Quantitative analysis of defective interfering particles in infectious pancreatic necrosis virus preparations

Infectious pancreatic necrosis virus exhibited an interference phenomenon that resulted in the survival of the infected cell with one hit kinetics. The responsible factor was found to co-purify with standard virus through a purification regime that employed two CsCl gradients and a sucrose gradient. This result suggested that a defective interfering (DI) viral particle was involved. It was possible to estimate the number of DI particles by a statistical method using the Poisson distribution that related cell survival to input DI/cell, which indicated that virus samples from dilute passage contained as many DI particles as samples from undiluted passage; this means that multiple undiluted virus passage did not increase the yield of DI particles. In isopycnic CsCl gradient centrifugation, the DI particles were found in a broad band superimposed over the standard virus peak and extending above it, such that the ratio DI/PFU varied from 0.3--20 in different fractions. These centrifugation methods did not completely separate DI particles from standard virus.