Maintenance of episomal SV40 genomes in GM637 human fibroblasts

Episomal SV40 (SV40: simian virus 40, Polyomavirus maccacae) has been reported in SV40-transformed human fibroblast cell lines the integrated SV40 sequences of which are unlikely to give rise to episomal copies by recombinational mechanisms. The levels of episomal viral DNA in these lines are high, being easily visualized by ethidium staining of agarose gels after electrophoresis. We find that the episomal mutant gmSV40 in GM637 cells represents a persistent lytic infection that can be cured by treatment with neutralizing antibody, leaving only the chromosomally integrated viral genomes. The finding that maintenance of the gmSV40 in GM637 cells is due to persistent infection raises a note of caution for SV40-transformed lines with episomal SV40 genomes because these lines often are used in studies of DNA replication and repair. An infective center assay that does not depend on plaque formation shows that gmSV40 is a host range mutant, with poor infectivity for CV-1 monkey kidney cells and greatly increased infectivity for human cells. Passage of gmSV40 through monkey kidney cells selects for variants with greatly increased infectivity for monkey cells and, independently, for cytopathic variants that produce plaques. Thus plaque assays can give very unreliable infective center values in studies of host range mutants.