Specific interference among strains of Newcastle disease virus: II. Comparison of interference by active and inactive virus

Abstract

High concentrations of NDV-B1, NDV-B1-UV, and NDV-L-UV interfere with the infection of chick embryo cells by NDV-L when added prior to or simultaneously with this virus. When time is allowed to elapse between infection by interfering virus and NDV-L there is a temperature-dependent increase in interference lasting about 90 minutes. The interference induced by ultraviolet (UV)-irradiated virus then begins to disappear immediately, while that induced by active virus remains constant for about 4 hours before increasing again. The second increase in interference exhibited by cells infected with active virus has been correlated with the multiplication of the interfering virus. The interfering capacity of a virus inoculum has been shown to be associated with the virus particle, and the degree of interference is proportional to the number of virus particles in the inoculum. UV irradiation initially reduces both infectivity and the simultaneous and early interfering capacities of the virus with the same single-hit kinetics. UV irradiation in excess of about three hits to the infectious capacity of the virus continues to inactivate infectivity but has no further effect on interfering capacity, and no effect at all on the viral hemagglutinin and RDE. NDV-B1-UV, NDV-L-UV, and heat-inactivated NDV-L all induce similar levels of simultaneous and early interference which are 5- to 50-fold lower than the levels of interference induced by active NDV-B1. Preliminary evidence suggests that different mechanisms may be responsible for the interference induced by active and inactive virus.