Papillomavirus pseudovirus: a novel vaccine to induce mucosal and systemic cytotoxic T-lymphocyte responses

Vaccine Strategies to Elicit Mucosal Immunity

Vaccines are essential tools to prevent infection and control transmission of infectious diseases that threaten public health. Most infectious agents enter their hosts across mucosal surfaces, which make up key first lines of host defense against pathogens. Mucosal immune responses play critical roles in host immune defense to provide durable and better recall responses. Substantial attention has been focused on developing effective mucosal vaccines to elicit robust localized and systemic immune responses by administration via mucosal routes. Mucosal vaccines that elicit effective immune responses yield protection superior to parenterally delivered vaccines. Beyond their valuable immunogenicity, mucosal vaccines can be less expensive and easier to administer without a need for injection materials and more highly trained personnel. However, developing effective mucosal vaccines faces many challenges, and much effort has been directed at their development. In this article, we review the history of mucosal vaccine development and present an overview of mucosal compartment biology and the roles that mucosal immunity plays in defending against infection, knowledge that has helped inform mucosal vaccine development. We explore new progress in mucosal vaccine design and optimization and novel approaches created to improve the efficacy and safety of mucosal vaccines.

A Comparative Study on Delivery of Externally Attached DNA by Papillomavirus VLPs and Pseudoviruses

Human papillomavirus (HPV) 16 capsids have been chosen as a DNA delivery vehicle in many studies. Our preliminary studies suggest that HPV58 capsids could be better vehicles than HPV16 capsids to deliver encapsidated DNA in vitro and in vivo. In the current study, we compared HPV16, HPV58, and the cottontail rabbit papillomavirus (CRPV) capsids either as L1/L2 VLPs or pseudoviruses (PSVs) to deliver externally attached GFP-expressing DNA. Both rabbit and human cells were used to test whether there was a species-specific effect. DNA delivery efficiency was determined by quantifying either GFP-expressing cell populations or mean fluorescent intensities (MFI) by flow cytometry. Interestingly, CRPV and 58-VLPs and PSVs were significantly more efficient at delivering attached DNA when compared to 16-VLPs and PSVs. A capsid/DNA ratio of 2:1 showed the highest efficiency for delivering external DNA. The PSVs with papillomavirus DNA genomes also showed higher efficiency than those with irrelevant plasmid DNA. HPV16L1/58L2 hybrid VLPs displayed increased efficiency compared to HPV58L1/16L2 VLPs, suggesting that L2 may play a critical role in the delivery of attached DNA. Additionally, we demonstrated that VLPs increased in vivo infectivity of CRPV DNA in rabbits. We conclude that choosing CRPV or 58 capsids to deliver external DNA could improve DNA uptake in in vitro and in vivo models.

Demystifying particle-based oral vaccines

Introduction: The oral route of vaccination is pain- and needle-free and can induce systemic and mucosal immunity. However, gastrointestinal barriers and antigen degradation impose significant hurdles in the development of oral vaccines. Live attenuated viruses and bacteria can overcome these barriers but at the risk of introducing safety concerns. As an alternative, particles have been investigated for antigen protection and delivery, yet there are no FDA-approved oral vaccines based on particle-based delivery systems. Our objective was to discover underlying determinants that can explain the current inadequacies and identify paradigms that can be implemented in future for successful development of oral vaccines relying on particle-based delivery systems.Areas covered: We reviewed literature related to the use of particles for oral vaccination and placed special emphasis on formulation characteristics and administration schedules to gain an insight into how these parameters impact production of antigen-specific antibodies in systemic and mucosal compartments.Expert opinion: Despite the long history of vaccines, particle-based oral vaccination is a relative new field with the first study published in 1989. Substantial variability exists between different studies with respect to dosing schedules, number of doses, and the amount of vaccine per dose. Most studies have not used adjuvants in the formulations. Better standardization in vaccination parameters is required to improve comparison between experiments, and adjuvants should be used to enhance the systemic and mucosal immune responses and to reduce the number of doses, which will make oral vaccines more attractive.

Cutaneous Papillomaviruses and Non-melanoma Skin Cancer: Causal Agents or Innocent Bystanders?

There is still controversy in the scientific field about whether certain types of cutaneous human papillomaviruses (HPVs) are causally involved in the development of non-melanoma skin cancer (NMSC). Deciphering the etiological role of cutaneous HPVs requires - besides tissue culture systems - appropriate preclinical models to match the obtained results with clinical data from affected patients. Clear scientific evidence about the etiology and underlying mechanisms involved in NMSC development is fundamental to provide reasonable arguments for public health institutions to classify at least certain cutaneous HPVs as group 1 carcinogens. This in turn would have implications on fundraising institutions and health care decision makers to force - similarly as for anogenital cancer - the implementation of a broad vaccination program against "high-risk" cutaneous HPVs to prevent NMSC as the most frequent cancer worldwide. Precise knowledge of the multi-step progression from normal cells to cancer is a prerequisite to understand the functional and clinical impact of cofactors that affect the individual outcome and the personalized treatment of a disease. This overview summarizes not only recent arguments that favor the acceptance of a viral etiology in NMSC development but also reflects aspects of causality in medicine, the use of empirically meaningful model systems and strategies for prevention.

Papilloma-pseudovirus eradicates intestinal tumours and triples the lifespan of ApcMin/+ mice

Inducing tumour-specific adaptive immunity, such as cytotoxic T lymphocyte (CTL) response, can result in promising antitumour effect against several human malignancies, especially in combination with immune checkpoint blockade strategies. However, little is known whether activation of innate immunity can lead to direct tumoricidal effect. Here, we develop a papilloma pseudovirus-based oral immunotherapeutic approach that shows strong tumoricidal effects in the gut, resulting in an almost tripled lifespan of Apc^Min/+ mice (an animal model of human intestinal tumorigenesis). Mechanistically, these pseudoviruses activate the NLRP3 and AIM2 inflammasomes, leading to caspase-1-mediated tumour regression that is dependent on neither cytotoxic T lymphocytes nor humoral immune response. Blocking caspase-1 activation abrogated the therapeutic effects of the pseudoviruses. Thus, targeting innate immune sensors in tumours by the pseudoviruses might represent a strategy to treat intestinal tumours.

Current Advances in Virus-Like Particles as a Vaccination Approach against HIV Infection

HIV-1 virus-like particles (VLPs) are promising vaccine candidates against HIV-1 infection. They are capable of preserving the native conformation of HIV-1 antigens and priming CD4+ and CD8+ T cell responses efficiently via cross presentation by both major histocompatibility complex (MHC) class I and II molecules. Progress has been achieved in the preclinical research of HIV-1 VLPs as prophylactic vaccines that induce broadly neutralizing antibodies and potent T cell responses. Moreover, the progress in HIV-1 dendritic cells (DC)-based immunotherapy provides us with a new vision for HIV-1 vaccine development. In this review, we describe updates from the past 5 years on the development of HIV-1 VLPs as a vaccine candidate and on the combined use of HIV particles with HIV-1 DC-based immunotherapy as efficient prophylactic and therapeutic vaccination strategies.

Human papillomavirus genotypes among females in Mexico: a study from the Mexican institute for social security

BACKGROUND

The aetiological relationship between human papillomavirus (HPV) infection and cervical cancer (CC) is widely accepted. Our goal was to determine the prevalence of HPV types in Mexican women attending at the Mexican Institute for Social Security from different areas of Mexico.

MATERIALS AND METHODS

DNAs from 2,956 cervical samples were subjected to HPV genotyping: 1,020 samples with normal cytology, 931 with low-grade squamous intraepithelial lesions (LGSIL), 481 with high grade HGSIL and 524 CC.

RESULTS

Overall HPV prevalence was 67.1%. A total of 40 HPV types were found; HPV16 was detected in 39.4% of the HPV-positive samples followed by HPV18 at 7.5%, HPV31 at 7.1%, HPV59 at 4.9%, and HPV58 at 3.2%. HPV16 presented the highest prevalence both in women with altered or normal cytology and HPV 18 presented a minor prevalence as reported worldwide. The prevalence ratio (PR) was calculated for the HPV types. The analysis of PR showed that HPV16 presents the highest association with CC, HPV 31, -33, -45, -52 and -58 also demonstrating a high association.

CONCLUSIONS

The most prevalent HPV types in cervical cancer samples were -16, -18, -31, but it is important to note that we obtained a minor prevalence of HPV18 as reported worldwide, and that HPV58 and -52 also were genotypes with an important prevalence in CC samples. Determination of HPV genotypes is very important in order to evaluate the impact of vaccine introduction and future cervical cancer prevention strategies.

Bovine papillomavirus-like particles presenting conserved epitopes from membrane-proximal external region of HIV-1 gp41 induced mucosal and systemic antibodies

Two conserved epitopes, located in the membrane-proximal external region (MPER) of the human immunodeficiency virus type 1 (HIV-1) gp41, are recognized by two HIV-1 broadly neutralizing antibodies 2F5 and 4E10, and are promising targets for vaccine design in efforts to elicit anti-HIV-1 broadly neutralizing antibodies. Since most HIV-1 infections initiate at mucosal surfaces, induction of mucosal neutralizing antibodies is necessary and of utmost importance to counteract HIV-1 infection. Here, we utilized a mucosal vaccine vector, bovine papillomavirus (BPV) virus-like particles (VLPs), as a platform to present HIV-1 neutralizing epitopes by inserting the extended 2F5 or 4E10 epitope or the MPER domain into D-E loop of BPV L1 respectively. The chimeric VLPs presenting MPER domain resembled the HIV-1 natural epitopes better than the chimeric VLPs presenting single epitopes. Oral immunization of mice with the chimeric VLPs displaying the 2F5 epitope or MPER domain elicited epitope-specific serum IgGs and mucosal secretory IgAs. The induced antibodies specifically recognized the native conformation of MPER in the context of HIV-1 envelope protein. The antibodies induced by chimeric VLPs presenting MPER domain are able to partially neutralize HIV-1 viruses from clade B and clade C.

Targeting the vaginal mucosa with human papillomavirus pseudovirion vaccines delivering simian immunodeficiency virus DNA

The majority of HIV infections occur via mucosal transmission. Vaccines that induce memory T and B cells in the female genital tract may prevent the establishment and systemic dissemination of HIV. We tested the immunogenicity of a vaccine that uses human papillomavirus (HPV)-based gene transfer vectors, also called pseudovirions (PsVs), to deliver SIV genes to the vaginal epithelium. Our findings demonstrate that this vaccine platform induces gene expression in the genital tract in both cynomolgus and rhesus macaques. Intravaginal vaccination with HPV16, HPV45, and HPV58 PsVs delivering SIV Gag DNA induced Gag-specific Abs in serum and the vaginal tract, and T cell responses in blood, vaginal mucosa, and draining lymph nodes that rapidly expanded following intravaginal exposure to SIV(mac251.) HPV PsV-based vehicles are immunogenic, which warrant further testing as vaccine candidates for HIV and may provide a useful model to evaluate the benefits and risks of inducing high levels of SIV-specific immune responses at mucosal sites prior to SIV infection.

Preventive and Therapeutic Vaccines against Human Papillomaviruses Associated Cervical Cancers

Cervical cancer is, globally known to be, one of the most common cancers among women especially in developing countries. More than 90% of cervical cancers are associated with high-risk human papillomaviruses (HPVs) particularly HPV types 16 and 18. Two major strategies have been developed for prevention and treatment of cervical cancer and other HPV-associated malignancies; the first one is based on HPV virus-like particles (VLPs) containing HPV structural proteins. VLP based vaccines can induce genotype specific virus neutralizing antibodies for preventing HPV infections. The other strategy is based on HPV early genes especially E6 and E7 for eliminating the established HPV infections; therefore they are classified as HPV therapeutic vaccines. This article reviews the preventive and therapeutic vaccines against HPV infections and cervical cancer.

DNA vaccines delivered by human papillomavirus pseudovirions as a promising approach for generating antigen-specific CD8+ T cell immunity

Background

Human papillomavirus (HPV) pseudovirions have recently been shown to deliver DNA efficiently in vivo, resulting in the priming of antigen-specific CD8+ T cells in vaccinated mice. In the current study, we compare the different preparation methods for the generation of HPV pseudovirions for their ability to efficiently infect cells. We also compare the antigen-specific CD8+ T cell immune responses generated by different DNA delivery methods and several commonly used forms of vaccination with that of HPV pseudovirions.

Results

We found that the preparation method of pseudovirions is important for the efficient delivery of encapsidated DNA. We have shown that vaccination with DNA encoding model antigen ovalbumin (OVA) delivered by HPV-16 pseudovirions was capable of generating therapeutic antitumor effects against OVA-expressing tumor. In addition, vaccination with DNA encoding OVA delivered by HPV-16 pseudovirions generated the highest number of OVA-specific CD8+ T cells in mice in our system compared to DNA delivered by other delivery methods. We also found that vaccination with OVA DNA delivered by HPV-16 pseudovirions generated the highest number of OVA-specific CD8+ T cells in mice compared to other forms of antigen-specific vaccines. Furthermore, HPV-16 pseudovirions were capable of carrying DNA vaccine encoding clinically relevant antigen, telomerase reverse transcriptase, to generate antigen-specific CD8+ T cell immune responses.

Conclusions

Our data suggest that DNA vaccines delivered by HPV-16 pseudovirions may be advantageous compared to other delivery methods and other forms of antigen-specific vaccines for application to antigen-specific immunotherapy.

Vaccine and adjuvant design for emerging viruses: mutations, deletions, segments and signaling

Vaccination is currently the most effective strategy to medically control viral diseases. However, developing vaccines is a long and expensive process, and traditional methods, such as attenuating wild-type viruses by serial passage, may not be suitable for all viruses and may lead to vaccine safety considerations, particularly in the case of the vaccination of particular patient groups, such as the immunocompromised and the elderly. In particular, developing vaccines against emerging viral pathogens adds a further level of complexity, as they may only be administered to small groups of people or only in response to a specific event or threat, limiting our ability to study and evaluate responses. In this commentary, we discuss how novel techniques may be used to engineer a new generation of vaccine candidates as we move toward a more targeted vaccine design strategy, driven by our understanding of the mechanisms of viral pathogenesis, attenuation and the signaling events which are required to develop a lasting, protective immunity. We will also briefly discuss the potential future role of vaccine adjuvants, which could be used to bridge the gap between vaccine safety, and lasting immunity from a single vaccination. 

AAV's anatomy: roadmap for optimizing vectors for translational success

Adeno-Associated Virus based vectors (rAAV) are advantageous for human gene therapy due to low inflammatory responses, lack of toxicity, natural persistence, and ability to transencapsidate the genome allowing large variations in vector biology and tropism. Over sixty clinical trials have been conducted using rAAV serotype 2 for gene delivery with a number demonstrating success in immunoprivileged sites, including the retina and the CNS. Furthermore, an increasing number of trials have been initiated utilizing other serotypes of AAV to exploit vector tropism, trafficking, and expression efficiency. While these trials have demonstrated success in safety with emerging success in clinical outcomes, one benefit has been identification of issues associated with vector administration in humans (e.g. the role of pre-existing antibody responses, loss of transgene expression in non-immunoprivileged sites, and low transgene expression levels). For these reasons, several strategies are being used to optimize rAAV vectors, ranging from addition of exogenous agents for immune evasion to optimization of the transgene cassette for enhanced therapeutic output. By far, the vast majority of approaches have focused on genetic manipulation of the viral capsid. These methods include rational mutagenesis, engineering of targeting peptides, generation of chimeric particles, library and directed evolution approaches, as well as immune evasion modifications. Overall, these modifications have created a new repertoire of AAV vectors with improved targeting, transgene expression, and immune evasion. Continued work in these areas should synergize strategies to improve capsids and transgene cassettes that will eventually lead to optimized vectors ideally suited for translational success.

Mucosal delivery of human papillomavirus pseudovirus-encapsidated plasmids improves the potency of DNA vaccination

Mucosal immunization may be important for protection against pathogens whose transmission and pathogenesis target the mucosal tissue. The capsid proteins of human papillomavirus (HPV) confer tropism for the basal epithelium and can encapsidate DNA during self-assembly to form pseudovirions (PsVs). Therefore, we produced mucosal vaccine vectors by HPV PsV encapsidation of DNA plasmids expressing an experimental antigen derived from the M and M2 proteins of respiratory syncytial virus (RSV). Intravaginal (IVag) delivery elicited local and systemic M-M2-specific CD8+ T-cell and antibody responses in mice that were comparable to an approximately 10,000-fold higher dose of naked DNA. A single HPV PsV IVag immunization primed for M-M2-specific-IgA in nasal and vaginal secretions. Based on light emission and immunofluorescent microscopy, immunization with HPV PsV-encapsidated luciferase- and red fluorescent protein (RFP)-expressing plasmids resulted in transient antigen expression (<5 days), which was restricted to the vaginal epithelium. HPV PsV encapsidation of plasmid DNA is a novel strategy for mucosal immunization that could provide new vaccine options for selected mucosal pathogens.

Papillomavirus pseudovirions packaged with the L2 gene induce cross-neutralizing antibodies

BACKGROUND

Current vaccines against HPVs are constituted of L1 protein self-assembled into virus-like particles (VLPs) and they have been shown to protect against natural HPV16 and HPV18 infections and associated lesions. In addition, limited cross-protection has been observed against closely related types. Immunization with L2 protein in animal models has been shown to provide cross-protection against distant papillomavirus types, suggesting that the L2 protein contains cross-neutralizing epitopes. However, vaccination with L2 protein or L2 peptides does not induce high titers of anti-L2 antibodies. In order to develop a vaccine with the potential to protect against other high-risk HPV types, we have produced HPV58 pseudovirions encoding the HPV31 L2 protein and compared their capacity to induce cross-neutralizing antibodies with that of HPV L1 and HPV L1/L2 VLPs.

METHODS

The titers of neutralizing antibodies against HPV16, HPV18, HPV31 and HPV58 induced in Balb/c mice were compared after immunization with L2-containing vaccines.

RESULTS

Low titers of cross-neutralizing antibodies were detected in mice when immunized with L1/L2 VLPs, and the highest levels of cross-neutralizing antibodies were observed in mice immunized with HPV 58 L1/L2 pseudovirions encoding the HPV 31 L2 protein.

CONCLUSIONS

The results obtained indicate that high levels of cross-neutralizing antibodies are only observed after immunization with pseudovirions encoding the L2 protein. HPV pseudovirions thus represent a possible new strategy for the generation of a broad-spectrum vaccine to protect against high-risk HPVs and associated neoplasia.

Listeria monocytogenes delivery of HPV-16 major capsid protein L1 induces systemic and mucosal cell-mediated CD4+ and CD8+ T-cell responses after oral immunization

Neutralizing antibodies are thought to be required at mucosal surfaces to prevent human papillomavirus (HPV) transmission. However, the potential for cell-mediated immunity in mediating protection against HPV infection has not been well explored. We generated recombinant Listeria monocytogenes (Lm) constructs that secrete listeriolysin O (LLO) fused with overlapping N-terminal (LLO-L1(1-258)) or C-terminal (LLO-L1(238-474)) fragments of HPV type 16 major capsid protein L1 (HPV-16-L1). Oral immunization of mice with either construct induced IFN-gamma-producing CD8+ and CD4+ T cells in the spleen and in the Peyer's patches with the C-terminal construct. Oral immunization with both constructs resulted in diminished viral titers in the cervix and uterus of mice after intravaginal challenge with vaccinia virus expressing HPV-16-L1.

Biological gene delivery vehicles: beyond viral vectors

Gene therapy covers a broad spectrum of applications, from gene replacement and knockdown for genetic or acquired diseases such as cancer, to vaccination, each with different requirements for gene delivery. Viral vectors and synthetic liposomes have emerged as the vehicles of choice for many applications today, but both have limitations and risks, including complexity of production, limited packaging capacity, and unfavorable immunological features, which restrict gene therapy applications and hold back the potential for preventive gene therapy. While continuing to improve these vectors, it is important to investigate other options, particularly nonviral biological agents which include bacteria, bacteriophage, virus-like particles (VLPs), erythrocyte ghosts, and exosomes. Exploiting the natural properties of these biological entities for specific gene delivery applications will expand the repertoire of gene therapy vectors available for clinical use. Here, we review the prospects for nonviral biological delivery vehicles as gene therapy agents with focus on their unique evolved biological properties and respective limitations and potential applications. The potential of these nonviral biological entities to act as clinical gene therapy delivery vehicles has already been shown in clinical trials using bacteria-mediated gene transfer and with sufficient development, these entities will complement the established delivery techniques for gene therapy applications.

Inhibition of cervical cancer cell growth by human papillomavirus virus-like particles packaged with human papillomavirus oncoprotein short hairpin RNAs

Overexpression of human papillomavirus (HPV E6 and HPV E7) oncogenes in human cervical cells results in the development of cancer, and E6 and E7 proteins are therefore targets for preventing cervical cancer progression. Here, we describe the silencing of E6 and E7 expression in cervical carcinoma cells by RNA interference. In order to increase the efficacy of the RNA interference, HPV pseudovirions coding for a short hairpin RNA (shRNA) sequence were produced. The results indicated the degradation of E6 and E7 mRNAs when shRNA against E6 or E7 were delivered by pseudovirions in HPV-positive cells (CaSki and TC1 cells). E6 silencing resulted in the accumulation of cellular p53 and reduced cell viability. More significant cell death was observed when E7 expression was suppressed. Silencing E6 and E7 and the consequences for cancer cell growth were also investigated in vivo in mice using the capacity of murine TC1 cells expressing HPV-16 E6 and E7 oncogenes to induce fast-growing tumors. Treatment with lentiviruses and HPV virus-like particle vectors coding for an E7 shRNA sequence both resulted in dramatic inhibition of tumor growth. These results show the ability of pseudovirion-delivered shRNA to produce specific gene suppression and provide an effective means of reducing HPV-positive tumor growth.

Targeting mucosal dendritic cells with microbial antigens from probiotic lactic acid bacteria

The use of vaccines against infectious microbes has been critical to the advancement of medicine. Vaccine strategies combined with, or without, adjuvants have been established to eradicate various bacterial and viral pathogens. A new generation of vaccines is being developed using specific strains of Gram-positive, lactic acid bacteria and, notably, some probiotic lactobacilli. These bacteria have been safely consumed by humans for centuries in fermented foods. Thus, they can be orally administered, are well tolerated by recipients and could be easily and economically provided to large populations. In this overview, we focus on mucosal immunity and how its cellular component(s), particularly dendritic cells, can be specifically targeted to deliver immunogenic subunits, such as the protective antigen from Bacillus anthracis (the causative agent of anthrax). An antigen-specific immune response can be elicited using specific strains of Lactobacillus acidophilus expressing the protective antigen. A mucosal, dendritic cell-targeted approach increases the bioavailability of an immunogen of interest when delivered orally by L. acidophilus. This provides an efficiently elegant natural strategy and serves a dual function as an immune-stimulating adjuvant in vivo.

Type-specific and cross-reactive antibodies induced by human papillomavirus 31 L1/L2 virus-like particles

The aim of this study was to determine whether antibodies induced by human papillomavirus (HPV) type 31 L1/L2 virus-like particles (VLPs) could cross-react with VLPs of the closely related HPV-16 and distantly related HPV-11, and to investigate the potential role of the L2 protein in L1/L2 VLPs in inducing cross-neutralizing antibodies. Antisera were prepared from rabbits immunized with intact or denatured HPV-31 L1/L2 VLPs. Cross-reaction and cross-neutralization were analysed by Western blotting and ELISA, and by haemagglutination inhibition, respectively. Western blotting results showed that H31 L1/L2 (D) antiserum (antiserum from a rabbit immunized with denatured HPV-31 L1/L2 VLPs) could cross-react with the L1 protein of HPV-11 and -16. HPV-31 L1/L2 VLP antiserum showed strong cross-reaction with and cross-neutralization of HPV-16 VLPs, but this was significantly less with HPV-11 VLPs. In addition, the cross-neutralizing activity against HPV-16 L1/L2 VLPs was slightly higher than that against HPV-16 L1 VLPs, although the difference was not statistically significant. Epitope-blocking ELISA showed that mAb H16.V5 could partially inhibit the cross-reaction of HPV-31 L1/L2 VLP antiserum with HPV-16 L1/L2 VLPs. These results suggested that (i) H31 L1/L2 (D) antiserum could cross-react with L1 protein from both closely related and distantly related HPV types, but HPV-31 L1/L2 VLP antiserum could only cross-neutralize closely related HPV types, (ii) surface-exposed epitopes of the L2 protein in L1/L2 VLPs may induce only low titres of cross-neutralizing antibodies and (iii) certain epitopes that cross-reacted with HPV-31 L1/L2 VLP antiserum are located close to the epitopes recognized by mAb H16.V5. These findings may provide suggestions for the design of multivalent VLP vaccines.

Oral vaccines: new needs, new possibilities

Vaccination is an important tool for handling healthcare programs both in developed and developing countries. The current global scenario calls for a more-efficacious, acceptable, cost-effective and reliable method of immunization for many fatal diseases. It is hoped that the adoption of oral vaccines will help to provide an effective vaccination strategy, especially in developing countries. Mucosal immunity generated by oral vaccines can serve as a strong first line of defense against most of the pathogens infecting through the mucosal lining. Advances in elucidating the mechanism of action of oral vaccines will facilitate the design of more effective, new generation vaccines. There are promising developments in the use of different agents to effectively deliver the vaccine candidate. It is hoped that ongoing research may be able to set another cardinal point, after polio vaccine, in eradicating infectious diseases.

Community-acquired pneumonia: paving the way towards new vaccination concepts

Despite the availability of antimicrobial agents and vaccines, community-acquired pneumonia remains a serious problem. Severe forms tend to occur in very young children and among the elderly, since their immune competence is eroded by immaturity and immune senescence, respectively. The main etiologic agents differ according to patient age and geographic area. Streptococcus pneumoniae, Haemophilus influenzae, respiratory syncytial virus (RSV) and parainfluenza virus type 3 (PIV-3) are the most important pathogens in children, whereas influenza viruses are the leading cause of fatal pneumonia in the elderly. Effective vaccines are available against some of these organisms. However, there are still many agents against which vaccines are not available or the existent ones are suboptimal. To tackle this problem, empiric approaches are now being systematically replaced by rational vaccine design. This is facilitated by the growing knowledge in the fields of immunology, microbial pathogenesis and host response to infection, as well as by the availability of sophisticated strategies for antigen selection, potent immune modulators and efficient antigen delivery systems. Thus, a new generation of vaccines with improved safety and efficacy profiles compared to old and new agents is emerging. In this chapter, an overview is provided about currently available and new vaccination concepts.

Novel ways for immune intervention in immunotherapy: mucosal allergy vaccines

Allergen-specific immunotherapy is currently the only curative treatment for allergy. Subcutaneous immunotherapy (SCIT) has been successfully used to treat patients who are allergic to insect venom, house dust mites, or tree or grass pollens. In the context of potentially severe, albeit infrequent, side effects associated with SCIT, mucosal routes of administration are being investigated to conduct allergenic desensitization. This article reviews recent developments in the field of nasal, oral, and sublingual immunotherapy as they relate to safety, clinical efficacy, and immune mechanisms of action. Implications for the design and development of improved allergy vaccines that could be used through such nonparenteral routes are discussed. Specifically, allergen presentation platforms and adjuvants facilitating the targeting of immune cells at mucosal surfaces to promote tolerance induction are reviewed.

Papillomavirus virus-like particles as vehicles for the delivery of epitopes or genes

Papillomaviruses (PVs) are simple double-strand DNA viruses whose virion shells are T = 7 icosahedrons and composed of major capsid protein L1 and minor capsid protein L2.L1 alone or together with L2 can self-assemble into virus-like particles (VLPs) when expressed in eukaryotic or prokaryotic expression systems. Although the VLPs lack the virus genome DNA, their morphological and immunological characteristics are very similar to those of nature papillomaviruses. PV VLP vaccination can induce high titers of neutralizing antibodies and can effectively protect animals or humans from PV infection. Moreover, PV VLPs have been good candidates for vehicles to deliver epitopes or genes to target cells. They are widely used in the fields of vaccine development, neutralizing antibody detection, basic virologic research on papillomaviruses, and human papillomavirus (HPV) screening. Besides the structural biology and immunological basis for PV VLPs used as vehicles to deliver epitopes or genes, this review details the latest findings on chimeric papillomavirus VLPs and papillomavirus pseudoviruses, which are two important forms of PV VLPs used to transfer epitopes or genes.

Induction of mucosal and systemic immune responses against human carcinoembryonic antigen by an oral vaccine

Carcinoembryonic antigen (CEA) is a tumor-associated antigen targeted for the development of colorectal tumor vaccines. In this study, we developed papillomavirus pseudoviruses encoding the truncated CEA without NH2-terminal signal peptide (PV-CEA) as an oral vaccine to induce CEA-specific CTL responses. In CEA transgenic (CEA-Tg) mice orally immunized with PV-CEA, the immunologic tolerance to CEA as a "self-antigen" was overcome and both mucosal and systemic CEA-specific cytolytic activities were detected by in vitro 51Cr release assays. In a tumor prevention model, the growth rate of CEA+ tumors was significantly delayed in CEA-Tg mice orally immunized with PV-CEA when compared with the control vaccine. Further, the IFN-gamma enzyme-linked ImmunoSPOT and in vitro 51Cr release assay results showed that HLA-A2-restricted, CEA-specific CTL responses were induced in both mucosal and systemic lymphoid tissues in A2 transgenic mice after oral immunization with PV-CEA. Finally, we showed that coadministration of papillomavirus pseudoviruses encoding interleukin-2 with PV-CEA enhanced the generation of A2-restricted, CEA-specific CTLs in aged CEA/A2 double transgenic mice, which were more clinically relevant. Our data suggest that PV-CEA pseudovirus vaccine is a promising oral CEA vaccine for humans to induce CEA-specific CTLs at the site of colorectal tumors (i.e., intestinal mucosa), which might efficiently eliminate CEA+ colorectal tumor cells in the mucosa.

Turning On/Off Tumor-Specific CTL Response during Progressive Tumor Growth

Therapeutic vaccinations used to induce CTLs and treat firmly established tumors are generally ineffective. To understand the mechanisms underlying the failure of therapeutic vaccinations, we investigated the fate of tumor-specific CD8+ T cells in tumor-bearing mice with or without vaccinations. Our data demonstrate that tumor-specific CD8+ T cells are activated at the early stage of tumor growth, tumor-specific CTL response reaches a maximal level during progressive tumor growth, and tumor-specific CD8+ T cells lose cytolytic function at the late stage of tumor growth. The early stage therapeutic vaccination induces efficient antitumor activity by amplifying the CTL response, whereas the late-stage therapeutic vaccination is invalid due to tumor-induced dysfunction of CD8+ T cells. However, at the late stage, tumor-specific CD8+ T cells are still present in the periphery. These tumor-specific CD8+ T cells lose cytolytic activity, but retain IFN-gamma secretion function. In contrast to in vitro cultured tumor cells, in vivo growing tumor cells are more resistant to tumor-specific CTL killing, despite an increase of tumor Ag gene expression. Both tumor-induced CD8+ T cell dysfunction at the late stage and immune evasion developed by in vivo growing tumor cells contribute to an eventual inefficacy of therapeutic vaccinations. Our study suggests that it is important to design a vaccination regimen according to the stages of tumor growth and the functional states of tumor-specific CD8+ T cells.

Mucosal immunity and vaccines

There is currently great interest in developing mucosal vaccines against a variety of microbial pathogens. Mucosally induced tolerance also seems to be a promising form of immunomodulation for treating certain autoimmune diseases and allergies. Here we review the properties of the mucosal immune system and discuss advances in the development of mucosal vaccines for protection against infections and for treatment of various inflammatory disorders.

Modified HIV envelope proteins with enhanced binding to neutralizing monoclonal antibodies

The target for neutralizing antibodies against human immunodeficiency virus (HIV) is the trimeric Env protein on the native virion. Conserved neutralizing epitopes of receptor binding sites are located in the recessed core of the Env protein, partially masked by glycosylations and variable loops. In this study, we have investigated the effects of modifications of the HIV Env protein by glycosylation site mutations, deletions of variable loops, or combinations of both types of mutations on their protein functions and reactivities with neutralizing antibodies. Modified Env proteins were expressed in insect or mammalian cells, and their reactivity with epitope-specific broadly neutralizing monoclonal antibodies (Mabs) was determined by flow cytometry. A unique mutant designated 3G with mutations in three glycosylation motifs within the V3/C3 domains surrounding the CD4 binding site showed higher levels of binding to most broadly neutralizing Mabs (b12 and 2F5) in both insect and mammalian expression systems. Mutants with a deletion of both V1 and V2 loop domains or with a unique combination of both types of mutations also bound to most neutralizing Mabs at higher levels compared to the wild-type control. Most mutants maintained the ability to bind CD4 and to induce syncytium formation at similar or higher levels as compared to that of the wild-type Env protein, except for a mutant with a combination of variable loop deletions and deglycosylation mutations. Our study suggests that modified HIV Env proteins with reduced glycosylation in domains surrounding the CD4 binding site or variable loop-deleted mutants expose important neutralizing epitopes at higher levels than wild type and may provide novel vaccine immunogens.

Preventative and therapeutic vaccines for cervical cancer

"High-risk" genotypes of the human papillomavirus (HPV), most commonly HPV genotype 16, are the primary etiologic agents of cervical cancer. Indeed HPV DNA is detected in 99% of cervical carcinomas. Thus, cervical cancer and other HPV-associated malignancies might be prevented or treated by the induction of the appropriate viral-antigen-specific immune responses. Transmission of papillomavirus may be prevented by the generation of antibodies to capsid proteins L1 and L2 that neutralize viral infection. HPV L1 virus-like particles (VLPs) show great promise as prophylactic HPV vaccines in ongoing clinical trials but L2-based preventative vaccines have yet to be tested in patients. Since the capsid proteins are not expressed at detectable levels by infected basal keratinocytes or in HPV-transformed cells, therapeutic vaccines generally target the nonstructural early viral antigens. Two HPV oncogenic proteins, E6 and E7, are critical to the induction and maintenance of cellular transformation and are co-expressed in the majority of HPV-containing carcinomas. Although other early viral antigens show promise for vaccination against papillomas, therapeutic vaccines targeting E6 and E7 may provide the best opportunity to control HPV-associated malignancies. Various candidate therapeutic HPV vaccines are currently being tested whereby E6 and/or E7 are administered in live vectors, as peptides or proteins, in nucleic acid form, as components of chimeric VLPs, or in cell-based vaccines. Encouraging results from experimental vaccination systems in animal models have led to several prophylactic and therapeutic vaccine clinical trials. Should this new generation of HPV preventative and therapeutic vaccines function in patients as demonstrated in animal models, oncogenic HPV infection and its associated malignancies could be controlled by vaccination. Importantly, recent advances in HPV detection and continued improvements in screening further enhance our opportunities to systematically eradicate HPV-associated malignancy.

Human immunodeficiency virus type 1 gag-specific mucosal immunity after oral immunization with papillomavirus pseudoviruses encoding gag

Mucosal surfaces are the primary portals for human immunodeficiency virus (HIV) transmission. Because systemic immunization, in general, does not induce effective mucosal immune responses, a mucosal HIV vaccine is urgently needed. For this study, we developed papillomavirus pseudoviruses that express HIV-1 Gag. The pseudoviruses are synthetic, nonreplicating viruses, yet they can produce antigens for a long time in the immune system. Here we show that oral immunization of mice by the use of papillomavirus pseudoviruses encoding Gag generated mucosal and systemic Gag-specific cytotoxic T lymphocytes that effectively lysed Gag-expressing target cells. Furthermore, the pseudoviruses generated Gag-specific gamma interferon-producing T cells and serum immunoglobulin G (IgG) and mucosal IgA. In contrast, oral immunization with plasmid DNA encoding HIV-1 Gag did not induce specific immune responses. Importantly, oral immunization with the pseudoviruses induced Gag-specific memory cytotoxic T lymphocytes and protected mice against a rectal mucosal challenge with a recombinant vaccinia virus expressing HIV-1 Gag. Thus, papillomavirus pseudoviruses encoding Gag are a promising mucosal vaccine against AIDS.

Induction of mucosal and systemic neutralizing antibodies against human immunodeficiency virus type 1 (HIV-1) by oral immunization with bovine Papillomavirus-HIV-1 gp41 chimeric virus-like particles

Human immunodeficiency virus type 1 (HIV-1) envelope-specific neutralizing antibodies are generated late after initial infection, and the neutralizing antibody response is weak in the infected individuals. Administration of neutralizing antibodies such as 2F5 to HIV-1-infected individuals resulted in reductions in viral loads. Because HIV-1 is transmitted mainly via mucosa and because HIV-specific neutralizing antibodies reduce HIV-1 in infected individuals, a vaccine that can induce both mucosal and systemic HIV-1-specific neutralizing antibodies may be used to prevent and to treat HIV-1 infection. In this study, we made a bovine papillomavirus (BPV) L1-HIV-1 gp41 fusion protein in which ELDKWA of gp41 was inserted into the N terminus of BPV L1 (amino acids 130 to 136). Expression of the fusion protein in insect cells led to the assembly of chimeric virus-like particles (CVLPs). The CVLPs had sizes similar to those of BPV particles and were able to bind to the cell surface and penetrate the cell membrane. Oral immunization of mice with CVLPs induced gp41-specific serum immunoglobulin G (IgG) and intestinal secretory IgA. However, intramuscular immunization with the CVLPs resulted in similar amounts of gp41-specific IgG but low levels of secretory IgA. The antibodies specifically recognized the fixed HIV-1 gp41 on the cell surface. Importantly, the sera and fecal extracts from mice orally immunized with the CVLPs neutralized HIV-1(MN) in vitro. Thus, BPV-HIV-1 gp41 CVLPs may be used to prevent and to treat HIV-1 infection.

Oral Administration with Papillomavirus Pseudovirus Encoding IL-2 Fully Restores Mucosal and Systemic Immune Responses to Vaccinations in Aged Mice

Infectious diseases are one of the major threats for the elderly because their immune system is often compromised, and vaccinations to prevent these infections are not effective. A major defect in their immune system seems to be the inability of T cells to produce IL-2. We used papillomavirus (PV) pseudoviruses (PSVs) as a model vaccine and a gene delivery vector to address how to enhance immune responses to vaccinations. We found that oral immunization with PV PSV induced minimal mucosal and systemic Abs and CTLs specific for the PSVs in aged mice compared with young adult mice. In addition, fewer specific Th cells were generated in the aged mice. When aged mice were immunized with PV PSVs encoding human IL-2, specific Th cells were generated, producing murine IL-2, IL-4, and IFN-gamma. Further, specific Abs and CTLs were induced, resulting in protection against mucosal viral challenge. Thus, this study provided a basis for clinical trials using PV PSVs encoding IL-2 for vaccination of the elderly.

Vaccination to prevent and treat cervical cancer

Human papillomaviruses (HPVs) are the primary etiologic agents of cervical cancer. Thus, cervical cancer and other HPV-associated malignancies might be prevented or treated by HPV vaccines. Transmission of papillomavirus may be prevented by the generation of antibodies to capsid proteins L1 and L2 that neutralize viral infection. However, because the capsid proteins are not expressed at detectable levels by infected basal keratinocytes or in HPV-transformed cells, therapeutic vaccines generally target nonstructural early viral antigens. Two HPV oncogenic proteins, E6 and E7, are critical to the induction and maintenance of cellular transformation and are coexpressed in the majority of HPV-containing carcinomas. Thus, therapeutic vaccines targeting E6 and E7 may provide the best option for controlling HPV-associated malignancies. Various candidate therapeutic HPV vaccines are currently being tested whereby E6 and/or E7 are administered in live vectors, as peptides or protein, in nucleic acid form, as components of chimeric virus-like particles, or in cell-based vaccines. Encouraging results from experimental vaccination systems in animal models have led to several prophylactic and therapeutic vaccine clinical trials. If these preventive and therapeutic HPV vaccines prove successful in patients, as they have in animal models, then oncogenic HPV infection and its associated malignancies may be controllable by vaccination.

Efficient intracellular assembly of papillomaviral vectors

Although the papillomavirus structural proteins, L1 and L2, can spontaneously coassemble to form virus-like particles, currently available methods for production of L1/L2 particles capable of transducing reporter plasmids into mammalian cells are technically demanding and relatively low-yield. In this report, we describe a simple 293 cell transfection method for efficient intracellular production of papillomaviral-based gene transfer vectors carrying reporter plasmids. Using bovine papillomavirus type 1 (BPV1) and human papillomavirus type 16 as model papillomaviruses, we have developed a system for producing papillomaviral vector stocks with titers of several billion transducing units per milliliter. Production of these vectors requires both L1 and L2, and transduction can be prevented by papillomavirus-neutralizing antibodies. The stocks can be purified by an iodixanol (OptiPrep) gradient centrifugation procedure that is substantially more effective than standard cesium chloride gradient purification. Although earlier data had suggested a potential role for the viral early protein E2, we found that E2 protein expression did not enhance the intracellular production of BPV1 vectors. It was also possible to encapsidate reporter plasmids devoid of BPV1 DNA sequences. BPV1 vector production efficiency was significantly influenced by the size of the target plasmid being packaged. Use of 6-kb target plasmids resulted in BPV1 vector yields that were higher than those with target plasmids closer to the native 7.9-kb size of papillomavirus genomes. The results suggest that the intracellular assembly of papillomavirus structural proteins around heterologous reporter plasmids is surprisingly promiscuous and may be driven primarily by a size discrimination mechanism.

Enhancement of mucosal immune responses by chimeric influenza HA/SHIV virus-like particles

To enhance mucosal immune responses using simian/human immunodeficiency virus-like particles (SHIV VLPs), we have produced novel phenotypically mixed chimeric influenza HA/SHIV VLPs and used them to immunize C57BL/6J mice intranasally. Antibody and cytotoxic T-cell (CTL) responses as well as cytokine production in both systemic and mucosal sites were compared after immunization with SHIV VLPs or chimeric HA/SHIV VLPs. By using enzyme-linked immunosorbent assay (ELISA), the levels of serum IgG and mucosal IgA to the HIV envelope protein (Env) were found to be highest in the group immunized with chimeric HA/SHIV VLPs. Furthermore, the highest titer of serum neutralizing antibody against HIV Env was found with the group immunized with chimeric HA/SHIV VLPs. Analysis of the IgG1/IgG2a ratio indicated that a T(H)1-oriented immune response resulted from these VLP immunizations. HA/SHIV VLP-immunized mice also showed significantly higher CTL responses than those observed in SHIV VLP-immunized mice. Moreover, a MHC class I restricted T-cell activation ELISPOT assay showed a mixed type of T(H)1/T(H)2 cytokines in the HA/SHIV VLP-immunized mice, indicating that the chimeric VLPs can enhance both humoral and cellular immune responses to the HIV Env protein at multiple mucosal and systemic sites. The results indicate that incorporation of influenza HA into heterotypic VLPs may be highly effective for targeting vaccines to mucosal surfaces.

Heterologous boosting increases immunogenicity of chimeric papillomavirus virus-like particle vaccines

Chimeric human papillomavirus virus-like particles (HPV cVLPs), containing the HPV16 non-structural protein E7, are potent vaccines for inducing antigen-specific protective immunity against HPV-transformed tumors in animal models. Previous data demonstrated that the effectiveness of cytotoxic T lymphocyte (CTL) induction after repetitive vaccination with the same cVLP, and thus vaccine efficacy, is limited by the presence of neutralizing antibodies induced after the first application. Here, we determined if altering the route of vaccine delivery or incorporation of the target antigen into VLPs of a heterologous papillomavirus type could overcome inhibition of MHC class I antigen presentation by neutralizing antibodies, resulting in a boosting of CD8(+) T-cell responses against the incorporated antigen, HPV16 E7. Mucosal delivery of cVLPs resulted in detection of systemic E7-specific CD8(+) T cells, however, these routes were not able to bypass the inhibitory effect of circulating antibodies against homologous VLP types. In contrast, mice immunized and boosted with heterologous cVLPs containing HPV16 E7 showed a higher frequency of E7-specific T cells in vitro and displayed reduced tumor growth in a therapeutic setting compared to mice treated with homologous cVLPs. The data indicate that the use of different cVLP types for prime/boost regimens is a promising strategy to increase the efficacy and usefulness of cVLP-based vaccines for the treatment of cervical neoplasia.

Mucosal immunisation and adjuvants: a brief overview of recent advances and challenges

Mucosal immunisation may be used both to prevent mucosal infections through the activation of anti-microbial immunity and to treat selected autoimmune, allergic or infectious-immunopathological disorders through the induction of antigen-specific tolerance. The development of mucosal vaccines, whether for prevention of infectious diseases or for immunotherapy, requires antigen delivery and adjuvant systems that can efficiently help to present vaccine or immunotherapy antigens to the mucosal immune system. Promising advances have recently been made in the design of more efficient mucosal adjuvants based on detoxified bacterial toxin derivatives or CpG motif-containing DNA, and perhaps even more striking progress has been done in the use of virus-like particles as mucosal delivery systems for vaccines and of cholera toxin B subunit as antigen vector for immunotherapeutic tolerance induction. However, it is a memento that two recently developed mucosal vaccines for human use against rotavirus diarrhoea and influenza were withdrawn after a short period in the market because of adverse reactions among the vaccinees, thus emphasising the difficult and challenging task also for mucosal immunisation of combining vaccine and adjuvant efficacy with safety and acceptability.

Oral vaccination with recombinant Listeria monocytogenes expressing human papillomavirus type 16 E7 can cause tumor growth in mice to regress

Listeria monocytogenes is a Gram-positive, facultative intracellular bacterium with the ability to present secreted proteins to the major histocompatibility complex class I pathway to stimulate cell-mediated immune response. In our study, we constructed the recombinant L. monocytogenes encoding human papillomavirus type 16 E7 gene (rLM-E7). When orally administered to syngeneic mice, rLM-E7 could induce a cytotoxic T-lymphocyte (CTL) response. Furthermore, in vitro flow cytometric assay and in vivo immune deficiency assays showed that rLM-E7 could prevent and eradicate tumor growth via CD8+-dependent CTLs. Hence, the potency of rLM-E7 as a therapeutic vaccine for cervical cancer is the result of the induction E7-specific cell-mediated immunity by L. monocytogenes. In addition to potency, this vaccine also offers ease of administration and reduced cost of production compared with other vaccines formulated for injection. Thus, L. monocytogenes encoding HPV-16 E7 may be a useful oral vaccine for cervical cancer treatment.

Recent advances in mucosal vaccines and adjuvants

Mucosal vaccines may be used both to prevent mucosal infections through the activation of antimicrobial immunity and to treat systemic inflammatory diseases through the induction of antigen-specific mucosal tolerance. New, efficient mucosal adjuvants for human use have been designed based on, amongst others, bacterial toxins and their derivatives, CpG-containing DNA, and different cytokines and chemokines, with the aim of improving the induction of mucosal Th1 and Th2 responses. Mucosal delivery systems, in particular virus-like particles, have been shown to enhance the binding, uptake and half-life of the antigens, as well as target the vaccine to mucosal surfaces. DNA vaccines are currently being developed for administration at mucosal surfaces. However, there have also been failures, such as the withdrawal of an oral vaccine against rotavirus diarrhea and a nasal vaccine against influenza, because of their potential side effects.