Induction of an HPV 6bL1-specific mucosal IgA response by DNA immunization

Vaginal delivery of vaccines

Recent estimates suggest that one in two sexually active individuals will acquire a sexually transmitted infection by age 25, an alarming statistic that amounts to over 1 million new infections per day worldwide. Vaccination against STIs is highly desirable for alleviating this global burden of disease. Vaginal immunization is a promising strategy to combat transmission via the vaginal mucosa. The vagina is typically considered a poor inductive site for common correlates of adaptive immunity. However, emerging evidence suggests that immune tolerance may be overcome by precisely engineered vaccination schemes that orchestrate cell-mediated immunity and establish tissue resident memory immune cells. In this review, we will discuss the unique immunological milieu of the vaginal mucosa and our current understanding of correlates of pathogenesis and protection for several common STIs. We then present a summary of recent vaginal vaccine studies and explore the role that mucosal adjuvants and delivery systems play in enhancing protection according to requisite features of immunity. Finally, we offer perspectives on the challenges and future directions of vaginal vaccine delivery, discussing remaining physiological barriers and innovative vaccine formulations that may overcome them.

Nasal vaccination against SARS-CoV-2: Synergistic or alternative to intramuscular vaccines?

It is striking that all marketed SARS-CoV-2 vaccines are developed for intramuscular administration designed to produce humoral and cell mediated immune responses, preventing viremia and the COVID-19 syndrome. They have a high degree of efficacy in humans (70-95%) depending on the type of vaccine. However, little protection is provided against viral replication and shedding in the upper airways due to the lack of a local sIgA immune response, indicating a risk of transmission of virus from vaccinated individuals. A range of novel nasal COVID-19 vaccines are in development and preclinical results in non-human primates have shown a promising prevention of replication and shedding of virus due to the induction of mucosal immune response (sIgA) in upper and lower respiratory tracts as well as robust systemic and humoral immune responses. Whether these results will translate to humans remains to be clarified. An IM prime followed by an IN booster vaccination would likely result in a better well-rounded immune response, including prevention (or strong reduction) in viral replication in the upper and lower respiratory tracts.

Development of HPV Vaccines for HPV-associated Head and Neck Squamous Cell Carcinoma

High-risk genotypes of the human papillomavirus (HPV), particularly HPV type 16, are found in a distinct subset of head and neck squamous cell carcinomas (HNSCC). Thus, these HPV-associated HNSCC may be prevented or treated by vaccines designed to induce appropriate HPV virus-specific immune responses. Infection by HPV may be prevented by neutralizing antibodies specific for the viral capsid proteins. In clinical trials, vaccines comprised of HPV virus-like particles (VLPs) have shown great promise as prophylactic HPV vaccines. However, given that capsid proteins are not expressed at detectable levels by infected basal keratinocytes, vaccines with therapeutic potential must target other non-structural viral antigens. Two HPV oncogenic proteins, E6 and E7, are important in the induction and maintenance of cellular transformation and are co-expressed in the majority of HPV-containing carcinomas. Therefore, therapeutic vaccines targeting these proteins may have potential to control HPV-associated malignancies. Various candidate therapeutic HPV vaccines are currently being tested whereby E6 and/or E7 is administered in live vectors, in peptides or protein, 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 they fulfill their promise, these vaccines may prevent HPV infection or control its potentially life-threatening consequences in humans.

Induction of mucosal immunity through systemic immunization: Phantom or reality?

Generation of protective immunity at mucosal surfaces can greatly assist the host defense against pathogens which either cause disease at the mucosal epithelial barriers or enter the host through these surfaces. Although mucosal routes of immunization, such as intranasal and oral, are being intensely explored and appear promising for eliciting protective mucosal immunity in mammals, their application in clinical practice has been limited due to technical and safety related challenges. Most of the currently approved human vaccines are administered via systemic (such as intramuscular and subcutaneous) routes. Whereas these routes are acknowledged as being capable to elicit antigen-specific systemic humoral and cell-mediated immune responses, they are generally perceived as incapable of generating IgA responses or protective mucosal immunity. Nevertheless, currently licensed systemic vaccines do provide effective protection against mucosal pathogens such as influenza viruses and Streptococcus pneumoniae. However, whether systemic immunization induces protective mucosal immunity remains a controversial topic. Here we reviewed the current literature and discussed the potential of systemic routes of immunization for the induction of mucosal immunity.

Development of amino acid substituted gemini surfactant-based mucoadhesive gene delivery systems for potential use as noninvasive vaginal genetic vaccination

Aim: Recently, we synthesized amino acid- and peptide-substituted gemini surfactants, ‘biolipids’ that exhibited high transfection efficiency in vitro. In this study, we developed these plasmid DNA and gemini surfactant lipid particles for noninvasive administration in vaginal cavity. Material & methods: Novel formulations of these gene delivery systems were prepared with poloxamer 407 to induce in situ gelling of the formulation and diethylene glycol monoethyl ether to improve their penetration across mucosal tissue. Results: Poloxamer at 16% w/v concentration in diethylene glycol monoethyl ether aqueous solution produced dispersions that gelled near body temperature and had a high yield value, preventing leakage of the formulation from the vaginal cavity. Intravaginal administration in rabbits showed that the glycyl-lysine-substituted gemini surfactant led to a higher gene expression compared with the parent unsubstituted gemini surfactant. Conclusion: This provides a proof-of-concept that amino acid substituted gemini surfactants can be used as noninvasive mucosal (vaginal) gene delivery systems to treat diseases associated with mucosal epithelia.

Prophylactic papillomavirus vaccines

Human papillomaviruses (HPV) are the causative agents of cervical cancer, the third most common cancer in women. The development of prophylactic HPV vaccines Gardasil® and Cervarix® targeting the major oncogenic HPV types is now the frontline of cervical cancer prevention. Both vaccines have been proven to be highly effective and safe although there are still open questions about their target population, cross-protection, and long-term efficacy. The main limitation for a worldwide implementation of Gardasil® and Cervarix® is their high cost. To develop more affordable vaccines research groups are concentrated in new formulations with different antigens including capsomeres, the minor capsid protein L2 and DNA. In this article we describe the vaccines' impact on HPV-associated disease, the main open questions about the marketed vaccines, and current efforts for the development of second-generation vaccines.

Codon usage roles in human papillomavirus

Human papillomavirus (HPV) genomes, similar to other virus genomes, frequently have a G + C content significantly different from their host species. The HPV genomes show a strong codon usage bias to 18 codons, with 14 showing T at the third position amongst degenerately encoded amino acids. The codon usage pattern in HPV genome plays an important role, which regulates low or non-translational expression of the viral capsid genes and results in very weak protein expression of oncogenes in a wide range of mammalian cells. Codon modification has been proved to be a powerful technology to overcome the translational blockage and weak expression of both HPV capsid genes and oncogenes in different expression systems. Furthermore, keratinocytes are the host cells of HPV infection; the codon usage in HPV capsid genes matches available aminoacyl-tRNAs in differentiated keratinocytes to modulate their protein expression. HPV DNA vaccines with codon optimization have been shown to have higher immunogenicity and induce both strong cellular and humoral responses in animal models, which may be a promising form of therapeutic HPV vaccines.

Repeated vaginal administration of trimeric HIV-1 clade C gp140 induces serum and mucosal antibody responses

Vaccine-mediated prevention of primary infection with human immunodeficiency virus (HIV) may require the sustained production of antibody at mucosal portals of entry. Here, we describe a novel approach of repeated mucosal immunization by delivering an HIV-1 envelope glycoprotein (gp) in a gel formulated for intravaginal delivery. Rabbits were immunized over one to three 19-day cycles of intravaginal dosing with soluble recombinant trimeric HIV-1 clade C gp140 administered in Carbopol gel. The formulation was well tolerated. A single immunization cycle induced immunoglobulin G (IgG) antibody detected in the serum and female genital tract, and titers were boosted on further immunization. Vaccine-induced serum antibodies neutralized the infectivity of a pseudovirus carrying a heterologous clade C envelope. Our data prove the concept that repeated exposure of the female genital tract to HIV envelope can induce mucosally detectable antibody.

Development of a novel viral DNA vaccine against human papillomavirus: AcHERV-HP16L1

In this study, we developed a novel DNA vaccine for HPV; a recombinant baculovirus bearing human endogenous retrovirus (HERV) envelope protein, which cannot replicate in mammals, was used as a nano-carrier for HPV-16L1 DNA vaccine (AcHERV-HP16L1). For in vivo test, mice were injected intramuscularly with 107 particles of the constructs, with two boosts at 2-week intervals. Compared with Gardasil (25 microL/dose), the AcHERV-HP16L1 immunized mice showed similar high levels of humoral immunity in IgG/IgA and in neutralization of HPV pseudovirions. Combined immunization (prime with AcHERV-HP16L1 and boost with Gardasil) induced slightly higher neutralizing activity. As compared to the group treated with Gardasil, the mice immunized with AcHERV-HP16L1 showed 450- and 490-fold increase in the IFN-gamma at 5 and 20 weeks after the first priming, respectively. The combined immunization conferred lower T cell immunity than AcHERV-HP16L1 treatment. The advantages of our novel AcHERV-HP16L1 vaccine over Gardasil include higher cellular immunogenicity, considerably lower production cost, and comparable safety. Therefore, we suggest that AcHERV-HP16L1 can be developed as an efficient prophylactic vaccine and therapeutic vaccine.

Human papillomavirus therapeutic vaccines in head and neck tumors

Head and neck cancer represents one of the most challenging diseases as the mortality remains high despite advances in early diagnosis and treatment. Human papillomavirus has been implicated in a third of head and neck squamous cell carcinomas and human papillomavirus type 16 is strongly associated with carcinomas arising from the oropharynx, the tonsil being the preferred infected site. Novel therapeutic approaches including immunotherapy are currently under investigation. Immune vaccines developed against human papillomavirus in the genital area are already available and could simultaneously protect other anatomical localizations; however, prophylactic vaccines are expected to be effective in reducing the incidence of tumors after many years and, therefore, there is an urgent need to improve therapeutic interventions, such as immunotherapy. To date, human papillomavirus therapeutic vaccines are either at the preclinical level or at early phase human trials for genital pathologies. Nevertheless, accumulating evidence from animal and clinical studies suggests that the enhancement of specific and innate immune responses is effective in clearance of the human papillomavirus infection, promoting a cautious optimism regarding the achievement of an efficacious immunotherapy. This article reviews what has been achieved and what remains to be done in the field for the development of future viral vaccines in head and neck tumors.

Intranasal vaccination with recombinant adeno-associated virus type 5 against human papillomavirus type 16 L1

Adeno-associated viruses (AAV) have been developed and evaluated as recombinant vectors for gene therapy in many preclinical studies, as well as in clinical trials. However, only a few approaches have used recombinant AAV (rAAV) to deliver vaccine antigens. We generated an rAAV encoding the major capsid protein L1 (L1h) from the human papillomavirus type 16 (HPV16), aiming to develop a prophylactic vaccine against HPV16 infections, which are the major cause of cervical cancer in women worldwide. A single dose of rAAV5 L1h administered intranasally was sufficient to induce high titers of L1-specific serum antibodies, as well as mucosal antibodies in vaginal washes. Seroconversion was maintained for at least 1 year. In addition, a cellular immune response was still detectable 60 weeks after immunization. Furthermore, lyophilized rAAV5 L1h successfully evoked a systemic and mucosal immune response in mice. These data clearly show the efficacy of a single-dose intranasal immunization against HPV16 based on the recombinant rAAV5L1h vector without the need of an adjuvant.

DNA vaccine against hamster oral papillomavirus-associated oral cancer

Previously we developed a carcinogenesis model involving the combination of 9,10-dimethyl-1,2-benzanthracene (DMBA) application with physical wounding of hamster lingual mucosa. The presence of a novel hamster oral papillomavirus (HOPV) was demonstrated and its genome sequenced. In the present study, this HOPV hamster model was used to test whether vaccination with the L1 gene could prevent the development of oral carcinoma. DNA plasmids encoding the L1 gene or the vector alone were injected intramuscularly into 20 vaccinated and 20 control hamsters, respectively. The lingual tips of the hamsters were painted with DMBA for 8 weeks. A portion of the lingual tips was excised, and the tips were then painted daily with DMBA until the animals were killed 13 days later. All control hamsters developed lingual carcinoma, whereas 12 of the L1-vaccinated hamsters showed no lesions. These results suggest that immunization with L1 DNA vaccines may prevent the development of papillomavirus-associated oral cancer.

Plasmid encoding papillomavirus Type 16 (HPV16) DNA constructed with codon optimization improved the immunogenicity against HPV infection

Human papillomavirus Type 16 (HPV16) infections can cause neoplasia, which is thought to be closely associated with the development of cervical cancers. In the study, we attempted to construct a DNA plasmid encoding a HPV16 capsid protein (L1) and a HPV16 oncoprotein (E7), which was capable of preventing HPV16 infection and eliminating HPV16-infected cells. A plasmid, L1E7hpSCA1, encoding the L1 and E7 genes with the codon usage optimized for mammalian cell expression, was constructed. Mutations were introduced into the E7 gene sequence for reducing its oncogenicity. C57BL/6 mice were intramuscularly immunized at tibialis anterior (TA) muscles with the newly constructed L1E7hpSCA1 plasmid. The immune responses induced by the L1E7hpSCA1 plasmid (with codon optimization) and a control L1E7pSCA1 plasmid (without codon optimization) were compared. It is shown that the L1E7hpSCA1 was able to induce much stronger immune responses than the L1E7pSCA1. Sera obtained from immunized animals were found to contain anti-HPV16 antibodies as detected by ELISA and hemagglutination inhibition (HAI) assays. Cytotoxicity and interferon-gamma assays showed that spleenocytes from immunized animals were able to recognize and lyze E7 expressing tumor TC-1 cells. Moreover, the growth of E7 expressing tumor mass was inhibited in vaccinated mice. In vivo tumor protection test indicated that tumor formation was prevented in the experimental animals (67%) after vaccination with L1E7hpSCA1, while for the control group injected with L1E7pSCA1 only and the animal group injected with pSCA1 only, tumor formation was observed in all experimental animals. Our results suggest that the L1E7h gene (with codon optimization) is more effective against HPV16 than the L1E7 gene (without codon optimization). The L1E7hpSCA1 plasmid was able to provide protection against E7 expressing tumor, and it might have the potential to be a vaccine candidate for HPV prevention.

Mucosal immunity: overcoming the barrier for induction of proximal responses

Vaccination represents one of the most efficacious and cost-effective medical interventions. It is the only medical intervention proven to eliminate disease at a global level. Many of the pathogens against which we most require adequate vaccines infect via the highly exposed mucosal surfaces. For this reason the mucosa is often considered the first, and sometimes only, line of defense. Therefore, responses that protect the local mucosa are vital. In this review, we first explore the immunological mechanisms that protect the mucosa. We then review the literature of mucosal vaccines within the principles of antigenic composition, dose, and danger, highlighting the need and niche for the next generation of mucosal vaccines.

Systemic and mucosal antibody responses following retroductal gene transfer to the salivary gland

Gene transfer to salivary glands by retrograde perfusion of the salivary duct has been shown to result in production of the encoded protein. We sought to determine if this technique would be useful for genetic immunization. In studies that compare delivery of DNA to either the salivary gland (SG) or muscle (im), mean plasma IgG and IgA titers obtained following SG delivery were 46- and 86-fold greater, respectively, than those following im delivery. We also tested the hypothesis that SG vaccination could generate mucosal responses in sites proximal and distal to DNA administration. SG-treated animals produced specific antibodies within saliva, vaginal fluid, and lung washes as well as demonstrating robust specific responses in Peyer's patches. In a test of functional immunity, animals vaccinated with DNA by SG retrograde perfusion were significantly more resistant to the effects of lethal anthrax challenge than im DNA-vaccinated animals. These data suggest that SG genetic immunization may offer advantages over conventional routes of vaccination.

Antibody response in the female rabbit reproductive tract to influenza haemagglutinin encoded by a recombinant myxoma virus

The antibody response in serum and the reproductive tract of female rabbits to a model antigen, influenza virus haemagglutinin (HA), encoded by a recombinant myxoma virus was investigated. Strong and lasting IgG antibody responses to HA were induced in serum following intradermal, intranasal, and intravaginal immunisations. HA IgG was also detected in reproductive tract fluids but was only about 1% the titer of that in serum. HA IgA was not detected in serum of any infected groups and was occasionally detected in reproductive tract fluids at a low titer only after infections through mucosal sites. HA IgM was also detected only in some of the reproductive tract fluids at very low levels. Induction of ovulation did not change these patterns and B cell homing to the reproductive tract was not profound. In contrast, HA IgG and IgM titers in ovarian follicular fluids were comparable to that in serum. These data suggest that if this virus is used to deliver an immunocontraceptive vaccine that requires a high-level antibody response, the target antigen needs to be accessible to serum antibody or in the ovary.

Enhanced mucosal and systemic immune responses following intravaginal immunization with human papillomavirus 16 L1 virus-like particle vaccine in thermosensitive mucoadhesive delivery systems

To develop more potent and convenient mucosal human papillomavirus (HPV) vaccines, we tested the effect of thermosensitive mucoadhesive vaginal vaccine delivery systems on the local and systemic antibody responses to HPV 16 L1 virus-like particles (VLP). HPV 16 L1 VLP expressed from recombinant baculovirus-infected Sf21 insect cells were delivered in phosphate-buffered saline (PBS) or thermosensitive mucoadhesive delivery systems, composed of poloxamers (Pol) and varying amounts of polyethylene oxide (PEO). Pol/PEO-based vaginal vaccine delivery systems existed in liquid form at room temperature, but gelled at 37 degrees C. The mucoadhesiveness of Pol/PEO-based delivery systems increased with PEO, but the formulations with PEO higher than 1.0% were too viscous to be administered into the vagina. Vaccine vehicles affected the vaginal and salivary immune responses to HPV 16 L1 VLP intravaginally administered into mice. At 42 days after the first intravaginal immunization of HPV 16 L1 VLP with cholera toxin, vaginal and salivary IgA titers were the highest in the group given in Pol/PEO 1.0% vehicle followed by Pol/PEO 0.4% and PBS vehicles. Intravaginal coadministration of HPV 16 L1 VLP and cholera toxin in Pol/PEO 1.0% showed 31- and 39-fold higher titers compared to the PBS-based HPV 16 L1 VLP groups administered by intravaginal and intramuscular routes, respectively. Following intravaginal administration, Pol/PEO 1.0%, but not Pol/PEO 0.4%, showed significantly higher HPV 16 L1 VLP-specific serum IgG titers as compared to the PBS vehicle. Our results indicate that the use of in situ-gelling vaginal vaccine delivery systems with increased mucoadhesiveness would be beneficial for more effective induction of mucosal and systemic immune responses to intravaginally administered HPV 16 L1 VLP vaccines.

T cell response to human papillomavirus 16 E7 in mice: comparison of Cr release assay, intracellular IFN-gamma production, ELISPOT and tetramer staining

Successful vaccination against infections by high-risk papillomaviruses aiming at the prevention of cervical cancer most likely requires the induction of neutralizing antibodies and human papillomavirus (HPV)-specific T cells directed against early viral proteins such as E7. Whereas the technology for detection of antibodies is well established, measurement of T cells is more cumbersome and standardization of assays is difficult. By using chromium release assay, ELISPOT, tetramer staining and intracellular IFN-gamma assay, we compared the levels of HPV 16 E7-specific T cells obtained after immunization of C57BL/6 mice with different DNA expression vectors. We found that all four assays gave highly comparable results. ELISPOT can be recommended for future studies as it indicates the presence of activated (i.e. IFN-gamma-secreting) T cells in a quantitative manner and combines high sensitivity with relatively low T cell demand.

Human papillomavirus therapy for the prevention and treatment of cervical cancer

Cervical carcinoma is associated with human papillomavirus infection. Proliferation of cancer cells depends on the continual expression of the E6 and E7 viral oncogenes. This article includes treatment strategies that can interfere with expression or function of the proteins and immunotherapeutic approaches that can eliminate cells that express E6 and E7 proteins.

Vaccines for viral diseases with dermatologic manifestations

Vaccines against infectious diseases have been available since the 1800s, when an immunization strategy against smallpox developed by Jenner gained wide acceptance. Until recently, the only vaccination strategies available involved the use of protein-based, whole killed, and attenuated live virus vaccines. These strategies have led to the development of effective vaccines against a variety of diseases with primary or prominent cutaneous manifestations. Effective and safe vaccines now used worldwide include those directed against measles and rubella (now commonly used together with a mumps vaccine as the trivalent MMR), chickenpox, and hepatitis B. The eradication of naturally occurring smallpox remains one of the greatest successes in the history of modern medicine, but stockpiles of live smallpox exist in the United States and Russia. Renewed interest in the smallpox vaccine reflects concerns about a possible bioterrorist threat using this virus. Yellow fever is a hemorrhagic virus endemic to tropical areas of South America and Africa. An effective vaccine for this virus has existed since 1937, and it is used widely in endemic areas of South America, and to a lesser extent in Africa. This vaccine is recommended once every 10 years for people who are traveling to endemic areas. Advances in immunology have led to a greater understanding of immune system function in viral diseases. Progress in genetics and molecular biology has allowed researchers to design vaccines with novel mechanisms of action (eg, DNA, vector, and VLP vaccines). Vaccines have also been designed to specifically target particular viral components, allowing for stimulation of various arms of the immune system as desired. Ongoing research shows promise in prophylactic and therapeutic vaccination for viral infections with cutaneous manifestations. Further studies are necessary before vaccines for HSV, HPV, and HIV become commercially available.

Synthesis and application of functional peptides as cell nucleus-directed molecules in the treatment of malignant diseases

The unique functions of biomolecules, including transport across biological membranes (e.g. the cell membrane, the nuclear envelope), modulation of protein function, gene transcription, reconstitution of the malignant transformation, and viral, bacterial and fungal activities underlie a high pharmaceutical potential. The development of combinatorial functional peptide modules in this important area has been slow, in contrast to the rapid development in the synthesis of small biopolymers. The conjugation of a short transmembrane transport peptide module with a cell nucleus address peptide module and with any substance is attractive for preparation of BioShuttle-based peptides because of the well-established automated synthesis of peptides. Variation of the different functional modules for drug targeting and the choice of substances can be combined to create novel bioconjugates with unique properties. This article provides an overview of previous work on the BioShuttle technology and outlines the promising use of this approach in combinatorial peptide synthesis and drug discovery.

Polynucleotide viral vaccines: codon optimisation and ubiquitin conjugation enhances prophylactic and therapeutic efficacy

Papillomavirus infection is a major antecedent of anogenital malignancy. We have previously established that the L1 and L2 capsid genes of papillomavirus have suboptimal codon usage for expression in mammalian cells. We now show that the lack of immunogenicity of polynucleotide vaccines based on the L1 gene can be overcome with codon modified L1, which induces strong immune responses, including conformational virus neutralising antibody and delayed type hypersensitivity. Conjugation of a ubiquitin gene to a hybrid gene incorporating L1 and the E7 non-structural papillomavirus protein improved E7 specific CTL responses, and induced protection against an E7 expressing tumour, but induced little neutralising antibody. However, a mixture of ubiquitin conjugated and non-ubiquitin conjugated polynucleotides induced virus neutralising antibody and E7 specific CD8 T cells. An optimal combined prophylactic/therapeutic viral vaccine might therefore comprise ubiquitin conjugated and non-ubiquitinated genes, to induce prophylactic neutralising antibody and therapeutic cell mediated immune responses.

Enhancement of capsid gene expression: preparing the human papillomavirus type 16 major structural gene L1 for DNA vaccination purposes

Expression of the structural proteins L1 and L2 of the human papillomaviruses (HPV) is tightly regulated. As a consequence, attempts to express these prime-candidate genes for prophylactic vaccination against papillomavirus-associated diseases in mammalian cells by means of simple DNA transfections result in insufficient production of the viral antigens. Similarly, in vivo DNA vaccination using HPV L1 or L2 expression constructs produces only weak immune responses. In this study we demonstrate that transient expression of the HPV type 16 L1 and L2 proteins can be highly improved by changing the RNA coding sequence, resulting in the accumulation of significant amounts of virus-like particles in the nuclei of transfected cells. Data presented indicate that, in the case of L1, adaptation for codon usage accounts for the vast majority of the improvement in protein expression, whereas translation-independent posttranscriptional events contribute only to a minor degree. Finally, the adapted L1 genes demonstrate strongly increased immunogenicity in vivo compared to that of unmodified L1 genes.

Nucleic acid vaccines: tasks and tactics

There are no adequate vaccines against some of the new or reemerged infectious scourges such as HIV and TB. They may require strong and enduring cell-mediated immunity to be elicited. This is quite a task, as the only known basis of protection by current commercial vaccines is antibody. As DNA or RNA vaccines may induce both cell-mediated and humoral immunity, great interest has been shown in them. However, doubt remains whether their efficacy will suffice for their clinical realization. We look at the various tactics to increase the potency of nucleic acid vaccines and divided them broadly under those affecting delivery and those affecting immune induction. For delivery, we have considered ways of improving uptake and the use of bacterial, replicon or viral vectors. For immune induction, we considered aspects of immunostimulatory CpG motifs, coinjection of cytokines or costimulators and alterations of the antigen, its cellular localization and its anatomical localization including the use of ligand-targeting to lymphoid tissue. We also thought that mucosal application of DNA deserved a separate section. In this review, we have taken the liberty to discuss these enhancement methods, whenever possible, in the context of the underlying mechanisms that might argue for or against these strategies.