Anti-KIT designer T cells for the treatment of gastrointestinal stromal tumor

The immunotherapy in gastrointestinal stromal tumors

Using Tyrosine Kinase Inhibitors (TKIs) for gastrointestinal stromal tumors (GIST) has significantly reduced the risk of recurrence and prolonged survival. Immunotherapy has demonstrated efficacy in multiple solid tumors, but its effectiveness in GIST remains uncertain. Although early clinical studies indicate good tolerability of immunotherapy in patients, the efficacy is not as desired. Therefore, identifying the subset of GIST patients who benefit from immunotherapy and coordinating the relationship between immunotherapy and TKI treatment are crucial issues to be explored. In this review, we aims to provide a retrospective analysis of relevant literature and find that GIST patients exhibit a rich presence of tumor-infiltrating immune cells, which play critical roles in the immune surveillance and evasion processes of tumors. This review incorporates a selection of 48 articles published between 2002 and 2023, sourced from PubMed, EBSCO, and Google Scholar databases.

Molecular characteristics and immune microenvironment of gastrointestinal stromal tumours: targets for therapeutic strategies

Gastrointestinal stromal tumours (GISTs) are the most common mesenchymal tumours, arising mainly from the interstitial cells of Cajal (ICCs) of the gastrointestinal tract. As radiotherapy and chemotherapy are generally ineffective for GISTs, the current primary treatment is surgical resection. However, surgical resection is not choice for most patients. Therefore, new therapeutic strategies are urgently needed. Targeted therapy, represented by tyrosine kinase inhibitors (TKIs), and immunotherapy, represented by immune checkpoint inhibitor therapies and chimeric antigen receptor T-cell immunotherapy (CAR-T), offer new therapeutic options in GISTs and have shown promising treatment responses. In this review, we summarize the molecular classification and immune microenvironment of GISTs and discuss the corresponding targeted therapy and immunotherapy options. This updated knowledge may provide more options for future therapeutic strategies and applications in GISTs.

Advances in immunology and immunotherapy for mesenchymal gastrointestinal cancers

Mesenchymal gastrointestinal cancers are represented by the gastrointestinal stromal tumors (GISTs) which occur throughout the whole gastrointestinal tract, and affect human health and economy globally. Curative surgical resections and tyrosine kinase inhibitors (TKIs) are the main managements for localized GISTs and recurrent/metastatic GISTs, respectively. Despite multi-lines of TKIs treatments prolonged the survival time of recurrent/metastatic GISTs by delaying the relapse and metastasis of the tumor, drug resistance developed quickly and inevitably, and became the huge obstacle for stopping disease progression. Immunotherapy, which is typically represented by immune checkpoint inhibitors (ICIs), has achieved great success in several solid tumors by reactivating the host immune system, and been proposed as an alternative choice for GIST treatment. Substantial efforts have been devoted to the research of immunology and immunotherapy for GIST, and great achievements have been made. Generally, the intratumoral immune cell level and the immune-related gene expressions are influenced by metastasis status, anatomical locations, driver gene mutations of the tumor, and modulated by imatinib therapy. Systemic inflammatory biomarkers are regarded as prognostic indicators of GIST and closely associated with its clinicopathological features. The efficacy of immunotherapy strategies for GIST has been widely explored in pre-clinical cell and mouse models and clinical experiments in human, and some patients did benefit from ICIs. This review comprehensively summarizes the up-to-date advancements of immunology, immunotherapy and research models for GIST, and provides new insights and perspectives for future studies.

Exploring the Dynamic Crosstalk between the Immune System and Genetics in Gastrointestinal Stromal Tumors

Gastrointestinal Stromal Tumors (GISTs) represent a paradigmatic model of oncogene addiction. Despite the well-known impact of the mutational status on clinical outcomes, we need to expand our knowledge to other factors that influence behavior heterogeneity in GIST patients. A growing body of studies has revealed that the tumor microenvironment (TME), mostly populated by tumor-associated macrophages (TAMs) and lymphocytes (TILs), and stromal differentiation (SD) have a significant impact on prognosis and response to treatment. Interestingly, even though the current knowledge of the role of immune response in this setting is still limited, recent pre-clinical and clinical data have highlighted the relevance of the TME in GISTs, with possible implications for clinical practice in the near future. Moreover, the expression of immune checkpoints, such as PD-L1, PD-1, and CTLA-4, and their relationship to the clinical phenotype in GIST are emerging as potential prognostic biomarkers. Looking forward, these variables related to the underlying tumoral microenvironment in GIST, though limited to still-ongoing trials, might lead to the potential use of immunotherapy, alone or in combination with targeted therapy, in advanced TKI-refractory GISTs. This review aims to deepen understanding of the potential link between mutational status and the immune microenvironment in GIST.

High-Grade Endometrial Stromal Sarcoma: Molecular Alterations and Potential Immunotherapeutic Strategies

Endometrial stromal tumor (EST) is an uncommon and unusual mesenchymal tumor of the uterus characterized by multicolored histopathological, immunohistochemical, and molecular features. The morphology of ESTs is similar to normal endometrial stromal cells during the proliferative phase of the menstrual cycle. ESTs were first classified into benign and malignant based on the number of mitotic cells. However, recently WHO has divided ESTs into four categories: endometrial stromal nodules (ESN), undifferentiated uterine sarcoma (UUS), low-grade endometrial stromal sarcoma (LG-ESS), and high-grade endometrial stromal sarcoma (HG-ESS). HG-ESS is the most malignant of these categories, with poor clinical outcomes compared to other types. With advances in molecular biology, ESTs have been further classified with morphological identification. ESTs, including HG-ESS, is a relatively rare type of cancer, and the therapeutics are not being developed compared to other cancers. However, considering the tumor microenvironment of usual stromal cancers, the advance of immunotherapy shows auspicious outcomes reported in many different stromal tumors and non-identified uterine cancers. These studies show the high possibility of successful immunotherapy in HG-ESS patients in the future. In this review, we are discussing the background of ESTs and the BCOR and the development of HG-ESS by mutations of BCOR or other related genes. Among the gene mutations of HG-ESSs, BCOR shows the most common mutations in different ways. In current tumor therapies, immunotherapy is one of the most effective therapeutic approaches. In order to connect immunotherapy with HG-ESS, the understanding of tumor microenvironment (TME) is required. The TME of HG-ESS shows the mixture of tumor cells, vessels, immune cells and non-malignant stromal cells. Macrophages, neutrophils, dendritic cells and natural killer cells lose their expected functions, but rather show pro-tumoral functions by the matricellular proteins, extracellular matrix and other complicated environment in TME. In order to overcome the current therapeutic limitations of HG-ESS, immunotherapies should be considered in addition to the current surgical strategies. Checkpoint inhibitors, cytokine-based immunotherapies, immune cell therapies are good candidates to be considered as they show promising results in other stromal cancers and uterine cancers, while less studied because of the rarity of ESTs. Based on the advance of knowledge of immune therapies in HG-ESS, the new strategies can also be applied to the current therapies and also in other ESTs.

CSPG4 Expression in GIST Is Associated with Better Prognosis and Strong Cytotoxic Immune Response

Simple Summary

Gastrointestinal stromal tumors (GIST) are the most frequent sarcomas of the gastrointestinal tract. Identification of novel prognostic and/or therapeutic targets is a major issue to overcome tyrosine kinase inhibitors resistances. CSPG4, a cell surface proteoglycan, emerged as a potential therapeutic target for immune therapy in different cancers, including sarcomas. CSPG4 expression has never been studied in GIST. In this work we analyzed CSPG4 mRNA expression in a large series of clinical GIST samples given the scarcity of disease (n = 309 patients). We find that high CSPG4 expression is independently associated with disease-free survival, and with an immune landscape favorable to induce strong cytotoxic immune response after NK cell stimulation. Our results suggest the potential value of CSPG4-specific chimeric antigen receptor-redirected cytokine-induced killer lymphocytes treatment in GIST, notably “CSPG4-high” tumors, and calls for preclinical validation, drug testing in vivo, then in clinical trials.

Abstract

The treatment of gastrointestinal stromal tumors (GIST) must be improved through the development of more reliable prognostic factors and of therapies able to overcome imatinib resistance. The immune system represents an attractive tool. CSPG4, a cell surface proteoglycan, emerged as a potential therapeutic target for immune therapy in different cancers, including cell therapy based on CSPG4-specific chimeric antigen receptor (CAR)-redirected cytokine-induced killer lymphocytes (CSPG4-CAR.CIKs) in sarcomas. CSPG4 expression has never been studied in GIST. We analyzed CSPG4 mRNA expression data of 309 clinical GIST samples profiled using DNA microarrays and searched for correlations with clinicopathological and immune features. CSPG4 expression, higher in tumors than normal digestive tissues, was heterogeneous across tumors. High expression was associated with AFIP low-risk, gastric site, and localized stage, and independently with longer postoperative disease-free survival (DFS) in localized stage. The correlations between CSPG4 expression and immune signatures highlighted a higher anti-tumor immune response in “CSPG4-high” tumors, relying on both the adaptive and innate immune system, in which the boost of NK cells by CSPG4-CAR.CIKs might be instrumental, eventually combined with immune checkpoint inhibitors. In conclusion, high CSPG4 expression in GIST is associated with better DFS and offers an immune environment favorable to a vulnerability to CAR.CIKs.

Benign tumors in TSC are amenable to treatment by GD3 CAR T cells in mice

Mutations underlying disease in tuberous sclerosis complex (TSC) give rise to tumors with biallelic mutations in TSC1 or TSC2 and hyperactive mammalian target of rapamycin complex 1 (mTORC1). Benign tumors might exhibit de novo expression of immunogens, targetable by immunotherapy. As tumors may rely on ganglioside D3 (GD3) expression for mTORC1 activation and growth, we compared GD3 expression in tissues from patients with TSC and controls. GD3 was overexpressed in affected tissues from patients with TSC and also in aging Tsc2+/^– mice. As GD3 overexpression was not accompanied by marked natural immune responses to the target molecule, we performed preclinical studies with GD3 chimeric antigen receptor (CAR) T cells. Polyfunctional CAR T cells were cytotoxic toward GD3-overexpressing targets. In mice challenged with Tsc2^–/– tumor cells, CAR T cells substantially and durably reduced the tumor burden, correlating with increased T cell infiltration. We also treated aged Tsc2+/^– heterozygous (>60 weeks) mice that carry spontaneous Tsc2^–/– tumors with GD3 CAR or untransduced T cells and evaluated them at endpoint. Following CAR T cell treatment, the majority of mice were tumor free while all control animals carried tumors. The outcomes demonstrate a strong treatment effect and suggest that targeting GD3 can be successful in TSC.

Adoptive T-cell immunotherapy in digestive tract malignancies: Current challenges and future perspectives

Multiple systemic treatments are currently available for advanced cancers of the digestive tract, but none of them is curative. Adoptive T-cell immunotherapy refers to the extraction, modification and re-infusion of autologous or allogenic T lymphocytes for therapeutic purposes. A number of clinical trials have investigated either non-engineered T cells (i.e., lymphokine-activated killer cells, cytokine induced killer cells, or tumor-infiltrating lymphocytes) or engineered T cells (T cell receptor-redirected T cells or chimeric antigen receptor T cells) in patients with digestive tract malignancies over the past two decades, with variable degrees of success. While the majority of completed trials have been primarily aimed at assessing the safety of T-cell transfer strategies, a new generation of studies is being designed to formally evaluate the antitumor potential of adoptive T-cell immunotherapy in both the metastatic and adjuvant settings. In this review, we provide an overview of completed and ongoing clinical trials of passive T-cell immunotherapy in patients with cancers of the digestive tract, focusing on present obstacles and future strategies for achieving potential success.

Beyond the Driver Mutation: Immunotherapies in Gastrointestinal Stromal Tumors

Gastrointestinal stromal tumors (GISTs) are a subtype of soft tissue sarcoma (STS), and have become a concept of oncogenic addiction and targeted therapies.The large majority of these tumors develop after a mutation in KIT or platelet derived growth factor receptor α (PDGFRα), resulting in uncontrolled proliferation. GISTs are highly sensitive to imatinib. GISTs are immune infiltrated tumors with a predominance of tumor-associated macrophages (TAMs) and T-cells, including many CD8+ T-cells, whose numbers are prognostic. The genomic expression profile is that of an inhibited Th1 response and the presence of tertiary lymphoid structures and B cell signatures, which are known as predictive to response to ICI. However, the microtumoral environment has immunosuppressive attributes, with immunosuppressive M2 macrophages, overexpression of indoleamine 2,3-dioxygenase (IDO) or PD-L1, and loss of major histocompatibility complex type 1. In addition to inhibiting the KIT oncogene, imatinib appears to act by promoting cytotoxic T-cell activity, interacting with natural killer cells, and inhibiting the expression of PD-L1. Paradoxically, imatinib also appears to induce M2 polarization of macrophages. There have been few immunotherapy trials with anti-CTLA-4 or anti-PD-L1drugs and available clinical data are not very promising. Based on this comprehensive analysis of TME, we believe three immunotherapeutic strategies must be underlined in GIST. First, patients included in clinical trials must be better selected, based on the identified driver mutation (such as PDGFRα D842V mutation), the presence of tertiary lymphoid structures (TLS) or PD-L1 expression. Moreover, innovative immunotherapeutic agents also provide great interest in GIST, and there is a strong rationale for exploring IDO targeting after disease progression during imatinib therapy. Finally and most importantly, there is a strong rationale to combine of c-kit inhibition with immune checkpoint inhibitors.

Immunotherapy Strategies for Gastrointestinal Stromal Tumor

Gastrointestinal stromal tumors (GIST) are the most common mesenchymal soft tissue sarcoma of the gastrointestinal tract. The management of locally advanced or metastatic unresectable GIST involves detecting KIT, PDGFR, or other molecular alterations targeted by imatinib and other tyrosine kinase inhibitors. The role of immunotherapy in soft tissue sarcomas is growing fast due to multiple clinical and pre-clinical studies with no current standard of care. The potential therapies include cytokine-based therapy, immune checkpoint inhibitors, anti-KIT monoclonal antibodies, bi-specific monoclonal antibodies, and cell-based therapies. Here we provide a comprehensive review of the immunotherapeutic strategies for GIST.

Synthetic immunology: T-cell engineering and adoptive immunotherapy

During the past decades, the rapidly-evolving cancer is hard to be thoroughly eliminated even though the radiotherapy and chemotherapy do exhibit efficacy in some degree. However, a breakthrough appeared when the adoptive cancer therapy [1] was developed, especially T cells armed with chimeric antigen receptors (CARs) showed great potential in tumor clinical trials recently. CAR-T cells successfully elevated the efficiency and specificity of cytotoxicity. In this review, we will talk about the design of CAR and CAR-included combinatory therapeutic applications in the principles of systems and synthetic immunology.

Latest advances in adult gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are the most common GI tract mesenchymal tumors. GIST patients are optimally managed by a precision medicine approach. Herein, we discuss the latest advances in precision medicine and ongoing clinical trials relevant to GIST. Circulating tumor DNA for detection of mutational changes could replace tissue biopsies and radiographic imaging once validated. Most GISTs are KIT/PDGFRα mutated, and despite the good clinical response to imatinib, treatment is generally not curative, more often due to secondary mutations. New mechanisms to bypass this resistance by inhibiting KIT downstream pathways and by targeting multiple KIT or PDGFRα mutations are being investigated. Immunotherapy for GIST patients is in its infancy. These approaches may lead to more effective, less toxic therapies.

The immune system and gastrointestinal stromal tumor: a wealth of opportunities

PURPOSE OF REVIEW

This article reviews the current literature on tumor-infiltrating immune cells in gastrointestinal stromal tumor (GIST), and the current status and prospects of effective immunotherapeutic strategies.

RECENT FINDINGS

Tumor-infiltrating immune cells populate the microenvironment of GISTs; the most numerous are tumor-associated macrophages (TAMs) and CD3 T cells. TAMs have not been shown to have a relationship with the biological behavior of GISTs; however, the number of CD3 T cells correlates with better outcomes. The prognostic significance of tumor-infiltrating neutrophils, natural killer cells, CD4 T cells, CD8 T cells, and Treg cells remains unknown.Imatinib mesylate achieves a clinical response in 80% of patients with GIST. Its antitumor mechanism is partially immune mediated. The combination of imatinib and interferon-α has been shown to be effective against GIST - it eradicates tumor cells including those that are drug resistant. Preclinical trials including cytotoxic T lymphocyte-associated antigen 4 blockade, anti-KIT antibody, and the generation of designer T cells have shown promising therapeutic effect in animal models of GIST.

SUMMARY

GIST contains many tumor-infiltrating immune cells and should be susceptible to immunotherapy; early clinical and preclinical trials have shown promising results that should lead to new investigations and effective forms of direct and synergistic therapies.

Cancer immunotherapy in clinical practice -- the past, present, and future

Considerable progress has been made in the field of cancer immunotherapy in recent years. This has been made possible in large part by the identification of new immune-based cellular targets and the development of novel approaches aimed at stimulating the immune system. The role played by the immunosuppressive microenvironment in the development of tumors has been established. The success of checkpoint-inhibiting antibodies and cancer vaccines has marked the beginning of a new era in cancer treatment. This review highlights the clinically relevant principles of cancer immunology and various immunotherapeutic approaches that have either already entered mainstream oncologic practice or are currently in the process of being evaluated in clinical trials. Furthermore, the current barriers to the development of effective immunotherapies and the potential strategies of overcoming them are also discussed.

Constitutive activation of oncogenic PDGFRα-mutant proteins occurring in GIST patients induces receptor mislocalisation and alters PDGFRα signalling characteristics

Background

Gastrointestinal stromal tumours (GIST) are mainly characterised by the presence of activating mutations in either of the two receptor tyrosine kinases c-KIT or platelet-derived growth factor receptor-α (PDGFRα). Most mechanistic studies dealing with GIST mutations have focused on c-KIT and far less is known about the signalling characteristics of the mutated PDGFRα proteins. Here, we study the signalling capacities and corresponding transcriptional responses of the different PDGFRα proteins under comparable genomic conditions.

Results

We demonstrate that the constitutive signalling via the oncogenic PDGFRα mutants favours a mislocalisation of the receptors and that this modifies the signalling characteristics of the mutated receptors. We show that signalling via the oncogenic PDGFRα mutants is not solely characterised by a constitutive activation of the conventional PDGFRα signalling pathways. In contrast to wild-type PDGFRα signal transduction, the activation of STAT factors (STAT1, STAT3 and STAT5) is an integral part of signalling mediated via mutated PDGF-receptors. Furthermore, this unconventional STAT activation by mutated PDGFRα is already initiated in the endoplasmic reticulum whereas the conventional signalling pathways rather require cell surface expression of the receptor. Finally, we demonstrate that the activation of STAT factors also translates into a biologic response as highlighted by the induction of STAT target genes.

Conclusion

We show that the overall oncogenic response is the result of different signatures emanating from different cellular compartments. Furthermore, STAT mediated responses are an integral part of mutated PDGFRα signalling.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-015-0096-8) contains supplementary material, which is available to authorized users.

Immune cells in primary and metastatic gastrointestinal stromal tumors (GIST)

We have previously described immune cells in untreated primary gastrointestinal stromal tumors (GIST). Here we compare immune cells in metastatic and primary GIST, and describe their chemoattractants. For this purpose, tissue microarrays from 196 patients, 188 primary and 51 metastasized GIST were constructed for paraffin staining. Quantitative analysis was performed for cells of macrophage lineage (Ki-M1P, CD68), T-cells (CD3, CD56) and B-cells (CD20). Chemokine gene-expression was evaluated by real-time RT-PCR. Immuno-localisation was verified by immunofluorescence. Ki-M1P+ cells were the predominant immune cells in both primary and metastatic GIST (2 8.8% ± 7.1, vs. 26.7% ± 6.3). CD68+ macrophages were significantly fewer, with no significant difference between primary GIST (3.6% ± 2.1) and metastases (4.6% ± 1.5). CD3+ T-cells were the most dominant lymphocytes with a significant increase in metastases (7.3% ± 2.3 vs. 2.2% ± 1.8 in primary GIST, P < 0.01). The percentage of CD56+ NK-cells was 1.1% ± 0.9 in the primary, and 2.4 ± 0.7 (P < 0.05) in the metastases. The number of CD20+ B-cells was generally low with 0.6% ± 0.7 in the primary and 1.8% ± 0.3 (P < 0.05) in the metastases. Analysis of the metastases showed significantly more Ki-M1P+ cells in peritoneal metastases (31.8% ± 7.4 vs. 18.2% ± 3.7, P < 0.01), whilst CD3+ T-cells were more common in liver metastases (11.7% ± 1.8 vs. 4.4% ± 2.6, P < 0.01). The highest transcript expression was seen for monocyte chemotactic protein 1 (MCP1/CCL2), macrophage inflammatory protein 1α (MIP-1α/CCL3) and the pro-angiogenic growth-related oncoprotein 1 (Gro-α/CXCL-1). Whilst the ligands were predominantly expressed in tumor cells, their receptors were mostly present in immune cells. This locally specific microenvironment might influence neoplastic progression of GIST at the different metastatic sites.

MicroRNA-218 inhibits gastrointestinal stromal tumor cell and invasion by targeting KIT

The objectives of this study were to detect the expressions of microRNA-218 (miR-218) in human gastrointestinal stromal tumor (GIST) tissues and cells and explore its effects on the biological features of GIST-T1 cells and the expression of its target gene KIT, so as to provide new insights for GIST treatment. Using quantitative real-time polymerase chain reaction (qRT-PCR), we detected the expressions of miR-218 in the tissues and adjacent tissues of GIST and in the GIST cell lines including GIST882, GIST430, GIST48, and GIST-T1. Forty-eight hours after the miR-218 mimic was transfected into the GIST-T1 cells, the expression of miR-218 in the GIST-T1 cells was detected by qRT-PCR. The effect of miR-218 on the GIST-T1 cell viability was detected using MTT. The effect of miR-218 on the proliferation and apoptosis of GIST-T1 cell was analyzed using flow cytometry. Transwell invasion chamber was applied to detect the effect of miR-218 on the invasion of GIST-T1 cells. KIT was identified to be a target gene of miR-218 by the luciferase reporter enzyme system, and the effect of miR-218 on the expression of KIT protein in cells was determined using Western blotting. As shown by qRT-PCR, compared with that in the GIST adjacent tissue, the expressions of miR-218 in the tumor tissue and GIST cell lines were significantly decreased (P < 0.0001). Compared with the control group, the expression of miR-218 increased significantly in GIST-T1 cells transfected with miR-218 mimic for 48 h (P < 0.01). MTT showed that the cell viability decreased significantly after the overexpression of miR-218 in the GIST-T1 cells (P < 0.01). Flow cytometry showed that the cell proliferation index significantly declined after the overexpression of miR-218 (P < 0.01); meanwhile, the apoptosis of cells also significantly increased (P < 0.01). Detection using the Transwell invasion chamber showed that the number of cells passing through the Transwell chamber significantly dropped after the enhanced expression of miR-218 (P < 0.01). Luciferase reporter gene assay showed that, compared with the control group, the relative luciferase activity significantly declined in the miR-218 mimic transfection group (P < 0.01). Compared with the control group, the expression of KIT protein in the GIST-T1 cells transfected with miR-218 mimic for 48 h significantly decreased (P < 0.01). In conclusion, the expression of miR-218 decreases in human GIST tissue and cell lines. miR-218 can negatively regulate the expression of KIT protein and inhibit the proliferation and invasion of GIST cells. Treatment based on the enhanced expression of miR-218 may be a promising strategy for GIST.

Immunotherapy for solid tumors--a review for surgeons

Immunotherapy has evolved considerably in the last decade and is becoming an integral component of the armamentarium for the treatment of patients with advanced solid tumors. It is important for clinicians, especially surgeons, to understand the basic principles of novel immunotherapies and the immune system. This review summarizes the evolution of the most relevant immunotherapies, their mechanisms of action, the data supporting their clinical use, and integration of immunotherapy into multidisciplinary management of solid tumors. This review should serve as a primer for clinicians and surgeons to understand the rapidly evolving field of immunotherapy.

Gene-engineered T cells for cancer therapy

T cells have the capacity to eradicate diseased cells, but tumours present considerable challenges that render T cells ineffectual. Cancer cells often make themselves almost 'invisible' to the immune system, and they sculpt a microenvironment that suppresses T cell activity, survival and migration. Genetic engineering of T cells can be used therapeutically to overcome these challenges. T cells can be taken from the blood of cancer patients and then modified with genes encoding receptors that recognize cancer-specific antigens. Additional genes can be used to enable resistance to immunosuppression, to extend survival and to facilitate the penetration of engineered T cells into tumours. Using genetic modification, highly active, self-propagating 'slayers' of cancer cells can be generated.