Generation of Defective Interfering Particles by Two Vaccine Strains of Measles Virus

A systematic study was made to measure the generation of defective interfering particles upon up to 13 serial passages of two measles vaccine strains, Edmonston and Edmonston–Zagreb, through either simian (Vero) or human (WI-38) cell lines. Results for the Vero cell passage were nearly identical for both viruses. Infectivity titers dropped by nearly 8 logs to undetectable levels at passage 4 and cycled between maximum and minimum levels every 4 passages. Samples with the lowest infectivity titers produced the greatest reduction in titer of standard virus and contained an approximately 900-nucleotide subgenomic RNA for the Edmonston strain and two subgenomic RNAs of 4300 and 3000 nucleotides for the Edmonston–Zagreb vaccine strain. A defective interfering RNA-specific reverse transcription-polymerase chain reaction (RT-PCR) detected subgenomic RNAs at all passage levels. In contrast, samples obtained after passage of these viruses in WI-38 did not reduce the yield of standard virus and did not contain subgenomic RNAs in both Northern blot and RT-PCR assays. These results clearly show that cell type rather than virus strain affects defective interfering particle generation for measles virus.