The effects of monospecific antibodies to the viral glycoprotein with hemagglutinating and neuraminidase activity (HN) and the viral glycoprotein with membrane-fusing activity (F) of the paramyxovirus simian virus 5 (SV5) on the spread of infection in two cell types have been investigated. In CV-1 cells, infection can spread by either released progeny virus adsorbing to and infecting other cells, or by fusion of an infected cell with an adjacent cell as a result of the cell-fusing activity of the F glycoprotein. In these cells, antibodies specific for the HN glycoprotein prevented the dissemination of infection by released infectious virus, but spread by cell fusion was not inhibited. Antibodies to the F glycoprotein completely prevented the spread of infection in these cells. In Madin-Darby bovine kidney cells, which are relatively resistant to SV5-induced fusion, antibodies to either the HN or F glycoproteins were capable of preventing the dissemination of infection. These results indicate that effective immunological prevention of the spread of paramyxovirus infection requires the presence of antibodies that inactivate the F glycoprotein. This requirement for anti-F antibodies has obvious implications for the design of effective paramyxovirus vaccines and provides an explanation for previous failures of formalin-inactivated paramyxovirus vaccines as well as additional insight into the possible immunopathological mechanisms involved in the atypical and severe infections that have occurred in individuals who received inactivated paramyxovirus vaccines and were subsequently infected by the virus.

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