Antiviral and Antitumor Immunologic Mechanisms Operative in the Shope Papilloma-Carcinoma System

In 1933 Shope described an infectious papilloma of wild cottontail rabbits. The tumor was regularly transmissible to domestic rabbits on primary inoculation but was not transmissible in indefinite series. He noted that the tumors of 2 animals regressed, whereas in most cases the papillomas persisted indefinitely. During the following 10 years, Rous, Kidd and Beard published extensive studies of these papillomas and of squamous cell carcinomas originating in them. Shope (1937) was the first to note that, although papillomas of domestic rabbits were commonly free of virus as judged by transmission experiments, they nevertheless contained viral antigen as judged by their capacity to stimulate the development of neutralizing antibody and resistance to reinfection following intraperitoneal injection of tissue suspensions. He spoke of “masked” virus and suggested that this might represent either virus rendered inactive by combination with neutralizing substance or by alteration to a non-infective phase. Carcinomas were likewise shown...