A replication-efficient mutant of West Nile virus is insensitive to DI particle interference

A previous report described the isolation of a mutant of West Nile virus (WNV) from culture fluid obtained from persistently infected genetically resistant C3H/RV mouse cells that replicates significantly more efficiently in cultures of C3H/RV cells than does the parental virus. This replication-efficient mutant, designated RE-WNV, has now been found to be insensitive to interference by WNV defective interfering (DI) particles. This characteristic was demonstrated by several means. The RE-WNV mutant was able to superinfect persistently infected cultures that were no longer producing detectable parental virus, while the parental virus was not. Good yields of the mutant virus were produced during six serial undiluted passages of RE-WNV in both resistant C3H/RV and congenic susceptible C3H/HE cells. In contrast, during passage of parental virus in C3H/RV cells, progeny virus could not be detected after the third passage, due to an enhanced interference by WNV DI particles with standard virus replication in these cells. The RE-WNV was also insensitive to interference by a pool of parental virus enriched for DI particles. Analysis of the mutant genome by oligonucleotide fingerprinting indicated that the genome RNA of the mutant differs by two unique spots from the parental RNA. The relevance of this mutant to the eventual understanding of the mechanism by which C3H/RV and C3H/HE cells manifest their flavivirus-specific difference in the efficiency of progeny virus production is discussed.