Persistent Infection of Continuous Line of Pig Kidney Cells with a Variant of the WSN Strain of Influenza A 0 Virus

Abstract

Summary

RES cells were infected with the RES₁₆ variant of the WSN strain of influenza A₀ virus and the culture medium changed at frequent intervals; five lines of cells were established in which infection was sustained for periods of 126 to 146 days. Each line of persistently-infected cells showed recurring cycles of cell degeneration and repopulation; these cultures did not remain in equilibrium with the virus indefinitely, however, and ultimately degenerated.

The presence of virus in the RES culture fluids was easily demonstrated for periods of 27 to 105 days in various cell lines using as the indicator intraallantoic inoculation of eggs with hemagglutination by the allantoic fluids. Subsequently this method failed to demonstrate virus, although the continued presence of CPE suggested that the cultures were still infected. By using as indicator of the presence of virus the production of CPE in normal RES or PS cultures inoculated with fluids from the infected lines, it was found that influenza A₀ virus was still present in the fluids, and that it was able to propagate in eggs without producing detectable hemagglutinins. Only modest infectivity titers were found in the nonhemagglutinating allantoic fluids; however the original WSN and WSN/RES₁₆ variants at the same infectivity titers readily produced detectable hemagglutination. This fact suggests that the virus had not simply lost its ability to multiply in eggs to titers sufficient to produce hemagglutination, but that at least part of the infectious virus did not hemagglutinate. Spontaneous reversion to the original hemag-glutinating state was noted in one infected line (VL-V) during the last days of infection, just before complete degeneration of the culture, and in two cases reversion followed serial passages in eggs (VL-V-63) and in eggs and PS cells (VL-IV-106).