Synchronization Temperature Determines the Location of RSV Fusion During Entry in Cultured Cells

Respiratory syncytial virus (RSV) is the most frequent cause of bronchiolitis in children under five years of age. No vaccines against this virus are currently available. RSV infection of a cell is initiated by fusion between the virion membrane and a cellular membrane, but it is not clear if the fusion process takes place at the plasma membrane or within an endosome. Most such experiments have been initiated at the traditional synchronization temperature of 4°C, an abnormal temperature for animal cells and one at which cellular homeostasis may be negatively affected. We have compared two synchronization temperatures (4°C and 22°C) to determine the kinetics of RSV entry into human bronchial epithelial cells. Following inoculation, virus entry was halted at different times by the addition of neutralizing antibody or temperature reduction to 4°C. We engineered a virus that encodes an extra viral gene, beta-lactamase fused to the viral phosphoprotein (P), to enable rapid detection after infection initiation. We found that the synchronization temperature used during inoculation determines the site of fusion. Transition from 4°C to 37°C resulted in RSV entry via the endosomal pathway but also induced F-actin disruption and plasma membrane blebbing, whether the cells were inoculated with RSV or not. Transition from 22°C to 37°C resulted in RSV entry by fusion at the plasma membrane and without the F-actin and plasma membrane disruptions. These results suggest that RSV normally enters cells by fusion at the plasma membrane and that the induction of endocytosis by infection synchronization at 4°C may be an artefact caused by distortion of the plasma membrane-supporting cytoskeleton. Author Summary In order to understand the overall mechanism driving infection, it is important to determine how the virus enters cells. The pathway that RSV uses to infect cells is unclear. It is a common practice to attach the virions at 4°C to synchronize the viral infection. In this report, we found that warming up primary cultures of undifferentiated normal human bronchial epithelial cells to 37°C from 4°C triggered dramatic changes in their cell membrane and cytoskeleton totally unrelated to the presence of the virions. The assessment of viral content delivery to the cytoplasm using RSV engineered to express BlaM allowed us to find that the virions attached at 4°C or 22°C fused their envelope with endosome or plasma membrane, respectively. Consequently, the entry via endosome after attachment at 4°C is an experimental artefact and RSV infects by fusing its envelope with the plasma membrane. The implications go beyond RSV since the entry of several virus species have been explored by synchronizing the infection after attachment at 4°C.