Prevalent types of tumors developing in neonatally thymectomized mice

Tumor development was observed for 24 months in neonatally thymectomized (nTx) and normal (C3H/HeMs x 129/J)F1 mice. Thymectomy was performed at 3 days of age. Ovarian (29%, with significant difference from the control at P less than 0.001), pituitary (6%, P less than 0.08), and lymphoreticular tumors (16%, P less than 0.05) were observed in higher incidence in nTx females compared with normal controls, resulting in a significant increase in overall tumor incidence (99 tumors in 114 nTx females vs. 37 tumors in 71 normal females, P less than 0.01). No apparent difference in overall tumor incidence was observed between nTx and non-Tx males. Also, there was no higher risk for lung, liver, and mammary tumors in males and females after neonatal thymectomy. The finding that increased tumor incidence was limited to endocrine and lymphoreticular tissues does not support the concept of immune surveillance of carcinogenesis, but rather suggests the importance of tumor-prone conditioning of endocrine or immune systems as a result of neonatal thymectomy.