TH17 Cells in Cancer Related Inflammation

Until 2005, T helper (CD4+) cells were proposed to be a binary system, consisting of TH1 and TH2 cells (Mosmann TR et al.,1986) , when a third T helper -cell subset, known as TH17 (interleukin-17 (IL-17) expressing cells), was identified (Harrington LE et al., 2005, Park H et al., 2005). This was followed up by the another independent discovery in three different laboratories of the differentiation factors cytokines such as interleukin (IL)-6 and transforming growth factor beta (TGF-┚), that simplified in vitro analysis of this T cell subset to a large extent (Veldhoen M et al., 2006, Bettelli E et al., 2006, Mangan et al., 2006). The discovery of these unique TH17 cells has opened up exciting new avenues for research into the etiology and therapeutics of a broad spectrum of human diseases and data on the biology of these cells have emerged at an astounding pace in just 5 years. The reason for these cells to receive considerable attention in these recent years is their emerging involvement as principal mediators of pathogenesis in several autoimmune and chronic inflammatory disorders. Many reviews of the field have already highlighted the important role of TH17 cells in the diverse group of human autoimmune and inflammatory diseases (Tesmer et al., 2008, Sallusto and Lanzavecchia 2009, Torrado and Cooper 2010, Kimura and Kishimoto 2011, Cosmi et al., 2011).


Introduction
Until 2005, T helper (CD4+) cells were proposed to be a binary system, consisting of T H 1 and T H 2 cells (Mosmann TR et al.,1986) , when a third T helper -cell subset, known as T H 17 (interleukin-17 (IL-17) expressing cells), was identified (Harrington LE et al., 2005, Park H et al., 2005. This was followed up by the another independent discovery in three different laboratories of the differentiation factors cytokines such as interleukin (IL)-6 and transforming growth factor beta (TGF-), that simplified in vitro analysis of this T cell subset to a large extent (Veldhoen M et al., 2006, Bettelli E et al., 2006, Mangan et al., 2006. The discovery of these unique T H 17 cells has opened up exciting new avenues for research into the etiology and therapeutics of a broad spectrum of human diseases and data on the biology of these cells have emerged at an astounding pace in just 5 years. The reason for these cells to receive considerable attention in these recent years is their emerging involvement as principal mediators of pathogenesis in several autoimmune and chronic inflammatory disorders. Many reviews of the field have already highlighted the important role of T H  With regards to cancer, the involvement of T H 17 cells in tumour immunology has raised their status as a target for cancer therapy. However based on the reported evidence on the potential anti-tumourigenic and pro-tumourigenic activities of T H 17 cells, their role as friends or foes, respectively is still under debate; could be because of a few studies have focused on primary T H 17 cells in the human tumour microenvironment (Wilke et al., 2011). The link between cancer development and inflammation is now widely accepted and cancer patients have local and systemic changes in inflammatory parameters (Chechlinska, et al., 2010). Tumours frequently display the characteristics of chronically inflamed tissue, including immune cell infiltration and an activated stroma , Mantovani et al., 2008. Indeed inflammation has been proposed as the seventh trait of cancer by supplementing Hanahan and Weinberg's model that identifies six hallmarks of cancer (Mantovani 2009). This chapter focuses on the role of T H 17 cells in cancer by understanding its links with chronic inflammation.

Association of cancer with inflammation
Inflammation is the first line of defence against various extracellular stimuli (microbes, trauma, chemicals, heat or any other phenomenon) and can be acute or chronic. Acute or physiological inflammation is when body cells respond to external stimuli for short periods of time. Normal inflammation, for example, inflammation associated with acute infections, injury, wound healing is usually self-limiting; however, dysregulation of any of the involved factors leads to abnormalities. If the stimulus sustains for longer time, it results in a pathological state known as chronic or pathological inflammation as seen in autoimmune and chronic inflammatory diseases such as atherosclerosis, multiple sclerosis, rheumatoid arthritis, allergic inflammation of the lung leading to asthma (Kanwar et al., 2001a, Kanwar 2005, Kanwar et al., 2009, Barreiro et al., 2010. Chronic inflammation is also the case during tumour progression in cancer. The patients with chronic inflammatory conditions have a greatly increased risk of cancer in the affected organs. Also chronic inflammation resulting from viral or bacterial infections can often lead to or hasten the development of malignancy (Coussens and Werb 2002, Kanwar et al., 2011). Table 1 summarizes the chronic inflammatory conditions associated with cancer.

Inflammatory Condition
Associated Cancer(s) When the control of cell proliferation, growth and cell death (apoptosis) is lost, we obtain a clone of cells known as benign tumour. By growing its own blood supply (angiogenesis), the tumour feeds itself, grows indefinitely and spreads (metastasizes) in the body thereby leads to malignant cancer. Tumour cells are known to produce various pro inflammatory cytokines such as IL-1 , IL-6, IL-23 and tumour necrosis factor (TNF)-and chemokines that attract inflammatory leukocytes which include neutrophils, dendritic cells, macrophages, eosinophils, mast cells and lymphocytes (Coussens and Werb 2002, . These cells further produce growth factors, various cytokines, chemokines, cytotoxic mediators like reactive oxygen species, matrix metalloproteinases (MMPs), membraneperforating agents and soluble mediators of cell killing such as TNF-, interleukins and interferons (Wahl et al., 1998, Kuper et al., 2000, Coussens and Werb 2002. The recruitment of dendritic cells capture antigen and stimulate anti-tumour immunity by T lymphocyte activation which kill cancer cells via cell mediated cytotoxicity (Kanwar et al., 1999). According to the immune surveillance theory, tumours arise only if cancer cells are able to escape immune surveillance, yet sometimes a robust immune response might result in a favourable effect that might be due to CD8+ cytotoxic T cells which have the capacity to kill tumour cells (Kanwar et al., 2001b) CD4+ T cell responses are also important as they help recruiting CD8+ cytotoxic T cell and generate an inflammatory response that chains the function of CTLs activity (Kanwar et al., 2003). The growth factors asnd cytokines released by inflammatory cells can also have pro-tumour actions. They can lead to proliferation, survival and migration of the tumour by promoting angiogenesis and lymphanogenesis, remodelling extracellular matrix to facilitate invasion, coating tumour cells to make available receptors for spreading cells via lymphatics and capillaries, and evading host mechanisms (Coussens and Werb 2002, Rigo et al., 2010). In this context tumour-associated macrophages (TAMs) have a significant role. After migration the monocytes, recruited largely by monocyte chemotactic protein (MCP) chemokine become the significant component of inflammatory infiltrates as TAMs in neoplastic tissues, and has a dual role in neoplasms. TAMs may kill neoplastic cells following activation by IL-2, interferon and IL-12 or potentiate neoplastic progression through the production of a number of potent angiogenic and lymphangiogenic growth factors, cytokines and proteases, all of which are mediators for tumour growth (Brigati et al., 2002, Tsung et al., 2002. Further TAMs and tumour cells also produce IL-10, which effectively blunts the anti-tumour response by cytotoxic T cells, and prevent maturation of anti-tumour dendritic cells in situ leading to immunosuppression and immune evasion (Coffelt et al., 2009). Increasing evidences have suggested that many types of cancer are closely associated with inflammation (Table 1). Thus, inflammation is a process used by immune cells to eliminate cancer and by cancer cells to promote tumour progression and metastasis.

CD4+ T cell subsets as essential regulators of immune responses and inflammatory diseases
Immune system consists of innate and adaptive immunity. Adaptive immunity is mediated by T and B cells. T helper cells/CD4+ cells are the key actors in establishing an immune response. Naive CD4 + T cells differentiate into different types of effector cells depending upon the combination of cytokines in milieu, antigen and the antigen presenting cell (APC). There are four types known so far ( Figure 1) and include T H 1, T H 2, T-regulatory (Treg) and T H 17. T H 1 cells, induced by IL-12, express T H 1 specific Transcription factors (T-bet) and produce IFN-as their signature cytokine and evoke cell-mediated immunity and phagocyte-dependent inflammation. Vigorous pro-inflammatory activities of T H 1 cells has been seen to cause tissue damage and elicit unwanted T H 1-dominated responses in the pathogenesis of organ-specific autoimmune/inflammatory disorders, Crohn's disease, sarcoidosis, acute kidney allograft rejection, and some unexplained recurrent abortions (Romagnani, 2000). T H 2 cells are induced by IL-4, express GATA 3 and produce IL-4, IL-5, IL-9, IL-10 and IL-13. These are associated with the humoral immunity and resistance against extracellular forms of pathogens. T-regulatory (Treg) cells, characterized by expression of FoxP3 (forkhead/winged helix transcription factor), produce TGF-(transforming growth factor-1). These distinct regulatory T cell subsets suppress adaptive T cell responses, have antiinflammatory role and are involved in maintaining tolerance to self components (prevent autoimmunity).   TGFβ1   IL2, IL4   IL4, IL0   IL21   TGFβ1   TGFβ1  IL3, IL4, IL5, IL9, IL13  IL2, IFN, TNF   IL6, IL10,

Association of T H 17 cells with chronic inflammation
Earlier, T H 1 phenotype was associated with inflammation and autoimmunity and now the T H 17 subset has also been described as pro-inflammatory to play a role in autoimunity and chronic inflammation. The findings that IFN- and IFN- receptor-deficient mice and mice lacking IL-12p35 and other molecules involved in T H 1 differentiation were not protected from experimental autoimmune encephalomyelitis (EAE), but rather developed more severe disease have challenged the concept that autoimmunity is a T H 1 driven disease process (Gran B et al., 2002 Table 1, the inflammatory conditions are present before a malignant change occurs. To understand the kinetics and targets of inflammation in a discussion of T H 17 cells and cancer, the relationship between T H 1-derived IFN, T H 17 cells and antigen-presenting cells (APCs) in humans was recently studied (Kryczek et al., 2008a). These authors demonstrated in a cutting edge study that IFN could rapidly induce elevated B7-H1 expression on APCs and stimulate their production of IL-1 and IL-23. B7-H1 signaling resulted in abrogation of the T H 1-polarizing capacity of APC, whereas IL-1 and IL-23 directed them toward a memory T H 17-expanding phenotype. These findings thus suggest that in the course of inflammation, that the acute T H 1-mediated response is attenuated by IFN-induced B7-H1 on APCs and is subsequently evolved toward T H 17-mediated chronic inflammation by APC derived IL-1 and IL-23. This study in addition to challenging the dogma that IFN suppresses T H 17 and enhances T H 1 development, also strengthens the notion that T H 17 kinetics depends strongly on the context of the ongoing immune reactions www.intechopen.com TH17 Cells in Cancer Related Inflammation 51 and the constituents of the cytokine milieu, both of which are influenced by disease progression (Figure 3).

T H 17 cells in cancer
Various studies have been carried out in the recent years with rapid progress on different cancer types to investigate the association of cancer and T H 17 cells. It has been seen that, T H 17 cells, might either promote tumour growth or regulate antitumour responses. This may be due to the irregular conflicting data based on the studies in humans versus those in mice and contradictory data from experiments in immunocompetent versus immunodeficient mice (Wilke et al., 2011). There is, however, a strikingly high frequency of tumour-infiltrating T H 17 cells in patients with diverse cancer types. These cells when examined in cancer patients, the findings reveal that human tumour-associated T H 17 cells express minimal levels of human leukocyte antigen (HLA)-DR, CD25 and granzyme B, suggesting that they are not a 'conventional' effector cell population (Wilke et al., 2011). On examining the associated mechanisms and clinical significance of T H 17 cells in 201 ovarian cancer patients, it was found that T H 17 exhibited a polyfunctional effector T-cell phenotype, were positively associated with effector cells, and were negatively associated with tumourinfiltrating Treg cells (Kryczek et al., 2009a). The study authors further reveal that for homing molecules, tumour-associated T H 17 highly express chemokine receptors CXCR4 and CCR6, c-type lectin receptor CD161 and the CD49 integrin isoforms c, d and e, while CCR2, CCR5 and CCR7 are not present on these cells (Figure 3).  According to few studies, IL-10 released by T reg cells negatively regulates differentiation of T H 17 cells and IL-2, a growth factor for most T cells promote FoxP3 expression in T H 17 cells and inhibit cellular differentiation to T H 17 cells (Wilson et al., 2007). Retinoic acid has been found to enhance TGF-signalling and decrease IL-6 signalling, thus, it might also be affecting the balance between T H 17 and T reg cells. Apart from this, it has also been seen that mouse peripheral mature T reg can be converted to T H 17 cells favoured by inflammation and IL-6 ('plasticity') (Yang et al., 2008a). The role of TGF-in the differentiation of both induced T reg cells as well as T H 17 cells, along with the documented interactions between ROR and FoxP3 that influence the two subsets, suggest a system that balances inflammation with tolerance ( Figure 3).

Evidences for the negative and positive roles of T H 17 in anti-tumour Immunity
Though reports have addressed the presence of T H 17 cells in experimental and human tumours but they lack regarding the clear indication about either a pro-tumoural or anti-tumoural activity of these cells (Bronte 2008).There are various biological functions of T H 17 cells and their effector molecules as mentioned earlier in the chapter that could be on the basis of experimental and clinical data, suggest T H 17 cells might either be positively or negatively co-related with cancer.

Negative role of T H 17 cells in anti-cancer
IL-17 produced by T H 17 cells is an angiogenic factor (Numasaki et al., 2003) which stimulates the migration and cord formation of vascular endothelial cells in vitro and elicits vessel formation in vivo which in turn promotes tumour growth and metastasis through de novo carcinogenesis and neovascularisation via STAT3 signalling. Another cytokine, IL-23 required for T H 17 activity has been identified as a cancer-associated cytokine because it promotes tumour incidence and growth (Langowski et al., 2006). It has been seen that T H 17 cells produce negligible levels of HLA-DR, CD 25, granzyme B, PD1 and FoxP3, all of which are involved in effector functions suggesting that they do not contribute to immune suppression in the tumour environment. Thus, as T H 17 cells produce pro-inflammatory cytokines and have been found to accumulate in tumour microenvironment and as inflammation is linked to cancer development and progression, it is reasonable to predict a positive relation between these cells and cancer progression. Also, the data from experiments on ovarian cancer suggest that T H 17 cells through TNF-are involved in the development or progression of cancer in mice and humans (Charles et al., 2009).

Further T H 17 cells might increase their own frequency in the tumour by both direct and indirect mechanisms (Zou and Restifo 2010). The induction of T H 17 cells in the human
tumour microenvironment through IL-1 production by the myeloid APCs may in turn promote dendritic cell trafficking into tumour-draining lymph nodes and the tumour environment by producing CCL20 (Kryczek et al., 2009a). Further as CCR6+ T H 17 cells are known to efficiently migrate towards CCL20 (Kryczek et al., 2008b, Kryczek et al., 2009a, and CCL20 can then lead to the recruitment of dendritic cells to the tumour-draining lymph nodes and tumour itself in a CCR6-dependent manner (Martin-Orozco et al., 2009). Compared with corresponding non-tumour regions, the levels of T H 17 cells were found to be significantly increased in tumours of HCC patients. Most of these intratumoural T H 17 cells exhibited an effector memory phenotype with increased expression of CCR4 and CCR6. Furthermore, the intratumoural cell density of T H 17 correlated with poor survival in HCC patients (Zhang et al., 2009). A study from Kuang and colleagues in 2010, has demonstrated predominantly enriched levels of IL-17-producing cells in peritumoural stroma of murine HCC tissues, where their levels correlated with monocyte/macrophage density. The level of murine hepatoma-infiltrating CD4+ IL-17+ cells as well as the tumour growth was reduced significantly when monocyte/macrophage inflammation in liver was inhibited via treatment with a Kupffer cell toxicant (gadolinium chloride).
Similar to humans, healthy mice has limited populations of T H 17 cells but these cells expanded in the blood, bone marrow and spleens but not in the tumour draining lymph nodes and largest populations were seen in tumour itself of mice with the aggressive B16 melanoma, fibrosarcoma and advanced head and neck cancers, The number of CD4+IL-17+ T cells gradually increased in the tumour microenvironment during tumour development but interestingly, the number of these cells remained limited during tumour development in the tumour draining lymph nodes, including advanced tumour stages. (Kryczek et al., 2007).On the other hand in nasopharyngeal carcinoma, data from human samples demonstrated no correlation of T H 17 cells with patient clinicopathological characteristics or survival outcomes . Studies with patient samples from lung adenocarcinoma or squamous cell carcinoma revealed that malignant pleural effusion from these patients was chemotactic for T H 17 cells, and this activity was partially abrogated by CCL20 and/or CCL22 blockade . Interestingly, higher infiltration of T H 17 cells in malignant pleural effusion predicted improved patient survival. In prostate cancer patients, a significant inverse correlation was seen between T H 17 cell differentiation and tumour progression (Sfanos et al., 2008). In addition to these evidences, it is known that IL-17 released by T H 17 cells promote dendritic cell maturation which might allow for better tumour antigen presentation and thereby leading to a stronger T cell response. Furthermore, direct mechanistic and functional evidence that T H 17 cells mediate antitumour immunity by promoting dendritic cell trafficking to tumour-draining lymph nodes, and to the tumour itself has also been provided (Martin-Orozco et al., 2009). tumourtumourMore recently, CTLA4 (cytotoxic T lymphocyte antigen 4) blockade was shown to increase T H 17 cells in patients with metastatic melanoma and IL-17 levels in tumour-associated ascites positively predicted patient survival (von Euw et al., 2009). To summarize the above data, there is strong evidence that T H 17 cells can have protective roles in tumour immunity but the exact nature of T H 17 cells in anti-tumour immunity remains to be explored.

Conclusions
Rapid and large advances in understanding the development, regulation and function of these cells have been made since T H 17 cells are originally identified as a third lineage of effector T helper cells in 2005. The study of T H 17 cells has been one of the fast-moving and exciting subject areas in immunology. This has been particularly true in the context of a diverse group of immune-mediated chronic inflammatory diseases and autoimmunity, where the pathogenic role of T H 17 cells has been well documented. With regards to cancer, T H 17 cells are found to be present in the tumour microenvironment though not as a predominant T cell subset within the tumour. Based on the evidence provided by both human and clinical studies data, T H 17 cells and T H 17-associated cytokines/effector molecules have been shown to have both pro-tumorigenic and anti-tumorigenic functions. On one hand it seems that the pro-inflammatory T H 17 cells might engineer the microenvironment around tumours, and contribute to the proliferation, migration and survival of cancer cells. On the other hand, it is possible that inflammatory cells and molecules play roles to initiate and maintain protective anti-tumour immunity as seen in the case of infectious diseases (Punj et al., 2003). The IL-17 dependent pro-tumorigenic or anti-tumorigenic activity might be due to inherent technical limitations for example source and dose of exogenous versus endogenous IL-17, in each of the studies (Zou and Restifo 2010). Further, based on the results from recent murine model studies, employing T H 17-polarized T cells for cancer therapy may appear to be to be a promising approach for translational research. It is also important to study futher the specific nature of inflammatory response and the tissue context, so that the positive or negative effects of T H 17 cells on tumour immunopathology can be determined. Equally important to understand is i) how the effector functions of T H 17cells are regulated?, ii) how do the regulators of T H 17-cell differentiation work? iii), do T H 17 play same role in different types and stages of cancer?, and iv) how T reg cells can be suppressed in chronic inflammatory or large tumour burdens to increase the T H 17 cells and later activation and proliferation of cytotoxic T cells to clear tumour cells? The answers will, help in designing future novel therapeutic vaccine approaches; specifically targeting inflammatory T H 17 cells for cancer therapy. Immunology is the branch of biomedical sciences to study of the immune system physiology both in healthy and diseased states. Some aspects of autoimmunity draws our attention to the fact that it is not always associated with pathology. For instance, autoimmune reactions are highly useful in clearing off the excess, unwanted or aged tissues from the body. Also, generation of autoimmunity occurs after the exposure to the non-self antigen that is structurally similar to the self, aided by the stimulatory molecules like the cytokines.

Abbreviations
Thus, a narrow margin differentiates immunity from auto-immunity as already discussed. Hence, finding answers for how the physiologic immunity turns to pathologic autoimmunity always remains a question of intense interest. However, this margin could be cut down only if the physiology of the immune system is better understood. The individual chapters included in this book will cover all the possible aspects of immunology and pathologies associated with it. The authors have taken strenuous effort in elaborating the concepts that are lucid and will be of reader's interest.