Production of a viral growth enhancing factor (enhancer) in eggs infected with influenza virus (PR8)

Abstract

A viral growth enhancing factor (enhancer) was produced in the chorioallantois of eggs infected with PR8 virus and released into the allantoic fluid. Enhancer was detected by its ability to enhance HA production by HVJ in roller tube cultures of shell-membrane pieces. The enhancement of HA production was obtained either by preincubation of the cultures with enhancer at 37° C for 24 hours before challenge of HVJ, or by its addition to the cultures simultanously with the challenge. The production and release of enhancer occurred in the early stages of infection with 104 to 105 EID50 of PR8 virus. Enhancer activity of the allantoic fluid and chorioallantois declined rapidly after 24 hours of infection and was followed by the appearance of interferon activity. The time when enhancer activity was increasing and maintained a high level corresponds with the time when virus was growing approximately at a linear slope. Interferon activity first appeared in the chorioallantoic membrane and allantoic fluid at 36 hours after infection with PR8 virus, when virus growth had already reached a maximal level. Subsequent rise in interferon titers was associated with decline of cellular virus titers.