Cell fusion studies identified multiple cellular factors involved in mouse hepatitis virus entry.

We have previously reported that certain murine cell lines are susceptible to mouse hepatitis virus (MHV) A59 strain infection but resistant to JHM strain, despite the expression of the viral receptor for both strains. This restriction on viral infection has been shown to occur at the virus entry level (Yokomori et al., Virology 196, 45-56, 1993). To study whether JHM resistance of these cell lines is due to a defective cellular factor necessary for JHM virus entry, or due to the presence of an inhibitor, hybrid cells were obtained by polyethylene glycol (PEG)-mediated fusion between two resistant cell lines, i.e., a Balb/c mouse-derived cell line, BC10 cells, which are resistant to JHM infection but express a viral receptor, and the simian cell line COS cells, which are resistant to JHM because of the absence of the viral receptor. JHM could replicate in the hybrid BC10-COS cells, indicating that the restriction of viral infection in BC10 cells could be complemented by COS cells. This result indicates that the resistance of BC10 cells to JHM infection is due to the defectiveness of a cellular factor rather than the presence of an inhibitor. JHM virus-binding and penetration into BC10 cells appeared to be normal. However, JHM internalization into BC10 cells by PEG-induced virus-cell fusion did not lead to viral replication, suggesting that the restriction of JHM infection in BC10 cells is at the level of viral uncoating. This restriction could be complemented by PEG-mediated fusion with other murine cell lines which have the virion-uncoating activity, but not by cell lines which lack this activity. Furthermore, the viral resistance of most of other murine cell lines, which express MHV receptors, could not be overcome by fusion with COS cells, suggesting that different murine cell lines are defective in different viral entry functions. Therefore, we conclude that JHM viral entry process requires multiple cellular factors secondary to the viral receptor.