ACQUISITION OF THE TS PHENOTYPE BY A CHEMICALLY MUTAGENIZED COLD-PASSAGED HUMAN RESPIRATORY SYNCYTIAL VIRUS-VACCINE CANDIDATE RESULTS FROM THE ACQUISITION OF A SINGLE MUTATION IN THE POLYMERASE (L) GENE

A cold-passaged (cp) temperature-sensitive (fs) mutant of human respir atory syncytial virus designated RSV cpts-248 was previously derived b y random chemical mutagenesis of the non-ts mutant cp-RSV that possess es one or more host range mutations. We previously demonstrated in rod ents and seronegative chimpanzees that the cpts-248 virus is more atte nuated than cp-RSV and is more stable genetically than previously isol ated RSV ts mutants. In the present study, we determined that the acqu isition of the ts phenotype and the increased attenuation of the cpts- 248 virus are associated with a single nucleotide substitution at nucl eotide 10,989 that results in a change in the coding region (amino aci d position 831) of the polymerase gene. The identification of this att enuating ts mutation is important because cpts-248 was used as the par ent virus for the generation of a number of further attenuated mutants that are currently being evaluated as candidate vaccine strains in cl inical trials in infants. Furthermore, technology now exists to ration ally design new vaccine candidates by incorporating multiple attenuati ng mutations, such as the one identified here, into infectious viruses that are genetically stable and appropriately attenuated.