Infection and growth of Newcastle disease virus (NDV) in cultures of chick embryo lung epithelium

Abstract

The present report constitutes a quantitative study of various stages of infection of chick embryo lung epithelium in vitro with Newcastle disease virus (NDV). The virus was assayed throughout these experiments by the plaque technique. The velocity constant for adsorption was determined as well as the true multiplicity of infection. One-step growth curves were carried out in which both the virus liberated into the medium, and that remaining associated with the cells (cell-associated virus or CAV) were measured. It was found that extracellular virus and CAV were equal in amount during the period of exponential virus increase, and the apparent release time (i.e. the time required for release of a newly formed virus particle into the medium after it has gained the property of infectivity) was calculated as about 80 minutes. Further study showed that practically all of this time was spent at the cell surface. In fact, there was no compelling evidence for infective virus at any place other than the cell surface. X-ray inactivation studies showed that both CAV and released virus occur as independent particles rather than in clumps. The small fraction of inoculated virus which retains its infectivity upon coming in contact with susceptible cells and which can be found associated with these cells during the latent period is largely resistant to serum inactivation. These particles are somehow protected from serum neutralization by their association with subcellular particulates, but do not participate in the infectious cycle unless liberated by homogenization and plated on another cell layer.