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Clinical Pharmacology

The Use of TLR7 and TLR8 Ligands for the Enhancement of Cancer Immunotherapy

^aVaccine & Infectious Disease Institute (VIDI), Laboratory of Experimental Hematology, Faculty of Medicine, University of Antwerp, Antwerp, Belgium; ^bCenter for Cellular Therapy and Regenerative Medicine, Antwerp University Hospital, Antwerp, Belgium

Key Words. TLR7 • TLR8 • TLR ligands • Cancer • Immunotherapy • Vaccine adjuvant

Correspondence: V. F. I. Van Tendeloo, Ph.D., VIDI, Laboratory of Experimental Hematology, University of Antwerp (UA), Antwerp University Hospital (UZA), Wilrijkstraat 10, B-2650 Antwerp, Belgium. Telephone 32-3-8213661; Fax: 32-3-8214456; e-mail: viggo.van.tendeloo{at}uza.be

Received April 16, 2008; accepted for publication July 14, 2008; first published online in THE ONCOLOGIST Express on August 13, 2008.

Disclosure: The content of this article has been reviewed by independent peer reviewers to ensure that it is balanced, objective, and free from commercial bias. No financial relationships relevant to the content of this article have been disclosed by the authors, planners, independent peer reviewers, or staff managers.

	Learning Objectives

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
After completing this course, the reader should be able to:

1. Describe the subtypes of Toll-like receptor 7 and 8 agonists and their effect on the different components of the antitumor immune response.
   
2. Argue why they are used as stand-alone immunotherapeutic agents.
   
3. Evaluate their potential to improve current approaches of active and passive immunotherapy.
   

	ABSTRACT

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
The importance of Toll-like receptors (TLRs) in stimulating innate and adaptive immunity is now well established. In view of this, TLR ligands have become interesting targets to use as stand-alone immunotherapeutics or vaccine adjuvants for cancer treatment. TLR7 and TLR8 were found to be closely related, sharing their intracellular endosomal location, as well as their ligands.

In this review, we describe the agonists of TLR7 and TLR8 that are known so far, as well as their contribution to antitumor responses by affecting immune cells, tumor cells, and the tumor microenvironment. The major benefit of TLR7/8 agonists as immune response enhancers is their simultaneous stimulation of several cell types, resulting in a mix of activated immune cells, cytokines and chemokines at the tumor site. We discuss the studies that used TLR7/8 agonists as stand-alone immunotherapeutics or cancer vaccine adjuvants, as well as the potential of TLR7/8 ligands to enhance antitumor responses in passive immunotherapy approaches.

	INTRODUCTION: CURRENT STATUS OF CANCER IMMUNOTHERAPY

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
The ultimate goal of cancer immunotherapy is the eradication of tumor cells by the immune system. Both the innate and the adaptive arm of the immune system can contribute to eradication of tumor cells, with natural killer (NK) cells and T cells, respectively, as key players.

Crucial in the adaptive immune response against tumor cells is the activation of CD8⁺ cytotoxic T lymphocytes (CTLs), able to exploit their cytotoxic potential against tumor cells after recognition of tumor-associated antigens (TAAs). Activation of naïve CD8⁺ cells occurs via antigen-presenting cells (APCs), with dendritic cells (DCs) considered as the most professional APC. These cells capture and process TAAs, presenting the epitopes at their membrane in complex with major histocompatibility complex (MHC) molecules. Maturation of APCs by danger signals is essential for the presentation of epitopes in a stimulatory way to T cells.

Peripheral T-cell tolerance against TAAs (mostly overexpressed self-antigens) results in tumor growth, despite the potential of TAA-specific T cells to eliminate tumor cells. The approaches to break this T-cell tolerance against TAAs can be divided into two groups: (a) active specific immunotherapy (also known as cancer vaccines) and (b) passive specific immunotherapy (by adoptive transfer of antitumor T cells or by monoclonal antibodies). Both strategies have been proven to be effective in various animal tumor models, but generally result in limited clinical effectiveness in humans, lacking immunological correlates of tumor protection in the majority of patients [1, 2]. More encouraging results have been obtained by passive immunotherapy approaches [3]. However, immunotherapy is still open to improvement.

Apart from peripheral tolerance against TAAs, the poor immunogenicity of tumor cells is another pitfall for cancer immunotherapy. This low immunogenicity is a result of the fact that TAAs are mostly self-antigens, and also because of the downregulation of human leukocyte antigen and costimulatory molecules on the membranes of tumor cells. Moreover, tumor cells actively inhibit the immune system by the secretion of immunosuppressive factors that interfere with DC and T-cell function.

For cancer vaccines, on one hand, efforts are now focused on methods to: (a) achieve superior in vivo activation of APCs, (b) create an inflammatory environment at the tumor site to promote the homing of effector lymphocytes to the tumor, (c) enhance CD4⁺ T-cell activation, (d) downregulate the suppressive effects of regulatory T (Treg) cells and tumor cells, (e) enhance the immunogenicity of tumor cells, and (f) prevent angiogenesis. For adoptive immunotherapy, on the other hand, total body irradiation (TBI) prior to the administration of TAA-specific T cells is an important strategy to improve effectiveness, because it leads to: (a) depletion of Treg cells, (b) removal of homeostatic cytokine sinks, and (c) activation of the innate immune system by leakage of gut microbes [4].

Recently, the family of Toll-like receptors (TLRs) was identified as primary sensors of microbial components, resulting in initiation of innate and adaptive immune responses. Members of the TLR family are largely expressed by several immune and nonimmune cells, each cell type expressing a different combination of TLRs. TLR ligands represent a promising class of immune-response enhancers with the potential to generate an effective antitumor immune response. In this review, we focus on how ligands of TLR7 and TLR8 can be a useful component of current immunotherapy approaches.

	TLR7 AND TLR8 LIGANDS

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
To date, several ligands have been characterized as TLR7 and/or TLR8 ligands, classified in synthetic compounds and natural nucleoside structures. Some synthetic compounds were already produced and used as immune activators before they were characterized as TLR7/TLR8 ligands. Recognition of these nucleoside structures by TLR7 or TLR8 activates intracellular pathways that culminate in the induction of proinflammatory cytokines, chemokines, and type I interferons (IFNs), and in the upregulation of costimulatory molecules. TLRs are type I membrane proteins, characterized by an ectodomain composed of leucine-rich repeats, responsible for recognition of pathogen-associated molecular patterns, and a cytoplasmic domain, called the Toll/interleukin-1 receptor (TIR) domain, which is required for downstream signaling. TLR7 and TLR8 are grouped together with TLR3 and TLR9 because these TLRs are predominantly located within endosomal compartments. Recognition of a ligand by TLR7 or TLR8 is followed by recruitment of the TIR domain–containing adaptor molecule myeloid differentiation primary response gene 88 (MyD88). The association of TLR7/8 and MyD88 stimulates the recruitment of members of the interleukin-1 receptor-associated kinase family. This results in the downstream activation of mitogen-activated protein kinases (MAPKs) and the IB kinase (IKK) complex. Members of the MAPK family phosphorylate and activate the transcription factor activator protein (AP)-1, whereas the IKK complex is involved in the nuclear translocation of the transcription factor nuclear factor (NF)-B. Both AP-1 and NF-B control the expression of proinflammatory cytokine genes. Furthermore, members of the interferon regulatory factor family of transcription factors are activated in a MyD88-dependent way, resulting in type I IFN induction. These signaling events are summarized in Figure 1. For a more profound overview of TLRs and their signaling pathways, we refer to a recent review published by Kawai and Akira [5]. Here, we give an overview of the different categories of TLR7/8 agonists and their specific features.

View larger version (22K): [in this window] [in a new window]   Figure 1. TLR7/8 signaling. Abbreviations: AP-1, activator protein 1; IKK, IB kinase; IRAK, interleukin-1 receptor-associated kinase; IRF, interferon regulatory factor; ISRE, interferon-stimulated response element; LRR, leucine-rich repeat; MAPK, mitogen-activated protein kinase; MyD88, myeloid differentiation primary response gene 88; NF-B, nuclear factor-B; TIR, Toll/interleukin-1 receptor; TLR, Toll-like receptor.

The first experiments with these molecules were focused on the guinea pig model of genital herpes, caused by herpes simplex virus (HSV) [6]. Imidazoquinolines exhibited a small effect on in vitro virus replication, but had a strong effect in vivo. It was shown that imidazoquinolines initiated immune cells to produce proinflammatory and regulatory cytokines, resulting in antiviral and antitumor immune responses. Therefore, this family of molecules is classified as immune response modifiers (IRMs) with low molecular weight.

Imiquimod (R-837, S-26308) was the first IRM to be licensed in 1997 by the U.S. Food and Drug Administration. Imiquimod 5% cream (Aldara®; Graceway Pharmaceuticals, LLC, Exton, PA) is available for the treatment of genital warts, caused by human papilloma virus infection. It is also used for the treatment of malignant tumors of the skin. Imiquimod is a ligand that preferentially binds to TLR7 and has a half-life of 2–3 hours in humans [7]. Resiquimod (R-848, S-28463) was shown to be more soluble and more potent in inducing cytokine expression than its family member imiquimod [8–11]. Resiquimod exerts its immunostimulatory activities via activation of mouse TLR7, human TLR7, and human TLR8 [12, 13]. Recently, some compounds were developed that specifically activate TLR7 or TLR8. CL097 (3M-001) [14] and 852A [15] induce TLR7-mediated activation of NF-B at much lower concentrations than those needed for TLR8-mediated activation of NF-B. The opposite is true for the compound CL075 (3M-002), consequently considered as a ligand that preferentially activates TLR8 [14].

Guanosine Analogues
Guanine ribonucleosides that are monosubstituted at the C8 position or disubstituted at the C8 and N7 positions were found to induce immunologic responses, including antitumor activities [16]. Therefore, they were also classified as IRMs with low molecular weight. Disubstituted guanosine analogues exert more potent immunostimulatory effects than their monosubstituted counterparts.

Loxoribine is a disubstituted guanosine analogue that was characterized as a ligand restricted to TLR7 [17, 18]. However, in a recent study by Jurk et al. [19], coincubation of loxoribine with the phosphorothioate oligo(deoxythymidine)₁₇ led to a complete switch from TLR7- to TLR8-mediated NF-B activation. The authors suggested that the presence of thymidine (T)-rich oligodeoxynucleotides (ODNs) might enhance the affinity of loxoribine to TLR8, although the basal affinity of loxoribine to TLR8 is too weak to detect in current biological assays.

Viral or Synthetic Single-Stranded RNA
Because of the structural similarity between synthetic TLR7 and TLR8 ligands and single-stranded (ss)RNA, whether or not ssRNA could be a natural ligand for these TLRs was investigated. Experiments have shown that it is useful to stabilize viral or synthetic ssRNA and to protect it against degradation by RNases when investigating the immunostimulatory properties of ssRNA. Stabilized or protected RNA can be obtained by one or more of the following processes: (a) in vitro encapsulation of ssRNA in cationic liposomes (e.g., DOTAP [20]), (b) condensation of ssRNA to cationic polymers or proteins (e.g., polyethylenimine [21] or protamine [22]), (c) chemical modification of the ssRNA backbone to a phosphorothioate backbone [23] (instead of phosphodiesterase), and (d) modification of the 3' and 5' RNA ends to diminish degradation by exonucleases [24].

Using stabilized RNA and viral infection, the natural ligand of mouse TLR7 and human TLR8 was identified as viral, guanosine (G)- and uridine (U)-rich ssRNA [21, 23, 25]. Since these early reports, it has been shown by others that several ssRNA viruses induce TLR7- and TLR8-mediated immune responses (reviewed in [26]). Stabilized synthetic RNA oligoribonucleotides (ORNs) of around 20 nucleotides were also found to be ligands for TLR7 and TLR8 [21, 23, 27]. Although the content of U or GU in the RNA molecule seems to be important for both TLR7 and TLR8 activation [21, 23, 27, 28], the specific RNA motif recognized by TLR7 and/or TLR8 has not yet been identified. It was suggested by Diebold et al. [29] that the uracil content and motifs in ORNs define the ability of the ORN to act as a TLR7 ligand, whereas the G content is important for TLR8 ligands. The finding of Peng et al. [28] that poly-G3 oligonucleotides are recognized by TLR8 fits into this hypothesis. However, further experiments are needed to unravel the ligand specificity of both TLRs and the differences between species, more specifically between mice and humans.

Other immunostimulatory ligands of TLR7/TLR8 that have been described are self-RNA [29, 30] and short interfering RNA [31–34]. Moreover, 2'-O-methyl modification of these RNA molecules inhibits TLR7/8-mediated immune activation, thereby paving the way to the development of TLR7/8 antagonists [35–38]. These antagonists might be potent agents for the treatment of autoimmune diseases, such as systemic lupus erythematosus (SLE). SLE is an autoimmune disease characterized by the production of autoantibodies to cellular macromolecules, for example, small nuclear ribonucleoprotein (snRNP). It was also shown that snRNPs have immunostimulatory capacity through TLR7 and TLR8 activation, making TLR7/8 antagonists interesting targets for SLE therapy [30].

Recent studies showed an unexpected plasticity in the ligand specificities of TLR7 and TLR8. It was shown that poly(T) ODNs could modulate the imidazoquinoline effects on TLR7 and TLR8. Simultaneous addition of poly(T) ODNs with TLR7 or TLR8 agonists inhibited TLR7 and enhanced TLR8 signaling events in TLR-transfected human embryonic kidney cells and in human primary peripheral blood mononuclear cells (PBMCs) [19, 39]. The study of Jurk et al. [13] further demonstrated a switch from TLR7- to TLR8-mediated NF-B activation when loxoribine was added together with poly(T) ODNs. Failure of activation of mouse TLR8 by expected TLR8 ligands such as R-848 [13] or ssRNA sequences [23] suggested that TLR8 is nonfunctional in mice or that TLR8 only plays a nonimmunological role, for example, in the mammalian nervous system during brain development [40]. However, the functionality of mouse TLR8 was recently shown by Gorden et al. [41]. In their study, primary mouse cells responded to the interaction between the TLR8 ligand 3M-002 and poly(T) ODNs by secreting tumor necrosis factor (TNF)-, thereby showing that mouse TLR8 can be functional under certain conditions. In conclusion, there is flexibility in the specificity of TLR7 and TLR8 for their ligands, depending on characteristics of the ligands and the environment that have not yet been completely elucidated.

	EFFECTS OF TLR7/8 LIGANDS ON DIFFERENT COMPONENTS OF THE ANTITUMOR IMMUNE RESPONSE

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
The effects of TLR7/8 agonists on various immune cells and on tumor cells are summarized and depicted in Figure 2.

View larger version (32K): [in this window] [in a new window]   Figure 2. Pleiotropic effects of TLR7/8 agonists on different components of the antitumor response. Black arrows, TLR7/8 agonists activate different components of the antitumor response; white arrows, cytokine and chemokine production by the activated cells. Abbreviations: CCL, chemokine (C-C motif) ligand; DC, dendritic cell; IFN-; interferon-; IL, interleukin; IP-10, interferon-inducible protein 10; MCP-1, monocyte chemotactic protein 1; NK, natural killer; TLR, Toll-like receptor; TNF-, tumor necrosis factor-.

Several experiments corroborated that mDCs and pDCs respond to TLR7/8 agonists not only by production of proinflammatory and regulatory cytokines but also by enhanced expression of costimulatory molecules and migration, thereby facilitating a T helper 1 (Th1)-type of T-cell response [21, 22, 44–49]. Next to the increase in the Th1-polarizing capacity of DCs, it was shown that TLR7/8 activation results in an enhanced potential of mDCs and pDCs to act as cytotoxic effector cells that can kill tumor cells [50]. Furthermore, it was shown by Sioud and Fløisand [51] that activation of TLR7 and TLR8 in CD34⁺ bone marrow–derived progenitors pushed their differentiation towards mDCs capable of inducing a Th1-mediated T-cell response. This differentiation was dependent on autocrine stimulation by TNF-.

To understand the mechanisms behind the success of topical treatment with imiquimod, studies were performed to determine the effects of TLR7/8 agonists on Langerhans cells, the DCs of the skin. Mice studies by Suzuki et al. [52] revealed that imiquimod enhanced Langerhans cell migration from the skin to the lymph nodes, without influencing the expression of costimulatory molecules. In vitro studies with human Langerhans cells showed that imidazoquinolines induced functional, but not phenotypic, maturation of these cells [11]. So, in contrast to mDCs and pDCs, Langerhans cells retain an immature phenotype after treatment with imidazoquinolines, although they also become functionally mature. Apart from migration of Langerhans cells from the skin to the lymph nodes, it was also observed that topical treatment with imiquimod resulted in the recruitment of pDCs and activated DCs to the skin [46, 53, 54]. Moreover, the immune activation observed was related to the number of pDCs recruited to the treatment site [53]. Furthermore, dermal mast cells were shown to be essential in the imiquimod-induced immune response. Both early cutaneous inflammation and migration of Langerhans cells were promoted by TLR7-activated mast cells, with mast cell–derived TNF- and IL-1 as the essential mediators [55].

In addition to the effects on DCs, it was also shown that resiquimod treatment mimics CD40-mediated B-cell activation [56]. This resulted in the induction of antibody and cytokine production, protection from apoptosis, and upregulation of B7 costimulatory molecules. Resiquimod acted in synergy with B-cell receptor activators, thereby specifically stimulating antigen-specific B cells. Furthermore, CD40-activated B cells were previously shown to act as professional APCs for the induction of immune responses [57]. In conclusion, the effect of TLR7/8 agonists on APCs is a profound enhancement of their immunostimulatory capacity by maturation, activation, and/or migration.

T Cells and NK Cells
Expression of TLR7 and/or TLR8 was found in T cells, although real-time polymerase chain reaction showed 100–1,000 times less expression than in monocytes [42, 43, 58]. In a study by Caron et al. [58], T cells were found to be sensitive to resiquimod treatment, especially in combination with T-cell receptor (TCR)-dependent and TCR-independent stimuli. This treatment resulted in increased T-cell proliferation and production of IFN-, IL-2, and IL-10, with no induction of IL-4 or Th2-attracting chemokines [58]. Moreover, it was found that memory T cells were more sensitive to this TLR-mediated activation than naïve T cells.

Another direct effect of TLR7/8 agonists on T cells is the reversal of CD4⁺ Treg cell function [28]. This effect was dependent on TLR8 signaling and independent of DCs. Antitumor immunity was enhanced by adoptive transfer of these TLR8-stimulated Treg cells. Also, for tumor-infiltrating T cells, it was shown that stimulation with TLR8 ligands abrogated their immunosuppressive activity in vitro and in vivo [59]. In general, administration of TLR7/8 agonists results in activation of CD4⁺ Th1 cells and enhanced CD8⁺ T-cell responses [60–62], at least partially mediated by the induced Th1-polarizing cytokine pattern [63, 64]. TLR7/8 agonists lead to better priming of CTLs by efficient maturation and activation of APCs [62, 65], and CD8⁺ T cells showed a higher expression level of perforin and cytotoxic granule markers [64, 66]. Imiquimod not only enhanced DC migration but also induced migration of human CD4⁺ and CD8⁺ T cells toward the place of topical treatment [54, 67, 68]. In this way, TLR7/8 agonists might be an indispensable aid for T cells to meet the three requirements for immunological destruction of established tumors by T cells, as described by Rosenberg et al. [1]: (a) the in vivo generation of sufficient numbers of T cells with highly avid recognition of tumor cells, (b) traffic to and infiltration of the tumor stroma, and (c) activation of T cells at the tumor site to manifest appropriate effector mechanisms such as direct lysis or cytokine secretion. Important to note is that the quality of the T-cell response was dependent on which TLR agonist was used, as shown in nonhuman primates [69].

For NK cells, whether or not they express TLR7 and/or TLR8 is still a matter of debate [42, 70–72]. In spite of conflicting data on TLR expression in NK cells, several groups have shown that TLR7/TLR8 agonists induce NK cell activation, as judged by CD69 expression, IFN- production, and exertion of their cytotoxic effector function. However, it was also observed that this TLR7/TLR8-induced NK cell activation is crucially dependent on contact with cytokines (e.g., IL-12 or type I IFNs) produced by accessory cells in peripheral blood [24, 70, 70, 71, 73–76]. Therefore, it is not completely clear yet if the NK activation observed after the addition of TLR7/8 agonists is partially caused by or dependent on direct activation of TLR7 and/or TLR8 in NK cells. However, it is clear that NK and NKT cells [77, 78] also become highly activated by IRM treatment. For NKT cells, it was shown that activation of this cell type contributes substantially to the antitumor immune response by enhancing activation of the other cell types of the innate and adaptive immune system [79].

Tumor Microenvironment
Administration of TLR7/8 agonists activates several cell types, such as DCs, monocytes, macrophages, fibroblasts, and human keratinocytes. This activation is accompanied by secretion of proinflammatory cytokines and chemokines, including TNF-, IL-1, IL-6, IL-8, IFN-inducible protein 10, monocyte chemotactic protein 1, IL-12p40, IL-12p70, and IL-18, with no strong induction of Th2 cytokines [8, 9, 14, 21, 23, 27, 63, 73, 77, 80–83]. The induced cytokines and chemokines play an important role in the effects of TLR7/8 agonists as described above. In the previous studies in which imiquimod was applied topically, this IRM led to local cytokine production, thereby altering the tumor microenvironment with a Th1-polarizing cytokine pattern that promotes effective antitumor immune responses [46, 83, 84]. Cytokines induced by TLR7/8 agonists also have additional functions, embedded in the complex process of immune activation, such as rendering CD4⁺ effector T cells refractory to Treg cell–mediated suppression [68, 85]. However, a prominent role for the effects of TLR7/8 agonists is attributed to IFN-.

In a guinea pig model of herpes, it was already shown in the 1980s that the titer of type I IFNs induced by imiquimod corresponded to the effectiveness [86]. It is now clear that the effects of TLR7/8 agonists on different types of cells can be explained to a considerable extent by the induction of type I IFNs [68]. This type of IFN is very important for effective antitumor immune responses, acting directly and indirectly, as already extensively reviewed by others [87, 88]. Type I IFNs affect tumor cells directly in different ways, for example, by suppression of tumor cell proliferation [89]. Some other effects of type I IFNs are: (a) upregulation of MHC molecules, (b) maturation of DCs, (c) enhancement of IFN- production in synergism with IL-12, and (d) stimulation of crosspriming by DCs. Therefore, type I IFNs are very important as a bridge between the innate and adaptive immune responses by stimulating DC maturation and crosspriming [90]. Because of their pleiotropic antitumor effects, type I IFNs have already been used for cancer treatment, for example, in the treatment of melanoma or chronic myeloid leukemia [87]. However, side effects, such as flu-like symptoms, are linked to the administration of high-dose exogenous IFN-. These negative effects could possibly be circumvented by the addition of a carefully defined dose of TLR7/8 agonists, in order to provoke endogenous IFN- production.

Angiogenesis
Angiogenesis is the formation of new capillary blood vessels from the existing vasculature. Inhibition of angiogenesis has a negative impact on tumor progression because tumors are dependent on angiogenesis for their growth [91]. A few studies in mice and humans have investigated the potential of imiquimod as an antiangiogenic agent, concluding that imiquimod acts as an inhibitor of angiogenesis [84, 92–94].

Majewski et al. [92] found that topically applied imiquimod strongly inhibited angiogenesis in murine tumor models. In these models, angiogenesis was induced by intradermal injection of human transformed keratinocytes or murine lung sarcoma cells. The antiangiogenic effect of imiquimod was mediated by several antiangiogenic cytokines, including IFN-, TNF-, and IL-18 [92]. In a mouse model of hemangioma, it was shown that topical administration of imiquimod inhibited vascular tumor growth [95]. This inhibition was associated with an increase in the expression of tissue inhibitor of matrix metalloproteinase 1 and a decrease in the activity of matrix metalloproteinase (MMP)-9. Decreasing the activity of MMP contributes to the inhibition of angiogenesis because the microenvironment is made suitable for tumor progression by MMP [96]. Another important inhibitor of angiogenesis is IFN-, which is induced by imiquimod and responsible for several of its antitumor mechanisms, as described above. In a study by Li and Li [94], the dose of imiquimod per patient was optimized, because high doses of imiquimod are not required to provide benefit from its antiangiogenic effects. In this way, antiangiogenic activity can be achieved with minimal discomfort associated with inflammation [94].

Tumor Cells
It was shown in vitro that imiquimod can directly induce apoptosis of tumor cells using several skin and bladder cancer cells lines, as well as tumor tissue cultures [97–101]. For the skin cancer cell lines, it was demonstrated that this proapoptotic activity was independent of membrane-bound death receptors. However, overexpression of Bcl-2 or inhibition of caspase activation made tumor cells resistant to imiquimod-induced apoptosis. Resiquimod induced markedly less apoptosis than imiquimod [97]. The apoptosis-inducing potential of imiquimod was also shown in vivo in basal cell carcinoma patients [97]. Similar results in vitro and in vivo were found for melanoma cells [99]. Treatment with EAPB0203, a member of the imidazoquinoxaline family, also induced growth inhibition and caspase-dependent apoptosis in a series of malignant T cells, but not in normal T cells. However, it is not yet clear whether the effects of imidazoquinoxalines are mediated via TLR7 and/or TLR8 [102]. In addition to direct induction of apoptosis, treatment with imidazoquinolines leads to infiltration of CTLs, NK cells, and DCs with cytotoxic potential, as described above. These immune cells also contribute to the increase in apoptotic tumor cells after treatment with imiquimod.

Another direct effect of TLR7/8 treatment on tumor cells is greater sensitivity to killing by chemotherapeutic agents or CTLs, as described for chronic lymphoid leukemia (CLL) cells. Lipopolysaccharide (LPS) pretreatment of human promonocytic THP-1 cells induces tolerance to secondary LPS stimulation, resulting in lower TNF- production. This phenomenon is called endotoxin tolerance. In analogy with endotoxin tolerance, Shi et al. [103] found that the amount of TNF- mRNA transcripts was greater in CLL cells after primary stimulation, but not after restimulation with the TLR7 agonist. These TLR7-tolerized CLL cells were more sensitive to chemotherapeutic agents, such as vincristine and doxorubicin [103]. Despite strong expression of costimulatory molecules and production of inflammatory cytokines, TLR7-tolerized CLL cells did not induce strong T-cell proliferation [104, 105]. However, treatment with TLR7/8 agonists for several days rendered primary CLL cells more sensitive to CTL-mediated killing in vitro [104, 106], in analogy with the greater sensitivity to chemotherapeutic agents. A successful way to increase the immunogenicity of CLL cells is the simultaneous addition of IL-2 and a protein kinase C (PKC) agonist together with TLR7 activation. This combination resulted in CLL cells with a DC-like phenotype and a high ability to stimulate T-cell proliferation [104, 105]. For primary acute myeloid leukemia (AML) cells, treatment with resiquimod resulted in greater expression of MHC class I molecules, production of proinflammatory cytokines, and greater immunogenicity (own data, manuscript submitted).

Data on the apoptosis of tumor cells caused by treatment with TLR7/8 agonists contrast sharply with studies showing that these agonists induce proliferation of certain tumor cell types. For multiple myeloma cells, it was shown that TLR ligands, such as loxoribine or R-848, potently inhibit apoptosis and promote the proliferation and survival of the malignant cells [107, 108]. These effects were partially mediated by the autocrine secretion of IL-6. Because proinflammatory cytokines like IL-6 can be crucial growth factors for certain tumor cell types, it is important to check whether the proinflammatory microenvironment created by TLR7/8 agonist treatment is not beneficial for tumor growth. In our hands, treatment of AML cells with resiquimod resulted in higher production of proinflammatory cytokines such as IL-6, but not in increased proliferation of AML cells (manuscript submitted). For CLL cells, TLR7 activation alone did not increase their proliferation, in contrast to simultaneous addition of IL-2 with the TLR7 ligand. This proliferation was blocked if a PKC agonist was added together with the TLR7 ligand and IL-2 [105]. In addition to greater proliferation, it was shown that activation of TLR4 and TLR9 in tumor cells can lead to resistance to killing by proinflammatory cytokines, and to enhanced invasiveness by activation of certain MMPs [109, 110]. Because of common mediators, these adverse effects also need to be evaluated when administering ligands of TLR7 and TLR8.

In conclusion, the direct effects of IRMs on tumor cells include induction of apoptosis, greater immunogenicity, and sensitization to killing mediated by CTLs and chemotherapeutic agents. On the other hand, TLR activation can be advantageous for the proliferation, invasiveness, and/or survival of tumor cells. These unwanted effects of TLR7/8 agonists on tumor cells largely depend on the tumor cell type, and need to be carefully taken into account in preclinical studies. The activation of additional signaling pathways in the tumor cell is a possible mechanism to block or reverse these undesirable effects.

	CURRENT USE OF TLR7/8 LIGANDS IN CANCER TREATMENT AND CANCER PREVENTION

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
After the discovery of the effectiveness of imiquimod to protect guinea pigs from herpes virus infection, imiquimod was also shown to be effective against several transplantable murine tumors [86]. Preclinical studies were followed by extensive testing of imiquimod in clinical trials for the treatment of cutaneous tumors. Clinical responsiveness to topical treatment with imiquimod (Aldara® 5% cream) was found to be effective for both primary skin tumors and cutaneous metastases, as reviewed by Schön and Schön [111]. In these reports, no TAAs were added, and the immune-enhancing effects of imiquimod were sufficient to elicit an antitumor response.

Furthermore, imiquimod was shown to be effective for the treatment of actinic keratoses (AKs). These are premalignant lesions appearing as rough and dry patches on the skin, associated with damage caused by UV radiation. If DNA damage results in inhibition of apoptosis of actinically damaged cells, further derangements might accumulate, resulting in the development of squamous cell carcinoma. Torres et al. [112] examined the changes in gene expression occurring in sun-exposed nonlesional skin and in AK lesions. They found that imiquimod treatment was associated with partial or total reversal of the aberrant expression of some genes in AK, thereby demonstrating the ability of imiquimod to prevent the development of cancer. This statement was confirmed by others, showing that imiquimod treatment results in clearing of AK lesions [113–116].

Most TAAs are (overexpressed) self-antigens, implying that peripheral tolerance against the TAA needs to be broken in order to obtain effective antitumor immune responses. Because TLR7/8 agonists are strong immune activators that are able to induce anti-TAA immune responses, there is also the risk of inducing autoimmune diseases against self-antigens. As reviewed by Krieg and Vollmer [117], there are no reports on the induction of a systemic autoimmune disease by topical imiquimod treatment. There are a few case reports where imiquimod treatment induced the localized autoimmune skin disorders vitiligo and pemphigus [118, 119] and exacerbated the immune-mediated disease psoriasis [120]. However, imiquimod did not aggravate pre-existing vitiligo [121] and was even successful in the treatment of discoid lupus erythematosus [122]. In conclusion, TLR7/8 agonists seem to have the quality of facilitating the induction of effective antitumor immune responses without inducing systemic autoimmune diseases. However, careful examination remains necessary to further evaluate the risk of inducing autoimmune diseases.

Building on the promising data with imiquimod, other synthetic TLR7/8 agonists were developed in order to refine the properties of these agonists in terms of pharmacokinetics, toxicity, amount of cytokines induced, and TLR specificity. These agonists (like stabilized ssRNA, R-848, and 852A) have now also been tested as IRMs in preclinical models [123] and in clinical trials [75, 124, 125]. Until now, TLR7/8 agonists were predominantly given by topical administration because of the side effects reported after oral and systemic therapy with IRMs [126–129]. The transient reduction in viral levels by oral administration of imiquimod to patients with chronic hepatitis C virus infection was accompanied by adverse effects similar to those seen with exogenous IFN- treatment (influenza-like symptoms such as fever, malaise, and nausea) [129]. Systemic administration of resiquimod, in turn, is associated with transient local immune insufficiency [130]. In recent reports by the groups of Miller and Urosevic [75, 125], i.v. administration of the TLR7 agonist 852A was tested in patients with different tumor types. This TLR7 agonist was well tolerated with transient adverse effects, and 852A induced systemic immune activation. However, i.v. administration of the specific TLR7 agonist 852A did not result in strong antitumoral activity against CLL cells [131], although leukemic skin infiltrates in a patient with CLL disappeared after treatment with imiquimod [106]. In conclusion, alteration of the structure of synthetic TLR7/8 agonists might result in molecules with pharmacological features that make them a suitable treatment method for tumors other than skin tumors. However, the route of administration and the choice of TLR7/8 agonist might be important factors defining the outcome of TLR7/8 administration on tumor survival.

	HOW CAN CANCER VACCINE APPROACHES BENEFIT FROM TLR7/8 LIGANDS?

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
To date, most reports on the effectiveness of TLR7/8 agonists used imiquimod as an immune stimulant for topical treatment of skin tumors. However, TLR7/8 agonists can also be administered as cancer vaccine adjuvants, together with TAAs. The inclusion of TLR7/8 agonists in cancer vaccine protocols could be very advantageous because they influence antitumor reactions in several ways simultaneously. These mechanisms include: (a) activation of APCs, (b) stimulation of crosspriming, (c) activation of CD4⁺ T cells, CD8⁺ T cells, and NK cells, (d) creation of an inflammatory tumor microenvironment, (e) downregulation of the suppressive mechanisms of Treg cells, (f) upregulation of tumor cell immunogenicity and apoptosis, and (g) prevention of angiogenesis (summarized in Fig. 2).

A variety of TLR7/8 agonists and administration routes have already been tested, as listed in Table 1. To date, the experiments on TLR7/8 agonists as vaccine adjuvants were predominantly performed in animal models. Imiquimod was used first as a vaccine adjuvant in a guinea pig model of HSV. Both imiquimod and the imiquimod–protein vaccine combination significantly reduced genital HSV recurrences, but the combination resulted in a longer duration and greater extent of protection for recurrences than with imiquimod alone [132, 133]. Also, in mouse and macaque models of HIV-1, i.m. or s.c. injection of TLR7/8 agonists as adjuvants resulted in enhanced CTL and Th1 antibody responses [69, 134–136]. In addition to models with viral antigens, s.c. injection of the TLR7/8 agonists imiquimod, resiquimod, poly(U), and ORNs resulted in enhanced antigen-specific T-cell activation and Th1 antibody responses in ovalbumin (OVA) mouse models [60, 137–139]. However, more attention has been paid to topical administration of imidazoquinolines, evaluated in OVA, HIV-1, and melanoma models. With this application method, effective anti-OVA or antitumor immune responses were also elucidated, even when OVA peptides were delivered transcutaneously together with imiquimod in an ointment [65, 140, 141]. Craft et al. [78] observed that the antitumor activity of imiquimod was not dependent on direct application of imiquimod to the tumor site, whereas Tomai et al. [142] reported that the greater Th1 antibody response was only found if resiquimod was applied at the vaccination site. Differences in their results can be explained by the use of different TLR7/8 agonists (imiquimod versus resiquimod), different models (a live recombinant Listeria monocytogenes vaccine in mice versus an OVA protein vaccination in rats), or the detection of different immune responses (antitumor activity versus antibody response).

In addition to topical administration of TLR7/8 agonists, i.p. injection of imidazoquinolines also resulted in protective antitumor immune responses in a transgenic mouse model of mammary carcinoma, in which the plasmid encoding the TAA was administered by a gene gun method [61].

However, despite reports of the successful induction of antigen-specific T-cell and Th1 antibody responses, Weeratna et al. [144] could not find a greater immune response against hepatitis B surface antigen when resiquimod was injected as an adjuvant, in contrast to the results obtained using the TLR9 ligand CpG as an adjuvant. Others also showed that CpG induced higher levels of T-cell responses than imiquimod or resiquimod [135, 145]. Two methods are known to further enhance the adjuvant power of TLR7/8 agonists: (a) conjugation of the TLR7/8 agonist to the antigen [135] and (b) combination of TLR7/8 agonists with other factors. Injection of an anti-CD40 antibody (to provide costimulation) in combination with TLR/8 ligands resulted in enhanced CTL activation with a reduction of lung metastases [146], effective induction of memory cells, and greater tumor protection [141].

In conclusion, reports on the use of TLR7/8 agonists as vaccine adjuvants (Table 1) revealed that the following factors might be crucial for their effectiveness: (a) the route of administration of TLR7/8 ligands, (b) the site of topical imiquimod or resiquimod administration [142], (c) the choice of TLR7/8 agonist, (d) the combination of TLR7/8 agonists with other factors like CD40 stimulation, (e) the route of administration of the antigens [147], and (f) the proximity of the agonist and the antigen [135]. It has not been completely clarified yet why different TLR7/8 agonists induce different responses. This is probably related to different binding affinities to TLR7 and/or TLR8 and to different pharmacokinetic properties. In view of this, high-resolution x-ray crystallography will provide more detailed knowledge on TLR7/8 ligand/receptor interactions [148], thereby facilitating the design of potent TLR7/8 agonists.

TLR7/8 agonists can also prove useful in cancer vaccines for the in vitro maturation of APCs. As described above, TLR7/8 agonists induce efficient maturation and activation of DCs, and these cells can be used in DC-based cancer vaccines. Therefore, R-848 has been included in maturation cocktails to generate mature clinical-grade DCs [149, 150]. Furthermore, DCs have been shown to exhibit a higher Th1-polarizing potential if TLR7/8 agonists are combined with other factors, like TLR3 or TLR4 agonists and CD40 ligand [151–155]. A study by Xu et al. [47] showed that R-848 acts as both a priming and maturation signal to obtain mature myeloid type 1–polarized DCs that secrete IL-12p70, leading to the in vitro generation of high-avidity antitumor T cells. From these data and the data on the combination of TLR7/8 agonists with anti-CD40 antibodies (Table 1), we deduce that the most potent cancer vaccines will combine TAA delivery with activation of TLR7/8, activation of TLR3 or TLR4, and stimulation of CD40. In addition, the topical administration of imiquimod was tested in mice and humans for the in vivo maturation of immature DCs (Table 1) [143, 156–158]. In vivo maturation of immature DCs by imiquimod enhanced their migratory capacity, immunostimulatory capacity, and antitumor activity to levels similar to or higher than those seen with ex vivo–matured DCs [156, 157]. Optimizing this protocol holds the promise of obtaining superior mature DCs, resulting in more effective antitumor immune stimulation with less manipulations than with ex vivo–matured DCs.

	HOW COULD CURRENT ADOPTIVE IMMUNOTHERAPY APPROACHES BENEFIT FROM TLR7/8 LIGANDS?

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
It has been shown for T-cell based immunotherapy that better cancer regression results are obtained if patients undergo TBI prior to T-cell transfer. One of the reasons for this is the activation of the innate immune system via bacterial translocation of commensal gut microbes, resulting in activation of the adoptively transferred T cells. Gut microbes bear several TLR ligands, with the TLR4 ligand LPS shown to be principally responsible for the effectiveness of TBI [159]. In mice, bacterial translocation resulted in an increase in splenic CD11c⁺CD86^hi DCs capable of activating adoptively transferred T cells. In addition, higher levels of proinflammatory cytokines were detected, as well as higher levels of IL-12 [159]. Paulos et al. [4] tried to mimic the effectiveness of lymphodepleting preparative regimens by the use of clinical reagents. LPS, however, cannot be used in clinical trials because of its toxicity [160]. Although administration of imiquimod did not enhance adoptive immunotherapy results in irradiated mice [4], the authors state that it might be effective for treating human patients because of the different expression pattern and functionality of TLRs between mice and humans.

In addition to mimicking commensal bacterial leakage, TLR7/8 agonists can enhance immune activation, as described above for cancer vaccines. Although TBI deprives a cancer patient of his/her own immune system, TLR7/8 agonists might enhance the persistence and responsiveness of the adoptively transferred antitumor T cells. Furthermore, the agonists influence the tumor cells themselves and create a tumor microenvironment that is more favorable for effective antitumor responses.

	CONCLUSION

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
Because of their pleiotropic effects, activating several immune cells simultaneously, TLR7/8 agonists are able to boost the immune system. It has now been profoundly examined whether this boost results in the induction of effective antitumor responses and long-lasting clearance of tumor cells. For the treatment of primary or secondary skin tumors, imiquimod has been proven effective as a stand-alone immunotherapeutic agent. It was found that direct induction of tumor cell apoptosis and greater sensitivity of tumor cells to killing contribute to the antitumor effects of TLR7/8 agonists, as well as the mix of chemokines and cytokines that are released by activated cells (not only immune cells but also tumor cells and keratinocytes) into the tumor microenvironment. In addition to their use as a stand-alone immunotherapeutic agent, TLR7/8 agonists hold promise as adjuvants in cancer vaccine or adoptive T-cell transfer protocols. Extension of the families of synthetic TLR7/8 agonists paves the way to the development of TLR7/8 agonists that are well tolerated, broadly applicable, and effective as adjuvants. The combination of TLR7/8 agonists with other factors, like agonists of TLR3 or TLR4 or anti-CD40 antibodies, amplifies their capacity to enhance immune responses. Ongoing and future preclinical and clinical studies will clarify whether or not TLR7/8 agonists can fulfill their promise as effective antitumor treatments and/or vaccine adjuvants.

	AUTHOR CONTRIBUTIONS

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
Conception/design: Evelien L.J.M. Smits

Manuscript writing: Evelien L.J.M. Smits, Viggo F.I. Van Tendeloo

Final approval of manuscript: Peter Ponsaerts, Zwi N. Berneman, Viggo F.I. Van Tendeloo

	ACKNOWLEDGMENTS

Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 
This work was supported by grants no. G.0370.08 and G.0082.08 of the Fund for Scientific Research, Flanders, Belgium (FWO-Vlaanderen), by research grants of the Foundation Against Cancer (Stichting tegen Kanker), by grant no. 802 of the Antwerp University Concerted Research Action (BOF-GOA), and by a part of the Interuniversity Attraction Poles (IAP) program financed by the Belgian Government. E.L.J.M. Smits holds a Ph.D. fellowship of the FWO-Vlaanderen.

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Top Learning Objectives Abstract Introduction: Current Status of... TLR7 and TLR8 Ligands Effects of TLR7/8 Ligands... Current Use of TLR7/8... How Can Cancer Vaccine... How Could Current Adoptive... Conclusion Author Contributions Acknowledgments References

 

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