MG-63 cells, a line derived from an osteosarcoma, produced high yields of interferon after superinduction with polyinosinic acid·polycytidylic acid, cycloheximide, and actinomycin D. Advantages of MG-63 cells over diploid fibroblasts as a substrate are: no requirement for aging between confluency and induction, no requirement for priming, and 3.7-fold higher yields per square centimeter of culture surface. Physicochemically and biologically, MG-63 cell interferon resembles fibroblast rather than leukocyte interferon.

Interferon treatment of murine C‐type oncornavirus carrier lines results in a drastic inhibition of extracellular virus formation. When such cells are subcultured in the presence of interferon, synthesis of extracellular C‐type virus remains depressed until the interferon treatement is discontinued. Cell‐associated virus as seen in the electron microscope or as detected by uridine incorporation is not proportionally inhibited. Several possibilities to explain this discrepancy are tested. The most likely hypothesis is that interferon treatment primarily inhibits virus release.

Double-stranded RNAs from Penicillium chrysogenum virus have been treated with RNAse 111, pancreatic RNAse A and RNAse T, and the degradation of the RNAs has been studied under different conditions. It was found that only the two former enzymes cut across both strands, RNase TI cannot cleave double strands.RNase 111 was shown to digest double-stranded RNA by a two step process: an initial phase of specific cleavage is followed by random degradation. In the first phase the enzyme exhibited a definite preference for some specific base pattern.Partial or complete degradation with pancreatic RNase A could also be achieved in media with high salt concentration provided that the enzyme : substrate ratio was increased together with the salt concentration. By combining different assay techniques, the process of degradation was followed from the early stages to complete digestion and the breakdown products were characterised. It is suggested that a structural change in the enzyme molecules enables them to act on double-stranded RNA.RNAse TI, being unable to cleave double strands, provides a useful tool for studying the secondary structure of RNA molecules. Treatment with different nucleases yielded some new information on the structure of different RNA species in Penicilliurn stoloniferurn virus.

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Human fibroblast interferon (F-interferon) purified by adsorption on controlled-pore glass was given intramuscularly to patients at daily dosages of up to 20 x 106 units. Serum levels of antiviral activity were low or undetectable. In contrast, reasonably high serum titers were found in patients receiving interferon prepared from leukocytes (L-interferon). Similarly, in rabbits lower serum titers were seen with F-interferon than with L-interferon. These results are at variance with those obtained earlier

The production and partial purification of human fibroblast interferon for performing clinical trials is described. The interferon was produced by superinduction (exposure to riboinosinic-ribocytidylic acid, cycloheximide, and actinomycin D) of large numbers of human diploid fibroblast cultures. The yield averaged 750 units per cm2 of culture area. The interferon was concentrated and purified by a two-step procedure involving acid desorption from controlled-pore glass beads and dialysis against polyethylene glycol. Human plasma protein was added as a stabilizer. The lyophilized end product had a specific activity of 0.5 x 106 to 1 x 106 units/mg of protein; it could be reconstituted for injection at a concentration of 2 x 106 units/ml. The composition of this interferon was characterized by crossed immunoelectrophoresis with polyspecific antibodies prepared against the principal sources of potential contaminants: human serum, calf serum, and normal human fibroblasts. Several components of each source were detected. Although the major component of calf serum, bovine serum albumin, was absent, other minor components were retained by the production and purification sequence. One of the main contaminants of fibroblast origin was found to be fibronectin.The interferon mechanism is considered as an important element of the natural defense of vertebrates against viruses and cancer (for a review see reference 17). Human interferon exists in at least three molecular variants: L-(leukocyte), F-(fibroblast), and immune interferons (8,12,15,18,19

The effect of interferon on the synthesis and release of A-, B- and C-type viruses by oncornavirus carrier lines was studied. Murine cell lines were selected which carry either of these viruses and are sensitive to the antiviral effect of interferon, as measured by inhibition of vesicular stomatitis virus. Release of C-type virus was found to be highly sensitive. Release of B-type virus, on the contrary, was only marginally inhibited. Synthesis of intracisternal A-type particles was finally not inhibited by interferon pretreatment. These differences between infectious C-type and non-infectious A- and B-type viruses may reflect fundamental differences in the synthesis of these viruses.

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