A colorimetric assay for defective interfering (DI) particles of respiratory syncytial (RS) virus was developed. This quantitative biological assay is based on neutral red dye uptake by DI particle-protected cells that survive standard virus challenge. This assay was more sensitive than the reduction of infectious yield (RIY) assay and was capable of detecting 1 × 10 to 2 × 10 DI particles/ml. The coefficient of variation for parallel, simultaneous replicates (n = 10) was 23%. Cell-protecting activity in the colorimetric assay appeared simultaneously with activity in the RIY assay on undiluted passage of plaque-purified virus. Both activities were particulate, were inactivated by RS virus antiserum and exhibited similar ultraviolet-inactivation kinetics. The absolute values of the slopes of dilution curves for both assays were similar, and using regression analysis both assays enabled estimation of similar numbers of active particles. These results suggest that both activities are mediated by the same DI particle. The mechanism of cell protection does not appear to involve extracellular interferon because the inclusion of interferon antibody in the assay did not diminish DI particle cell protection. Finally, the colorimetric assay was used to reveal alternating cycles of infectious and DI virus production on serial undiluted passage.


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