Differences in the response of normal and transformed BHK21 cells to dual virus infection. Conditions affecting synergism between vesicular stomatitis virus and newcastle disease virus

Polyoma-transformed BHK21 cells were less sensitive than untransformed BHK21 cells to plaque production by vesicular stomatitis virus (VSV) although neither the adsorption of the input virus to transformed cells nor the burst size were found to be reduced. Infection of either cell type with an avirulent strain (BI) of Newcastle disease virus (NDV) resulted in only a single cycle of noninfectious NDV hemagglutinin production. Neither BHK21 cells nor a polyoma-transformed clonal derivative (Cl-I) produced detectable interferon in response to NDV infection. On the contrary, dual infection with NDV and VSV resulted in a paradoxical enhancement of VSV plaque number and size in Cl-I but not in BHK21 cells. The degree of synergism was affected by the density of Cl-I cell cultures, by NDV multiplicity, and by the time interval between NDV and VSV infection. These findings suggest that alteration of cell functions affecting cellular sensitivity to single (VSV) and also dual (NDV-VSV) infection may be associated with the virus-transformed state.