Peritumoral CpG DNA elicits a coordinated response of CD8 T cells and innate effectors to cure established tumors in a murine colon carcinoma model

Challenges and prospects of immunotherapy as cancer treatment

The concept of cancer immunotherapy stems from the proposed function of the immune system, called immunosurveillance, to protect against growing tumors. Due to genetic aberrations, tumor cells display an altered repertoire of MHC-associated peptides that can lead to the activation of immune cells able to eliminate the transformed cells. In some instances, under the pressure of the immune system, both the tumor and its microenvironment are shaped and immune-resistant tumor variants are selected initiating the process of cancer immunoediting. This can impair not only host-generated immunosurveillance, but also attempts to harness the immune response for therapeutic purposes, namely immunotherapy. Rather than being an exhaustive review of the different approaches of cancer immunotherapy, the focus of this review is to provide the reader with future challenges of the field by proposing 'second generation' immunotherapy approaches that take into account immunosubversive mechanisms adopted by tumor cells. After an introduction on the process of immunosurveillance and immunoescape we will analyze why current immunotherapy approaches have not fulfilled their promise and will finish by summarizing what are the challenges for future approaches.

Bacterial DNA containing methylated CpG motifs retains immunostimulatory activity in synergy with modified lipopolysaccharides

We previously described the immunostimulatory activity of CIA07, a combination of bacterial DNA fragments and modified LPS, and demonstrated that CIA07 has antitumor activity in a mouse bladder cancer model. In this study, we investigated whether methylation of the CpG motifs on the bacterial DNA fragments affects the immunostimulatory potential of CIA07. E. coli DNA fragments were methylated with CpG methylase, and then combined with modified LPS for experiments. Our results revealed that methylated CIA07 (mCIA07) and unmethylated CIA07 were equally active in inducing cytokine secretion from human whole blood cells. In addition, both methylated DNA fragments and mCIA07 retained the ability to activate expression and nuclear translocation of NF-kappaB in RAW 264.7 cells. Finally, methylated DNA fragments and mCIA07 exhibited an antitumor activity comparable to those of their unmethylated counterparts in our mouse bladder cancer model. These data demonstrate that CpG methylation of E. coli DNA does not abrogate the immunostimulatory activity of DNA fragments or CIA07, suggesting that the synergistic activity by bacterial DNA in combination with LPS may be independent of the methylation status of CpG motifs.

Improved systemic pharmacokinetics, biodistribution, and antitumor activity of CpG oligodeoxynucleotides complexed to endogenous antibodies in vivo

CpG oligodeoxynucleotides (CpG-ODNs) fail to elicit antitumor immunity after intravenous administration presumably due to their rapid renal clearance and low tumor accumulation. To address this issue, we tested the hypothesis that endogenous IgG can be used as systemic drug carriers to improve the pharmacokinetics, tumor accumulation, and antitumor activity of intravenously administered CpG-ODNs. To this end, tritium-labeled CpG-ODNs conjugated with one or two dinitrophenyl (DNP) haptens (DNP- and DNP(2)-[(3)H]-CpG-ODN) were intravenously dosed into DNP-immunized Balb/c mice bearing subcutaneous CT26 colorectal tumors. Serum and tissue samples for pharmacokinetic and biodistribution profiling were collected at predetermined timepoints and analyzed by liquid scintillation. In antitumor efficacy studies, DNP-immunized, CT26 tumor-bearing mice were intravenously dosed with PBS, CpG-ODN, or DNP-CpG-ODN every five days. Tumor volumes and macroscopic and histological examination of resected solid tumors were used to quantitatively and qualitatively assess tumor growth inhibition. Relative to [(3)H]-CpG-ODN, dinitrophenylated [(3)H]-CpG-ODNs displayed substantial increases in systemic exposure (900-1650 fold) and half-life (100-300 fold), marked decreases in systemic clearance (750-1500 fold) and volume of tissue distribution (13-37 fold), as well as substantial and sustained tumor accumulation (approximately 30% vs. <2% injected dose/g). Antitumor efficacy studies demonstrated that DNP-CpG-ODN inhibited tumor growth by up to 60% relative to PBS control whereas CpG-ODN treatment had no apparent effect. Macroscopic and histological examination of harvested tumors at various timepoints revealed the presence of regions of necrotic tissue only in tumors from mice treated with DNP-CpG-ODN. Collectively, these results show the potential of endogenous IgG to mediate the systemic delivery of CpG-ODN to solid tumors and to enhance their antitumor activity following intravenous administration.

Immunostimulatory properties of CpG-oligonucleotides are enhanced by the use of protamine nanoparticles

The aim of this paper was to investigate if the immunostimulatory effects of CpG-oligonucleotides (CpG-ODN) can be enhanced by the use of biodegradable protamine nanoparticles (proticles). We analyzed size, surface charge, and morphology of protamine nanoparticles containing CpG-ODN with photon correlation spectroscopy and transmission electron microscopy. Immunostimulatory effects of these nanoparticles on B cells, plasmacytoid dendritic cells (PDC), peripheral blood mononuclear cells, and whole blood were studied. Cytokine production, activation of the cells in terms of upregulation of surface molecules and uptake of nanoparticles were examined. We found that the use of protamine nanoparticles significantly increased (20-fold) CpG-ODN mediated interferon (IFN)-alpha production of PDC. ODN uptake in PDC was only marginally enhanced. CpG-ODN mediated IP-10 production in whole blood was strongly enhanced by the use of nanoparticles. Apart from a slight increase in CpG-ODN-induced interleukin (IL)-6 production in B cells, other parameters like the CpG-mediated activation of B cells and PDC as well as tumor necrosis factor (TNF)-alpha production of PDC remained largely unchanged. The use of control ODN indicated that the protamine nanoparticles themselves have no immunostimulatory properties. These results strongly support the use of particulate delivery systems like biodegradable protamine nanoparticles for the development of CpG-ODN-based therapeutics.

Macrophages are essential for antitumour effects against weakly immunogenic murine tumours induced by class B CpG-oligodeoxynucleotides

We explored the mechanisms of class B CpG-oligodeoxynucleotide-induced antitumour effects against weakly immunogenic tumours. Treatment with CpG-oligodeoxynucleotide 1826 (CpG) induced similar antitumour effects in B16 melanoma-bearing immunocompetent C57BL/6 mice and T-cell-deficient severe combined immunodeficient (SCID) mice, and NXS2 neuroblastoma-bearing T-cell-depleted A/J mice. Both macrophages (Mphi) and natural killer (NK) cells from CpG-treated C57BL/6 mice could mediate cytotoxicity in vitro, suggesting that these cell types might control tumour growth in vivo. However, CpG treatment of SCID/beige mice or T-cell-depleted and NK-cell-depleted A/J mice still induced antitumour effects in vivo, arguing against a major role of NK cells in the antitumour effects of CpG in the absence of T cells. In contrast, CpG treatment of interferon-gamma knockout (IFN-gamma(-/-)) C57BL/6 mice resulted in no antitumour effects in vivo and no Mphi-mediated tumoristasis in vitro despite unaltered cytolytic function of NK cells in vitro. Moreover, Mphi inactivation by silica substantially reduced CpG-induced suppression of tumour growth in vivo, revealing an important role of Mphi in CpG-induced antitumour effects. The in vitro tumouritoxicity by CpG-stimulated Mphi (CpG-Mphi) correlated with tumour cell mitochondria dysfunction and involved nitric oxide (NO), tumour necrosis factor-alpha (TNF-alpha) and IFN-gamma, whereas interleukin-1alpha (IL-1alpha), IL-1beta, IFN-alpha, TNF-related apoptosis-inducing ligand and Fas ligand played insignificant roles in CpG-Mphi tumouritoxicity. Taken together, our results indicate that the growth control of weakly immunogenic tumours during CpG-immunotherapy is mediated predominantly by Mphi, rather than T cells or NK cells.

MUC1-specific immune therapy generates a strong anti-tumor response in a MUC1-tolerant colon cancer model

A MUC1-based vaccine was used in a preclinical model of colon cancer. The trial was conducted in a MUC1-tolerant immune competent host injected with MC38 colon cancer cells expressing MUC1. The vaccine included: MHC class I-restricted MUC1 peptides, MHC class II-restricted pan-helper-peptide, unmethylated CpG oligodeoxynucleotide, and granulocyte macrophage-colony stimulating factor. Immunization was successful in breaking MUC1 self-tolerance, and in eliciting a robust anti-tumor response. The vaccine stimulated IFN-gamma-producing CD4(+) helper and CD8(+) cytotoxic T cells against MUC1 and other undefined MC38 tumor antigens. In the prophylactic setting, immunization caused complete rejection of tumor cells, while in the therapeutic regimen, tumor burden was significantly reduced.

Synergy between in situ cryoablation and TLR9 stimulation results in a highly effective in vivo dendritic cell vaccine

Dendritic cells (DC) are professional antigen-presenting cells that play a pivotal role in the induction of immunity. Ex vivo-generated, tumor antigen-loaded mature DC are currently exploited as cancer vaccines in clinical studies. However, antigen loading and maturation of DC directly in vivo would greatly facilitate the application of DC-based vaccines. We have previously shown that in situ tumor destruction by ablative treatments efficiently delivers antigens for the in vivo induction of antitumor immunity. In this article, we show that although 20% of the draining lymph node DCs acquire intratumorally injected model antigens after in situ cryoablation, only partial protection against a subsequent tumor rechallenge is observed. However, we also show that a combination treatment of cryoablation plus TLR9 stimulation via CpG-oligodeoxynucleotides is far more effective in the eradication of local and systemic tumors than either treatment modality alone. Analysis of the underlying mechanism revealed that in situ tumor ablation synergizes with TLR9 stimulation to induce DC maturation and efficient cross-presentation in tumor-bearing mice, leading to superior DC function in vivo. Therefore, in situ tumor destruction in combination with CpG-oligodeoxynucleotide administration creates a unique "in situ DC vaccine" that is readily applicable in the clinic.

Oligodeoxynucleotide CpG 7909 Delivered as Intravenous Infusion Demonstrates Immunologic Modulation in Patients With Previously Treated Non-Hodgkin Lymphoma

Oligodeoxynucleotides containing CpG motifs (CpG ODN) can alter various immune cell subsets important in antibody therapy of malignancy. We undertook a phase I trial of CPG 7909 (also known as PF-3512676) in patients with previously treated lymphoma with the primary objective of evaluating safety across a range of doses, and secondary objectives of evaluating immunomodulatory effects and clinical effects. Twenty-three patients with previously treated non-Hodgkin lymphoma received up to 3 weekly 2-hour intravenous (IV) infusions of CPG ODN 7909 at dose levels 0.01 to 0.64 mg/kg. Evaluation of immunologic parameters and clinical endpoints occurred for 6 weeks. Infusion-related toxicity included grade 1 nausea, hypotension, and IV catheter discomfort. Serious adverse hematologic events observed more than once included anemia (2=Gr3, 2=Gr4), thrombocytopenia (4=Gr3), and neutropenia (2=Gr3), and were largely judged owing to progressive disease. Immunologic observations included: (1) The mean ratio of NK-cell concentrations compared with pretreatment at day 2 was 1.44 (95% CI=0.94-1.94) and at day 42 was 1.53 (95% CI=1.14-1.91); (2) NK activity generally increased in subjects; and (3) Antibody-dependent cellular cytotoxicity activity increased in select cohorts. No clinical responses were documented radiographically at day 42. Two subjects demonstrated late response. We conclude CpG 7909 can be safely given as a 2-hour IV infusion to patients with previously treated non-Hodgkin lymphoma at doses that have immunomodulatory effects.

CpG oligodeoxynucleotides are potent enhancers of radio- and chemoresponses of murine tumors

BACKGROUND AND PURPOSE

Synthetic oligodeoxynucleotides (ODNs) containing unmethylated cytosine-guanine (CpG) motifs bind to Toll-like receptor 9 (TLR9) and stimulate both innate and adaptive immune reactions and possess anti-tumor activity. We recently reported that CpG ODN 1826 strongly enhances radioresponse of both immunogenic [Milas L, Mason K, Ariga H, et al. CpG oligodeoxynucleotide enhances tumor response to radiation. Cancer Res 2004;64:5074-7] and non-immunogenic [Mason KA, Ariga H, Neal R, et al. Targeting toll-like receptor-9 with CpG oligodeoxynucleotides enhances tumor response to fractionated radiotherapy. Clin Cancer Res 2005;11:361-9] murine tumors. Using two immunogenic murine tumors, a fibrosarcoma (FSa) and a mammary carcinoma (MCa-K), the present study explored whether CpG ODN 1826 also improves the response of murine tumors to the chemotherapeutic agent docetaxel (DOC).

MATERIALS AND METHODS

CpG ODN 1826 (100 microg) was given sc three times: when leg tumors were 6mm, when they grew to 8mm and again 1 week later. DOC (33 mg/kg iv) and local tumor radiation (10Gy) were given when tumors were 8mm. Effects of the treatments were assayed by tumor growth delay, defined as days for tumors to grow from 8 to 12 mm in diameter.

RESULTS

Treatment with CpG ODN 1826 resulted in strongly enhanced response of FSa tumors to radiation and MCa-K tumors to the chemotherapeutic agent DOC. Enhancement of tumor treatment response was demonstrated by a strong prolongation in the primary tumor treatment endpoint, tumor growth delay. Coincidentally, this treatment also resulted in a higher rate of tumor cure than that observed after tumor radiotherapy or chemotherapy alone. When all three agents were combined the effect was comparable to that of the combination of CpG ODN 1826 with radiation in the case of FSa or of the combination of CpG ODN 1826 with DOC in the case of MCa-K.

CONCLUSION

Overall results show that CpG ODN 1826 can markedly improve tumor response to radiation and chemotherapy (DOC), suggesting that CpG ODNs have potential to be beneficial when used singly or in combination with other standard treatment modalities such as taxane chemotherapy, radiotherapy or both.

Therapeutic efficacy of antigen-specific vaccination and toll-like receptor stimulation against established transplanted and autochthonous melanoma in mice

Malignant melanoma is an attractive model disease for the development of antigen-specific immunotherapy because many antigens recognized by tumor-specific T cells have been identified. In C57BL/6 mice, genetic immunization with recombinant adenovirus encoding xenogeneic human tyrosinase-related protein 2 (Ad-hTRP2) induces protective but not therapeutic cellular immunity against growth of transplanted B16 melanoma cells. Here, we additionally applied CpG DNA and synthetic double-stranded RNA, which activate the innate immune system via Toll-like receptors (TLR). Both adenoviral vaccination and peritumoral injections of TLR ligands were required for rejection of established B16 melanoma in the skin. To more closely mimic the clinical situation in patients with melanoma, we evaluated this combined immunotherapeutic strategy in genetically modified mice, which overexpress hepatocyte growth factor (HGF) and carry an oncogenic mutation in the cyclin-dependent kinase 4 (CDK4)(R24C). HGF x CDK4(R24C) mice rapidly develop multiple invasive melanomas in the skin following neonatal carcinogen treatment, which spontaneously metastasize to lymph nodes and lungs. Vaccination with Ad-hTRP2 followed by injections of TLR ligands resulted in delayed growth of autochthonous primary melanomas in the skin and reduction in the number of spontaneous lung metastases but did not induce tumor regression. Carcinogen-treated HGF x CDK4(R24C) mice bearing multiple autochthonous melanomas did not reject transplanted B16 melanoma despite treatment with Ad-hTRP2 and TLR ligands, suggesting the development of tumor immunotolerance. Further investigations in our novel genetic melanoma model may help to better understand the role of the immune system in the pathogenesis and treatment of this life-threatening disease.

Antitumor efficacy of the novel chemotherapeutic agent coramsine is potentiated by cotreatment with CpG-containing oligodeoxynucleotides

Coramsine is a novel chemotherapeutic agent isolated from Solanum linnaeanum (devil's apple). Topical treatment provides clinical benefit for skin tumors. To evaluate the potential broader applicability of the drug, its in vivo anticancer efficacy in a murine model of malignant mesothelioma and its mode of action were investigated. Systemic administration of coramsine slowed tumor growth and prolonged survival time. Importantly, the antitumor efficacy of coramsine was enhanced when treatment was combined with stimulation of innate immunity using unmethylated CpG-containing oligodeoxynucleotides (ODNs). Combination treatment further slowed tumor growth and provided a survival benefit. Coramsine seems to kill tumor cells by direct cell lysis. Using 2 different assays to detect apoptosis (caspase activation and DNA fragmentation), we found no evidence that coramsine induces any form of programmed cell death. The fact that the efficacy of coramsine is potentiated by CpG ODNs suggests that coramsine-induced cell death is an immunologic null event.

Immunotherapy with dendritic cells and CpG oligonucleotides can be combined with chemotherapy without loss of efficacy in a mouse model of colon cancer

Although immunotherapy has shown promising results in the treatment of cancer, clinical studies assessing immunologic approaches in patients with advanced cancer will seldom be conducted in the absence of conventional treatment strategies such as chemotherapy. Here we investigate the combination of chemotherapy with CpG oligonucleotide and dendritic cell-based immunotherapy in the C26 mouse model of colon carcinoma. The coinjection of antigen-pulsed, mature dendritic cells and CpG oligonucleotides together with a peritumoral injection of CpG oligonucleotides elicits a CD8 T-cell response resulting in tumor rejection and long-term protection in the C26 model. Tumor-bearing mice were treated weekly for 4 weeks by this immunotherapy protocol, by 5-fluorouracil plus leucovorin or irinotecan, or by the combination of immunotherapy and chemotherapy. We observed that immunotherapy was more effective in reducing tumor growth and increasing survival than 5-fluorouracil or irinotecan. Immunotherapy was well tolerated, whereas therapeutic doses of 5-fluorouracil or irinotecan were associated with dose-limiting toxicity. Furthermore, the efficacy of immunotherapy combined with either 5-fluorouracil or irinotecan was similar to that of immunotherapy alone. Addition of immunotherapy to either 5-fluorouracil or irinotecan treatment strongly decreased the toxicity of chemotherapy. Immunotherapy both with and without chemotherapy generated a memory immune response, leading to tumor rejection in mice rechallenged with C26 tumor cells up to several months after treatment. In summary, immunotherapy with a combination of dendritic cells and CpG oligonucleotides is superior to chemotherapy in the C26 tumor model. This immunotherapy protocol can be combined with current chemotherapy agents with no loss in therapeutic activity.

A combination of E. coli DNA fragments and modified lipopolysaccharides as a cancer immunotherapy

The use of Escherichia coli DNA or lipopolysaccharide (LPS) as an immunotherapy is often associated with unacceptable toxicity and insufficient therapeutic effects. In this study, we investigated the efficacy of using a combination of bacterial DNA fragments and LPS as an anticancer agent. LPS was isolated from an E. coli strain expressing short-carbohydrate-chain-containing LPS and subjected to alkaline hydrolysis to remove lipid A. The ability to induce tumor necrosis factor-alpha (TNF-alpha) release in human whole blood cells was significantly lower for the LPS devoid of lipid A than for its parent form. The immunostimulating activity of E. coli DNA fragments of various sizes were tested. Those of 0.2-0.5 kb in size exhibited the highest activity in whole blood assays, whereas those of size 0.5-2.0 kb exhibited the highest adjuvant activity in mice. A combination of 0.5-2.0-kb DNA fragments and modified LPS at a ratio of 100:1, designated CIA07, exhibited higher immunostimulating activity than each substance alone, and its antitumor activity was significantly higher than that of Bacillus Calmette-Guerin in a mouse bladder cancer model. An intraperitoneal injection of CIA07 at a dose of 25mg/kg body weight caused no apparent adverse effects in mice and guinea pigs. Taken together, these data demonstrate that CIA07 exhibits potent immunostimulating activity with no apparent toxicity, and therefore warrant the further development of CIA07 as an immunotherapy for cancer treatment.

Progress in drug development of immunostimulatory CpG oligodeoxynucleotide ligands for TLR9

Oligodeoxynucleotides (ODNs) with unmethylated deoxycytosine-deoxyguanosine (CpG) motifs are recognised by Toll-like receptor (TLR)9 expressed in specialised cell subsets of the human immune system, B cells and plasmacytoid dendritic cells. TLR9-mediated stimulation of the immune system leads to a plethora of directed effects linking innate to adaptive immune responses. This allows the use of TLR9 agonists as highly effective targeted immunomodulatory drugs with broad potential applications as vaccine adjuvants, stand-alone therapy or in combination with other therapies in cancer, infectious diseases or asthma and allergy. TLR9 agonists represent a new class of small (8-30 bases long), easily synthesised, non-antisense ODN pharmaceuticals.

Inhibitory effects of unmethylated CpG oligodeoxynucleotides on MHC class I-deficient and -proficient HPV16-associated tumours

Unmethylated oligodeoxynucleotides containing guanine-cytidine dimers (CpG ODN) have been described as potent inducers of selected antitumour immune responses and the immunotherapeutic efficacy of CpG ODN has been examined either alone or as a vaccine adjuvant. We hypothesized that CpG ODN therapy could be an effective tool for immunotherapy of not only conventional MHC class I(+) tumours but also of those tumours that have lost MHC class I expression during their progression. To address this hypothesis, we employed the animal model resembling MHC class I-proficient and -deficient human papilloma virus (HPV) 16-associated tumours. A cell line transformed with HPV16 E6 and E7 oncogenes, TC-1, as a prototype of MHC class I-positive line, and its MHC class I-deficient sublines TC-1/A9 and TC-1/P3C10 were injected into syngeneic C57BL/6 mice and the growing tumours were subjected to immunotherapy with CpG ODN 1826. The therapy started either 1 day after the challenge with the tumour cells or later, when the tumours had reached a palpable size. In both settings, CpG ODN 1826 significantly reduced the growth of MHC class I-proficient and -deficient tumours. Furthermore, we demonstrated that CpG ODN 1585, whose mechanism of action preferably involves indirect activation of the natural killer cells, induced regression of the MHC class I-deficient tumours TC1/A9 but not of the MHC class I-proficient tumours TC-1. This study infers that synthetic CpG ODN have a potential for the therapy of both MHC class I-proficient and -deficient tumours and thus could be also used against tumours that tend to down-regulate their MHC class I expression.

G3139 and Other CpG-Containing Immunostimulatory Phosphorothioate Oligodeoxynucleotides Are Potent Suppressors of the Growth of Human Tumor Xenografts in Nude Mice

Several phosphorothioate antisense oligodeoxynucleotides (ODN) are developed to target factors potentially involved in tumor growth and apoptosis suppression. Among them, the 18-mer G3139 (Oblimersen), which targets Bcl-2, is currently being tested in phase II and phase III clinical trials for various tumors in combination with chemotherapy. On the other hand, ODNs containing CpG dinucleotides (CpG-ODN) within specific-sequence contexts (CpG motifs) have been shown to activate rodent or primate immune cells via toll-like receptor 9 (TLR9) and have demonstrated remarkable T cell-dependent antitumor efficacy in a series of murine tumor models. However, immune cell activation by CpG-ODN is largely diminished upon C-5 methylation at CpG cytosine. As G3139 contains CpG motifs, we questioned whether the antitumor effects seen in human tumor xenografts might be abrogated by cytosine C-5 methylation of G3139, which retained the ability of G3139 to suppress Bcl-2 expression in tissue culture, or by similar derivatization of other phosphorothioate ODNs developed for the immune activation of rodent or human cells. The in vivo antitumor efficacy of the immunostimulatory H1826 and H2006 ODNs was compared with that of G3139. Bcl-2 suppression achieved by G3139 purportedly sensitizes tumor cells toward cytotoxic agents, and some of the experiments employed combinations of ODN with such drugs as cisplatin or etoposide. H1826, H2006, and G3139 all produced similar, striking, growth inhibitory effects on either H69 SCLC, A2780 ovarian carcinoma, or A549 lung adenocarcinoma human tumor xenografts at doses of 0.3 mg/kg and 1 mg/kg (H1826, H2006) or 12 mg/kg (G3139) per day. In contrast, the H2006-mC (1 mg/kg) or G3139-mC (12 mg/kg) derivatives demonstrated no significant antitumor effects. The combination of G3139 (12 mg/kg) with cisplatin produced some additive antitumor efficacy, which was not seen in combinations of G3139-mC (12 mg/kg) or H1826 (1 mg/kg) with cisplatin. G3139, at a dose of 12 mg/kg, alone induced extensive enlargement of the spleen. Immunostimulation was evaluated in vitro by flow cytometric measurements of the CD80 and CD86 activation markers found on CD19+ murine splenocytes. The CpG-ODN producing strong antitumor effects in vivo also induced these activation markers in vitro, in contrast to the in vivo inactive G3139-mC. Our data indicate a significant contribution of the immunostimulatory properties of CpG-ODN (including G3139) to the antitumor effects observed in nude mouse xenograft models. This is in contrast to previous data presented by other authors indicating that the activity of G3139 in human tumor xenografts was Bcl-2 specific. Furthermore, as nude mice are devoid of T cells, a T cell-mediated immune response apparently is not required for the potent antitumor responses observed here; innate immune responses are sufficient.

Virus or TLR agonists induce TRAIL-mediated cytotoxic activity of plasmacytoid dendritic cells

Among dendritic cells, plasmacytoid dendritic cells (PDC) represent a functionally distinct lineage. Regarding innate immunity, PDC secrete large amounts of type I IFN upon viral exposure or stimulation by microbial products such as unmethylated CpG-motif containing oligo-DNA due to their selective expression of TLR7 and TLR9. We asked whether they could acquire cytotoxic functions during the early phases of infection or after activation with TLR7 or TLR9 agonists. In the present study, we describe a human PDC cell line called GEN2.2, derived from leukemic PDC, that shares most of the phenotypic and functional features of normal PDC. We show that after contact with the influenza virus, GEN2.2, as well as normal PDC, acquires TRAIL and killer activity against TRAIL-sensitive target cells. Moreover, we show that activation of GEN2.2 cells by CpG-motif containing oligo-DNA or R848 also induces TRAIL and endows them with the ability to kill melanoma cells. Therefore, PDC may represent a major component of innate immunity that could participate to the clearance of infected cells and tumor cells. This phenomenon could be relevant for the efficacy of TLR7 or TLR9 agonists in the therapy of infectious disease and cancer.

CpG oligonucleotide therapy cures subcutaneous and orthotopic tumors and evokes protective immunity in murine bladder cancer

Bacillus Calmette-Guerin (BCG) instillation is standard immunotherapy for superficial bladder carcinoma. However, many patients become refractory to BCG, giving impetus to the development of alternative therapies. CpG oligodeoxynucleotide (ODN) therapy has been shown to promote T(H)1-oriented antitumor responses in various tumor models. To investigate its therapeutic effect in bladder cancer, we used different CpG ODNs to treat C57BL/6 mice bearing the subcutaneous murine bladder tumor MB49. CpG type B ODN 1668 was superior at inhibiting tumor growth, leading to complete regression of large tumors. More importantly, CpG ODN 1668 also regressed orthotopically growing MB49 tumors for the first time. Rechallenge of CpG ODN-cured mice with MB49 showed that a majority of the mice were protected long term, demonstrating that CpG ODN therapy evokes a memory response. Adenoviral vectors (Ad) encoding CD40L, tumor necrosis factor-related activation-induced cytokine, lymphotactin, interleukin (IL) 2, and IL-15 were also investigated. AdCD40L and AdIL-15 transduction could abolish MB49 tumorigenicity, and these vectors were combined with CpG ODN 1668 to investigate any enhanced effects. No such effects were seen. All groups of mice treated with CpG ODNs, alone or in combination with adenoviral vector, exhibited increased serum concentrations of IL-12, indicative of a T(H)1 response. Our results show that CpG ODN therapy cures established subcutaneous and orthotopic bladder cancer via a T(H)1-mediated response and provides long-lasting protective immunity.

Immunostimulatory CpG oligonucleotides reduce tumor burden after intravesical administration in an orthotopic murine bladder cancer model

Bacillus Calmette-Guérin is established in the prophylaxis of recurrent intermediate and high-risk superficial bladder cancer and induces an unspecific, Th1-biased local immune response. Small CpG oligonucleotides (CpG ODN) containing a central unmethylated CpG motif are able to mimic the immunostimulatory activity of bacterial DNA. The purpose of the present study was to evaluate the antineoplastic properties of intravesically administered CpG ODN in an orthotopic murine bladder cancer model. MB49 tumor cell suspension was instilled transurethrally in female C57/BL6 mice on day 0. Mice were divided in three groups of 12 animals. Four mice in each group received either stimulative CpG ODN, non-stimulative GpC ODN or PBS intravesically: group I on day 3, group II on day 5, group III on day 7. After sacrifice 7 days after treatment, bladders were removed and histological examinations were performed. Single instillation of CpG ODN revealed antineoplastic effects in every group demonstrated by significantly lower bladder weight compared with non-stimulative GpC ODN- and PBS-treated mice. Histological examination showed extensive infiltration of macrophages and lymphocytes in CpG ODN-treated mice, whereas PBS- and GpC ODN-treated mice showed solid tumor growth with only few leucocytes. Intravesically applied immunostimulative DNA demonstrated antitumoral activity in an orthotopic murine bladder cancer model. A single instillation seems to be sufficient to reduce tumor load.

B-cell lymphomas differ in their responsiveness to CpG oligodeoxynucleotides

Human B cells detect CpG motifs within microbial DNA via TLR9. Synthetic CpG oligodeoxynucleotides are currently being tested in clinical trials for the therapy of different types of B cell non-Hodgkin's lymphoma. However, there is only limited information on the CpG oligodeoxynucleotide sensitivity of primary malignant B cells of different non-Hodgkin's lymphoma entities. Here we found that most B-cell malignancies except plasmacytoma respond to CpG oligodeoxynucleotides by up-regulating expression of costimulatory and antigen-presenting molecules, by increasing expression of CD20, and by proliferation. In an in vitro analysis of 41 individual patient-derived primary tumor samples, B-cell chronic lymphocytic leukemia (B-CLL) and marginal zone lymphoma showed the strongest activation upon stimulation with CpG oligodeoxynucleotides. Small lymphocytic lymphoma, follicular lymphoma, mantle cell lymphoma, and large cell lymphoma showed an intermediate response. Consistent with CpG oligodeoxynucleotides sensitivity, TLR9 mRNA was present in B-CLL but absent in plasmacytoma. Although CpG oligodeoxynucleotides induced proliferation in all CpG oligodeoxynucleotide-sensitive types of B-cell malignancies, proliferation was weaker than in normal B cells and at least for B-CLL was followed by increased apoptosis. In conclusion, B-cell malignancies show significant differences in their responsiveness to CpG oligodeoxynucleotides. Focusing clinical studies on patients with highly CpG oligodeoxynucleotide-sensitive B-cell malignancies may improve the clinical outcome of such trials.

Serum alters the uptake and biologic activity of CpG oligodeoxynucleotides in B cell chronic lymphocytic leukemia

Immunostimulatory CpG-containing oligodeoxynucleotides (CpG ODN) have a number of effects on B cells, including upregulation of immunogenic molecules, and, therefore, appear attractive as potential components of immunotherapy for B cell chronic lymphocytic leukemia (B-CLL). Previous in vitro studies investigating the effect of CpG ODN on B-CLL cells used serum-low conditions and did not account for the longer-half life of CpG ODN in vitro. The present study was designed to explore how the presence of serum and exposure time affect CpG ODN-mediated changes on B-CLL cells. The optimal concentration for CpG ODN-mediated effects in the presence of 100% serum or plasma was higher (10-20 microg/ml) than for serum-low conditions. Maximal CpG ODN-mediated effects required the presence of ODN for no longer than 3 hours. The inhibition of CpG ODN-mediated effects by serum correlated with lower uptake of ODN into B-CLL cells in the presence of serum. A threshold effect on biologic response was observed, with a given amount of ODN internalized, resulting in phenotypic changes. In conclusion, systemic short-term application of CpG ODN appears to be sufficient to induce phenotypic changes, but higher doses of CpG ODN than previously thought may be necessary because of inhibition of their uptake by serum.

Therapeutic vaccination with papillomavirus E6 and E7 long peptides results in the control of both established virus-induced lesions and latently infected sites in a pre-clinical cottontail rabbit papillomavirus model

This study was performed to test the therapeutic efficacy of overlapping long E6 and E7 peptides, containing both CD4+ T-helper and CD8+ CTL epitopes, on CRPV-induced lesions, which is an appropriate pre-clinical model for HPV diseases, including recurrent respiratory papillomatosis (RRP). Therapeutic peptide vaccination was able to significantly control wart growth (p < 0.01) and abrogate latent CRPV infection (p = 0.0006) compared to controls. Vaccination was associated with a T(H)1 T cell response, as suggested by a strong DTH skin test, antigen-specific proliferation of PBMC and a minimal IgG antibody response. Thus, this study shows promise for treatment of RRP by vaccination with long peptides.

Cancer immunotherapy based on killing of Salmonella-infected tumor cells

A major obstacle for the development of effective immunotherapy is the ability of tumors to escape the immune system. The possibility to kill tumor cells because they are recognized as infected rather than as malignant could help overcome immune escape mechanisms. Here we report a conceptually new approach of cancer immunotherapy based on in vivo infection of tumors and killing of infected tumor cells. Attenuated but still invasive, Salmonella typhimurium can be successfully exploited to invade melanoma cells that can present antigenic determinants of bacterial origin and become targets for anti-Salmonella-specific T cells. However, to fully appreciate the anticancer therapeutic properties of S. typhimurium, tumor-bearing mice need to be vaccinated against S. typhimurium before intratumoral Salmonella injection. Tumor infection when coupled to anti-Salmonella vaccination leads to 50% to 100% tumor-free mice with a better outcome on larger tumors. Invasive Salmonella also exert an indirect toxic effect on tumor cells through the recruitment of inflammatory cells and the cross-presentation of tumor antigens, which allow induction of tumor-specific immune response. This is effective in retarding the growth of untreated established distant tumors and in protecting the mice from subsequent tumor challenges.

Redirecting in vivo elicited tumor infiltrating macrophages and dendritic cells towards tumor rejection

A hostile tumor microenvironment interferes with the development and function of the adaptive immune response. Here we report the mechanisms by which large numbers of tumor-infiltrating macrophages and dendritic cells (DC) can be redirected to become potent effectors and activators of the innate and adaptive immunity, respectively. We use adenoviral delivery of the CCL16 chemokine to promote accumulation of macrophages and DC at the site of preestablished tumor nodules, combined with the Toll-like receptor 9 ligand CpG and with anti-interleukin-10 receptor antibody. CpG plus anti-interleukin-10 receptor antibody promptly switched infiltrating macrophages infiltrate from M2 to M1 and triggered innate response debulking large tumors within 16 hours. Tumor-infiltrating DC matured and migrated in parallel with the onset of the innate response, allowing the triggering of adaptive immunity before the diffuse hemorrhagic necrosis halted the communication between tumor and draining lymph nodes. Treatment of B6>CXB6 chimeras implanted with BALB/c tumors with the above combination induced an efficient innate response but not CTL-mediated tumor lysis. In these mice, tumor rejection did not exceed 25%, similarly to that observed in CCR7-null mice that have DC unable to prime an adaptive response. The requirement of CD4 help was shown in CD40-KO, as well as in mice depleted of CD4 T cells, during the priming rather than the effector phase. Our data describe the critical requirements for the immunologic rejection of large tumors: a hemorrhagic necrosis initiated by activated M1 macrophages and a concomitant DC migration to draining lymph nodes for subsequent CTL priming and clearing of any tumor remnants.

Enhanced HIV-1 specific immune response by CpG ODN and HIV-1 immunogen-pulsed dendritic cells confers protection in the Trimera murine model of HIV-1 infection

We have recently developed a novel small animal model for HIV-1 infection (Ayash-Rashkovsky et al., http://www.fasebj.org/cgi/doi/10.1096/fj.04-3184fje; doi:10.1096/fj.04-3184fje). The mice were successfully infected with HIV-1 for 4-6 wk with different clades of either T- or M-tropic isolates. HIV-1 infection was accompanied by rapid loss of human CD4+ T cells, decrease in CD4/CD8 ratio, and increased T cell activation. HIV specific human humoral and cellular immune responses were observed in all HIV-1 infected animals. In the present study, HIV specific human immune responses, both humoral and cellular, were generated in noninfected Trimera mice, after their immunization with gp120-depleted HIV-1 antigen, presented by autologous human dendritic cells. Addition of CpG ODN to the antigen-pulsed DCs significantly enhanced (by 2- to 30-fold) the humoral and cellular HIV-1 specific immune responses. Only mice immunized with the HIV-1 immunogen and CpG were completely protected from infection with HIV-1 after challenge with high infection titers of the virus. This novel small animal model for HIV-1 infection may thus serve as an attractive platform for rapid testing of candidate HIV-1 vaccines and of adjuvants and may shorten the time needed for the development and final assessment of protective HIV-1 vaccines in human trials.

CpG oligodeoxynucleotides inhibit tumor growth and reverse the immunosuppression caused by the therapy with 5-fluorouracil in murine hepatoma

AIM: To investigate the effect of CpG-containing oligodeoxynucleotides (CpG ODN) alone or in combination with the chemotherapeutic agent 5-fluorouracil (5-FU) on tumor growth and whether CpG ODN can reverse the immunosuppression caused by the chemotherapy with 5-FU in murine hepatoma model.

METHODS: Hepatoma model was established by subcutaneous inoculation with hepatoma-22 (H₂₂) cells into the right flank of BALB/c mice. Mice with tumor were treated by peritumoral injection of CpG ODN alone or in combination with subcutaneous injection of 5-FU. Tumor size was quantified regularly. Serum levels of IL-12 and IFN-γ in mice were assayed by enzyme-linked immunosorbent assay (ELISA). The lytic capacity of splenic NK cells was tested by lactate dehydrogenase release assay.

RESULTS: Peritumoral injection of CpG ODN alone or in combination with subcutaneous injection of 5-FU, and the treatment with 5-FU alone all led to significant inhibition of hepatoma growth. The mean tumor volumes fell by 46.66% in mice injected with CpG ODN, 68.34% in the 5-FU treated mice, and 70.23% in mice treated with the combination of CpG ODN and 5-FU than in controls. There was no significant difference in tumor size between 5-FU-treated mice and mice treated with the combination of 5-FU and CpG ODN (P>0.05). The serum levels of IL-12 and IFN-γ of mice treated with CpG ODN alone (IL-12: 464.50±24.37 pg/mL; IFN-γ: 134.20±25.76 pg/mL) or with the co-administration of CpG ODN and 5-FU (IL-12: 335.83±28.74 pg/mL; IFN-γ: 111.00±5.33 pg/mL) were significantly higher than that of controls (IL-12: 237.50±45.31 pg/mL; IFN-γ: 56.75±8.22 pg/mL). The production of IL-12 and IFN-γ was suppressed moderately in 5-FU-treated mice (IL-12: 166.67±53.22 pg/mL; 53.33±16.98 pg/mL) compared to control mice (P>0.05), whereas the combination of CpG ODN and 5-FU significantly increased the serum levels of IL-12 and IFN-γ compared to 5-FU alone (P<0.05). The NK cell killing activity in CpG ODN-treated mice (44.04±1.38%) or the mice treated with CpG ODN combined with 5-FU (30.67±1.28%) was significantly potentiated compared to controls (19.22±0.95%, P<0.05). The co-administration of CpG ODN and 5-FU also significantly enhanced the lytic activity of NK cells when compared with the treatment with 5-FU alone (12.03±1.42%, P<0.05).

CONCLUSION: The present data suggests that CpG ODN used as single therapeutic agent triggers anti-tumor immune response to inhibit the growth of implanted hepatoma and reverses the immunosuppression caused by the chemotherapy with 5-FU.

Sequence-specific potent induction of IFN-alpha by short interfering RNA in plasmacytoid dendritic cells through TLR7

Short interfering RNA (siRNA) is used in RNA interference technology to avoid non-target-related induction of type I interferon (IFN) typical for long double-stranded RNA. Here we show that in plasmacytoid dendritic cells (PDC), an immune cell subset specialized in the detection of viral nucleic acids and production of type I IFN, some siRNA sequences, independent of their GU content, are potent stimuli of IFN-alpha production. Localization of the immunostimulatory motif on the sense strand of a potent IFN-alpha-inducing siRNA allowed dissection of immunostimulation and target silencing. Injection into mice of immunostimulatory siRNA, when complexed with cationic liposomes, induced systemic immune responses in the same range as the TLR9 ligand CpG, including IFN-alpha in serum and activation of T cells and dendritic cells in spleen. Immunostimulation by siRNA was absent in TLR7-deficient mice. Thus sequence-specific TLR7-dependent immune recognition in PDC needs to be considered as an additional biological activity of siRNA, which then should be termed immunostimulatory RNA (isRNA).

Potentiation of a Dendritic Cell Vaccine for Murine Renal Cell Carcinoma by CpG Oligonucleotides

Purpose: An ideal vaccine therapy for tumors should activate both effector and memory immune responses against tumor-specific antigens. Here we investigated the effect of CpG oligodeoxynucleotides (CpG-ODN) for their ability to potentiate the activity of tumor antigen–pulsed bone marrow–derived dendritic cells (DC) in a vaccine model for the treatment of murine renal cell carcinoma (RENCA).

Experimental Design: First we evaluated the effects of a murine renal cell carcinoma (RENCA) on immune cell activity in a mouse model using in vitro assays for T-cell proliferation and natural killer cell activation. To overcome the immune suppression of the tumor, we s.c. injected groups of 10 mice with dendritic cells and tumor cells. We compared the effect of different conditioning regimens of the DCs with RENCA antigen and/or CpG-ODNs before injection by measuring tumor size twice a week.

Results: Tumor growth was shown to negatively affect spleen cell and T-cell proliferation, IFN-γ production, natural killer cell activity, and NF-κB activation in T cells. In this model, we have shown that RENCA-pulsed CpG-ODN-treated DCs were able not only to significantly reduce tumor growth but also to prevent tumor implantation in 60% of mice. Tumor-free mice were resistant to tumor challenge and the immunity conferred by the vaccine was transferable and tumor specific.

Conclusions: This data show that RENCA down-modulates the immune response, and DC vaccine therapy, in conjunction with CpG-ODN, can restore tumor-specific immunity.

CpG immunostimulatory oligodeoxynucleotide 1826 enhances antitumor effect of interleukin 12 gene-modified tumor vaccine in a melanoma model in mice

PURPOSE

The effectiveness of interleukin (IL)-12-secreting tumor vaccines in the treatment of mouse tumors could be enhanced by concurrent application of cytokines and costimulatory molecules. We investigated the therapeutic potential of IL-12 gene-transduced melanoma vaccine in combination with CpG immunostimulatory oligodeoxynucleotide (ODN) 1826, an adjuvant known to favor development of Th1-biased immune response, in a B78-H1 (B78) melanoma model in mice.

EXPERIMENTAL DESIGN

Mice injected with B78 melanoma cells were treated with irradiated IL-12 gene-transduced B78 cells [B78/IL-12(X)] and/or ODN 1826. Mechanisms responsible for the antitumor effects of the treatment were investigated using fluorescence-activated cell sorter analysis, a standard (51)Cr releasing assay, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, and ELISA.

RESULTS

Single injection of B78/IL-12(X) cells had no effect on tumor growth, whereas seven consecutive daily injections of ODN 1826 markedly inhibited tumor progression with occasional curative effects. When used in combination, B78/IL-12(X) cells and ODN 1826 caused additional tumor growth reduction and eradication of tumors in 62% of treated mice. The combined treatment activated local inflammatory response against tumor but also induced systemic antitumor immunity. In vitro studies have shown that when used together, B78/IL-12(X) cells and ODN 1826 induced a potent Th1 response and suggested the role of IFN-gamma in activation of the host immune response. The antitumor effects in double-treated mice were accompanied by the development of cytotoxic effectors in the spleen and activation of macrophages.

CONCLUSIONS

The results provided the evidence that the combination of IL-12 gene-modified melanoma vaccine and ODN 1826 induces synergistically systemic and local antitumor immunity.

Targeting Toll-like Receptor 9 with CpG Oligodeoxynucleotides Enhances Tumor Response to Fractionated Radiotherapy

Synthetic oligodeoxynucleotides containing unmethylated CpG motifs detected by Toll-like receptor 9 of dendritic cells and B cells have potent immunomodulatory effects. CpG oligodeoxynucleotides induce cytokines, activate natural killer cells, and elicit T-cell responses leading to antitumor effects, including improved efficacy of chemotherapeutic agents and, as we reported recently, synergy between CpG oligodeoxynucleotide 1826 and single-dose radiotherapy of an immunogenic mouse fibrosarcoma. The present study extends this finding to the fractionated radiotherapy of the fibrosarcoma tumor and assesses the ability of CpG oligodeoxynucleotide 1826 to increase the radioresponse of a tumor (nonimmunogenic fibrosarcoma). The experiments used a murine immunogenic fibrosarcoma tumor, fibrosarcoma growing in the leg of mice, and response to radiotherapy was assessed by tumor growth delay and tumor cure rate (TCD50, radiation dose yielding 50% tumor cure). Multiple s.c. peritumoral or i.t. administrations of CpG oligodeoxynucleotide 1826 at a dose of 100 μg per mouse were given when established tumors were 6 mm in diameter. Local tumor irradiation was initiated when tumors grew to 8 mm in diameter; radiation was delivered in 1 to 9 Gy fractions given twice daily separated by 6 to 7 hours for 5 consecutive days to achieve a total dose of 10 to 90 Gy. CpG oligodeoxynucleotide 1826, given as a single agent, had only a small antitumor effect, but it dramatically enhanced fibrosarcoma response to radiotherapy. Although 83.1 (79.2-90.0) Gy total dose were needed to achieve tumor cures in 50% of mice treated with radiotherapy alone, only 23.0 (11.5-32.7) Gy total dose were needed in mice treated with both CpG oligodeoxynucleotide 1826 and radiotherapy. The magnitude of potentiation of tumor radioresponse at the TCD50 level was by a factor of 3.61, a much higher value than that (a factor of 1.93) that we reported for single-dose radiotherapy. Mice cured of their tumors by combined CpG oligodeoxynucleotide 1826 plus radiotherapy were highly resistant to s.c. tumor take or development of tumor nodules in the lung from i.v. injected tumor cells when rechallenged with fibrosarcoma cells 100 to 120 days after the treatment, suggesting the development of a memory response. CpG oligodeoxynucleotide 1826 also increased radioresponse of the nonimmunogenic fibrosarcoma tumor by a factor of 1.41 and 1.73 when CpG oligodeoxynucleotide 1826 was given s.c. and i.t., respectively. These findings show that CpG oligodeoxynucleotides are highly potent enhancers of tumor response to both single-dose and fractionated radiation and as such have potential to improve clinical radiotherapy.

CpG-DNA as immune response modifier

Toll-like receptors (TLRs) have been recognized to play a fundamental role in the recognition of microbial pathogens and the activation of innate immunity. However, it has also turned out that triggering of TLRs with purified compounds or synthetic ligands represents a powerful means to modulate innate as well as adaptive immune responses. Among the substances currently under investigation as immune response modifiers CpG-DNA has gained particular interest. One reason for this is the feasibility to easily synthesize and modify immunostimulatory CpG-containing oligodeoxynucleotides (CpG-ODNs). This review deals with the structural needs of CpG-ODNs. CpG-ODNs are discussed as being composed of distinct building blocks (e.g. sequence strings, backbone modifications) giving rise to a concept of structural modules within CpG-ODNs.

Enhanced tumor-specific long-term immunity of hemagglutinating [correction of hemaggluttinating] virus of Japan-mediated dendritic cell-tumor fused cell vaccination by coadministration with CpG oligodeoxynucleotides

Immunization with dendritic cells (DCs) using various Ag-loading approaches has shown promising results in tumor-specific immunotherapy and immunoprevention. Fused cells (FCs) that are generated from DCs and tumor cells are one of effective cancer vaccines because both known and unknown tumor Ags are presented on the FCs and recognized by T cells. In this study, we attempted to augment antitumor immunity by the combination of DC-tumor FC vaccination with immunostimulatory oligodeoxynucleotides containing CpG motif (CpG ODN). Murine DCs were fused with syngeneic tumor cells ex vivo using inactivated hemagglutinating virus of Japan (Sendai virus). Mice were intradermally (i.d.) immunized with FCs and/or CpG ODN. Coadministration of CpG ODN enhanced the phenotypical maturation of FCs and unfused DCs, and the production of Th1 cytokines, such as IFN-gamma and IL-12, leading to the induction of tumor-specific CTLs without falling into T cell anergy. In addition, immunization with FCs + CpG ODN provided significant protection against lethal s.c. tumor challenge and spontaneous lung metastasis compared with that with either FCs or CpG ODN alone. Furthermore, among mice that rejected tumor challenge, the mice immunized with FCs + CpG ODN, but not the mice immunized with FCs or CpG ODN alone, completely rejected tumor rechallenge, indicating that CpG ODN provided long-term maintenance of tumor-specific immunity induced by FCs. Thus, the combination of DC-tumor FCs and CpG ODN is an effective and feasible cancer vaccine to prevent the generation and recurrence of cancers.

CpG DNA and cancer immunotherapy: orchestrating the antitumor immune response

PURPOSE OF REVIEW

Cancer treatment is entering an era of targeted approaches. One such approach is use of the immune system to recognize and eliminate malignant cells. Synthetic CpG oligonucleotides (CpG DNA) are a relatively new class of agents that have the ability to stimulate a potent, orchestrated tumor-specific immune response. This review provides an overview of the immunologic effects of CpG DNA and summarizes the results of preclinical investigations that have led to ongoing development of CpG DNA as a component of clinical cancer immunotherapy.

RECENT FINDINGS

New studies demonstrate that at least three classes of CpG DNA sequences exist, each with different physical characteristics and biologic effects. Preliminary studies in several animal models of cancer suggest that CpG DNA have the ability to induce tumor regression by activating innate immunity, enhancing antibody dependent cellular cytotoxicity, and serving as potent vaccine adjuvants that elicit a specific, protective immune response.

SUMMARY

Animal models suggest that CpG DNA may have many uses in cancer immunotherapy. Early clinical trials suggest that CpG DNA can be administered safely to humans, and studies are ongoing to understand how these agents may play a role in cancer immunotherapy.

Immunoprevention of basal cell carcinomas with recombinant hedgehog-interacting protein

Basal cell carcinomas (BCCs) are driven by abnormal hedgehog signaling and highly overexpress several hedgehog target genes. We report here our use of one of these target genes, hedgehog-interacting protein (Hip1), as a tumor-associated antigen for immunoprevention of BCCs in Ptch1^+/− mice treated with ionizing radiation. Hip1 mRNA is expressed in adult mouse tissues at levels considerably lower than those in BCCs. Immunization with either of two large recombinant Hip1 polypeptides was well tolerated in Ptch1^+/− mice, induced B and T cell responses detectable by enzyme-linked immunosorbent assay, Western blot, delayed type hypersensitivity, and enzyme-linked immunospot assay, and reduced the number of BCCs by 42% (P < 0.001) and 32% (P < 0.01), respectively. We conclude that immunization with proteins specifically up-regulated by hedgehog signaling may hold promise as a preventive option for patients such as those with the basal cell nevus syndrome who are destined to develop large numbers of BCCs.

CpG oligodeoxynucleotide enhances tumor response to radiation

CpG oligodeoxynucleotides (ODNs) are synthetic DNA sequences containing unmethylated cytosine-guanine motifs with potent immunomodulatory effects. Via Toll-like receptor 9 agonism of dendritic cells and B cells, CpG ODNs induce cytokines, activate natural killer cells, and elicit vigorous T-cell responses that lead to significant antitumor effects, including improved efficacy of chemotherapeutic agents. On the basis of these properties of CpG ODNs, we tested whether they also could enhance tumor response to radiotherapy. Using an immunogenic mouse tumor, designated FSa, the response to radiotherapy was assayed by tumor growth delay and tumor cure rate (TCD(50), radiation dose yielding 50% tumor cure rate). Treatments were initiated when established tumors were either 6 or 8 mm in diameter. CpG ODN as a single agent given s.c. peritumorally had little effect on tumor growth; however, it dramatically enhanced tumor growth delay in response to single-dose radiation by a factor of 2.58-2.65. CpG ODN also dramatically improved tumor radiocurability, reducing the TCD(50) by a factor of 1.93, from 39.6 (36.1-43.1) Gy to 20.5 (14.3-25.7) Gy. The CpG ODN-induced enhancement of tumor radioresponse was diminished in tumor-bearing mice immunocompromised by sublethal whole-body radiation. Tumors treated with CpG ODN and radiation showed histologic changes characterized by increased necrosis, heavy infiltration by host inflammatory cells (lymphocytes and granulocytes), and reduced tumor cell density. These results show that CpG ODNs are potent enhancers of tumor radioresponse and as such have potential to improve clinical radiotherapy.

Co-administration of a DNA vaccine encoding the prostate specific membrane antigen and CpG oligodeoxynucleotides suppresses tumor growth

Background

Prostate-specific membrane antigen (PSMA) is a well characterized prostate-specific tumor associated antigen. Its expression is elevated in prostate carcinoma, particularly in metastatic and recurrent lesions. These observations suggest that PSMA can be used as immune target to induce tumor cell-specific recognition by the host and, consequently tumor rejection. We utilized a DNA-based vaccine to specifically enhance PSMA expression. An immune modulator, such as CpG oligodeoxynucleotides which promote Th1-type immune responses was combined to increase the efficacy of tumor recognition and elimination.

Methods

A eukaryotic expression plasmid pCDNA3.1-PSMA encoding full-length PSMA was constructed. C57BL/6 mice were immunized with endotoxin-free pCDNA3.1-PSMA alone or in combination with CpG oligodeoxynucleotides by intramuscular injection. After 4 immunizations, PSMA specific antibodies and cytotoxic T lymphocyte reactivity were measured. Immunized C57BL/6 mice were also challenged subcutaneously with B16 cells transfected with PSMA to evaluate suppression of tumor growth.

Results

Vaccine-specific cytotoxic T lymphocytes reactive with B16 cells expressing PSMA could be induced with this treatment schedule. Immune protection was observed in vaccinated mice as indicated by increased tumor growth in the control group (100%) compared with the groups vaccinated with DNA alone (66.7%) or DNA plus CpG oligodeoxynucleotides (50%) respectively. Average tumor volume was smaller in vaccinated groups and tumor-free survival time was prolonged by the vaccination.

Conclusion

The current findings suggest that specific anti-tumor immune response can be induced by DNA vaccines expressing PSMA. In addition, the suppression of in vivo growth of tumor cells expressing PSMA was augmented by CpG oligodeoxynucleotides. This strategy may provide a new venue for the treatment of carcinoma of prostate after failure of standard therapy.

Development of immunomodulatory six base-length non-CpG motif oligonucleotides for cancer vaccination

We have previously described a novel family of immunomodulatory synthetic oligonucleotides characterized by a phosphodiester backbone, a length of six bases and a 5'G3xG23' sequence, where x is A, C, G or T. In the present study, we have evaluated whether these 5'G3xG23' oligonucleotides possess additional activities essential for adequate cancer vaccination. Immunization for the treatment of cancer requires an adjuvant, a source of tumor-associated antigen(s), for example apoptotic cancer cells, and a way to overcome the escape of tumor cells from the immune system, for example the up-regulation of Fas ligand (FasL) on the surface of cancer cells. The results show that phosphodiester 5'G3AG23' and 5'G3TG23' oligonucleotides have a direct activity on a number of different cancer cells by inducing apoptosis (release of cytochrome C, activation of caspase-3, cleavage of poly [ADP-ribose] polymerase, degradation of nuclear mitotic apparatus protein and translocation of phophatidylserine at the cell surface). In addition, the 5'G3AG23', 5'G3CG23', and 5'G3TG23' oligonucleotides were found to down-regulate the levels of FasL on the surface of cancer cells. These immunomodulatory phosphodiester six base-length oligonucleotides, which are capable of inducing apoptosis in cancer cells as well as downregulating the expression of FasL at their cell surface, may have application as cancer cell vaccines.

CpG motifs as proinflammatory factors render autochthonous tumors permissive for infiltration and destruction

In a transgenic mouse model expressing SV40 T Ag (Tag) as a de novo tumor Ag, immune surveillance fails and islet cell carcinomas grow progressively. To develop an anticancer strategy that would be effective in eradicating solid, autochthonously growing tumors, we evaluated the effectiveness of immunostimulatory oligodeoxynucleotides (ODN) with cytosine-guanine-rich (CpG) motifs (CpG-ODN). In a classical vaccination protocol, Tag was administered with CpG-ODN as adjuvant. The antitumor vaccination, however, was only effective in a prophylactic setting, despite the successful activation of a Tag-specific CTL response in vivo. Histological examination demonstrated that even primed immune cells failed to infiltrate tumors once a malignant environment was established. To ensure that effector cells were not limiting, highly activated tumor Ag-specific T cells were transferred into tumor-bearing mice. However, this treatment also failed to result in tumor infiltration and rejection. Therefore, we further tested the efficacy of CpG-ODN as a proinflammatory agent in combination with the transfer of preactivated Tag-specific CD4(+) and CD8(+) T cells. Indeed, this combination therapy proved to be highly effective, because CpG-ODN rendered insulinomas permissive for massive infiltration and destruction. The opening of tumor tissue correlated with uptake of CpG-ODN by tissue-resident macrophages and a strong up-regulation of adhesion molecules such as ICAM and VCAM on blood vessel endothelia. These data demonstrate that systemic application of proinflammatory reagents drastically enhances extravasation of effector cells into tumor tissue, an observation that is of general importance for immunotherapy of solid tumors in a clinical setting.

Human B cells express functional TRAIL/Apo-2 ligand after CpG-containing oligodeoxynucleotide stimulation

CpG-containing oligodeoxynucleotides (CpG ODN) have broad-ranging immunostimulatory effects, including the generation of antitumor immune responses. Analysis of different CpG ODN have identified two classes: CpG-A ODN, which stimulate high levels of IFN-alpha production from plasmacytoid dendritic cells and weakly activate B cells, and CpG-B ODN, which strongly activate B cells but stimulate low production of IFN-alpha from plasmacytoid dendritic cells. Previously, we observed that CpG-B ODN (2006) induces TRAIL/Apo-2 ligand (Apo-2L)-mediated killing of tumor cells by CD14(+) PBMC. In this study, we extend our investigation of CpG ODN-induced TRAIL/Apo-2L expression and activity in PBMC to include CpG-A ODN. Of the two classes, IFN-alpha production and TRAIL/Apo-2L-mediated killing of tumor cells was greatest with CpG-A ODN. Surprisingly, CD3(+), CD14(+), CD19(+), and CD56(+) PBMC expressed high levels of TRAIL/Apo-2L following CpG-A ODN stimulation. When isolated, the CD19(+) PBMC (B cells) were able to kill tumor cells in a TRAIL/Apo-2L-dependent manner. As with CD14(+) PBMC, CD19(+) sorted B cells were capable of up-regulating TRAIL/Apo-2L expression when stimulated with IFN-alpha alone. Interestingly, agonist anti-CD40 mAb further enhanced the IFN-alpha-induced TRAIL/Apo-2L expression on CD19(+) B cells. These results are the first to demonstrate human B cell-mediated killing of tumor cells in a TRAIL/Apo-2L-dependent fashion.

Peritumoral CpG oligodeoxynucleotide treatment inhibits tumor growth and metastasis of B16F10 melanoma cells

Although melanoma mostly affects the skin, it is notorious for its propensity to easily develop metastasis. Metastatic melanoma is highly resistant to a variety of therapies. We examined the anti-metastatic potential of peritumoral monotherapy against murine cutaneous B16F10 melanoma with synthetic oligodeoxynucleotides (ODN) containing unmethylated CpG motifs. We demonstrated that repeated peritumoral injections of CpG ODN significantly reduced skin tumor size. Peritumoral CpG ODN-treatment of skin tumors prevented the development of pulmonary B16F10 colonies. Adoptive transfer of splenocytes obtained from CpG ODN-treated mice markedly reduced the number of previously established pulmonary colonies in recipient naïve mice. T-lymphocyte depletion studies indicated that the anti-metastatic effect was dependent on both CD4+ and CD8+ T cells. These results suggest that CpG ODN are promising as a preventive and therapeutic anti-metastatic measure against melanoma.

Combination of a CpG-oligodeoxynucleotide and a topoisomerase I inhibitor in the therapy of human tumour xenografts

The study was conducted to investigate the effects of a novel therapeutic approach, i.e. the combination of chemotherapy and immunotherapy, against a human prostate carcinoma xenograft. A topoisomerase I inhibitor, topotecan, and CpG-containing oligodeoxynucleotides (CpG-ODN) were combined. Athymic mice bearing the PC-3 human prostate carcinoma were treated with the maximum tolerated dose (MTD) of topotecan (3 weekly treatments) and with repeated treatments of CpG-ODN (40 and 20 microg/mouse); tumour growth and lethal toxicity were monitored. Topotecan effect on CpG-ODN-induced production of interleukin (IL) 12, interferon (IFN)-gamma and tumour necrosis factor-alpha was also assessed. Since topotecan pretreatment differentially influenced CpG-ODN-induced production of IL-12 and IFN-gamma, the antitumour effects of the two therapies were investigated in a sequential (full topotecan regimen followed by CpG-ODN) or in an alternating sequence (starting with CpG-ODN). Topotecan inhibited PC-3 tumour growth, inducing 95% tumour volume inhibition. All combined treatments resulted in a significant delay in tumour growth, compared to the effects in topotecan-treated mice (P<0.01, by analysis of tumour growth curves). The combination regimens were well tolerated, except for the alternating sequence of 40 microg CpG-ODN and topotecan, which resulted in three out of eight toxic deaths. This alternating sequence was highly toxic even when another cytotoxic drug (doxorubicin) was used in healthy mice. In conclusion, the combination of topotecan and CpG-ODN increased antitumour effects over chemotherapy alone in the growth of a human prostate carcinoma xenograft. Administration sequence was critical to the combination toxicity: the complete regimen of the cytotoxic drug followed by repeated administrations of the immunomodulator seemed the most promising for further investigations.

CpG-ODN-stimulated dendritic cells act as a potent adjuvant for E7 protein delivery to induce antigen-specific antitumour immunity in a HPV 16 E7-associated animal tumour model

We previously reported that both E7 and CpG-oligodeoxynucleotide (ODN) are required for protecting animals from human papillomavirus (HPV) 16 E7-associated tumour challenge. Here we investigate dendritic cells (DC)-based approach in this protection. In the study, we isolated bone marrow-derived DC and stimulated DC with E7 and ODN. In vitro stimulation of DC with E7 plus ODN resulted in more production of interleukin-12, as compared to that with E7 or ODN alone. Further injection with E7+ODN-stimulated DC resulted in more significant tumour protection, as compared to stimulation with E7 or ODN alone. We further evaluated the levels of immune responses induced by DC stimulated with E7+ODN. We observed little enhancement of E7-specific antibody and T helper cell proliferative responses by E7+ODN stimulation, as compared to E7 stimulation. However, there was some enhancement of interferon-gamma (IFN-gamma) production from CD4+ T cells and a more significant production of IFN-gamma from CD8+ T cells by E7+ODN stimulation, as compared to E7 stimulation alone. This was consistent with intracellular IFN-gamma staining levels of CD8+ T cells. Tumour protection further appeared to be mediated by CD8+ T cells, as determined by in vivo T-cell depletion. Thus, these data suggest that upon ODN stimulation DC might function as a potent adjuvant for E7 protein delivery for induction of protective cellular immunity against HPV E7-associated tumour challenge.

Activation of Dendritic Cells and Induction of T Cell Responses by Hpv 16 L1/E7 Chimeric Virus-Like Particles are Enhanced by Cpg ODN or Sorbitol

Chimeric human papillomavirus-like particles, consisting of human papillomavirus (HPV) 16 L1-E7 fusion proteins [HPV 16 L1/E7 chimeric virus-like particles (CVLP)], are a vaccine candidate for treatment and prevention of cervical cancer. Although in preclinical studies CVLPs were shown to induce neutralizing antibodies and L1-and E7-specific T cell responses, the results of a recent clinical trial emphasized the need of improved immunogenicity of CVLPs. Here we studied the interaction of HPV 16 L1/E7 CVLPs with mouse bone marrow-derived dendritic cells (BMDCs) activated with different immune adjuvants. We found that lipopolysaccharides (LPS), unmethylated CpG motifs (CpG ODN) and sorbitol enhanced CVLP-induced stimulation of C57BL/6 mouse BMDCs as revealed by increased levels of CD40, CD80, MHC II and CD54 at the cell surface. CpG ODN and sorbitol also enhanced the presentation of Db-restricted cytotoxic T lymphocyte epitopes to HPV 16 L1- or E7-specific T lymphocytes after loading of CVLPs onto BMDCs. Treatment of BMDCs with CpG ODN in combination with CVLPs improved in vitro priming of naive T lymphocytes by CVLP-loaded BMDCs. In vivo, CVLP-loaded BMDCs were more immunogenic as compared with injection of CVLPs alone. CpG ODN and sorbitol further enhanced priming of antigen-specific T cell responses. Our data demonstrate that CpG ODN- or sorbitol-activated BMDCs substantially increase the immunogenicity of CVLPs. Implementing our results in clinical trial protocols may lead to improved activity of therapeutic HPV vaccines for the treatment of HPV-induced cancer.

Antitumor applications of stimulating toll-like receptor 9 with CpG oligodeoxynucleotides

Tumor immunotherapy has evolved from the use of crude bacterial extracts to chemically synthesized ligands for specific immune receptors, such as Toll-like receptors (TLRs). One of the most promising targets for therapeutic immune activation is TLR9, which detects unmethylated CpG dinucleotides present in viral and prokaryotic genomes, which are generally methylated in host DNA. This review describes the immune effects of synthetic CpG oligonucleotides as TLR9 ligands and their applications in cancer immunotherapy.

Ionizing radiation inhibition of distant untreated tumors (abscopal effect) is immune mediated

PURPOSE

Ionizing radiation can reduce tumor growth outside the field of radiation, known as the abscopal effect. Although it has been reported in multiple malignancies, the abscopal effect remains a rare and poorly understood event. Ionizing radiation generates inflammatory signals and, in principle, could provide both tumor-specific antigens from dying cells and maturation stimuli that are necessary for dendritic cells' activation of tumor-specific T cells. We therefore tested the hypothesis that the abscopal effect elicited by radiation is immune mediated. This was directly tested by enhancing the number of available dendritic cells using the growth factor Flt3-Ligand (Flt3-L).

METHODS AND MATERIALS

Mice bearing a syngeneic mammary carcinoma, 67NR, in both flanks were treated with Flt3-L daily for 10 days after local radiation therapy (RT) to only 1 of the 2 tumors at a single dose of 2 or 6 Gy. The second nonirradiated tumor was used as indicator of the abscopal effect. Data were analyzed using repeated measures regression.

RESULTS

RT alone led to growth delay exclusively of the irradiated 67NR tumor, as expected. Surprisingly, growth of the nonirradiated tumor was also impaired by the combination of RT and Flt3-L. As control, Flt3-L had no effect without RT. Importantly, the abscopal effect was shown to be tumor specific, because growth of a nonirradiated A20 lymphoma in the same mice containing a treated 67NR tumor was not affected. Moreover, no growth delay of nonirradiated 67NR tumors was observed when T cell deficient (nude) mice were treated with RT plus Flt3-L.

CONCLUSIONS

These results demonstrate that the abscopal effect is in part immune mediated and that T cells are required to mediate distant tumor inhibition induced by radiation.

Exosomes as Potent Cell-Free Peptide-Based Vaccine. II. Exosomes in CpG Adjuvants Efficiently Prime Naive Tc1 Lymphocytes Leading to Tumor Rejection

Ideal vaccines should be stable, safe, molecularly defined, and out-of-shelf reagents efficient at triggering effector and memory Ag-specific T cell-based immune responses. Dendritic cell-derived exosomes could be considered as novel peptide-based vaccines because exosomes harbor a discrete set of proteins, bear functional MHC class I and II molecules that can be loaded with synthetic peptides of choice, and are stable reagents that were safely used in pioneering phase I studies. However, we showed in part I that exosomes are efficient to promote primary MHC class I-restricted effector CD8(+) T cell responses only when transferred onto mature DC in vivo. In this work, we bring evidence that among the clinically available reagents, Toll-like receptor 3 and 9 ligands are elective adjuvants capable of triggering efficient MHC-restricted CD8(+) T cell responses when combined to exosomes. Exosome immunogenicity across species allowed to verify the efficacy of good manufactory procedures-manufactured human exosomes admixed with CpG oligonucleotides in prophylactic and therapeutic settings of melanoma in HLA-A2 transgenic mice. CpG adjuvants appear to be ideal adjuvants for exosome-based cancer vaccines.

CpG-A and CpG-B oligonucleotides differentially enhance human peptide-specific primary and memory CD8+ T-cell responses in vitro

Two distinct types of CpG oligodeoxynucleotide (ODN) have been identified that differ in their capacity to stimulate antigen-presenting cells: CpG-A induces high amounts of interferon-alpha (IFN-alpha) and IFN-beta in plasmacytoid dendritic cells (PDCs), whereas CpG-B induces PDC maturation and is a potent activator of B cells but stimulates only small amounts of IFN-alpha and IFN-beta. Here we examined the ability of these CpG ODNs to enhance peptide-specific CD8+ T-cell responses in human peripheral blood mononuclear cells (PBMCs). The frequency of influenza matrix-specific "memory" CD8+ T cells was increased by both types of CpG ODN, whereas the frequency of Melan-A specific "naive" CD8+ T cells increased on stimulation with CpG-B but not with CpG-A. The presence of PDCs in PBMCs was required for this CpG ODN-mediated effect. The expanded cells were cytotoxic and produced IFN- on peptide restimulation. Soluble factors induced by CpG-A but not CpG-B increased the granzyme-B content and cytotoxicity of established CD8+ T-cell clones, each of which was IFN-alpha/-beta dependent. In conclusion, CpG-B seems to be superior for priming CD8+ T-cell responses, and CpG-A selectively enhances memory CD8+ T-cell responses and induces cytotoxicity. These results demonstrate distinct functional properties of CpG-A and CpG-B with regard to CD8 T cells.

The intratumoral application of poly-G-oligodeoxynucleotides does not augment the naturally induced antitumoral CD8-T-cell response in P815 mastocytomas

DNA sequences containing CpG have been described to induce a strong immune reaction by acting on a variety of immune cells including a strong and pronounced antitumoral response. Poly-G-oligodeoxynucleotides (ODNs) on the other hand have been attributed the preferential induction of CD8-T-cell proliferation when used in vitro. This activity led us to the investigation of the possible antitumoral properties of poly-G-ODNs in an established CD8-dependent tumor eradication model. We used the well described poly-G-ODN 1628 in its capacity to enhance antitumoral CD8 response in the cutaneous mastocytoma P815. When injecting 30 microg of the purified phosphothioate-modified oligo into the tumor bearing area of P815 challenged mice for up to 12 consecutive days we did not observe increased tumor rejection as compared to the group of mice injected with a control oligo. The 1628-injected mice did not produce higher numbers of P815-specific CD8 cells as measured by P1A-, and P1E-tetramer staining and Immunoscope analysis. Furthermore, tumor-specific CD8 cells in 1628 did not show enhanced antitumoral cytotoxicity when analyzing lymphocyte-tumor cell co-cultures or transcription of the cytotoxic CD8-cell associated molecules interferon gamma, FAS ligand, perforin, or granzyme B by quantitative real-time RT-PCR. These experiments show that there is no enhanced induction of an antitumoral CD8 response after in situ administration of poly-G-ODNs in the P815 mastocytoma model.

Immunostimulatory CpG oligodeoxynucleotides and antibody therapy of cancer

Synthetic oligodeoxynucleotides containing CG motifs (CpG ODN) have potent immunostimulatory properties, and have potential as immunotherapeutic agents in cancer. Animal models suggest CpG ODN can activate a variety of immune effector cells such as natural killer (NK) cells, and also enhance the efficacy of tumor immunization when used as immune adjuvants or to directly activate antigen-presenting cells. CpG ODN are also capable of altering the expression of a number of antigens by malignant B-cells, including those targeted by monoclonal antibodies (moAbs) and those involved in communication with T cells. The ability of CpG ODN to activate the immune effector cells that participate in antibody-dependent cellular cytotoxicity (ADCC), upregulate target antigen, and perhaps induce development of an active immune response, suggest these agents may be capable of enhancing the efficacy of antitumor moAb therapy. Such enhanced efficacy has been demonstrated in animal models and is now undergoing evaluation in clinical trials.