Biological characteristics of a stimulatory factor for viral replication detected in egg fluids

Yolk sac and allantoic fluids and albumen from uninfected chicken eggs contain a low-molecular weight factor which, after 90 minutes of contact with cell cultures, significantly enhances viral replication. Of several viruses tested, Semliki Forest virus exhibited the highest (10⁵-fold) responsiveness to this enhancing factor. Maximal enhancement was obtained with cells subjected to low multiplicities of infection. The stimulation of viral replication was linear with the age of the cells in culture. The enhancing factor did not operate through an anti-interferon mechanism. However, it exerted a boosting effect on the low-grade cellular metabolism of ageing cell monolayers. Some of the physico-chemical features of the enhancing factor were determined and conjectures concerning its chemical makeup are discussed.