Effects of hemagglutinin-neuraminidase protein mutations on cell-cell fusion mediated by human parainfluenza type 2 virus

The monoclonal antibody M1-1A, specific for the hemagglutinin-neuraminidase (HN) protein of human parainfluenza type 2 virus (HPIV2), blocks virus-induced cell-cell fusion without affecting the hemagglutinating and neuraminidase activities. F13 is a neutralization escape variant selected with M1-1A and contains amino acid mutations N83Y and M186I in the HN protein, with no mutation in the fusion protein. Intriguingly, F13 exhibits reduced ability to induce cell-cell fusion despite its multistep replication. To investigate the potential role of HPIV2 HN protein in the regulation of cell-cell fusion, we introduced these mutations individually or in combination to the HN protein in the context of recombinant HPIV2. Following infection at a low multiplicity, Vero cells infected with the mutant virus H-83/186, which carried both the N83Y and M186I mutations, remained as nonfused single cells at least for 24 h, whereas most of the cells infected with wild-type virus mediated prominent cell-cell fusion within 24 h. On the other hand, the cells infected with the mutant virus, carrying either the H-83 or H-186 mutation, mediated cell-cell fusion but less efficiently than those infected with wild-type virus. Irrespective of the ability to cause cell-cell fusion, however, every virus could infect all the cells in the culture within 48 h after the initial infection. These results indicated that both the N83Y and M186I mutations in the HN protein are involved in the regulation of cell-cell fusion. Notably, the limited cell-cell fusion by H-83/186 virus was greatly promoted by lysophosphatidic acid, a stimulator of the Ras and Rho family GTPases.