The immune response to a DNA vaccine can be modulated by co-delivery of cytokine genes using a DNA prime-protein boost strategy

DNA Vaccines: Their Formulations, Engineering and Delivery

The concept of DNA vaccination was introduced in the early 1990s. Since then, advancements in the augmentation of the immunogenicity of DNA vaccines have brought this technology to the market, especially in veterinary medicine, to prevent many diseases. Along with the successful COVID mRNA vaccines, the first DNA vaccine for human use, the Indian ZyCovD vaccine against SARS-CoV-2, was approved in 2021. In the current review, we first give an overview of the DNA vaccine focusing on the science, including adjuvants and delivery methods. We then cover some of the emerging science in the field of DNA vaccines, notably efforts to optimize delivery systems, better engineer delivery apparatuses, identify optimal delivery sites, personalize cancer immunotherapy through DNA vaccination, enhance adjuvant science through gene adjuvants, enhance off-target and heritable immunity through epigenetic modification, and predict epitopes with bioinformatic approaches. We also discuss the major limitations of DNA vaccines and we aim to address many theoretical concerns.

Influenza virus-like particles composed of conserved influenza proteins and GPI-anchored CCL28/GM-CSF fusion proteins enhance protective immunity against homologous and heterologous viruses

Influenza viruses cause significant morbidity and mortality and pose a substantial threat to public health. Vaccination represents the principle means of preventing influenza virus infection. Current vaccine approaches are hindered by the need to routinely reformulate vaccine compositions in an effort to account for the progressive antigenic changes that occur as influenza viruses circulate in the human population. In this study, we evaluated chimeric virus-like particle (cVLP) vaccines containing conserved elements of influenza proteins (HL5M2e (HA stem gene with 5M2e gene inserted) and NP), with or without glycosylphosphatidylinositol-anchored CCL28 (GPI-CCL28) and/or GM-CSF (GPI-GM-CSF) fusion proteins as molecular adjuvants. cVLPs elicited strong humoral and cellular immune responses against homologous and heterologous viruses, and improved survival following lethal challenge with both homologous and heterologous viruses. Inclusion of GPI-anchored adjuvants in cVLP vaccines augmented the generation of influenza-specific humoral and cellular immune responses in mice in comparison to the non-adjuvanted cVLP vaccines. VLPs containing GPI-anchored adjuvants reduced morbidity and improved survival to lethal challenge with homologous and heterologous influenza viruses. This work suggests that VLP vaccines incorporating conserved influenza virus proteins and GPI-anchored molecular adjuvants may serve as a platform for a broadly protective "universal" influenza vaccine.

Time-Course Study of the Transcriptome of Peripheral Blood Mononuclear Cells (PBMCs) from Sheep Infected with Fasciola hepatica

Fasciola hepatica is a parasitic trematode that infects a wide range of mammalian hosts, including livestock and humans, in temperate and tropical regions globally. This trematode causes the disease fascioliasis, which consists of an acute phase (≤ 12 weeks) during which juvenile parasites migrate through the host liver tissues, and a chronic phase (> 12 weeks) following the establishment of adult parasites in the liver bile ducts. Few studies have explored the progression of the host response over the course of Fasciola infection in the same animals. In this study, we characterized transcriptomic changes in peripheral blood mononuclear cells (PBMCs) collected from sheep at three time points over the first eight weeks of infection relative to uninfected controls. In total, 183 and 76 genes were found to be differentially transcribed at two and eight weeks post-infection respectively. Functional and pathway analysis of differentially transcribed genes revealed changes related to T-cell activation that may underpin a Th2-biased immune response against this parasite. This first insight into the dynamics of host responses during the early stages of infection improves the understanding of the pathogenesis of acute fascioliasis, informs vaccine development and presents a set of PBMC markers with diagnostic potential.

Effect of AcHERV-GmCSF as an Influenza Virus Vaccine Adjuvant

Introduction

The first identification of swine-originated influenza A/CA/04/2009 (pH1N1) as the cause of an outbreak of human influenza accelerated efforts to develop vaccines to prevent and control influenza viruses. The current norm in many countries is to prepare influenza vaccines using cell-based or egg-based killed vaccines, but it is difficult to elicit a sufficient immune response using this approach. To improve immune responses, researchers have examined the use of cytokines as vaccine adjuvants, and extensively investigated their functions as chemoattractants of immune cells and boosters of vaccine-mediated protection. Here, we evaluated the effect of Granulocyte-macrophage Colony-Stimulating Factor (GmCSF) as an influenza vaccine adjuvant in BALB/c mice.

Method and Results

Female BALB/c mice were immunized with killed vaccine together with a murine GmCSF gene delivered by human endogenous retrovirus (HERV) envelope coated baculovirus (1×10⁷ FFU AcHERV-GmCSF, i.m.) and were compared with mice immunized with the killed vaccine alone. On day 14, immunized mice were challenged with 10 median lethal dose of mouse adapted pH1N1 virus. The vaccination together with GmCSF treatment exerted a strong adjuvant effect on humoral and cellular immune responses. In addition, the vaccinated mice together with GmCSF were fully protected against infection by the lethal influenza pH1N1 virus.

Conclusion

Thus, these results indicate that AcHERV-GmCSF is an effective molecular adjuvant that augments immune responses against influenza virus.

Hydatid disease: vaccinology and development of the EG95 recombinant vaccine

Hydatid disease is a zoonotic parasitic disease that is distributed widely around the world and causes substantial human morbidity and mortality, particularly in developing countries. Reduction of human hydatid disease using anthelmintics, together with changes in human lifestyle and animal management practices, have been unsuccessful in some developing countries where the disease still persists. Substantial progress has been made towards developing a practical, recombinant vaccine in sheep, to interrupt the lifecycle of Echinococcus granulosus and to prevent subsequent transmission from dogs to humans. This review focuses on the scientific advances in the development of a recombinant vaccine for hydatid disease and the remaining challenges facing the widespread use of the vaccine for control of hydatid disease in endemic areas.

Enhanced immune response against HIV-1 induced by a heterologous DNA prime-adenovirus boost vaccination using mannosylated polyethyleneimine as DNA vaccine adjuvant

Background

The heterologous deoxyribonucleic acid (DNA) prime-adenovirus (AdV) boost vaccination approach has been widely applied as a promising strategy against human immunodeficiency virus (HIV)-1. However, the problem of inefficient delivery and lack of specificity of DNA vaccine remains a major issue. In this paper, to improve the transfection of DNA vaccine and realize dendritic cell targeting, we used mannosylated polyethyleneimine (man-PEI) as a DNA vector carrier.

Method

The DNA plasmid encoding antigen HIV gag fragment was constructed by polymerase chain reaction. Then the DNA plasmid was complexed with man-PEI. The in vitro transfection efficiency of man-PEI/DNA was analyzed on DC 2.4 cells. Mice were primed with 25 μg pVAX1-HIV gag plasmid complexed with man-PEI, 100 μg naked pVAX1-HIV gag plasmid, or empty pVAX1 vector and boosted by AdV encoding the same antigen. The antibody titer, CD4⁺ and CD8⁺ T-cell response, as well as interferon-γ and interleukin-4 levels in serum and in splenocytes culture were analyzed using flow cytometry or enzyme-linked immunosorbent assay to evaluate the immune response. To test a long-term effect of the vaccination regimen, CD8⁺ memory T-cell was also detected by flow cytometry.

Results

The pVAX1-HIV gag was constructed successfully. The in vitro transfection efficiency in dendritic cells was significantly higher than naked DNA plasmid. Compared with 100 μg naked DNA/AdV group, the immunoglobulin G2a antibody titer, T-cell response percentage, and cytokine production level induced by man-PEI/DNA/AdV group were significantly higher at a lower DNA dose. Also, the man-PEI/DNA could stimulate a memory CD8⁺ T-cell response.

Conclusion

Owing to the adjuvant effect of man-PEI, the man-PEI/pVAX1-HIV gag priming plus AdV boosting strategy proved to be a potent vaccine candidate against HIV, which could induce a stronger immune response with a lower DNA dose.

Biological activity of ovine IL-23 expressed using a foot-and-mouth disease virus 2A self-cleaving peptide

Hetero-dimeric cytokines often require equi-molar expression of both subunits to achieve biological activity. Previously, we expressed ovine IL-12 p40 and p35 linked using a self-cleaving 2A peptide from foot-and-mouth disease virus (FMDV). We now generated a new improved vector for the expression of hetero-dimeric cytokines and demonstrate the more general applicability of this strategy by cloning and expressing ovine IL-23 using the 2A peptide to link IL-12/IL-23 p40 and p19. The resulting protein was shown to be biologically active when expressed in mammalian COS cells. IL-23 plays a significant role in the differentiation of Th17 cells as well as autoimmunity and the regulation of inflammatory processes. As such this reagents will be invaluable in the unravelling of regulation of the ovine immune system for both veterinary and human animal model applications.

Induction of T helper 1 response by immunization of BALB/c mice with the gene encoding the second subunit of Echinococcus granulosus antigen B (EgAgB8/2)

A pre-designed plasmid containing the gene encoding the second subunit of Echinococcus granulosus AgB8 (EgAgB8/2) was used to study the effect of the immunization route on the immune response in BALB/c mice. Mice were immunized with pDRIVEEgAgB8/ 2 or pDRIVE empty cassette using the intramuscular (i.m.), intranasal (i.n.) or the epidermal gene gun (g.g.) routes. Analysis of the antibody response and cytokine data revealed that gene immunization by the i.m. route induced a marked bias towards a T helper type 1 (Th1) immune response as characterized by high IFN-γ gene expression and a low IgG1/IgG2a reactivity index (R.I.) ratio of 0.04. The i.n. route showed a moderate IFN-γ expression but a higher IgG1/IgG2a R.I. ratio of 0.25 indicating a moderate Th1 response. In contrast, epidermal g.g. immunization induced a Th2 response characterized by high IL-4 expression and the highest IgG1/IgG2a R.I. ratio of 0.58. In conclusion, this study showed the advantage of genetic immunization using the i.m. route and i.n. over the epidermal g.g. routes in the induction of Th1 immunity in response to E. granulosus AgB gene immunization.

Increased in vitro Cell Proliferation by Chitosan/pGM-CSF Complexes

Granulocyte macrophage colony stimulating factor, a potent hematopoietic cytokine, has been shown to stimulate production of white blood cells following chemotherapy. Therefore, the granulocyte macrophage colony stimulating factor gene is a potential candidate for the treatment of different pathological conditions. The purpose of this study is to investigate the suitability of chitosan as carrier for pORF-hGMCSF plasmid encoding granulocyte macrophage colony stimulating factor gene and also to study the effect of complexes on protein production and cell proliferation. Chitosan/pGM-CSF complexes were prepared using different (+/-) ratios (from 0.01/1 to 5/1). Complex formation was checked with agarose gel electrophoresis. The size and zeta potential values were measured. Enzyme and serum stability of complexes were studied. In vitro transfection properties of complexes were studied in HeLa cells. According to agarose gel electrophoresis, full complexation was obtained at 0.1/1 and higher chitosan/pGM-CSF ratios. Complexes having about 132 nm size and +13.7 mV zeta potential value were obtained. Chitosan complexes protected plasmid against enzymatic and serum effects. The gene expression-dependent cell proliferation after transfection of chitosan/pGM-CSF complexes at 72 h was markedly increased in comparision with the level of control group. These results indicate that the effect of chitosan/pGM-CSF complexes on cell proliferation was changed with N/P ratio and time-dependently. For GM-CSF therapy, chitosan/pGM-CSF complexes may be used as alternative to conventional protein treatments. Chitosan may be a good carrier for pORF-hGMCSF. Further, in vivo study is ongoing.

Coexpression of PPE 34.9 Antigen of Mycobacterium avium subsp. Paratuberculosis with Murine Interferon Gamma in HeLa Cell Line and Study of Their Immunogenicity in Murine Model

Mycobacterium avium subsp. paratuberculosis (Map) is the causative agent of johne's disease whose immunopathology mainly depends on cell mediated immuneresponse. Genome sequencing revealed various PPE (Proline-Proline-Glutamic acid) protein family of Map which are immunologically importance candidate genes In present study we have developed a bicistrionic construct pIR PPE/IFN containing a 34.9 kDa PPE protein (PPE 34.9) of Map along with a cytokine gene encoding murine gamma Interferon gene (IFNγ) and a monocistrionic construct pIR PPE using a mammalian vector system pIRES 6.1. The construct were transfected in HeLa cell line and expression were studied by Western blot as well as Immunefluroscent assay using recombinant sera. Further we have compared the immunereactivity of these two constructs in murine model by means of DTH study, LTT, NO assay and ELISA. DTH response was higher in pIR PPE/IFN than pIR PPE group of mice, similar finding also observed in case of LTT and NO production assay . ELISA titer of the pIR PPE/IFN was less than that with PPE only. These preliminary finding can revealed a CMI response of this PPE protein of Map and IFNγ having synergistic effect on this PPE protein to elicit a T cell based immunity in mice.

Comparison on in vitro characterization of fucospheres and chitosan microspheres encapsulated plasmid DNA (pGM-CSF): formulation design and release characteristics

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a cytokine used in the treatment of serious conditions resulting from chemotherapy and bone marrow transplantation such as neutropenia and aplastic anemia. Despite these effects, GM-CSF has a very short biological half-life, and it requires frequent injection during the treatment. Therefore, the cytokine production is possible in the body with plasmid-encoded GM-CSF (pGM-CSF) coding for cytokine administered to the body. However, the selection of the proper delivery system for the plasmid is important. In this study, two different delivery systems, encapsulated plasmid such as fucoidan-chitosan (fucosphere) and chitosan microspheres, were prepared and the particle physicochemical properties evaluated. Fucospheres and chitosan microspheres size ranges are 151-401 and 376-681 nm. The zeta potential values of the microspheres were changed between 8.3-17.1 mV (fucosphere) and +21.9-28.9 mV (chitosan microspheres). The encapsulation capacity of fucospheres changed between 84.2% and 94.7% depending on the chitosan molecular weight used in the formulation. In vitro plasmid DNA release from both delivery systems exhibited slower profiles of approximately 90-140 days. Integrity of released samples was checked by agarose gel electrophoresis, and any additional band was not seen. All formulations were analyzed kinetically. The calculated regression coefficients showed a higher r2 value with zero-order kinetics. In conclusion, the characterizations of the microspheres can be modulated by changing the formulation variables, and it can be concluded that fucospheres might be a potential carrier system for the controlled delivery of GM-CSF encoding plasmid DNA.

The effects of gene gun delivered pIL-3 adjuvant on skin pathology and cytokine expression

The aim of this study was to investigate skin immunopathology following gene gun delivery of plasmid-encoding interleukin 3 (pIL-3) and hence explore the possible mechanisms of its adjuvant activity. Using the sheep as the experimental model, expressible pIL-3 was administered to the epidermis and the dermal/epidermal junction and its effects on the skin were assessed by histopathology, immunohistology and quantitative RT-PCR for a range of pro-inflammatory and immune response polarizing cytokines. Delivery of both functional and non-functional plasmids caused an acute inflammatory response with the infiltration of neutrophils and micro-abscess formation; however, the response to pIL-3 was more severe and was also associated with an early (24 h) infiltration of B cells and a later accumulation of CD172a-/CD45RA+ dendritic cells (DC). In terms of cytokine transcript expression, an early TNFalpha response was stimulated by gene gun delivery of plasmid-associated gold beads, which coincided with an immediate infiltration of neutrophils. However, only pIL-3 triggered the short-lived expression of IL-3 (peaking at 6 h) and significant long-term increases in both TNFalpha and IL-1beta. pIL-3 did not affect the expression of the immune response polarizing cytokines, IL-10 and IL-12.

The effect of gene gun-delivered pGM-CSF on the immunopathology of the vaccinated skin

The aim of this study was to investigate the skin immunopathology of gene gun-delivered plasmid-encoded granulocyte-macrophage colony-stimulating factor (pGM-CSF) and hence explore the possible mechanisms of its adjuvant activity. Using sheep as the experimental model, expressible pGM-CSF was administered to the epidermis and the dermal/epidermal junction and its effects on the skin were assessed by histopathology, immunohistology and quantitative RT-PCR for a range of pro-inflammatory and immune response-polarizing cytokines. Both functional and non-functional plasmids caused an acute inflammatory response with the infiltration of neutrophils and micro-abscess formation; however, the response to pGM-CSF was more severe and was also associated with the accumulation of eosinophils, immature (CD1b(-)/CD172a(-)) dendritic cells and B cells. In terms of cytokine expression, an early TNF-alpha response was stimulated by gene gun delivery of plasmid-associated gold beads, which coincided with an immediate infiltration of neutrophils. However, only pGM-CSF triggered the short-lived expression of GM-CSF (peaking at 4 h) and significant long-term increases in both TNF-alpha and IL-1beta. pGM-CSF did not affect the expression of the immune response-polarizing cytokines, IL-10 and IL-12.

Combined DNA vaccination against three animal viruses elicits decreased immunogenicity of a single plasmid in mice

In order to investigate whether combined DNA vaccines are an ideal way to combine antigens in a single vaccine formulation, we immunized mice with three plasmids (pVSG, pVgD and pVE2), respectively, encoding the antigen of foot-and-mouth disease virus (FMDV), pseudorabies virus (PRV) and classic swine fever virus (CSFV), either alone or in a combined vaccine regimen. We also investigated the immune responses induced by a series of mixtures in which three plasmids were mixed in pairs. Then we further immunized mice with three different plasmids in separate sites and preformed an adoptive transfer experiment. While being given alone, each of the vaccine plasmids induced significant virus-specific antibody responses and splenocytes proliferative activity. But reduced immunogenicity of the pVSG plasmid was found in combined DNA vaccination, no matter whether it was injected in a single or a separate site. Removal single plasmid (pVgD or pVE2) from combined DNA vaccine led to significant increase in the immunogenicity of the pVSG plasmid (P<0.05). And the induction of immune suppression was not mediated by suppressor T cells, as demonstrated by an adoptive transfer experiment. Furthermore, by boosting with whole virus protein of FMDV, mice primed with either pVSG alone or combined DNA vaccine produced statistically significant increase in the FMDV-specific antibody titers (P<0.05). But after boosting, FMDV-specific splenocytes proliferative activity of mice primed with combined DNA vaccine was even lower than that of mice primed with pVSG alone (P<0.05). Taken together, this study reflected the immunogenicity of a single plasmid may be decreased in combined DNA immunization strategy, which still needs to be carefully evaluated before practical application.

Co-delivery of plasmid-encoded cytokines modulates the immune response to a DNA vaccine delivered by in vivo electroporation

In this study, in vivo electroporation of a DNA vaccine adjuvanted with plasmids encoding different cytokines was investigated in large animals. Sheep were injected intramuscularly with a DNA vaccine encoding an antigen of Haemonchus contortus (pNPA) and plasmids encoding different cytokines followed by in vivo electroporation. Plasmids (pCI) carrying the genes of different cytokines including ovine IL-4(pCI-IL4), IL-10(pCI-IL10), GM-CSF(pCI-GMCSF), and MCP-1alpha(pCI-MCP1alpha), and pCI-IL4+pCI-GMCSF were co-delivered with pNPA. The results showed that co-delivery of pCI-GMCSF or pCI-IL4+pCI-GMCSF significantly enhanced both antibody responses and T cell proliferation responses to the antigen after two DNA immunisations compared to co-delivery of pCI. In contrast, antibody responses of the sheep that received pCI-IL10 were decreased significantly. Other cytokine expressing plasmids did not significantly alter the measured immune responses. Furthermore, co-delivery of pCI-GMCSF increased IgG2 response more than IgG1 responses, suggesting a Th1 bias. However, the increase in IgG2 over IgG1 was less apparent when co-delivery of pCI-IL4 with pCI-GMCSF. Interestingly, the co-delivery of pCI-IL4 alone did not increase the IgG1 titre, suggesting that both pCI-GMCSF and pCI-IL4 are required for optimal IgG1 production. Thus, co-delivery of plasmid-encoded cytokine genes with in vivo electroporation has the ability to effectively modulate immune responses to a DNA vaccine in a large animal.

Enhanced protective immunity against pneumococcal infection with PspA DNA and protein

The effect of priming and boosting with pspA/EF5668 and purified recombinant PspA/EF5668 was examined. With this strategy CBA/N mice were protected against fatal challenge with Streptococcus pneumoniae EF5668. Anti-PspA antibody titers were elevated, and Western analysis with the immune serum demonstrated cross-reactivity with PspA from several different pneumococcal isolates, representing different PspA clades. Immune serum localized cross-reactive epitopes to the alpha-helical domain of PspA/Rx1 and PspA/EF5668. We demonstrated that DNA/protein prime-boost immunizations can enhance protective immunity against pneumococcal challenge.

Immunisation of the male tammar wallaby (Macropus eugenii) with spermatozoa elicits epididymal antigen-specific antibody secretion and compromised fertilisation rate

Immunocontraception has been proposed as an effective and humane means of controlling overabundant kangaroo populations in Australia. We have examined the feasibility of using a sperm-based vaccine for this purpose using a model macropod species, the tammar wallaby (Macropus eugenii). This study has demonstrated immunocontraception in a marsupial species following immunisation of males with homologous spermatozoa. Serum anti-sperm IgG titres were associated with a significant reduction in fertilisation rates following mating with superovulated female wallabies. Antigen-specific IgG penetrated the reproductive tract at the rete testis and bound spermatozoa in vivo. IgG was detected bound to the acrosome and midpiece regions of both epididymal and ejaculated spermatozoa. The absence of adverse testicular pathology and sperm movement effects suggests that contraception may have been achieved by antibody-mediated blocking of sperm surface antigens essential for fertilisation. This study demonstrates that a contraceptive vaccine targeting sperm antigens has potential for fertility control in male macropods.

DNA vaccines in sheep: CTLA-4 mediated targeting and CpG motifs enhance immunogenicity in a DNA prime/protein boost strategy

DNA vaccines have proven to be an efficient means of inducing immune responses in small laboratory animals; however, their efficacy in large out-bred animal models has been much less promising. In addressing this issue, we have investigated the ability of ovine cytotoxic lymphocyte antigen 4 (CTLA-4) mediated targeting and ruminant specific CpG optimised plasmids, both alone and in combination, to enhance immune responses in sheep to the pro cathepsin B (FhCatB) antigen from Fasciola hepatica. In this study, CTLA-4 mediated targeting enhanced the speed and magnitude of the primary antibody response and effectively primed for a potent memory response compared to conventional DNA vaccination alone, which failed to induce a detectable immune response. While the CpG-augmentation of the CTLA-4 targeted construct did not further enhance the magnitude or isotype profile of the CTLA-4 induced antibody titres, it did result in the induction of significant antigen-specific, lymphocyte-proliferative responses that were not observed in any other treatment group, showing for the first time that significant cellular responses can be induced in sheep following DNA vaccination. In contrast, CpG-augmentation in the absence of CTLA-4 mediated targeting failed to induce a detectable immune response. This is the first study to explore the potential adjuvant effects of ruminant specific CpG motifs on DNA vaccine induced immune responses in sheep. The ability of CpG-augmented CTLA-4 mediated targeting to induce both humoral and cellular immune responses in this study suggests that this may be an effective approach for enhancing the efficacy of DNA vaccines in large out-bred animal models.

Veterinary applications of cytokines

In the recent past a large variety of cytokines have been cloned for most important veterinary species and more is planned with development of a coordinated approach to cytokine reagents production. Application of these cytokines in veterinary species can be found in the development of effective diagnostics, with the IFN-gamma-based detection of tuberculosis as a prime example. In addition, cytokines have been used to determine which immune responses are essential for immune protection with flow-on effects for the development of novel ways to induce these specific immune responses. The realisation that the murine immune system is quite different from the human, together with the increased availability of cytokine reagents for many large animals plus unique experimental approaches only available in these animals, has lead to an explosion in the use of veterinary species as models for human diseases.

Immunization with plasmid DNA encoding a truncated, secreted form of the bovine viral diarrhea virus E2 protein elicits strong humoral and cellular immune responses

The major protective antigen of bovine viral diarrhea virus (BVDV), the E2 protein, is cell-associated and not expressed on the cell surface. In this study we evaluated a DNA vaccine encoding various secreted versions of E2. In vitro analysis demonstrated that deletion of the transmembrane anchor and addition of the signal sequence of bovine herpesvirus-1 (BHV-1) (gDsDeltaE2) resulted in efficient secretion of E2 into the culture medium. In contrast, full-length E2, either without or with gDs (gDsE2), as well as truncated E2 without gDs (DeltaE2), remained entirely cell-associated. Mice immunized with plasmid encoding gDsDeltaE2 developed significantly higher IgG and virus neutralizing antibody titres compared to animals vaccinated with plasmid encoding E2, DeltaE2 or gDsE2. To optimize secretion of E2, the efficiency of gDs was compared with that of the tissue plasminogen activator signal (tPAs) sequence. In addition, the effect of the plasmid backbone was assessed by comparing two vectors. Four plasmids, pMASIA-gDsDeltaE2, pMASIA-tPAsDeltaE2, pSLKIA-gDsDeltaE2 and pSLKIA-tPAsDeltaE2, were constructed and administered intradermally to mice. The mice immunized with pMASIA-tPAsDeltaE2 developed the strongest and most balanced immune responses. Vaccination of cattle confirmed that pMASIA-tPAsDeltaE2 elicited both strong humoral and cellular immune responses and thus could be a candidate DNA vaccine against BVDV.

Coimmunization with an Optimized IL-15 Plasmid Results in Enhanced Function and Longevity of CD8 T Cells That Are Partially Independent of CD4 T Cell Help

DNA vaccines are a promising technology for the induction of Ag-specific immune responses, and much recent attention has gone into improving their immune potency. In this study we test the feasibility of delivering a plasmid encoding IL-15 as a DNA vaccine adjuvant for the induction of improved Ag-specific CD8(+) T cellular immune responses. Because native IL-15 is poorly expressed, we used PCR-based strategies to develop an optimized construct that expresses 80-fold higher than the native IL-15 construct. Using a DNA vaccination model, we determined that immunization with optimized IL-15 in combination with HIV-1gag DNA constructs resulted in a significant enhancement of Ag-specific CD8(+) T cell proliferation and IFN-gamma secretion, and strong induction of long-lived CD8(+) T cell responses. In an influenza DNA vaccine model, coimmunization with plasmid expressing influenza A PR8/34 hemagglutinin with the optimized IL-15 plasmid generated improved long term CD8(+) T cellular immunity and protected the mice against a lethal mucosal challenge with influenza virus. Because we observed that IL-15 appeared to mostly adjuvant CD8(+) T cell function, we show that in the partial, but not total, absence of CD4(+) T cell help, plasmid-delivered IL-15 could restore CD8 secondary immune responses to an antigenic DNA plasmid, supporting the idea that the effects of IL-15 on CD8(+) T cell expansion require the presence of low levels of CD4 T cells. These data suggest a role for enhanced plasmid IL-15 as a candidate adjuvant for vaccine or immunotherapeutic studies.

DNA immunization with Na+-K+ ATPase (Sseat-6) induces protective immunity to larval Strongyloides stercoralis in mice

Strongyloides stercoralis causes chronic asymptomatic infections which can be maintained in the human host for many decades. Identification and treatment of S. stercoralis-infected individuals is required because immunosuppression can lead to fatal hyperinfection. In this study, human immunoglobulin G (IgG) that had previously been shown to transfer protective immunity to mice was used to identify potential protective antigens. Three antigens or genes from S. stercoralis larvae were identified as tropomyosin (Sstmy-1), Na+-K+ ATPase (Sseat-6), and LEC-5 (Sslec-5). The genes were cloned into plasmids for DNA immunization, and mice were immunized intradermally with the three plasmids individually in combination with a plasmid containing murine granulocyte-macrophage colony-stimulating factor. Only Na+-K+ ATPase induced a significant reduction in larval survival after DNA immunization. Immunization with a combination of all three plasmids, including Na+-K+ ATPase, did not induce protective immunity. Serum from mice immunized with DNA encoding Na+-K+ ATPase was transferred to naive mice and resulted in partial protective immunity. Therefore, DNA immunization with Na+-K+ ATPase induces protective immunity in mice, and it is the first identified vaccine candidate against infection with larval S. stercoralis.

Recombinant GRA4 or ROP2 protein combined with alum or the gra4 gene provides partial protection in chronic murine models of toxoplasmosis

The efficacy of vaccination with Toxoplasma gondii recombinant GRA4 (rGRA4) and ROP2 (rRPO2) proteins and a mix of both combined with alum were evaluated in C57BL/6 and C3H mice. In C57BL/6 mice, rGRA4 and rGRA4-rROP2 immunizations generated similar levels of immunoglobulin G1 (IgG1) and IgG2a isotypes against GRA4, whereas immunizations with rROP2 and the mix induced a predominant IgG1 production against ROP2. All groups of C3H vaccinated mice exhibited higher levels of IgG1 than IgG2a. rGRA4-stimulated splenocytes from vaccinated mice produced primarily gamma interferon while those stimulated with rROP2 produced interleukin-4. Challenge of rGRA4- or rGRA4-rROP2-vaccinated mice from both strains with ME49 cysts resulted in fewer brain cysts than the controls, whereas vaccination with rROP2 alone only conferred protection to C3H mice. Immunization with a plasmid carrying the entire open reading frame of GRA4 showed a protective level similar to that of rGRA4 combined with alum. These results suggest that GRA4 can be a good candidate for a multiantigen anti-T. gondii vaccine based on the use of alum as an adjuvant.

Strategies for improved formulation and delivery of DNA vaccines to veterinary target species

Interest in DNA immunization of animals continues, despite the fact that immune responses induced by DNA vaccines are generally lower than those elicited by conventional vaccines. In attempts to enhance the immune response to DNA vaccines, individuals have tried a variety of immune modulators, cytokines, and costimulatory molecules, but these only boost immune responses marginally. These results clearly demonstrate that the major challenge to improving DNA-based vaccines is to improve the transfection efficiency. Gene gun and electroporation can increase transfection and improve immune responses significantly, but these technologies have not yet advanced to the stage of routine use in livestock. Hopefully, transfection efficiency can be increased further in a user-friendly manner to ensure that the benefits of using DNA vaccines become a reality.

Priming of piglets against enterotoxigenic E. coli F4 fimbriae by immunisation with FAEG DNA

Early vaccination is necessary to protect pigs against postweaning diarrhoea caused by enterotoxigenic Escherichia coli (ETEC). However, at present no commercial vaccine allows successful vaccination. This is partly due to the presence of maternally derived antibodies. Since DNA vaccines are suggested to be superior to protein vaccines in young animals with maternal antibodies, we determined whether the fimbrial adhesin (FaeG) of F4ac(+) ETEC could be used as a plasmid DNA vaccine to prime piglets in a heterologous prime-boost approach. Hereto, pcDNA1/faeG19 was constructed and expression of rFaeG in Cos-7 cells was demonstrated. Thereafter, pigs were immunised (days 0, 21 and 42) intramuscularly by injection or intradermally by gene gun and humoral and cellular immune responses were analysed. Even though responses were low, results demonstrated that intramuscular injection was superior to gene gun delivery for priming the humoral immune response since higher antibody titres were raised, whereas gene gun delivery better induced a cellular response, evaluated by a lymphocyte proliferation assay. Effective priming of the humoral immune response was evidenced by high IgG titres 1 week after a protein boost with purified F4. The low responses to the pcDNA1/faeG19 DNA vaccination suggest that delivery of the DNA and/or the expression of the faeG gene should be improved.

In vivo electroporation improves immune responses to DNA vaccination in sheep

In vivo electroporation was utilised to enhance plasmid DNA expression in sheep muscle to improve the immune response to DNA vaccination. DNA encoding enhanced green fluorescence protein expressed at higher levels in sheep muscle following in vivo electroporation which caused minimal muscle damage. Groups of seven sheep were then given three intramuscular injections of plasmids encoding two Haemonchus contortus Ag, with and without electroporation at 0, 3 and 7 weeks. Humoral responses were enhanced in electroporated sheep. Four weeks after vaccination, all groups were injected subcutaneously with recombinant Ag formulated in Quil A. Induction of vaccine-specific immune memory was demonstrated in DNA-vaccinated sheep.

A DNA-prime/protein-boost vaccination regimen enhances Th2 immune responses but not protection following Schistosoma mansoni infection

DNA immunization represents a promising vaccine strategy that has been reasonably successful, and will likely play an even greater role in vaccine development as these vaccines continue to be improved. We have developed a partially protective DNA vaccine against schistosome infection based on a 23-kDa integral membrane protein, Sm23. The focus of this study was to compare immunogenicity and efficacy of vaccination regimens utilizing Sm23 DNA vaccine alone vs. regimens that utilized both Sm23 DNA and Sm23 in recombinant protein form. We found that priming and boosting with the Sm23 DNA construct (Sm23-pcDNA) resulted in a significant level of protection against challenge infection (36-44%). In contrast, altering this protocol by changing the boost from Sm23 DNA to boosting with recombinant Sm23 protein (rSm23) formulated in aluminium hydroxide (alum) failed to induce a significant reduction in worm burdens. Similarly, mice primed and boosted with the rSm23 in alum also did not develop significant levels of protection against challenge infection. We hypothesize that the differences in the ability to drive protective immunity using the DNA prime-DNA boost strategy and the inability to do so when recombinant Sm23 in alum was substituted for Sm23 DNA is due to driving of different immune responses. In support of this, we found that mice primed and boosted with Sm23-pcDNA had Th1-type immune responses characterized by low anti-Sm23 IgG1 : IgG2a antibody isotype ratios, whereas mice boosted with rSm23 had higher IgG1 : IgG2a ratios. In addition, priming and boosting with rSm23 elicited mainly IgG1 antibodies with no detectable IgG2a, indicative of a polarized Th2-type immune response. Thus, similar to our earlier work, the results of this study show that protective vaccination using Sm23 is associated with a Th1 immune response, and efficacy is diminished using protocols that diminish this Th1 bias. In our study, this was likely due to the use of the Th2-driving adjuvant alum, and future studies are planned where we will compare the protective efficacy of rSm23 administered with Th1-type adjuvants.

Protective effects of vaccination with bovine leukemia virus (BLV) Tax DNA against BLV infection in sheep

A DNA vaccination trial was performed on sheep to determine whether vaccination with bovine leukemia virus (BLV) transactivator Tax DNA is effective against BLV infection. Immunization was carried out with cationic liposomes containing the Tax-expressing plasmid DNA and subsequently all sheep were challenged with BLV. BLV titers in peripheral blood mononuclear cell (PBMC) determined by syncytium formation assay and BLV provirus load detected by genomic PCR analysis showed higher levels of virus titers in control sheep than those in Tax-vaccinated sheep. Higher levels of IFN-gamma mRNA expression have been demonstrated in vaccinated sheep after the challenge. These results suggested that Th1 type immune response induced by Tax DNA vaccine inhibited BLV propagation in vaccinated sheep at the early phase of infection.

Technological advances and genomics in metazoan parasites

Molecular biology has provided the means to identify parasite proteins, to define their function, patterns of expression and the means to produce them in quantity for subsequent functional analyses. Whole genome and expressed sequence tag programmes, and the parallel development of powerful bioinformatics tools, allow the execution of genome-wide between stage or species comparisons and meaningful gene-expression profiling. The latter can be undertaken with several new technologies such as DNA microarray and serial analysis of gene expression. Proteome analysis has come to the fore in recent years providing a crucial link between the gene and its protein product. RNA interference and ballistic gene transfer are exciting developments which can provide the means to precisely define the function of individual genes and, of importance in devising novel parasite control strategies, the effect that gene knockdown will have on parasite survival.

DNA vaccines and their application against parasites--promise, limitations and potential solutions

DNA or nucleic acid vaccines are being evaluated for efficacy against a range of parasitic diseases. Data from studies in rodent model systems have provided proof of principle that DNA vaccines are effective at inducing both humoral and T cell responses to a variety of candidate vaccine antigens. In particular, the induction of potent cellular responses often gives DNA vaccination an immunological advantage over subunit protein vaccination. Protection against parasite challenge has been demonstrated in a number of systems. However, application of parasite DNA vaccines in large animals including ruminants, primates and humans has been compromised by the relative lack of immune responsiveness to the vaccines, but the reasons for this hyporesponsiveness are not clear. Here, we review DNA vaccines against protozoan parasites, in particular vaccines for malaria, and the use of genomic approaches such as expression library immunization to generate novel vaccines. The application of DNA vaccines in ruminants is reviewed. We discuss some of the approaches being evaluated to improve responsiveness in large animals including the use of cytokines as adjuvants, targeting molecules as delivery ligands, electroporation and CpG oligonucleotides.

Vaccines for prevention of cysticercosis

Neurocysticercosis due to Taenia solium infection is an important cause of human morbidity and mortality. Despite the availability of effective anthelmintics, the disease remains prevalent in many parts of the world and there is a need for new and improved measures for control of the infection. An effective vaccine to prevent infection in pigs, the parasite's natural intermediate host, would be a valuable new option to assist with T. solium control. Several approaches are being used currently towards the development of a T. solium vaccine and these approaches are reviewed briefly, with emphasis on the use of recombinant oncosphere antigens. Highly effective vaccines have been developed against cysticercosis in sheep and cattle caused by Taenia ovis and Taenia saginata, respectively. This success has encouraged the adoption of a similar strategy for T. solium. The recent finding that one oncosphere antigen, TSOL18, can induce complete protection against T. solium infection in pigs, highlights the potential for development of a practical vaccine. A vision is proposed for the development of a safe, effective, inexpensive vaccine for pigs, which can be administered in an edible form. Through an international collaborative effort, research is progressing towards the realisation of such a vaccine and its use to reduce the global burden of neurocysticercosis.

Vaccine adjuvant technology: from mechanistic concepts to practical applications

Distinct types of immune responses are required for efficient elimination of different pathogens. Programming of the desired type of immune response by safe nonreplicating vaccines requires suitable vaccine adjuvants. Adjuvants largely determine the magnitude and quality of immune responses specific for the coadministered antigen. Unfortunately, rational vaccine design requiring a rational choice of vaccine adjuvant, is hampered by a lack of knowledge about the mechanism(s) of vaccine adjuvant activity. The current review addresses different critical immunological processes possibly explaining adjuvant functions. In addition, we discuss traditional vaccine adjuvant formulations and their possible mode of action. Finally, we reflect on the latest technologies for the identification of novel adjuvants using molecular analysis of immune activation and functional genomics.

HBV DNA vaccine with adjuvant cytokines induced specific immune responses against HBV infection

AIM: To seek for an effective method to improve the immune responses induced by DNA vaccine expressing HBV surface antigen (pCR3.1-S) in Balb/c mice (H-2^d).

METHODS: The pCR3.1-S plasmid and the eukaryotic expression vectors expressing murine IL-2 (pDOR-IL-2) or IL-12 (pWRG3169) were injected into mice subcutaneously. The immune responses to pCR3.1-S and the adjuvant effect of the cytokines plasmid were studied. Meanwhile the effect of pCR3.1-S on anti-translated subcutaneous tumor of P815 mastocytoma cells stably expressing HBsAg (P815-HBV-S) was also studied. Anti-HBs in serum was detected by enzyme-linked immunoadsordent assay (ELISA) and HBsAg specific cytotoxic T lymphocytes (CTLs) activity was measured by ⁵¹Cr release assay. After three weeks of DNA immunization, the cells of P815-HBV-S were inoculated into mice subcutaneously and the tumor growth was measured every five days. The survival rate and living periods of mice were also calculated.

RESULTS: After 8 wk DNA immunization, the A 450 nm values of sera in mice immunized with pCR3.1, pCR3.1-S and pCR3.1-S codeliveried with IL-2 or IL-12 plasmids were 0.03 ± 0.01, 1.24 ± 0.10, 1.98 ± 0.17 and 1.67 ± 0.12 respectively. Data in mice codeliveried pCR3.1-S with IL-2 or IL-12 plasmids were significantly higher than that of mice injected pCR3.1 or pCR3.1-S only. The HBsAg specific CTL activities in mice coinjected with pCR3.1-S and IL-2 or IL-12 eukaryotic expression vectors were (61.9 ± 7.1)% and (73.3 ± 8.8)%, which were significantly higher than that of mice injected with pCR3.1 (10.1 ± 2.1)% or pCR3.1-S (50.5 ± 6.4)%. The HBsAg specific CTL activities in mice injected with pCR3.1, pCR3.1-S, pCR3.1-S combined with IL-2 or IL-12 eukaryotic expression vectors decreased significantly to (3.2 ± 0.8)%, (10.6 ± 1.4)%, (13.6 ± 1.3)% and (16.9 ± 2.3)% respectively after the spleen cells were treated by anti-CD8⁺ monoclonal antibody, but presented no significant change to anti-CD4⁺ monoclonal antibody or unrelated to monoclonal antibody. The HBV-S DNA vaccine (pCR3.1-S) could evidently inhibit the tumor growth, prolong the survival period of mice and improve the survival rate of mice and these effects could be improved by IL-12 gene codeliveried.

CONCLUSION: HBV DNA vaccine has a strong antigenicity in humoral and cellular immunities, which can be promoted by plasmid expressing IL-2 or IL-12. CD8⁺ cells executed the CTL activities. DNA vaccine may be useful for both prophylaxis and treatment of HBV infection.

Evaluation of protection against Chlamydophila abortus challenge after DNA immunization with the major outer-membrane protein-encoding gene in pregnant and non-pregnant mice

The protective effect of DNA vaccination with the gene encoding the major outer-membrane protein (MOMP) of Chlamydophila abortus has been studied in non-pregnant and pregnant mouse models after chlamydial challenge. OF1 outbred mice were vaccinated intramuscularly three times every 3 weeks, mated and challenged with C. abortus 2 weeks after the last injection of DNA. In non-pregnant mice, the MOMP DNA vaccine elicited a specific humoral response with predominantly IgG2a antibodies, suggesting a Th1-type immune response. The induced antibodies showed no in vitro neutralizing effect on C. abortus infectivity. Moreover, immunization with the momp gene showed no reduction in the mean splenic bacterial counts of non-pregnant or pregnant mice or in the mean placental bacterial counts of pregnant mice after the C. abortus challenge. Nevertheless, the MOMP DNA immunization induced a non-specific and partial protection in fetuses against challenge.

Immunocontraceptive vaccines for human use

The development of safe, effective, reversible contraceptive vaccines for the regulation of human fertility would be a significant addition to our contraceptive armamentarium. However, because we are such an out-bred species, immunological responsiveness to any given vaccine is certain to exhibit a high level of inter-individual variation that will impact upon the efficacy, reversibility and feasibility of the approach. Nevertheless a role for vaccines undoubtedly exists as an aid to birth spacing, particularly in developing countries, and as a non-surgical means of inducing sterility in men. Currently vaccines are being researched that target one of two strategic points in the reproductive process, fertilization and the maternal recognition of pregnancy. Our ability to engineer vaccines that target fertilization is hampered by deficiencies in our knowledge of the molecular mechanisms that regulate this process. However, anti-hCG vaccines have advanced to the stage of clinical trials and appear promising.

Genetic immunization elicits antigen-specific protective immune responses and decreases disease severity in Trypanosoma cruzi infection

Immunity to Trypanosoma cruzi requires elicitation of humoral and cell-mediated immune responses to extracellular trypomastigotes and intracellular amastigotes. In this study, the effectiveness of the T. cruzi trans-sialidase family (ts) genes ASP-1, ASP-2, and TSA-1 as genetic vaccines was assessed. Immunization of mice with plasmids encoding ASP-1, ASP-2, or TSA-1 elicited poor antigen-specific cytotoxic-T-lymphocyte (CTL) activity and T. cruzi-specific antibody responses. Codelivery of interleukin-12 and granulocyte-macrophage colony-stimulating factor plasmids with antigen-encoding plasmids resulted in a substantial increase in CTL activity and antibody production and in increased resistance to T. cruzi infection. In pooled results from two to four experiments, 30 to 60% of mice immunized with antigen-encoding plasmids and 60 to 80% of mice immunized with antigen-encoding plasmids plus cytokine adjuvants survived a lethal challenge with T. cruzi. In comparison, 90% of control mice injected with empty plasmid DNA died during the acute phase of infection. However, the pool of three ts genes provided no greater protection than the most effective single gene (ASP-2) either with or without coadministration of cytokine plasmids. Importantly, the extent of tissue parasitism, inflammation, and associated tissue damage in skeletal muscles during the chronic phase of T. cruzi infection in mice immunized with antigen-encoding plasmids plus cytokine adjuvants was remarkably reduced compared to mice immunized with only cytokine adjuvants or empty plasmid DNA. These results identify new vaccine candidates and establish some of the methodologies that might be needed to develop effective vaccine-mediated control of T. cruzi infection. In addition, this work provides the first evidence that prophylactic genetic immunization can prevent the development of Chagas' disease.

Simultaneous GeneGun immunisation with plasmids encoding antigen and GM-CSF: significant enhancement of murine antivenom IgG1 titres

GeneGun DNA immunisation is a potent means of inducing antibody-dominant immune responses that we are exploiting to generate venom toxin-specific antibodies to improve the therapy of systemic envenoming by snakes. Here, we report that mice immunised with DNA encoding the carboxyl domain (JD9) of a haemorrhagic Zn metalloprotease (Jararhagin) in venom of the South American pit viper, Bothrops jararaca, and a plasmid expressing murine cytokine granulocyte/macrophage-colony stimulating factor (GM-CSF) raised significantly higher antigen-specific IgG1 titres than mice immunised with JD9 DNA alone. Serological responses to GeneGun JD9 DNA immunisation were shown to be dominated by IgG1, an IgG subclass associated with T lymphocyte helper 2 (Th2) immune responses. Further significant enhancement of JD9-specific IgG1 titres was achieved by increasing the number of immunisations. This report illustrates that DNA immunisation protocols to achieve high-titre, venom toxin-specific antibody production are well advanced and encourage the development of a DNA-based approach to antivenom production.

Strategies for designing and optimizing new generation vaccines

Although the field of immunology developed in part from the early vaccine studies of Edward Jenner, Louis Pasteur and others, vaccine development had largely become the province of virologists and other microbiologists, because the model for classic vaccines was to isolate the pathogen and prepare a killed or attenuated pathogen vaccine. Only recently has vaccinology returned to the realm of immunology, because a new understanding of immune mechanisms has allowed translation of basic discoveries into vaccine strategies.