Effect of transient immunosuppression on host response to neonatally introduced oncogenic virus.

induced tumors in experimental animals (16, 21, 24). Nevertheless, there are some exceptions to this generalization. The effect of neonatal thymectomy on the incidence of viral-induced leukemias in mice is inhibitory (17). Immuno depression also inhibits the development of tumors of the mouse mammary gland induced by the MTV. There have been several reports that neonatal thymectomy decreases mammary tumor incidence or increases the latent period in MTV-infected mice (11, 22, 23), and this laboratory has recently reported that a similar decrease in the incidence of MTV-induced mammary tumors is observed after treatment with ALS (15). It has been suggested (18) that the inhibiting effect of thymectomy on the development of MTV-induced mammary tumors could result from either an alteration in the hormonal milieu, the necessity of the thymus for the growth and multiplication of MTV, or a relationship between oncogenesis and the immunological responsiveness of the host. Evidence of such hormonal (22) and immunological (10) changes has been reported. This communication deals with the 1st part of an investigation of the possibility that the similar inhibiting effect of treatment with ALS or ATS on mammary tumor development might be the result of an enhanced immunological reactivity to MTV or to cells altered by MTV following recovery of the mice from the transient immunodepression. Treatment with ATS is herein reported to