MULTIPLE VIRULENCE DETERMINANTS OF FOOT-AND-MOUTH-DISEASE VIRUS IN CELL-CULTURE

Hypervirulent variants of foot-and-mouth disease virus (FMDV) of serot ype C arise upon serial cytolytic or persistent infections in cell cul ture. A specific mutation in the internal ribosome entry site of persi stent FMDV was previously associated with enhanced translation initiat ion activity that could contribute to the hypervirulent phenotype for BHK-21 cells. Here we report that several hypervirulent FMDV variants arising upon serial cytolytic passage show an invariant internal ribos ome entry site but have a number of mutations affecting structural and nonstructural viral proteins. The construction of chimeric type O-typ e C infectious transcripts has allowed the mapping of a major determin ant of hypervirulence to the viral capsid. Tissue culture-adapted FMDV displayed enhanced affinity for heparin, but binding to cell surface heparan sulfate moieties was not required for expression of the hyperv irulent phenotype in Chinese hamster ovary (CHO) cells. Virulence was identical or even higher for glycosaminoglycan-deficient CHO cells tha n for wild-type CHO cells. FMDV variants with decreased affinity for h eparin were selected from a high-binding parental population and analy zed. Substitutions associated with decreased heparin binding were loca ted at positions 173 of capsid protein VP3 and 144 of capsid protein V P1. These substitutions had a moderate effect on virulence for BHK-21 cells but completely abrogated infection of CHO cells. The comparative results with several FMDV isolates show that (i) increased affinity f or heparin and alterations in cell tropism may be mediated by a number of independent sites on the viral capsid and (ii) the same capsid mod ifications may have different effects on different cell types.