Temperature-sensitive phenotype of the human parainfluenza virus type 3 candidate vaccine strain (cp45) correlates with a defect in the L gene.

We have previously demonstrated that the temperature sensitivity of a human parainfluenza virus type 3 (HPIV-3) candidate vaccine strain (cp45), which is currently under evaluation in humans, is associated with poor transcriptional activity of the virus at the nonpermissive temperature (R. Ray, K. Meyer, F. Newman, and R. B. Belshe, J. Virol. 69:1959-1963, 1995). In this study, the temperature sensitivity of cp45 virus was further investigated by the complementation of a specific gene function. CV-1 cells were transfected with cloned genes from wild-type HPIV-3 encoding the large protein (L), phosphoprotein (P), and nucleocapsid protein (NP), alone or together, for the expression of biologically active proteins. Only cells expressing the L gene were able to rescue cp45 replication when incubated at the nonpermissive temperature (39.5 degrees C), whereas cells transiently expressing NP or P were incapable of rescuing the virus. The virus titers obtained following complementation of the L protein were 190 to 2,300 PFU/ml of culture medium, compared with the undetectable growth of the cp45 temperature-sensitive mutant at the nonpermissive temperature. Rescued progeny virus still maintained the temperature-sensitive phenotype. Results from this study suggest that the temperature sensitivity of the cp45 candidate vaccine strain is associated primarily with L-protein function and that the defect can be complemented by transient expression of the wild-type protein. This study underscores the importance of the L protein in RNA polymerase activity and its critical role in virus replication.