Journal of Immunotherapy:
Cancer and Innate Immune System Interactions: Translational Potentials for Cancer Immunotherapy
Liu, Yanan; Zeng, Gang
Department of Urology, David Geffen School of Medicine at UCLA, Los Angeles, CA
Reprints: Gang Zeng, Department of Urology, David Geffen School of Medicine at UCLA, Los Angeles, CA 90095-1738 (e-mail: firstname.lastname@example.org).
Received February 2, 2012
Accepted February 22, 2012
Passive immunotherapy, including adoptive T-cell therapy and antibody therapy, has shown encouraging results in cancer treatment lately. However, active immunotherapy of solid cancers remains an elusive goal. It is now known that the human innate immune system recognizes pathogen-associated molecular patterns conserved among microbes or damage-associated molecular patterns released from tissue injuries to initiate adaptive immune responses during infection and tissue inflammation, respectively. In contrast, how the innate immune system recognizes endogenously arising cancer remains poorly understood at the molecular level, which poses a significant roadblock to the development of active cancer immunotherapy. We hereby review the current knowledge of how solid cancers directly and indirectly interact with cells of the human innate immune system, with a focus on the potential effect of such interactions to the resultant adaptive immune responses against cancer. We believe that understanding cancer and innate immune system interactions may allow us to better manipulate the adaptive immune system at the molecular level to develop effective active immunotherapy against cancer. Current and future perspectives in clinical development that exploits these molecular interactions are discussed.
Despite a predominantly immunosuppressive tumor microenvironment,1,2 spontaneous T-cell and antibody responses against tumor-associated antigens (TAAs) can be induced in tumor-bearing hosts.3–5 In a small fraction of patients, antitumor immunity may lead to spontaneous tumor regression or control of tumor expansion, with perhaps the most compelling evidence documented in patients with melanoma3 and paraneoplastic neurological disorders.6
The ultimate goal of active cancer immunotherapy is to achieve the antitumor immunity that has been demonstrated in the sporadic examples of spontaneous tumor regression/containment and recent success of passive immunotherapy such as adoptive T-cell therapy and antibody therapy.7–10 Recent advances in basic science have defined several ligand/receptor interactions and molecular pathways that have significant effect on subsequent adaptive immune responses in various circumstances. For example, it is now known that the human innate immune system, through its cell-surface pattern recognition receptors, recognizes pathogen-associated molecular patterns conserved among microbes or damage-associated molecular patterns (DAMPs) released from tissue injuries to initiate adaptive immune responses during infection and tissue inflammation, respectively.11,12 Despite this wealth of knowledge, how spontaneous antitumor immune responses are initiated is still poorly understood at the molecular level, which poses a major obstacle in developing effective active immunotherapy.
DIRECT CANCER AND INNATE IMMUNE SYSTEM INTERACTIONS
The major effector cells of the immune system that directly target cancer cells include natural killer cells (NK), dendritic cells (DCs), macrophages, polymorphonuclear leukocytes (PMN including neutrophils, eosinophils, and basophils), mast cells, and cytotoxic T lymphocytes. NK cells, DCs, PMN, mast cells, and macrophages are first-line effectors to damaged cells and cancer cells. NK T cells and γδ T cells play roles as both innate and adaptive components, through close interactions with cells of the adaptive immune system, such as CD4+ and CD8+ T lymphocytes with cytotoxic effects and memory.13 The importance of innate immune system in limiting cancer progression has been highlighted recently with the following direct molecular interactions between cancers and innate immune effector cells.
NK cells constitute the primary innate immune cell type responsible for killing nonmajor histocompatibility complex (MHC) expressing cancer cells, releasing small cytotoxic proteins such as perforin and granzyme that cause apoptosis in target cells. There are 2 functional types of receptors on the NK-cell surface: stimulatory receptors and inhibitory receptors. Natural killer group 2D (NKG2D) molecule is perhaps the best known stimulatory receptor.14 The ligands on tumor cells for NKG2D include MHC class-I-chain-related protein A,15 MHC class-I-chain-related protein B,16,17 UL16 binding protein18 in human, and minor histocompatibility molecule H60, retinoic acid early transcript 1 protein, UL16 binding protein-like transcript 1 protein in mice.19–22 Figure 1 shows the interactive diagram of such interactions in humans. The binding of the previously mentioned stress-related ligands with NKG2D stimulate NK cells, leading to secretion of interferon-γ (IFN-γ) and perforin, release of inflammatory cytokines, and the induction of apoptosis in cancer cells. Other NK stimulatory receptors have also been characterized, such as NKp30,23 NKp44,24 and NKp4625 in humans, NK-cell receptor protein 1,26,27 Ly49d/h,28,29 and NKG2C/E-CD94 in mice,14,30 and DNAX accessory molecule31 in both humans and mice. The inhibitory receptors of NK cells consist of the human killer-cell immunoglobulin-like receptors (KIRs),32,33 the mouse Ly49a/c/g2,34–36 and NKG2A-CD94 lectin-like receptors shared by both humans and mice.37 The nonclassical MHC class I molecule, HLA-G, on tumors also functions as a ligand for KIRs that can inhibit cytotoxicity mediated by NK cells. Ly49 family receptors specifically recognize MHC class I or MHC class I-like molecules. The nonclassical MHC class I molecule HLA-E is the ligand for human NKG2A/CD94 heterodimer receptors.38
Tumor necrosis factor (TNF) family ligands are widely expressed on the NK-cell surface: TNF, TNF-related apoptosis-inducing ligand (TRAIL), lymphotoxin, Fas ligand, 4-1BB ligand, lymphotoxin-like inducible protein that competes with glycoprotein D for binding herpesvirus entry mediator (HVEM) on T cells (LIGHT), OX40 ligand, CD40 ligand, CD30 ligand, and CD27 ligand. In parallel, the TNF family of receptors, TNF receptor, TRAIL receptor, lymphotoxin receptor, Fas, 4-1BB, HVEM/LTβ receptor, OX40, CD40, CD30, and CD27 are expressed in many tumor cell lines.39–43 The complementary binding between TNF ligands and TNF receptors can efficiently induce tumor cell apoptotic death. Hence, engineered or induced expression of TNF family receptors on cancer cells represents 1 avenue being actively pursued for active immunotherapy. Moreover, LIGHT/HVEM (LTβR) signaling helps develop the adaptive immune response through priming and recruiting tumor-specific T cells.44–46 NK cells, activated by LIGHT, produce IFN-γ to directly promote the expansion and differentiation of T cells. Studies from mouse LIGHT tumor model suggest that intratumoral NK cells and local IFN-γ are required for priming cytotoxic T cells and tumor rejection.46
Tumors coated with antibodies against cell-surface molecules can be directly recognized by several innate immune cells through Fc receptors (FcR), the receptors for immunoglobulin. The FcR for IgG (FcγR) include 2 functional types of receptors, activating and inhibitory receptors. Antibody-coated tumor cells can be killed by NK cells or macrophages with activating FcγR, termed antibody-dependent cell-mediated cytotoxicity.47,48 NK cells solely express the activating FcγR CD16 for IgG49 without inhibitory FcγR detected.
Apoptotic tumor cells can be efficiently eliminated by macrophages to avoid autoimmunity. These tumor cells express the so-called “eat me” molecules at cell surface (Fig. 1) for recognition and phagocytosis by macrophages. These signals include lipid phosphatidylserine (PS), oxidized PS, oxidized low-density lipoprotein, and the multifunctional protein calreticulin (CRT).50 These molecules are translocated or redistributed to expose at the tumor cell surface during apoptosis.51,52 CRT is also associated with the CD91 receptor on macrophages and involved in the engulfment of apoptotic cells through interaction with soluble complement protein C1q and its ligand PS. Scavenger receptors, such as SR-A, CLA-1, CD36, CD68, LOX-1, and stabilin-2, can bind oxidized PS and oxidized low-density lipoprotein motifs on apoptotic tumor cells. T-cell immunoglobulin mucin (TIM) proteins (TIM-1, TIM-3, and TIM-4) were recently identified as critical receptors for PS to mediate uptake of apoptotic cells.53–55 CD36 may also form complex receptors with αvβ3 integrin on macrophages, whereas CD14 on macrophages can serve as the receptor for intercellular adhesion molecule-3, and trigger phagocytosis and clearance of apoptotic cells.56 Under normal circumstances in the tumor environment, the interaction between apoptotic tumor cells and macrophage phagocytes leads to immune tolerance without provoking significant proinflammatory cytokines. Unlike NK cells, macrophages express both activating and inhibitory FcγR simultaneously. Activating FcγR stimulate cytotoxicity to tumor cells. In contrast, FcγRIIB is the only inhibitory receptor on macrophages in mice, which is responsible for inhibitory effects on macrophage including inhibition of phagocytosis, decreased cytokine release, superoxide production, and blocking Toll-like receptor 4 (TLR4) signaling pathway.57
In the tumor milieu, macrophages are believed to be major contributors to the chronic inflammation that renders an immune suppressive environment benefiting tumor growth.2 Direct and indirect interactions of macrophages and cancer cells in the previously mentioned and following sections provide molecular mechanisms underlying such effects.
DCs are perhaps the most potent professional antigen-presenting cells, and bridge between innate and adaptive immune system. The 2 major groups of DCs are known as the myeloid DCs and the plasmacytoid DCs. Functional subsets of myeloid DCs in the skin, epidermal Langerhans cells, and dermal interstitial DCs are also characterized with distinct immune induction potentials. Activated epidermal Langerhans cells secret interleukin 15 (IL-15) and induce CD4+ and CD8+ T-cell priming to elicit cellular immunity. Dermal interstitial DCs stimulate B-cell priming to produce humoral immunity.58,59 Engaging DCs using different receptors and subpopulations may stimulate different inflammation responses, producing multiple T-cell outcomes including T-helper cells of type 1 (Th1), Th2, Th17, Th21, and T-regulatory cells.
With respect to direct interactions with cancer cells, DCs phagocytose apoptotic cancer cells using αvβ5 integrin and CD36 receptors.60 Similar to macrophages, DCs can recognize the so-called “eat me” signals on apoptotic cells through endocytotic receptors, scavenger receptors, and TIM receptors. In addition, the apoptotic cell marker PS can be captured by TAM receptor protein tyrosine kinases (TYRO3, AXL, and MER) on DCs and macrophages using molecular linkers Gas6/protein S, and through αvβ3 integrin using linker MFG-E8. TAM receptors promote phagocytosis of apoptotic tumor cells and inhibit inflammation in DCs and macrophages.61–63 The integrin αvβ3 complex is able to mediate engulfment of apoptotic cancer cells.64,65 Similar to macrophages, phagocytosis of apoptotic tumor cells by DCs in the absence of danger signals generally leads to immune tolerance.
DCs also express both activating and inhibitory FcγR. Comparing to other fashions of antigen uptake, antibody-coated tumor cells are more efficiently internalized into DCs through activating FcγR, leading to more efficient MHC class I and II-restricted antigen presentation and induction of tumor-specific effector and memory T cells.66 Therefore, inflammation and adaptive immune response could be trigged by DC-cancer cell encountering through activating FcγR signaling pathway, and this process is negatively modulated by coexpression of inhibitory FcγRIIB and TAM receptors on DCs. However, it is necessary to note that uptake of antigens does not accompany induction of effector T cells. The induction of active adaptive immunity requires danger signals or maturation of DCs during antigen encountering as discussed in the following sections.
PMN and Mast Cells
Tumor-associated PMN and mast cells can have a significant role in tumorigenesis and metastasis.67 However, fewer studies have been focused on the direct molecular recognition between tumor cells and PMN. The known examples are activating and inhibiting FcγR on PMN and mast cells to interact with antibody-coated antigens on tumor cells. Activating FcγR induces neutrophils to release cytokines and chemoattractants which influence recruitment and activation of DCs and macrophages in tumor environment.48,68,69 Activation of inhibitory FcγRIIB on neutrophils decreases products of reactive oxygen species, which are cytotoxic against tumors. Although in mast cells, stimulating FcγRIIB can decrease IgE-mediated release of granular molecules, IL-4 cytokine, and histamine which trigger inflammatory response in tumor environment.57 One study has shown that increased direct contact between tumor cells and PMN plus macrophages in mice is responsible for resisting lethal doses of cancer cells.70,71 However, the molecular mechanism for such efficacy remains unclear.
CLINICAL DEVELOPMENT BASED ON DIRECT CANCER AND INNATE IMMUNE SYSTEM INTERACTIONS
A few NK–cell-based cancer therapies are now being tested in clinical trials, most of which utilize direct cytotoxic activity of NK cells against cancer, such as activation of NK–cell-surface stimulatory receptors or blocking surface inhibitory receptors. On the basis of preclinical studies showing tumor regression induced through genetic overexpression of NKG2D, several drugs that selectively upregulate NKG2D ligands on tumor cells are introduced to complement chemotherapy such as DNA damage-inducing cisplatin and 5-fluorouracil,72 the histone deacetylase inhibitor sodium valproate.73 Low-dose proteasome inhibitor bortezomib has also been applied in human breast cancer74 and hepatocellular carcinoma75 to increase NK-activating ligands and subsequent tumor lysis. TRAIL on NK cells can efficiently trigger cancer cell apoptosis even after chemotherapy, which induces resistance to intrinsic apoptotic process in cancer. Thus, modulating TRAIL pathway on NK cells is also a new approach combining NK–cell-based therapy with chemotherapy.76,77 In addition to activation of NK-surface stimulatory receptors, therapeutic monoclonal antibodies such as the anti-KIR monoclonal antibody blocking inhibitory signaling in NK cells have been tested in clinical trials on acute myeloid leukemia and multiple myeloma patients.78
Several clinically useful monoclonal antibodies have now been approved for lymphoma and leukemia, with some functioning in part through antibody-dependent cell-mediated cytotoxicity, such as B-lymphocyte antigen CD20-targeted humanized monoclonal antibodies rituximab, tositumomab, and veltuzumab.79,80
CANCER AND INNATE IMMUNE SYSTEM INTERACTIONS THROUGH DAMPs AND THEIR PARTNER RECEPTORS
In addition to the direct cancer/innate immune system interactions, a large number of molecules released due to cancer cell death, may function as DAMPs and interact with innate immune cells (Table 1). Such cancer-derived DAMPs include both intracellular molecules and extracellular matrix (ECM) molecules released from apoptotic and necrotic tumor cells. Intracellular molecules that can function as DAMPs include heat shock proteins (HSPs), high-mobility group box-1 protein (HMGB1), adenosine triphosphate (ATP), mitochondrial formyl peptides, mitochondrial DNA, and uric acid. Special attention is given to NY-ESO-1 and possibly others, which are initially identified as TAAs but lately have been recognized with similar properties as DAMPs. ECM danger molecules include hyaluronan and heparan sulfate fragments, S100 family proteins, fibronectin, surfactant protein A, biglycan, versican, and so on. TLRs on innate immune cells represent the major pattern recognition receptors sensing DAMP-related danger signals.11 Other receptors such as cytoplasm NOD-like receptors and RIG-I-like receptors also play significant roles in responding to DAMPs derived from cancers.115
The exact nature of these DAMPs in the cancer microenvironment and their contributions to the cancer-associated inflammation and immunity are yet to be clearly understood, which are now an active area of investigation. Nevertheless, it is believed that cancer-derived DAMPs and their partner receptors represent new molecular targets with potentially significant immunological outcomes upon intervention.
HSPs are house-keeping proteins that are widely expressed in most cells, and are molecular chaperones under normal and stressed conditions. HSPs from necrotic tumor cells display immunological properties characterized by induction of DC maturation, inflammatory cytokine production, and stimulation of NK-cell cytotoxicity.116 Some of these activities are related to promoting tumor growth,117 whereas others contribute to antitumor immunity. HSP90, Gp96, CRT, HSP70, HSP110, and Grp170 can function as chaperones of polypeptides in cancer. Tumor-derived HSP-peptide complexes can be taken up by antigen-presenting cells such as macrophages and DCs and cross-presented by MHC class I molecules, which makes HSPs excellent carriers for cancer vaccines. Scavenger receptors and CD91 are common recognition receptors for HSPs on macrophage and DC surface.81 Among various HSP family members, HSP70, GRP78, and Gp96 have been found immunogenic in cancer patients, and also qualify as TAAs.118–120 TLR2/TLR4 have been indicated as the major receptors involved in HSP70-mediated and Gp96-mediated DC activation through the MyD88/NF-κB pathway,82 although conflicting data suggested that stimulation of TLR2 or TLR4 could be caused by microbial contaminants in the purified HSP preparations. Other cell-surface receptors are also indicated in HSP signaling, such as CD14 and CD40 in HSP70-mediated DC activation and scavenger receptor LOX-1 in HSP70-mediated antigen cross-presentation.83
HMGB1 is a widely expressed protein normally located in the cell nucleus and functions as a DNA-binding transcriptional factor. However, it can be released as a secreted protein from necrotic and apoptotic cancer cells.121 In necrotic cell death, emitted HMGB1 contributes to inflammation in activating DC/macrophage to secret IFN-α, TNF-α, IL-12, and IFN-γ, upregulate CD80 and CD86 costimulatory molecules, and induce adaptive CD8+ T cells.121,122 In contrast, oxidized HMGB1 delivers tolerogenic signals during apoptosis. Extracellular HMGB1 usually associates with other molecules correlating with differential binding to DC/macrophage cell surface receptors. For example, HMGB1/DNA/RNA complex signals through receptor for advanced glycation end products.84 HMGB1/IL-1β associates with the IL-1R/IL-1RAcP complex.85 HMGB1 and lipopolysaccharide complex can activate TLR4,86,87 whereas HMGB1/nucleosome preferentially engages TLR2.88 HMGB1/CXCL12 associates with receptors CXCR4, TLR4, and receptor for advanced glycation end products.89 HMGB1 has also been reported to directly bind to triggering receptor expressed on myeloid cell-1.90 Proinflammatory responses are usually caused by the previously mentioned HMGB1 and associated partners, whereas several binding receptors of HMGB1 suppress its proinflammatory effects, such as CD24 and thrombospondin.91
Multiple ECM components are upregulated or degraded in cancer, serving as proinflammatory mediators mostly through pattern recognition receptors TLR2 or TLR4 or both. Biglycan, an ECM proteoglycan liberated during inflammation, activates p38, ERK, and NF-κB signaling pathway through receptors TLR2 and TLR4 in macrophage and induces the production of inflammatory cytokines TNF-α and chemokine macrophage inflammatory protein-2.92 ECM degradation product of polysaccharide fragments derived from hyaluronic acid93 and heparan sulfate94 have revealed new roles for immunomodulatory signals eliciting DC maturation using TLR4. S100A8/S100A9 proteins, another family of endogenous DAMP molecules, can specifically interact with the TLR4-MD2 complex on phagocytes, which results in elevated expression of TNF-α and stimulation of chemotactic response. This includes the secretion of proinflammatory chemokines IL-8, upregulation of adhesion molecule intercellular adhesion molecule-1 and adhesion receptor CD11b/CD18.95,96 Fibronectin and surfactant protein A may also be recognized by TLR4 promoting expression of genes involved in the inflammatory response.97,98 Recent studies suggest that versican, a large ECM proteoglycan that accumulates in the mouse Lewis lung carcinoma microenvironment, stimulates tumor infiltrating macrophages (using TLR2, and coreceptors TLR6 and CD14) to produce IL-6 and TNF-α, and accelerates Lewis lung carcinoma metastasis.99 Versican is also accumulated in stroma surrounding human skin tumors induced by UV, colocalizing with infiltrating neutrophils.100
Recent evidence show that high levels of extracellular ATP can function as an endogenous danger signal and proinflammatory factor.101 High concentrations of extracellular ATP are quickly detected after tumor death induced by stress and chemotherapeutic agents.102 ATP is believed to play an important role in rendering the “immunogenic” death of tumor (late-stage apoptosis and necrosis) and induction of anticancer immune response accompanied with chemotherapy.103 After chemotherapy, ATP emitted from dying cancer cells engages the purinergic receptor P2X7 on immature DCs, activating the NOD-like receptor family, pyrin domain containing-3 protein (NLRP3) inflammasome, and driving the secretion of IL-1β. IL-1β then contributes to adaptive immunity against cancers, including priming IFNγ-producing CD8+ T cells.104
Mitochondrial DAMPs are newly identified intracellular DAMPs that can be released into the circulation from shock-injured tissues, which can elicit significant immune consequences.123 Among them, mitochondrial formyl peptides activate human PMN through formyl peptide receptor-1105; mitochondrial DNA, which are evolutionarily derived from bacteria, is recognized by innate immune system using TLR9, that similarly binds bacterial DNA. Mitochondrial DAMPs promote PMN Ca2+ flux, activate p38 mitogen-activated protein kinase107 and p44/42 mitogen-activated protein kinase,106 and induce PMN to secrete IL-8 and matrix metalloproteinase-9. This has lead to PMN migration, degranulation, and contribute to systemic inflammatory responses in vivo. Dying tumor cells may also release mitochondrial debris containing formyl peptides and DNA, producing similar immune outcomes.
Uric acid is a by-product of nucleic acid metabolism, which can be released from dying tumor cells and serve as a DAMP alert, shaping both the innate and adaptive immune responses.108 First, uric acid crystals may form in tumor cells with high contents of nucleic acids, which are able to upregulate costimulatory molecules on immature DCs and subsequently prime CD8+ T cells.109 Second, in cooperation with NF-κB activation (such as that caused by lipopolysaccharide), uric acid crystals have recently been shown to induce DCs to secrete IL-1α/β, IL-6, and IL-23, which subsequently drive proinflammatory Th17 differentiation of naive CD4+ T cells.110 IL-1 then binds to the IL-1 R and signals through MyD88 to amplify proinflammatory responses, including neutrophil recruitment.111 The effect of Th17 differentiation is dependent on the NLRP3 inflammasome, and cytokines IL-1α/β and IL-18. The receptor that identifies uric acid crystals is not clear. The binding of uric acid crystals with immature DCs seems not to be mediated by a specific receptor on the cell surface, but instead depends on directly engaging the cholesterol-rich membrane lipid rafts and Syk kinase activation.112
TAAs and DAMPs
TAAs are usually defined based on their recognition by spontaneous T-cell and antibody responses in cancer patients. When encountering antigen-presenting cells, TAAs themselves are generally perceived as by-standers that rely on the previously referenced “danger signals” to initiate adaptive immune responses. According to this paradigm, TAAs will be mostly resulted from the neopeptides of genetic mutations in cancer cells. However, human TAAs identified to date are commonly seen as nonmutated self-proteins.3 It is speculated that direct interactions may exist between some TAAs and the innate immune cells, which may play a role in the initiation of adaptive antitumor immunity in vivo. In search of intrinsic factors derived from TAAs that contribute to antitumor immune responses, our laboratory has been focused on NY-ESO-1, a nonmutated cancer/testis antigen with distinctively strong immunogenicity.124 Spontaneous antibody and T-cell immune responses against NY-ESO-1 are readily detectable in a wide spectrum of cancer patients with NY-ESO-1-expressing tumors, including older patients with late-stage cancers, whose immune systems are known to be less responsive. The immunogenicity of NY-ESO-1 is not due to its higher level of expression compared with other TAAs. Indeed, at least in melanoma, the expression of NY-ESO-1 is much lower than that of melanocyte differentiation antigens such as gp100, MART-1, TRP-1, and TRP-2, as well as other cancer/testis antigens, such as MAGE-1 and MAGE-3.125 Our recent investigation of the specific interaction between polymeric NY-ESO-1 and TLR4/CRT on the surface of immature DCs, macrophages, and monocytes indicates a unique interaction between NY-ESO-1 and the innate immune system.113,114 Although the exact signaling events of NY-ESO-1/DC interactions still need to be elucidated, NY-ESO-1 is shown to serve as an endogenous molecular adjuvant in antitumor immune responses. Expression plasmids encoding NY-ESO-1 fused with TAA carbonic anhydrase 9 generated robust antibody responses against the otherwise nonimmunogenic protein in mice.114
NY-ESO-1 thus represents the first example of a cancer/testis antigen that is also a DAMP. In contrast, antibody (and maybe T cell) responses against well-known protein DAMPs, such as HSP70, GRP78, and HMGB1 are present in various cancer patients.118–120 These DAMPs are thus also TAAs, supporting the cross-over roles between TAAs and DAMPs, that is, certain TAAs may serve as DAMPs and certain protein DAMPs may serve as TAAs.
CLINICAL DEVELOPMENT BASED ON INTERACTIONS OF CANCER-DERIVED DAMPs AND THEIR RECEPTORS
Current strategies in clinical development include: (1) TLR functional blockade using neutralizing antibodies and antagonists; (2) TLR signaling pathway inhibitors; and (3) the use of TLR agonists alone or as vaccine adjuvants.126–129 We emphasize on TLR agonists in immunotherapy of solid cancers in the following paragraph.
Because of complicated and sometimes adverse immune effects of TLR agonists, their overall use as cancer monotherapies is limited locally but not systematically. So far, TLR agonists approved by the Food and Drug Administration for clinical use in cancer treatment consist of the classic Bacillus Calmette-Guein (mycobacterium mixture) targeting TLR2, TLR4, and TLR9 for bladder cancer,130 imiquimod (small-molecule single-stranded RNA) targeting TLR7 for superficial basal cell carcinoma,131,132 and the AS04 adjuvant system (detoxified lipid A on aluminum hydroxide) targeting TLR4 for human papillomavirus as a prophylactic cervical cancer vaccine.127 Several other TLR agonists, such as CpG oligodeoxynucleotides targeting TLR7, polyriboinsinic-polyribocytidylic acid targeting TLR9, and flagellin-protein fusions targeting TLR5 are being actively evaluated as adjuvants in multiple cancer indications.133 For example, a small single-stranded RNA molecule-based TLR7 agonist, 852A, stimulates immature DCs to produce multiple cytokines including IFNα in vitro and in vivo. It is now being evaluated in a phase II clinical trial for treatment of inoperable melanoma.134 There are also numerous efforts to discover new TLR agonists with low toxicities and improved systemic antitumor effects from natural product extracts analysis and structural modifications. TLR agonists are being exploited as adjuvants in cancer vaccines based on their ability to induce maturation of antigen-presenting cells.133 They can also combine with chemotherapy, radiotherapy, or monoclonal antibodies to improve efficacy.
Molecular Adjuvant Effect of HSPs and Other DAMPs
HSPs have been applied as carriers/adjuvants for cancer vaccines in clinical trials. The most commonly used approaches include autologous tumor-derived HSP-polypeptide complexes and chimeric HSP-TAA fusion proteins. Promising effects are being obtained in clinical trials using Gp96 complex purified from patients’ own cancers including glioma, renal cell carcinoma, melanoma, and pediatric neurological cancer patients. For example, in a phase II trial carried out in stage IV melanoma patients treated with autologous tumor-derived Gp96, 28 among 39 patients had residual measurable disease, whereas 11 were disease free after surgery.135 In another phase II study of HSP-polypeptide complex for patients with metastatic renal cell carcinoma, 2 patients had a partial remission, 1 had a complete remission and 18 had stable disease, among 61 patients treated. These HSP-based vaccines exhibit minimal toxicity and promising antitumor activity.83 Phase III clinical trials have been initiated in advanced melanoma and kidney cancer with earlier stage disease.136
Preclinical studies have indicated potential advantages in cancer vaccine-induced helper T cells and cytotoxic T cells generated through activating immature DCs directly with DAMPs rather than indirectly using proinflammatory or activating cytokines provided by neighboring cells.137,138 In particular, after the recognition of the mechanism of immunogenicity, HMGB1 and NY-ESO-1 are being studied in preclinical investigations as immune adjuvants with perspectives as potential vaccine adjuvants in human trials in the future.114,139 DAMPs, due to its limited toxicity comparing with bacterial and viral products, are attractive candidates of molecular adjuvant development.
Other areas of clinical development exploiting cancer/innate immune cell interactions, such as blocking DAMPs that are associated with chronic inflammation for the prevention and treatment of cancer, blocking or enhancing cytokines/chemokines in cancer biotherapy, utilization of growth factors to increase the number of DCs, and other antigen-presenting cells, have been the subject of other review articles1,2,140 and not explicitly discussed here.
Spontaneous immune responses against cancer are complex and can be well summarized in the immune editing model.5 In most patients present at the clinic, chronic inflammation and immune suppression are the dominant effects in the tumor microenvironment. However, this does not exclude the existence of cancer-derived intrinsic factors that may have a powerful activation effect to the immune system. By dissecting the molecular details of cancer and innate immune system interactions as summarized in Figure 1 and Table 1, we hope to individually identify cancer-derived intrinsic factors involved in this complex network and point to areas with the potential of tipping the balance through immunological interventions. These factors are composed of certain cancer-derived DAMPs as well as their partner receptors on the immature DCs, which represent new molecular targets for immunotherapy of cancer in the future.
The authors are thankful to the support of the NIHR21CA137651 Grant under the American Recovery and Reinvestment Act and the Research Scholar Award (#RSG-08-070-01-LIB) from the American Cancer Society. Robert M. Prins, PhD and David H. Nguyen, PhD of UCLA provided helpful discussions for the draft of this manuscript.
CONFLICTS OF INTEREST/FINANCIAL DISCLOSURES
This study was funded by the National Institutes of Health (NIHR21CA137651) and the American Cancer Society (RSG-08-070-01-LIB).
The authors declare that there are no financial conflicts of interest in regard to this work.
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