INFLUENZA TYPE-A VIRUS NEURAMINIDASE DOES NOT PLAY A ROLE IN VIRAL ENTRY, REPLICATION, ASSEMBLY, OR BUDDING

We have used a neuraminidase-deficient influenza virus, NWS-Mvi, which ,vas selected by supplying bacterial neuraminidase in the medium (C. L iu and G. M. Air, Virology 194:403-407, 1993), to define the role of n euraminidase in influenza virus replication, Electron microscopy showe d that virions of the NWS-Mvi mutant assembled normally and formed lar ge aggregates associated with cell surfaces. The NWS-MVi virus grown i n the absence of neuraminidase was able to carry out a second round of replication in MDCK cells without added neuraminidase, indicating tha t the virus particles contained in these aggregates were infectious, A ggregates of virus were also found in cytoplasmic vacuoles, When virus -infected cells were incubated in the presence of ferritin, such aggre gates were found to be labeled with ferritin, indicating that they are derived from uptake at the cell surface. When the neuraminidase-defic ient virus was administered intranasally to C57BL/6 mice, low titers o f virus were recovered from the lungs and major histocompatibility com plex class I-restricted cytotoxic T cells were generated: evidence tha t cells were infected in vivo. In C57BL/6 nu/nu mice, the low level of virus persisted for at least 28 days but never increased, These resul ts suggest that neuraminidase is not required for influenza virus entr y, replication, or assembly in cell culture or in mice,