The Ability of TwoListeria monocytogenesVaccines Targeting Human Papillomavirus-16 E7 to Induce an Antitumor Response Correlates with Myeloid Dendritic Cell Function

Exploring the Molecular Pathogenesis, Pathogen Association, and Therapeutic Strategies against HPV Infection

The human papillomavirus (HPV), commonly documented as the cause of warts, has gained much interest recently due to its possible links to several types of cancer. HPV infection is discussed in this review from multiple angles, including its virology, epidemiology, etiology, immunology, clinical symptoms, and treatment. Recent breakthroughs in molecular biology have led to the development of new methods for detecting and treating HPV in tissue. There is no cure for HPV, and although vaccines are available to prevent infection with the most common HPV viruses, their utilization is limited. Destruction and excision are the primary treatment modalities. This review sheds light on the epidemiology, molecular pathogenesis, the association of several other pathogens with HPV, the latest treatment strategies available to treat the same, and an overview of the progress made and the obstacles still to be overcome in the fight against HPV infection.

The Role of Immunotherapy in the Treatment of Anal Cancer and Future Strategies

OPINION STATEMENT

Despite being markedly sensitive to chemoradiotherapy, patients with locally advanced (T3-4 and/or node-positive) squamous cell carcinoma of the anal canal (SCCA) still present high rates of disease recurrence, which is characterized by meaningful morbidity and mortality. Abdominoperineal resection as salvage surgery may be considered for patients with local recurrence, but with an important negative impact in the quality of life. Systemic therapy of advanced SCCA is an unmet clinical need. Palliative chemotherapy for the management of unresectable or metastatic disease yields approximately 60% of objective response rate; however, it still portends a grim prognosis. Based on the recently published InterAACT trial, carboplatin plus paclitaxel has become the standard of care of advanced disease; modified DCF (docetaxel, cisplatin, and 5-fluorouracil) may also be considered for fit patients amenable to intensive therapy. There are no FDA-approved therapies for the treatment of chemorefractory patients. Nevertheless, both nivolumab and pembrolizumab may be considered for these patients with promising results, regardless of PD-L1 expression or other predictive biomarkers. It is estimated that approximately 1 out of 5 patients with SCCA will derive large benefit from PD-1 inhibitors, which may produce considerable durations of response. Ongoing clinical trials exploring the combination of chemotherapy plus immune checkpoint inhibitors in the first-line therapy, combination of anti-PD-1/PD-L1 plus anti-CTLA-4, and emerging immunotherapeutic approaches, such as adoptive T cell therapies, are eagerly awaited and may bring practice-changing results in the next few years for the treatment of this challenging disease.

Combination immunotherapy with two attenuated Listeria strains carrying shuffled HPV-16 E6E7 protein causes tumor regression in a mouse tumor model

Cervical cancer continues to impose a heavy burden worldwide, and human papilloma virus (HPV) infection, especially persistent infection with type 16 (HPV-16), is known to be the primary etiological factor. Therapeutic vaccines are urgently needed because prophylactic vaccines are ineffective at clearing pre-existing HPV infection. Here, two recombinant Listeria strains (LMΔ-E6E7 & LIΔ-E6E7) with deletions of the actA and plcB genes, expressing the shuffled HPV-16 E6E7 protein were constructed. The strains were delivered into the spleen and liver by intravenous inoculation, induced antigen-specific cellular immunity and were eliminated completely from the internal organs several days later. Intravenously treating with single strain for three times, or with both strains alternately for three times significantly reduced the tumor size and prolonged the survival time of model mice. Combination immunotherapy with two strains seemed more effective than immunotherapy with single strain in that it enhanced the survival of the mice, and the LMΔ-E6E7-prime-LIΔ-E6E7-boost strategy showed significant stronger efficacy than single treatment with the LIΔ-E6E7 strain. The antitumor effect of this treatment might due to its ability to increase the proportion of CD8⁺ T cells and reduce the proportion of T regulatory cells (Tregs) in the intratumoral milieu. This is the first report regarding Listeria ivanovii-based therapeutic vaccine candidate against cervical cancer. Most importantly we are the first to confirm that combination therapy with two different recombinant Listeria strains has a more satisfactory antitumor effect than administration of a single strain. Thus, we propose a novel prime-boost treatment strategy.

ADXS11-001 LM-LLO as specific immunotherapy in cervical cancer

Human papillomavirus (HPV) infection is a well-known cause of cervical cancer. Therapeutic cancer vaccines are part of the current therapeutic options for HPV-associated cancers. Axalimogen filolisbac (ADXS11-001) is an immunotherapy based on live attenuated Listeria monocytogenes-listeriolysin O (Lm-LLO), designed by biological engineering to secrete an antigen-adjuvant fusion protein, composed of a truncated fragment of LLO fused to HPV. The proposed mechanism of action is that Lm-based vectors infect antigen-presenting cells (APC) and secrete HPV-LLO fusion proteins within the APC cytoplasm, these proteins are processed and presented to cytotoxic T lymphocytes (CTL), thus generating a new population of CTLs specific to HPV antigens. These HPV-specific CTLs destroy HPV infected cells. ADXS11-001 has demonstrated safety results in phase I-II studies in women with cervical cancer and is being assessed in clinical trials in patients with HPV-positive anal canal and head and neck cancers.

Sticholysins, pore-forming proteins from a marine anemone can induce maturation of dendritic cells through a TLR4 dependent-pathway

Sticholysins (Sts) I and II (StI and StII) are pore-forming proteins (PFPs), purified from the Caribbean Sea anemone Stichodactyla helianthus. StII encapsulated into liposomes induces a robust antigen-specific cytotoxic CD8⁺ T lymphocytes (CTL) response and in its free form the maturation of bone marrow-derived dendritic cells (BM-DCs). It is probable that the latter is partially supporting in part the immunomodulatory effect on the CTL response induced by StII-containing liposomes. In the present work, we demonstrate that the StII's ability of inducing maturation of BM-DCs is also shared by StI, an isoform of StII. Using heat-denatured Sts we observed a significant reduction in the up-regulation of maturation markers indicating that both PFP's ability to promote maturation of BM-DCs is dependent on their conformational characteristics. StII-mediated DC maturation was abrogated in BM-DCs from toll-like receptor (TLR) 4 and myeloid differentiation primary response gene 88 (MyD88)-knockout mice but not in cells from TLR2-knockout mice. Furthermore, the antigen-specific CTL response induced by StII-containing liposomes was reduced in TLR4-knockout mice. These results indicate that StII, and probably by extension StI, has the ability to induce maturation of DCs through a TLR4/MyD88-dependent pathway, and that this activation contributes to the CTL response generated by StII-containing liposomes.

A phase II study of axalimogene filolisbac for patients with previously treated, unresectable, persistent/recurrent loco-regional or metastatic anal cancer

Squamous cell carcinoma of the anorectal canal (SCCA) is a rare HPV-related malignancy that is steadily increasing in incidence. A high unmet need exists for patients with persistent loco-regional and metastatic disease. Axalimogene filolisbac (ADXS11-001) is an investigational immunotherapy that stimulates tumor-specific responses against HPV-associated cancers, and has demonstrated benefit in metastatic cervical cancer. We conducted this single-arm, multicenter, phase 2 trial in patients with persistent/recurrent, loco-regional or metastatic SCCA. Patients received ADXS11-001, 1 × 10⁹ colony-forming units intravenously every 3 weeks. A Simon 2-stage design was used to test primary co-endpoints of overall response rate (ORR) and 6-month progression-free survival (PFS) rate. Study would proceed to full enrollment if ORR ≥ 10% or 6-month PFS rate ≥ 20%. Thirty-six patients were treated; 29 patients were evaluable for response. One patient had a prolonged partial response (3.4% ORR). The 6-month PFS rate was 15.5%. Grade 3 adverse event were noted in 10 patients, with the majority being cytokine-release symptoms; one grade 4 adverse event was noted. No grade 5 adverse events occurred. ADXS11-001 was safe and well-tolerated in patients with SCCA. However, this study did not meet either primary endpoint. ADXS11-001 may be more beneficial when administered in combination with other cytotoxic or targeted agents.

A suicidal strain of Listeria monocytogenes is effective as a DNA vaccine delivery system for oral administration

In this study we determined the in vivo activity of model ovalbumin vaccines delivered by direct intramuscular delivery of plasmid DNA or oral delivery using a recombinant suicidal Listeria monocytogenes strain (rsΔ2). In a previous report we described how rsΔ2 is capable of delivering luciferase, as protein or DNA, in vitro, into non-dividing intestinal epithelial cells (Kuo et al., 2009). This is achieved by engineering a dual expression shuttle vector, pDuLX-Luc, that replicates in E. coli and rsΔ2 and drives gene expression from the Listeria promoter (Phly) as well as the eukaryotic cytomegalovirus promoter (CMV), thereby delivering both protein and plasmid DNA to the cell cytoplasm. For the current in vivo study rsΔ2 containing pDuLX-OVA was used to deliver both ovalbumin protein and the mammalian expression plasmid by the oral route. Controls were used to investigate the activity of this system versus positive and negative controls, as well as quantifying activity against direct intramuscular injection of expression plasmids. Oral administration of rsΔ2(pDuLX-OVA) produced significant titres of antibody and was effective at inducing targeted T-cell lysis (approximately 30% lysis relative to an experimental positive control, intravenous OVA-coated splenocytes+lipopolysaccharide). Intramuscular injection of plasmids pDuLX-OVA or p3L-OVA (which lacks the prokaryotic promoter) also produced significant CTL-mediated cell lysis. The delivery of the negative control rsΔ2 (pDuLX-Luc) confirmed that the observed activity was induced specifically by the ovalbumin vaccination. The data suggest that the oral activity of rsΔ2(pDuLX-OVA) is explained by delivery of OVA protein, expressed in rsΔ2 from the prokaryotic promoter present in pDuLX-OVA, but transfection of mammalian cells in vivo may also play a role. Antibody titres were also produced by oral delivery (in rsΔ2) of the p3L-OVA plasmid in which does not include a prokaryotic promoter.

Agonist anti-GITR antibody significantly enhances the therapeutic efficacy of Listeria monocytogenes-based immunotherapy

Background

We previously demonstrated that in addition to generating an antigen-specific immune response, Listeria monocytogenes (Lm)-based immunotherapy significantly reduces the ratio of regulatory T cells (Tregs)/CD4⁺ and myeloid-derived suppressor cells (MDSCs) in the tumor microenvironment. Since Lm-based immunotherapy is able to inhibit the immune suppressive environment, we hypothesized that combining this treatment with agonist antibody to a co-stimulatory receptor that would further boost the effector arm of immunity will result in significant improvement of anti-tumor efficacy of treatment.

Methods

Here we tested the immune and therapeutic efficacy of Listeria-based immunotherapy combination with agonist antibody to glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) in TC-1 mouse tumor model. We evaluated the potency of combination on tumor growth and survival of treated animals and profiled tumor microenvironment for effector and suppressor cell populations.

Results

We demonstrate that combination of Listeria-based immunotherapy with agonist antibody to GITR synergizes to improve immune and therapeutic efficacy of treatment in a mouse tumor model. We show that this combinational treatment leads to significant inhibition of tumor-growth, prolongs survival and leads to complete regression of established tumors in 60% of treated animals. We determined that this therapeutic benefit of combinational treatment is due to a significant increase in tumor infiltrating effector CD4⁺ and CD8⁺ T cells along with a decrease of inhibitory cells.

Conclusion

To our knowledge, this is the first study that exploits Lm-based immunotherapy combined with agonist anti-GITR antibody as a potent treatment strategy that simultaneously targets both the effector and suppressor arms of the immune system, leading to significantly improved anti-tumor efficacy. We believe that our findings depicted in this manuscript provide a promising and translatable strategy that can enhance the overall efficacy of cancer immunotherapy.

A combination of anti-PD-L1 mAb plus Lm-LLO-E6 vaccine efficiently suppresses tumor growth and metastasis in HPV-infected cancers

PD‐1/PD‐L1 immunotherapy is viewed as having clinical benefits in advanced cancers but is effective in only a few patients, suggesting that an efficient combination approach is needed to improve efficacy. Immunohistochemistry analysis indicated that PD‐L1 expression was correlated with the E6 expression in tumors from 122 lung cancer patients. The poorest survival occurred in PD‐L1‐positive/E6‐positive tumor. PD‐L1 expression was increased by the expression of E6, but not the E7, oncoprotein in lung and cervical cancer cells. PD‐L1 expression was responsible for E6‐mediated colony formation and soft agar growth. Therefore, PD‐L1 secreted from tumor cells may directly promote tumor progression, particularly in E6‐positive tumors. Immune deficiency nude mice were used to test the possibility that combining anti‐PD‐L1 mAb with Lm‐LLO‐E6 vaccine could have a higher antitumor activity compared with anti‐PD‐L1 mAb or Lm‐LLO‐E6 vaccine alone. A greater antitumor activity was obtained with anti‐PD‐L1 mAb + Lm‐LLO‐E6 vaccine than with anti‐PD‐L1 mAb or Lm‐LLO‐E6 alone in subcutaneous and metastatic tumors induced by TL‐1 and SiHa cells. The longest survival time for nude mice was observed in the anti‐PD‐L1 mAb + Lm‐LLO‐E6 vaccine group. In conclusion, an anti‐PD‐L1 mAb + Lm‐LLO‐E6 vaccine may be an efficient treatment for suppression of tumor growth and metastasis induced by HPV‐infected cells.

A Genetically Modified attenuated Listeria Vaccine Expressing HPV16 E7 Kill Tumor Cells in Direct and Antigen-Specific Manner

Attenuated Listeria monocytogenes (L. monocytogenes, LM) induces specific CD8⁺ and CD4⁺ T cell responses, and has been identified as a promising cancer vaccine vector. Cervical cancer is the third most common cancer in women worldwide, with human papillomavirus (HPV), particularly type 16, being the main etiological factor. The therapeutic HPV vaccines are urgently needed. The E7 protein of HPV is necessary for maintaining malignancy in tumor cells. Here, a genetically modified attenuated LM expressing HPV16 E7 protein was constructed. Intraperitoneal vaccination of LM4Δhly::E7 significantly reduced tumor size and even resulted in complete regression of established tumors in a murine model of cervical cancer. We provided evidence that recombinant LM strains could enter the tumor tissue and induce non-specific tumor cell death, probably via activation of reactive oxygen species and increased intracellular Ca²⁺ levels. LM4Δhly::E7 effectively triggered a strong antigen-specific cellular immunity in tumor-bearing mice, and elicited significant infiltration of T cells in the intratumoral milieu. In summary, these data showed LM4Δhly::E7 to be effective in a cervical cancer model and LM4Δhly::E7 induced an antitumor effect by antigen-specific cellular immune responses and direct killing of tumor cells, indicating a potential application against cervical cancer.

Local endothelial complement activation reverses endothelial quiescence, enabling t-cell homing, and tumor control during t-cell immunotherapy

Cancer immunotherapy relies upon the ability of T cells to infiltrate tumors. The endothelium constitutes a barrier between the tumor and effector T cells, and the ability to manipulate local vascular permeability could be translated into effective immunotherapy. Here, we show that in the context of adoptive T cell therapy, antitumor T cells, delivered at high enough doses, can overcome the endothelial barrier and infiltrate tumors, a process that requires local production of C3, complement activation on tumor endothelium and release of C5a. C5a, in turn, acts on endothelial cells promoting the upregulation of adhesion molecules and T-cell homing. Genetic deletion of C3 or the C5a receptor 1 (C5aR1), and pharmacological blockade of C5aR1, impaired the ability of T cells to overcome the endothelial barrier, infiltrate tumors, and control tumor progression in vivo, while genetic chimera mice demonstrated that C3 and C5aR1 expression by tumor stroma, and not leukocytes, governs T cell homing, acting on the local endothelium. In vitro, endothelial C3 and C5a expressions were required for endothelial activation by type 1 cytokines. Our data indicate that effective immunotherapy is a consequence of successful homing of T cells in response to local complement activation, which disrupts the tumor endothelial barrier.

Mechanistic insights into ADXS11-001 human papillomavirus-associated cancer immunotherapy

Immune responses to the facultative intracellular bacterium Listeria monocytogenes (Lm) are robust and well characterized. Utilized for decades as a model of host-disease immunology, Lm is well suited for use as an immunotherapeutic bacterial vector for the delivery of foreign antigen. Genetic modification of Lm has been undertaken to create an attenuated organism that is deficient in its master transcriptional regulator, protein-related factor A, and incorporates a truncated, nonhemolytic version of the listeriolysin O (LLO) molecule to ensure its adjuvant properties while also preventing escape of the live organism from the phagolysosome. Delivery of a vaccine construct (Lm-LLO-E7; axalimogene filolisbac [AXAL] or ADXS11-001) in which the modified LLO molecule is fused with the E7 oncoprotein of human papillomavirus type 16 (HPV-16) consistently stimulates strong innate and E7 antigen-specific adaptive immune responses, resulting in reduction of tumor burden in animal cancer models. In the clinical setting, AXAL has shown early promise in phase I/II trials of women with cervical cancer, and several more trials are currently underway to assess the efficacy and safety of this antitumor vaccine in patients with HPV-positive head and neck and anal cancers.

Attenuated Listeria monocytogenes: a powerful and versatile vector for the future of tumor immunotherapy

For over a century, inactivated or attenuated bacteria have been employed in the clinic as immunotherapies to treat cancer, starting with the Coley's vaccines in the 19th century and leading to the currently approved bacillus Calmette-Guérin vaccine for bladder cancer. While effective, the inflammation induced by these therapies is transient and not designed to induce long-lasting tumor-specific cytolytic T lymphocyte (CTL) responses that have proven so adept at eradicating tumors. Therefore, in order to maintain the benefits of bacteria-induced acute inflammation but gain long-lasting anti-tumor immunity, many groups have constructed recombinant bacteria expressing tumor-associated antigens (TAAs) for the purpose of activating tumor-specific CTLs. One bacterium has proven particularly adept at inducing powerful anti-tumor immunity, Listeria monocytogenes (Lm). Lm is a gram-positive bacterium that selectively infects antigen-presenting cells wherein it is able to efficiently deliver tumor antigens to both the MHC Class I and II antigen presentation pathways for activation of tumor-targeting CTL-mediated immunity. Lm is a versatile bacterial vector as evidenced by its ability to induce therapeutic immunity against a wide-array of TAAs and specifically infect and kill tumor cells directly. It is for these reasons, among others, that Lm-based immunotherapies have delivered impressive therapeutic efficacy in preclinical models of cancer for two decades and are now showing promise clinically. In this review, we will provide an overview of the history leading up to the development of current Lm-based immunotherapies, the advantages and mechanisms of Lm as a therapeutic vaccine vector, the preclinical experience with Lm-based immunotherapies targeting a number of malignancies, and the recent findings from clinical trials along with concluding remarks on the future of Lm-based tumor immunotherapies.

Listeria monocytogenes: a promising vehicle for neonatal vaccination

Vaccination as a medical intervention has proven capable of greatly reducing the suffering from childhood infectious disease. However, newborns and infants in particular are age groups for whom adequate vaccine-mediated protection is still largely lacking. With the challenges that the neonatal immune system faces and the required highest level of stringency for safety, designing vaccines for early life in general and the newborn in particular poses great difficulty. Nevertheless, recent advances in our understanding of neonatal immunity and its responses to vaccines and adjuvants suggest that neonatal vaccination is a task fully within reach. Among the most promising developments in neonatal vaccination is the use of Listeria monocytogenes (Lm) as a delivery platform. In this review, we will outline key properties of Lm that make it such an ideal neonatal and early life vaccine vehicle, and also discuss potential constraints of Lm as a vaccine delivery platform.

Interferon γ-induced intratumoral expression of CXCL9 alters the local distribution of T cells following immunotherapy with Listeria monocytogenes

The ability of Listeria monocytogenes-based anticancer vaccines to induce tumor regression depends on the responsiveness of malignant cells to interferon γ (IFNγ). Inhibition of IFNγ limits the recruitment of T cells to the tumors of vaccinated mice. We hypothesized that vaccination with immunotherapeutic L. monocytogenes induces the IFNγ-dependent production of chemokines that regulate the migration of tumor-infiltrating T cells. To gain further insights into this issue, we examined the chemokine responses of a transplantable, human papillomavirus (HPV)-immortalized murine tumor model (TC-1) following the administration of a L. monocytogenes-based immunotherapeutic agent that expresses E7 from HPV-16. Here, we report that the administration of L. monocytogenes-based anticancer vaccines increases the secretion of chemokine (C-X-C motif) ligand 9 (CXCL9), and CXCL10 by tumors, hence favoring the recruitment of T cells bearing the cognate chemokine (C-X-C motif) receptor 3 (CXCR3). Furthermore, the expression of CXCL9, but not CXCL10, in TC-1 tumors was significantly reduced upon anti-IFNγ antibody treatment. CXCL9 was highly expressed by TC-1 cells following the administration of IFNγ and tumor necrosis factor α (TNFα), in vitro. Moreover, the inhibition of CXCL9 in TC-1 cells reduced the proportion of CD8⁺ T cells infiltrating tumors in vaccinated mice, while increasing that of CD4⁺ T cells, thus altering T-cell subset distribution. We conclude that the administration of L. monocytogenes-based anticancer vaccines regulates T_H1 chemokine responses and that malignant cells are an important source of these chemokines.

Listeriolysin O as a strong immunogenic molecule for the development of new anti-tumor vaccines

The pore-forming toxin listeriolysin O (LLO), which is produced by Listeria monocytogenes, mediates bacterial phagosomal escape and facilitates bacterial multiplication during infection. This toxin has recently gained attention because of its confirmed role in the controlled and specific modulation of the immune response. Currently, cancer immunotherapies are focused on conquering the immune tolerance induced by poorly immunogenic tumor antigens and eliciting strong, lasting immunological memory. An effective way to achieve these goals is the co-administration of potent immunomodulatory adjuvant components with vaccine vectors. LLO, a toxin that belongs to the family of cholesterol-dependent cytolysins (CDCs), exhibits potent cell type-non-specific toxicity and is a source of dominant CD4(+) and CD8(+) T cell epitopes. According to recent research, in addition to its effective cytotoxicity as a cancer immunotherapeutic drug, the non-specific adjuvant property of LLO makes it promising for the development of efficacious anti-tumor vaccines.

Using macrophage activation to augment immunotherapy of established tumours

Background:

Successful immunotherapy will require alteration of the tumour microenvironment and/or decreased immune suppression. Tumour-associated macrophages (TAMs) are one major factor affecting tumour microenvironment. We hypothesised that altering TAM phenotype would augment the efficacy of immunotherapy.

Methods:

We and others have reported that 5,6-Dimethylxanthenone-4-acetic-acid (DMXAA, Vadimezan) has the ability to change TAM phenotypes, inducing a tumour microenvironment conducive to antitumour immune responses. We therefore combined DMXAA with active immunotherapies, and evaluated anti-tumour efficacy, immune cell phenotypes (flow cytometry), and tumour microenvironment (RT–PCR).

Results:

In several different murine models of immunotherapy for lung cancer, DMXAA-induced macrophage activation significantly augmented the therapeutic effects of immunotherapy. By increasing influx of neutrophils and anti-tumour (M1) macrophages to the tumour, DMXAA altered myeloid cell phenotypes, thus changing the intratumoural M2/non-M2 TAM immunoinhibitory ratio. It also altered the tumour microenvironment to be more pro-inflammatory. Modulating macrophages during immunotherapy resulted in increased numbers, activity, and antigen-specificity of intratumoural CD8⁺ T cells. Macrophage depletion reduced the effect of combining immunotherapy with macrophage activation, supporting the importance of TAMs in the combined effect.

Conclusion:

Modulating intratumoural macrophages dramatically augmented the effect of immunotherapy. Our observations suggest that addition of agents that activate TAMs to immunotherapy should be considered in future trials.

Listeria monocytogenes-derived listeriolysin O has pathogen-associated molecular pattern-like properties independent of its hemolytic ability

There is a constant need for improved adjuvants to augment the induction of immune responses against tumor-associated antigens (TAA) during immunotherapy. Previous studies have established that listeriolysin O (LLO), a cholesterol-dependent cytolysin derived from Listeria monocytogenes, exhibits multifaceted effects to boost the stimulation of immune responses to a variety of antigens. However, the direct ability of LLO as an adjuvant and whether it acts as a pathogen-associated molecular pattern (PAMP) have not been demonstrated. In this paper, we show that a detoxified, nonhemolytic form of LLO (dtLLO) is an effective adjuvant in tumor immunotherapy and may activate innate and cellular immune responses by acting as a PAMP. Our investigation of the adjuvant activity demonstrates that dtLLO, either fused to or administered as a mixture with a human papillomavirus type 16 (HPV-16) E7 recombinant protein, can augment antitumor immune responses and facilitate tumor eradication. Further mechanistic studies using bone marrow-derived dendritic cells suggest that dtLLO acts as a PAMP by stimulating production of proinflammatory cytokines and inducing maturation of antigen-presenting cells (APC). We propose that dtLLO is an effective adjuvant for tumor immunotherapy, and likely for other therapeutic settings.

Listeria monocytogenes and its products as agents for cancer immunotherapy

This review covers the use of Listeria monocytogenes and its virulence factors as cancer immunotherapeutics. We describe their development as vectors to carry protein tumor antigen and eukaryotic DNA plasmids to antigen-presenting cells and efforts to harness their tumor-homing properties. We also describe their use as vectors of angiogenic molecules to induce an immune response that will destroy tumor vasculature. The background knowledge necessary to understand the biology behind the rationale to develop Listeria as a vaccine vector for tumor immunotherapy is included as well as a brief summary of the major therapies that have used this approach thus far.

Prime-boost vaccination with heterologous live vectors encoding SIV gag and multimeric HIV-1 gp160 protein: efficacy against repeated mucosal R5 clade C SHIV challenges

We sought to induce primate immunodeficiency virus-specific cellular and neutralizing antibody (nAb) responses in rhesus macaques (RM) through a bimodal vaccine approach. RM were immunized intragastrically (i.g.) with the live-attenuated Listeria monocytogenes (Lm) vector Lmdd-BdopSIVgag encoding SIVmac239 gag. SIV Gag-specific cellular responses were boosted by intranasal and intratracheal administration of replication-competent adenovirus (Ad5hr-SIVgag) encoding the same gag. To broaden antiviral immunity, the RM were immunized with multimeric HIV clade C (HIV-C) gp160 and HIV Tat. SIV Gag-specific cellular immune responses and HIV-1 nAb developed in some RM. The animals were challenged intrarectally with five low doses of R5 SHIV-1157ipEL-p, encoding a heterologous HIV-C Env (22.1% divergent to the Env immunogen). All five controls became viremic. One out of ten vaccinees was completely protected and another had low peak viremia. Sera from the completely and partially protected RM neutralized the challenge virus > 90%; these RM also had strong SIV Gag-specific proliferation of CD8⁺ T cells. Peak and area under the curve of plasma viremia (during acute phase) among vaccinees was lower than for controls, but did not attain significance. The completely protected RM showed persistently low numbers of the α4β7-expressing CD4⁺ T cells; the latter have been implicated as preferential virus targets in vivo. Thus, vaccine-induced immune responses and relatively lower numbers of potential target cells were associated with protection.

A live attenuated Listeria monocytogenes vaccine vector expressing SIV Gag is safe and immunogenic in macaques and can be administered repeatedly

Listeria monocytogenes (Lm) is known to induce strong cellular immune responses. We constructed a live-attenuated Lm vector, Lmdd-BdopSIVgag, which encodes SIVmac239 gag. Intragastric (i.g.) administration of 3 × 10(12) bacteria to rhesus macaques was safe and induced anti-Gag cellular but no humoral immune responses. Boosting of Gag-specific cellular responses was observed after i.g. administration of Lmdd-BdopSIVgag to previously vaccinated RM despite preexisting anti-Lm immunity shown by lymphoproliferative responses. Surprisingly, anti-Lm cellular responses were also detected in non-vaccinated controls, which may reflect the fact that Lm is a ubiquitous bacterium. The novel, live-attenuated Lmdd-BdopSIVgag may be an attractive platform for oral vaccine delivery.

Recombinant E. coli LLO/OVA induces murine BMDCs maturation via TLR4 and NOD1 receptor and promotes specific cytotoxic T cell immunity

OBJECTIVE

To explore the immune stimulation effect of recombinant E.coli LLO/OVA on mice bone marrow-derived dendritic cells (BMDCs) and T lymphocytes in vitro.

METHODS

After BMDCs stimulated by E.coli LLO/OVA, their Toll-like receptor (TLR) and nucleotide-binding oligomerization domain (NOD) receptor signalling pathway were examined by superarray hybridization; and the priming effect of the vaccine activated BMDCs on CD4(+)T and CD8(+)T was determined by [3H]thymidine uptake and ELISA, the tumor cytotoxic effect of activated CD8(+)T cells was determined by cytotoxic assay.

RESULTS

After BMDCs were activated by E. coli LLO/OVA via TLR4, NOD1 receptor and NF-κB signalling pathway, the expression of their surface molecules including MHC class I, MHC class II, CD40, CD80 and CD86 significantly up-regulated; the secretion of IL-12 and IFN-γ increased also. The mature BMDCs stimulated the allergic CD4(+)T and CD8(+)T cells proliferation and their IL-2 and IFN-γ secretion, and the activated CD8(+)T cells effectively killed B16-OVA melanoma cells and RMA-S/OVA lymphoma cells in vitro.

CONCLUSION

E.coli LLO/OVA is effective in inducing BMDCs maturation via activating TLR4 and NOD1 receptor signalling pathway and promoting specific anti-tumor T cell immunity in vitro.

Costimulator B7-DC attenuates strong Th2 responses induced by Nippostrongylus brasiliensis

The caliber and magnitude of T cell responses are regulated by costimulatory molecules following the engagement of TCRs and MHC molecules. B7-DC has the highest homology with B7-H1 in the B7 family, and both of them bind an immunoregulatory molecule, programmed death 1. Previous studies have demonstrated that B7-DC stimulates T cell proliferation and CTL generation, which sharply contrasts the inhibitory role of B7-H1. Th2 cytokines prompt B7-DC expression, which in turn enhances Th1 responses. In this study, we used an intestinal nematode, Nippostrongylus brasiliensis, to induce strong Th2 responses and to evaluate B7-DC function under Th2-polarizing conditions in vivo. By either blocking B7-DC expression during N. brasiliensis infection or by examining N. brasiliensis-infected B7-DC knockout mice, we observed enhanced eosinophilia, the overproduction of serum IgE, and increased Th2 cytokine production along with decreased Th1 cytokine production (particularly IFN-gamma production), indicating that B7-DC inhibits Th2 responses. Our results further demonstrate that the inhibition of Th2 responses by B7-DC occurs independently of programmed death 1 but conceivably acts through an as yet unknown alternative receptor that enhances Th1 responses. Although the deficiency of B7-DC expression that enhanced the production of IL-13 paradoxically resulted in better protection against N. brasiliensis infection, our results show that B7-DC plays an important role in bolstering a robust Th1 response that is required for effective antiviral and anticancer immunity, even under a strong Th2-polarizing environment induced by N. brasiliensis infection.

Multiple effector mechanisms induced by recombinant Listeria monocytogenes anticancer immunotherapeutics

Listeria monocytogenes is a facultative intracellular gram-positive bacterium that naturally infects professional antigen presenting cells (APC) to target antigens to both class I and class II antigen processing pathways. This infection process results in the stimulation of strong innate and adaptive immune responses, which make it an ideal candidate for a vaccine vector to deliver heterologous antigens. This ability of L. monocytogenes has been exploited by several researchers over the past decade to specifically deliver tumor-associated antigens that are poorly immunogenic such as self-antigens. This review describes the preclinical studies that have elucidated the multiple immune responses elicited by this bacterium that direct its ability to influence tumor growth.

Engineering bacterial vectors for delivery of genes and proteins to antigen-presenting cells

Bacterial vectors offer a biological route to gene and protein delivery with this article featuring delivery to antigen-presenting cells (APCs). Primarily in the context of immune stimulation against infectious disease or cancer, the goal of bacterially mediated delivery is to overcome the hurdles to effective macromolecule delivery. This review will present several bacterial vectors as macromolecule (protein or gene) delivery devices with both innate and acquirable (or engineered) biological features to facilitate delivery to APCs. The review will also present topics related to large-scale manufacture, storage, and distribution that must be considered if the bacterial delivery devices are ever to be used in a global market.

Dendritic cell targeting of survivin protein in a xenogeneic form elicits strong CD4+ T cell immunity to mouse survivin

To determine whether strong CD4+ T cell immunity could be induced to a nonmutated self protein that is important for tumorigenesis, we selectively targeted the xenogeneic form of survivin, a survival protein overexpressed in tumors, to maturing dendritic cells in lymphoid tissues. Dendritic cell targeting via the DEC205 receptor in the presence of anti-CD40 and poly(I:C) as maturation stimuli, induced strong human and mouse survivin-specific CD4+ T cell responses, as determined by IFN-gamma, TNF-alpha, and IL-2 production, as well as the development of lytic MHC class II-restricted T cells and memory. Immunity was enhanced further by depletion of CD25+foxp3+ cells before vaccination. anti-DEC205-human survivin was superior in inducing CD4+ T cell responses relative to other approaches involving survivin plasmid DNA or survivin peptides with adjuvants. However, we were unable to induce CD8+ T cell immunity to survivin by two doses of DEC205-targeted survivin or the other strategies. Therefore, significant CD4+ T cell immunity to a self protein that is overexpressed in most human cancers can be induced by DEC205 targeting of the Ag in its xenogeneic form to maturing DCs.

Adjuvant properties of listeriolysin O protein in a DNA vaccination strategy

The use of infectious agents as vaccine adjuvants has shown utility in both prophylactic and therapeutic vaccinations. Listeria monocytogenes has been used extensively as a vaccine vehicle due to its ability to initiate both CD4(+) and CD8(+) immune responses. Previous work from this laboratory has used transgenic Listeria to deliver vaccine constructs. A chimeric protein composed of tumor antigen and a non-hemolytic variant of the Listeria protein, listeriolysin O (LLO), has demonstrated effective tumor protection beyond that of antigen alone expressed in the same system. To address the question of how fusion with LLO improves vaccine efficacy, we constructed a number of DNA plasmid vaccines to isolate this effect in the absence of other endogenous Listeria effects. Here we have analyzed the ability of these vaccines to induce the regression of previously established tumors. A vaccine strategy using DNA vaccines bearing the tumor antigen either alone or in combination with LLO in addition to plasmids encoding MIP-1alpha and GM-CSF was examined. Further, LLO was used either as a chimera or in a bicistronic construct to address the importance of fusion between these elements. Notably, the strategies employing both chimeric and bicistronic vaccines were effective in reducing tumor burden suggesting that LLO can act as an adjuvant that does not require fusion with the tumor antigen to mediate its effect.

Fusion to Listeriolysin O and delivery by Listeria monocytogenes enhances the immunogenicity of HER-2/neu and reveals subdominant epitopes in the FVB/N mouse

Five overlapping fragments of rat HER-2/neu have been expressed in recombinant Listeria monocytogenes. Each fragment of HER-2/neu is secreted as a fusion protein with a truncated, nonhemolytic form of listeriolysin O (LLO). Lm-LLO-EC1, Lm-LLO-EC2, and Lm-LLO-EC3 overlap the extracellular domain of HER-2/neu, whereas Lm-LLO-IC1 and Lm-LLO-IC2 span the intracellular domain. All five strains controlled the growth of established NT-2 tumors, a rat HER-2/neu-expressing tumor line derived from a spontaneously arising mammary tumor in a FVB/N HER-2/neu-transgenic mouse. The antitumor effect of each of these vaccine constructs was abrogated by the in vivo depletion of CD8(+) T cells, although only one known epitope has been defined previously and is present in Lm-LLO-EC2. Anti-HER-2/neu CTL responses were generated by each of the rLm vaccine constructs. With the use of a panel of 3T3 cell lines expressing overlapping fragments of HER-2/neu, regions of HER-2/neu with potential CD8(+) T cell epitopes have been defined. DNA vaccines expressing either a fragment or full-length HER-2/neu were constructed in LLO-fused and non-LLO-fused forms. CTL analysis of the DNA vaccines revealed a broadening in the regions of HER-2/neu recognizable as targets when the target Ag is fused to LLO. These studies show the efficacy of L. monocytogenes-based HER-2/neu vaccines in a murine model of breast cancer and also that the immunogenicity of self-Ags can be increased by fusion to LLO and delivery by L. monocytogenes revealing subdominant epitopes.

What is needed for effective antitumor immunotherapy? Lessons learned using Listeria monocytogenes as a live vector for HPV-associated tumors

As a vaccine vector, Listeria monocytogenes targets the innate immune system, resulting in a cytokine response that enhances antigen-presenting cell function as well as inducing a Th1 profile. It also enhances cell-mediated immunity by targeting antigen delivery in antigen-presenting cells to both the MHC class I pathway of exogenous presentation that activates CD8 T cells and the MHC class II pathway that processes antigen endogenously and presents it to CD4 T cells. In this review, we describe the development of vaccine constructs that target the human papillomavirus 16 (HPV-16) E7 antigen, and we characterize their effects on tumor regression as well as various immune parameters both innate and adaptive. In particular, we describe the effect on tumor angiogenesis, induction of antitumor suppressor factors like CD4+CD25+ T cells and regulatory cytokines TGF-beta and IL-10, homing and infiltration of antigen-specific CD8+ T cells to the tumor, and also effects of the vaccines on antigen-presenting cells, especially focusing on dendritic cell maturation and ability to influence tumor regression. We believe that the identification of several immune parameters that correlate with antitumor efficacy, and of some that have a negative correlation, may have wider application for other cancer immunotherapeutic approaches.

Tumor sensitivity to IFN-gamma is required for successful antigen-specific immunotherapy of a transplantable mouse tumor model for HPV-transformed tumors

PURPOSE

Many human tumors lose responsiveness to IFN-gamma, providing a possible mechanism for the tumor to avoid immune recognition and destruction. Here we investigate the importance of tumor responsiveness to IFN-gamma in the successful immunotherapy of TC1 tumors that were immortalized with human papillomavirus proteins E6 and E7.

METHODS

To investigate the role of IFN-gamma in vivo, we constructed a variant of TC1, TC1.mugR, that is unresponsive to IFN-gamma due to overexpression of a dominant negative IFN-gamma receptor.

RESULTS

Using recombinant Listeria monocytogenes that express HPV-16 E7 (Lm-LLO-E7) to stimulate an antitumor response, we demonstrate that sensitivity to IFN-gamma is required for therapeutic efficacy in that Lm-LLO-E7 induces regression of TC1 tumors but not TC1.mugR. In addition, we show that tumor sensitivity to IFN-gamma is not required for inhibition of tumor angiogenesis by Lm-LLO-E7 or for trafficking of CD4+ and CD8+ T cells to the tumor. However, it is required for penetration of lymphocytes into the tumor mass in vivo.

CONCLUSIONS

Our findings identify a role for IFN-gamma in immunity to TC1 tumors and show that loss of tumor responsiveness to IFN-gamma poses a challenge to antigen-based immunotherapy.

CD4+CD25+ regulatory T cells that secrete TGFbeta and IL-10 are preferentially induced by a vaccine vector

CD4CD25 T cells generated in a vaccine scenario can play a critical role in limiting antitumor therapy, thus having widespread implications for the immunotherapy-based treatment of cancer. The authors previously used Listeria monocytogenes to develop two vaccine constructs that express HPV-16 E7 protein and induce strong cellular immunity to HPV-E7-expressing tumors. Immunization of mice bearing established E7-expressing tumors with Lm-LLO-E7 induced regression of the tumors, whereas Lm-E7 showed little or no tumor regression. To investigate the possibility that regulatory CD4CD25 T-cell populations may be responsible for the differences in tumor regression, the authors characterized the role of these cells generated by the two vaccine systems. The authors compared the prevalence of CD4CD25 T cells in tumor-bearing vaccinated mice and demonstrate that Lm-E7-vaccinated mice have significantly increased numbers of CD4CD25 T cells in both the spleen and tumor-infiltrating lymphocytes compared with Lm-LLO-E7-vaccinated mice. The authors confirm that these increased numbers of CD4CD25 T cells are indeed suppressor in function by in vitro suppression assays and that the mechanism of action of the tumor-infiltrating cells involves the production of suppressor cytokines interleukin-10 and transforming growth factor beta. These results show that it is possible for a tumor vaccine system to generate tumor-infiltrating CD4CD25 regulatory T cells that critically affect tumor regression and the overall success of vaccine therapy.