Visualization of the fate of inactive influenza viruses in Daudi cells by electron microscopy

The replication of active and inactivated influenza viruses in Daudi lymphoma cells was studied by immunofluorescence and electron microscopy. In a previous study, we demonstrated that active and heat-inactivated X47 (H3N2) virus arrested Daudi cell growth by inhibiting cellular DNA synthesis while formalin-treated X47 virus did not. Transmission electron microscopic studies revealed that both the active and the heat-inactivated X47 virus penetrated into the cells. Only the active X47 (XA) virus replicated completely in Daudi cells and produced new viral particles by budding. The formalin-treated X47 virus did not damage or change the cells, and although the viral particles remained adsorbed to the cells, there was little penetration. The heat inactivated X47 virus (which was the most effective, non-virulent, oncolytic agent we studied) was visualized as large aggregates of particles adsorbed to the cell surface by electron microscopy. The cells themselves formed clumps. The viral aggregation and cell clumping likely resulted from the loss of viral neuraminidase activity due to heat treatment. The penetration of heat-inactivated viral particles was massive and involved numerous particles. Production of new viral particles was not demonstrated in this study even though nucleocapsids from the original virus were found in the cytoplasm. Thus, it appears that the massive penetration of the viral particles into cells damages the plasma membrane and may be responsible for the oncolytic potential of the heat-inactivated virus on Daudi cells.