Nonspecific Inhibition of Tumor Growth in the Central Nervous System: Observations of Intracerebral Ependymoblastoma in Mice With Chronic Toxoplasma Infection

Abstract

Resistance against growth of tumor (ependymoblastoma) in the central nervous system was studied in mice infected with Toxoplasma gondii because such mice have been shown to be resistant to tumors outside the central nervous system and to have macrophages cytotoxic to tumor target cells in vitro. Mice used were C57BL/6J females. Groups of infected and un infected control mice were killed 4–53 days after tumor implantation, and their brains were removed for histologic examination. An inflammatory reaction consisting of a diffuse cellular infiltrate and microglial nodules was present in brains of Toxoplasma-infected mice but not in controls, whether or not they had been given injections of ependymoblastoma. An intense inflammatory cellular reaction surrounded tumor growth in the brains of infected mice, and the amount of tumor growth was less than that in brains of uninfected mice. In marked contrast, no inflammatory reaction was seen in response to tumors in brains of control animals. Tumors in brains of the Toxoplasma-infected mice showed more necrosis per unit area than did brain tumors in control mice. Growth of ependymoblastoma in a subcutaneous location was also significantly less in Toxoplasma-infected mice, suggesting a common mechanism of resistance to both subcutaneous and intracerebral tumor growth. A possible role for macrophages was suggested by the fact that, whereas peritoneal macrophages from Toxoplasma-infected mice were cytotoxic in vitro to ependymoblastoma target cells, those from control mice were not.