In this work we have characterized the virus (RSV48) present in passage 48 of a respiratory syncytial virus persistently infected murine macrophage-like cell culture. This virus was noncytopathic in macrophages and had a low-fusogenic activity in RSV-permissive cell lines, although the level of this activity varied among the different cell lines tested. The fusogenic activity of RSV48 in Vero cells, as evaluated by the number and size (nuclei per syncytium) of syncytia, was lower than that shown in cells H358. However, the syncytia formed by RSV48 in Vero cells increased significantly when the virus was treated with trypsin previous to cell infection and the protease was left in the medium during the development of polykarions. Moreover, the fusogenic activity of RSV48 was increased by a brief acidic pH treatment of infected cells. These results imply that the RSV48 F protein was inefficiently activated by intracellular proteases in Vero cells and exposure to low pH favours membrane fusion. Analysis of the nucleotide and the deduced amino acid sequences of the RSV48 F protein showed nine amino acid residue differences with respect to the RSVwt sequence, some of which mapped to positions that suggest they might be responsible for the low-fusogenic activity observed for the RSV48 F protein.
