Biochemical characterizations of two temperature-sensitive and attenuated strains of respiratory syncytial virus subgroup B

Cold-adapted, temperature-sensitive (ts), attenuated strains of respiratory syncytial virus have been developed from a B subgroup clinical isolate for potential use as vaccine candidates. The replication of two B subgroup ts mutant viruses (2B33F and 2B20L) at the permissive and nonpermissive temperatures have been compared with that of the parental 2B virus to establish differences that may account for their ts and/or attenuated phenotypes. We have shown that the ts restriction at 39 degrees C in the replication of the two mutant viruses in tissue culture occurs at a step after virus adsorption but before or including initiation of virus-specific mRNA transcription. At the permissive temperature of 32 degrees C a 12- to 24-h delay in the accumulation of mRNA for both mutant viruses in comparison to that of the parental 2B virus was exhibited. This effect was mirrored by equivalent delays in viral protein synthesis and production of infectious virus. By 36 h postinfection both mutants had produced levels of viral mRNA, protein, and infectious virus that were similar to those of the parent virus at 32 degrees C. ts+ revertant viruses derived from both mutants have also reverted in their viral mRNA, protein, and infectious virus production kinetics at 32 degrees C to rates more like those exhibited by the parental 2B virus. This suggests a positive correlation between the ts step in the replication of the mutant viruses and the initial delay in mRNA production that occurs at the permissive temperature.