Interference between inactivated and active influenza viruses in the chick embryo: II. Interference by incomplete forms of influenza virus

Abstract

A comparison was made of the interfering capacities of several types of ultraviolet-irradiated incomplete and of standard influenza virus preparations on the basis of similar hemagglutinating activities. Serial decimal dilutions of challenge virus were employed, and the results were evaluated in terms of yields of infectious progeny. Incomplete virus derived from chick embryos by undiluted passage series or injection of partially heat-inactivated standard seed (37° for several days) interfered to about the same extents as complete virus, as long as the amount of internal S antigen did not fall below a certain minimum. Virus components with smaller concentrations of internal S, as obtained from allantoic membranes in a third undiluted passage, revealed a definite, but not complete loss of interfering activity. The S-free HA components separated from standard virus after exposure to ether failed to induced interference, as did the incomplete virus particles derived from infected HeLa cultures. No evidence has been found to indicate that the latter incorporate S antigen, although it is produced in the cells. These results are compatible with the hypothesis that S antigen, or its RNA (ribonucleic acid) part, constitutes the actual agent responsible for inducing interference. However, S antigen per se could not be shown to interfere. This does not necessarily deny its role in interference, since its entry into the cells under the experimental conditions has not been established. The fact that the two S-free HA preparations, which possessed full receptor-destroying-enzyme activity, failed to cause interference, reaffirms previous conclusions that the phenomenon is not referable to the receptor mechanism.