Lack of oncogenicity with immunosuppressive therapy.

The effect of immunosuppressive therapy on the incidence of malignancy was examined in BALB/c mice. In a short (31-week) protocol study, malignancy was induced by inoculating animals with 10(2.1) ID50 of lymphatic leukemia virus. Antilymphocyte antiserum and azathioprine increased the mortality and shortened the latency period of leukemia. Combining these two agents had a synergistic effect. The concept of immunological surveillance was investigated in a long-term protocol using the identical animal and immunosuppressive systems. Mice did not receive leukemia virus and were observed for development of spontaneous malignancy. Significant immunodepression was demonstrated after 348 days of immunosuppressive therapy. After two years, all surviving animals were killed and examined for neoplasia. There was no evidence that immunodepression increased the incidence of spontaneous malignancy. Immunosuppression adversely influenced exogenously administered, virus-induced murine leukemia. However, immunosuppressive therapy was not innately oncogenic and the concept of immunological surveillance was not confirmed.