Coronavirus IBV-induced membrane fusion occurs at near-neutral pH

The lysosomotropic agent NH4Cl caused a reduction of 80-95% in the number of chick kidney (CK) cells and Vero cells infected by infectious bronchitis virus (IBV) strain Beaudette, as determined by immunofluorescence at the end of the first replication cycle. Inhibition only occurred when NH4Cl was present during the first 2 h after infection. Syncytium formation was studied during replication of IBV-Beaudette in Vero cells. Some cell-cell fusion occurred at pH 7.0 and pH 6.5 but it was optimal at pH 6.7. IBV strain UK/123/82 did not replicate in Vero cells and was studied in CK cells in which it grew well but without forming syncytia. In contrast to IBV-Beaudette, NH4Cl had virtually no effect on the replication of UK/123/82. The results show that the IBV spike glycoprotein induces membrane fusion at near neutral pH although some IBV strains may require a mildly acidic environment for the efficient uncoating of the virion RNA.