Immunologic mechanisms in the induction and regression of Shope papilloma virus-induced epidermal papillomas of rats.

Shope papilloma virus can readily induce epidermal papillomas in rabbits but rats show a high level of resistance to tumor induction. Tumors have not been induced in the skin of adult rats but when fetal rat skin is infected and grafted to syngeneic adult recipients (Lewis strain), papillomas develop in about 7–10% of the grafts. These tumors invariably regress within one week. The regression is always accompanied by an intensive lymphocytic infiltration. If this papilloma regression is mediated by an immune response, then it should be inhibited by immunosuppressive agents. For this purpose, adult rats were treated by a variety of methods. In the first series, varying doses of methylprednisolone were used. In the second series, anti-rat lymphocyte sera prepared in sheep or horses, were used either alone or in combination with adult thymectomy. All of the immunosuppressive methods resulted in prolongation of survival of the pappillomas. The best results were obtained with a combination of adult thymectomy and horse anti-rat lymphocyte serum. The original low incidence of papillomas (7–10%) was not increased by immunosuppression. It is concluded that an immunological mechanism is probably operative in the papilloma regression but that other mechanisms of resistance are responsible for the low initial incidence of papillomas.