Scrapie Agent

Although scrapie has been studied for many years, the nature of the scrapie agent remains obscure (14). In spite of a number of unusual properties, this agent has been generally considered to be a virus (9, 13). In a previous study, repeated daily injections of 6,000 units of interferon failed to protect mice challenged intracerebrally (ic) with about 5,000 median lethal dose (LD50) of the scrapie agent (10). In another study repeated injections of statolon, an interferon stimulator, failed to protect mice from intraperitoneal (ip) challenge with a 10% scrapie mouse brain suspension which usually contains an extremely large challenge dose. However only 12 and 6 mice were included in the statolon treated and nontreated groups, respectively (6). In both of these experiments a very large challenge dose of the scrapie agent was used, and it seems possible that even a moderate protective effect of interferon might have been overwhelmed by these large challenge doses (2, 8). This is particularly true since the scrapie agent was injected ic in the first experiment, and, at least in the rabbit, there exists a strong blood-brain barrier to interferon (4). In addition, small numbers of mice were used in the second experiment. Because of this possibility, experiments were undertaken to determine the effect of longterm administration of a potent interferon stimulator, polyinosinic polycytidylic acid (In. Cn), on the development of scrapie in mice challenged peripherally with low doses of the scrapie agent. The ability of scrapie-infected mice to respond to interferon stimulators was also evaluated, and an attempt was made to determine whether interferon could be detected in the brains, serum, or spleens of scrapie-infected mice. A titered mouse brain pool of the scrapie agent was obtained from C. J. Gibbs, Jr., National Institutes of Health; this agent had initially been isolated from a sheep and was subsequently passaged in goats in England. It was further passaged eight times through Swiss mice prior to these experiments. Double-stranded In * Cnribonucleic acid was prepared as described previously (7). Final concentration of this material was 0.5 mg per 1.0 ml. Interferon titers were determined as the reciprocal of the highest dilution of the test fluid which inhibited the hemagglutinin yield of GD-VII virus during a single growth cycle in mouse L cells by 0.5 log10 (H. Oie et al., Proc. Soc. Exp. Biol. Med., in press). Titers were adjusted in accordance with the titer of a laboratory reference interferon which was titered in each assay. The international reference mouse serum interferon titered 104 units/ml. In the first experiment each experimental group consisted of 20 Swiss female mice, 4 to 6 weeks of age. Mice in groups 1 and 2 were injected ip on day zero with about 6 LD50 of the scrapie agent, and mice in groups 3 and 4 received ip about 100 LD50. Mice receiving In Cn (groups 1 and 3) received ip 100 ,ug in 0.2 ml of saline on day -1 and three times a week for 6 weeks; after this they received ip 50 ,ug three times a week until mice developed clinical evidence of scrapie. Mice in group 5 received just In*Cn according to the schedule outlined. Mice from group 5 were bled for serum interferon determinations at 1, 3, and 5 months after In Cn was begun; serum interferon levels, 6 hr after the previous dose of In Cn, were 1,000, 300, and 500 units/ml, respectively, at these times. The results of this experiment are shown in Fig. 1. Treatment with In Cn did not significantly affect the time of death of the mice receiving