Growth Characteristics and Virology of Regressing Rous Sarcomas2

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Abstract
Tumors were produced in 95 percent (42 of 44) of 4- to 8-week-old White Leghorn chickens inoculated intramuscularly with 100 Focus-Forming units of Rous sarcoma virus (RSV), of which 40 percent (17) began to regress. Regressing and progressing tumors had mean latent periods of 10.2 and 8.8 days, respectively. Regressing tumors did not grow to the same size as progressive ones during a comparable period. Four of 4 progressive tumors transplanted successfully to the chorioallantoic membrane (CAM) of chick embryos, yielding virus, but 3 of 5 regressing tumors did not transplant; the remaining 2 grew, though less than progressive ones, and eventually yielded RSV. Regressing tumors contained very little RSV in tumor homogenates or in tissue culture Ruids assayed over a long period. No evidence of interferon was Found in either tumor homogenates or tissue culture Ruids of 7 regressing tumors. Three of 5 regressing tumors contained an interfering virus in either tumor homogenates or tissue culture Ruids, or both. Only 1 of 5 regressing tumor tissue cultures rechallenged with RSV yielded RSV at a level higher than a parallel uninoculated control culture of the tumor, but at a level so low as to be of doubtful significance. Two tumors, which had regressed, recurred at the original site after rechallenge inoculation with homologous virus in the opposite wing. RSV was recovered from tissue culture Ruids and CAM-passaged tumor of one, but no RSV was recovered from the other.