Cytotoxic substances from tropical plants.

Thymectomy at 3 days of age in several inbred strains of mice and in an F1 hybrid resulted in strikingly increased frequencies of neoplasms following parenteral injection of various “strains” of polyoma virus. Age susceptibility to the oncogenic effects of virus was extended to the adult period. The usual stigmate associated with thymectomy at birth were not observed in the 3-day thymectomized mice. Nevertheless the methods used to restore immunologically deficient neonates were also effective in restoring the capacity to resist tumor induction by polyoma virus. Growth curves of polyoma virus in kidney, salivary glands and liver were found to be similar in the thymectomized and intact littermates; hemagglutination-inhibiting (HI) antibodies determined periodically up to 30 days were also similar in both groups. A high frequency of parotid gland tumors was observed in C3H female mice thymectomized at 3 days of age. In these mice “spontaneous” infection with polyoma virus occurred between 2 and 4 months. Intact littermates similarly infected have remained free of polyoma-type tumors. In contrast, typical mammary adenocarcinomas appeared later and in lower frequency in thymectomized C3H females, and the incidence and latent period of methylcholanthrene (MCA)-induced fibrosarcomas was not influenced by thymectomy. The immunologic deficit in 3-day thymectomized mice was indeed subtle and not easily detectable by the usual methods; yet these mice were rendered strikingly sensitive to polyoma virus. The results are discussed in terms of a concept involving virus-specific “tumor” antigens and the immunologic status of the animal.