Induction of resistance to ectromelia virus infection by corynebacterium parvum in murine peritoneal macrophages.

An in vitro model has been developed to study the replication of ectromelia virus in murine macrophages (M phi). Infection of mineral oil-elicited peritoneal M phi cultures with either the virulent (Moscow) or attenuated (Hampstead) strain of ectromelia virus led to productive infections. The kinetics of virus synthesis was similar to those seen following infection of murine fibroblasts. In contrast, peritoneal M phi s activated by intraperitoneal injection of Corynebacterium parvum vaccine were found to be totally refractory to infection by the attenuated strain and significantly more resistant to the virulent strain of ectromelia virus. Administration of C. parvum doses as small as 7 micrograms were sufficient to induce antiviral activity. M phi resistance became maximal at 5-9 days after C. parvum administration; however, M phi resistance was unstable during in vitro culture. Decay of antiviral activity was detected within the first 24 hr of culture and complete virus susceptibility returned after 5 days in culture. Peritoneal exudate cells (PEC) from C. parvum-immunized mice could induce resistance in susceptible M phi cultures during overnight cocultivation. In addition, cell-free culture supernatants from C. parvum-immune PEC could also induce resistance in susceptible M phi cultures, suggesting that a soluble factor, induced by C. parvum immunization and possessing interferon activity, may account for the intrinsic resistance to ectromelia virus by activated M phi s.