Homologous interference of lymphocytic choriomeningitis virus: detection and measurement of interference focus-forming units

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Abstract
Lymphocytic choriomeningitis (LCM) virus defective interfering (DI) particles form foci of protected cells [mouse L fibroblasts] in a monolayer under an agarose-containing overlay medium. Foci originate from 1 cell dually infected with at least 1 interference focus-forming unit and infectious virus. As a result, an interfering factor is produced and released which interacts with neighboring cells, thereby protecting them against cytopathic lysis by challenge virus. The property of individual LCM virus DI particles to induce countable foci was made the basis of a quantitative assay that is comparable in every respect to the plaque assay of infectious virus and is much more sensitive and probably more accurate than other procedures used to measure LCM virus DI particles. LCM virus was other procedures used to measure LCM virus DI particles. LCM virus was passaged, undiluted, 10 times in cell cultures. When yields were analyzed as to concentrations of PFU [plaque-forming units] and interference focus-forming units, both entities were found to fluctuate with the pattern expected from theoretical considerations.