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Investigation of the Anti-viral Mechanism of Poly I and Poly C Against Encephalomyocarditis Virus Infection in the Absence of Interferon Induction in Mice

Anti-Virals Department, Searle Research Laboratories, Lane End Road, High Wycombe, Bucks., U.K.

Protection of mice against EMC virus infection by poly C and poly I has already been distinguished from interferon mediated protection in several ways. Transfer of serum from EMC virus infected and poly C or poly I treated mice to donor mice that were then infected shows that the anti-viral effect of the single-stranded poly-nucleotides is not due to boosting interferon produced by infection itself in the way that interferon can be ‘primed’ in vitro. Mice surviving infections of more than 1 x LD₁₀₀ as a result of poly C or poly I treatment show no protection against re-infection 15 days after the first infection, indicating no long-term stimulation of immune responses to the virus. Mice treated with an immunosuppressive regime of cytosine arabinoside can be protected against EMC virus infection with poly C and poly I treatment and athymic ‘nude’ mice can also be protected. The possibility of IgM stimulation by poly C and poly I seems unlikely from experiments in which serum was transferred from mice treated with the polynucleotides and an inactivated EMC ‘vaccine’ to recipient mice which were then challenged with infectious virus.

Protection of mice against EMC virus by the single-stranded polynucleotides is abolished by administration of silica to the mice, implying an involvement of macrophages in the protective effects of poly C and poly I. The possibility that the polynucleotides stimulate clearance of virus particles, at least from immunologically responsive regions of the mouse, has been discounted by the inability of polynucleotide treatment to suppress ‘vaccine’ mediated protection of mice. These results indicate that macrophages are involved in the anti-viral effects of poly C and poly I either because they inhibit replication of the virus in macrophages or because direct anti-viral properties of macrophages are activated by the polynucleotides.

Received 7 November 1975; accepted 2 March 1976.