Biochemistry and pathogenicity of echovirus 9. II. Mutants of echovirus 9, strain Hill, with altered capsid surface properties

The two echovirus 9 strains Hill and Barty have been shown previously not only to differ in pathogenicity for newborn mice but also in a number of in vitro characteristics which depend on viral capsid structure. Three spontaneously occurring mutants of the mouse-apathogenic echovirus 9 prototype strain Hill being resistant to an inhibitor of plaque formation present in agar were isolated and compared biochemically and biophysically to their parent strain and to the mouse-pathogenic echovirus 9 strain Barty, which is resistant to this inhibitor. The mutants differ from their apathogenic parent strain Hill in most of the same in vitro characteristics as strain Barty, namely adsorption to cells in culture, sedimentation behavior in low salt sucrose gradients, distribution of mutant virus particles in isoelectric focusing, and antigenic determinants inducing neutralizing antibodies. For two of the three mutants, Ag2 and Ag3, evidence was obtained from fingerprinting that they differ from their parent strain Hill in VP1; thus, the observed in vitro properties may be caused by the change in this capsid protein. All mutants, however, were found to be apathogenic for newborn mice and do not replicate in the tissues of these animals. It is concluded that the observed changes in capsid structure do not covary with virulence.