Anti-viral effects of single-stranded polynucleotides against avirulent Semliki Forest virus infection of mice and avirulent infection of rats with encephalomyocarditis virus.

Single-stranded polynucleotide preparations, which neither induce detectable interferon nor affect immune responses, suppress development of antiviral antibodies in mice infected with an avirulent strain of SFV. On a weight basis the antiviral activity of a mixture of poly(I) and poly(ho5C)-copolymer is greater than that of tRNA and similar antiviral effects are observed against a related virulent strain of SFV. EMC virus causes and avirulent infection of rats and development of EMC virus antibodies (routinely determined by assaying the protective effect of rat serum against EMC virus infection of mice) is suppressed when the rats are treated with tRNA or the mixture of poly(I) and poly(ho5C)-copolymer. This suppression of antibodies to EMC virus appears to reflect reduction of virus replication. Treatments of 6 mg/rat i.p. or i.v. 6 hours before infection confer essentially the same antiviral effect as 3 times these polynucleotide doses administered during 3 days immediately post infection. These results with avirulent infections indicate that the previously reported antiviral effects of the single-stranded polynucleotides are not simply due to modifications of the tissue pathology which leads to death in the case of virulent virus infections.