Splenectomy, chemically-induced mammary tumors and parathymic lymph nodes in rats: experimental and morphological studies.

The objective of this study was to examine whether splenectomy alters the tumorigenic response of rats to chemically-induced mammary tumors and to determine the role played by the parathymic lymph nodes (PTLN) in the antitumor immune response. Female rats were splenectomized and then exposed to 9,10-dimethyl-1,2-benz(a)anthracene (DMBA) to induce mammary tumors. Splenectomy decreased the rate of tumor appearance. The latent period for appearance of the first tumors in splenectomized rats was 12.0 +/- 0.9 week compared to 9.7 +/- 0.5 week in intact controls. The latent periods for appearance of all tumors including those with malignancy were similar for both rat groups. Splenectomy caused a significant increase in the size of the PTLN as well as changes in their structure. The cortex became thick, better formed and enriched with lymph elements of follicles and a wide venous net. CD8+ cells infiltrated the paracortex whereas CD4+ cells were located around the blood vessels. In conclusion, splenectomy inhibits the early stages of tumorigenesis, but does not prevent the progress of carcinogenesis. Different responses of splenectomized (operated) and intact rats to the effect of DMBA can be explained by an increase in the nonspecific resistance of splenectomized rats as a result of the operation. PTLN actively participate in this compensatory reaction of the immune system of animals.