Inhibition of mammary tumors by incomplete T-cell depletion.

The effects of neonatal and perinatal thymectomy on mammary tumorigenesis in (C57BL X I)F1fC3H hybrid female mice were determined. When hybrid females were neonatally thymectomized by controlled suction, a procedure that removes thymic lobes completely, a large proportion of animals developed stigmas of a fulminant wasting disease and died before tumors developed. However, when hybrid females were subjected to neonatal thymectomy by continuous suction, a procedure that resulted in retention of thymic remnants, they survived and manifested a significant prolongation of latent period before tumorigenesis. When complete removal of the thymus was carried out in the perinatal period, the effect on mammary tumorigenesis was critically dependent on the age at surgery. The procedure was without effect when performed at 1, 3, and 8 weeks of age. However, when it was performed at 9-12 days of age, there was a delay or a decrease in the appearance of mammary tumors. The extent of T-cell depletion and/or its timing in relationship to the introduction of murine mammary tumor virus appeared to play a critical role in determining the effect on eventual tumor development.