RAPID SELECTION IN MODIFIED BHK-21-CELLS OF A FOOT-AND-MOUTH-DISEASE VIRUS VARIANT SHOWING ALTERATIONS IN CELL TROPISM

With persistent foot-and-mouth disease virus (FMDV) in BHK-21 cells, t here is coevolution of the cells and the resident virus; the virulence of the virus for the parental BHK-21 cells is gradually increased, an d the cells become partially resistant to FMDV. Here we report that va riants of FMDV C(3)Arg/85 were selected in a single infection of parti ally resistant BHK-21 cells (termed BHK-Rb cells). Indirect immunofluo rescence showed that the BHK-Rb cell population was heterogeneous with regard to susceptibility to C(3)Arg/85 infection. Infection of BHK-Rb cells with C(3)Arg/85 resulted in an early phase of partial cytopatho logy which was followed at 6 to 10 days postinfection by the shedding of mutant FMDVs, termed C-3-Rb. The selected C-3-Rb variants showed in creased virulence for BHK-21 cells, were able to overcome the resistan ce of modified BHK-21 cells to infection, and had acquired the ability to bind heparin and to infect wild-type Chinese hamster ovary (CHO) c ells. A comparison of the genomic sequences of the parental and modifi ed viruses revealed only two amino acid differences, located at the su rface of the particle, at the fivefold axis of the viral capsid (Asp-9 -->Ala in VP3 and either Gly-110-->Arg or His-108-Arg in VP1). The sam e phenotypic and genotypic modifications occurred in a highly reproduc ible manner; they were seen in a number of independent infections of B HK-Rb cells with viral preparation C(3)Arg/85 or with clones derived f rom it. Neither amino acid substitutions in other structural or nonstr uctural proteins nor nucleotide substitutions in regulatory regions we re found. These results prove that infection of partially permissive c ells can promote the rapid selection of virus variants that show alter ations in cell tropism and are highly virulent for the same cells.