Monoclonal antibodies identify the NS5 yellow fever virus non-structural protein in the nuclei of infected cells.

Eight monoclonal antibodies (MAbs) derived using yellow fever (YF) virus (French viscerotropic virus strain) labelled the nuclei (wild-type strains) and/or the nucleoli (vaccine strains) of cells infected with different strains of YF virus. The specificity of these antibodies for YF virus-infected cells was confirmed using MAbs that bind only the YF virus envelope glycoprotein. The characteristics of fluorescent labelling in the nuclei and nucleoli of both normal cells and of nuclei separate from cell cytoplasm confirmed that the antigen was inside the nucleus rather than on the outer surface of the nuclear membrane. Virus-specific antigen was also observed in the cytoplasm of infected cells. One additional virus envelope-specific antibody, derived at the same time, identified cytoplasmic antigen exclusively. The eight antibodies that identified nuclear antigen showed no activity in neutralization, haemagglutination inhibition or mouse protection tests. Analysis of their molecular specificities by radioimmunoprecipitation of virus-infected cell lysates showed that they identified the non-structural NS5 antigen of YF virus. These results support the possibility of nuclear involvement in the replicative stages of YF virus infection.