Characterization of the cord-like structures emerging from the surface of influenza C virus-infected cells

When HMV-II cells (a human malignant melanoma cell line) infected with a newly isolated influenza C strain (Yamagata/1/88) were examined by simple light microscopy, it was found that a large number of cord-like structures which had lengths up to about 500 μm or greater were emerging from the cell surface. The existence of viral glycoproteins (hemagglutinin-esterase, HE) on the surface of these huge structures was confirmed by hemadsorption experiments with erythrocytes from a variety of species as well as by immunofluorescent staining with anti-HE monoclonal antibody. Furthermore, electron microscopy revealed that numerous filamentous particles in the process of budding, each covered with a layer of surface projections ∼ 13 nm in length, aggregated with their long axes parallel to form a cord-like structure visible under a light microscope. An electron-dense layer, which presumably consists of membrane protein (M), was seen in cross-sections of all filamentous virions whereas internal nucleocapsids were rarely seen. SDS-polyacrylamide gel electrophoresis of the purified cords also showed that they contained HE and M polypeptides but not nucleoprotein, confirming that long filamentous particles are mostly devoid of nucleocapsids. The emergence of cords on the cell surface was observed in various cell cultures infected with C/Yamagata/1/88 though their number and length varied markedly depending on cell type. The production of cord-like structures was also evident in HMV-II cells infected with any of several different influenza C strains, which suggests that the cord formation is a common feature of influenza C virus group.