Biological Properties of Three Variants of Rous Sarcoma Virus

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Abstract
The responses to 3 antigenic variants of Rous sarcoma virus have been compared in 4 host systems. The strains are designated RSV(B), RSV(H), and RSV(29). On the chorio-allantoic membrane, RSV(B) induced relatively opaque pocks about 1.5 times the diameter of those initiated by the other 2 strains. Histologic examination showed that pocks were qualitatively similar with all 3 strains, but a greater proportion of those induced by RSV(B) showed involvement of the mesoderm and entoderm. Cornification of the ectoderm was more frequent with RSV(29) than with the other strains. Tissue cultures of chick embryo fibroblasts infected with RSV(B) or RSV(29) developed typical round cell foci, and plating efficiency in the tissue culture assay was high. With RSV(H), tissue cultures developed diffuse foci of elongated cells, which were impossible to count accurately, and plating efficiency was 10 percent or less compared with the chorio-allantoic membrane assay. In 7- to 8-week-old turkeys, RSV(B) and RSV(29) induced tumors that increased in size throughout a 10-week period. By 10 weeks, more than 50 percent of the birds infected with RSV(B) had gross tumors at sites other than the point of inoculation, but none of the turkeys infected with RSV(29) developed secondary tumors in this period. Turkey tumors induced by RSV(H) grew for about 2 weeks, then rapidly regressed, and gross metastases were not seen. The responses of 1-week-old Brown Leghorn chickens to all 3 strains were similar with respect to latent period of tumor response after use of both high and low doses of virus. All tumor growth rates were the same, although RSV(B) tumors reached somewhat larger size. The virus content of RSV(B) tumors was greater than that of tumors induced by the other 2 strains. Histologically, tumors induced by the 3 viruses were identical.