Enhancing the immunogenicity of bioengineered Listeria monocytogenes by passaging through live animal hosts

Current and future direction in treatment of HPV-related cervical disease

Human papillomavirus (HPV) is the most common sexually transmitted virus in the world. About 70% of cervical cancers are caused by the most oncogenic HPV genotypes of 16 and 18. Since available prophylactic vaccines do not induce immunity in those with established HPV infections, the development of therapeutic HPV vaccines using E6 and E7 oncogenes, or both as the target antigens remains essential. Also, knocking out the E6 and E7 oncogenes in host genome by genome-editing CRISPR/Cas system can result in tumor growth suppression. These methods have shown promising results in both preclinical and clinical trials and can be used for controlling the progression of HPV-related cervical diseases. This comprehensive review will detail the current treatment of HPV-related cervical precancerous and cancerous diseases. We also reviewed the future direction of treatment including different kinds of therapeutic methods and vaccines, genome-editing CRISPR/Cas system being studied in clinical trials. Although the progress in the development of therapeutic HPV vaccine has been slow, encouraging results from recent trials showed vaccine-induced regression in high-grade CIN lesions. CRISPR/Cas genome-editing system is also a promising strategy for HPV cancer therapy. However, its safety and specificity need to be optimized before it is used in clinical setting.

Cervical Cancer Immunotherapy: Facts and Hopes

It is a sad fact that despite being almost completely preventable through human papillomavirus (HPV) vaccination and screening, cervical cancer remains the fourth most common cancer to affect women worldwide. Persistent high-risk HPV (hrHPV) infection is the primary etiologic factor for cervical cancer. Upward of 70% of cases are driven by HPV types 16 and 18, with a dozen other hrHPVs associated with the remainder of cases. Current standard-of-care treatments include radiotherapy, chemotherapy, and/or surgical resection. However, they have significant side effects and limited efficacy against advanced disease. There are a few treatment options for recurrent or metastatic cases. Immunotherapy offers new hope, as demonstrated by the recent approval of programmed cell death protein 1-blocking antibody for recurrent or metastatic disease. This might be augmented by combination with antigen-specific immunotherapy approaches, such as vaccines or adoptive cell transfer, to enhance the host cellular immune response targeting HPV-positive cancer cells. As cervical cancer progresses, it can foster an immunosuppressive microenvironment and counteract host anticancer immunity. Thus, approaches to reverse suppressive immune environments and bolster effector T-cell functioning are likely to enhance the success of such cervical cancer immunotherapy. The success of nonspecific immunostimulants like imiquimod against genital warts also suggest the possibility of utilizing these immunotherapeutic strategies in cervical cancer prevention to treat precursor lesions (cervical intraepithelial neoplasia) and persistent hrHPV infections against which the licensed prophylactic HPV vaccines have no efficacy. Here, we review the progress and challenges in the development of immunotherapeutic approaches for the prevention and treatment of cervical cancer.

Novel Therapeutics for Recurrent Cervical Cancer: Moving Towards Personalized Therapy

While screening programs and HPV vaccination have decreased the incidence of cervical cancer, still over 13,000 cases occur in the USA annually. Early-stage cervical cancer has an excellent long-term prognosis, with 5-year survival for localized disease being > 90%. Survival decreases markedly for both locally advanced and metastatic disease, and both are associated with a higher risk of recurrence. Few effective treatment options exist for persistent, recurrent, or metastatic cervical cancer. In 2014, the anti-VEGF antibody bevacizumab was approved in combination with chemotherapy based on the results of the Phase III GOG-240 study. As the majority of cervical cancers have a viral etiology, which impairs the immune system, immunotherapy using checkpoint inhibitors and other agents, appears to be a promising approach. In June 2018, the US FDA approved the anti-PD1 antibody pembrolizumab for recurrent or metastatic cervical cancer with PD-L1 expression that progressed after one or more lines of chemotherapy. Another anti-PD1 antibody, cemiplimab also shows potential in this setting, either as monotherapy or combined with radiotherapy, and it is currently being evaluated in a Phase III trial. Additional checkpoint inhibitors including nivolumab, durvalumab, atezolizumab, and camrelizumab are in different stages of clinical development for the disease. Finally, an additional targeted approach being pursued involves PARP inhibitors (rucaparib and olaparib are both in Phase II) based on earlier study results.

Therapeutic Vaccines Against Human Papilloma Viruses: Achievements and Prospects

Human papillomaviruses of high carcinogenic risk (HR HPVs) are major etiological agents of malignant diseases of the cervix, vulva, penis, anal canal, larynx, head, and neck. Prophylactic vaccination against HPV, which mainly covers girls and women under 25, does not prevent vertical and horizontal HPV transmission in infants and children and does not have a therapeutic effect. As a result, a significant proportion of the population is not protected from the HPV infection and development of HPV-associated neoplastic transformation and cancer, which indicates the need for development and introduction of therapeutic HPV vaccines. Unlike prophylactic vaccines aimed at the formation of virus-neutralizing antibodies, therapeutic vaccines elicit cellular immune response leading to the elimination of infected and malignant cells expressing viral proteins. The ideal targets for vaccine immunotherapy are highly conserved HR HPV oncoproteins E6 and E7 expressed in precancerous and tumor tissues. Here, we describe expression of these proteins during different stages of HPV infection, their antigenic and immunogenic properties, and T-cell epitopes, the response to which correlates with natural regression of HPV-induced neoplastic changes. The review describes patterns of E6 and E7 oncoproteins presentation to the immune system as components of candidate vaccines along with the results of the most promising preclinical trials and animal models used in these trials. Special attention is paid to vaccine candidates which have shown efficacy in clinical trials in patients with HPV-associated neoplastic changes.

Current strategies against persistent human papillomavirus infection (Review)

Human papillomavirus (HPV) is the most common sexually transmitted infection, exhibiting a tropism for the epidermis and mucosae. The link between persistent HPV infection and malignancies involving the anogenital tract as well as the head and neck has been well‑established, and it is estimated that HPV‑related cancers involving various anatomical sites account for 4.5% of all human cancers. Current prophylactic vaccines against HPV have enabled the prevention of associated malignancies. However, the sizeable population base of current infection in whom prophylactic vaccines are not applicable, certain high‑risk HPV types not included in vaccines, and the vast susceptible population in developing countries who do not have access to the costly prophylactic vaccines, put forward an imperative need for effective therapies targeting persistent infection. In this article, the life cycle of HPV, the mechanisms facilitating HPV evasion of recognition and clearance by the host immune system, and the promising therapeutic strategies currently under investigation, particularly antiviral drugs and therapeutic vaccines, are reviewed.

Current status and future prospects for human papillomavirus vaccines

Cervical cancer is the fourth most frequent cancer among women worldwide. Human papillomaviruses (HPVs) cause almost all cervical cancers in low-income countries. Three prophylactic HPV virus-like particle-based vaccines have been licensed to date, and they have all shown high efficacy and reliable safety profiles. However, isolated safety issues have resulted in a reluctance to use these vaccinations. In addition, the high prices of the vaccinations have caused the inequitable distribution of the vaccine: the prices are unaffordable for low-income countries. Meanwhile, great effort has been put into the development of therapeutic HPV vaccines, including protein/peptide-, live vector-, DNA- and cell-based vaccines. These new vaccines have considerable therapeutic potential but limited practical use. The development of immune checkpoint inhibitors and personalized immunotherapy remain challenges for future study. In this article, the current status of the licensed vaccines, therapeutic HPV vaccines and biosimilars, and new platforms for HPV vaccines, are reviewed, and safety issues related to the licensed vaccines are discussed. In addition, the prospects for HPV vaccines are considered.

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.

Effect of continuous sub-culturing on infectivity of Clostridium perfringens ATCC13124 in mouse gas gangrene model

Clostridium perfringens is a Validated Biological Agent and a pathogen of medical, veterinary, and military significance. Gas gangrene is the most destructive of all the clostridial diseases and is caused by C. perfringens type A strains wherein the infection spreads quickly (several inches per hour) with production of gas. Influence of repeated in vitro cultivation on the infectivity of C. perfringens was investigated by comparing the surface proteins of laboratory strain and repository strains of the bacterium using 2DE-MS approach. In order to optimize host-pathogen interaction during experimental gas gangrene infection, we also explored the role of particulate matrix on ability of C. perfringens to cause gas gangrene.

The current state of therapeutic and T cell-based vaccines against human papillomaviruses

Human papillomavirus (HPV) is known to be a necessary factor for many gynecologic malignancies and is also associated with a subset of head and neck malignancies. This knowledge has created the opportunity to control these HPV-associated cancers through vaccination. However, despite the availability of prophylactic HPV vaccines, HPV infections remain extremely common worldwide. In addition, while prophylactic HPV vaccines have been effective in preventing infection, they are ineffective at clearing pre-existing HPV infections. Thus, there is an urgent need for therapeutic and T cell-based vaccines to treat existing HPV infections and HPV-associated lesions and cancers. Unlike prophylactic vaccines, which generate neutralizing antibodies, therapeutic, and T cell-based vaccines enhance cell-mediated immunity against HPV antigens. Our review will cover various therapeutic and T cell-based vaccines in development for the treatment of HPV-associated diseases. Furthermore, we review the strategies to enhance the efficacy of therapeutic vaccines and the latest clinical trials on therapeutic and T cell-based HPV vaccines.

Perspectives for therapeutic HPV vaccine development

Background

Human papillomavirus (HPV) infections and associated diseases remain a serious burden worldwide. It is now clear that HPV serves as the etiological factor and biologic carcinogen for HPV-associated lesions and cancers. Although preventative HPV vaccines are available, these vaccines do not induce strong therapeutic effects against established HPV infections and lesions. These concerns create a critical need for the development of therapeutic strategies, such as vaccines, to treat these existing infections and diseases.

Main Body

Unlike preventative vaccines, therapeutic vaccines aim to generate cell-mediated immunity. HPV oncoproteins E6 and E7 are responsible for the malignant progression of HPV-associated diseases and are consistently expressed in HPV-associated diseases and cancer lesions; therefore, they serve as ideal targets for the development of therapeutic HPV vaccines. In this review we revisit therapeutic HPV vaccines that utilize this knowledge to treat HPV-associated lesions and cancers, with a focus on the findings of recent therapeutic HPV vaccine clinical trials.

Conclusion

Great progress has been made to develop and improve novel therapeutic HPV vaccines to treat existing HPV infections and diseases; however, there is still much work to be done. We believe that therapeutic HPV vaccines have the potential to become a widely available and successful therapy to treat HPV and HPV-associated diseases in the near future.

Current state in the development of candidate therapeutic HPV vaccines

The identification of human papillomavirus (HPV) as an etiological factor for HPV-associated malignancies creates the opportunity to control these cancers through vaccination. Currently, available preventive HPV vaccines have not yet demonstrated strong evidences for therapeutic effects against established HPV infections and lesions. Furthermore, HPV infections remain extremely common. Thus, there is urgent need for therapeutic vaccines to treat existing HPV infections and HPV-associated diseases. Therapeutic vaccines differ from preventive vaccines in that they are aimed at generating cell-mediated immunity rather than neutralizing antibodies. The HPV-encoded early proteins, especially oncoproteins E6 and E7, form ideal targets for therapeutic HPV vaccines since they are consistently expressed in HPV-associated malignancies and precancerous lesions, playing crucial roles in the generation and maintenance of HPV-associated disease. Our review will cover various therapeutic vaccines in development for the treatment of HPV-associated lesions and cancers. Furthermore, we review strategies to enhance vaccine efficacy and the latest clinical trials on therapeutic HPV vaccines.

The potential of plants for the production and delivery of human papillomavirus vaccines

The available vaccines against human papillomavirus have some limitations such as low coverage due to their high cost, reduced immune coverage and the lack of therapeutic effects. Recombinant vaccines produced in plants (genetically engineered using stable or transient expression systems) offer the possibility to obtain low cost, efficacious and easy to administer vaccines. The status on the development of plant-based vaccines against human papillomavirus is analyzed and placed in perspective in this review. Some candidates have been characterized at a preclinical level with interesting outcomes. However, there is a need to perform the immunological characterization of several vaccine prototypes, especially through the oral administration route, as well as develop new candidates based on new chimeric designs intended to provide broader immunoprotection and therapeutic activity.

Tertiary prevention of cervical cancer

Human pappilomavirus (HPV) has been recognized as the most common sexually transmitted disease in the world and over 100 different HPV types have been identified. Persistent HPV infection has been closely linked to the development of invasive cervical cancer. Although surgical and ablative therapies have been the mainstay of treatment, vaccination against the main oncogenic type of HPV is a reasonable preventive strategy for HPV-induced cervical cancer.

Perspective for prophylaxis and treatment of cervical cancer: an immunological approach

As the second most common cause of cancer-related death in women, human papilloma virus (HPV) vaccines have been a major step in decreasing the morbidity and mortality associated with cervical cancer. An estimated 490,000 women are diagnosed with cervical cancer each year. Increasing knowledge of the HPV role in the etiology of cervical cancer has led to the development and introduction of HPV-based vaccines for active immunotherapy of cervical cancer. Immunotherapies directed at preventing HPV-persistent infections. These vaccines are already accessible for prophylaxis and in the near future, they will be available for the treatment of preexisting HPV-related neoplastic lesions.

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.

Development of a Listeria monocytogenes-based vaccine against hepatocellular carcinoma

Live attenuated Listeria monocytogenes (LM) is a promising bacterial vector able to induce a T-cell response to tumor-associated antigens and demonstrates great potential for use in vaccine development. A novel recombinant LM-based vaccine (Lmdd (LM ΔdalΔdat)-MPFG (multiple peptide fusing genes)) was developed with the ability to express and secrete hepatocellular carcinoma (HCC)-related tumor-associated antigens fragments due to the insertion of hepatitis B virus (HBV)-X protein (HBx)-derived epitopes HBx(52-60) and HBx(140-148), the universal T-helper epitope, alpha-fetoprotein (AFP) epitope AFP(158-166), and melanoma antigen gene (MAGE)-3(271-279) into the HBV core protein. Following immunization with the Lmdd-MPFG vaccine, macrophages exhibited uptake of the bacteria; the vaccine was then nearly cleared 3 days after the first administration. It disappeared even more quickly following subsequent vaccinations. However, recombinant Lmdd-MPFG allowed for the full development of an antitumor response towards the human leukocyte antigen (HLA)-A0201 epitopes of MPFG. Each epitope stimulated an augmented T-cell proliferation and enhanced the supernatant level of interferon (IFN)-γ in vitro. In addition, IFN-γ-producing CD8(+) T cells as well as in vivo cytolytic activity were significantly increased in HLA-A2 transgenic mice. Additionally, the Lmdd-MPFG developed a strong antitumor response, as indicated by the significant resistance of immunized mice to MPFG-positive Hepa1-6 cell challenge in both a prophylactic and therapeutic setting. Tumor regression was accompanied by an enhanced cytotoxic T lymphocyte response and a decrease of regulatory T cells in the tumor. Collectively, these results suggest that utilizing attenuated LM as a vaccine vector, able to carry the MPFG gene, presents a potentially feasible strategy for prevention of HCC.

Heterologous prime/boost immunization with p53-based vaccines combined with toll-like receptor stimulation enhances tumor regression

The p53 gene product is overexpressed in approximately 50% of cancers, making it an ideal target for cancer immunotherapy. We previously demonstrated that a modified vaccinia Ankara (MVA) vaccine expressing human p53 (MVA-p53) was moderately active when given as a homologous prime/boost in a human p53 knock in (Hupki) mouse model. We needed to improve upon the inefficient homologous boosting approach, because development of neutralizing immunity to the vaccine viral vector backbone suppresses its immunogenicity. To enhance specificity, we examined the combination of 2 different vaccine vectors provided in sequence as a heterologous prime/boost. Hupki mice were evaluated as a human p53 tolerant model to explore the capacity of heterologous p53 immunization to reject human p53-expressing tumors. We employed attenuated recombinant Listeria monocytogenes expressing human p53 (LmddA-LLO-p53) in addition to MVA-p53. Heterologous p53 immunization resulted in a significant increase in p53-specific CD8 and CD4 T cells compared with homologous single vector p53 immunization. Heterologous p53 immunization induced protection against tumor growth but had only a modest effect on established tumors. To enhance the immune response we used synthetic double-strand RNA (polyinsosinic:polycytidylic acid) and unmethylated CpG-containing oligodeoxynucleotide to activate the innate immune system via Toll-like receptors. Treatment of established tumor-bearing Hupki mice with polyinsosinic:polycytidylic acid and CpG-oligodeoxynucleotide in combination with heterologous p53 immunization resulted in enhanced tumor rejection relative to treatment with either agent alone. These results suggest that heterologous prime/boost immunization and Toll-like receptor stimulation increases the efficacy of a cancer vaccine, targeting a tolerized tumor antigen.

Listeria and Salmonella bacterial vectors of tumor-associated antigens for cancer immunotherapy

This review covers the use of the facultative intracellular bacteria, Listeriamonocytogenes and Salmonella enterica serovar typhimurium as delivery systems for tumor-associated antigens in tumor immunotherapy. Because of their ability to infect and survive in antigen presenting cells, these bacteria have been harnessed to deliver tumor antigens to the immune system both as bacterially expressed proteins and encoded on eukaryotic plasmids. They do this in the context of strong innate immunity, which provides the required stimulus to the immune response to break tolerance against those tumor-associated antigens that bear homology to self. Here we describe differences in the properties of these bacteria as vaccine vectors, a summary of the major therapies they have been applied to and their advancement towards the clinic.

Perspectives for preventive and therapeutic HPV vaccines

Human Papillomavirus (HPV) has been associated with several human cancers, including cervical cancer, vulvar cancer, vaginal and anal cancer, and a subset of head and neck cancers. Thus effective vaccination against HPV provides an opportunity to reduce the morbidity and mortality associated with HPV. The Food and Drug Administration of the United States has approved two preventive vaccines to limit the spread of HPV. However, these are unlikely to impact upon HPV prevalence and cervical cancer rates for many years. Furthermore, preventive vaccines do not exert therapeutic effects on pre-existing HPV infections and HPV-associated lesions. In order to further impact upon the burden of HPV infections worldwide, therapeutic vaccines are being developed. These vaccines aim to generate a cell-mediated immune response to infected cells. This review discusses current preventive and therapeutic HPV vaccines and their future directions.

RasGRP1 regulates antigen-induced developmental programming by naive CD8 T cells

Ag encounter by naive CD8 T cells initiates a developmental program consisting of cellular proliferation, changes in gene expression, and the formation of effector and memory T cells. The strength and duration of TCR signaling are known to be important parameters regulating the differentiation of naive CD8 T cells, although the molecular signals arbitrating these processes remain poorly defined. The Ras-guanyl nucleotide exchange factor RasGRP1 has been shown to transduce TCR-mediated signals critically required for the maturation of developing thymocytes. To elucidate the role of RasGRP1 in CD8 T cell differentiation, in vitro and in vivo experiments were performed with 2C TCR transgenic CD8 T cells lacking RasGRP1. In this study, we report that RasGRP1 regulates the threshold of T cell activation and Ag-induced expansion, at least in part, through the regulation of IL-2 production. Moreover, RasGRP1(-/-) 2C CD8 T cells exhibit an anergic phenotype in response to cognate Ag stimulation that is partially reversible upon the addition of exogenous IL-2. By contrast, the capacity of IL-2/IL-2R interactions to mediate Ras activation and CD8 T cell expansion and differentiation appears to be largely RasGRP1-independent. Collectively, our results demonstrate that RasGRP1 plays a selective role in T cell signaling, controlling the initiation and duration of CD8 T cell immune responses.

A novel human Her-2/neu chimeric molecule expressed by Listeria monocytogenes can elicit potent HLA-A2 restricted CD8-positive T cell responses and impact the growth and spread of Her-2/neu-positive breast tumors

PURPOSE

The aim of this study was to efficiently design a novel vaccine for human Her-2/neu-positive (hHer-2/neu) breast cancer using the live, attenuated bacterial vector Listeria monocytogenes.

EXPERIMENTAL DESIGN

Three recombinant L. monocytogenes-based vaccines were generated that could express and secrete extracellular and intracellular fragments of the hHer-2/neu protein. In addition, we generated a fourth construct fusing selected portions of each individual fragment that contained most of the human leukocyte antigen (HLA) epitopes as a combination vaccine (L. monocytogenes-hHer-2/neu chimera).

RESULTS

Each individual vaccine was able to either fully regress or slow tumor growth in a mouse model for Her-2/neu-positive tumors. All three vaccines could elicit immune responses directed toward human leukocyte antigen-A2 epitopes of hHer-2/neu. The L. monocytogenes-hHer-2/neu chimera was able to mimic responses generated by the three separate vaccines and prevent spontaneous outgrowth of tumors in an autochthonous model for Her-2/neu-positive breast cancer, induce tumor regression in transplantable models, and prevent seeding of experimental lung metastases in a murine model for metastatic breast cancer.

CONCLUSION

This novel L. monocytogenes-hHer-2/neu chimera vaccine proves to be just as effective as the individual vaccines but combines the strength of all three in a single vaccination. These encouraging results support future clinical trials using this chimera vaccine and may be applicable to other cancer types expressing the Her-2/neu molecule such as colorectal and pancreatic cancer.

From bench to cageside: Risk assessment for rodent pathogen contamination of cells and biologics

Many newly developed animal models involve the transfer of cells, serum, or other tissue-derived products into live rodents. These biologics can serve as repositories for adventitious rodent pathogens that, when used in animal studies, can alter research outcomes and result in endemic outbreaks. This review includes a description of some of the biologics that have inadvertently introduced infectious agents into in vivo studies and/or resulted in endemic outbreaks. I also discuss the points of potential exposure of specific biologics to adventitious rodent pathogens as well as the importance of acquiring a complete developmental and testing history of each biologic introduced into a barrier facility. There are descriptions of specific cases of mycoplasma and lactate dehydrogenase–elevating virus (LDHV), two of the most common organisms that contaminate cells and cell byproducts. The information in this article should help investigators and animal resource program personnel to perform an appropriate risk assessment of biologics before their use in in vivo studies that involve rodents.

Cancer immunotherapy targeting the high molecular weight melanoma-associated antigen protein results in a broad antitumor response and reduction of pericytes in the tumor vasculature

The high molecular weight melanoma-associated antigen (HMW-MAA), also known as melanoma chondroitin sulfate proteoglycan, has been used as a target for the immunotherapy of melanoma. This antigen is expressed on the cell surface and has a restricted distribution in normal tissues. Besides its expression in a broad range of transformed cells, this antigen is also found in pericytes, which are important for tumor angiogenesis. We generated a recombinant Listeria monocytogenes (Lm-LLO-HMW-MAA-C) that expresses and secretes a fragment of HMW-MAA (residues 2,160-2,258) fused to the first 441 residues of the listeriolysin O (LLO) protein. Immunization with Lm-LLO-HMW-MAA-C was able to impede the tumor growth of early established B16F10-HMW-MAA tumors in mice and both CD4(+) and CD8(+) T cells were required for therapeutic efficacy. Immune responses to a known HLA-A2 epitope present in the HMW-MAA(2160-2258) fragment was detected in the HLA-A2/K(b) transgenic mice immunized with Lm-LLO-HMW-MAA-C. Surprisingly, this vaccine also significantly impaired the in vivo growth of other tumorigenic cell lines, such as melanoma, renal carcinoma, and breast tumors, which were not engineered to express HMW-MAA. One hypothesis is that the vaccine could be targeting pericytes, which are important for tumor angiogenesis. In a breast tumor model, immunization with Lm-LLO-HMW-MAA-C caused CD8(+) T-cell infiltration in the tumor stroma and a significant decrease in the number of pericytes in the tumor blood vessels. In conclusion, a Lm-based vaccine against HMW-MAA can trigger cell-mediated immune responses to this antigen that can target not only tumor cells but also pericytes in the tumor vasculature.

Construction and characterization of an attenuated Listeria monocytogenes strain for clinical use in cancer immunotherapy

Listeria monocytogenes has been exploited previously as a vaccine vector for the delivery of heterologous proteins such as tumor-specific antigens for active cancer immunotherapy. However, for effective use of live vector in clinics, safety is a major concern. In the present study, we describe an irreversibly attenuated and highly immunogenic L. monocytogenes platform, the L. monocytogenes dal-, dat-, and actA-deleted strain that expresses the human prostate-specific antigen (PSA) using an antibiotic resistance marker-free plasmid (the dal dat DeltaactA 142 strain expressing PSA). Despite limited in vivo survival, the dal dat DeltaactA 142 strain was able to elicit efficient immune responses required for tumor clearance. Our results showed that immunization of mice with the dal dat DeltaactA 142 strain caused the regression of the tumors established by the prostate adenocarcinoma cell line expressing PSA. An evaluation of immunologic potency indicated that the dal dat DeltaactA 142 strain elicits a high frequency of PSA-specific immune responses. Interestingly, immunization with the dal dat DeltaactA 142 strain induced significant infiltration of PSA-specific T cells in the intratumoral milieu. Collectively, our data suggest that the dal dat DeltaactA 142 strain is a safe and potent vector for clinical use and that this platform may be further exploited as a potential candidate to express other single or multiple antigens for cancer immunotherapy.

Development of a Listeria monocytogenes based vaccine against prostate cancer

Prostate specific antigen (PSA) is a likely immunotherapeutic target antigen for prostate cancer, the second leading cause of cancer-related death in American men. Previously, we demonstrated that attenuated strains of Listeria monocytogenes (Lm) can be used as effective vaccine vectors for delivery of tumor antigens causing regression of established tumors accompanied by strong immune responses toward these antigens in murine models of cancer. In the present study, we have developed and characterized a recombinant live attenuated L. monocytogenes/PSA (Lm-LLO-PSA) vaccine with potential use for the treatment of pCa. Human PSA gene was cloned into and expressed by an attenuated Lm strain. This recombinant bacterial vaccine, Lm-LLO-PSA was tested for stability, virulence, immunogenicity and anti-tumor effects in a murine model for pCa. Immunization with Lm-LLO-PSA was shown to lower the number of tumor infiltrating T regulatory cells and cause complete regression of over 80% of tumors formed by an implanted genetically modified mouse prostate adenocarcinoma cell line, which expressed human PSA. Lm-LLO-PSA was immunogenic in C57BL/6 mice and splenocytes from mice immunized with Lm-LLO-PSA showed significantly higher number of IFN-gamma secreting cells over that of the naïve animals in response to a PSA H2Db-specific peptide, as measured by both, ELISpot and intracellular cytokine staining. In addition, using a CTL assay we show that the T cells specific for PSA were able to recognize and lyse PSA-peptide pulsed target cells in vitro. In a comparison study with two other PSA-based vaccines (a pDNA and a vaccinia vaccine), Lm-LLO-PSA was shown to be more efficacious in regressing established tumors when used in a homologues prime/boost regimen. Together, these results indicate that Lm-LLO-PSA is a potential candidate for pCa immunotherapy and should be further developed.

Live attenuated Listeria monocytogenes expressing HIV Gag: immunogenicity in rhesus monkeys

Induction of strong cellular immunity will be important for AIDS vaccine candidates. Natural infection with wild-type Listeria monocytogenes (Lm), an orally transmitted organism, is known to generate strong cellular immunity, thus raising the possibility that live attenuated Lm could serve as a vaccine vector. We sought to examine the potential of live attenuated Lm to induce cellular immune responses to HIV Gag. Rhesus macaques were immunized with Lmdd-gag that expresses HIV gag and lacks two genes in the D-alanine (D-ala) synthesis pathway. Without this key component of the bacterial cell wall, vaccine vector replication critically depends on exogenous D-ala. Lmdd-gag was given to animals either solely orally or by oral priming followed by intramuscular (i.m.) boosting; D-ala was co-administered with all vaccinations. Lmdd-gag and D-ala were well tolerated. Oral priming/oral boosting induced Gag-specific cellular immune responses, whereas oral priming/i.m. boosting induced systemic as well as mucosal anti-Gag antibodies. These results suggest that the route of vaccination may bias anti-Gag immune responses either towards T-helper type 1 (Th1) or Th2 responses; overall, our data show that live attenuated, recombinant Lmdd-gag is safe and immunogenic in primates.

A DNA prime-oral Listeria boost vaccine in rhesus macaques induces a SIV-specific CD8 T cell mucosal response characterized by high levels of alpha4beta7 integrin and an effector memory phenotype

In this study in Rhesus macaques, we tested whether IL-12 or IL-15 in a DNA prime-oral Listeria boost amplifies the SIV-Gag-specific CD8 mucosal response. SIV-specific CD8 T cells were demonstrated in the peripheral blood (PB) in all test vaccine groups, but not the control group. SIV-Gag-specific CD8 T cells in the PB expressed alpha4beta7 integrin, the gut-homing receptor; a minor subset co-express alphaEbeta7 integrin. SIV-Gag-specific CD8 T cells were also detected in the gut tissue, intraepithelial (IEL) and lamina propria lymphocytes (LPL) of the duodenum and ileum. These cells were characterized by high levels of beta7 integrin expression and a predominance of the effector memory phenotype. Neither Il-12 nor IL-15 amplified the frequency of SIV-specific CD8 T cells in the gut. Thus, the DNA prime-oral Listeria boost strategy induced a mucosal SIV-Gag-specific CD8 T cell response characterized by expression of the alpha4beta7 integrin gut-homing receptor.

In vivo bactofection: listeria can function as a DNA-cancer vaccine

The development of an effective therapeutic vaccine to induce cancer-specific immunity remains an unsolved yet pressing priority requiring novel vaccine strategies. Here we have generated a series of vaccines in which bacteria deliver a plasmid encoding a tumor antigen under the control of a mammalian promoter in an attempt to induce an antitumor immune response. Utilizing a plasmid release mechanism involving the suicide of the carrier bacteria, we were able to engineer Listeria monocytogenes to induce antitumor immunity to a physiologically relevant tumor antigen, the cervical cancer oncoprotein E7. In a mouse model of cervical cancer, we were able to slow tumor growth and induce an effector CD8(+) T-cell response against the immunodominant epitope for E7. The CD8(+) T cells generated could both home to and penetrate the tumor. This is the first demonstration of in vivo efficacy of bactofection vectors in treating solid tumors. However, although this delivery system was more effective than administering plasmid alone, it was not as effective as L. monocytogenes engineered to deliver the E7 protein in impacting on established tumor growth.

Listeria monocytogenes-based antibiotic resistance gene-free antigen delivery system applicable to other bacterial vectors and DNA vaccines

Plasmids represent a powerful tool to rapidly introduce genes into bacteria and help them reach high expression levels. In vaccine development, with live vaccine vectors, this allows greater flexibility and the ability to induce larger antigen amounts through multiple gene copies. However, plasmid retention often requires antibiotic resistance markers, the presence of which has been discouraged in clinical applications by the Food and Drug Administration. Therefore, we developed a Listeria monocytogenes-Escherichia coli shuttle plasmid that is retained by complementation of D-alanine racemase-deficient mutant strains both in vitro and in vivo. Our technology potentially allows the production of antibiotic resistance marker-free DNA vaccines as well as bacterial vaccine vectors devoid of engineered antibiotic resistances. As a proof of concept, we applied the D-alanine racemase complementation system to our Listeria cancer vaccine platform. With a transplantable tumor model, we compared the efficacy of the new Listeria vector to that of an established vector containing a conventional plasmid carrying a tumor-specific antigen. Both vaccine vector systems resulted in long-term regression of established tumors, with no significant difference between them. Thus, the Listeria vaccine vector presented here potentially complies with Food and Drug Administration regulations and could be developed further for clinical use.

Vaccination strategies for the treatment and prevention of cervical cancer

PURPOSE OF REVIEW

Immunotherapy of HPV-induced premalignant anogenital lesions and cervical cancer has made impressive progress. HPV as causative agent is targeted by prophylactic and therapeutic vaccination strategies. Preclinical and clinical studies have shown induction of natural and/or vaccine-induced immune responses. This review will summarize the status of vaccine development and clinical testing published since March 2003.

RECENT FINDINGS

For prophylactic vaccines there is first clinical evidence of effectivity (ie, 100% protection from HPV infection and dysplasia by virus-like particle (VLP) vaccine-induced neutralizing antibodies). Also, therapeutic vaccines have entered clinical evaluation. While prophylactic VLP vaccines are immunogenic per se, therapeutic vaccines will need further adjuvants to guide T cell differentiation, expansion, survival, and homing to tumor sites. To enhance clinical outcome of successful T cell induction in patients, the susceptibility of the tumor cells for lysis must be addressed in the future, since tumor immune evasion is a severe problem in cervical cancer.

SUMMARY

While successful prophylactic HPV vaccines have entered large clinical trials, therapeutic HPV vaccines, in spite of T cell induction, lack clinical responses due to the problem of tumor immune evasion. Adjuvants for systemic and local immune modulation will be mandatory for effective therapy.

The induction of HIV Gag-specific CD8+ T cells in the spleen and gut-associated lymphoid tissue by parenteral or mucosal immunization with recombinant Listeria monocytogenes HIV Gag

The induction of mucosal immunity is crucial in controlling viral replication during HIV infection. In this study we compare the ability of a recombinant Listeria monocytogenes that expresses and secretes the HIV Ag Gag to induce CD8(+) T cells against this Ag in the spleen, mesenteric lymph nodes, and Peyer's patches and the ability to provide effector Gag-specific CD8(+) T cells to the lamina propria after i.v., oral, or rectal administration of the vaccine. The levels of Ag-specific CD8(+)-activated T cells were measured ex vivo using intracellular cytokine staining for IFN-gamma and H-2K(d) Gag peptide tetramer staining. We found that all routes of immunization induced Gag-specific CD8(+) T cells in the spleen. After secondary infection, we observed substantial increases in splenic levels of CD8(+) T cells, and levels of Gag-specific cells were similar to those against listeriolysin O, the immunodominant Ag of L. monocytogenes. Both primary and secondary oral immunization resulted in abundant Gag-specific CD8(+)-activated T cells in the lamina propria that constituted approximately 35% of the CD8 compartment. However, significant levels of Gag and listeriolysin O-specific CD8(+) T cells were observed in mucosal lymphoid tissue only after two immunizations, perhaps because they had already entered the lamina propria compartment after a single immunization. In the context of HIV, a mucosally administered vaccine seems best calculated to prompt an immune response that is capable of preventing infection. The data presented in this report demonstrate that mucosally administered Listeria can prompt such a response and that booster doses can maintain this response.