Electron microscopic study of measles virus infection: cell fusion and hemadsorption

Virus-induced cell fusion has been studied after infection of Vero cells with measles virus. Scanning
and transmission electron microscopy were combined with immunoperoxidase labeling of measles
antigens to correlate viral production and distribution of virus-induced erythrocyte binding sites with
progress of fusion-Release of infectious virus started before syncytia were detected and decreased
while the number and size of syncytia were increasing. Most virions were seen budding from
mononucleated cells or from the periphery of syncytia where cells were being recruited. Moving
inward, the surfaces of syncytia were covered with numerous ridges containing viral antigen, but few
viral buds were seen, suggesting that syncytia might be sites of defective viral formation.
Hemadsorption occurred predominantly within the confines of syncytia. Erythrocytes were scattered
sparsely over immature syncytia but were densely packed in the center of mature syncytia. Active
binding sites for erythrocytes were located on cell villi and ridges covered with measles antigens.
Hemadsorption was completely inhibited in measles virus-infected cultures pretreated with virus-
specific immunoglobulin G for 1 h at 4°C. However, when these cultures were shifted to 37°C,
hemadsorbing sites were recovered at the periphery of enlarging syncytia. Virus-induced sites for
erythrocyte adsorption were found to move centripetally on syncytium membranes as fusion
progressed.