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Science of Cancer Immunology

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Cancer as an Immunologic Disorder

Since Paul Ehrlich in 1909 first proposed the idea that nascent transformed cells arise continuously in our bodies and that the immune system scans for and eradicates these transformed cells before they are manifested clinically, immune surveillance has been a controversial topic in tumor immunology.(1) In the mid-20th century, experimental evidence that tumors could be repressed by the immune system came from tumor transplantation models. The findings from these models strongly suggested the existence of tumor-associated antigens and formed the basis of immune surveillance, which was postulated by Burnet and Thomas.(2) After that the functional role of antigen-presenting cells in cross-priming for T-cell activation was demonstrated, and the cancer immune surveillance model was developed. However, the idea of cancer immune surveillance resisted widespread acceptance until the 1990s when experimental animal models using knockout mice validated the existence of cancer immune surveillance in both chemically induced and spontaneous tumors. The central roles of immune effector cells, such as B, T, natural killer (NK) and natural killer T (NKT) cells, and of type I and II interferons (IFNs), and perforin (pfp) have since been clarified in cancer immune surveillance. (3,4) As part of the current concept of cancer immunoediting leading from immune surveillance to immune escape, three essential phases have been proposed(3) (1) elimination; (2) equilibrium; and (3) escape. Nascent transformed cells can be eliminated initially by immune effector cells such as NK cells and by the secreted IFN-γ in an innate immune response. Elimination of transformed cells results in immune selection and immune sculpting, which induce tumor variants that decrease immunogenicity and become resistant to immune effector cells in the equilibrium phase. Eventually, during tumor progression, when the increased tumor size can be detected by imaging diagnosis, tumor-derived soluble factors (TDSFs) can induce several mechanisms for escape from immune attack in the tumor microenvironment.(5) In this review, a general overview is provided and the basic principles of immunoediting from immune surveillance to escape, and the central role of immune effector cells in the process of immunoediting are discussed. A better understanding of the mechanisms of immunoediting during tumor progression may provide new insights for improving cancer immunotherapy.



Clinical evidence for immune surveillance


Tumor-infiltrating lymphocytes (TILs)

It is readily accepted that TILs in tumors can attack and eradicate tumor cells in the cancer patient. In fact, the presence of intratumor TILs is important evidence for an immune response between tumor cells and immune effector cells. Several previous studies have shown that the high-grade density of CD8+ T cells in cancer cell nests was correlated with prognosis, and the presence of TILs was able to predict a better survival as an independent prognostic factor in various types of cancers including colon cancer(40,41), oesophageal cancer(42), oral squamous cell carcinoma(43), breast cancer(44), ovarian cancer(45), and malignant melanoma.(46) Of importance, the T lymphocytes recruited around the tumor site (peritumor site) do not always contribute to the antitumor immune response but rather intratumor T lymphocytes are important for eradicating tumor cells.(40) Other studies have shown a similar positive correlation between NK cell infiltration and the survival for gastric cancer(47), colorectal cancer(48) and squamous cell lung cancer.(49) Thus, significant evidence has been presented for a link between the presence of TILs and increased survival in cancer patients.



Organ transplant-related cancer

The theory that cancer may arise under conditions of reduced immune capacity is supported by observations in humans with immune deficiencies such as those that occur following organ transplants. Increased relative risk ratios for various types of cancers have been observed in immuno-suppressed transplant recipients that have no apparent viral origin. Information on 5692 Nordic recipients of renal transplants in 1964–82 was linked to the national cancer registries in 1964–86 and to population registries.(50) Significant overall excess risks of two- to five-fold were seen in both sexes for cancers of the colon, larynx, lung and bladder, and in men for cancers of the prostate and testis. Notable high risks ranging from 10- to 30-fold above expectations, were associated with cancers of the lip, skin (non-melanoma), kidney and endocrine glands, non-Hodgkin’s lymphoma, and in women with cancers of the cervix and vulva–vagina.(50) Kidney transplantation increases the risk of cancer in the short term and in the long term, consistent with the theory that an impaired immune system allows carcinogenic factors to act. The other study on the development of solid-organ tumors after cardiac transplantation reported that 38 solid tumors were identified in 36 (5.9%) of 608 cardiac transplant recipients who survived more than 30 days. The tumors included the following types: skin, lung, breast, bladder, larynx, liver, parotid, testicle, uterus and melanoma.(51) A recent review reported a high frequency of skin cancers and lympho-proliferative diseases in renal transplant recipients.(52) However, a survey of the literature showed that the relative frequency of malignancy after renal transplantation varied widely between different geographical regions. The type of malignancy is different in various countries and dependent on genetic and environmental factors. The hypothesis that the action of immunosuppressive drugs is responsible for the increased incidence of cancers in transplant recipients is supported by the observation that patients also develop cancers if they receive immunosuppressive therapy for conditions other than transplantation, e.g. rheumatoid arthritis or systemic lupus erythematosus.
The other possibility for organ transplant-related cancer is the transmission of a tumor via micrometastasis of an undiagnosed malignancy in the donor after transplantation. According to data from the Organ Procurement and Transplantation Network/UNOS, 21 donor-related malignancies were reported out of 108062 transplant recipients. (53) Except for 15 tumors that existed in the donor at the time of transplantation, six tumors were de novo donor-derived tumors that developed in transplanted hematogenous or lymphoid cells of the donor. Similar donor-derived tumors have been reported in allografts obtained from donors with breast cancer and malignant melanoma.(54) These de novo tumors could be activated by the use of immunosuppressive drugs in the recipient even though the de novo tumors might also be inactivated by immune surveillance in the donor before transplantation.



Non-immunological surveillance

The concept of cancer immune surveillance has been formulated based on the hypothesis that cancer cells are recognized as non-self and are capable of inducing a rejection reaction. The immune system contributes to the surveillance of spontaneously developing tumors as well as of virally induced tumors. Given that the immune system alone is not responsible for protecting primary tumor formation, there is still a need for an intrinsic non-immune surveillance system that regulates the growth of tumor cells. There are two major forms of non-immune surveillance. One is DNA repair as intracellular surveillance, which is observed in the increased incidence of tumors in xeroderma pigmentosum, in which there are several deficiencies of mismatch repair enzymes. The other is intracellular surveillance, which is well documented in apoptotic cell death elicited by DNA damage or the activation of oncogenes. Since the definition of non-immune surveillance is control of tumor cell growth and tumor progression, escape from nonimmune surveillance in the early stages and from subsequent immune surveillance in the late stages is associated with an increased resistance to apoptosis. The p53 pathway is a well-known example of genetic surveillance. Upon DNA damage, wild-type p53 is up-regulated and binds DNA to induce growth arrest, allowing DNA repair.(55) Since the p53 gene is inactivated in about 50% of human cancers that impair the DNA binding capacity of the protein,(56) cell growth can continue despite DNA damage, resulting in tumor development. Inherited mutations in p53 seen in the Li–Fraumeni syndrome are associated with increased susceptibility to malignant diseases.(57) The relative contributions of non-immune surveillance compared with immune surveillance remain to be elucidated, but it is likely that they are complementary and not redundant. In fact, p53-deficient mice are susceptible to the formation of spontaneous tumors,(58) as demonstrated when mice produced by the crossing of p53-deficient mice and pfp-deficient mice showed disseminated lymphomas, indicating a direct involvement of cytotoxic lymphocytes in cancer immune surveillance.28 Mice lacking p53 and IFN-γ receptor, or p53 and Stat1, showed a wider spectrum of tumors than those lacking only p53.(23)

As for other types of non-immune surveillance aside from the two major forms already discussed, the existence of intercellular surveillance has been documented. This is elicited by the interaction of cancer cells and surrounding normal cells in the tumor microenvironment that influences the probability of disseminated tumor cell growth.(59) In addition, recent studies suggest that there is a genetically determined variation in the stringency of chromatin imprinting. More relaxed imprinting may lead to increased cancer risk, and has been termed epigenetic surveillance. The immune evasion by tumors that is mediated by non-mutational epigenetic events involving chromatin and epigenetics collaborates with mutations in determining tumor progression.(60)

Cancer immune surveillance


Historical background

In the early 20th century, Ehrlich first proposed the existence of immune surveillance for eradicating nascent transformed cells before they are clinically detected.(1) In the mid-20th century, 50 years later, Burnet and Thomas postulated that the control of nascent transformed cells may represent an ancient immune system, which played a critical role in surveillance against malignant transformation. (2) This idea was supported by experimental results showing strong immune-mediated rejection of transplanted tumors in mice. Although there is excellent evidence to support the belief that immune surveillance mechanisms prevent the outgrowth of tumor cells induced by horizontally transmitted, ubiquitous, potential oncogenic viruses, there is much less evidence for immune surveillance acting against chemically induced tumors in syngeneic mice.(6) However using genetically identical mice, tumor-specific protection was generated from methylcholanthrene (MCA) and virally induced tumors.(7,8) These results from mouse models strongly suggested the existence of tumor-associated antigens and immune surveillance for protection from transformed cells in the host, as was postulated by Burnet and Thomas.(2,9,10) Despite the fact that several lines of evidence from experimental mouse models showed that the immune system played a critical role in dealing with transformed cells, there was no increased incidence of spontaneous or chemically induced tumors in athymic nude mice, as compared to wild-type animals.(11,12) This suggested that immune surveillance in mice targeted transforming viruses but not tumors.(2) It is now known that athymic nude mice have NK cells and fewer T cells that can contribute to immune surveillance than wild-type mice. Further, they have detectable populations of functional T-cell receptor-αβ (TCR-αβ) -bearing lymphocytes.(13,14) Nevertheless, when MCA was injected into nude and control mice with different doses of MCA, nude mice formed more tumors than controls.(15) Similarly, tumor formation induced by MCA was greater in immunodeficient (SCID) mice than in wild-type BALB/c mice.(16)



Experimental evidence for immune surveillance


NK, NKT and γδ T cells

During the mid-1970s to the 1990s, several experimental studies have attempted to demonstrate the immune surveillance concept. The discovery of NK cells provided a considerable stimulus for the possibility that they functioned as the effectors of immune surveillance(17), even though a precise definition and understanding of these cells had not been confirmed. Later, and in the 2000s however, gene-targeted and lymphocyte subset-depleted mice were used to establish the relative importance of NK and NK1.1+ T (natural killer T, NKT) cells in protecting against tumor initiation and metastasis. In these models, CD3+ NK cells were responsible for tumor rejection and protection from metastasis in models where control of major histocompatibility complex (MHC) class I-deficient tumors was independent of interleukin-12 (IL-12).(18) C57BL/6 mice that were depleted of both NK and NKT cells by the anti-NK1.1 monoclonal antibody, which can eliminate both NK and NKT cells, were two to three times more susceptible to MCA-induced tumor formation than control mice.(19) A similar result was observed in C57BL/6 mice treated with anti-asiaro-GM1, which selectively eliminates NK but not NKT cells, even though antiasiaro- GM1 can also eliminate activated macrophages. A protective role for NKT cells was only observed when tumor rejection required endogenous IL-12 activity. In particular, studies in TCR Ja281 gene-targeted mice confirmed a critical function for NKT cells in protecting against spontaneous tumors initiated by the chemical carcinogen, MCA. Ja281–/– mice, lacking Va14Ja281- expressing invariant NKT cells, formed MCA-induced sarcomas at a higher frequency than wild-type mice.(18) Another study showed that mice treated with the NKT cell-activating ligand a-galactosylceramide throughout MCA-induced tumorigenesis exhibited a reduced incidence of tumors and displayed a longer latency period to tumor formation than control mice.(20) Mice lacking γδ T cells were highly susceptible to multiple regimens of cutaneous carcinogenesis. After exposure to carcinogens, skin cells expressed Rae-1 and H60, MHC-related molecules that structurally resemble human MHC class I chain-related A (MICA). Each of these is a ligand for NKG2D, a receptor expressed by cytolytic T cells and NK cells. In vitro, skin-associated NKG2D+ γδ T cells killed skin carcinoma cells by a mechanism that was sensitive to blocking NKG2D engagement.(21) The localization of γδ T cells within epithelia may contribute to the down-regulation of epithelial malignancies.



Interferon-γ

Endogenously produced interferon-γ (IFN-γ) protected the host against transplanted tumors and the formation of chemically induced and spontaneous tumors. When the mice were treated with neutralizing monoclonal antibody to IFN-γ, the growth of immunogenic sarcomas transplanted into mice grew faster than in the control mice.(22) Overexpression of the truncated dominant negative form of the murine IFN-γ receptor a-subunit (IFNGR1) in Meth A fibrosarcoma completely abrogated tumor sensitivity to IFN-γ, and the tumors showed enhanced tumorigenicity and reduced immunogenicity when they were transplanted into syngeneic BALB/c mice.(22) These results showed that IFN-γ had direct effects on tumor cell immunogenicity and played an important role in promoting tumor cell recognition and elimination. In a study of MCA-induced tumor formation, compared with wild-type mice, mice lacking sensitivity to either IFN-γ (IFNGR-deficient mice) or all IFN family members (Stat1-deficient mice; Stat1 being the transcription factor that is important in mediating IFNGR signaling) developed tumors more rapidly and with greater frequency when challenged with different doses of the chemical carcinogen MCA. In addition, IFN-γ-insensitive mice developed tumors more rapidly than wild-type mice when bred onto a background that was deficient for the p53 tumor-suppressor gene.(23) IFN-γ-insensitive p53–/– mice also developed a broader spectrum of tumors compared with mice lacking p53 alone. The importance of this experiment lay in the discovery that certain types of human tumors become selectively unresponsive to IFN-γ. Thus, IFN-γ forms the basis for an extrinsic tumor-suppressor mechanism in immunocompetent hosts. Using experimental (B6, RM-1 prostate carcinoma) and spontaneous (BALB/c, DA3 mammary carcinoma) models of metastatic cancer, mice deficient in both pfp and IFN-γ were significantly less proficient than pfp-deficient or IFN-γ-deficient mice in preventing metastasis of tumor cells to the lung. Both pfp-deficient and IFN-γ-deficient mice were equally as susceptible as mice depleted of NK cells in both tumor metastasis models, and IFN-γ appeared to play an early role in protection from metastasis.(24) Further analysis demonstrated that IFN-γ, but not pfp, controlled the growth rate of sarcomas arising in these mice, and that host IFN-γ and direct cytotoxicity mediated by cytotoxic lymphocytes expressing pfp independently contributed antitumor effector functions that together controlled the initiation, growth and spread of tumors in mice. In another study, both IFN-γ and pfp were critical for the suppression of lymphomagenesis, but the level of protection afforded by IFN-γ was strain specific. Lymphomas arising in IFN-γ-deficient mice were very non-immunogenic compared with those derived from pfp-deficient mice, suggesting a comparatively weaker immune selection pressure by IFN-γ.(25) A significant incidence of late onset adenocarcinomas observed in both IFN-γ-deficient and pfp-deficient mice indicated that some epithelial tissues were also subject to immune surveillance.



Perforin and Fas/FasL system

Perforin and Fas/Fas ligand (FasL) are the other important factors involved in immune surveillance. In general, cell-mediated cytotoxicity attributed to cytotoxic T lymphocytes (CTLs) and NK cells are derived from either the granule exocytosis pathway or the Fas pathway. The granule exocytosis pathway utilizes pfp to direct the granzymes to appropriate locations in target cells, where they cleave critical substrates that initiate apoptosis. Granzymes A and B induce death via alternate, non-overlapping pathways. The Fas/FasL system is responsible for activation-induced cell death but also plays an important role in lymphocyte-mediated killing under certain circumstances. (26) The interplay between these two cytotoxic systems provides opportunities for therapeutic interventions to control malignant disease, but over suppression of these pathways also leads to decreased tumor cell killing. In fact, C57BL/6 mice lacking pfp (i.e. pfp–/–) were more susceptible for MCA-induced tumor formation. In MCA-induced tumor formation, pfp–/– mice developed significantly more tumors compared with pfp-sufficient mice treated in the same manner.(24,25) In addition, a previous study showed that pfp-dependent cytotoxicity is not only a crucial mechanism of both CTL-dependent and NK-dependent resistance to injected tumor cell lines, but also operates during viral and chemical carcinogenesis that were induced by MCA, or 12-O-tetradecanoylphorbol- 13-acetate (TPA) plus 7,12-dimethylbenzanthracene (DMBA), or by injection of oncogenic Moloney sarcoma virus in vivo.(27) Experiments addressing the role of Fas dependent cytotoxicity by studying resistance to tumor cell lines that were stably transfected with Fas failed to detect a major role for Fas in tumor control, but cannot exclude a minor contribution of Fas in tumor surveillance. (27) Another study showed that pfp–/– mice have a high incidence of malignancy in distinct lymphoid cell lineages (T, B, NKT), indicating a specific requirement for pfp in protection against lymphomagenesis.28 The susceptibility to lymphoma was enhanced by the simultaneous lack of expression of the p53 gene. Mice that were pfp–/– were at least 1000-fold more susceptible to these lymphomas when transplanted, compared with immunocompetent mice, in which tumor rejection was controlled by CD8+ T lymphocytes.(28) Taken together, these results indicate that components of the immune system were involved in controlling primary tumor development, and showed the differential role of pfp and IFN-γ in protecting tumor formation between lymphoid and epithelial malignancies.



Lymphocytes



Although evidence had accumulated that the immune surveillance of cancer was dependent on both IFN-γ and lymphocytes, the critical demonstration for the involvement of lymphocytes came from the use of gene-targeted mice lacking the recombination activating gene 1 (Rag1) or Rag2. Homozygous mutants of Rag-2 are viable but fail to produce mature B or T lymphocytes.(29) Loss of the Rag2 function in vivo results in a total inability to initiate VDJ rearrangement, leading to a novel SCID phenotype. Rag2 function and VDJ recombinase activity, per se, are not required for the development of cells other than lymphocytes. Since nude mice do not completely lack functional T cells and the two components of the immune system, IFN-γ and pfp, to prevent tumor formation in mice, an elegant study using a Rag2–/– and Stat1–/– mouse model showed for the first time that lymphocytes and IFN-γ collaborate to prevent the formation of carcinogen-induced sarcomas and spontaneous epithelial carcinomas.(30) Both the wild-type and Rag2–/– mice had the same genetic background and were injected with MCA and monitored for tumor formation. Rag2–/– mice formed tumors earlier than wild-type mice and with greater frequency. After 160 days, nine out of 15 Rag2–/– mice but only two out of 15 wild-type mice had formed MCA-induced tumors. The increased tumor formation in Rag2–/– mice was comparable to findings in IFN-γ-insensitive mice that lacked either the Ifngr1 (12 out of 20) or Stat1 (17 out of 30) genes versus wild-type mice (11 out of 57). In the collaboration between the lymphocyte- and IFN-γ/Stat1-dependent tumor suppressor mechanisms, mice lacking both genes, i.e. Rag2–/– · Stat1–/– mice (RkSk mice) showed increased susceptibility to MCA-induced tumors with 13 out of 18 mice being susceptible compared to 11 out of 57 wildtype mice. However, they did not show a significantly increased incidence compared to mice that lacked either Rag2 or Stat1. Thus, these findings indicated that T, NKT and/or B cells are essential to suppress the formation of chemically induced tumors, and also indicated the presence of an extensive overlap between lymphocytes and Stat1-dependent IFN-γ-signaling. As for the effect of tumor suppressor mechanisms on spontaneous tumors, nine out of 11 wild-type mice were free of malignant disease, two had adenomas, but none had cancer. In contrast, all 12 Rag2–/– mice showed malignant lesions in the intestinal tract and elsewhere. Half of these mice developed malignant diseases: three cecal adenocarcinoma; one ileocecal adenocarcinoma; one small intestinal adenocarcinoma; one lung adenocarcinoma. In addition, six out of 11 RkSk mice developed mammary carcinomas including two adenocarcinomas and a distinct adenocarcinoma in the breast and cecum in one mouse. The other five RkSk mice did not show palpable masses but the following were found at necropsy: two cecal adenocarcinomas; one cecal and lung adenocarcinoma; two intestinal adenomas. Overall, 82% of the RkSk mice formed spontaneous cancers. Thus, these findings indicate that the lack of lymphocytes, either alone or in combination with the IFN-γ-signaling defect, causes significantly more susceptibility to spontaneous epithelial tumor formation than is found in their wild-type counterparts. Moreover, RkSk mice form more spontaneous cancers than Rag2–/– mice, suggesting that the overlap of the tumor suppressor mechanisms mediated by lymphocytes and IFN-γ/ Stat1-signaling may only be partially effective.(30) In another report, the relative contributions of αβ and γδ T cells in blocking tumor formation by chemical carcinogens such as MCA, DMBA and TPA, and the injection of the squamous cell carcinoma cell line PDV in TCR-β–/–, TCR-δ–/– and TCR-β–/– TCR-δ–/– mice, which lack αβ , γδ, and all T cells, were studied.(21) Comparing tumor formation using PDV cells between wild-type and TCRd–/– mice, 41 out of 110 sites developed tumors in TCRδ–/– mice, whereas 13 out of 134 sites developed tumors in wild-type mice. Although tumor latency accounted for a minor reduction, γδ T cells reduced the number of tumors formed. In contrast, in TCRβ–/– and TCRβ–/– TCRδ–/– mice, nearly 100% of sites showed tumor formation and the latency was substantially reduced. These findings indicate that αβ T cells and γδ T cells regulate the tumor growth of PDV cells in a distinct fashion, and that the lack of γδ T cells is not compensated by the presence of αβ T cells and NK cells. In addition, as to the role of γδ T cells in the development of MCA-induced sarcomas and spindle cell carcinomas, an increase in the number of tumors formed in TCRδ–/– and TCRβ–/– mice after MCA injection was observed compared to FVB mice(21). In naturally occurring human carcinomas induced by DMBA and TPA, 67% of TCRδ–/– mice showed tumor formation with increased tumor burden compared to 16% of wild-type mice. In contrast, TCRβ–/– and wild-type mice were equally susceptible to DMBA-induced and TPA-induced carcinogenesis. In addition, TCRδ–/– mice also showed a higher incidence of progression of papillomas into carcinomas. These findings indicate a distinct additional contribution to the regulation of tumor growth in γδ T cells and αβ T cells. In turn, it seems that gd T cells act to inhibit initial tumor formation that converts to malignant progression, whereas αβ T cells directly inhibit tumor progression by using their cytotoxic mechanisms to kill tumor cells. Thus, both the previous and the recent data support the following basic concept of cancer immune surveillance originally proposed by Burnet and Thomas: that the naturally existing immune system can recognize nascent transformed cells and can eliminate primary tumor formation by lymphocytes and secreted cytokines, both of which are important protective mechanisms in the host. The studies that used inbred mouse lines targeting disruptions in genes encoding critical components of the immune system support the control of tumor formation by the immune systems of both innate and adaptive immune compartments in cancer immune surveillance.



Type I interferons

Much less is known about the involvement in the cancer immunoediting process of the type I interferons (IFN-αβ), which regulate immunological functions and induce the same biological effects as IFN-γ. Some of the previous studies suggested a potential antitumor function for endogenously produced IFN-αβ. This was demonstrated by showing that neutralization of IFN-αβ using polyclonal antibodies in mice enhanced the growth of transplanted, syngeneic tumor cells in immunocompetent mice(31,32) and the rejection was abrogated in the allografts or tumor xenografts.(33) In a recent study on the potential function of endogenously produced IFN-a/b in cancer immunoediting for tumor transplantation and primary tumor formation, endogenously produced IFN-αβ rejected highly immunogenic and syngeneic mouse sarcomas. (4) Furthermore, although tumor cell immunogenicity was not influenced by the sensitivity to IFN-αβ, the requirement for IFN-αβ sensitivity in the antitumor immune response for a host-protective effect depends on the level of hematopoietic cells. The host-protective effect of IFN-αβ was not completely overlapped by that of IFN-γ, indicating that IFN-αβ clearly played an important role and was a critical component in the process of cancer immunoediting. In this report, endogenously produced IFN-αβ rejected the tumor formation of highly immunogenic MCA-induced sarcomas and also inhibited the outgrowth of primary carcinogen-induced tumors in immunocompetent mice. Furthermore, MCA-induced sarcomas derived from IFN-α receptor 1-deficient (Ifnαr1–/–) mice were rejected in a lymphocyte- dependent manner in wild-type mice. This suggested that tumors formed in the absence of IFN-αβ responsiveness are more immunogenic than those formed in immunocompetent mice, which differs from the poor immunogenicity in tumors derived from Ifnγr1–/– mice. Unlike the case of IFN-αβ, this poor immunogenicity can be rendered highly immunogenic and can be rejected when IFN-γ sensitivity is recovered by enforced expression of Ifngr1.(23,34) Thus, the finding that the functions of IFN-αβ and IFN-γ for cancer immunoediting do not completely overlap is supported by the differential effects of these cytokines on tumor cell immunogenicity. Type I interferons are considered to be an important link between innate and adaptive immunity(35) and this function acts primarily on several different bone marrow derived cell subsets for tumor elimination. IFN-αβ has been shown to activate dendritic cells (DCs)(36) and to increase the cytotoxic activity of NK cells through the induction of tumor necrosis factor-related apoptosis inducing ligand (TRAIL).(37) In addition, IFN-α-expressing tumor cells can promote antitumor immunity by preventing apoptotic cell death after stimulation of T lymphocytes.(38) Also, type I IFNs promote the development of memory-phenotype CD8+ T (but not CD4+) cells through the induction of IL-15.(39) These findings indicate that the editing function of the immune system during tumor progression is served not only by lymphocytes and IFN-γ but also by IFN-αβ. Nevertheless, the involvement of the endogenously produced IFN-αβ in a host protective function for naturally occurring antitumor immune responses to spontaneous tumor formation remains to be elucidated. Previous experimental and clinical studies in which exogenous IFN-αβ was administered showed that it may serve as an important immunostimulator to enhance antitumor immune responses that contribute to tumor reduction. Whether type I interferon is actively and continuously induced during tumor progression by specific cells such as tumors or non-tumorous components in host protection is still not known. Further molecular and cellular analyses to identify type I IFNs and for determining their responsiveness in cancer immunoediting will be needed.



Cancer immunoediting


Elimination

Elimination is the hallmark of the original concept in cancer immune surveillance for the successful eradication of developing tumor cells, working in concert with the intrinsic tumor suppressor mechanisms of the non immunogenic surveillance process. The process of elimination includes innate and adaptive immune responses to tumor cells. For the innate immune response, several effector cells such as NK, NKT, and γδ T cells are activated by the inflammatory cytokines, which are released by the growing tumor cells, macrophages and stromal cells surrounding the tumor cells. The secreted cytokines recruit more immune cells, which produce other pro-inflammatory cytokines such as IL-12 and IFN-γ. Perforin-, FasL- and TRAIL-mediated killing of tumor cells by NK cells releases tumor antigens (TAs), which lead to adaptive immune responses.(28,61,62) In the crosstalk between NK cells and DCs(63), NK cells promote the maturation of DCs and their migration to tumor draining lymph nodes (TDLNs), resulting in the enhancement of antigen presentation to naive T cells for clonal expansion of CTLs. The TA-specific T lymphocytes are recruited to the primary tumor site, and directly attack and kill tumor cells with the production of cytotoxic IFN-γ.

The following four phases have been proposed for the elimination process.(3) (1) Recognition of tumor cells by innate immune cells and their limited killing: when a solid tumor has grown to more than 2–3 mm, it requires a blood supply and stromal remodelling for tumor progression, which in turn induces pro-inflammatory signals leading to the recruitment of innate immune cells such as NK, NKT, γδ T cells, macrophages and DCs into the tumor site.(64,65) The transformed cells can be recognized by infiltrating lymphocytes such as NK, NKT and γδ T cells, which produce IFN-γ.(66,67) (2) Maturation and migration of DCs and cross-priming for T cells: IFN-γ exerts a limited cytotoxicity via antiproliferative (68) and anti-angiogenic effects(69) and induces apoptosis.(70) Some of the chemokines derived from tumors and surrounding non-tumorous tissues block the formation of new blood vessels even while continuing to induce tumor cell death.(3,71) Necrotic tumor cells are ingested by immature DCs (iDCs), which have matured under pro-inflammatory conditions, and have migrated to TDLNs. (3) Generation of TA-specific T cells: the recruited tumor-infiltrating NK and macrophages produce IL-12 and IFN-γ, which kill more tumor cells by activating cytotoxic mechanisms such as perforin, TRAIL and reactive oxygen.(72,73) In the TDLNs, the migrated DCs present TAs to naive CD4+ T cells that differentiate to CD4+ T cells, which develop TA-specific CD8+ T cells that lead to clonal expansion. (4) Homing of TA-specific T cells to tumor site and elimination of tumor cells. Tumor antigen-specific CD4+ and CD8+ T cells home to the primary tumor site, where the CTLs eliminate the remaining TA-expressing tumor cells; this is enhanced by the secreted IFN-γ, but also selects for tumor cells with reduced immunogenicity.(30) Regarding the recognition of tumor cells, how the unmanipulated immune system can be activated in a developing tumor has been controversial, even though tumor-specific antigens may be expressed as distinct recognition molecules on the surface of tumor cells. As a hypothesis of danger theory, it was considered that cellular transformation did not provide sufficient proinflammatory signals to activate the immune system in response to a developing tumor. In the absence of such signals, there is often no immune response and tolerance may develop.(65) However, recent studies indicate that danger signals such as uric acid (74), the potential toll-like receptor ligands such as heat-shock proteins(75) or a ligand transfer molecule in the signaling cascade induced by CpG DNA(76), and extracellular matrix (ECM) derivatives, (77) may induce pro-inflammatory responses that activate innate immune responses to foreign pathogens. Danger signals are thought to act by stimulating the maturation of DCs so that they can present foreign antigens and stimulate T lymphocytes. Dying mammalian cells have also been found to release danger signals of unknown identity. Of note, although local limited inflammation may be involved in initiating immune responses, excessive inflammation may promote tumor progression in steady-state conditions.(78) This may be in part because of the anti-inflammatory reactions in antigen-presenting cells, which release anti-inflammatory cytokines such as IL-10 and transforming growth factor-β (TGF-β) that inhibits the activation of effector cells.(79)

Equilibrium

The next step in cancer immunoediting proceeds to the equilibrium phase in which a continuous sculpting of tumor cells produces cells resistant to immune effector cells. This process leads to the immune selection of tumor cells with reduced immunogenicity. These cells are more capable of surviving in an immunocompetent host, which explains the apparent paradox of tumor formation in immunologically intact individuals. Although random gene mutations may occur within tumors that produce more unstable tumors, these tumor cell variants are less immunogenic, and the immune selection pressure also favors the growth of tumor cell clones with a non-immmunogenic phenotype. Several experimental studies using mice with different deficiencies of effector molecules indicated various degrees of immune selection pressure. The lymphomas that formed in pfp deficient mice were more immunogenic than those in IFN-γ-deficient mice, suggesting that pfp may be more strongly involved in the immune selection of lymphoma cells than IFN-γ.(25) In contrast, MCA-induced sarcomas in IFN-γ-receptor-deficient mice are highly immunogenic.(30) Furthermore, chemically induced sarcomas in both nude and severe combined immunodeficiency (SCID) mice were more immunogenic than similar tumors from immunocompetent mice.(16,18,30,80) These findings suggest that the original tumor cells induced in normal mice and selected by a T-cell-mediated selection process have been adapted to grow in a host with a functional T-cell system, which has eliminated highly immunogenic tumor cells, leaving non-immunogenic tumor cells to grow. There is however, no connection between this loss of immunogenicity and the loss of MHC class I expression. Furthermore, two important issues can be suggested. One is that perforin-mediated cytotoxicity in T cells contributes more to the elimination of lymphoma cells than epithelial tumor cells, whereas IFN-γ-mediated cytotoxicity is directed more to the elimination of mesenchymal tumor cells such as sarcomas. The other is the higher immunogenicity of the tumors derived from immunodeficient mice than those from immuno-competent mice indicated less immune selection pressure in the tumors derived from immunodeficient mice than in those of immunocompetent mice. Thus, T-cell-mediated elimination has adapted to highly immunogenic tumors, such as chemically and virally induced tumors. On the other hand, the immune selection pressure induces less immunogenic tumor variants that survive and grow in the tumor microenvironment. In cases of spontaneous tumors appearing for a long period of time, the immunogenic sculpting also produces fewer immunogenic tumors than chemically and virally induced tumors. Since the equilibrium phase involves the continuous elimination of tumor cells and the production of resistant tumor variants by immune selection pressure, it is likely that equilibrium is the longest of the three processes in cancer immunoediting and may occur over a period of many years.(81) In this process, lymphocytes and IFN-γ play a critical role in exerting immune selection pressure on tumor cells. During this period of Darwinian selection, many tumor variants from the original are killed but new variants emerge carrying different mutations that increase resistance to immune attack. Since the equilibrium model persists for a long time in the interaction between cancer cells and the host, the transmission of cancer during organ transplantation can be considered. One report described the appearance of metastatic melanoma 1–2 years after transplantation in two patients receiving renal transplants from the same donor. The donor had been previously treated for melanoma 16 years earlier and was considered tumor-free.82 Several similar observations have been reported in recipients of allografts from those considered as healthy donors.(83–85) In speculating on the appearance of the transmitted cancer, tumors may have been kept in equilibrium in the donor, and conceivably activated by the continuous administration of immunosuppressive drugs that facilitated the growth of occult cancer.(81)



Escape


Alterations in signal transduction molecules on effector cells

Given the lack of TA recognition, which is mediated by alterations of effector molecules and which is important for the recognition and activation by the immune system, the loss of signal transducer CD3-ξ chain (CD3-ξ) of TILs has been attributed to immune evasion in the co-operation of immunosuppressive cytokines and local impairment of TILs.86 The loss of CD3-ξ is reported to be correlated with increased levels of IL-10 and TGF-b, and down-regulation of IFN-γ. The CD3-ξ chain is located as a large intracytoplasmic homodimer in the TCR that forms part of the TCR–CD3 complex, which functions as a single transducer upon antigen binding. Since the TCR signal transduction through the formation of the CD3 complex is one of three important signals for initiating a successful immune response as well as the expression of tumor antigen and T helper 1 polarization, any alterations in the CD3-ξ chain that are associated with the absence of p56lck tyrosine kinase, but not CD3-ε, produce the changes in the signaling pathway for T-cell activation. The alterations of TCR-ξ in several types of tumors such as pancreatic cancer(87), uveal malignant melanoma(88), renal cell cancer(89), ovarian cancer(90) and oral cancer(43) have been shown to be attributed to immune invasion that links to poor prognosis. Tumor-derived macrophages or tumor-derived factors led to a selective loss of TCR-ξ compared with CD3-ε.(90) Given that the TCR/CD3-signaling led to lymphocyte proliferation, the poor proliferative responses of TILs could be explained by the defect in TCR-ξ expression. TIL underwent marked spontaneous apoptosis in vitro, which was associated with down-regulation of the anti-apoptotic Bcl-xL and Bcl-2 proteins.(91) Furthermore, because TCR-ξ is a substrate of caspase 3 leading to apoptosis(92) tumor cells can trigger caspase-dependent apoptotic cascades in T lymphocytes, which are not effectively protected by Bcl-2.(93) In oral squamous cell carcinoma, a high proportion of T cells in the tumor undergo apoptosis, which correlates with FasL expression on tumor cells. FasL-positive micro-vesicles induced caspase-3 cleavage, cytochrome c release, loss of mitochondrial membrane potential, and reduced TCR-ξ chain expression in target lymphocytes.(94)



Tumor-derived soluble factors

Tumors evolve mechanisms to escape immune control by a process called immune editing, which provides a selective pressure in the tumor microenvironment that can lead to malignant progression. A variety of tumor derived soluble factors contribute to the emergence of complex local and regional immunosuppressive networks, including vascular endothelial growth factor (VEGF)(95), IL-10(96) TGF-β97) prostaglandin-E2(98), soluble phosphatidylserine(79), soluble Fas(99), soluble FasL(100) and soluble MICA.(101) Although deposited at the primary tumor site, these secreted factors can extend immunosuppressive effects into local lymph nodes and the spleen, thereby promoting invasion and metastasis.(5) VEGF plays a key role in recruiting immature myeloid cells from the bone marrow to enrich the microenvironment as tumor-associated immature DCs (iDCs) and macrophages (TiDCs and TAMs).(102) Accumulation of TiDCs may cause roving DCs and T cells to become suppressed through activation of indoleamine 2,3-dioxygenase (103) and arginase-I(104) by tumor-derived growth factors. VEGF prevents DC differentiation and maturation by suppressing the nuclear factor-jB in hematopoietic stem cells.(105) Blocking nuclear factor-jB activation in hematopoietic cells by tumor-derived factors is considered to be a mechanism by which tumor cells can directly down-regulate the ability of the immune system to generate an antitumor response.(105) In addition, because VEGF can be activated by signal transducers and activators of transcription 3 (Stat3)(106) and because DC differentiation requires decreasing activity of Stat3, neutralizing antibody specific for VEGF or dominant-negative Stat3 and its inhibitors can prevent Stat3 activation and promote DC differentiation and function.(107,108) The increased serum levels of VEGF in cancers have been reported to be correlated with poor prognosis, which involves not only its angiogenic properties but also its ability to induce immune evasion leading to tumor progression.(109,110)

Soluble FasL and soluble MICA products also play important roles in immune evasion, by inhibiting Fas mediated and NKG2D-mediated killing of immune cells.(111,112) Soluble phosphatidylserine, another TDSF, acts as an inducer of an anti-inflammatory response to TAMs, resulting in the release of anti-inflammatory mediators such as IL-10 and TGF-β—that inhibit immune responses of DCs and T cells.(79) The altered tumor surface antigen, such as FasL, also causes immune evasion by counterattacking immune cells, resulting in cell death.(113) In addition, the soluble forms of FasL and MICA are able to inhibit Fas and the NKG2D-mediated death of immune cells.(114,115) Thus it is likely that TDSFs play pivotal roles in constituting immunosuppressive networks that aid tumor progression and metastasis. Indeed, the immunosuppressive networks derived from these TDSFs can be a critical factor in causing unsatisfactory clinical responses that are usually seen in immunotherapy of advanced cancer, and they remain an important obstacle to be overcome in the interaction between tumors and the immune system in the tumor microenvironment. (5,116–118)



Immunological ignorance and tolerance in tumors

A tumor-specific immune response is regulated by tumor antigen levels and maturation stages of antigen presenting cells such as DCs. Many solid tumors, such as sarcomas and carcinomas, express tumor-specific antigens that can serve as targets for immune effector T cells. Nevertheless, the overall immune surveillance against such tumors seems relatively inefficient. Tumor cells are capable of inducing a protective cytotoxic T-cell response if transferred as a single-cell suspension. However, if they are transplanted as small tumor pieces, tumors readily grow because the tumor antigen level can be modulated in the tumor microenvironment.(119) Thus tumor cells are surrounded by non-tumor cells, including bone marrow-derived cells such as iDCs and non-bone-marrow-derived cells such as fibroblasts, endothelium and ECM. The ECM binds tumor antigen(120) and fibroblasts and endothelial cells compete with DCs for the antigen(121), whereby many tumor antigens are down-regulated, thereby allowing tumor progression.(122) Furthermore, these stromal cells increase interstitial fluid pressure in the tumor, resulting in escape from immune attack by effector cells.(123) In these situations, insufficient levels of tumor antigen are largely ignored by T cells, even though T-cell function is suppressed by iDCs in the tumor microenvironment. In addition, iDCs stimulate CD4+ CD25+ regulatory T cells, which inhibit T-cell activation.(124,125) It is known that sufficient levels of tumor antigen can produce an immune response, which is mediated by mature DCs presenting tumor antigens to T cells by cross-priming. However, even in the presence of sufficient levels of tumor antigen, iDCs inhibit the maturation of DCs and T-cell activation, resulting in immunological tolerance.(126) Thus, it is likely that tumor immune evasion is mediated not only by immunological ignorance as a result of decreased levels of tumor antigen but also by immunological tolerance because of inhibition of T-cell activation by iDCs. Many important events and the central roles of effector cells in the process of immunoediting from immune surveillance to escape are summarized in the following diagram.



Conclusion

Owing to the abundant experimental and clinical evidence there is no longer any doubt for the existence of cancer immunoediting from immune surveillance to escape. Cancer cells are gradually able to gain several mechanisms of immune evasion during tumor progression, even though they are being pursued by the initial and continuing phases of immune surveillance. Rather, immunological sculpting contributes to immune selection pressure, which produces tumor cell variants that are resistant to immune effector cells because of their low immunogenicity. In advanced cancers, the marked shifting to immunosuppressive conditions as the result of the constitution of the immunosuppressive network in tumors makes it more difficult to provoke an immune activation to eliminate cancer cells. Given that adoptive immunotherapy using peptide vaccine and DC transfer is not sufficient to reduce tumor volume and their elimination by direct priming for T cells in such conditions, indirect cross-priming for T cells, which can be induced by massive cell death in combination with anticancer drugs, will be required. Indeed, not only modulation of anticancer drug-induced cell death, but also activation of antitumor immune responses by using molecular targeting drugs such as antibodies and small molecules may provide remarkable enhancement of chemotherapeutic effects in cancer therapy. Further studies on cellular and molecular mechanisms to contribute to antitumor immune responses will be needed.

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