Specific immune responses against malignant brain tumors have been difficult to demonstrate. Moreover, immunotherapy has met with little success, despite using lymphocytes with high levels of cytotoxicity against brain tumor cells. Lymphokine-activated killer (LAK) cells that nonspecifically kill brain tumor cells are produced by stimulating resting precursors with high concentrations of interleukin-2 (IL-2). Cytotoxic T lymphocytes that specifically kill brain tumor cells are produced by stimulating antigen receptor-positive immune-cell precursors with tumor cells. In an attempt to gain insight into immune cell function against brain tumors, the present study compared the in vitro and in vivo activities of LAK cells and cytotoxic T lymphocytes produced against RT2, a fast-growing rat glioma cell line. Lymphokine-activated killer cells were produced by stimulating normal rat spleen cells with 1000 units of IL-2, and RT2-specific cytotoxic T lymphocytes were produced by priming them in vivo with RT2 and Corynebacterium parvum and restimulating primed spleen cells with RT2 in vitro. Lymphokine-activated killer cells were highly cytotoxic for a panel of syngeneic and allogeneic brain tumor and non-brain tumor target cells, including RT2, as measured in a 4-hour 51Cr release assay. Cytotoxic T lymphocytes were highly cytotoxic only for syngeneic brain tumor target cells. Lymphokine-activated killer cells and cytotoxic T lymphocytes were tested for in vivo antitumor activity against intracerebral RT2 by intravenous adoptive transfer of activated lymphocytes. Untreated rats died in approximately 2 weeks. Lymphokine-activated killer cells plus IL-2 failed to affect survival when treatment was initiated as early as 1 day following tumor inoculation. Cytotoxic T lymphocytes and IL-2 administered as late as Day 5 rejected progressing intracerebral tumor. Thus, although both cytotoxic T lymphocytes and LAK cells exhibited high levels of in vitro killing of glioma cells, only cytotoxic T lymphocytes rejected progressing intracerebral tumors.

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Brain tumors are highly resistant to therapy. Their diffuse infiltrative nature and the relative inaccessibility of brain tissue to blood and lymph are barriers to surgical and cytotoxic treatments alike. The purpose of this study was to produce immune cells specifically reactive with an anaplastic rat glioma (RT2) and determine whether those cells could affect tumor progression in the brain. RT2-specific cytotoxic cells were prepared by priming rats in vivo with RT2 tumor cells and Corynebacterium parvum and stimulating the primed lymphocytes in vitro with irradiated RT2 tumor cells and interleukin-2 (IL-2). Cultured cells exhibited a high level of cytotoxicity against RT2, but not C6 (an allogeneic glioma), 3M2N (a syngeneic mammary tumor), or CSE (a syngeneic fibrosarcoma) tumor cells. To generate a model for therapy, rats were injected intracerebrally with RT2, generating progressing brain tumors, which killed untreated rats in approximately 2 weeks. To test the therapeutic potential of the effector cells, tumor-bearing rats were treated by intravenous injection of lymphocytes on Day 5 of tumor growth. Treated rats also received a 5-day course of systemic IL-2 beginning on Day 5. Treatment with IL-2 alone, RT2-primed spleen cells, or RT2-primed spleen cells stimulated in vitro with C6 did not affect rat survival. However, tumor-bearing rats treated with RT2-stimulated lymphocytes exhibited increased survival or were cured. Systemic IL-2 was an essential adjunct, because survival was not affected by treatment with effector cells alone. Therapy initiated on Day 8 of tumor progression lacked effect on survival.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytotoxic T lymphocytes specific for tumor-associated antigens are produced by exposing animals to tumor cells and stimulating lymphocytes from animals immunized in vitro with tumor cells and small amounts of interleukin-2 (IL-2). This study was designed to determine whether a fast-growing immunogenic avian sarcoma virus-induced glioma produces primed cytotoxic T lymphocyte precursors during its progression. Lymphocytes from intracerebral glioma-bearing rats generally failed to proliferate in vitro in response to immunization with tumor cells and IL-2 and, when proliferative responses were observed, the lymphocytes were not cytotoxic for glioma cells. However, when the same tumor was growing subcutaneously, lymphocytes proliferated and exhibited glioma-specific cytotoxicity when stimulated in vitro with autologous tumor cells and IL-2. Subcutaneous immunization of intracerebral glioma-bearing rats with tumor cells and adjuvant induced strong cytotoxic T lymphocyte responses. The results demonstrated that, while intracerebral tumor progression itself does not induce an anti-glioma immune response, immune responses to tumor-associated antigens may be induced by systemic immunization of tumor-bearing animals. The results suggest that the immunogenicity of brain tumors is masked by the immunologically privileged status of the brain, not by the induction of generalized immune suppression during tumor progression.