Oncogenicity of the R-35 Rat Mammary Tumor Virus

Summary

R-35 mammary tumor virus (R-35MTV), a C-type rat virus originally isolated from the R-35 mammary adenocarcinoma, was inoculated subcutaneously (sc) into neonatal Sprague-Dawley-derived GFRC:SPF(SD) females and males to determine its oncogenic potential in the species of origin. During a 20-month observation period, overall mammary tumor incidence in the R-35MTVinoculated female population was 29.5%; it was 18.5% in uninoculated controls. Although benign tumor incidence in both populations was similar, malignant tumor incidence in the virus-inoculated group (10.8%) was 6 times greater than in controls (1.8%). Malignant tumors, all mammary carcinomas or adenocarcinomas, also appeared earlier (4-5 months) in virus-treated than in control (9–13 months) animals. Estrogen imbalance, induced by sc implantation of 17β-estradiol pellets, resulted in a mammary tumor incidence of 35% in virus-treated animals during a 13- month observation period, whereas no tumors were detected in the control group during that time. Of 8 tumors examined from animals given virus, all were found positive for R-35MTV antigen and 3 for C-type particles by electron microscopy. Five primary tumors from controls were negative for both R-35MTV antigens and C-type particles. None of the 315 virus-inoculated and 247 control animals, of either sex, developed leukemia. Though morphologically it is a C-type particle, R-35MTV given neonatally seemed associated with an increased incidence of mammary neoplasms of epithelial rather than of connective tissue origin.