Liposome-mediated DNA Immunisation via the Subcutaneous Route

Strong immunogenicity & protection in mice with PlaCCine: A COVID-19 DNA vaccine formulated with a functional polymer

DNA- based vaccines have demonstrated the potential as a safe and effective modality. PlaCCine, a DNA-based vaccine approach described subsequently relies on a synthetic DNA delivery system and is independent of virus or device. The synthetic functionalized polymer combined with DNA demonstrated stability over 12 months at 4C and for one month at 25C. Transfection efficiency compared to naked DNA increased by 5-15-fold in murine skeletal muscle. Studies of DNA vaccines expressing spike proteins from variants D614G (pVAC15), Delta (pVAC16), or a D614G + Delta combination (pVAC17) were conducted. Mice immunized intramuscular injection (IM) with pVAC15, pVAC16 or pVAC17 formulated with functionalized polymer and adjuvant resulted in induction of spike-specific humoral and cellular responses. Antibody responses were observed after one immunization. And endpoint IgG titers increased to greater than 1x 105 two weeks after the second injection. Neutralizing antibodies as determined by a pseudovirus competition assay were observed following vaccination with pVAC15, pVAC16 or pVAC17. Spike specific T cell immune responses were also observed following vaccination and flow cytometry analysis demonstrated the cellular immune responses included both CD4 and CD8 spike specific T cells. The immune responses in vaccinated mice were maintained for up to 14 months after vaccination. In an immunization and challenge study of K18 hACE2 transgenic mice pVAC15, pVAC16 and pVAC17 induced immune responses lead to decreased lung viral loads by greater than 90 % along with improved clinical score. These findings suggest that PlaCCine DNA vaccines are effective and stable and further development against emerging SARS-CoV-2 variants is warranted.

Cationic Nanostructures for Vaccines Design

Subunit vaccines rely on adjuvants carrying one or a few molecular antigens from the pathogen in order to guarantee an improved immune response. However, to be effective, the vaccine formulation usually consists of several components: an antigen carrier, the antigen, a stimulator of cellular immunity such as a Toll-like Receptors (TLRs) ligand, and a stimulator of humoral response such as an inflammasome activator. Most antigens are negatively charged and combine well with oppositely charged adjuvants. This explains the paramount importance of studying a variety of cationic supramolecular assemblies aiming at the optimal activity in vivo associated with adjuvant simplicity, positive charge, nanometric size, and colloidal stability. In this review, we discuss the use of several antigen/adjuvant cationic combinations. The discussion involves antigen assembled to 1) cationic lipids, 2) cationic polymers, 3) cationic lipid/polymer nanostructures, and 4) cationic polymer/biocompatible polymer nanostructures. Some of these cationic assemblies revealed good yet poorly explored perspectives as general adjuvants for vaccine design.

Immune-Enhancing Effects of Taishan Pinus massoniana Pollen Polysaccharides on DNA Vaccine Expressing Bordetella avium ompA

Bordetella avium is the causative agent of bordetellosis, which remains to be the cause of severe losses in the turkey industry. Given the lack of vaccines that can provide good protection, developing a novel vaccine against B. avium infection is crucial. In this study, we constructed a eukaryotic expression plasmid, which expressed the outer membrane protein A (ompA) of B. avium, to prepare a B. avium recombinant ompA-DNA vaccine. Three concentrations (low, middle, and high) of Taishan Pinus massoniana pollen polysaccharides (TPPPS), a known immunomodulator, were used as adjuvants, and their immune conditioning effects on the developed DNA vaccine were examined. The pure ompA-DNA vaccine, Freund’s incomplete adjuvant ompA-DNA vaccine, and the empty plasmid served as the controls. The chickens in each group were separately inoculated with these vaccines three times at 1, 7, and 14 days old. Dynamic changes in antibody production, cytokine secretion, and lymphocyte count were then determined from 7 to 49 days after the first inoculation. Protective rates of the vaccines were also determined after the third inoculation. Results showed that the pure DNA vaccine obviously induced the production of antibodies, the secretion of cytokines, and the increase in CD⁴⁺ and CD⁸⁺ T lymphocyte counts in peripheral blood, as well as provided a protective rate of 50% to the B. avium-challenged chickens. The chickens inoculated with the TPPPS adjuvant ompA-DNA vaccine and Freund’s adjuvant ompA-DNA vaccine demonstrated higher levels of immune responses than those inoculated with pure ompA-DNA vaccine, whereas only the ompA-DNA vaccine with 200 mg/mL TPPPS completely protected the chickens against B. avium infection. These findings indicate that the B. avium ompA-DNA vaccine combined with TPPPS is a potentially effective B. avium vaccine.

Evaluation of the humoral immune response and cross reactivity against Mycobacterium tuberculosis of mice immunized with liposomes containing glycolipids of Mycobacterium smegmatis

Mycobacterium smegmatis (Ms) is a nonpathogenic mycobacteria of rapid growth, which shares many characteristics with Mycobacterium tuberculosis (MTB), the major causative agent of tuberculosis. MTB has several cell wall glycolipids in common with Ms, which play an important role in the pathogenesis of tuberculosis and the induction of a protective immune response against MTB infection in some animal models. In this study, the humoral immune response and cross reactivity against MTB, of liposomes containing a mixture of cell wall glycolipids of Ms and commercial lipids was evaluated, in order to study its possible use as a component of a vaccine candidate against tuberculosis. Liposomes containing total lipids extracted from Ms, distearoyl phosphatidyl choline and cholesterol were prepared by the dehydration-rehydration technique. Balb/c mice were immunized with the liposomes obtained and the antibody response and cross reactivity against MTB were tested by ELISA. Total lipids extract from Ms showed the presence of several polar glycolipids in common with MTB, such as phosphatidylinositol mannosides. Liposomes that contained glycolipids of Ms were capable of inducing a specific IgG antibody response that allowed the recognition of surface antigens of MTB. The results of this study demonstrated the presence of immunogenic glycolipids in Ms, which could be included to enhance the protective effects of subunit vaccine formulations against tuberculosis.

Vaccines

Vaccines continue to offer the key line of protection against a range of infectious diseases; however, the range of vaccines currently available is limited. One key consideration in the development of a vaccine is risk-versus-benefit, and in an environment of perceived low risk, the benefit of vaccination may not be recognised. To address this, there has been a move towards the use of subunit-based vaccines, which offer low side-effect profiles but are generally weakly immunogenic. This can be compensated for by the development of effective adjuvants. Nanotechnology offers key attributes in this field through the ability of nanoparticulates to incorporate and protect antigens from rapid degradation, combined with their potential to effectively deliver the antigens to appropriate cells within the immune system. These characteristics can be exploited in the development of new adjuvants. This chapter will outline the applications of nanosystems in vaccine formulations and consider the mechanisms of action behind a range of formulations.

Protective immune response against Toxoplasma gondii elicited by a recombinant DNA vaccine with a novel genetic adjuvant

Previous immunological studies from our laboratory have demonstrated the potential role of Toxoplasma gondii antigens SAG1 and GRA2 as vaccine candidates. To further evaluate the vaccine's effects, a series of recombinant DNA vaccines pVAX1-SAG1, pVAX1-GRA2 and pVAX1-SAG1-GRA2, termed pSAG1, pGRA2 and pSAG1-GRA2, respectively, were constructed. A plasmid pVAX1-S/PreS2, termed pSPreS2 encoding hepatitis B virus (HBV) surface antigen (HBsAg) S and PreS2 as a novel genetic adjuvant, was also constructed. The expression abilities of those DNA plasmids were examined in HFF cells by Western blotting. Then BALB/c mice were intramuscularly immunized with DNA plasmids and followed by challenging with the highly virulent T. gondii RH strain. The results demonstrated that the recombinant DNA vaccine pSAG1-GRA2 was capable of eliciting high levels of antibodies, a Th1 type of immune response with significant production of IFN-γ and low levels of IL-4 or IL-10 in BALB/c mice, and partial protection against the acute phase of toxoplasmosis as compared to pSAG1, pGRA2 and controls. In addition, the adjuvant pSPreS2 formulated with DNA vaccine induced a Th1 type of immune response and therefore might be a novel genetic adjuvant to DNA vaccine for further investigation.

Exploring the correlation between lipid packaging in lipoplexes and their transfection efficacy

Whilst there is a large body of evidence looking at the design of cationic liposomes as transfection agents, correlates of formulation to function remain elusive. In this research, we investigate if lipid packaging can give further insights into transfection efficacy. DNA lipoplexes composed of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) or 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE) in combination with 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or 1,2-stearoyl-3-trimethylammonium-propane (DSTAP) were prepared by the lipid hydration method. Each of the formulations was prepared by hydration in dH2O or phosphate buffer saline (PBS) to investigate the effect of buffer salts on lipoplex physicochemical characteristics and in vitro transfection. In addition, Langmuir monolayer studies were performed to investigate any possible correlation between lipid packaging and liposome attributes. Using PBS, rather than dH2O, to prepare the lipoplexes increased the size of vesicles in most of formulations and resulted in variation in transfection efficacies. However, one combination of lipids (DSPE:DOTAP) could not form liposomes in PBS, whilst the DSPE:DSTAP combination could not form liposomes in either aqueous media. Monolayer studies demonstrated saturated lipid combinations offered dramatically closer molecular packing compared to the other combinations which could suggest why this lipid combination could not form vesicles. Of the lipoplexes prepared, those formulated with DSTAP showed higher transfection efficacy, however, the effect of buffer on transfection efficiency was formulation dependent.

Cationic liposomes as vaccine adjuvants

The application of cationic liposomes as vaccine delivery systems and adjuvants has been investigated extensively over the last few decades. However, cationic liposomes are, in general, not sufficiently immunostimulatory, which is why the combination of liposomes with immunostimulating ligands has arisen as a strategy in the development of novel adjuvant systems. Within the last 5 years, two novel adjuvant systems based on cationic liposomes incorporating Toll-like receptor or non-Toll-like receptor immunostimulating ligands have progressed from preclinical testing in smaller animal species to clinical testing in humans. The immune responses that these clinical candidates induce are primarily of the Th1 type for which there is a profound unmet need. Furthermore, a number of new cationic liposome-forming surfactants with notable immunostimulatory properties have been discovered. In this article we review the recent progress on the application of cationic liposomes as vaccine delivery systems/adjuvants.

Effect of vesicle size on tissue localization and immunogenicity of liposomal DNA vaccines

The formulation of plasmid DNA (pDNA) in cationic liposomes is a promising strategy to improve the potency of DNA vaccines. In this respect, physicochemical parameters such as liposome size may be important for their efficacy. The aim of the current study was to investigate the effect of vesicle size on the in vivo performance of liposomal pDNA vaccines after subcutaneous vaccination in mice. The tissue distribution of cationic liposomes of two sizes, 500 nm (PDI 0.6) and 140 nm (PDI 0.15), composed of egg PC, DOPE and DOTAP, with encapsulated OVA-encoding pDNA, was studied by using dual radiolabeled pDNA-liposomes. Their potency to elicit cellular and humoral immune responses was investigated upon application in a homologous and heterologous vaccination schedule with 3 week intervals. It was shown that encapsulation of pDNA into cationic lipsomes resulted in deposition at the site of injection, and strongest retention was observed at large vesicle size. The vaccination studies demonstrated a more robust induction of OVA-specific, functional CD8+ T-cells and higher antibody levels upon vaccination with small monodisperse pDNA-liposomes, as compared to large heterodisperse liposomes or naked pDNA. The introduction of a PEG-coating on the small cationic liposomes resulted in enhanced lymphatic drainage, but immune responses were not improved when compared to non-PEGylated liposomes. In conclusion, it was shown that the physicochemical properties of the liposomes are of crucial importance for their performance as pDNA vaccine carrier, and cationic charge and small size are favorable properties for subcutaneous DNA vaccination.

Comparison of the depot effect and immunogenicity of liposomes based on dimethyldioctadecylammonium (DDA), 3β-[N-(N',N'-Dimethylaminoethane)carbomyl] cholesterol (DC-Chol), and 1,2-Dioleoyl-3-trimethylammonium propane (DOTAP): prolonged liposome retention mediates stronger Th1 responses

The immunostimulatory capacities of cationic liposomes are well-documented and are attributed both to inherent immunogenicity of the cationic lipid and more physical capacities such as the formation of antigen depots and antigen delivery. Very few studies have however been conducted comparing the immunostimulatory capacities of different cationic lipids. In the present study we therefore chose to investigate three of the most well-known cationic liposome-forming lipids as potential adjuvants for protein subunit vaccines. The ability of 3β-[N-(N',N'-dimethylaminoethane)carbomyl] cholesterol (DC-Chol), 1,2-dioleoyl-3-trimethylammonium propane (DOTAP), and dimethyldioctadecylammonium (DDA) liposomes incorporating immunomodulating trehalose dibehenate (TDB) to form an antigen depot at the site of injection (SOI) and to induce immunological recall responses against coadministered tuberculosis vaccine antigen Ag85B-ESAT-6 are reported. Furthermore, physical characterization of the liposomes is presented. Our results suggest that liposome composition plays an important role in vaccine retention at the SOI and the ability to enable the immune system to induce a vaccine specific recall response. While all three cationic liposomes facilitated increased antigen presentation by antigen presenting cells, the monocyte infiltration to the SOI and the production of IFN-γ upon antigen recall was markedly higher for DDA and DC-Chol based liposomes which exhibited a longer retention profile at the SOI. A long-term retention and slow release of liposome and vaccine antigen from the injection site hence appears to favor a stronger Th1 immune response.

Liposomal delivery of p-ialB and p-omp25 DNA vaccines improves immunogenicity but fails to provide full protection against B. melitensis challenge

Background

We have previously demonstrated protective efficacy against B. melitensis using formulations of naked DNA vaccines encoding genes ialB and omp25. The present study was undertaken to further understand the immune response generated by the protective vaccination regimens and to evaluate cationic liposome adsorption as a delivery method to improve vaccine utility.

Methods

The protective efficacy and immunogenicity of vaccines delivered as four doses of naked DNA, a single dose of naked DNA or a single dose of DNA surface adsorbed to cationic liposomes were compared using the BALB/c murine infection model of B. melitensis. Antigen-specific T cells and antibody responses were compared between the various formulations.

Results

The four dose vaccination strategy was confirmed to be protective against B. melitensis challenge. The immune response elicited by the various vaccines was found to be dependent upon both the antigen and the delivery strategy, with the IalB antigen favouring CD4+ T cell priming and Omp25 antigen favouring CD8+. Delivery of the p-ialB construct as a lipoplex improved antibody generation in comparison to the equivalent quantity of naked DNA. Delivery of p-omp25 as a lipoplex altered the profile of responsive T cells from CD8+ to CD4+ dominated. Under these conditions neither candidate delivered by single dose naked DNA or lipoplex vaccination methods was able to produce a robust protective effect.

Conclusions

Delivery of the p-omp25 and p-ialB DNA vaccine candidates as a lipoplex was able to enhance antibody production and effect CD4+ T cell priming, but was insufficient to promote protection from a single dose of either vaccine. The enhancement of immunogenicity by lipoplex delivery is a promising step toward improving the practicality of these two candidate vaccines, and suggests that this lipoplex formulation may be of value in situations where improvements to CD4+ responses are required. However, in the case of Brucella vaccine development it is suggested that further modifications to the candidate vaccines and delivery strategies will be required in order to deliver sustained protection.

Protective effect of DNA-mediated immunization with liposome-encapsulated GRA4 against infection of Toxoplasma gondii

The dense granule protein 4 (GRA4) is a granular protein from Toxoplasma gondii, and is a candidate for vaccination against this parasite. In this study, the plasmid pcDNA3.1-GRA4 (pGRA4), encoding for the GRA4 antigen, was incorporated by the dehydration-rehydration method into liposomes composed of 16 mmol/L egg phosphatidylcholine (PC), 8 mmol/L dioleoyl phosphatidylethanolamine (DOPE), and 4 mmol/L 1,2-diodeoyl-3-(trimethylammonium) propane (DOTAP). C57BL/6 mice and BALB/c mice were immunized intramuscularly three times with liposome-encapsulated pGRA4 to determine whether DNA immunization could elicit a protective immune response to T. gondii. Enzyme-linked immunosorbent assay (ELISA) of sera from immunized mice showed that liposome-encapsulated pGRA4 generated high levels of IgG antibodies to GRA4. Production of primary interferon (IFN)-gamma and interleukin (IL)-2 in GRA4-stimulated splenocytes from vaccinated mice suggested a modulated Th1-type response. 72.7% of C57BL/6 mice immunized with liposome-encapsulated pGRA4 survived the challenge with 80 tissue cysts of ME49 strain, whereas C57BL/6 mice immunized with pGRA4 had only a survival rate of 54.5%. When immunized BALB/c mice were intraperitoneally challenged with 10(3) tachyzoites of the highly virulent RH strain, the survival time of mice immunized with liposome-encapsulated pGRA4 was markedly longer than that of other groups. Our observations show that liposome-encapsulated pGRA4 enhanced the protective effect against infection of T. gondii.

Toxicology and biodistribution study of CIGB-230, a DNA vaccine against hepatitis C virus

CIGB-230, a mixture of a DNA plasmid expressing hepatitis C virus (HCV) structural antigens and a HCV recombinant capsid protein, has demonstrated to elicit strong immune responses in animals. The present study evaluated the plasmid biodistribution after the administration of CIGB-230 in mice, as well as toxicity of this vaccine candidate in rats. In the biodistribution study, mice received single or repeated intramuscular injections of CIGB-230, 50 μg of plasmid DNA mixed with 5 μg of Co.120 protein. Plasmid presence was assessed in ovaries, kidney, liver, pancreas, mesenteric ganglion, blood, and muscle of the injection site by a qualitative polymerase chain reaction. The toxicology evaluation included treatment groups receiving doses 5, 15, or 50 times higher, according to the body weight, than the expected therapeutic clinical dose. During the first hour after repeated inoculation, a promiscuous distribution was observed. However, 3 months later, plasmid could not be detected in any tissue. There was an absence of detectable adverse effects on key toxicology parameters and no damage evidenced in inspected organs and tissues. These results indicate that CIGB-230 is nontoxic at local and systemic levels and no concerns about persistence are observed, which support clinical testing of this vaccine candidate against HCV.

Opportunities and challenges in vaccine delivery

This report is a distillation of the workshop 'Opportunities and Challenges in Vaccine Delivery', organised by EUFEPS/FIP and co-sponsored by AAPS and CRS, in Archamps, France, September 2008. The aim of this workshop was to bridge knowledge gaps between the different disciplines involved in the delivery of vaccines. Here, key challenges include target identification, mapping the needs and target population, the development and harmonisation of predictive read-out systems and surrogate markers for protection, and improving antigen immunogenicity, delivery and stability. The workshop underlined the need and possibilities of a multidisciplinary approach to meet these challenges. This involves increasing our understanding of immunological mechanisms, the development of advanced delivery systems and adjuvant technologies, and insight into the regulatory guidelines and target population. Based upon this knowledge, future vaccinology can increasingly focus on rational design of antigens, adjuvants and delivery systems, which will lead to new and improved vaccines.

Vaccine adjuvant systems: enhancing the efficacy of sub-unit protein antigens

Vaccination remains a key tool in the protection and eradication of diseases. However, the development of new safe and effective vaccines is not easy. Various live organism based vaccines currently licensed, exhibit high efficacy; however, this benefit is associated with risk, due to the adverse reactions found with these vaccines. Therefore, in the development of vaccines, the associated risk-benefit issues need to be addressed. Sub-unit proteins offer a much safer alternative; however, their efficacy is low. The use of adjuvanted systems have proven to enhance the immunogenicity of these sub-unit vaccines through protection (i.e. preventing degradation of the antigen in vivo) and enhanced targeting of these antigens to professional antigen-presenting cells. Understanding of the immunological implications of the related disease will enable validation for the design and development of potential adjuvant systems. Novel adjuvant research involves the combination of both pharmaceutical analysis accompanied by detailed immunological investigations, whereby, pharmaceutically designed adjuvants are driven by an increased understanding of mechanisms of adjuvant activity, largely facilitated by description of highly specific innate immune recognition of components usually associated with the presence of invading bacteria or virus. The majority of pharmaceutical based adjuvants currently being investigated are particulate based delivery systems, such as liposome formulations. As an adjuvant, liposomes have been shown to enhance immunity against the associated disease particularly when a cationic lipid is used within the formulation. In addition, the inclusion of components such as immunomodulators, further enhance immunity. Within this review, the use and application of effective adjuvants is investigated, with particular emphasis on liposomal-based systems. The mechanisms of adjuvant activity, analysis of complex immunological characteristics and formulation and delivery of these vaccines are considered.

Liposome-nucleic acid immunotherapeutics

Cationic liposome-nucleic acid complexes, which were originally developed for use as non-viral gene delivery vectors, may now have an equally important application as immunotherapeutic drugs. Recent studies have highlighted the ability of cationic liposomes to potently activate the innate immune system when used to deliver certain Toll-like receptor (TLR) agonists. The immune-enhancing properties of cationic liposomes have been most clearly demonstrated when combined with nucleic acid agonists for endosomally located TLRs, including TLR3, TLR7/8 and TLR9. Immune potentiation by cationic liposomes likely results from the combined effects of endosomal targeting, protection of nucleic acids from extracellular degradation, and from signaling via newly identified cytoplasmic receptors for nucleic acids. The potent innate immune stimulatory properties of liposome-nucleic acid complexes make them particularly attractive as non-specific immunotherapeutics and as vaccine adjuvants. Liposome-nucleic acid complexes have demonstrated impressive anticancer activity in a number of different animal tumor models. Moreover, liposome-nucleic acid complexes have also been shown to be effective for immunotherapy of acute viral and bacterial infections, as well as chronic fungal infections. When used as vaccine adjuvants, liposome-nucleic acid complexes target antigens for efficient uptake by dendritic cells and are particularly effective in eliciting CD8(+) T-cell responses to protein antigens. Thus, liposome-nucleic acid complexes form a potent and versatile immunotherapeutic platform.

Cationic liposomes as vaccine adjuvants

Cationic liposomes are lipid-bilayer vesicles with a positive surface charge that have re-emerged as a promising new adjuvant technology. Although there is some evidence that cationic liposomes themselves can improve the immune response against coadministered vaccine antigens, their main functions are to protect the antigens from clearance in the body and deliver the antigens to professional antigen-presenting cells. In addition, cationic liposomes can be used to introduce immunomodulators to enhance and modulate the immune response in a desirable direction and, thereby, represent an efficient tool when designing tailor-made adjuvants for specific disease targets. In this article we review the recent progress on cationic liposomes as vehicles, enhancing the effect of immunomodulators and the presentation of vaccine antigens.

Mechanism of adjuvant activity of cationic liposome: phosphorylation of a MAP kinase, ERK and induction of chemokines

Cationic liposome has been effectively used as a delivery system for DNA and protein vaccines. Recently, we discovered that strong anti-tumor immunity could be generated when a peptide antigen (E7) was incorporated into 1,2-dioleoyl-3-trimethylammonium-propane (chloride salt) (DOTAP) cationic liposome. Therefore, DOTAP liposome exhibits not only efficient delivery capacity, but also a potent adjuvant activity. In this report, the molecular mechanism of the adjuvanticity was studied both in vitro and in vivo. Microarray of mRNA analysis demonstrated that several chemokine genes are up-regulated by DOTAP liposome, including CCL2, CCL3 and CCL4, upon treatment of dendritic cells (DC) with DOTAP liposomes. CCL2 induction was mediated through extracellular-signal-regulated kinase (ERK) pathway, demonstrated by specific inhibitors of ERK pathway and siRNA approaches. Furthermore, DOTAP-induced CCL2 expression is negatively regulated by the p38 pathway. Consistently, ERK activation by DOTAP is also negatively regulated by p38. Moreover, PI-3 kinase was shown to be involved in both activation of ERK and induction of CCL2 by DOTAP. DOTAP- induced CCL2 release was also confirmed in the draining lymph nodes. More importantly, inhibition of ERK pathway completely abolishes the CCL2 accumulation in the draining lymph nodes and attenuates anti-tumor activity of DOTAP/E7. In conclusion, DOTAP is an active lipid stimulator for DC resulting in ERK activation and CC chemokine induction. Our data elucidated one important mechanism of adjuvant activity of cationic liposome and could facilitate rational design of synthetic lipid based adjuvants.

Chemical adjuvants for plasmid DNA vaccines

Plasmid DNA vaccines are a promising modality for immunization against a variety of human pathogens. Immunization via multiple routes with plasmid DNA can elicit potent cellular immune responses, and these immunogens can be administered repeatedly without inducing anti-vector immunity. Nonetheless, the immunogenicity of plasmid DNA vaccines has been limited by problems associated with delivery. A number of adjuvants have been designed to improve plasmid DNA immunogenicity, either by directly stimulating the immune system or by enhancing plasmid DNA expression. Chemical adjuvants for enhancing plasmid DNA expression include liposomes, polymers, and microparticles, all of which have shown promise for enhancing the expression and immunogenicity of plasmid DNA vaccines in animal models. Micro- and nanoparticles have not been shown to enhance immune responses to plasmid DNA vaccines. However, formulation of plasmid DNA with some non-particulate polymeric adjuvants has led to a statistically significant enhancement of immune responses. Further development of these technologies will significantly improve the utility of plasmid DNA vaccination.

Particulate delivery systems for vaccines: what can we expect?

In our attempts to thwart the unwanted attentions of microbes by prophylactic and therapeutic vaccination, the knowledge of interactions at the molecular level may prove to be an invaluable asset. This article examines how particulate delivery systems such as liposomes and polymer microspheres can be applied in the light of recent advances in immunological understanding. Some of the biological interactions of these delivery systems are discussed with relevance for antigen trafficking and molecular pathways of immunogenicity and emphasis on the possible interaction of liposomal components. In particular, traditional concepts such as antigen protection, delivery to antigen presenting cells and depot formation remain important aspects, whilst the inclusion of selected co-adjuvants and enhanced delivery of these moieties in conjunction with antigen now has a firm rationale.

Protective immune responses to a multi-gene DNA vaccine against Staphylococcus aureus

To investigate the strategy of using a multivalent polyprotein DNA vaccine against Staphylococcus aureus, a series of plasmids was used to immunize mice followed by infectious challenge. The plasmid vaccines expressed Clumping factor A (Clfa), fibronectin binding protein A (FnBPA) and the enzyme Sortase (Srt) as single proteins or combined as a polyprotein. All animals produced a mixed Th1 and Th2 response including functional antigen-specific, mostly IgG2a antibodies, sustained production of IFN-gamma and a predominantly CD8+ T-cell response. Upon challenge with a virulent S. aureus isolate (Sa042), after 21 days, 55% of the multi-gene vaccinated mice survived infection compared to only 15% of the control groups. Vaccinated mice showed no signs of arthritis when challenged with the less virulent "Newman" strain that caused reactive arthritis in the controls. The results suggest that a multi-gene polyprotein-expressing nucleic acid vaccine alone produces a combined Th1 and Th2 response that can contribute to protection against the complex pathogenesis of S. aureus.