Abstract
Maturing flaviviruses undergo pH-dependent conformational changes, transitioning from trimeric pre-membrane protein prM and envelope protein E (prM-E) complexes to dimeric E with cleaved prM. However, little is known about the intermediate maturation stages and whether immature virions contribute to infection. Here we demonstrated that immature prM-containing particles of tick-borne flaviviruses—tick-borne encephalitis virus (TBEV), Langat virus, and louping ill virus—exhibited infectivity in mammalian cells under physiological conditions. This finding challenges the long-standing assumption that prM-containing flavivirus particles are non-infectious, as is the case for immature mosquito-borne flaviviruses, including Usutu virus, Zika virus, and West Nile virus (WNV). In a mouse model, prM-TBEV exhibited high pathogenicity, contrasting with the reduced pathogenicity of prM-WNV. Structural modelling, functional validation, and site-directed mutagenesis revealed interactions within the prM-E complex that influence pH sensitivity, furin cleavage site accessibility, and virus infectivity. This study, therefore, elucidates the distinctive dynamics of infectivity of tick- and mosquito-borne flaviviruses.