Immune responses to malignancies

Immune responses to tumor-associated antigens (TAs) are often detectable in tumor-bearing hosts, but they fail to eliminate malignant cells or prevent the development of metastases. Patients with cancer generate robust immune responses to infectious agents (bacteria and viruses) perceived as a "danger signal" but only ineffective weak responses to TAs, which are considered as "self." This fundamental difference in responses to self versus nonself is further magnified by the ability of tumors to subvert the host immune system. Tumors induce dysfunction and apoptosis in CD8+ antitumor effector cells and promote expansion of regulatory T cells, myeloid-derived suppressor cells, or both, which downregulate antitumor immunity, allowing tumors to escape from the host immune system. The tumor escape is mediated by several distinct molecular mechanisms. Recent insights into these mechanisms encourage expectations that a more effective control of tumor-induced immune dysfunction will be developed in the near future. Novel strategies for immunotherapy of cancer are aimed at the protection and survival of antitumor effector cells and also of central memory T cells in the tumor microenvironment. Immune responses to tumor-associated antigens (TAs) are often detectable in tumor-bearing hosts, but they fail to eliminate malignant cells or prevent the development of metastases. Patients with cancer generate robust immune responses to infectious agents (bacteria and viruses) perceived as a "danger signal" but only ineffective weak responses to TAs, which are considered as "self." This fundamental difference in responses to self versus nonself is further magnified by the ability of tumors to subvert the host immune system. Tumors induce dysfunction and apoptosis in CD8+ antitumor effector cells and promote expansion of regulatory T cells, myeloid-derived suppressor cells, or both, which downregulate antitumor immunity, allowing tumors to escape from the host immune system. The tumor escape is mediated by several distinct molecular mechanisms. Recent insights into these mechanisms encourage expectations that a more effective control of tumor-induced immune dysfunction will be developed in the near future. Novel strategies for immunotherapy of cancer are aimed at the protection and survival of antitumor effector cells and also of central memory T cells in the tumor microenvironment. Evidence accumulated over the last few years convincingly shows that the host immune system is involved in cancer development and progression, as well as control of metastasis. The presence of antitumor cellular responses, humoral responses, or both to tumor-associated antigens (TAs) has been observed in many, but not all, patients with cancer.1Romero P. Cerottini J.C. Speiser D.E. The human T cell response to melanoma antigens.Adv Immunol. 2006; 92: 187-224Crossref PubMed Scopus (55) Google Scholar, 2Dunn G.P. Koebel C.M. Schreiber R.D. Interferons, immunity and cancer immunoediting.Nat Rev Immunol. 2006; 6: 836-848Crossref PubMed Scopus (1128) Google Scholar The evidence for such pre-existing antitumor immunity in patients with cancer confirms that the tumor-bearing host is capable of mounting an immune response to TAs. Tumor progression from a single transformed cell to a mass of malignant cells is a multistep process involving a series of genetic changes occurring in human subjects over a period of months or years and culminating in the established tumor.3Zhang L. Zhou W. Velculescu V.E. Kern S.E. Hruban R.H. Hamilton S.R. et al.Gene expression profiles in normal and cancer cells.Science. 1997; 276: 1268-1272Crossref PubMed Scopus (1220) Google Scholar During this period, neither the host immune system nor the developing tumor are idle: those newly emerging tumor cells that are recognized by the immune system are eliminated only to be replaced by genetic tumor variants resistant to immune intervention and giving rise to a heterogenous population of malignant cells found in any tumor. Tumors are genetically unstable, and the emergence of new genetic variants, which is responsible for the tumor heterogeneity, ensures that the tumor survives in the face of the host immune system. Only the tumor cells that manage to avoid recognition escape and survive, whereas those that are recognized by the immune system are eliminated as soon as they arise. The tumor development involves a prolonged series of checks and balances between the host attempting to curtail tumor growth and the tumor benefiting from genetic changes, altering its microenvironment and avoiding immune elimination. Thus the tumor becomes resistant to immune effector cells. The interactions between the host and the tumor have been referred to as "immune surveillance," a concept that originated many years ago with F. M. Burnett and that introduced his vision of a vigilant host immune system able to spot, recognize, and eliminate tumor cells. A modern version of the immune surveillance theory not only emphasizes the ability of the host immune system to recognize and destroy tumor cells but also its contribution to "immune selection" of resistant tumor variants. Thus the "immune editing" hypothesis2Dunn G.P. Koebel C.M. Schreiber R.D. Interferons, immunity and cancer immunoediting.Nat Rev Immunol. 2006; 6: 836-848Crossref PubMed Scopus (1128) Google Scholar, 4Dunn G.P. Bruce A.T. Ikeda H. Old L.J. Schreiber R.D. Cancer immunoediting: from immunosurveillance to tumor escape.Nat Immunol. 2002; 3: 991-998Crossref PubMed Scopus (3455) Google Scholar has been advanced to suggest that by means of elimination of tumor cells sensitive to immune intervention, the host immune system edits for survival of tumors that become resistant to immune cells. An alternative hypothesis allows for the progressing tumor to develop immunosuppressive mechanisms that will thwart any attempt of immune tumor elimination and in effect will induce a state of tumor-specific tolerance.5Whiteside T.L. The tumor microenvironment and its role in promoting tumor growth.Oncogene. 2008; 27: 5904-5912Crossref PubMed Scopus (1354) Google Scholar In the first instance the immune system initiates the selection of resistant tumor variants, and in the second the tumor becomes a perpetrator of immune unresponsiveness. Central to the paradigms of immune selection or immune editing and immune suppression is the premise that the tumors acquiring new mutations are able to avoid immune intervention and are capable of both escaping and disabling the host immune system. Neither of the 2 hypotheses has been completely accepted today, and there are those who believe that tumors progress because of the genetic instability and others who favor tumor-specific tolerance of the immune system, which enables the tumor to take advantage of the tissue microenvironment regardless of the immune system and benefit from it. This controversy regarding the significance of the immune system in tumor development and progression underscores the complexity of interactions between the tumor and the immune cells. It surmises that these interactions might be bidirectional, are influenced by the local microenvironment, and not infrequently might result in demise not of the tumor but of tumor-reactive immune cells. In this chapter the nature and components of the host immune response against tumors will be discussed, including the reasons for the failure of the immune system to contain tumor growth and metastasis. It is this latter aspect of the immunobiology of human malignancies that will be emphasized, largely because it directly affects cancer immunotherapy. A relatively recent realization that tumors have devised multiple and remarkably effective mechanisms for disarming the host immune system has opened a way for the introduction of novel therapeutic strategies aimed at eliminating tumor escape. If the tricks tumors use for protection from immune intervention by the host are responsible for their progression, then it could be surmised that a limited success of current immune therapies for cancer can be reversed by therapies that target the escape mechanisms, and because these escape mechanisms might be unique for each tumor rather than generalized, the future challenge will be to identify the "immunologic signature" of each tumor and then use selective therapies to eliminate the tricks and restore vigorous antitumor immunity. There are several lines of evidence that point to an early, as well as late, involvement of the immune system in tumor development. Early tumor lesions, and even premalignant foci, such as melanocytic nevi, are frequently infiltrated with hematopoietic cells, including lymphocytes, macrophages, and occasionally granulocytes.6Kornstein M.J. Brooks J.S. Elder D.E. Immunoperoxidase localization of lymphocyte subsets in the host responses to melanoma and nevi.Cancer Res. 1983; 43: 2749-2753PubMed Google Scholar, 7Von Kleist S. Berling J. Bohle W. Wittekind C. Immunohistochemical analysis of lymphocyte subpopulations infiltrating breast carcinomas and benign lesions.Int J Cancer. 1987; 40: 18-23Crossref PubMed Scopus (35) Google Scholar The presence of immune cells in the tumor at later stages of development (ie, the abundance of tumor-infiltrating lymphocytes [TILs]) has been associated with improved patient survival in several early studies (reviewed in Whiteside8Whiteside T.L. Tumor infiltrating lymphocytes in human malignancies.Austin (TX): R.G. Landes Co. 1993; Google Scholar). More recently, studies by Fridman's group performed a comprehensive multivariate analysis of cellular interactions in the tumor microenvironment based on the type, density, localization, and function of immune cells present within human colorectal cancer and demonstrated that immune reactivity at the tumor site influences clinical outcome.9Galon J. Costes A. Sanchez-Cabo F. Kirilovsky A. Mlecnik B. Lagorce-Pages C. et al.Type, density, and location of immune cells within human colorectal tumors predict clinical outcome.Science. 2006; 313: 1960-1964Crossref PubMed Scopus (4384) Google Scholar, 10Galon J. Fridman W.-H. Pages F. The adaptive immunologic microenvironment in colorectal cancer: a novel perspective.Cancer Res. 2007; 67: 1883-1886Crossref PubMed Scopus (306) Google Scholar, 11Pages F. Berger A. Camus M. Sanchez-Cabo F. Costes A. Molidor R. et al.Effector memory T cells, early metastasis and survival in colorectal cancer.N Engl J Med. 2005; 353: 2654-2666Crossref PubMed Scopus (1586) Google Scholar Thus increased densities of T-cell infiltrates with a high proportion of CD8+ T cells within primary colorectal carcinomas were associated with a significant protection against tumor recurrence.11Pages F. Berger A. Camus M. Sanchez-Cabo F. Costes A. Molidor R. et al.Effector memory T cells, early metastasis and survival in colorectal cancer.N Engl J Med. 2005; 353: 2654-2666Crossref PubMed Scopus (1586) Google Scholar Furthermore, the same group also showed that coexpression of genes mediating cytotoxicity and TH1 adaptive immune responses accurately predicted survival in patients with colorectal carcinoma independently of the metastatic status.12Camus M. Tosolini M. Mlecnik B. Pages F. Kirilovsky A. Berger A. et al.Coordination of intratumoral immune reaction and human colorectal cancer recurrence.Cancer Res. 2009; 69: 2685-2693Crossref PubMed Scopus (197) Google Scholar In aggregate these multiparameter analyses of tumor-infiltrating cells in situ suggest that immune cells can and indeed often do play a role in tumor control but that both intrinsic and extrinsic factors in the tumor microenvironment alter the balance required for optimal control.12Camus M. Tosolini M. Mlecnik B. Pages F. Kirilovsky A. Berger A. et al.Coordination of intratumoral immune reaction and human colorectal cancer recurrence.Cancer Res. 2009; 69: 2685-2693Crossref PubMed Scopus (197) Google Scholar In many patients with cancer, it is possible to expand in culture and in vitro test functions of tumor-specific cytolytic T lymphocytes (CTLs) from the peripheral blood or TILs.8Whiteside T.L. Tumor infiltrating lymphocytes in human malignancies.Austin (TX): R.G. Landes Co. 1993; Google Scholar This finding, which has been reproduced in many laboratories, suggests that precursors of such CTLs exist in the circulation or at the tumor site in patients with cancer and can be induced to proliferate when autologous dendritic cells (DCs) pulsed with relevant tumor epitopes and used as antigen-presenting cells (APCs). More recent experiments, using tetramers and flow cytometry, have directly demonstrated the presence of tumor peptide–specific T cells in the circulation of patients with cancer.1Romero P. Cerottini J.C. Speiser D.E. The human T cell response to melanoma antigens.Adv Immunol. 2006; 92: 187-224Crossref PubMed Scopus (55) Google Scholar, 13Lee P.P. Yee C. Savage P.A. Fong L. Brockstedt D. Weber J.S. et al.Characterization of circulating T cells specific for tumor-associated antigens in melanoma patients.Nat Med. 1999; 5: 677-685Crossref PubMed Scopus (955) Google Scholar, 14Pittet M.J. Speiser D.E. Lienard D. Valmore D. Guillaume P. Dutoit V. et al.Expansion and functional maturation of human tumor antigen-specific CD8 + T cells after vaccination with antigenic peptide.Clin Cancer Res. 2001; 7: 796s-803sPubMed Google Scholar Furthermore, the frequency of such peptide-specific T cells appears to be higher in the circulation of patients with cancer than in healthy subjects.15Hoffmann T.K. Donnenberg A.D. Finkelstein S.D. Donnenberg V.S. Friebe-Hoffmann F. Myers E.N. et al.Frequencies of tetramer+ T cells specific for the wild-type sequence p53264-272 peptide in the circulations of patients with head and neck cancer.Cancer Res. 2002; 62: 3521-3529PubMed Google Scholar Finally, the SEREX technology, based on the presence of tumor-specific antibodies in sera of patients with cancer, has been successfully used for tumor-antigen discovery in many laboratories.16Sahin U. Tureci O. Pfreundschuh M. Serologic identification of human tumor antigens.Curr Opin Immunol. 1997; 9: 709-716Crossref PubMed Scopus (276) Google Scholar These findings, as well as recent identification of numerous TAs that appear to be immunogenic in that they induce humoral immune responses, cellular immune responses, or both in vitro by using human immune cells and in vivo in animal models of tumor growth, strongly support the notion that the host immune system recognizes the presence of the tumor and responds to it by generating both local and systemic immune responses. If the tumors are not ignored by the immune system, why do they progress? Several answers to this question can be considered. First, there is the old argument for the lack of a "danger signal"17Matzinger P. An innate sense of danger.Semin Immunol. 1998; 10: 399-415Crossref PubMed Scopus (617) Google Scholar in tumors akin to those presented by pathogens invading tissues during an infection. Recognition by DCs of pathogen-associated molecular patterns through the ubiquitous Toll-like receptors leads to efficient DC activation and maturation. It promotes generation of vigorous cellular and antibody responses to bacterial or viral antigens, presumably because the immune system perceives an infection as a danger signal17Matzinger P. An innate sense of danger.Semin Immunol. 1998; 10: 399-415Crossref PubMed Scopus (617) Google Scholar benefiting the host. However, functional Toll-like receptors are known to be expressed by many human solid tumors,18Szczepanski M.J. Czystowska M. Szajnik M. Harasymczuk M. Boyiadzis M. Kruk-Zagajewska A. et al.Triggering of toll-like receptor 4 expressed on human head and neck squamous cell carcinoma promotes tumor development and protects the tumor from immune attack.Cancer Res. 2009; 69: 3105-3113Crossref PubMed Scopus (195) Google Scholar and recent data indicate that tumors use them to promote their own growth; for protection from spontaneous, immune-mediated, or drug-induced apoptosis; or both.18Szczepanski M.J. Czystowska M. Szajnik M. Harasymczuk M. Boyiadzis M. Kruk-Zagajewska A. et al.Triggering of toll-like receptor 4 expressed on human head and neck squamous cell carcinoma promotes tumor development and protects the tumor from immune attack.Cancer Res. 2009; 69: 3105-3113Crossref PubMed Scopus (195) Google Scholar, 19Uematsu S. Akira S. Toll-like receptors and innate immunity.J Mol Med. 2006; 84: 712-725Crossref PubMed Scopus (341) Google Scholar Second, TAs are perceived by the immune system as "self" or "altered self" antigens, which evoke weak immune responses because tolerance prevents generation of immune responses to self. The only "unique" TAs are mutated antigens, and these are strongly immunogenic and elicit robust immune responses.20Hollstein M. Sidransky D. Vogelstein B. Harris C. p53 mutations in human cancers.Science. 1991; 253: 49-53Crossref PubMed Scopus (7400) Google Scholar However, only a handful of such mutated TAs are known, and the vast majority of TAs are poorly immunogenic or simply tolerogenic. In this context cancer can be viewed as an autoimmune phenomenon in which tolerance to self prevents effective immune responses to TAs Patients with cancer who have not been treated with chemotherapy or radiotherapy generally have normal immune responses to viral or bacterial antigens, yet they are unable to respond to their own TAs. Except for late-stage disease, they generally have normal delayed-type hypersensitivity responses to recall antigens but are anergic to autologous TAs. Although tolerance to self is a detriment to the generation of antitumor responses in patients with cancer, another factor that exerts an overwhelming effect on these responses is the tumor microenvironment. Each tumor creates its own milieu characterized by the presence of immunosuppressive factors and by the excess of TAs produced and released by the growing tumor. Evidence suggests that tumors produce a broad array of immunoinhibitory factors, which exert either local or systemic effects on the host antitumor immune responses.5Whiteside T.L. The tumor microenvironment and its role in promoting tumor growth.Oncogene. 2008; 27: 5904-5912Crossref PubMed Scopus (1354) Google Scholar Therefore it is not surprising that antitumor immunity might be weak, inefficient, or even absent in patients with cancer, depending on the nature of tumor-host interactions, as well as the robustness of regulatory mechanisms in control of immune tolerance. Immune antitumor responses could be influenced by the gradual deterioration of the immune system with age.21Provinciali M. Immunosenescence and cancer vaccines.Cancer Immunol Immunother. 2009; 58: 1959-1967Crossref PubMed Scopus (22) Google Scholar The increased incidence of cancer present in the elderly might be due to immunosenescence (ie, progressive remodeling of the immune system with a reduced ability of immune cells to respond to activating stimuli and increased responsiveness to tolerogenic signals).21Provinciali M. Immunosenescence and cancer vaccines.Cancer Immunol Immunother. 2009; 58: 1959-1967Crossref PubMed Scopus (22) Google Scholar Immunosenescence can significantly interfere with the effectiveness of cancer immunotherapies, and it has been suggested that clinical trials testing immunopotentiating agents in patients with cancer should be conducted in elderly subjects.21Provinciali M. Immunosenescence and cancer vaccines.Cancer Immunol Immunother. 2009; 58: 1959-1967Crossref PubMed Scopus (22) Google Scholar Recent multiparameter analyses of primary and metastatic human tumors (eg, colorectal carcinoma) recognize several major immune "coordination profiles," the presence of which is influenced by the balance between tumor escape and immune antitumor responses and that are subject to host-tumor cross-talk.12Camus M. Tosolini M. Mlecnik B. Pages F. Kirilovsky A. Berger A. et al.Coordination of intratumoral immune reaction and human colorectal cancer recurrence.Cancer Res. 2009; 69: 2685-2693Crossref PubMed Scopus (197) Google Scholar In this context it is important to consider differences between primary and metastatic tumors. Not only are metastatic tumors more immunosuppressive, but also they appear to be less readily recognized by TA-specific immune effector cells. The latter could be due to defects in the expression levels of antigen-processing machinery (APM) components, MHC molecules, or both in the tumor and its metastases.22Ferrone S. Whiteside T.L. Tumor microenvironment and immune escape.Surg Oncol Clin North Am. 2007; 16: 755-774Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar Because different copy numbers of distinct trimolecular peptide–β2-microglobulin (β2 m)–MHC complexes presented on the tumor surface might lead to differential T-cell recognition, this aspect of tumor–immune cell interactions is critical.22Ferrone S. Whiteside T.L. Tumor microenvironment and immune escape.Surg Oncol Clin North Am. 2007; 16: 755-774Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar, 23Ferris R.L. Whiteside T.L. Ferrone S. Immune escape associated with functional defects in antigen processing machinery in head and neck cancer.Clin Cancer Res. 2006; 12: 3890-3895Crossref PubMed Scopus (160) Google Scholar A recent comparison of primary renal cell carcinoma, renal cell carcinoma metastases, and normal renal tissue with respect to HLA ligand presentation and gene expression demonstrated a greater similarity between primary tumor and metastasis than between the tumor and normal tissue.24Stickel J.S. Weinzieri A.O. Hillen N. Drews O. Schuler M.M. Hennenlotter J. et al.HLA ligand profile of primary renal cell carcinoma maintained in metastases.Cancer Immunol Immunother. 2009; 58: 1407-1417Crossref PubMed Scopus (39) Google Scholar This observation provides a good rationale for peptide-based immunotherapy because it is likely to preferentially target the tumor and its metastases and not the normal tissue. Antitumor immune responses can be innate (natural) or acquired (adaptive). Innate immunity is mediated by cells or soluble factors that naturally exist in tissues or body fluids and can interfere with tumor growth or survival. Among hematopoietic cells, macrophages, granulocytes, natural killer (NK) cells (CD3−CD56+), non–MHC-restricted T cells (CD3+CD56−), and γδ T cells have the natural capability to eliminate tumor cell targets.21Provinciali M. Immunosenescence and cancer vaccines.Cancer Immunol Immunother. 2009; 58: 1959-1967Crossref PubMed Scopus (22) Google Scholar In addition, natural antibodies with specificities directed at surface components of tumor cells might be present in the sera of patients with cancer.16Sahin U. Tureci O. Pfreundschuh M. Serologic identification of human tumor antigens.Curr Opin Immunol. 1997; 9: 709-716Crossref PubMed Scopus (276) Google Scholar Other serum factors, including complement components, C-reactive protein, mannose-binding protein, and serum amyloid protein, also play a role in innate immunity.25Fearon D.T. Locksley R.M. The instructive role of innate immunity in the acquired immune response.Science. 1996; 272: 50-54Crossref PubMed Scopus (1416) Google Scholar Adaptive immune responses to tumors are mediated by CD3+ T-cell receptor (TCR+) T cells when they recognize tumor-derived peptides bound to self-MHC molecules expressed on APCs. Little is currently known about the molecular signals and cellular steps involved in directing APCs, such as DCs, to execute a tolerogenic versus immunogenic program in response to antigens. As indicated above, tumors can also serve as APCs, although low levels of MHC class I molecule expression, MHC class II molecule expression, or both on the surface of tumor cells makes this an inefficient process.22Ferrone S. Whiteside T.L. Tumor microenvironment and immune escape.Surg Oncol Clin North Am. 2007; 16: 755-774Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar More likely, TAs are taken up by DCs present at the tumor site, processed, and cross-presented to T cells in the tumor-draining lymph nodes in the form of the trimolecular peptide–β2m–MHC complexes.23Ferris R.L. Whiteside T.L. Ferrone S. Immune escape associated with functional defects in antigen processing machinery in head and neck cancer.Clin Cancer Res. 2006; 12: 3890-3895Crossref PubMed Scopus (160) Google Scholar For adaptive immune response to occur, T cells expressing correct (cognate) TCRs have to be present. Recognition of the peptide and its binding to the variable domains of the TCR initiates signaling (signal 1) that leads to T-cell activation.26Irving B.A. Weiss A. The cytoplasmic domain of the T cell receptor zeta chain is sufficient to couple to receptor-associated signal transduction pathways.Cell. 1991; 64: 891-901Abstract Full Text PDF PubMed Scopus (617) Google Scholar This requirement implies prior sensitization and a clonal expansion of memory T cells in response to a cognate tumor epitope (anamnestic or recall responses). Alternatively, precursor T cells expressing the TCR can be primed by the cognate peptide–MHC ligands presented on APCs, and the subsequent development of antitumor effector cells is viewed as a primary immune response. In either case costimulatory molecules (signal 2) are necessary for an immune response to proceed,27Greenwald R.J. Freeman G.J. Sharpe A.H. The B7 family revisited.Annu Rev Immunol. 2005; 23: 515-548Crossref PubMed Scopus (1884) Google Scholar and once T-cell proliferation is initiated, appropriate cytokines (signal 3) become essential for sustaining the response.28Kalinski P. Hilkens C.M. Wierenga E.A. Kapsenberg M.L. T-cell priming by type-1 and type-2 polarized dendritic cells: the concept of a third signal.Immunol Today. 1999; 20: 561-567Abstract Full Text Full Text PDF PubMed Scopus (795) Google Scholar Recent findings stress the key importance of signal 3 for the development of immune responses and for their contraction.28Kalinski P. Hilkens C.M. Wierenga E.A. Kapsenberg M.L. T-cell priming by type-1 and type-2 polarized dendritic cells: the concept of a third signal.Immunol Today. 1999; 20: 561-567Abstract Full Text Full Text PDF PubMed Scopus (795) Google Scholar Like all immune responses, those that are TA specific do not go on forever but peak and then contract, restoring the preactivation balance. The precise mechanisms responsible for immune contraction are not yet defined, and regulatory T (Treg) cells, as well as other mechanisms, have been proposed to regulate immune reactivity, but it is clear that events in the environment play a dominant role in this respect. Immune responses to malignant cells can be categorized as locoregional or systemic. In situ or local responses refer mainly to TILs, which accumulate in most human solid tumors and the role of which in tumor progression remains highly controversial. Long considered by some an effector arm of antitumor responses, TILs are viewed by others as victims of the tumor microenvironment because their effector functions are often impaired, presumably by tumor-derived factors.29Whiteside T.L. Immune suppression in cancer: effects on immune cells, mechanisms and future therapeutic interventions.Semin Cancer Biol. 2006; 16: 3-15Crossref PubMed Scopus (354) Google Scholar A failure of local antitumor responses mediated by TILs is thought to contribute to tumor progression. Systemic immunity to tumors, as measured by delayed-type hypersensitivity responses or by various ex vivo assays of T-cell responses in the peripheral circulation of patients with cancer, are difficult to demonstrate, and TA-specific responses have been particularly elusive. Nevertheless, by using highly sensitive multicolor flow cytometry, it has been possible to detect and measure the frequency of TA-specific CD8+ and CD4+ T cells in the peripheral circulation of patients with cancer.1Romero P. Cerottini J.C. Speiser D.E. The human T cell response to melanoma antigens.Adv Immunol. 2006; 92: 187-224Crossref PubMed Scopus (55) Google Scholar Although the response levels vary, TA-specific and nonspecific proliferative or cytotoxic responses of peripheral lymphocytes in patients with cancer appear to be at least partially impaired.29Whiteside T.L. Immune suppression in cancer: effects on immune cells, mechanisms and future therapeutic interventions.Semin Cancer Biol. 2006; 16: 3-15Crossref PubMed Scopus (354) Google Scholar, 30Reichert T.E. Strauss L. Wagner E.M. Gooding W. Whiteside T.L. Signaling abnormalities and reduced proliferation of circulating and tumor-infiltrating lymphocytes in patients with oral carcinoma.Clin Cancer Res. 2002; 8: 3137-3145PubMed Google Scholar, 31Uzzo R.G. Clark P.E. Rayman P. Bloom T. Rybicki L. Novick A.C. et al.Alterations in NFκB activation in T lymphocytes of patients with renal cell carcinoma.J Natl Cancer Inst. 1999; 91: 718-721Crossref PubMed Scopus (70) Google Scholar Data indicate that the same functional impairments seen in TILs are found in both circulating and lymph node lymphocytes of patients with cancer.29Whiteside T.L. Immune suppression in cancer: effects on immune cells, mechanisms and future therapeutic interventions.Semin Cancer Biol. 2006; 16: 3-15Crossref PubMed Scopus (354) Google Scholar, 32Reichert T.E. Rabinowich H. Johnson J.T. Whiteside T.L. Human immune cells in the tumor microenvironment: mechanisms responsible for signaling and functional defects.J Immunother. 1998; 21: 295-306Crossref PubMed Scopus (120) Google Scholar Thus it has been concluded that, in general, human tumors exert profound suppressive effects on both local and systemic antitumor immunity in these patients. In contrast to the failure of antitumor immune responses to control tumor progression in human subjects, a large body of experimental evidence derived from preclinical animal models of cancer suggests t