Selection of foot-and-mouth disease viruses by passage in bovine and swine kidney cell cultures

Two uncloned populations of foot-and-mouth disease virus, one pathogenic for adult mice and the other nonpathogenic, were passaged in cultures of primary bovine kidney (BK) cells and a line of pig kidney (MVPK) cells. Within 10 passages in MVPK cells, the nonpathogenic virus became pathogenic for adult mice, but similar passages of this virus in BK cells did not affect its pathogenicity. In contrast, passage of the pathogenic virus in MVPK cells resulted in a decrease in pathogenicity, but again passage in BK cells had no effect on this characteristics. Neither of the viruses changed in pathogenicity for infant mice during the four passages that were tested. The nonpathogenic virus passaged in BK cells was more infectious for BK than for MVPK cells, but after passage in MVPK cells, this virus was about equally infectious for the two types of cells. Infectivity of the pathogenic virus was relatively unchanged by passage in either type of cell. The parent nonpathogenic virus and the 10th BK cell passage of this virus were much more resistant to adsorption with homogenized mouse kidney than were the MVPK cell passages of nonpathogenic virus. The parent and passaged pathogenic viruses were readily adsorbed. The results demonstrated that passage of the two viruses in MVPK cells had a pronounced selective effect.