Interferon is produced in cultures of rabbit leukocytes in response to infection with Newcastle disease virus or in the absence of known viral infection. The macrophage appears to be the responsible producing cell. Cultures prepared from sterile peritoneal exudates, which contained about 90% macrophages, are at least as efficient as cultures of rabbit kidney (RK) cells in their capacity to synthesize NDV-induced interferon. Interferon can be detected in the medium by 2 hr after viral infection and the titers usually reach a peak of 10,000 PDD50/ml by 4–6 hr. Exposure to actinomycin prior to or shortly after viral induction effectively blocks interferon synthesis by cells of both types. However, macrophages become refractory to actinomycin by 30–60 min compared with 607–120 min for RK cells, a finding which suggests earlier and more rapid transcription of interferon-specific messenger RNA in macrophages. Macrophages harvested from the peritioneal cavity of rabbits injected intravenously with NDV 48 hr previously also exhibit slightly reduced capacity to synthesize interferon, but this tolerant state is less marked than is tolerance to production of circulating interferon in intact rabbits.

Interferon is also synthesized by rabbit macrophages not infected with virus but simply incubated at 37°C in medium with or without added bacterial endotoxin. Uninfected polymorphonuclear leukocytes, rabbit kidney and spleen cells produced no detectable interferon under similar conditions of cultivation. No interferon was released by intact macrophages incubated at 4°C or by ultrasonically disrupted macrophages incubated at 37°C. Although interferon titers were found to be higher when uninfected cultures were exposed to 10–100 µg/ml of E. coli lipopolysaccharide, unavailability of suitable pyrogen-free maintenance media precluded answering the question whether macrophages can continually synthesize and release interferon spontaneously. Interferon yields from uninfected macrophages were only 1% or less of the yields from NDV-infected macrophages, but the rates of synthesis were similar under both conditions. Studies with actinomycin and puromycin revealed that sequential transcriptive and translational events are required for de novo interferon synthesis by uninfected cells in a manner similar to virus-induced interferon synthesis.

The physical properties and molecular weights of these rabbit interferons are discussed in the following report (12).

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