunized animals after challenge. Despite the absence of expressed hemolytic syndrome in immunized C. aethiops TBE vaccine did not prevent the reproduction of OHFV in CNS and visceral organs. Subcutaneous inoculation of M. fascicularis with two TBEV strains led to a febrile disease with well expressed viremia, fever, and virus reproduction in spleen, lymph nodes and CNS. The optimal terms for estimation of the viral titers in CNS were defined as 8–16 days post infection. We characterized two animal models similar to humans in their susceptibility to tick-borne flaviviruses and found the most optimal scheme for evaluation of efficacy of preventive and therapeutic preparations. We also identified M. fascicularis to be more susceptible to TBEV than C. aethiops.