C57BL/6 mice that are highly susceptible to infection with mouse hepatitis virus type 3 were protected against intraperitoneal viral infection by simultaneous intraperitoneal injection of Corynebacterium parvum. No protection was observed when C. parvum was given intravenously or when it was injected intraperitoneally 3 days before viral infection. Protective effects were, however, consistently found when C. parvum was given 2 h before or 2 h after viral infection. Activity was seen only against 10 50% lethal doses and not against 100 50% lethal doses. C. parvum also caused a significant decrease of virus type 3. These data suggest a direct effect of C. parvum on virus-susceptible cells. Injection of C. parvum in mice caused activation of natural killer (NK) cells and of interferon production. However, these two effects were equally demonstrable at high and low doses of C. parvum, whereas protection against mouse hepatitis virus type 3 was not demonstrable at low doses of C. parvum. Thus, antiviral protection may be dissociated from activation of NK cells and induction of interferon.
BHK-21 cells persistently infected with either vaccinia or foot and mouth disease virus were used to study the efficacy of antiviral compounds. The results of the persistent infection cell culture (PICC) test were compared with those obtained by the plaque reduction (PR) test. The comparison showed that: (1) the PICC test is more informative than the PR test; (2) stimulative as well as inhibitory activities of compounds are detectable, and (3) since the PICC test can be carried on for several weeks or even months this test is especially well suited to study the problem of drug resistance in cell cultures.
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