Suppressive Effects on Simian Virus 40-induced Oncogenesis of Several Immunosuppressive Agents and Hormonal Modifications Applied during the Latent Period¹

The long latent period required for tumor induction with Simian virus 40 (SV40) in the subcutis of hamsters can be used to investigate whether host factor(s) participate in oncogenesis. Several treatments were applied during this period, and the results were compared with those of the same treatment applied in another series of experiments, homologous SV40 tumor grafting in hamsters. The results obtained were as follows: (a) the latent period for SV40 tumor induction in the female was shorter than that in the male; tumor development was delayed significantly by oophorectomy but was little affected by orchiectomy. Tumor development was markedly delayed in animals of both sexes by estrone given at birth but was accelerated by testosterone given in the adult male; (b) by each of these hormonal modifications, growth of transplanted SV40 tumors was influenced in a different way from that of tumor induction; (c) immunosuppressive treatments, such as thymectomy, administration of antilymphocyte sera, cyclophosphamide, or cortisone acetate delayed and decreased tumor development when applied in the latent period, and the degree or pattern of this effect varied from one procedure to another, depending on the sex and age of the animals; (d) in contrast, tumor growth was markedly accelerated in thymectomized or cortisone-treated hosts irrespective of sex.

The different and almost reverse effect of the same procedure in the two phases of tumorigenesis may indicate two discriminating mechanisms operating in the host during these phases. This different effect may be due to virtual absence of any “mature” neoplastic cell in the latent period, except for a few weeks before the appearance of a palpable tumor. These results suggest that the long period of latency may be spent to complete SV40-induced neoplastic conversion of cells, receiving some help by host factors.