Ann Thorac Surg 2003;76:1319-1326
© 2003 The Society of Thoracic Surgeons
Review
Active, specific immunotherapy for lung cancer: hurdles and strategies using genetic modification
Robert J. Korst, MD*a,c,
Ronald G. Crystal, MDb,c
a Division of Thoracic Surgery, Department of Cardiothoracic Surgery, New York, New York USA,
b Division of Pulmonary and Critical Care Medicine, New York, New York USA
c Department of Genetic Medicine, Weill Medical College of Cornell University, New York, New York, USA
* Address reprint requests to Dr Korst, Department of Cardiothoracic Surgery, Room M-404, Weill Medical College of Cornell University, 525 East 68th St, New York, NY, USA 10021
e-mail: rjk2002{at}med.cornell.edu
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Abstract
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Active immunotherapy for lung cancer has been a challenge because of the poor antigenic characterization of these tumors and their ability to escape the immune response. However, knowledge of the mechanisms of anti-tumor immunity has expanded significantly over the past decade, leading to the development of more novel, specific strategies for augmenting the immune response. Genetic manipulation of tumor cells, immune cells, or both, may help overcome some of the previously encountered difficulties of immunotherapy. Laboratory and clinical investigations are currently ongoing to evaluate the feasibility and potential benefit of these novel approaches.
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Introduction
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Lung cancer is the leading cause of cancerous death in the United States, resulting in more deaths than colon, prostate, and breast cancer combined [1]. Only 12% of patients presenting with lung cancer will ever be cured of their disease, with the vast majority of patients dying of widely metastatic disease [1]. Recurrence after curative resection for localized disease is usually distant [2], implying that more effective systemic therapy is a priority for patients with lung cancer.
Systemic therapeutic options for patients with lung cancer are currently limited to cytotoxic agents, unless the patient is enrolled in a clinical trial. Although newer agents are continually being developed, the limitations of conventional cytotoxics include nonselectivity resulting in systemic toxicity as well as development of resistant tumor cell clones. Response rates to modern combination chemotherapy in patients with nonsmall cell lung cancer have been modest at best [3], and minimal survival benefit is appreciated when given as an adjuvant to surgical resection [4].
Immunotherapy represents a treatment strategy that has the potential to be both tumor specific as well as tumor systemic in nature. Active, specific immunotherapy refers to the induction of tumor-specific immunity in the tumor-bearing host. Tumor vaccines, in their many forms, represent the most common type of specific, active immunotherapy strategy.
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History of active, specific immunotherapy for lung cancer
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Although multiple clinical trials evaluating nonspecific immune stimulation (using agents including the bacille Calmette-Guérin, levamisole, and various cytokines) for the treatment of lung cancer were reported in previous decades [5–8] of which most were negative trials, reportsdescribing active, specific immunotherapy for patients with lung cancer are uncommon. In the evaluation of adjuvant, postoperative immunotherapy consisting of soluble tumor associated antigens isolated from allogeneic tumor cell membranes combined with complete Freund’s adjuvant, an early phase II, and a small phase III trial reported a survival advantage for patients with completely resected nonsmall cell lung cancer in the immunotherapy arm [9, 10]. However a clear survival benefit could not be demonstrated in two additional randomized trials evaluating these reagents, leaving the efficacy of this approach unsubstantiated [11, 12]. Studies using autologous or allogeneic tumor cell vaccines also have been investigated for patients with nonsmall cell lung cancer, again with no consistent survival benefit [13]. A more recent, novel approach to achieving specific, active immunotherapy involved administration of anti-idiotypic antibodies (mimicking the ganglioside GD3) to patients with either limited stage or extensive stage small cell lung cancer [14]. Although not a randomized trial, this approach seemed safe as tested in 15 patients, and survival was enhanced when compared with historical controls, prompting the investigators to evaluate this strategy in a multi-institutional, randomized trial.
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Overview of tumor immunity
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Subsequent to the immunotherapy trials of the 1970s and 1980s, significant advances have been made in understanding the mechanisms of tumor immunity. The immune response to tumors is generally thought to be cell mediated [15, 16]. Although tumor-bearing hosts have been demonstrated to produce a humoral response (antibodies) to some tumor antigens (eg, Epstein-Barr virus antigens present on Epstein-Barr virus associated lymphomas), the evidence that such a humoral response protects the host from tumor progression is lacking. In contrast, the concept of cell-mediated tumor immunity as protective is supported by evidence from experimental animals in which complete tumor eradication has been achieved by activated cytotoxic T lymphocytes (CTLs), as well as humans, in which CTLs have been isolated from patients with cancer that are able to lyse autologous tumor cells. However, even though CTL generation correlates with tumor regression in animal models, it is important to recognize that the relationship between the presence of CTLs and clinical tumor regression in humans is less clear.
The basic tenets of the current concepts of cell-mediated immune response to solid tumors are now understood (Fig 1).
For reviews of tumor immunity, see references 15 and 16. Tumor associated antigens (TAAs) synthesized by tumor cells are presented on the cell surface in association with class I molecules of the major histocompatability complex (MHC). The MHC represents a set of genes encoding proteins involved in the distinction of self versus nonself. Naive CD8+ T cells (pre-CTLs) recognize TAAs presented in the context of MHC class I and then proliferate and differentiate into activated CTLs. A requirement for this CTL generation is the presence of co-stimulation, which originates from either co-stimulatory molecules on antigen presenting cells, or cytokines secreted by the antigen presenting cells (interleukin-12) or CD4+ (helper) T cells (interleukin-2, interferon-
). Also in the tumor, "professional" antigen presenting cells, referred to as dendritic cells (DCs), ingest TAAs from dead and dying tumor cells, process these antigens, and display them on their surface associated with MHC class II molecules. Dendritic cells then become activated, and the MHC and TAA complex is presented to naive CD4+ T lymphocytes. Cross-priming can also occur, in which the DCs present exogenous antigens in the context of class I to CD8+ T cells. Dendritic cell activation is characterized by heightened secretion of immunostimulatory cytokines (most notably interleukin-12) and upregulation of surface molecules that act as co-stimulators of T cells (CD40, B7.1, B7.2) as well as MHC molecules.
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Fig 1. Cellular anti-tumor immunity. Tumor cells present tumor-associated antigens (TAAs) to naive CD8+ T lymphocytes in the context of the major histocompatability complex (MHC) class I molecules. In the presence of either co-stimulation (B7.1/B7.2) from activated antigen-presenting cells (APCs) or cytokine "help" (interleukin-12) from activated CD4+ T cells, these CD8+ cells become activated, TAA-specific cytotoxic T lymphocytes (CTLs). In the absence of co-stimulation and cytokines, anergy (tolerance) is thought to occur. CD4+ T cells become activated when soluble TAAs, which have been engulfed and processed by naive APCs, are presented to naive CD4+ cells through MHC class II (with B7 co-stimulation). Another pathway for CTL activation involves "crosspriming" by APCs, in which processed TAAs are presented to naive CD8+ cells using MHC class I. In this scenario cytokine help for CD4+ cells is not needed, because the activated APCs possess the necessary co-stimulatory molecules. CD40/CD40 ligand interactions further activate APCs to allow more efficient TAA presentation. Activated CTLs proliferate and migrate to tumors, bind to tumor cells through MHC class I, and induce apoptosis of the target tumor cell through several potential mechanisms (perforin, Fas ligand).
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Similar to the activation of CD8+ cells, CD4+ cells require co-stimulation to become activated, which occurs through the B7(DC) and CD28(T cell) interaction. Without co-stimulation, anergy or tolerance is thought to occur. In addition, once CD4+ cells become activated they express high levels of CD40 ligand (CD40L), which interacts with the CD40 receptor on DCs acting as a positive feedback loop to further activate DCs, thereby amplifying the immune response. Cytokines secreted by both activated DCs and CD4+ cells have the ability to function as co-stimulators of CD8+ cells, allowing them to differentiate into CTLs. Similar to CD4+ cell activation, the absence of co-stimulation results in anergy (tolerance).
In addition to the specific anti-tumor immunity characterized by CTLs, there are innate immune mechanisms that mediate tumor rejection [15, 16]. Natural killer cells can be activated by either direct recognition of tumor cells (in an MHC-independent fashion), or can respond to cytokines secreted by T cells (primarily interleukin-12). Natural killer cells use the same mechanism of cell killing as CTLs. The role of natural killer cells becomes more significant in situations in which tumors express little or no MHC class I molecules, a situation representing one mechanism by which tumors can escape killing by CTLs. Macrophages are also potentially important cellular mediators of tumor immunity, which may be due to the production of tumor necrosis factor by these cells.
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Hurdles in generating the anti-tumor immune response
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Heterogeneity of TAAs
Tumor cells differ from normal cells in that they are more likely to be recognized as foreign by the immune system, secondary to both qualitative and quantitative differences in gene expression in cancer cells compared with normal cells [17]. These aberrations in gene expression usually take the form of either mutations of normal genes, in which case an abnormal protein product is produced, or a dysregulated gene expression, involving the production of nonmutated proteins that are not normally expressed. Classification schemes for the types of TAAs that are recognized by CTLs have been proposed [17], which can be summarized by the following listed groups, beginning with the most tumor-specific and ending with the antigens that are shared by multiple tissues or least tumor-specific: - Products of random mutations in cellular genes not involved in oncogenesis. The transformation process involves DNA damage not only in genes resulting in the malignant phenotype, but also in totally unrelated, somatic genes. Because these mutations are random events, they are both patient and tumor-specific.
- Antigens (referred to as tumor-specific antigens) found exclusively in tumor cells, but not normal cells. These specific TAAs may be shared by different patients, and include both the products of mutated oncogenes (eg, ras), and tumor suppressor genes (eg, p53), as well as transforming viral gene products (eg, Epstein-Barr virus and lymphoma; human papillomavirus and cervical cancer).
- Products of genes expressed by tumors, but silent in the majority of normal tissues. The best example of these antigens are the cancer-testis antigens (eg, MAGE, BAGE, GAGE, NY-ESO-1), which are expressed in multiple tumor types as well as normal testis.
- Products of tissue-specific genes that are shared by tumors and the normal tissue from which the tumors arose. The best-known examples of these antigens include those that are specific to melanomas as well as normal melanocytes (eg, tyrosinase, gp100, MART-1), as well as carcinoembryonic antigen, which is expressed on both normal epithelial tissues and some corresponding carcinomas.
Because TAAs from groups 1 to 3 are the most tumor-specific, it would seem that immunization with these would result in a more effective CTL generation than immunization with those in group 4, because some degree of self-tolerance is expected to antigens shared by tumors and a significant amount of normal tissue, although this has not been proven. In this context, a variety of TAAs that have been associated with CD8+ CTL responses in humans (usually in melanoma patients), also have been detected to some degree in human lung cancer (tumor specimens and cell lines) (Table 1)
[18–34]. At times, wide ranges of expression are reported because of the detection methods and reagents used, as well as variable interpretations of a positive result. This marked heterogeneity of TAA expression represents a severe hurdle in developing antigen-specific, CTL-inducing vaccines for lung cancer.
Low immunogenicity of TAAs
Although TAAs are present on malignant cells, their ability to induce a strong CTL response in the host may not be sufficient to cause rejection of the tumor. The most striking evidence supporting this concept is that CTLs have been demonstrated in many cancer patients, but clinical regression of the tumor does not occur [35]. Whether this is caused by inherently low antigenicity of TAAs or other phenomena, such as secretion of immunoinhibitory substances by tumor cells, is not clear. Because the demonstration of CTL activity requires at least one step of ex vivo stimulation of T cells by tumor cells (co-culture), the question arises as to whether these CTL responses are artificial with little clinical relevance.
Selection of antigen-loss variants
Even if CTLs are successfully generated against a defined TAA, tumor cell populations may be selected that have lost the ability to express the antigen mediating the anti-tumor effect. This phenomenon, called selection of tumor antigen-loss variants, has been described in melanoma patients receiving antigen-specific vaccines [36], and potentially presents difficulties in any TAA-specific lung cancer vaccination approach.
Tolerance
The immune system does not normally become activated against antigens that it recognizes as self. This phenomenon, termed tolerance, is a complicated process that involves both the ability of naive T lymphocytes to ignore self antigens, as well as the active elimination of autoreactive T cells. In the context of establishing active, specific anti-tumor immunity, the issue of tolerance is significant for two reasons.
First, many of the known TAAs (Table 1) are nonmutated and shared with normal tissues, implying that the host has established tolerance to these antigens. Therefore, is it feasible to overcome this preestablished tolerance and induce CTLs against these shared antigens? Data from clinical trials in some human cancers have suggested that CTLs can indeed be generated against such TAAs, including those shared by tissues in multiple organ systems (such as Her-2/neu, carcinoembryonic antigen, and MUC-1) [37, 38]. Whether these CTLs are able to be generated in lung cancer patients and whether they will result in meaningful clinical responses remains to be determined.
Second, will immunization with specific TAAs result in the induction of tolerance to the antigen used in the vaccine, possibly enhancing tumor growth? To our knowledge, this phenomenon has yet to be described in human vaccination trials. However, in at least two specific murine models, tolerance to the vaccinated antigen has been observed [39, 40], suggesting that as human immunotherapy trials become more frequent, tolerance induction may become apparent.
Immune downregulation by tumors
Tumor cells have been demonstrated to actively change their phenotype to evade the immune response. Malignant cells are thought to do this by way of several mechanisms, including downregulation of MHC molecules on the tumor cell surface, secretion of immunoinhibitory factors, and direct lysis of CTLs through Fas/Fas ligand interactions [41–44]. Theoretically, downregulation of MHC class I molecules on the tumor cell surface should impair TAA presentation to naive CD8+ pre-CTLs. Such downregulation has been demonstrated in multiple human tumor types, including lung cancers [41]. What remains unknown is the quantitative effect of this process on the magnitude of the CTL response.
Transforming growth factor-ß is an immunoinhibitory cytokine that inhibits a variety of lymphocyte and macrophage functions and is secreted by many types of human tumors [42, 43]. Transforming growth factor-ß has been shown to inhibit proliferation of T lymphocytes, inhibit the maturation of CTLs and the activation of macrophages [42, 43]. Similarly, interleukin-10 can also inhibit T cell proliferation and can be secreted by tumors [43]. The role of these inhibitory cytokines in clinical immunotherapy strategies remains to be elucidated. Although probably not the dominant mechanism, CTLs (expressing Fas ligand) may kill target cells (expressing Fas receptor) by Fas/Fas ligand interactions and subsequent apoptosis. However, some recent in vitro data have suggested that tumor cells may express Fas ligand, which could induce CTLs to undergo apoptosis [44]. Whether or not this occurs in vivo is not clear.
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Gene transfer and immunotherapy
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With the recent explosion in the understanding and applications of molecular biology and recombinant DNA technology, active immunotherapy for cancer involving genetic manipulation (immunogene therapy) has become an area of intense investigation. Genetic manipulation is achieved by the transfer of genetic material (DNA, RNA) into various target cells to affect specific protein production. Targets for gene transfer include tumor cells (established tumors or cells cultured ex vivo), antigen presenting cells (dendritic cells), or in vivo tissues remote from the tumor site (usually skin or skeletal muscle). Transferred genetic material includes sequences encoding TAAs, as well as immunologically and relevant genes such as cytokines, chemokines, transcription factors, and surface activation molecules that facilitate antigen presentation to T cells.
Gene transfer to such diverse cell and tissue types has been successfully achieved using both viral and nonviral (plasmid) vector systems in both experimental animal models as well as clinical trials. For a detailed discussion of gene transfer vectors, several reviews are available [45, 46]. The ideal gene transfer vector for oncologic applications should efficiently and selectively transduce the target cell resulting in high levels of transgene expression and possess limited vector related toxicity. Although no currently used vector system is successful in uniformly achieving these goals, significant advances have been made.
Tumor immunity achieved using transfer of genes encoding specific TAAs
Because CTLs have been demonstrated that recognize specific TAAs in cancer patients, one strategy aimed at enhancing CTL generation is the transfer of genes encoding one or more defined TAAs to the tumor-bearing host with the hope that increased host TAA expression will result in enhanced antigen presentation to the immune system. One potential advantage of gene transfer over peptide vaccination in this paradigm is that gene transfer results in prolonged, although transient, expression of the desired antigen [47]. Such vaccination strategies have been successful in inducing anti-tumor immunity in pre-clinical tumor models with defined TAAs [48–52]. The most common sites of TAA gene transfer have been skeletal muscle, skin, and subcutaneous tissue, with the skin being potentially advantageous because of the high numbers of immature DCs present in the dermis (Langerhans cells). Indeed, successful transduction of resident DCs has been demonstrated after transcutaneous administration of plasmid DNA [53]. Anti-tumor immunity also has been achieved with intravenous vector delivery [52]. Vectors used for delivery of specific TAAs have included naked DNA strategies (plasmid DNA alone, or mixing with cationic lipids, or coated onto metallic particles before injection with a gene gun), as well as viral vectors (vaccinia, adenovirus, fowl pox, canary pox), and even RNA [48–52, 54]. Clinical trials using this approach are underway involving many TAAs, including genes encoding MUC-1, carcinoembryonic antigen, prostate-specific antigen, MAGE-1 and 3, p53, and melanoma-specific TAAs (eg, gp 100, MART-1, tyrosinase) for a variety of advanced-stage solid tumors [55–58]. Although the majority of these trials are phase I assessments of toxicity, CTLs have been demonstrated in some patients [58].
An obvious limitation of this approach in patients with lung cancer is that TAAs in this disease are poorly defined and inconsistently expressed. To enhance the probability of success it would be useful to screen the patient’s tumor before vaccination to determine if specific TAAs are expressed. Because a significant amount of tissue is needed to screen for TAA expression, these strategies are potentially limited to patients who have previously undergone resection, or have a tumor such that a significant amount of tissue that can be obtained with biopsy alone (a minority of patients with lung cancer). Other potential problems include the relative lack of co-stimulatory molecule expression required for generation of the cell mediated immune response when the vectors are given intravenously or at a remote site. To help overcome this challenge, gene transfer-based TAA expression could be combined with other strategies to enhance co-stimulation and antigen presentation. Another problem is that immunity would be lost if tumor cells downregulate the specific, vaccinated TAA. This problem may potentially be circumvented by the use of vaccines encoding multiple TAAs (multivalent vaccines).
Tumor cells genetically modified ex vivo
One strategy that has received significant attention in both past and ongoing studies has been the use of genetically modified autologous tumor cells as the source of TAAs for vaccination. Cells are typically harvested from the host, cultured ex vivo, genetically altered, irradiated to prevent further cell division, and administered back to the host as a vaccine [59]. This strategy is based on the concept that tumor cells express a broad array of potentially immunogenic proteins, but the tumor cells lack the ability to present these TAAs to T cells to induce immunity. Genetically engineering these cells to express immunostimulatory molecules such as cytokines, chemokines, co-stimulatory molecules, or antigen-presentation molecules, may augment the antigen presentation process by two mechanisms: (1) attracting inflammatory cells to the local tumor milieu where TAAs are being released by dying tumor cells, and (2) tumor cells engineered to express co-stimulatory or MHC molecules may function as antigen presenting cells. Preclinical studies have confirmed the ability of tumor cell-based vaccines to induce anti-tumor immunity, regression of established tumors, and long-lasting memory [59–61]. Early clinical trials have been reported, which have used this approach in patients with melanoma, renal cell carcinoma, neuroblastoma, as well as colon and prostate cancer. Although CTLs have been detected in 10% to 30% of patients in these trials, clinical response rates were lower [58, 62, 63].
Because tumor cells are a source of a broad array of TAAs, a theoretical advantage of these cell-based vaccines is that no prior knowledge of specific TAAs is necessary. In addition, because multiple antigens are expressed by tumors, immunity may not be confined to a single, specific antigen that could be downregulated by the tumor, and toxicity may be limited because the vector is not directly administered to the host. However, these strategies require the ability to culture patients’ tumors ex vivo, which has been difficult to achieve on a consistent basis for many human tumor types, including lung cancer. In addition, since a significant amount of tumor tissue is needed to prepare the vaccine, these cell-based strategies may be only applicable to lung cancer patients who have undergone resection, and these patients represent a minority.
Direct in vivo genetic modification of tumors
Because tumors are an abundant source of TAAs, transfer of immunomodulatory genes directly to established tumors in vivo to achieve systemic anti-tumor immunity has been widely investigated in preclinical models [60, 64–66]. This approach may be more appropriate for patients with malignancies such as lung cancer because (1) the tumor itself is the source of tumor antigens, eliminating the previously mentioned difficulties with TAA expression in lung cancer, (2) the potential exists for immunity to be established against multiple antigen epitopes (multivalent vaccine), and (3) there is no need for ex vivo culture of tumor cells. A requirement for this approach is that the vector needs to be administered directly into the tumor, or in some cases, it has been given peritumorally [67]. However, with modern techniques of endoscopic, as well as image-guided transcutaneous needle injection, access to lung tumors can be performed in most cases in a minimally invasive fashion [68]. Results from early clinical trials have been reported, and other trials are underway, mainly in melanoma, bladder cancer, or mixed advanced solid tumors, with some patients experiencing regression of both primary and metastatic lesions [55, 57, 58, 69].
Dendritic cell-based vaccines
Dendritic cells are the most efficient antigen-presenting cells. For DC-based vaccines, DCs are harvested from the tumor-bearing host and propagated ex vivo, at which time they are manipulated (exposed to TAAs; engineered to express immunomodulatory proteins, or both) and then administered back to the host. Dendritic cells are capable of being transduced with both plasmid as well as viral vectors [70]. Adenovirus vectors possess some distinct advantages for DC-based vaccines over other gene transfer techniques, including: (1) the ability to transduce both immature and mature DCs [70], (2) high levels of transgene expression with little toxicity to the DCs [71], (3) limited production of anti-adenovirus neutralizing antibodies when adenovirus-transduced DCs are administered in vivo [72], and (4) adenovirus vectors, even without a therapeutic transgene, will induce modest levels of DC activation and have been shown to enhance antigen presentation and migration of DCs to regional lymph nodes [71].
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Two fundamental DC-based vaccination strategies currently in existence
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DCs modified to express TAAs
Tumor-associated antigens can be loaded into cultured DCs (antigen priming) by either exposing them directly to TAAs themselves (antigen or peptide pulsing), or by transducing the DCs with genes coding for TAAs [73]. These modified DCs are then administered to the tumor-bearing host, usually by either subcutaneous or intravenous routes. Using gene transfer to accomplish antigen priming has theoretical advantages over pulsing DCs with protein antigens, which include higher and potentially more prolonged levels of protein expression, as well as direct processing of the gene product(s) through the class I pathway, which may enhance CTL stimulation [70]. Dendritic cells have also been pulsed with RNA to induce TAA expression [74]. TAA gene-transduced DCs have been successful in inducing anti-tumor CTLs in animal models [75, 76], resulting in the initiation of clinical trials for patients with multiple tumor types [55, 57, 77].
DC modified to express immunomodulatory molecules
Another DC vaccination strategy involves administration of genetically modified DCs through intratumoral injection, using the tumor itself as the source of TAAs. This approach may be especially useful for tumors with a heterogeneous and inconsistent pattern of TAA expression, including lung cancer. Dendritic cells, engineered to express a variety of immunostimulatory molecules and administered in this fashion, have been shown to induce anti-tumor CTLs in multiple animal models [78, 79]. In addition, DCs transduced with genes, in which the sole purpose was to activate the DCs ex vivo, were successful in eliciting tumor-specific immunity when injected intratumorally in preclinical models [80, 81].
Given their crucial involvement in antigen presentation to T lymphocytes, DCs may have a distinct advantage over other immunization strategies. In addition, it is now possible to amplify large numbers of DCs from peripheral blood of cancer patients [82]. Despite this, the addition of a cellular component to the vaccine adds a level of complexity and uncertainty to the vaccination process. Dendritic cells are heterogeneous with regard to their lineage and maturation states, and it is currently unclear as to which population should be used for vaccination. However, as DC biology is more completely understood, genetic manipulation of these cells will most certainly play a prominent role in future approaches to immunotherapy.
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Comment
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In summary, lung cancer is difficult to treat by immunotherapy because of its poor antigenic characterization and ability to circumvent the immune response. However, in the decades since the early active immunotherapy trials for lung cancer, significant inroads have been made into the mechanisms of anti-tumor immunity. It is generally accepted that the anti-tumor immune response is cell-mediated, involving the presentation of TAAs to the immune system, with CD8+ CTLs as the immune effector cells. This process requires a complex interaction of cells, secreted cytokines, as well as cell surface molecules. In addition, more specific and sophisticated assays have been developed to measure these responses in cancer patients. As a result of this explosion in knowledge, contemporary active immunotherapy strategies focus on enhancing antigen presentation to the immune system, with a subsequent increase in tumor-specific CTL generation. The use of gene transfer to manipulate both tumors as well as immune cells possesses some distinct advantages over nongenetic immunotherapy approaches that include prolonged, high level expression of transgenes (TAAs and immunomodulatory molecules), as well as the enhanced ability to localize expression of certain genes (cytokines, costimulatory molecules) in the tumor, potentially reducing systemic toxicity.
Despite these advances, objective responses in clinical trials (mainly performed in patients with melanoma and other tumors with better defined TAAs) using these strategies have been limited, although CTL generation has indeed been augmented in certain instances. As a result, future investigations will need to assess how these strategies can be made to work more efficiently, perhaps by combining them with other agents that may augment the immune response. One example is the use of demethylating agents to upregulate TAA expression on tumor cells [83], which may allow immunomodulatory therapies to work better. Another example is the combination of immunotherapy with other therapies, including chemotherapy. In this regard, cisplatin has been demonstrated to enhance Fas receptor expression on lung cancer cells [84]. Preliminary studies from our laboratory have shown that this increased expression of Fas results in enhanced CTL-mediated tumor cell killing in an animal lung cancer model, augmenting the anti-tumor effect of an immunotherapeutic strategy. With continued advances, it remains hopeful that immunotherapy may become part of the clinician’s armamentarium, along with chemotherapy, radiotherapy, and surgery, perhaps working in synergy with these other treatments as a minimally toxic, targeted therapy for lung cancer.
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Abstract
Introduction
