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The Community Oncologist

Cancer and Immune Response: Old and New Evidence for Future Challenges

^aMedical Oncology Department, Virgen de la Macarena University Hospital, Seville, Spain; ^bPfizer Medical Department, Madrid, Spain

Key Words. Cancer • Tumor-infiltrating lymphocytes • Immune tolerance • Cancer vaccines • CTLA-4 • GM-CSF • IL-2

Correspondence: Luis de la Cruz-Merino, M.D., Servicio de Oncología Médica, Hospital Universitario Virgen de la Macarena, Avenida Doctor Fedriani, 3, 41071 Sevilla, Spain. Telephone: 0034-955008934/955008932; Fax: 0034-954902219; e-mail: lucme12{at}yahoo.es

Received August 1, 2008; accepted for publication October 28, 2008; first published online in THE ONCOLOGIST Express on December 4, 2008.

Disclosure: Employment/leadership position: Enrique Grande-Pulido, Pfizer Pharmaceuticals; Intellectual property rights/inventor/patent holder: None; Consultant/advisory role: None; Honoraria: None; Research funding/contracted research: None; Ownership interest: Enrique Grande-Pulido, Pfizer Pharmaceuticals; Expert testimony: None; Other: None.

The content of this article has been reviewed by independent peer reviewers to ensure that it is balanced, objective, and free from commercial bias. No financial relationships relevant to the content of this article have been disclosed by the authors, planners, independent peer reviewers, or staff managers.

	LEARNING OBJECTIVES

Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

 
After completing this course, the reader should be able to:

1. Discuss the current scientific background of immunotherapy applied to cancer treatment.
   
2. Suggest lines of future investigation in the immunotherapy field.
   
3. Explain the rationale for developing and discuss the current status of new immunotherapeutic approaches in solid tumors.
   

	ABSTRACT

Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

 
Cancer may occur as a result of abnormal host immune system tolerance. Recent studies have confirmed the occurrence of spontaneous and induced antitumor immune responses expressed as the presence of tumor-infiltrating T cells in the tumor microenvironment in some cancer models. This finding has been recognized as a good prognostic factor in several types of tumors. Some chemotherapy agents, such as anthracyclines and gemcitabine, are effective boosters of the immune response through tumor-specific antigen overexpression after apoptotic tumor cell destruction. Other strategies, such as GM-CSF or interleukin-2, are pursued to increase immune cell availability in the tumor vicinity, and thus improve both antigen presentation and T-cell activation and proliferation. In addition, cytotoxic T lymphocyte antigen 4–blocking monoclonal antibodies enhance immune activity by prolonging T-cell activation. Strategies to stimulate the dormant immune system against tumors are varied and warrant further investigation of their applications to cancer therapy in the future.

	INTRODUCTION

Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

 
Cancer is essentially considered a complex cell disease. However, in recent decades increasing research of the tumoral microenvironment has revealed the crucial role of the host's immune system in interrupting the neoplastic phenotype. Therefore, cancer could be explained, at least in part, as an abnormal immune system tolerance to uncontrolled cells. Those cells that are usually recognized as tumor cells and are eliminated occasionally escape this control for several reasons (i.e., self-antigens, self-tolerance). The knowledge of, and interference with, the mechanisms that allow this "tumor immune tolerance" is the objective of cancer immunotherapy.

The first studies by Coley in the 19th century demonstrated tumor responses using bacteria stimuli. Based on evidence for spontaneous tumor regressions, Paul Ehrlich communicated the "specific tumor antigens" hypothesis in 1908. The "magic bullet" theory proposed that toxins could be targeted to cancer. In 1950, Thomas and McFarlane Burnet also proposed the tumor immunosurveillance theory, giving rise to a new era of clinical trials focused on the immunological host response [1].

Generally speaking, cancer immunotherapy has been classified as either passive or active. Passive immunotherapy refers mainly to monoclonal antibodies (mAbs) raised against an antigen commonly expressed in a tumor lineage (e.g., anti–human epidermal growth factor receptor [HER]-2/neu in breast cancer or anti-CD20 in B-cell lymphomas). Antibody production is performed ex vivo and then the patient is passively infused. Another example of passive immunotherapy is adoptive immunotherapy, where lymphocytes are isolated and expanded with immunogenic antigens in vitro, and then infused into patients. This approach has recently demonstrated promising results in melanoma [2]. In active immunotherapy, the objective is a full (but rough) immune system stimulation (unspecific immunotherapy) or stimulation of a specific tumor antigen(s) through the patient's own immune machinery (specific immunotherapy or tumor vaccines). Different vaccine preparations have been attempted to obtain a huge number of therapeutic vaccines that include purified tumor antigens or professional antigen-presenting cell (APC)-based vaccines, costimulator-enhanced vaccines (based on tumor cells or APCs transfected with cytokine genes), DNA vaccines (with plasmids encoding tumor antigens), viral vectors, and killed tumor-based vaccines. In spite of the great enthusiasm following some immunogenic effects of specific immunotherapy based on tumor lymphocyte infiltration, including the loss of certain tumor antigens and some minor clinical effects, the absence of real clinical efficacy is the rule [3]. Recently, Rosenberg et al. [4] reviewed the effect of 541 different cancer vaccines in 440 individuals with solid tumors (422 metastatic melanomas). Employing conventional oncological response criteria (World Health Organization or the Response Evaluation Criteria in Solid Tumors), an objective response rate of only 2.6% was achieved [4]. Despite the bad prognosis of this selected population (metastatic solid tumors, 67% with visceral disease), several mechanisms may explain the low clinical effectiveness reported. One of the main reasons lies in the inability of immune cells to infiltrate and become activated after an encounter with tumor antigen in vivo. Moreover, it seems that solid tumors do not express costimulatory molecules or produce the inflammatory microenvironment necessary to activate effector cells with the ability to eradicate tumors [5]. Therefore, the development of methods to activate antitumor immune cells by stimulating APCs and generate long-term memory cells, probably with the aid of costimulators, is one of the future challenges for definitively integrating tumor vaccines into the antineoplastic arsenal.

Compared with classic chemotherapy or radiotherapy, immunotherapy development presents two potential advantages: (a) specificity to the target cell, thus reducing adverse effects on normal tissues and (b) less interference with other therapies, making it an appropriate adjuvant treatment to chemotherapy or radiotherapy.

In spite of the expectations and exponential growth of immunotherapy trials [6], clinical benefits have not been obtained to date from most of the models used, leading the scientific community to a somewhat reluctant and skeptical attitude toward this topic [7].

Recently, however, new immune-mediated antitumor mechanisms have been proposed. For example, specific chemotherapy agents could facilitate immune stimulation against cancer. Also, the use of some cytokines, such as GM-CSF or interleukin (IL)-2, shows an immune active effect. The synergistic link between classic chemotherapy and new cancer treatments is an exciting arena to focus on in future cancer research strategies.

In this manuscript, we review some recent studies and new concepts that assess the efficacy of the natural antitumor immune response and the strategies developed to stimulate it from a clinical point of view.

	TUMOR MICROENVIRONMENT

Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

 
The immune system is an active and effective cancer "gatekeeper." Antitumor immune activity is primarily mediated by the innate immune system, mainly through circulating effector cells, particularly natural killer (NK) cells, neutrophils, and macrophages. Antigen-specific B and T lymphocytes comprise adaptive immunity, which is rather more specific and has the ability to generate memory cells. Several mechanisms have been proposed for tumor evasion of the immune recognition process, acting both locally—in the tumor microenvironment—and systemically [8]. Cancer development could be described as the success of the tumor's evasive mechanism over antitumor immune forces. This battlefield has been of high interest for cancer researchers, with interesting studies published on this topic.

The role of the cancer microenvironment has been evaluated in histopathological findings from several tumors. In 2003, Zhang et al. [9] published a study on 186 frozen specimens from advanced-stage ovarian carcinoma patients in which they assessed the distribution of tumor-infiltrating lymphocytes (TILs). TILs were detected within tumor cell islets in 54.8% of the specimens, were undetectable in 38.7% of the tumors, and were not evaluated in the remaining 6.5%. The 5-year overall survival rate was 38.0% among patients whose tumors contained T cells and 4.5% among patients whose tumors contained no T cells in the islets (p < .001). Furthermore, the progression-free survival duration was longer in patients with intratumoral T cells—22.4 versus 5.8 months (p < .001). Patients with TIL infiltrating tumors undergoing surgery had a better prognosis than those without TIL infiltration (67.4% versus 29%). The authors concluded that ovarian cancer shows a clear interaction between the host immune system and the tumor, and that this correlates with better clinical outcome, as happens in other neoplasias, such as breast cancer, hepatocellular carcinoma, lymphoma, melanoma, esophageal carcinoma, hereditary nonpolyposis colorectal cancer, and endometrial cancer [10–14]. However, results are not homogeneous because there are studies that relate TIL infiltration to a worse outcome in other cancer types (e.g., kidney cancer). TILs are lymphocytes isolated from the inflammatory infiltrates present in and around surgical resection samples of solid tumors and are composed of tumor-specific cytotoxic T lymphocytes (CTLs) and NK cells, which are in turn enhanced by cytokines such as IL-2 and GM-CSF. In any case, within the TILs, there are also regulatory T cells (T regs), which regulate activation of other T cells and are necessary to maintain peripheral tolerance to self-antigens. The imbalance between CTLs/NK cells and T regs might explain the different outcomes observed in histopathological studies. T regs are a subset of immune T cells specialized in the control of self-responsiveness. These cells mediate peripheral tolerance by suppressing self-antigen-reactive T cells and have been directly involved in tumor-induced immunosuppression [15]. T regs are characterized by the expression of CD4 and CD25 in their cell membrane. In addition, they also express FoxP3, a transcriptional factor critical for their development and function [16].

T regs inhibit the antitumor functions of tumor-specific T cells by direct cell-to-cell contact and also by a soluble-dependent mechanism [17]. Cytokines, like IL-10 and transforming growth factor (TGF)-β, and membrane antigens, like cytotoxic T lymphocyte antigen (CTLA)-4, have been implicated in the immunosuppression mechanism [18]. T regs accumulate at the tumor site, where they appear to directly suppress cytotoxic T cell responses against tumors [19] (Fig. 1). The higher levels of T regs in both metastatic lymph nodes and peripheral blood have been associated with a poor prognosis in several tumor types. Indeed, depletion of CD4⁺CD25⁺ FoxP3 T cells was found to improve tumor immunity and induce effective tumor rejection [20].

View larger version (43K): [in this window] [in a new window]   Figure 1. Inhibitory signals by T regs. Abbreviations: IL, interleukin; PDGF, platelet-derived growth factor; T reg, regulatory T cell; VEGF, vascular endothelial growth factor.

Welsh et al. [21] reported that the macrophage and mastocyte ratio in tumor cell islets correlated with longer survival in operated non-small cell lung cancer patients (5-year overall survival [OS] rate, 52.9% versus 7.7%; p < .001). Either way, as happens with TILs, we must take into account the fact that the host's immune system may also contribute to the development of some tumors. In fact, chronic inflammation is recognized as a risk factor predisposing to cancer in some tissues. Although mechanisms by which chronic inflammation can promote tumor development are not fully understood, there are some theories about it. Cells of the innate immune system are considered the most direct tumor-promoting agents among immune cells. Chronic activation of innate immune cells, particularly macrophages, is characterized by tissue remodeling and angiogenesis, both of which favor tumor formation. Innate immune cells can also contribute to malignant transformation of cells by generating free radicals or secreting soluble factors that promote cell cycle progression and survival of tumor cells. The adaptive immune system can indirectly enhance the chronic activation of innate immune cells [22].

The active relationship between the host's immune cells and some tumors is also supported by clinical observations and prognosis evolution. For example, it is well known that there is a higher lymphoma incidence—especially high-grade non-Hodgkin's lymphoma (NHL)—in patients with congenital or acquired immunodeficient diseases [23–28]. Curiously, Hodgkin's disease shows a better global prognosis than NHL and this could be related to its intense inflammatory cellular infiltration, mainly consisting of lymphocytes, hystiocytes, eosinophils, and plasmatic cells against <1% of tumor cells infiltrated (formerly Hodgkin and Reed-Sternberg cells). Recently, Diepstra et al. [29] reported HLA class II complex loss of expression in the Reed-Sternberg cell membrane as an independent factor predictive of a poor prognosis.

Another paradoxical correlation between pathological findings and clinical features occurs in medullar breast cancer. This disease has a markedly better prognosis than invasive ductal cancer (10-year OS rate, 84% versus 63%), but the histopathological feature of medullar breast cancer is a high anaplastic grade and mitotic index, which has been traditionally considered as characteristic of aggressive tumors. On the other hand, these tumors show an intense lymphoplasmacytic infiltrate composed mainly of CD8 and CD4 T lymphocytes and plasmatic cells. It has been proposed that these infiltrates are inducers of a better prognosis. The usual presence of activated granzyme-B⁺ T cells and plasmatic cells in the surroundings of medullar breast cancer apoptotic cells strengthens the theory on the immunogenicity of certain apoptosis pathways in some tumor types, whether mediated or not by cytotoxic drugs [30].

	IMMUNE RESPONSE TO TUMOR INDUCED BY CHEMOTHERAPY

Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

 
Cellular death can be achieved by two different mechanisms. Programmed death, or apoptosis, is a methodical process in which cells are dismantled through the activation of caspases and exposure of phosphatidylserine residues in the outer leaflet of the cell. This well-controlled process has been traditionally considered to be nonimmunogenic and occurring in the absence of inflammation. On the other hand, nonapoptotic death is characterized by inflammatory signs and immune system activation. This process is more chaotic, and uric acid release from destroyed cells is considered one of the most powerful mediators of not only the inflammatory process but also immune system activation, acting as a danger signal that stimulates dendritic cells (DCs) [31].

Although based on classic considerations, apoptosis is accepted nowadays as a heterogeneous process in which innate immunity can, in specific circumstances, be activated in several degrees ranging broadly from a nonimmunogenic, or "bland," to a powerful immune response [32].

Chemotherapy cell death is an apoptosis-mediated process traditionally considered as nonimmunogenic or bland, but new data suggest the presence of specific mechanisms according to the type of chemotherapy agent used. It has been proposed that some of them, such as gemcitabine and anthracyclines, act as powerful mediators of immune system activation [33], opening up a new paradigm in cancer therapy. Interestingly, immune system activation by chemotherapy may be masked by its common side effects, such as lymphopenia, which is detrimental to any immune response [32].

In this sense, the overexpression of proapoptotic Fas ligand or tumor necrosis factor (TNF)-related receptors and interferon- and TNF- release seem to be related to tumor cell destruction mediated by CD8 T cells. When massive cell destruction occurs, the mechanisms of controlled apoptosis are overwhelmed and a secondary necrosis occurs, which triggers an inflammatory response mediated by intracellular inflammatory mediator release. As previously mentioned, uric acid is a powerful inflammatory mediator, but heat shock proteins are also potent immune stimulators through DC activation and upregulation, CD80 and CD86 expression, and release of cytokines such as IL-12.

There are preclinical studies supporting immune stimulation "mediated" by chemotherapy: Nowak et al. [34, 35] reported an 80% tumor shrinkage rate in a solid tumor murine model combining gemcitabine and immunotherapy. In addition, Casares et al. [36] reported caspase-dependent apoptosis mediated by doxorubicin in a murine colon cancer model. A powerful immune response was assessed across DCs when doxorubicin was employed, but not when a different antineoplastic agent, such as mitomycin-C, was used [36]. The explanation for this selective immune activation mediated by drugs such as doxorubicin or gemcitabine is increased CD8 T lymphocyte expansion and an increased number of TILs mediated by an effective major histocompatibility complex (MHC) class I crosspresentation of tumor antigens released and phagocytosed. Other mechanisms that facilitate apoptotic cell antigen presentation to APCs have been reported, such as the translocation of the intracytoplasmic protein calreticulin to the cell surface mediated by anthracyclines [37, 38].

Crosspresentation is a mechanism favored by some antineoplastic drugs, such as anthracyclines and gemcitabine. These drugs allow tumor antigens to be presented to the MHC class I pathway through APCs, a mechanism previously thought to be restricted to the class II pathway [32]. This mechanism allows tumor antigen presentation to both CD4 and CD8 T cells, which subsequently identify and destroy the remaining tumor cells. Also, it is well known that some products of tumor cells may suppress antitumor immune responses; this is the case for some immune-inhibitory molecules, such as IL-10 and TGF-β. TGF-β is secreted in large quantities in many tumors and inhibits the proliferation and effector functions of lymphocytes and macrophages. Many of these cytokines can be downregulated as a result of the use of chemotherapy.

Locally advanced breast cancer constitutes an excellent situation for the investigation of clinical, pathological, and molecular interactions in cancer, because it is rather easy to recognize its genotypic and phenotypic features before and after treatment. Demaria et al. [39] found a change in the frequency of TILs after treatment with paclitaxel. More importantly, responses were correlated with TIL density, suggesting that apoptosis induced by paclitaxel is a powerful immunogenic stimulus, especially through DC activation. This process could be boosted by the effects of GM-CSF [39] (see below). Melanoma is also a model with a close relationship with the host immune system. Gogas et al. [40] reported a better outcome in 26% of the patients included in one study, who developed autoimmune signs such as vitiligo or autoantibodies. The study enrolled 200 patients treated with adjuvant interferon following Kirkwood's protocol. Patients with autoimmune features did not reach the median relapse-free survival (RFS) duration of 16 months reached in patients without autoimmune processes. Survival also was better in the first group (median OS time, not reached versus 37.6 months) [40]. The authors concluded that autoimmunity was an independent prognostic marker for longer RFS and OS. This effect was also confirmed in patients treated with anti-CTLA-4 (see below).

	CTLA-4 ANERGY AND IMMUNOTOLERANCE

Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

 
The immune system is a downregulated network, to avoid overstimulation and subsequent cell self-damage (autoimmunity). One of the keys for this control lies in leukotrine receptor recognition of the HLA-antigenic peptide complex. This process can result in T-cell activation or death by apoptosis. Such an interaction is indeed complex, and several ligands are involved. One such molecule, CD40L, is expressed on the surface of T cells shortly after their activation. This molecule is key for the production of B cell antibodies mediated by T cells and for the activation of APCs, which are essential players in cellular immune activation.

The interaction between CD40L and its receptor, CD40, which is expressed on B cells and APCs, stimulates the upregulation of these cells through both CD80 and CD86.

CD80 and CD86 are two surface proteins belonging to the B7 family, which costimulate T cells through interaction with the CD28 receptor. As a safeguarding process to limit self-damage, CD80 and CD86 interaction with CTLA-4 leads to immune tolerance or anergy. CTLA-4 (CD152) is produced and mobilized from the internal side of the cell membrane to the immune synapsis 2–3 days after T-cell activation has taken place. Once in the T-cell membrane, CTLA-4 is bound to either one of the costimulatory molecules CD80 and CD86. CTLA-4 expression turns the activated T cell into an inhibited T cell. This change in T-cell expression is explained by the higher affinity (ranging from 500- to 2500-fold) of CTLA-4 compared with CD28 [41]. The T-cell grade of activation/inhibition is a tight process depending on the grade of expression of both CD28 and CTLA-4 (Fig. 2). CD80 and CD86 preferential binding to CTLA-4 reduces the IL-2 release subsequent to T-cell activation. A delay in CTLA-4 expression favors T-cell activation and could be a pathway to improve or expand the immune response against tumors [41].

View larger version (29K): [in this window] [in a new window]   Figure 2. T-cell activation/inhibition. The MHC class II receptor and antigen complex is presented to the TCR. The activation pathway is shown in green and the inhibitory pathway is shown in red. For a detailed description, see the text. B7 includes both the CD80 and CD86 APC T-cell activators. Abbreviations: APC, antigen-presenting cell; CTLA-4, cytotoxic T lymphocyte antigen 4; MHC, major histocompatibility complex; TCR, T-cell receptor.

	GM-CSF AS AN ANTITUMOR IMMUNE BOOSTER

Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

 
Costimulators are required to initiate T-cell antitumor responses, and the induction of tumor-specific T-cell responses often requires crosspriming by DCs, which express costimulators and class II molecules. GM-CSF is a molecule that stimulates the proliferation and differentiation of granulocyte and macrophage colonies and related cells, such as monocytes/macrophages or DCs. It also acts as a TIL prolonged survival inducer, improving the immune system's antitumor activity [42]. Currently, GM-CSF is available only in the U.S. and is used as a supportive therapy in the treatment of deep hematological cytopenias derived from intensive chemotherapy regimens; however, its application as an immune "booster" in the treatment of solid tumors has been evaluated in some clinical trials (Table 1) [43–47].

These differences per number of treatment cycles cannot be justified only by the chemotherapy effect. Moreover, the large number of overexpressed tumoral antigens (HER-2/neu, carcinoembryonic antigen, mucin 1, etc.) represent an excellent target for an immune environment boosted by GM-CSF. This situation favors effective antigen presentation to APCs and better subsequent CD8 T-cell activation against tumor cells [48]. An additional effect of GM-CSF that improves antigen presentation to T cells is selective DC maturation and activation, and also the booster effect of a cancer vaccine when its gene is transduced into the vaccine cells [49].

Recently Cartron et al. [46] communicated the results of an immunotherapy combination schedule with rituximab and GM-CSF in patients with relapsed follicular lymphoma. There was a high response rate, which compares favorably with the results of rituximab in monotherapy, especially regarding complete responses (39% versus 6%). This immunotherapy combination schedule activates numerous immune cells, particularly granulocytes and monocytes [46].

Other therapeutic strategies that support immune system implication in reaching a better therapeutic outcome for cancer are the dose-intensive regimens supported with colony-stimulating factors used in breast cancer, lymphoma, or lung cancer [50–52]. This may be explained by the fact that the boosting effect of cytokines can alter and enhance tumor cell apoptosis and antigen release.

Finally, tumor-specific mAbs against certain antigens have become a reality in daily oncological practice (Table 2). Currently, there are >100 mAbs under clinical research, some of them with promising results. In addition to apoptosis induction by inactivation of specific intracellular pathways, the immune effector mechanisms by which antitumor antibodies may eradicate tumors include opsonization, phagocytosis, and activation of the complement system. Recently, a study by Musolino et al. [53] highlighted the crucial role that antibody-dependent cell-mediated cytotoxicity modulation plays in trastuzumab's antitumor activity, revealing that the ability to block the erbB-2 network is not the only mechanism for mAbs [53–55]. Likewise, a large number of studies focus on the syngenic antitumor activity of chemo- and immunotherapy. Interestingly, the powerful effect of these mAbs is enhanced when combined with chemotherapy, as evidenced by the priming of APC tumor antigen presentation and T-cell activation.

	PRACTICAL ISSUES

Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

1. Evaluation of the immunogenic potential of new chemotherapy agents in preclinical studies.
   
2. Prognosis correlation of tumors with their pathological findings, after systematic evaluation of the tumor microenvironment in excised tumors (TILs, macrophages).
   
3. Clinical trials to evaluate the immunopotential of chemotherapy agents such as anthracyclines and gemcitabine, especially in the neoadjuvant setting.
   
4. Identification of high immunogenic tumor antigens, and especially their efficacy in triggering the immune system as a "danger signal."
   
5. Identification of antitumor immune markers after chemotherapy, such as serologic markers, DCs or activated lymphocytes, inflammatory mediators like uric acid, or lymphocyte-labeled gammagraphy.
   
6. Evaluation of the "boosting" concept as an adjuvant mediator of the immune response against the tumor, including promising agents such as cytokines (GM-CSF or IL-2) or anti-CTLA-4 mAbs.
   
7. Assessment, in preclinical and clinical studies of solid tumors, of the potential synergy of combining chemotherapy with the ability to induce immunogenic apoptosis, with cytokines like GM-CSF and IL-2 (boosting) and anti CTLA-4 (avoiding tolerance).
   

This review suggests that the immune system can still be active when a cancer is at an advanced stage, but tumor immunotolerance mechanisms are able to keep it dormant. New immunotherapeutic approaches will try to awaken it.

	AUTHOR CONTRIBUTIONS

Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

 
Conception/design: Luis de la Cruz-Merino

Manuscript writing: Luis de la Cruz-Merino, Enrique Grande-Pulido, Ana Albero-Tamarit, Manuel Eduardo Codes-Manuel de Villena

Final approval of manuscript: Luis de la Cruz-Merino, Enrique Grande- Pulido, Ana Albero-Tamarit, Manuel Eduardo Codes-Manuel de Villena

	ACKNOWLEDGMENTS

Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

 
We thank S. Demaria, M.D., Assistant Professor, Department of Pathology of the New York University School of Medicine (NYU), and J.L. Villar-Rodríguez, M.D., Assistant Professor, Department of Pathology of the Hospital Universitario Virgen de la Macarena, for critical reading and helpful discussion.

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Top Learning Objectives Abstract Introduction Tumor Microenvironment Immune Response to Tumor... CTLA-4 Anergy and... GM-CSF as an Antitumor... Practical Issues Author Contributions References

 

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