Diversity and generation of defective interfering influenza virus particles

Abstract

A number of individually isolated, plaque-purified clones of ts-52 WSN influenza virus were analyzed for their ability to generate defective interfering (DI) virus particles. Clonal stocks, when passaged four or five times serially in MDBK cells at high multiplicity produced DI viruses each with a characteristic set of DI RNA segments. Aliquots of different viral passages from a given clone (VP1 through VP3), when passaged independently produced the same DI virus suggesting that DI viruses are already present in VP1. Furthermore, additional experiments suggested that DI viruses characteristic of a given clone were even present in the clonal stock virus and became amplified during undiluted passages. When aliquots of a virus clone (directly isolated from a plaque) were passaged independently some clones produced identical DI viruses at VP4 suggesting that DI viruses were already present in the plaque whereas aliquots of another clone produced different DI viruses suggesting that DI viruses were absent in the clone and were generated subsequently during passages. Thus DI virus can be generated even in the plaque produced by a single infectious virus. Large quantities of a particular DI virus could be produced by coinfection with a helper infectious virus. However, continued amplification of DI viruses for a large number of passages by a helper infectious virus resulted in disappearance of some DI RNA segments with the appearance of possible new DI RNA segments. Mixed infections of two different DI viruses in the presence of helper virus resulted in the production of both types of major DI viruses. Furthermore, we have developed an assay for quantitating DI virus directly using an infectious center reduction method. Our data suggest that a single DI virus particle can inhibit infectious center formation by standard virus and that standard virus is replaced almost entirely by DI virus after four undiluted passages.