Prolonged infection of L cells with vesicular stomatitis virus. Defective interfering forms and temperature-sensitive mutants as factors in the infection.

When L_y cells were treated with 100 units per milliliter of mouse interferon and then infected with vesicular stomatitis virus (VSV) at a multiplicity of 10–60 PFU/cell, a prolonged infection of cultures ensued, lasting in one case longer than 60 days. After four passages in L_y cells at high multiplicities of VSV from a prolonged infection, there was no inhibition of virus growth, and virus from the prolonged infection had the same distribution of intracellular and extracellular RNA forms as was found in wild-type virus passed four times in L_y cells. Also, the intracellular and extracellular RNA forms of virus taken directly from a prolonged infection of L_y cells were indistinguishable from those of the wild-type virus. A morphological examination of VSV from a prolonged infection did not show a significant number of small defective viral forms after four passages in L_y cells at a high multiplicity of infection. There was, however, evidence for the emergence of temperature-sensitive mutants of VSV during the course of the prolonged infection. Changing the incubation temperature of the cultures from 37° to 32° resulted in an increase in virus production and virus-induced cytopathology. The virus produced during prolonged infections grew to higher titers at 32° than at 37° and its plaque size at 39° progressively decreased with the length of time that the infection persisted. Furthermore, interferon production also seemed to have a role in the persistence of infections such as the treatment of cultures with rabbit anti-mouse interferon globulin and resulted in a significant rise in virus production and viral cytopathology in the cell cultures.