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THE POTENT oncogenic compound ethylnitrosourea (ENU) selectively induces a high incidence of neurogenic neoplasms in the offspring of pregnant rats after i.v. administration late in gestation (Ivankovic & Druckrey, 1968; Koestner et al., 1971). The ENU-treated offspring survive a latency period of 100 to several hundred days and then succumb either to malignant gliomas of the brain or spinal cord, or to malignant Schwannomas located predominantly in Cranial Nerve V or spinal nerve roots. Whether these gliomas and Schwannomas are immunogenic, and whether the gliomas, located in the relativelv immunologically privileged brain (Scheinberg et al., 1964, 1965) are protected from immune surveillance comprise two questions pertaining to the role of immunity in the pathogenesis of brain tumours. Among several experimental approaches to these questions, one consists of immunizing against carcinogenesis (Prehn, 1961) i.e., inoculating the offspring of ENU-treated rats with glioma or Schwannoma antigens to determine whether this type of immunization increases the lifespan, or reduces the incidence of tumours, in susceptible animals. The feasibility of attempting to immunize against chemical carcinogenesis in animal tumour systems was established by the successful work of Taranger et al. (1 972) who demonstrated that immunization of Fischer rats with syngeneic bladdertumour cells significantly decreased the incidence of papillomas induced by subsequent implantation of methylcholanthrene in the bladder.Encouraged by these observations, we investigated whether immunization with tumour cells during the latent period affects the process of ENU neuro-oncogenesis in rats, and present our findings in this report. Twenty pregnant F-344 rats (Tyler's Laboratories, Bellevue, Washington) received single i.v. injections of ENU (gift of Dr T. Lloyd Fletcher, University of Washington) at 50 mg/kg body wt between the 16th and 19th days of gestation (Koestner et al., 1971). The 181 offspring (90 males and 91 females) at 5 weeks of age were divided into 6 groups and immunized as indicated in Table I. The glioma-cell immunizing inocula administered to Group I rats were prepared from 4 separate ENU-induced syngeneic glioma lines, previously generated in our laboratory by the alternate culture and transplantation method (Benda et al., 1971). These glioma lines originated from a cerebral glioblastoma multiforme, a cerebral mixed oligodendroglioma-astrocytoma, and 2 astrocytomas, one cerebral and the other from spinal cord. Earlygeneration stocks of the 4 glioma lines maintained in vitro in Waymouth's medium (Hellstrom & Hellstrom, 1971) were washed wvith phosphate-buffered

Immunization against ethylnitrosourea-induced autochthonous neurogenic rat tumours