Cross-protection from St. Louis encephalitis virus and Usutu virus disease by human West Nile virus convalescent plasma in mice

West Nile virus (WNV), Saint Louis encephalitis virus (SLEV), and Usutu virus (USUV) are emerging mosquito-borne flaviviruses. These viruses are phylogenetically closely related and belong to the Japanese encephalitis serocomplex group. Similar to other flaviviruses, these viruses are enveloped, with genomes comprising positive-sense, single-stranded RNA approximately 11 kb in length. Upon translation, a single polyprotein is produced, consisting of three structural and seven non-structural proteins. These proteins function in virus binding to the cell membrane, entry into cells, replication, immune evasion, and the production of new virus progeny. Typically, these viruses are maintained in a sylvatic cycle involving avian hosts, such as passerine birds, and mosquitoes. However, they can accidentally spill over to humans through mosquito bites or wildlife exposure. Although humans generally remain asymptomatic and do not support sufficient viral replication for transmission, they can develop febrile disease and, in some cases, severe neuroinvasive diseases, especially among the elderly or immunocompromised individuals. Due to their co-circulation in the same geographical areas and sharing similar hosts and vectors, individuals in Italy and Germany have been detected as seropositive for WNV and USUV, while seropositivity for WNV and SLEV has been observed in the Americas.

Viruses in the Japanese encephalitis virus serocomplex group exhibit significant antigenic similarity. The envelope protein alone contains 12 distinct epitopes and at least three highly conserved epitopes among the JEV serocomplex. Consequently, infection with one member of the JEV serocomplex group, such as WNV, induces WNV-specific antibodies and heterotypic antibodies that can cross-neutralize other members of the JEV serocomplex group, such as USUV and SLEV. Therefore, cross-reactive epitopes can protect against heterologous virus challenges to varying extents, depending on the accessibility of the antibodies to the epitopes. Prior infection with WNV or its envelope domain III (EDIII) or non-structural protein 1 (NS1) protected mice from lethal JEV challenges. Vaccination against WNV protected mice from lethal USUV challenges, and vice versa. Immunity to JEV or SLEV protected hamsters from lethal WNV challenges. Although human sera immune to WNV cross-neutralized USUV and SLEV in vitro during serodiagnosis, the actual mechanism of cross-protection among WNV, USUV, and SLEV remains poorly characterized.

Therefore, this study aims to understand the mechanism of cross-protection. Specifically, this research investigated whether human plasma immune to WNV could cross-protect mice from encephalitis caused by SLEV or USUV. Initially, WNV-specific human convalescent plasma and mouse WNV convalescent serum (as a positive control) neutralized WNV and cross-neutralized USUV and SLEV in vitro in a neutralization test. Subsequently, immunocompetent mice were intraperitoneally injected with human WNV convalescent plasma, human normal plasma, mouse WNV convalescent serum, or mouse normal serum the day before being challenged with WNV, SLEV, or USUV via footpad injection. We found that human WNV convalescent plasma provided mice with strong protection against neuroinvasive encephalitis caused by WNV. Additionally, human WNV convalescent plasma reduced the viremia titers of SLEV and USUV for several days during acute infection. Human WNV convalescent serum also demonstrated a trend towards protecting mice from SLEV-induced encephalitis, as evidenced by lower SLEV titers in the brain and histopathology scores.

These findings will aid in decoding the mechanisms of cross-protection among the JEV serovars, developing therapeutic strategies against WNV, SLEV, and USUV, and anticipating potential disease outcomes, especially in regions where multiple viruses of the JEV serocomplex are endemic.