CBMS-7 IGF1/N-cadherin/Clusterin signaling axis mediates adaptive radioresistance of glioma stem cells

Glioblastoma (GBM) is composed of a variety of tumor cell populations including those with stem cell properties, known as glioma stem cells (GSCs). GSCs are innately less sensitive to radiation than the tumor bulk and are believed to drive GBM formation and recurrence following repeated irradiation. However, it is unclear how GSCs adapt to avoid the toxicity of repeated irradiation used in clinical practice. We established radioresistant human and mouse GSCs by exposing them to repeated rounds of irradiation in order to uncover critical mediators of adaptive radioresistance. Surviving subpopulations acquired strong radioresistance in vivo, which was accompanied by increased cell-cell adhesion, slower proliferation, an elevation of stemness properties and N-cadherin expression. Increasing N-cadherin expression rendered parental GSCs radioresistant, reduced their proliferation, and increased their stemness and intercellular adhesive properties. Conversely, radioresistant GSCs reduced their acquired phenotypes upon CRISPR/Cas9-mediated knockout of N-cadherin. Mechanistically, elevated N-cadherin expression resulted in the accumulation of β-catenin at the cell surface, which decreased Wnt/ β-catenin proliferative signaling, reduced neural differentiation, and protected against apoptosis through Clusterin secretion. Restoration of wild type N-cadherin, but not mutant N-cad lacking β-catenin binding region, led to increased radioresistance in N-cadherin knockout GSCs, indicating the importance of the binding between N-cadherin and β-catenin. We also demonstrated that N-cadherin upregulation was induced by radiation-induced IGF1 secretion, and the radiation resistance phenotype can be reversed with picropodophyllin (PPP), a clinically applicable blood-brain-barrier permeable IGF1 receptor inhibitor, supporting clinical translation. Moreover, the elevation of N-cad and Clusterin are related to prognosis of GBM in the TCGA dataset. In conclusion, our data indicate that IGF1R inhibitor can block the N-cadherin-mediated resistance pathway. Our research provides a deeper understanding of adaptive radioresistance after repeated irradiation, and validates the IGF1/N-cadherin/β-catenin/Clusterin signaling axis as a novel target for radio-sensitization, which has direct therapeutic applicability.

CBIO-12. THE ROLES OF lncRNAs IN GBM RADIATION RESISTANCE AND TUMOR RECURRENCE

Glioblastoma multiforme (GBM) almost invariably recurs and tumors exhibit resistance to conventional therapies. One mechanism of resistance is enhanced DNA damage response (DDR) to alkylating chemotherapy or radiation therapy (RT). We have generated 8 novel GBM patient-derived xenograft (PDX) models of tumor recurrence following serial in vivo irradiation (6 x 2Gy fractions over 2 weeks for 6+ rounds). RNA sequencing has revealed enrichment of a number of DDR pathways in the RT selected (RTS) PDX. Differential enrichment across the RTS PDX suggests multiple molecular routes to acquired RT resistance. We have also identified differential enrichment of molecular signatures for cell cycle progression, stemness, and chromatin remodeling all suggesting decreased proliferation, increased stemness, and more compacted chromatin states associated with RTS PDX. We have identified altered kinase signaling in RTS PDX that may suggest targetable signaling pathways using small molecule inhibitors. Integrated ‘–omics’ analysis has identified SRC family kinases and altered expression of collagens related to the RTS profile. Long non-coding RNAs (lncRNA) have the potential to regulate molecular phenotypes through nucleic acid binding. We have identified 269 lncRNAs significantly differentially expressed in the RTS condition. We have determined that a number of these transcripts have DNA binding potential in gene regulatory regions proximal to kinase, DDR, cell cycle, stemness, and chromatin remodeling genes. Analysis of lncRNAs and genes proximal to their binding sites has revealed regulatory networks potentially governing cell fate and phenotype. We have observed complex correlations of some of these transcripts, such as ZFAS1, which has a positive correlation with expression of stemness-promoting genes and simultaneous inverse correlation with cell cycle genes. This suggests ZFAS1 could be a phenotypic switch between a RT-sensitive proliferating cell and a RT-resistant non-proliferating stem-like cell. LncRNAs may represent a novel therapeutic target for the treatment of therapy resistant, recurrent GBM.

Reactive species balance via GTP cyclohydrolase I regulates glioblastoma growth and tumor initiating cell maintenance

Background

Depending on the level, differentiation state, and tumor stage, reactive nitrogen and oxygen species inhibit or increase cancer growth and tumor initiating cell maintenance. The rate-limiting enzyme in a pathway that can regulate reactive species production but has not been thoroughly investigated in glioblastoma (GBM; grade IV astrocytoma) is guanosine triphosphate (GTP) cyclohydrolase 1 (GCH1). We sought to define the role of GCH1 in the regulation of GBM growth and brain tumor initiating cell (BTIC) maintenance.

Methods

We examined GCH1 mRNA and protein expression in patient-derived xenografts, clinical samples, and glioma gene expression datasets. GCH1 levels were modulated using lentiviral expression systems, and effects on cell growth, self-renewal, reactive species production, and survival in orthotopic patient-derived xenograft models were determined.

Results

GCH1 was expressed in GBMs with elevated but not exclusive RNA and protein levels in BTICs in comparison to non-BTICs. Overexpression of GCH1 in GBM cells increased cell growth in vitro and decreased survival in an intracranial GBM mouse model. In converse experiments, GCH1 knockdown with short hairpin RNA led to GBM cell growth inhibition and reduced self-renewal in association with decreased CD44 expression. GCH1 was critical for controlling reactive species balance, including suppressing reactive oxygen species production, which mediated GCH1 cell growth effects. In silico analyses demonstrated that higher GCH1 levels in glioma patients correlate with higher glioma grade, recurrence, and worse survival.

Conclusions

GCH1 expression in established GBMs is pro-tumorigenic, causing increased growth due, in part, to promotion of BTIC maintenance and suppression of reactive oxygen species.

Abstract 1679: Combined efficacy of Cediranib and Quinacrine in glioma is enhanced by hypoxia and is associated with autophagic vacuole accumulation

Despite robust vascularity of malignant gliomas, anti-angiogenic therapy largely fails to induce durable responses. We have previously reported that efficacy with Cediranib (Ced), a VEGF/PDGF receptor tyrosine kinase inhibitor, is synergistically enhanced via combination with late-stage autophagy inhibitor quinacrine (Quin), in intracranial 4C8 mouse glioma, resulting in decreased tumor vascularization and growth, increased tumor necrosis and improved mouse survival (#1905, AACR 2012). Our present study was aimed at investigating the role of autophagy (a cellular catabolic pathway that promotes tumor cell survival under hypoxic/nutrient stress), in this synergistic efficacy. MTS assays revealed dose-dependent reductions in cell viability for Ced and Quin under normal (Nrm) and hypoxic (0.5%O2, Hyp) conditions: IC50s(μM) under Nrm and Hyp were 2.7±0.1 and 2.4±0.2 for Ced, and 3.2±0.2 and 2.4±0.04 for Quin, respectively. Greater than additive combined efficacy for Ced+Quin occurred only under Hyp (cell viability reductions for 1μM C + 2.5μM Q: 78±7%(Hyp) vs. 31±3%(Nrm), p<0.05). Western blotting for cleaved caspase 3 also indicated a marked increase in apoptosis with Ced+Quin/Hyp versus all other groups. Western blotting for autophagic vacuole (AV) associated LC3-II protein, which increases with increased autophagic flux and/or late stage autophagic inhibition, not only indicated increases with Quin, but also with Ced, suggesting that Ced may increase autophagic flux. Hypoxia potentiated LC3-II increases in the presence of either Quin or Ced, with the largest increases occurring with Ced+Quin/Hyp, suggesting that hypoxia-induced autophagic flux stimulation combined with late stage autophagic inhibition can trigger AV accumulation and cell death. Bafilomycin A1 (Baf), another late-stage autophagy inhibitor, also decreased cell viability. However, the efficacy of combined Baf and Ced was only additive and lacked substantial potentiation by Hyp (cell viability reductions for 1μM Ced/5ηM Baf: 54±8% (Hyp) vs 37±11% (Nrm)). LC3-II accumulation with Baf was substantially lower than with Quin, either with Hyp or Nrm, suggesting that in addition to inhibiting late-stage autophagy, Quin may also stimulate autophagic flux, consistent with its reported inhibition of PI3K/Akt/mTOR signaling. Thus, a key role for AV accumulation is consistent with its increase with Quin versus Baf in tandem with the improved efficacy of Quin over Baf, in combination with Ced. Our results suggest that the unique cytotoxic efficacy of Ced+Quin we previously reported in vivo, could be associated with increased AV accumulation within hypoxic tumor cells, induced by Ced and possibly Quin, in combination with late-stage autophagic inhibition by Quin. These findings provide a rationale for a careful evaluation of a Ced+Quin combination therapy in patients with malignant gliomas.

Citation Format: Merryl Lobo, Peter Kurre, Matthias Schabel, Yancey Gillespie, Randall Woltjer, Martin Pike. Combined efficacy of Cediranib and Quinacrine in glioma is enhanced by hypoxia and is associated with autophagic vacuole accumulation. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1679. doi:10.1158/1538-7445.AM2013-1679

Cross-species genomics matches driver mutations and cell compartments to model ependymoma

Understanding the biology that underlies histologically similar but molecularly distinct subgroups of cancer has proven difficult because their defining genetic alterations are often numerous, and the cellular origins of most cancers remain unknown. We sought to decipher this heterogeneity by integrating matched genetic alterations and candidate cells of origin to generate accurate disease models. First, we identified subgroups of human ependymoma, a form of neural tumour that arises throughout the central nervous system (CNS). Subgroup-specific alterations included amplifications and homozygous deletions of genes not yet implicated in ependymoma. To select cellular compartments most likely to give rise to subgroups of ependymoma, we matched the transcriptomes of human tumours to those of mouse neural stem cells (NSCs), isolated from different regions of the CNS at different developmental stages, with an intact or deleted Ink4a/Arf locus (that encodes Cdkn2a and b). The transcriptome of human supratentorial ependymomas with amplified EPHB2 and deleted INK4A/ARF matched only that of embryonic cerebral Ink4a/Arf(-/-) NSCs. Notably, activation of Ephb2 signalling in these, but not other, NSCs generated the first mouse model of ependymoma, which is highly penetrant and accurately models the histology and transcriptome of one subgroup of human supratentorial tumour. Further, comparative analysis of matched mouse and human tumours revealed selective deregulation in the expression and copy number of genes that control synaptogenesis, pinpointing disruption of this pathway as a critical event in the production of this ependymoma subgroup. Our data demonstrate the power of cross-species genomics to meticulously match subgroup-specific driver mutations with cellular compartments to model and interrogate cancer subgroups.

Abstract 2981: Regulation of Snail by DDX3 and ATM in response to DNA damage

Epithelial-mesenchymal transition (EMT) is the initial step for cancer invasion and metastasis. The hallmark of EMT is the loss of expression of E-cadherin, a cell-cell adhesion protein, allowing cancer cells to gain mobility leaving the site of primary tumor to invade adjacent tissues. Snail, as a transcription repressor, binds to the E-boxes of E-cadherin and then inhibits E-cadherin expression. Thus, Snail is essential to trigger EMT during tumor progression and it has been implicated in the acquisition of invasive growth phenotype of tumors. There is a strong correlation of Snail over-expression with tumor invasion and metastasis. However, how Snail expression is regulated is not fully known. Here we report that Snail is regulated by DDX3, a DEAD box protein family member which contributes to cancer progression. Knock-down of DDX3 by shRNA reduces the basal level of Snail, and this is associated with reduced cell proliferation and migration. Snail protein and mRNA levels are increased in the presence of the HDAC inhibitors sodium butyrate or trichostatin A, and the increases are attenuated in cells with DDX3 knocked down. Treatment of cells with a DNA damaging agent camptothecin also enhances Snail protein levels in a DDX3 dependent manner. Therefore we conclude that DDX3 is required for the basal level and the increase in Snail induced by HDAC inhibitors or camptothecin, indicating that this action of DDX3 may contribute to its promotion of the progression of some cancers. This conclusion is supported by our clinical investigations that analysis of 31 glioblastoma patient samples has revealed a strong correlation between the levels of DDX3 and Snail. Snail is a phosphor-protein, and kinases responsible for phosphorylation include Pak1 and GSK3-β. We also find Pak-1 and GSK-3 independent Snail phosphorylation in response to DNA damage. Further we find that inhibition of the activity of the ATM kinase, a critical element in DNA damage responses, eliminated DNA damage-induced Snail stabilization. Taken together, these observations have highlighted a comprehensive signaling network regulating Snail in response to DNA damage and indicated a critical role of optimal DNA damage responses in tumor invasion and metastasis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2981.

A comprehensive view of the structure and expression of the ependymoma genome at presentation and relapse

2073

Background: Although pediatric and adult ependymomas are associated with significant mortality and morbidity, little is known about the biology of these tumors. To identify underlying genetic alterations and cellular pathways that drive this disease, we conducted a genomic study of 200 adult and pediatric ependymomas. Methods: Using 500k single nucleotide polymorphism arrays, U133 Affymetrix gene and microRNA (miRNA) expression microarrays, and appropriate bioinformatics, we characterized 56 supratentorial (ST), 104 posterior fossa (PF), and 40 spinal (SP) ependymomas. Real-Time polymerase chain reaction and fluorescence in situ hybridization validated observed genetic events. Results: Gene expression profiles segregated tumors by site and identified disease subgroups within each anatomical region (4 ST, 4 PF, 1 SP). miRNA expression profiles identified these same subgroups, indicating that they are biologically distinct. Subgroup-specific gene expression profiles were dictated partly by developmental regulatory genes and partly by large chromosomal gains (eg. 1q, 5p, 16p) and losses (eg. 9p, 22q). Integrated genetic and expression mapping revealed key candidate tumor suppressor (TSG) and onco- genes, likely drivers of these large alterations. While large chromosomal changes occurred more frequently in SP tumors (p < 0.0001), ST tumors averaged more focal changes (n = 13.2) than PF (n = 6.2) or SP tumors (n = 3.0) (p < 0.0001). A total of 29 and 33 non-random focal amplifications and deletions, respectively, encompassing 402 known genes and miRNA clusters, were validated, of which 80 displayed copy number driven expression. These genetic alterations targeted specific cellular functions (e.g., cell adhesion, cell-cycle, neuronal development) and pathways (e.g., NOTCH, EPHRIN, TP53). Our cohort also included five sample sets consisting of primary tumor and at least two corresponding relapses. Genomic analysis of these tumors identified large chromosomal alterations as well as focal gains and losses associated with disease relapse. Conclusions: We present a highly comprehensive view of the ependymoma genome, including 80 previously unrecognized candidate TSG and oncogenes that may afford diagnostic and therapeutic targets.

No significant financial relationships to disclose.

The miR-17/92 polycistron is up-regulated in sonic hedgehog-driven medulloblastomas and induced by N-myc in sonic hedgehog-treated cerebellar neural precursors

Medulloblastoma is the most common malignant pediatric brain tumor, and mechanisms underlying its development are poorly understood. We identified recurrent amplification of the miR-17/92 polycistron proto-oncogene in 6% of pediatric medulloblastomas by high-resolution single-nucleotide polymorphism genotyping arrays and subsequent interphase fluorescence in situ hybridization on a human medulloblastoma tissue microarray. Profiling the expression of 427 mature microRNAs (miRNA) in a series of 90 primary human medulloblastomas revealed that components of the miR-17/92 polycistron are the most highly up-regulated miRNAs in medulloblastoma. Expression of miR-17/92 was highest in the subgroup of medulloblastomas associated with activation of the sonic hedgehog (Shh) signaling pathway compared with other subgroups of medulloblastoma. Medulloblastomas in which miR-17/92 was up-regulated also had elevated levels of MYC/MYCN expression. Consistent with its regulation by Shh, we observed that Shh treatment of primary cerebellar granule neuron precursors (CGNP), proposed cells of origin for the Shh-associated medulloblastomas, resulted in increased miR-17/92 expression. In CGNPs, the Shh effector N-myc, but not Gli1, induced miR-17/92 expression. Ectopic miR-17/92 expression in CGNPs synergized with exogenous Shh to increase proliferation and also enabled them to proliferate in the absence of Shh. We conclude that miR-17/92 is a positive effector of Shh-mediated proliferation and that aberrant expression/amplification of this miR confers a growth advantage to medulloblastomas.

A phase I trial of intratumoral administration of reovirus in patients with histologically confirmed recurrent malignant gliomas

Reovirus is an oncolytic virus with activity in in vivo models of malignant gliomas (MGs). The primary aims were to determine the dose-limiting toxicity (DLT) and maximum tolerated dose (MTD) of intratumoral administration of reovirus in patients with recurrent MGs. Response, survival, and time to progression (TTP) were secondary aims. Patients were adults, had Karnofsky Performance score > or = 60, received prior radiotherapy with or without chemotherapy, and had up to the third recurrence of MG. Reovirus was administered intratumorally stereotactically at 1 x 10(7), 1 x 10(8), or 1 x 10(9) tissue culture infectious dose 50 (TCID50) in a volume of 0.9 ml. Twelve patients were treated at three dose levels (3, 6, and 3 patients, respectively). Seven were men, median Karnofsky Performance score was 80, and median age was 53.5 years. There were no grade III or IV adverse events (AEs) definitely or probably related to treatment. Ten patients had tumor progression, one had stabilization, and one was not evaluable for response. Median survival was 21 weeks (range, 6-234), and one is alive 54 months after treatment. Median TTP was 4.3 weeks (range, 2.6-39). An MTD was not reached. The intratumoral administration of the genetically unmodified reovirus was well tolerated using these doses and schedule, in patients with recurrent MG.

Use of chlorotoxin for targeting of primary brain tumors

Gliomas are primary brain tumors that arise from differentiated glial cells through a poorly understood malignant transformation. Although glioma cells retain some genetic and antigenic features common to glial cells, they show a remarkable degree of antigenic heterogeneity and variable mutations in their genome. Glioma cells have recently been shown to express a glioma-specific chloride ion channel (GCC) that is sensitive to chlorotoxin (CTX), a small peptide purified from Leiurus quinquestriatus scorpion venom [N. Ullrich et al, Neuroreport, 7: 1020-1024, 1996; and N. Ullrich and H. Sontheimer, Am. J. Physiol. (Cell Physiol.), 270: C1511-C1521, 1996]. Using native and recombinant 125I-labeled CTX, we show that toxin binding to glioma cells is specific and involves high affinity [dissociation constant (Kd)=4.2 nM] and low affinity (Kd=660 nml) binding sites. In radioreceptor assays, 125I-labeled CTX binds to a protein with Mr=72,000, presumably GCC or a receptor that modulates GCC activity. In vivo targeting and biodistribution experiments were obtained using 125I- and (131)I-labeled CTX injected into severe combined immunodeficient mice bearing xenografted gliomas. CTX selectively accumulated in the brain of tumor-bearing mice with calculated brain: muscle ratios of 36.4% of injected dose/g (ID/g), as compared to 12.4% ID/g in control animals. In the tumor-bearing severe combined immunodeficient mice, the vast majority of the brain-associated radioactivity was localized within the tumor (tumor:muscle ratio, 39.13% ID/g; contralateral brain:muscle ratio, 6.68%ID/g). Moreover, (131)I-labeled CTX distribution, visualized through in vivo imaging by gamma ray camera scans, demonstrates specific and persistent intratumoral localization of the radioactive ligand. Immunohistochemical studies using biotinylated and fluorescently tagged CTX show highly selective staining of glioma cells in vitro, in situ, and in sections of patient biopsies. Comparison tissues including normal human brain, kidney, and colon were consistently negative for CTX immunostaining. These data suggest that CTX and CTX-conjugated molecules may serve as glioma-specific markers with diagnostic and therapeutic potential.

Cell populations of tendon: a simplified method for isolation of synovial cells and internal fibroblasts: confirmation of origin and biologic properties

Tendons transmit the force of muscle contraction to bone to effect limb movement. Special structural and biological properties of tendon have developed to facilitate force transmission. The tendon has a complex organization of cells surrounding the collagen bundles inside tendon as well as at the tendon surface. Internal cells may act to maintain the bulk of the collagen in tendon. External cells in the epitenon may provide lubrication for tendon gliding. To develop better understanding of these processes and the roles the cell populations play, we isolated cells from the surface and interior of tendon and studied them in vitro. Flexor tendons from 8-week-old white Leghorn chickens were separated into two distinct cell populations: the outer synovial cells and the fibroblasts more internal in tendon. These cell populations were discernible by their locations in the intact tendon, determined by sequential enzymatic and physical release from their substrata. Initially, some cells eluted in Hanks' salt solution (HSS) (population 1); then synovial cells were released after a 2-min treatment with 0.5% collagenase (population 2). Next, a population of synovial cells was released in high yield by treatment with 0.25% trypsin (step III, population 3). Step III, population 3 cells were used as synovial cells (SCs). Next, a population of SCs and fibroblasts were released by scraping with a rubber policeman (population 4). Subsequently, fibroblasts were released after incubation with 0.5% collagenase (population 5). A more direct procedure (procedure 2) to isolate the synovial and internal tendon cells involved treatment in 0.5% collagenase followed by sedimentation at 900 g. Cells that sedimented were largely fibroblasts, whereas the cells that remained at the top of the tube were largely SCs. Cells designated as SCs, isolated by procedure 2, most likely contained surface cells from epitenon and internal interfascicular cells from endotenon and paratenon. Surface tendon cells separated by sequential enzymatic and physical release from their substrata (by procedure 1) had all the following characteristics: distinct subpopulations of cells based on morphology; presence of cytoplasmic, lipid-containing vesicles; decreased sensitivity to trypsin; and reduced generation time as compared with that of internal fibroblasts. Conversely, the internal fibroblasts (IFs) appeared to represent a more uniform population based on morphological characteristics.

Tendon synovial cells secrete fibronectin in vivo and in vitro

The chemistry and cell biology of the tendon have been largely overlooked due to the emphasis on collagen, the principle structural component of the tendon. The tendon must not only transmit the force of muscle contraction to bone to effect movement, but it must also glide simultaneously over extratendonous tissues. Fibronectin is classified as a cell attachment molecule that induces cell spreading and adhesion to substratum. The external surface of intact avian flexor tendon stained positively with antibody to cellular fibronectin. However, if the surface synovial cells were first removed with collagenase, no positive reaction with antifibronectin antibody was detected. Analysis of immunologically stained frozen sections of tendon also revealed fibronectin at the tendon synovium, but little was associated with cells internal in tendon. The staining pattern with isolated, cultured synovial cells and fibroblasts from the tendon interior substantiated the histological observations. Analysis of polyacrylamide gel profiles of 35S-methionine-labeled proteins synthesized by synovial cells and internal fibroblasts indicated that fibronectin was synthesized principally by synovial cells. Fibronectin at the tendon surface may play a role in cell attachment to prevent cell removal by the friction of gliding. Alternatively, fibronectin, with its binding sites for hyaluronic acid and collagen, may act as a complex for boundary lubrication.

Response of glioma cells to interferon-gamma: increase in class II RNA, protein and mixed lymphocyte reaction-stimulating ability

Previous results by ourselves and others demonstrated that brain cells and cell lines express major histocompatibility complex class II antigens. We examined interferon-gamma (IFN-gamma)-mediated induction of human class II antigen expression on the glioma cells. Purified IFN-gamma induced the expression of HLA-DR antigens on the surface of the glioma cell lines U-373 MG and U-105 MG. Concomitant increase of HLA-DR alpha- and HLA-DC beta-specific RNA in the cytoplasm was also observed after treatment with IFN-gamma. Increases of class II antigen paralleled the increased level of class II-specific RNA. The effect of IFN-gamma on the induction of human class II antigen expression was dose and time dependent. A marked induction of human class II antigen expression was observed when glioma cells were cultured with more than 100 U/ml of IFN-gamma. Little or no induction was observed with less than 50 U/ml of IFN-gamma. Compared to human blood monocytes, glioma cells needed higher concentrations of IFN-gamma for the induction of class II antigen expression. In allogenic mixed lymphocyte cultures, the glioma cell line U-373 MG stimulated a mixed lymphocyte response (MLR). MLR-stimulating capacity was augmented by IFN-gamma. The concomitant augmentation of class II antigen levels and MLR-stimulating capacity suggests that the most relevant factor for MLR stimulation may be antigen density. This is the first report of MLR stimulation by a glioma cell line.