The influenza virus-induced fusion of erythrocyte ghosts does not depend on osmotic forces

The role of osmotic forces and cell swelling in the influenza virus-induced fusion of unsealed or resealed ghosts of human erythrocytes was investigated under isotonic and hypotonic conditions using a recently developed fluorescence assay (Hoekstra, D., De Boer, T., Klappe, K., Wilschut, J. (1984) Biochemistry 23, 5675-5681). The method is based on the relief of fluorescence selfquenching of the fluorescent amphiphile octadecyl rhodamine B chloride (R18) incorporated into the ghost membrane as occurs when labeled membranes fuse with unlabeled membranes. No effect neither of the external osmotic pressure nor of cell swelling on virally mediated ghost fusion was established. Influenza virus fused unsealed ghosts as effectively as resealed ghosts. It is concluded that neither osmotic forces nor osmotic swelling of cells is necessary for virus-induced cell fusion. This is supported by microscopic observations of virus-induced fusion of intact erythrocytes in hypotonic and hypertonic media. A disruption of the spectrin-actin network did not cause an enhanced cell fusion at acidic pH of about 5 or any fusion at pH 7.4.