Protection by grafts of embryonal rat tissues (teratomas) against induction and transplantation of malignant tumors.

Clinical observations and experimental studies have shown that pregnancy may have inhibitory effects on tumor growth rather than invariably aggravate neoplastic disease as believed previously. It has been suggested that circulating factors of maternal or fetal origin may protect against tumor growth during pregnancy. The previously created experimental model of teratomas provides the means of having an adult animal bearing a permanent graft of embryonal tissues. To investigate the potential effects of embryonal factors on the growth of malignant neoplasms, rats carrying grafts of embryonal tissues were subjected to the induction or transplantation of carcinomas and lymphomas. Finely minced embryo tissues or cell suspensions injected in homologous rat recipients formed permanent benign teratomas composed of a variety of well differentiated tissues. One injection of N-methyl-N-nitrosourea, a potent carcinogen administered to all rats, induced fatal mammary adenocarcinoma in 50-60% of control rats but in none of the rats bearing a grafted teratoma. Transplantation of N-methyl-N-nitrosourea-induced mammary adenocarcinoma or Gross virus-induced lymphoma killed 100% of control rats but resulted in smaller, later appearing tumors in only 25-61% of teratoma-bearing rats. The present experiments showed that rats bearing grafts of embryonal tissues in the form of teratomas were partially or totally protected against the induction and transplantation of malignant tumors that killed 100% of controls. These results suggest that the embryonal tissues are a source of tumor-inhibitory factors, which may be a part of mechanisms controlling the growth and detecting the aberrations of embryonal tissues. Their identification and analysis may provide knowledge about embryonal growth and possibly about new substances with antineoplastic activity.