Laboratory Investigation Parent phenotype and age dependence, on rat glioma tumor rejection Alicia Gonzalez-Martin 1 , Daniel Mun˜oz-Espı´n 1 , Andre´s M. Alonso 2 and Marta Izquierdo 1 1 Departamento de Biologı´a Molecular-Centro de Biologı´a Molecular Severo Ochoa; 2 Departamento de Matema ´ticas, Facultad de Ciencias, Universidad Auto ´noma de Madrid, Madrid, Spain Key words: glioma, host rejection, neonatal rats, parental phenotype, tumor transplantation Summary Background: The brain, despite the blood-brain barrier, does not escape to the highly variable host rejection response mediated by a very strong and complex immune reaction when rat glioma cells are transplanted into the adult animal. Methods: Crosses were performed among parents that are able or enable to reject a well-known brain tumor cell line (C6). Newborn animals were also challenged with rat glioma cells both in the brain and the side flanks. Results: The percentage of susceptibility or resistance to develop a lethal glioma can be estimated knowing the parental phenotypes. When both parents had rejected an induced tumor, 63% of the progeny will also reject it. Similarly, if both parents died as a consequence of the tumor, 70% of the progeny would also be unable to reject the challenge of glioma C6 cells. Newborn animals do not have a mature immune system and they tolerate transplanted cells much better than adults. We found no rejection to glioma C6, at both brain and side flank sites, in 1-day-old neonatal Wistar rats. Tumors were beginning to be eliminated if the cells are inoculated at day 3 from birth on the flanks, and at 1 week from birth on the brain. Conclusions: There is a genetic component conferring susceptibility or resistance to the lethal effect of tumor development and progression depending on the parental phenotype of the adult rats. Neonatal rats represent a much more reliable model than adults to study experimental therapies against gliomas. Introduction Gliomas are brain tumors that differ from most other cancers by their diffuse invasion of the sur- rounding normal tissue and their notorious recurrence following all forms of therapy (surgery, radiation, chemotherapy or gene therapy). The rat C6 glioma cell line was cloned in the 1960s [1] and remains as a very popular model for diverse ther- apies of the most frequent human glioma: glio- blastoma multiforme. A small deposit of C6 cells (5 · 10 5 ), inoculated into the brain of immune- competent rats, guarantees the development of a large brain tumor, visualized by magnetic reso- nance [2], that would be as fatal as it is for hu- mans. The intracraneal tumor displays the typical histological features of human glioblastoma, including necrosis, vascular proliferation, pseud- opallisadin, frequent mitotic figures and infiltrate growth. But we found, as others have also found [3,4] that a variable number of the inoculated animals can reject the tumor once formed, com- promising the usefulness of the therapy under study. The origin of C6 cells is not well docu- mented in the literature, and different rat strains have been used as supposedly syngenic hosts. It was found that the C6 tumors regressed sponta- neously in the BDX rat strain in three out of five animals, as a result of an allogeneic response that might have represented accelerated graft rejection [5]. We were interested in characterizing this observation, mainly at the genetic level, to find whether there was a genetic combination that gave higher resistance to developing tumors. If the inoculated cells were identical for all the animals, we wanted to know why some, but not all Journal of Neuro-Oncology 70: 29–34, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands.