A critical determinant of neurological disease associated with highly pathogenic tick-borne flavivirus in mice.

UNLABELLED Tick-borne encephalitis virus (TBEV) and Omsk hemorrhagic fever virus (OHFV) are highly pathogenic tick-borne flaviviruses; TBEV causes neurological disease in humans, while OHFV causes a disease typically identified with hemorrhagic fever. Although TBEV and OHFV are closely related genetically, the viral determinants responsible for these distinct disease phenotypes have not been identified. In this study, chimeric viruses incorporating components of TBEV and OHFV were generated using infectious clone technology, and their pathological characteristics were analyzed in a mouse model to identify virus-specific determinants of disease. We found that only four amino acids near the C terminus of the NS5 protein were primarily responsible for the development of neurological disease. Mutation of these four amino acids had no effect on viral replication or histopathological features, including inflammatory responses, in mice. These findings suggest a critical role for NS5 in stimulating neuronal dysfunction and degeneration following TBEV infection and provide new insights into the molecular mechanisms underlying the pathogenesis of tick-borne flaviviruses. IMPORTANCE Tick-borne encephalitis virus (TBEV) and Omsk hemorrhagic fever virus (OHFV) belong to the tick-borne encephalitis serocomplex, genus Flavivirus, family Flaviviridae. Although TBEV causes neurological disease in humans while OHFV causes a disease typically identified with hemorrhagic fever. In this study, we investigated the viral determinants responsible for the different disease phenotypes using reverse genetics technology. We identified a cluster of only four amino acids in nonstructural protein 5 primarily involved in the development of neurological disease in a mouse model. Moreover, the effect of these four amino acids was independent of viral replication property and did not affect the formation of virus-induced lesions in the brain directly. These data suggest that these amino acids may be involved in the induction of neuronal dysfunction and degeneration in virus-infected neurons, ultimately leading to the neurological disease phenotype. These findings provide new insight into the molecular mechanisms of tick-borne flavivirus pathogenesis.