September - December 2005: 
Volume 18, Issue 3

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Lung cancer: Immunosurveillance, immunoediting or immunoepigenetics?
Lung cancer is amongst the most frequent and malignant of neoplasms. Its mortality rates place it first of all types of cancer with more than 1.000.000 deaths annually worldwide. It is characterized by a series of biological and clinical features that position it in the epicenter of current research efforts. To this end, it is noted that this cancer responds poorly to various immunotherapeutic interventions as opposed to other malignancies.
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In general, immunotherapeutic attempts are far from being considered clinically efficacious4,5. However, despite the low clinical response rate, the fact that some cancer patients do respond to these treatments, indicates their potential and justifies the persistent and large efforts to device more inventive approaches6. This unrestricted effort aims to overcome obstacles continuously placed by newly discovered interrelations between cancer and the immune system. The critical role of innate immunity mechanisms7,8, the acceptance of at least some principles of the Danger Theory9, the yet fully comprehended role of regulatory T cells (Tregs)10 as well as the revision of various steps involved in the development of immunological memory11-13 are some of the aspects that, in a short period of time, propelled major reconsiderations of cancer immunotherapy.


Theoretically, the framework of all the above mentioned efforts remains the same, and is no other than the cancer immunosurveillance theory, initially proposed in the mid '50s by McFarlane Burnet and Lewis Thomas. Despite the fact that the first attempts towards its experimental validation led to contradictory results that contested and abandoned this theory, the coming of knock-out mouse technology resurrected it, in the early '80s. It became undoubtedly evident that the immune system encompasses mechanisms that confront and destroy malignantly transforming cells continuously produced in our body, as a result of the plethora of spontaneous mutations taking place during one's lifetime. It is exactly these mechanisms that immunotherapy attempts to use and enhance in order to achieve tumor destruction. The problem, however, is that tumor cells are capable of developing escape mechanisms enabling deviation from this immune attack rendering inactive any benefit that stems from immunotherapy. The mechanisms by which tumor cells achieve tolerance to, and/or even cause damage to the immune system represent a topical and challenging research area and are also the focus of immunotherapeutic approaches as well14-16.


In this context, recognition of tumor cells by the immune system and the subsequent response against them are fairly comprehensible. Tumor cells produce danger signals and express tumor antigens that are recognized as foreign, resulting in the induction of immune responses14. In contrast, the cancer immunosurveillance theory is least explanatory of the biological events responsible for tumor escape.


It has long been suggested that immune functions during oncogenesis, result in the selection of tumor variants, capable of surviving within an immunologically intact environment, very much like various pathogens. This view was strongly supported by the transplantation of tumors induced in immunocompetent or immunodeficient mice, into mice with an intact or deficient immune system17,18. These experiments demonstrated that tumor immunogenicity, as assessed by the fate of the transplanted tumors, was in direct relation to the functional integrity of the donor immune system19.The genetic instability of cancer cells allows the induction of several alterations with the final consequences being the overcoming of immune effector mechanisms and the shaping of tumor immunogenicity. It is estimated that, only as a result of chromosomal instability, 25-50% of tumor cell alleles can be lost20. Convincing evidence suggests that targets of such damage and loss could, at least, be genes that code for various tumor antigens, elements that participate in the processing and presentation of these antigens to T cells of the host21, or factors that intervene with the IFN-γ receptor signal transduction22.


On the basis of these findings, Robert D. Schreider and team formulated, at the beginning of the decade, a different and more comprehensive theory to relate cancer and the immune system, by introducing the term "cancer immunoediting"23-26. Accordingly, this relation follows an evolutionary course comprised by three steps that eventually lead to carcinogenesis ("the three Es of cancer immunoediting"). The initial step represents the time during which the immune system employs effective immunosurveillance by destroying rapidly transforming cells and impeding the formation of tumors (elimination). During the second step, the cancer cells that begin to prevail are those they have developed the ability to deviate from immune attack (equilibrium). Obviously, the surviving cancer cells develop an immunogenic phenotype mirroring the immunoreactive mechanisms of the host. At some point thereafter, the balance between immunosurveillance and oncogenesis, developed earlier, begins to shift towards oncogenesis. Hence, the process now enters the third step, where by immunosurveillance is not capable anymore in controlling the proliferation of rapidly transforming cells, resulting in their complete domination and the development and clinical manifestation of the tumor (escape).


The core of this theory embraces the concept that the immune system on the one hand protects the body from cancer and on the other it shapes the immunogenicity of these cancers. Bearing in mind that the issue is the development of cancer in immunocompetent individuals, it is extrapolated that immunogenicity of each cancer reflects the fine differences that quite possibly might exist between immunocompetent hosts. Another equally important concept offered by this theory, is that tumor escape mechanisms do not develop during the clinically evident period of the disease but rather they represent the final outcome of a long lasting interrelation between rapidly transforming cells and the immune system, during the second step of the immunoediting process.


It is apparent that the immunoediting theory introduces a completely different perception, not only for the relation between the immune system and cancer, but also for carcinogenesis in general. This conceptualization can radically influence the current approaches and potential future applications of cancer immunotherapy provided, of course, that unequivocal supporting evidence will continue to accumulate. Pragmatically, however, this is not so easy, since the focus of immunotherapeutic efforts will have to move towards the long lasting preclinical phase, during which selection of immunoresistant tumor clones takes place.


At the same time, the immunoediting theory raises several questions with respect to the interrelation between the immune system and cancer. As discussed earlier, the ability of cancer cells to sequentially develop the necessary oncogenetic immunoresistant phenotype (mutator phenotype), stems from their characteristic heterogeneity and genetic instability. What are, however, the evolutionary "presses" exercised by the immune system ("editor") and determine each time how this phenotype will be developed?




All efforts to-date towards the investigation of the interplay between the immune system and cancer as well as all immunotherapeutic approaches, assume an immunocompetent host. Undoubtedly, primary immunodeficiencies cannot constitute a similar investigational framework, since their incidence is far lesser than that of cancer. Accordingly, post-transplantation or experimentally induced immunosupression, despite their decisive contribution in the relevant research areas, are far from being considered as immunological microenvironments equivalent to those within which carcinogenesis naturally evolves.


On the other hand, however, it is well established that the immune system presents with an enhanced structural and functional diversity between individuals as a result of its tremendous internal complexity. Naturally, these do not necessarily develop into clinically evident immunodeficiencies due to the balancing effect of the defensive mechanisms. At the immunogenetic level, for example, a large number of alleles with convincingly decreased expression, rarely lead to overt immunodeficiency, or at least susceptibility to disease of immune origin (i.e. autoimmune)27. Moreover, some of the immunological aberrations that have been detected in cancer, like the reduction of IFN-γ production28 or decreased expression of the T cell ζ chain29, cannot be easily attributed to escape mechanisms or tumor mediated immune actions30. Thus, the emerging question is whether the combined presence of such differences shapes an immune micro-millieu that promotes or affects, differently amongst individuals, cancer cell immunoediting.


Theoretically, the above hypothesis could be tested utilizing quantitative approaches (i.e. immunogenomic). The immune micro-millieu, however, is extremely dynamic not only at the phenotypic level. The immune gene expression is under the control of numerous nuclear events, such as the epigenetic, amongst many others31-33. These, in turn, undergo themselves various alterations under unidentified as yet external situations. Therefore, it could be extrapolated that the immune microenvironment, within which immunoediting is taking place, is in fact much more inconstant than indicated even by its enormous genomic complexity.


Should the above reflections focus on the possible epigenetic alterations of immune genes, the question posed is "do cancer cells have the ability to induce them?" and, if affirmative, "from what stage of carcinogenesis?" In other words, "is immunoediting a one way path that carcinogenic processes must imperatively follow, or is it about a bidirectional relation that initiates long before tumor escape immune mechanisms can be detected?"


This question becomes particularly important if the following are taken into account. First, DNA-methylation is decisively implicated in the ontogeny of many immune response processes, such as the HLA antigen expression, the maturation and differentiation of T cells as well as TcR rearrangement31-33. Secondly, dramatic alterations of DNA-methylation are detected in cancer that could account for its etiology but their origin is unclear34-37. Thus, why should these alterations be restricted to cancer cells or related genes and not to other cells, including those of the immune system, as a result of a more generalized/systemic damage imposed by common causes?


There are very few reports indicating that, in cancer patients, peripheral lymphocytes appear with aberrant promoter methylations of their cancer related genes, e.g. the p53 in lung cancer38,39. Moreover, it has been observed in experimental animals, that a methylation-dependent progressive reduction of Fas expression exists in tumor antigen specific cytolytic T cells40.


A broader reasoning of the above question would have to consider the relation between cancer, autoimmunity and ageing. Autoreactive responses not only against tumor antigens but also against normal nuclear and cytoplasmic antigens are frequently detected in cancer41. Despite these responses being considered as secondary outcomes, there is absence of conclusive supporting evidence42. On the other hand it is well documented that the incidence of both viral and non viral tumors is increased among patients with autoimmune diseases43,44. In parallel, recent accumulating evidence indicates that epigenetic alterations of certain immune genes (e.g. CD11a/CD18) are implicated in the pathogenesis of autoimmune diseases45,46. Henceforth, by combining these data it is intriguing to postulate that cancer could be accompanied by DNA methylation alterations of immune genes which are unexplored as yet. The definitive justification of this postulation is the fact that cancer and autoimmunity represent the morbidity phenotype of ageing and immunosenescense, conditions that have also been linked with epigenetic alterations of genes implicated either in carcinogenesis (e.g. estrogen receptor gene, IGF2, etc.) or in the immune response47,48.


Beyond any doubt the experimental formulation of this opinion would require an in-depth reorientation of current cancer research approaches. In the event that such contemporary supporting evidence is gathered, many of the torturous aspects about cancer, in general, would be reconsidered. For example, certain forms of cancer (e.g. myelodysplastic syndrome) respond satisfactory to cyclosporine Α49,50. The question, however, remains as to whether this is a result of the immunosuppressive action of the drug, or the inhibition of the hepatic metabolism of the concomitantly given cytotoxics, or whether it is by-passing the expression of multidrug resistance genes51. Moreover, the recent introduction of demethylating agents in the therapeutic weaponry against cancer, will most likely be influenced52. Theoretically, the effectiveness of these agents, is based on hypermethylation induced silencing of tumor suppressive genes, which has been assumed as an oncogenetic mechanism. Bearing in mind the non specific effect of demethylating agents, it is questionable whether they induce enhancement of global hypomethylation, being also a methylation aberration characterizing cancer, and, even more, if their demethylating effect could affect or "be extended against" immune genes53.


Furthermore, since several lines of evidence indicate that epigenetics, in general, are implicated in the molecular pathways of environmental carcinogenesis, such as those induced by smoking or alcohol54,55, potential involvement of immune gene epigenetic alterations lends to our postulate an additional dimension. This dimension, in particular, could prove to be more important under the light of the ability of external factors, considered yet as non carcinogenic, to induce DNA-methylation (epimutagens)47,56.


In conclusion, the above presented approach adds to the cancer immunoediting theory, as well as the current perception of the immune system and cancer interrelation. Based on our hypothesis, the immunoresistant cancer cell phenotype is not shaped by the immune system acting as a steady and rigid evolutionary pressure, but as a rather dynamic situation forcing cancer cells towards a, necessary for their survival, adaptation process.




Lung cancer was one of the first cancers where hypermethylation of suppressor oncogenes was observed. A significant number of studies have confirmed that, as with all malignancies, lung cancer is characterized by extensive methylation disturbances of tumor-related genes, such as APC, CDH13, RARβ, FHIT, RASSFIA, p16, p14 and others57,58. Consequently, in lung cancer, according to the above described hypothesis, such a generalized, yet of unknown origin, epigenetic alteration can be extended to immune cells and genes therein.


Warm interest for the particular type of cancer is originating from its relation with smoking and lung inflammatory diseases, and in particular with chronic obstructive pulmonary diseases (COPD). Despite smoking constituting the main risk factor for both diseases59, they are highly associated, beyond what would be expected from smoking alone60,61. Most studies about the role of smoking on DNA methylation, concern cancer related genes and were performed within the context of defining precancerous alterations. Limited studies suggest, however, that smoking causes disturbances of DNA-methylation even in non-cancerous tissues62.


The subject of epigenetic alterations that accompany COPD is much more complicated. Chronic inflammation is associated with increased DNA-methylation of genes that is probably specific for particular types of inflammation63. Accordingly, these alterations lead to aberrations of histone acetylation which, in turn, involves the activation of inflammatory mediators in lung epithelial cells. Moreover, oxidative stress and the mediators of inflammation characterizing COPD promote histone acetylation in the same cells, possibly through MAP-kinase signaling pathways, which perpetuates the inflammatory process64. On the other hand, a series of molecular events that become activated during chronic inflammation, have been incriminated for lung carcinogenesis65.


From the above, it becomes evident that lung cancer represents the convergence of a series of events of different etiology that, through different paths, promote epigenetic alterations. Consequently, in lung cancer, more perhaps than for other forms of cancer, it is likely that these disturbances can be extended to genes that participate in the immune response. This potential immunoepigenetic alteration can contribute to the creation of an exceptionally dynamic framework that configures the immunogenic cancer phenotype. This argument represents a promising aspect for the immunoediting theory and, if verified, it can open new avenues in the hot area of cancer immunotherapeutics, in general.




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