Abstract
Infection of L929 cells by Newcastle disease virus (NDV) did not induce cell fusion, whereas addition of anti-L929 cell antiserum to the culture medium did result in cell fusion. When the antiserum was added to the culture medium of NDV-infected cells at the start of incubation, polykaryocytes appeared after 18 h post-infection (p.i.). On the other hand, when the antiserum was added to the culture medium at 18 h p.i., no polykaryocytes appeared, suggesting that the presence of the antiserum at the stage of NDV replication was necessary for the antiserum-induced cell fusion. Since anti-F polypeptide antiserum inhibited the cell fusion, functional F protein was essential for the antiserum-induced cell fusion. All the anti-L929 cell xenogenic sera tested were able to induce the cell fusion, whereas anti-L929 cell allogenic sera did not induce cell fusion. Anti-MCA cell serum also induced the cell fusion. Little replication of NDV was observed in the presence of anti-L929 cell serum and synthesis of virus-specific polypeptides, especially of M protein, was suppressed. However, there was no difference in expression of virus-specific glycoproteins on the surface membranes between cells treated with normal rabbit serum and those treated with anti-L929 cell serum. On the basis of the above results, the mechanism of induction of cell fusion by anti-host cell serum is discussed.