Low-pathogenic virus induced immunity against TBEV protects mice from disease but not from virus entry into the CNS

Tick-borne encephalitis virus (TBEV) is a leading cause of vector-borne viral encephalitis with expanding endemic regions across Europe. Although currently used inactivated whole virus vaccines are effective, vaccination breakthroughs have been reported for which the reasons are unclear. In this study we tested in mice the efficacy of pre-infection with a closely related low-virulent flavivirus, Langat virus (LGTV strain TP21), or a naturally avirulent TBEV strain (TBEV-280) in providing protection against lethal infection with the highly virulent TBEV strain TBEV-Hypr (referred to as TBEV-Hypr). LGTV has been evaluated as an experimental live vaccine against TBE, but further development was abandoned due to too high residual pathogenicity of a LGTV-based vaccine. Here we show that prior infection with TP21 or TBEV-280 is efficient in protecting mice from lethal TBEV-Hypr challenge. Histopathological analysis of brains from non-immunized control mice revealed neuronal TBEV infection and necrosis. Neuroinflammation, gliosis and neuronal necrosis was however also observed in some of the TP21 and TBEV-280 pre-infected mice although at reduced frequency as compared to the non-immunized TBEV-Hypr infected control mice. Interestingly, qPCR detected the presence of viral RNA in the brains and spinal cord of both TP21 and TBEV-280 immunized mice after TBEV-Hypr challenge, but significantly reduced compared to mock-immunized mice. Our results indicate that although TBEV-Hypr infection is effectively controlled in the periphery upon immunization with low-virulent LGTV or naturally avirulent TBEV-280, it may still enter the CNS of these animals. These findings improve our understanding of potential causes for vaccine failure in individuals vaccinated with TBE vaccines.