Tumor-specific Cytotoxic Lymphocytes Research Articles

Abstract 22: Development and characterization of human tumor specific cytotoxic lymphocytes for preclinical drug development

Abstract Heterogeneity within a tumor allows it to better adapt to changing conditions and can have therefore a negative impact on a patient’s prognosis, response to treatment and overall health. Breast cancer is a highly heterogeneous disease caused by distinct genetic mutations in mammary epithelial cells, resulting in significantly different forms of the disease and a need for highly personalized treatment schemes for patients. Hence, it is important to develop and improve (immuno)therapies for breast cancer patients. Adoptive T-cell therapy with autologous CD8+ cytotoxic T cell lymphocytes (CTL) is a promising method that has already shown promising effects in melanoma patients. Unfortunately, there is almost no information about the use in breast cancer patients. We developed an in vitro protocol to generate tumor-specific CTLs by priming on HER2+ breast cancer cell lines JIMT-1 and SKBR-3. Subsequently, these primed CTLs were tested in 2D and 3D immune cell killing assays in a life cell imaging device in combination with an endpoint viability assay. The activity was compared with unspecific CD8+T cells activated by phytohemagglutinin (PHA) or αCD2/αCD3/αCD28, HER2-peptide specific T cells and HER2-CAR T cells serving as a positive control. We investigated the feasibility to freeze CTLs for use in subsequent studies and validated the relevance of HER2 as the main druggable target in the tumor lines using an siRNA approach. The CTLs were most effective on JIMT-1 in a 3D setting, whereas the SKBR-3 CTLs displayed similar activity in 2D and in 3D. The respective CTLs were tumor line specific, as SKBR-3 CTLs did not kill efficiently the JIMT-1 cells and vice versa. The same CTLs displayed the same efficacy pattern after thawing but less pronounced. Thus, within one experimental set-up it is recommended to use either one. The Her2 knockdown in the target cells reduced the killing activity of the CTLs slightly. However, this effect was not statistically significant proving the importance of CTLs as treatment option as well as neoantigens as targets in drug development. In summary, the CTLs proved to capture the respective tumor heterogeneity showing high killing rates in both SK-BR3 and JIMT-1 being more active than HER2-peptide specific T cells. The availability of CTLs in a preclinical setting facilitates screening approaches for combinational studies. Beyond that they can serve as starting material to develop cancer vaccines and identify highly immunogenic neoantigens. Citation Format: Ina Rohleff, Lea Gröne, Kanstantsin Lashuk, Julia B. Schueler. Development and characterization of human tumor specific cytotoxic lymphocytes for preclinical drug development [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 22.

Abstract 4224: The application of sophisticated image analysis to a T cell killing assay in 3D deciphers the dynamics of the different components of the tumor microenvironment

Abstract We developed an in vitro protocol to generate tumor-specific cytotoxic lymphocytes (CTL) by priming T cells from healthy donors on HER2+ breast cancer cell line SKBR-3. Subsequently, these primed CTLs were tested in a 3D immune cell killing assays in a life cell imaging device. An image analysis workflow was applied to understand the T cell movement in spatial relation to the tumor and the kinetic of tumor cell killing. The activity of the CTLs was compared with non-activated CD8+T cells, staurosporin and CD8+ T cells, unspecifically activated by αCD2/αCD3/αCD28 beads. The tumor cells were expressing mKate whereas the T cells were label-free. By analyzing a combination of different masks, we could computationally measure tumor spheroid characteristics and dynamic attributes such as changes in shape, percent of cells expressing red fluorescentce, and relative average distance of T cells from the spheroid in each timelapse assay. This process was applied at each frame to determine the average T cell distance from its corresponding spheroid center and subsequently calculate the average T cell flux over the course of each assay by linking data between image frames. Additional measurements allowed for the quantification of tumor debris accumulation over time both relative to the primary tumor mass and in absolute terms. Our analyses revealed that bead-activated T cells approached the tumor spheroid with the highest speed and infiltrated the spheroid homogeneously. In contrast the CTLs approached the spheroid slower and started tumor cell killing from the rim of the spheroid towards the center. The reduction of spheroid area over time displayed a very similar kinetic when co-cultured with CTL or bead-activated T cells. At the end of the experiment the CTLs induced a more pronounced shrinkage of spheroid size. Interestingly the size of the spheroid and the red fluorescence intensity were not necessarily aligned. The positive control Staurosporin induced a fast reduction of the mKate signal, whereas the spheroid area did not change over time. In summary, the bead-activated T cell approached the spheroid fast, infiltrated the complete area and induced tumor cell killing homogenously across the tumor area. The CTL approached the spheroids slower but started killing earlier from the outside towards the center of the spheroid. The observed differences in infiltration and killing characteristics of the investigated T cell types lead to a better understanding of T cell biology in the context of the tumor biology, which can be translated into applications during dug development. Taken together, with the applied image analysis algorithm we were able to deconvolute the biological processes in the assay in much more detail. The additional read-outs increase the translational value of the assay tremendously in the context of drug development as well as precision medicine. Citation Format: Ryan Kerwin, Ina Rohleff, Martin Blanchard, Kanstantsin Lashuk, Bryan Walker, Julia B. Schueler. The application of sophisticated image analysis to a T cell killing assay in 3D deciphers the dynamics of the different components of the tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4224.

Abstract 6638: Pulsed electric fields combined with anti-PD1 prolongs survival and triggers an adaptive immune response in an IO-non-responsive orthotopic mouse model

Abstract Background: Although anti-PD1 therapy is standard of care in multiple tumor types, unmet need remains for those non-responsive to immunotherapy. Pulsed Electric Field (PEF) treatment may alter the tumor micro-environment, converting an immune “cold” tumor into an immune “hot” tumor, triggering an adaptive immune response and abscopal effect. We evaluated PEF in combination with anti-PD1 in a highly aggressive orthotopic triple negative breast tumor mouse model. Methods: Murine mammary 4T1 cells (2 × 105) non-responsive to anti-PD1 therapy were orthotopically implanted in mammary fat of 48 female Balb/C mice. Once tumors were established (5 mm diameter), mice were randomized into six groups: Sham+IgG controls, aPD-1-only, PEF (1X), PEF (2X), aPD-1 + PEF (1X), and aPD-1 + PEF (2X). Groups 4 and 6 received two PEF treatments, five days apart. Anti-PD1 and isotype matched control IgG was administered (200 ug, intraperitoneal) weekly, starting 3 days after PEF treatment. Tumor volumes were recorded 3x/week and standard survival analyses were conducted per IACUC protocol. In the cohort of tumors collected 30 days after PEF, flow cytometry (FC) evaluated T-cell immune infiltrates and associated markers. Blood was collected at different time points to evaluate systemic immunological response. Tumor-specific cytotoxic lymphocytes (CTL) were isolated from tumor-infiltrating and circulating lymphocytes by gp70 MHC tetramer formation assay. Results: Single PEF reduced tumor growth compared to IgG and aPD-1-only groups by 65% and 62%, respectively. Double PEF treatment reduced tumor growth by 74% and prolonged survival. FC demonstrated circulating helper T-cells and CTL frequency was increased two weeks post-PEF compared to the control group. The tetramer detected substantial increases in antigen-specific T-cell populations in both single and double PEF treatment groups. PEF groups had significant tumor and spleen infiltration of CD8 and CD4 T-cells, including increased tetramer levels. Anti-PD1-only animals showed no meaningful reductions in tumor growth, prolonged survival, or significant changes in CD8, CD4 T cells or in the tetramer assay. Conclusions: PEF treatments prior to initiation of anti-PD1 in orthotopic 4T1 mouse model triggers a tumor-specific adaptive immune response in an immune “cold” tumor model, irrespective of anti-PD-1 therapy. PEF increases circulating and tumor-infiltrating T-cells and antigen-specific T-cells, with these effects observed after a single or two treatments with PEF. Collectively, our results demonstrate that PEF treatment prior to initiation of anti-PD1 treatment can reduce tumor growth, prolong survival, and activate an adaptive immune response in an immune “cold” tumor. These findings suggest that PEF treatment merits further evaluation in IO-non-responsive cancer patients. Citation Format: Ebtesam Nafie, Mukta Wagh, Chiara Pastori, Mo Trikha, Robert Neal. Pulsed electric fields combined with anti-PD1 prolongs survival and triggers an adaptive immune response in an IO-non-responsive orthotopic mouse model. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6638.

Personalized tumor vaccine for pancreatic cancer.

Pancreatic cancer is a highly lethal malignancy often presenting with advanced disease and characterized by resistance to standard chemotherapy. Immune-based therapies such checkpoint inhibition have been largely ineffective such that pancreatic cancer is categorized as an immunologically "cold tumor". In the present study, we examine the therapeutic efficacy of a personalized cancer vaccine in which tumor cells are fused with dendritic cells (DC) resulting in the broad induction of antitumor immunity. In the KPC spontaneous pancreatic cancer murine model, we demonstrated that vaccination with DC/KPC fusions led to expansion of pancreatic cancer specific lymphocytes with an activated phenotype. Remarkably, vaccination led to a reduction in tumor bulk and near doubling of median survival in this highly aggressive model. In a second murine pancreatic model (Panc02), vaccination with DC/tumor fusions similarly led to expansion of tumor antigen specific lymphocytes and their infiltration to the tumor site. Having shown efficacy in immunocompetent murine models, we subsequently demonstrated that DC/tumor fusions generated from primary human pancreatic cancer and autologous DCs potently stimulate tumor specific cytotoxic lymphocyte responses. DC/tumor fusions induce the activation and expansion of tumor reactive lymphocytes with the capacity to infiltrate into the pancreatic cancer tumor bed.

2-methoxyestradiol inhibits melanoma cell growth by activating adaptive immunity

Background: The overall survival of melanoma patients remains poor despite advancements in surgical treatment and targeted therapies. Therefore, there is a need to develop new therapeutic strategies for melanoma. 2-methoxyestradiol (2-ME) is a major metabolite of estrogen that has been shown to have anti-tumor effects against many malignancies. However, the effects and mechanisms of action of 2-ME against melanoma remain unclear. Materials and methods: Melanoma cells (B16) were treated with 2-ME in vitro. Cell proliferation was detected by CCK8 and clone formation, transwell was carried out to measure the migration of B16 cells with or without 2-ME. Flow cytometry was performed to measure the apoptosis and cell cycle. C57BL/6 mice were used for tumor-bearing of B16 cells, tumor volumes were measured once a day, and sacrificed after it was over 2000 mm3, then immunofluorescence was implemented to examine the marker of CD3, CD8 and PD-L1. Results: In our study, we found that 2-ME significantly affected the proliferation, migration, apoptosis, and cell cycle of melanoma in vitro. Our results also showed that 2-ME had strong anti-tumor effects against melanoma in vivo and increased the infiltration of tumor-specific cytotoxic lymphocytes CD8+ T cells in the tumor microenvironment. Besides, PD-L1 expression in tumor cells was significantly higher in the 2-ME-treated group than in the control group, indicating that 2-ME could exhibit stronger anti-tumor effects against melanoma if combined with PD-1 blockade therapy. Conclusion: 2-ME suppresses melanoma in vivo and in vitro and is a promising synergistic enhancer of PD-1 blockade immunotherapy.

Tumor cells endowed with professional antigen-presenting cell functions prime PBLs to generate antitumor CTLs.

Intrinsic genetic instability of tumor cells leads to continuous production of mutated proteins referred to as tumor-specific neoantigens. Generally, they are recognized as nonself products by the host immune system. However, an effective adaptive response clearing neoantigen-expressing cells is lost in tumor diseases. Most advanced therapeutic strategies aim at inducing neoantigen-specific immune activation through personalized approaches. They include tumor cell exome sequencing, human leukocyte antigen (HLA) typing, synthesis, and injection of peptides/RNA with adjuvants. Here, we propose an innovative method to induce a CD8+ T cytotoxic lymphocyte (CTL) immune response against tumor neoantigens bypassing the steps needed in current therapeutic strategies of personalized vaccination. We assumed that tumor cells can be the most efficient and precise factory of major histocompatibility complex (MHC) class I-associated, tumor neoantigen-derived peptides. Hence, endowing tumor cells with professional antigen-presenting functions would prime CD8+ T lymphocytes towards a response against nonself tumor antigens. To explore this possibility, both adenocarcinoma and melanoma human cells were engineered to express both CD80 and CD86 costimulatory molecules. HLA-matched lymphocytes were then primed through cocultivation with the engineered tumor cells. The generation of tumor-specific CD8+ T lymphocytes was tested through the combined analysis of cell activation markers, formation of immunologic synapses, generation of tumor antigen-specific CD8+ T lymphocytes, and cytotoxic activity. Our data consistently indicate that tumor cells endowed with professional antigen-presenting functions can generate an effective tumor-specific CTL immune response. This finding may open avenues towards the development of innovative antitumor immunotherapies. KEY MESSAGES: We established a novel method to induce antitumor CTLs without a need to identify TAAs and/or tumor neoantigens. This strategy relies on transducing tumor cells with a retroviral vector expressing both CD80 and CD86. In this way, tumor cells prime naïve CD8+ T lymphocytes in a way that CTLs killing the same tumor cells are generated. These findings open the way towards preclinical assays in the perspective to introduce this antitumor immunotherapy strategy in clinic.

CAR T-клеточная терапия как современный метод лечения онкологических заболеваний

The annual increase in cases of oncological diseases dictates the need to search for and develop new methods for diagnosing and treating cancer. The article presents the results of the analysis of modern literature sources devoted to CAR T–cell therapy, which has become a breakthrough trend in the treatment of a number of hematological tumors. The basis of CAR T–cell therapy is the creation of a pool of tumor–specific cytotoxic lymphocytes by introducing an ex vivo transgene encoding a chimeric antigen receptor (CAR). The article outlines the basic principles, possible complications, the advantages and disadvantages of CAR T–cell therapy.

Natural killer cells as helper cells in dendritic cell cancer vaccines.

Vaccine-based cancer immunotherapy has generated highly variable clinical results due to differing methods of vaccine preparation and variation in patient populations among other lesser factors. Moreover, these clinical responses do not necessarily correspond with the induction of tumor-specific cytotoxic lymphocytes. Here, we review the participation of natural killer (NK) cells as alternative immune components that could cooperate in successful vaccination treatment. NK cells have been described as helper cells in dendritic cell-based cancer vaccines, but the role in other kinds of vaccination strategies (whole cells, peptide, or DNA-based vaccines) is poorly understood. In this article, we address the following issues regarding the role of NK cells in cancer vaccines: NK cell anti-tumor action sites, and the loci of NK cell interaction with other immune cells; descriptions of new data on the memory characteristics of NK cells described in infectious diseases; and finally phenotypical and functional changes after vaccination measured by immunomonitoring in preclinical and clinical settings.

Vaccination with dendritic cells pulsed in vitro with tumor antigen conjugated to cholera toxin efficiently induces specific tumoricidal CD8 + cytotoxic lymphocytes dependent on cyclic AMP activation of dendritic cells

We investigated the development of CD8 + tumor-specific cytotoxic lymphocytes (CTL) and protection against tumor growth after vaccination with bone marrow-derived dendritic cells (DC) pulsed with a model protein ovalbumin conjugated to cholera toxin (OVA-CT) in B6 mice using E.G7 tumor cells expressing OVA 257–264 peptide (SIINFEKL) as target cells in vitro and in vivo. Vaccination with OVA-CT-pulsed DC concurrently induced strong CTL in vitro activity and anti-E.G7 tumor protection in vivo in WT, NK-depleted and CD4-deficient mice as well as in IL-12−/− and IFN-γ−/− mice but not in CD8-deficient mice. Importantly, activation of CTL by OVA-CT-pulsed DC was dependent on CT-induced activation of adenylate cyclase and increased cAMP production by DC associated with increased expression of MHC class I and co-stimulatory molecules (CD80, CD86 and CD40). These results show that vaccination with DC pulsed with antigens (Ag) conjugated to CT induces a strong CTL response and suggest that conjugation of tumor Ag to CT for DC vaccination represents a promising approach for tumor vaccination and immunotherapy.

Isolation and Characterization of PAGE-1 andGAGE-7: NEW GENES EXPRESSED IN THE LNCaP PROSTATE CANCER PROGRESSION MODEL THAT SHARE HOMOLOGY WITH MELANOMA-ASSOCIATED ANTIGENS

The LNCaP progression model of human prostate cancer consists of lineage-related sublines that differ in their androgen sensitivity and metastatic potential. A differential display polymerase chain reaction was employed to evaluate mRNA expression differences between the LNCaP sublines in order to define the differences in gene expression between the androgen-sensitive, nontumorigenic LNCaP cell line and the androgen-insensitive, metastatic LNCaP sublines, C4-2 and C4-2B. An amplicon, BG16.21, was isolated that showed increased expression in the androgen-independent and metastatic LNCaP sublines, C4-2 and C4-2B. Hybridization screening of a lambda gt11 expression library with BG16.21 revealed two transcripts, both homologous to BG16.21 at the 3' end. A GenBankTM data base search using the GCG Wisconsin software package revealed the shorter approximately 600-bp transcript (designated GAGE-7) to be a new member of the GAGE family. The second approximately 700-bp transcript was a novel gene (designated PAGE-1, "prostate associated gene") with only 45% homology to GAGE gene family members. RNA blot analysis demonstrated that GAGE-7 mRNA was expressed at equal levels in all lineage related prostate cancer cell sublines, while PAGE-1 mRNA levels were elevated 5-fold in C4-2 and C4-2B as compared with LNCaP cells. Neither GAGE-7 nor PAGE-1 demonstrated any regulation by androgens in the prostate cancer cell lines used in this study. PAGE-1 and GAGE-7 expression was found to be restricted to testes (high) and placenta (low) on human multiple tissue Northern blots. As GAGE/MAGE antigens were reported previously to be targets for tumor-specific cytotoxic lymphocytes in melanoma, these results suggest that PAGE-1 and GAGE-7 may be related to prostate cancer progression and may serve as potential targets for novel therapies.

T-CELL RECEPTOR V-ALPHA AND V-BETA GENE USAGE IN INTERLEUKIN-2-CULTURED TUMOR-INFILTRATING LYMPHOCYTES FROM PATIENTS WITH BREAST-CANCER

Tumor-infiltrating lymphocytes (TIL) are often found in malignant breast tumors, and have been claimed to be of prognostic value. It has been proposed that TIL may represent an enriched population of tumor-specific cytotoxic lymphocytes, reacting with antigenic determinants on the tumor cell surface through the T cell receptor (TCR) complex. We have studied the phenotype, cytotoxicity, and expression of TCR variable (V) alpha and beta chain on in vitro IL-2-cultured TIL isolated from primary malignant breast tumors from 11 patients. 10/11 cultures were dominated by CD4(+) (T-helper) cells. The different TIL cultures exhibited varying levels of cytotoxicity against the natural killer (NK)-sensitive cell line K562 and breast cancer cell line T47D. The level of clonality, as measured by PCR-based analyses of usage of the different V segments was low, as only a few tumors showed patterns of restricted V gene expression. The mean number of V alpha segments per TIL culture was higher than the number of V beta segments per culture. A significant negative correlation was observed between the number of CD4+ cells and the number of V beta segments per culture, and no other correlations between phenotypes and expression of any particular V segments were found. Neither was there any correlation between the expression of specific V alpha/V beta segments and cytotoxicity against allogeneic tumor cells.

Cell-mediated immunity to chemically xenogenized tumors. V. Failure of novel antigens to increase the frequency of tumor-specific cytotoxic T cells

Xenogenized variant cells (L5178Y/DTIC) of a murine lymphoma line confer a high degree of specific protection against subsequent challenge of mice with parental L5178Y cells. In an attempt to better define the effect of DTIC-induced determinants on parental antigen recognition and the mechanisms involved in this protection, we evaluated the frequency of anti-parental tumor cytotoxic T lymphocyte precursors following priming of mice with xenogenized cells in vivo. In addition, we tested the effect of host sensitization with the immunogenic, retrovirus-related proteins that are precipitated from the surface of L5178Y/DTIC cells by means of specific antibody. Our results indicated that the novel determinants induced by DTIC treatment on L5178Y cells do not act as helper determinants for the generation of tumor-specific cytotoxic responses. Therefore, increased frequency of tumor-specific cytotoxic lymphocytes does not seem to be a major mechanism of anti-parental tumor immunity induced by xenogenized variant cells.

Detection of novel antigenic specificities on murine xenogenized lymphomas by means of specific antibodies

Xenogenized variant cells (L5178Y/DTIC) of a murine lymphoma line confer a high degree of specific protection against subsequent challenge of mice with parental L5178Y cells. In an attempt to better define the effect of DTIC-induced determinants on parental antigen recognition and the mechanisms involved in this protection, we evaluated the frequency of anti-parental tumor cytotoxic T lymphocyte precursors following priming of mice with xenogenized cells in vivo. In addition, we tested the effect of host sensitization with the immunogenic, retrovirus-related proteins that are precipitated from the surface of L5178Y/DTIC cells by means of specific antibody. Our results indicated that the novel determinants induced by DTIC treatment on L5178Y cells do not act as helper determinants for the generation of tumor-specific cytotoxic responses. Therefore, increased frequency of tumor-specific cytotoxic lymphocytes does not seem to be a major mechanism of anti-parental tumor immunity induced by xenogenized variant cells.

In vitro generation of tumor-specific cytotoxic lymphocytes. Secondary allogeneic mixed tumor lymphocyte culture of normal murine spleen cells.

In vivo or in vitro immunity to murine leukemia virus (MuLV)-induced leukemia cells which do not effectively produce virus, has been difficult to demonstrate. Because immunizations with allogeneic murine leukemia cells have been used to confer syngeneic tumor immunity to virus- producing cells, we attempted to generate lymphocytes, cytotoxic to syngeneic nonproducer leukemia cells, by stimulating normal murine spleen cells with allogeneic nonproducer leukemia cells in mixed tumor lymphocyte culture (MTLC) reactions in vitro. Secondary allogeneic MTLC of normal C57BL/6 or DBA/2 spleen cells effectively produced syngeneic tumor-specific cytotoxic lymphocytes. Target cells lysed in lymphocyte- mediated cytolysis (LMC) assays, included both Friend and Rauscher virus- induced syngeneic murine leukemia cells and chemically-induced hematopoietic tumor cells. Syngeneic tumor cells were lysed regardless of whether they produced infectious MuLV or expressed viral antigens gp-71, p-30, or p-12 at the cell surface. Syngeneic normal cells (thymus, lymph node, or Concanavalin A-stimulated spleen cells) used as targets in LMC assays were uneffected by lymphocytes harvested from secondary allogeneic MTLC. Several other in vitro culture treatments including secondary syngeneic MTLC and repetitive mixed lymphocyte culture stimulations were incapable of generating tumor-specific cytotoxic lymphocytes. Based upon these results, we propose that secondary MTLC stimulation of normal spleen cells with allogeneic nonproducer leukemia cells selects for the proliferation of two subpopulations of antigen-specific cytotoxic lymphocytes. The population capable of effecting syngeneic tumor cell lysis is directed against tumor-associated cell surface antigens which may be distinct from viral structural proteins or glycoproteins. The growth of these tumor-specific cytotoxic lymphocytes may be enhanced by a soluble allogeneic effect factor produced by the proliferation of the second subpopulation of lymphocytes generated in repetitive allogeneic MTLC, namely those lymphocytes with specificities directed against differing histocompatibility antigens.

Concomitant Presence of Tumor-Specific Cytotoxic and Inhibitor Lymphocytes in Patients with Osteogenic Sarcoma

The lack of detectable tumor-specific cytotoxicity by the peripheral blood lymphocytes of patients with cancer may be due to a lack of cytotoxic lymphocytes or the presence of suppressor lymphocytes that inhibit cytotoxic cells. Unfractionated peripheral blood lymphocytes from 12 of 28 patients with osteogenic sarcoma were cytotoxic to osteogenic sarcoma cells in vitro (P less than 0,001). When the peripheral blood lymphocytes from patients whose lymphocytes were not cytotoxic underwent fractionation, a tumor-specific cytotoxic subpopulation was isolated from 11 of 13 patients (P less than 0.0001). Lymphocytes that inhibited cytotoxic activity of autologous tumor-specific cytotoxic lymphocytes were found in four of 10 patients with osteogenic sarcoma but not in six normal controls. Inhibitor lymphocytes form rosettes with sheep erythrocytes and adhere to nylon, whereas cytotoxic lymphocytes have a receptor for C3 but no surface immunoglobulin. The lack of tumor-specific lymphocytotoxicity in some patients can be due to inhibitor lymphocytes.