Reduced pathogenicity of velogenic NDV strain AF22420-I via site-directed mutagenesis of V gene

Abstract

Newcastle disease virus (NDV), an avian paramyxovirus, has the potential to be used as an anti-cancer therapeutic vaccine due to its oncolytic and immunostimulatory activities. The virus can be categorised into three pathotypes: lentogenic, mesogenic, and velogenic; of the three pathotypes, the lentogenic strains such as the La Sota are the preferred pathotype for vaccine development due to their low virulence to birds. On the other hand, the translation of the virus to clinic of the velogenic strain AF2240-I is hindered by its virulence towards birds although it exhibits strong oncolysis with significant outcomes both in vitro and in vivo. This study aims to reduce the pathogenicity of AF2240-I yet retaining the anti-cancer properties of the virus. To achieve this, the V protein that acts as an interferon antagonist was chosen to be mutated. It is a non-structural protein that does not interfere with the binding and infection of the virus; hence, mutation of this virulence factor was deducted to be able to reduce harm to the avian species but retain its anti-cancer properties as much as possible. The V protein, which was produced from the insertion of an additional G into a conserved editing site of the P gene, was mutated by substituting the G nucleotide at position 411 from the start of P gene to a T nucleotide. This mutation will produce a premature stop codon from the V mRNA, resulting in a truncated V protein; but only causes a silent mutation in the P protein. The recombinant virus was recovered by the use of BHK cells stably expressing the phage T7 RNA polymerase. The pathogenicity of the mutated virus was determined in 9- to 11-day-old embryonated SPF chicken eggs. The mean death time (MDT) was determined to be 73.6 hours at the minimal lethal dose of 10-7, resembling to that of a mesogenic strain. The virulence of the mutated virus has been successfully reduced where it could be potentially used as the vector for the development of recombinant oncolytic virus for cancer treatment.