A spontaneous temperature sensitive mutant of Japanese encephalitis virus: preliminary characterization

Temperature-sensitive mutations for live-attenuated Rift Valley fever vaccines: implications from other RNA viruses

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to the African continent. RVF is characterized by high rate of abortions in ruminants and hemorrhagic fever, encephalitis, or blindness in humans. RVF is caused by the Rift Valley fever virus (RVFV: genus Phlebovirus, family Bunyaviridae). Vaccination is the only known effective strategy to prevent the disease, but there are no licensed RVF vaccines available for humans. A live-attenuated vaccine candidate derived from the wild-type pathogenic Egyptian ZH548 strain, MP-12, has been conditionally licensed for veterinary use in the U.S. MP-12 displays a temperature-sensitive (ts) phenotype and does not replicate at 41°C. The ts mutation limits viral replication at a specific body temperature and may lead to an attenuation of the virus. Here we will review well-characterized ts mutations for RNA viruses, and further discuss the potential in designing novel live-attenuated vaccines for RVF.

Yellow fever 17DD vaccine virus is temperature sensitive when grown in mosquito C6-36 cells

Twenty-three yellow fever (YF) vaccine viruses and three wild-type YF viruses were propagated independently in human adenocarcinoma (SW13) cells and mosquito Aedes albopictus C6-36 cells. The three YF 17DD vaccine viruses were found to be slightly temperature sensitive (ts) at 39.5 degrees C versus 37.0 degrees C (efficiency of plaquing 0.04 to 0.1) following propagation in C6-36 but not in SW13 cells. A plaque-purified preparation of the 17DD vaccine manufactured in Brazil was ts when grown in C6-36 cells and remained ts when passaged back into the SW13 cell line.

Temperature-sensitive mutants of Japanese encephalitis virus

Ten stable temperature-sensitive mutants of Japanese encephalitis virus were isolated after mutagenesis by growth of cloned wild-type virus in the presence of the nucleic acid precursor analogs 5-fluorouracil and 5-azacytidine. Mutants were selected which grew at least 100-fold better at 33 degrees C than at 41 degrees C. The 5-fluorouracil was found to be more effective at inducing temperature-sensitive mutations than was 5-azacytidine. Analysis of the virus-specific RNA and proteins synthesized by each mutant at the nonpermissive temperature was used to determine biochemical phenotypes. The mutants were analyzed for abilities to complement in mixed infections. Although inefficient and sometimes nonreciprocal, complementation occurred at higher levels than previously reported for flavivirus mutants. Interference between mutants in some mixed infections was also observed. Seven complementation groups were defined. Three groups contained mutants incapable of synthesizing virus-specific RNA at the nonpermissive temperature, whereas the remaining complementation groups displayed an RNA+ phenotype. Levels of protein synthesis comparable to that of wild type were observed at the nonpermissive temperature in three groups. Two other groups were represented by mutants which synthesized only low levels of virus-specific proteins at the higher temperature. Mutants in the remaining two groups did not produce detectable levels of proteins under nonpermissive conditions.

Characterization of west nile virus persistent infections in genetically resistant and susceptible mouse cells. II. Generation of temperature-sensitive mutants

Long-term persistent infections were established with the flavivirus, West Nile virus (WNV), strain E101, in embryofibroblast cultures derived from susceptible C3H/HE and congenic-resistant C3H/RV mice. Cultures were initially maintained by weekly subculture at 37 degrees, but at passage 6 sister cultures were shifted to 32 degrees. Virus progeny titers were observed to increase after the shift to 32 degrees indicating the possible presence of temperature-sensitive mutants. Temperature-sensitive mutants were found to arise in cultures of both susceptible and resistant cells. However, only in the resistant cultures did temperature-sensitive virus become the majority population. Temperature-sensitive mutants did not appear to be essential for either initiation or maintenance of WNV-persistant infections. The resistant cells appear to provide an environment which is advantageous for the amplification of temperature-sensitive mutants.

St. Louis encephalitis virus temperature-sensitive mutants. I. Induction, isolation and preliminary characterization

Nine temperature-sensitive (ts) mutants of St. Louis encephalitis virus were isolated after "forced mutagenesis" with 5-fluorouracil or 5-azacytidine. The ts mutants could be grouped on the basis of RNA synthesis at 40 degrees C, the nonpermissive temperature and complementation analysis. Four complementation groups were identified. Members of two of the groups were negative for RNA synthesis at 40 degrees C while the remainder were positive.

Dengue-2 vaccine: preparation from a small-plaque virus clone

The S-1 clone of dengue type 2 virus was used for the preparation of a live-attenuated vaccine after passage in DBS-FRhL-2 cell culture. The vaccine virus had a relatively higher replicative capacity at superoptimal temperatures than its precursor virus, S-1, passaged in primary green monkey kidney cells (S-1 PGMK). There was also a tendency for the S-1 vaccine virus to exhibit leakiness at increased temperatures. Another in vitro marker, replication in monkey peripheral blood leukocytes, indicated less host restriction for the S-1 vaccine in comparative assays with S-1 PGMK virus. Mouse virulence appeared to remain stable on passage in DBS-FRhL-2 cells, whereas monkey immunogenicity decreased. Cautious trials of the dengue type 2 S-1 vaccine in humans are indicated.