Studies on Defective Interfering Particles of West Nile Virus.

The studies undertaken in this thesis described the biological characteristics of defective interfering (DI) particles of West Nile (WN) virus both in vitro and in vivo. A reliable and reproducible assay was developed to measure the interfering activity of DI virus (the optimum interference assay [OIA]) based on ability of DI WN virus to cause a reduction in the yield of infectious standard (STD) WN virus. The sensitivity of the assay was found to be dependent on several parameters of which the cell type used in the assay was the most crucial. High-multiplicity serial passages of STD WN virus in Vero and LLC-MK2 cells generated and amplified a large quantity of DI WN virus as measured by the OIA and their presence was detected as early as after two serial passages. Propagation of pre-formed DI virus in seven cell lines demonstrated that SW13 cells would propagate STD WN virus only and no detectable DI virus was propagated in this cell line. Intraperitoneal inoculation of adult mice with DI WN virus protected them from a lethal challenge by the neurotropic STD WN virus inoculated by the same route. The multiplication of infectious virus in the brains of mice treated with live DI virus was greatly reduced compared to mice inoculated with STD virus and infectivity was rapidly eliminated. Evidence was produced to show that DI virus was propagated in the brain of mice protected by live DI virus. A minority of protected mice sampled at 19 days post infection had infectious virus in their brains suggesting that DI virus can modulate the acute WN virus infection and cause it to become persistent. Virus isolates collected from the protected mice demonstrated three categories of virulence for adult mice: as virulent as the parent virus, intermediate virulence and greatly attenuated. DI virus derived from three different strains of WN virus demonstrated heterotypic interference both in vitro and in vivo. Interference was limited between different members of the WN subgroup of the Flaviviridae and the extent of heterotypic interference was found to be consistent with the antigenic classification of flaviviruses. Thus, interference by flavivirus DI particles may be useful in the classification of flaviviruses. In conclusion, DI WN virus was generated with properties similar to those of DI virus reported for other RNA virus systems. However, the ability of the DI WN virus to protect mice was superior to that of any other virus systems described to date. Thus, DI WN virus may be an excellent system with which to study the biological properties of DI virus