Interleukin-2-activated lymphocytes from brain tumor patients. A comparison of two preparations generated in vitro

Cellular and functional characterization of immunoresistant human glioma cell clones selected with alloreactive cytotoxic T lymphocytes reveals their up-regulated synthesis of biologically active TGF-beta

Two immunoresistant (IR) glioma cell variants, 13-06-IR29 and 13-06-IR30, were cloned from 13-06-MG glioma cell populations after receiving continuous immunoselective pressure from multiple alloreactive cytotoxic T lymphocyte (aCTL) preparations. Reapplication of aCTL immunoselective pressure to the IR clones, displaying a partial regain in sensitivity to aCTL after removal of the selective pressure, restored the resistance. The IR variants exhibited cross-resistance to non-human leukocyte antigen (HLA)-restricted effector cells and gamma-irradiation, but not to carmustine. The IR clones were characterized for factors that might contribute to the immunoresistance. The aCTL adhesion to extracellular matrix extracts derived from either the IR clones or the parental cells was similar and not impaired. Furthermore, aCTL binding to parental cells and IR clones was equal. Down-regulation of the cell recognition molecules, class I HLA or intercellular adhesion molecule-1 (ICAM-1), that would inhibit their recognition by aCTL was not observed on the IR clones. The down-regulation of Fas by the IR clones correlated with their resistance to FasL-induced apoptosis. HLA-G or FasL that might provide an immunotolerant environment or provide a means of counterattack to aCTL, respectively, were not associated with the IR phenotype. The aCTL, coincubated with the IR clones and parental cells, displayed up-regulation of multiple secreted cytokines. A significant up-regulation of bioactive transforming growth factor (TGF)-beta was observed in the IR clones compared with the parental cells. These data suggest that increased secretion of bioactive TGF-beta may inhibit aCTL lysis of the IR clones. Disruption of the TGF-beta signaling pathway may circumvent the resistance.

Intracavitary placement of autologous lymphokine-activated killer (LAK) cells after resection of recurrent glioblastoma

This study was performed to obtain safety and survival data for patients with histologically confirmed recurrent glioblastoma multiforme (GBM) who received intralesional lymphokine-activated killer (LAK) cells following surgery. LAK cells were generated by incubating peripheral blood mononuclear cells with interleukin-2 for 3 to 5 days in vitro. Forty patients with pathologic confirmation of GBM at surgery had placement of autologous LAK cells into the tumor cavity. The 23 men and 17 women had a median age of 48 years (range 21-76). The median interval from the original diagnosis of glioma to LAK treatment was 10.9 months. Patients received an average of 2.0 +/- 1.0 x 10(9) LAK cells, with viability of 91 +/- 6.8%. Treatment was well tolerated; there was one death within 60 days. At a median follow-up of 2.3 years, median survival post-LAK was 9.0 months; 1-year survival was 34%. Gender, age, location of tumor, LAK cell lytic activity, number of cells implanted, and inclusion of interleukin-2 at cell instillation were not correlated with outcome. Median survival from the date of original diagnosis for 31 patients who had GBM at initial diagnosis was 17.5 months versus 13.6 months for a control group of 41 contemporary GBM patients (p2 = 0.012). This treatment is safe and feasible. The median survival rates are higher than reported in most published series of patients who underwent reoperation for recurrent GBM. A randomized trial would be needed to establish therapeutic benefit.

Combining cytotoxic and immune-mediated gene therapy to treat brain tumors

Glioblastoma (GBM) is a type of intracranial brain tumor, for which there is no cure. In spite of advances in surgery, chemotherapy and radiotherapy, patients die within a year of diagnosis. Therefore, there is a critical need to develop novel therapeutic approaches for this disease. Gene therapy, which is the use of genes or other nucleic acids as drugs, is a powerful new treatment strategy which can be developed to treat GBM. Several treatment modalities are amenable for gene therapy implementation, e.g. conditional cytotoxic approaches, targeted delivery of toxins into the tumor mass, immune stimulatory strategies, and these will all be the focus of this review. Both conditional cytotoxicity and targeted toxin mediated tumor death, are aimed at eliminating an established tumor mass and preventing further growth. Tumors employ several defensive strategies that suppress and inhibit anti-tumor immune responses. A better understanding of the mechanisms involved in eliciting anti-tumor immune responses has identified promising targets for immunotherapy. Immunotherapy is designed to aid the immune system to recognize and destroy tumor cells in order to eliminate the tumor burden. Also, immune-therapeutic strategies have the added advantage that an activated immune system has the capability of recognizing tumor cells at distant sites from the primary tumor, therefore targeting metastasis distant from the primary tumor locale. Pre-clinical models and clinical trials have demonstrated that in spite of their location within the central nervous system (CNS), a tissue described as 'immune privileged', brain tumors can be effectively targeted by the activated immune system following various immunotherapeutic strategies. This review will highlight recent advances in brain tumor immunotherapy, with particular emphasis on advances made using gene therapy strategies, as well as reviewing other novel therapies that can be used in combination with immunotherapy. Another important aspect of implementing gene therapy in the clinical arena is to be able to image the targeting of the therapeutics to the tumors, treatment effectiveness and progression of disease. We have therefore reviewed the most exciting non-invasive, in vivo imaging techniques which can be used in combination with gene therapy to monitor therapeutic efficacy over time.

Interactions of the allogeneic effector leukemic T cell line, TALL-104, with human malignant brain tumors

TALL-104 is a human leukemic T cell line that expresses markers characteristic of both cytotoxic T lymphocytes and natural killer cells. TALL-104 cells are potent tumor killers, and the use of lethally irradiated TALL-104 as cellular therapy for a variety of tumors has been explored. We investigated the interactions of TALL-104 cells with human brain tumor cells. TALL-104 cells mediated increased lysis of a panel of brain tumor cells at low effector-to-target ratios over time. We obtained evidence that TALL-104 cells injured glioma cells by both apoptotic and necrotic pathways. A 7-amino actinomycin D flow cytometry assay revealed that the percentages of both apoptotic and necrotic glioma cells increased after TALL-104 cell/glioma cell coincubations. Fluorescent microscopy studies and a quantitative morphologic assay confirmed that TALL-104 cell/glioma cell interactions resulted in tumor cell apoptosis. Cytokines are secreted when TALL-104 cells are coincubated with brain tumor cells; however, morphologic analysis assays revealed that the soluble factors contained within clarified supernates obtained from 4 h coincubates added back to brain tumor cell cultures did not trigger the glioma apoptosis. TALL-104 cells do not express Fas ligand, even upon coincubation with glioma targets, which suggests that the Fas/Fas ligand apoptotic pathway is not likely responsible for the cell injury observed. We obtained evidence that cell injury is calcium dependent and that lytic granule exocytosis is triggered by contact of TALL-104 cells with human glioma cells, suggesting that this pathway mediates glioma cell apoptosis and necrosis.

Human alloreactive CTL interactions with gliomas and with those having upregulated HLA expression from exogenous IFN-gamma or IFN-gamma gene modification

By flow cytometry, a panel of 18 primary glioma cell explants exhibited high expression of class I HLA-A, B, C, but class II HLA-DR expression was absent. Freshly isolated normal brain cells displayed little or no HLA antigens. Alloreactive cytotoxic T lymphocytes (aCTL), sensitized to the HLA of the patient, were generated in a one-way mixed lymphocyte response (MLR). The specificity of aCTL was confirmed to be to target cells (patient glioma cells or lymphoblasts) expressing the relevant HLA antigens. However, nontumor patient-specific aCTL did not lyse normal brain cells. Titration of antibodies to HLA class I into cytotoxicity assays blocked lysis of gliomas by aCTL, confirming aCTL T cell receptor (TCR) interactions with the class I antigen on gliomas. Furthermore, aCTL interactions with glioma cells caused their apoptosis. Coincubations of aCTL with gliomas resulted in upregulated cytokine secretion. Importantly, dexamethasone, an immunosuppressive steroid used for brain edema, did not affect aCTL lytic function against tumor, indicating that steroid-dependent patients may benefit from the immunotherapy. We also explored the use of interferon-gamma (IFN-gamma) to increase aCTL tumor recognition. Coincubation of gliomas with exogenous IFN-gamma (500 U/ml, 48 h) caused a 3-fold upregulation of HLA class I and a slight induction of class II antigen expression. Gene-modified glioma cells producing IFN-gamma similarly displayed upregulated HLA expression. Glioma cells incubated with exogenous IFN-gamma or IFN-gamma-transduced glioma cells were more susceptible to lysis by aCTL than their parental counterparts, thus supporting the concept of combining IFN-gamma cytokine gene therapy with adoptive aCTL immunotherapy for brain tumor treatment.

Lymphokine activated killer cells from umbilical cord blood show higher antitumor effect against anaplastic astrocytoma cell line (U87) and medulloblastoma cell line (TE671) than lymphokine activated killer cells from peripheral blood

OBJECTS

The aims of this study were to assess the cytotoxic capability of lymphokine-activated killer (LAK) cells from umbilical cord blood (UCB), to compare them with those of peripheral blood (PB)-derived cells against anaplastic astrocytoma cell line (U87) and medulloblastoma cell line (TE671), and to identify which mechanism and genes were involved in cytotoxicity.

METHODS

The effector cells were generated by interleukin-2 from UCB and PB. The antitumor property of effector cells against the target cells (U87, TE671) were estimated using a visual survival cell assay. The mixed target and effector (UCB) cells were analyzed for whether DNA fragmentation was present or not. Reverse transcription polymerase chain reaction analysis was then performed to estimate the statement of the perforin and FasL genes in activated and inactivated cells from UCB.

RESULTS

The higher in vitro antitumor properties of the LAK cells from UCB were observed in comparison to the LAK cells from PB against the U87 and the TE671 ( p<0.05). Apoptosis may be one of the lysis mechanisms of target cells by the LAK cells from UCB. The contributing genes could be FasL and perforin.

CONCLUSIONS

This study suggests that UCB may be used as a source of LAK cells in adults and children suffering from anaplastic astrocytoma or medulloblastoma.

Changes in the immunologic phenotype of human malignant glioma cells after passaging in vitro

Although immunotherapeutic strategies against glioblastomas have been promising both in vitro and in animal models, similar successes have not been realized in human clinical trials. One reason may be that immunotherapeutic strategies are based on prior studies that primarily have used human glioblastoma cell lines passaged in vitro, which may not accurately reflect the in vivo properties of glioblastoma cells. In this report, we used flow cytometry to quantify the expression of immunological cell surface molecules on human glioblastomas directly ex vivo (prior to any in vitro culturing) and after varying passages in vitro. Furthermore, we used ELISA to quantitate cytokine secretion after various passages in vitro. We demonstrate that in vitro culturing of established cell lines led to increases in the cell surface expression of MHC class I and ICAM-1 and secretion of IL-6 and TGF-beta(2). Furthermore, there were significant changes in the expression of MHC class I, MHC class II, B7-2, ICAM-1, and FasL when comparing ex vivo tumor cells to those after a single passage in vitro. After passaging once in vitro, there were also significant changes in the secretion of TGF-beta(2) and IL-10. This report indicates that in vitro culturing leads to significant changes in both cell surface molecules and secreted cytokines, which are known to affect the ability of immune cells to initiate an anti-tumor immune response. These changes in the immunological phenotype of glioblastomas after in vitro culturing may in part explain the limited success of immunotherapeutic strategies against glioblastomas in human clinical trials.

Immunologic approaches to therapy for brain tumors

Malignant brain tumors are notoriously invasive. Although surgical debulking can relieve the patient of the main mass of tumor, adjuvant treatments are needed to target the glioma cells that infiltrate through normal parenchyma as single cells or pockets of tumor cells from which recurrent tumors arise. Successful adjuvant cellular therapy of brain tumors, or activation of endogenous immune cells, requires that either cell effectors make direct contact with tumor cells or come within close proximity to them and exert an indirect effect. This review examines current clinical trials aimed at direct lysis of glioma cells and trials making gliomas more visible to the endogenous immune system.

Glioma immunology and immunotherapy

OBJECTIVE

Despite advances in conventional therapy, the prognosis for most glioma patients remains dismal. This has prompted an intensive search for effective treatment alternatives. Immunotherapy, one such alternative, has long been recognized as a potentially potent cancer treatment but has been limited by an inadequate understanding of the immune system. Now, increased insight into immunology is suggesting more rational approaches to immunotherapy. In this article, we explore key aspects of modern immunology and discuss their implications for glioma therapy.

METHODS

A thorough literature review of glioma immunology and immunotherapy was undertaken to inquire into the basic immunology, central nervous system immunology, glioma immunobiology, standard glioma immunotherapy, and recent immunotherapeutic advances in glioma treatment.

RESULTS

Although gliomas express tumor-associated antigens and appear potentially sensitive to immune responses, many factors work together to inhibit antiglioma immunity. Not surprisingly, most clinical attempts at glioma immunotherapy have met with little success to date. However, novel immunostimulatory strategies, such as immunogene therapy, directed cytokine delivery, and dendritic cell manipulation, have recently yielded dramatic preclinical results in glioma models. This suggests that glioma-derived immunosuppression can be overcome.

CONCLUSION

Modern molecular biology and immunology techniques have yielded a wealth of new data about glioma immunobiology. Armed with this information, many investigators have proposed novel means to stimulate antiglioma immune responses. Although definitive clinical results remain to be seen, the current renaissance in glioma immunology and immunotherapy shows great promise for the future.

Immune defects observed in patients with primary malignant brain tumors

Malignant glioblastomas (gliomas) account for approximately one third of all diagnosed brain tumors. Yet, a decade of research has made little progress in advancing the treatment of these tumors. In part this lack of progress is linked to the challenge of discovering how glial tumors are capable of both modulating host immune function and neutralizing immune-based therapies. Patients with gliomas exhibit a broad suppression of cell-mediated immunity. The impaired cell-mediated immunity observed in patients with gliomas appears to result from immunosuppressive factor(s) secreted by the tumor. This article reviews what has been elucidated about the immune defects of patients harboring glioma and the glioma-derived factors which mediate this immunosuppression. A model involving systemic cytokine dysregulation is presented to suggest how the immune defects arise in these individuals.

Intracranial administrations of single or multiple source allogeneic cytotoxic T lymphocytes: chronic therapy for primary brain tumors

Previous investigations by our group demonstrated the efficacy of single source allogeneic cytotoxic T lymphocytes (CTLs) given multiple times in reducing or curing tumor burden in the rat 9L gliosarcoma model. In this study, the lack of toxicity to normal brain when single source allogeneic CTLs were intracranially administered multiple times is documented. Additionally, the efficacy and lack of toxicity of allogeneic CTLs from multiple sources, each given once is documented. CTLs sensitized to Fischer antigen were prepared from major histocompatibility complex incompatible DA, PVG, Sprague-Dawley and Wistar-Furth rat lymphocytes. CTLs from multiple donors were administered one time each to Fischer rats bearing established 9L tumor at staggered intervals over a two week period and survival was monitored in relation to a sham treated group. Additional groups of nontumor-bearing rats received either multiple source allogeneic CTLs or single source DA anti Fischer CTLs in the same treatment regimen. Histological evaluation of the nontumor-bearing brains receiving either single or multiple source allogeneic CTL infusions showed minimal localized brain damage confined to the cannulation tract. No neuronal loss or inflammatory reaction was seen either adjacent to or remote from the administration site. Brains from the long-term survivors of the tumor-bearing animals showed no residual neoplasm; the instillation site had focal sterile abscesses; gliosis and neuronal loss did not extend into adjacent brain. The safety and potential of chronic, local allogeneic CTL administration, derived from multiple donors, as adjuvant local therapy for brain tumors was demonstrated.

Systemic chemotherapy combined with local adoptive immunotherapy cures rats bearing 9L gliosarcoma

Survival of Fischer rats bearing 9L gliosarcoma in the brain was measured to determine the efficacy of 1) systemically administered chemotherapy with local adoptive immunotherapy (chemo-adoptive immunotherapy) or 2) systemically administered chemo-immunotherapy. Winn assays, where tumor instillation coincided with the start of treatment, and one-week established tumor assays were conducted. Survival of chemo-adoptive immunotherapy treated groups given intraperitoneal cyclophosphamide and intracranial lymphokine activated killer cells and recombinant Interleukin-2 was significantly extended when compared to sham treated control groups, to groups given chemotherapy with intraperitoneal cyclophosphamide, and to groups treated by local adoptive immunotherapy with intracranial lymphokine activated killer cells and Interleukin-2. The killer cells were generated from spleens of donor rats that either had or had not been given cyclophosphamide 24 h earlier. Long-term survivors (9/39), sacrificed at day 70, were obtained only in the chemo-adoptive immunotherapy treated groups; 7/39 had no histologic evidence of tumor and had focal sterile abscesses at the site of killer cell instillation. Average group weight plotted over time showed that there was acceptable toxicity with chemo-adoptive immunotherapy; the toxicity was identical to that obtained with systemic cyclophosphamide treatment. In contrast, survival of chemo-immunotherapy treated groups given systemic cyclophosphamide and Interleukin-2 was not significantly extended from groups which were sham treated or treated only with systemic Interleukin-2. Rapid decline of average group weight plotted over time and early deaths following chemo-immunotherapy treatment indicated that the regimen was toxic. The effect of cyclophosphamide administration on the splenocytes of donor rats and the LAK cells generated from them was determined by in vitro studies analyzing cell number, viability, phenotypic expression and cytotoxicity against 9L tumor. In the treatment of this intracranial neoplasm, the beneficial effects of cyclophosphamide were determined to occur in situ in the tumor-bearing host. No benefit resulted from cyclophosphamide treatment of donor rats that supplied splenocytes for LAK cell production.

Therapy of recurrent high grade gliomas with surgery, and autologous mitogen activated IL-2 stimulated killer (MAK) lymphocytes: I. Enhancement of MAK lytic activity and cytokine production by PHA and clinical use of PHA

Nineteen patients with recurrent high grade gliomas were treated in a phase I/II trial with aggressive debulking of the tumor, mitogen activated IL-2 stimulated peripheral blood lymphocytes (MAK cells), and rIL-2. Phytohemagglutin (PHA) was introduced into the tumor site in 16 patients prior to implanting MAK cells and IL-2 in an attempt to trigger more effective lysis of the tumor in vivo. In vitro both TNF bioactivity and cytolytic activity of long term cultured MAK (LMAK) cells were dramatically enhanced by adding PHA to the cultures of these activated PBL. Three of eleven patients (27%) had a decrease in size of the enhancing lesion on CT and/or MRI. Seven (37%) patients clinically improved. Median survival after therapy was 30 weeks. PHA was shown to be safe in vivo and more effective than IL-2 triggering enhanced effector function in vitro.

The autocrine role of tumor necrosis factor in the proliferation and functional differentiation of human lymphokine-activated T killer cells (T-LAK) in vitro

The autocrine role of tumor necrosis factor alpha (TNF) in the proliferation and functional differentiation of human lymphokine-activated T-killer cells (T-LAK) in vitro was investigated. Human peripheral blood lymphocytes initially stimulated with IL-2 and phytohemagglutinin-P (PHA) for 48 h will proliferate for long periods in vitro in the presence of IL-2. These T-LAK cells have been shown to be 95% CD3 positive. Employing ELISA techniques, greater than 500 pg/ml of TNF was found to be released in the supernatants of these cells during the first 5 days of culture. However, the levels dropped to 100-200 pg/ml by days 7-10. T-LAK cells grown from days 7 to 10 in the presence of IL-2 and rabbit anti-TNF were significantly growth inhibited (up to 23%). The cytolytic activity of T-LAK cells grown from days 0 to 7 in the presence of anti-TNF was also decreased (up to 75%). Phenotypic analysis of these anti-TNF treated T-LAK cells revealed a decrease in CD8 expression (up to 12%) and increase in CD4 expression (up to 27%) when compared with control cells. The data suggest that TNF has a regulatory role in the growth and functional differentiation of these human T-LAK cells.

Characterization of a continuous human glioma cell line DBTRG-05MG: growth kinetics, karyotype, receptor expression, and tumor suppressor gene analyses

The establishment of a new glioma cell line, DBTRG-05MG, in a modified RPMI 1640 medium is described. The cells were derived from an adult female with glioblastoma multiforme who had been treated with local brain irradiation and multidrug chemotherapy; the tumor showed substantial change in histologic appearance compared to the original biopsy 13 mo. previously. The line has been successfully cryopreserved and passaged up to 20 times. The karyotype of the cells demonstrated it as a hypotetraploid line; the DNA index of 1.9 confirmed the karyotype analyses. By immunocytochemical analysis, the cell line reacted with polyclonal antibodies to vimentin, S100, and neuron specific enolase, reflecting its primitive neuroectodermal character. Positive immunostaining for epidermal growth factor receptor correlated with the excess of chromosome 7 seen in the karyotype. The cell line reacted negatively to antibodies against platelet-derived growth factor and its receptor, neuronal cell adhesion molecule, and glial fibrillary acidic protein. By flow cytometry, the cells were major histocompatibility class I antigen positive and class I antigen negative. Growth kinetic studies demonstrated an approximate population doubling time of 34 to 41 h and a colony forming efficiency of 71.4%. Western blot analysis showed the presence of low levels of normal-sized retinoblastoma protein. When compared to the patient's lymphocyte DNA, no loss of heterozygosity of the p53 tumor suppressor gene was observed in the DBTRG-05MG cell line DNA.

Allogeneic tumor-specific cytotoxic T lymphocytes

We report the development of cytotoxic T lymphocytes specific for an allogeneic brain tumor in a rat model. DA strain cytotoxic T cell precursors stimulated by an allogeneic tumor (9L gliosarcoma) from the Fischer rat could generate a population of cytotoxic T lymphocytes that lysed the allogeneic 9L tumor but failed to lyse other targets, including Fischer concanavalin-A(ConA)-stimulated lymphoid blast targets. DA T cells depleted of reactivity to the Fischer haplotype (DA-F) retained reactivity to the 9L tumor, demonstrating that T cell precursors with specificity for normal Fischer alloantigens were not required for the generation of a response to the 9L Fischer tumor. The preferential lysis of the tumor target did not simply reflect a higher density of Fischer target antigens on the tumor than that found on normal Fischer ConA blast targets. First, the relative densities of class I antigen on the 9L tumor and normal Fischer ConA blasts were comparable. Second, cytotoxic T cells could not be generated from DA-F precursors when Fischer ConA blasts were used as stimulators. If DA-F T cells were simply responding to the higher density of Fischer antigen found on 9L tumor, it would have been expected that the ConA blasts expressing comparable levels of antigen to that found on the tumor would have generated cytotoxicity for both the 9L and ConA targets. We conclude that the cytotoxic T cells are specific for a determinant expressed only by the tumor. Such tumor-specific cytotoxic T cells could be useful in vivo for adoptive immunotherapy of brain tumors.

Generation of stimulated, lymphokine activated T killer (T-LAK) cells from the peripheral blood of normal donors and adult patients with recurrent glioblastoma

Peripheral blood mononuclear cells (PBM) from normal donors and patients with recurrent glioma were activated initially for 48-72 h with phytohemagglutinin-P (PHA) and recombinant human interleukin-2 (IL-2), and then proliferated in vitro for up to 5 months with IL-2. These cells are termed mitogen-stimulated lymphokine-activated T killer (T-LAK) cells. We measured patterns of T-LAK cell growth, in vitro cytolytic activity on a panel of continuous and primary tumor cells, and the phenotypes of the cells in these cultures. Lymphocyte viability declined dramatically over the first 3-5 days; and then the remaining cells in these cultures began to divide and maintained a constant 30-36 h doubling time for long periods in vitro. Phenotyping revealed that cells in the initial few days of culture were heterogeneous, but became almost totally CD3 T cells after 7-10 days in culture. The T-LAK cells from individual normal donors and cancer patients demonstrated a non-genetically restricted cytolytic ability against a panel of both continuous cell lines and primary autologous and allogeneic glioblastoma cells in vitro. This technique provides a method of generating large numbers of autologous cytolytic T cells with non-restricted anti-tumor activity that can be derived from peripheral blood mononuclear cells.

Analysis of interleukin 2 and various effector cell populations in adoptive immunotherapy of 9L rat gliosarcoma: allogeneic cytotoxic T lymphocytes prevent tumor take

Recombinant interleukin 2 (rIL-2) and various effector cell populations were used for adoptive immunotherapy in the Fischer strain 9L rat gliosarcoma model. The in vivo cytotoxicities of nonspecifically activated lymphocytes and specifically activated cytotoxic T lymphocytes (CTLs) were assessed in a modified in vivo neutralization (Winn) assay. Effector cells (10(6)) and 9L tumor cells (10(5] were combined with 10(4) units of rIL-2 and stereotactically implanted into the right frontal centrum semiovale of the Fischer (F344) rat. At 7 and 14 days, additional effector cells (10(6] and rIL-2 (10(4) units) were administered through the same burr hole. Nonspecifically activated splenocytes were lymphokine-activated killer (LAK) cells, both plastic-adherent and nonadherent, whereas specifically activated CTLs were either syngeneic (genetically identical) or allogeneic (genetically dissimilar). Syngeneic CTLs were T lymphocytes from Fischer rats primed in vivo with 9L cells and restimulated in vitro. Allogeneic CTLs were generated by exposing DA rat lymphocytes either to irradiated Fischer lymph node cells or to 9L Fisher tumor cells in vitro. Control groups included rats bearing 9L tumor who were untreated, those who received peripheral (i.p. or s.c.) administration of rIL-2, or those who received syngeneic unstimulated T lymphocytes and rIL-2. For a set of animals given the same inoculum of 9L tumor, significantly improved survival was shown for groups treated with nonadherent or adherent LAK cells (P less than or equal to 0.0003), syngeneic CTLs (P = 0.0327), or allogeneic CTLs (P = 0.0025) over untreated control animals by using Mantel-Haenzel nonparametric logrank equations. Only treatment with allogeneic CTLs prevented tumor take.