Antigenic variants of foot-and-mouth disease virus (FMDV) of serotype C (isolate C-S8c1) were selected upon serial passage of the virus in cell culture in the absence of anti-FMDV antibodies. The variants rose from frequencies of < 10 in the initial plaque-purified FMDV C-S8c1 preparation, to 0.1 to 1 in three passaged populations. The proportion of antigenic variants was quantified using a new plaque immunotest. A nitrocellulose filter is applied to the agar overlay of a FMDV plaque assay, and allows recovery of infectious virus from individual plaques. A second filter is placed directly on the cell monolayer and binds enough virus to permit colorimetric visualization of plaques by an enzyme-linked assay using monoclonal antibodies (MAbs). Either all or a fraction of plaques from passaged FMDV failed to react with MAb 4G3, an antibody that recognizes an epitope located within residues 144 to 150 of capsid protein VP1. Some variants rapidly dominated the viral population, and others were maintained at low levels. RNA from unreactive viruses included mutations that resulted in amino acid substitutions at the epitope recognized by MAb 4G3. We discuss models for the selection of antigenic variants of FMDV in the absence of antibodies, and implications for the antigenic diversification of RNA viruses.


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