The combination of DNA vectors expressing IL-12 + IL-18 elicits high protective immune response against cutaneous leishmaniasis after priming with DNA-p36/LACK and the cytokines, followed by a booster with a vaccinia virus recombinant expressing p36/LACK

Global distribution of treatment resistance gene markers for leishmaniasis

Background

Pentavalent antimonials (Sb(V)) such as meglumine antimoniate (Glucantime®) and sodium stibogluconate (Pentostam®) are used as first‐line treatments for leishmaniasis, either alone or in combination with second‐line drugs such as amphotericin B (Amp B), miltefosine (MIL), methotrexate (MTX), or cryotherapy. Therapeutic aspects of these drugs are now challenged because of clinical resistance worldwide.

Methods

We reviewedthe recent original studies were assessed by searching in electronic databases such as Scopus, Pubmed, Embase, and Web of Science.

Results

Studies on molecular biomarkers involved in drug resistance are essential for monitoring the disease. We reviewed genes and mechanisms of resistance to leishmaniasis, and the geographical distribution of these biomarkers in each country has also been thoroughly investigated.

Conclusion

Due to the emergence of resistant genes mainly in anthroponotic Leishmania species such as L. donovani and L. tropica, as the causative agents of ACL and AVL, respectively, selection of an appropriate treatment modality is essential. Physicians should be aware of the presence of such resistance for the selection of proper treatment modalities in endemic countries.

Vaccination in Leishmaniasis: A Review Article

Leishmaniasis is caused by protozoan Leishmania parasites that are transmitted through female sandfly bites. The disease is predominantly endemic to the tropics and semi-tropics and has been reported in more than 98 countries. Due to the side effects of anti-Leishmania drugs and the emergence of drug-resistant isolates, there is currently no encouraging prospect of introducing an effective therapy for the disease. Hence, it seems that the key to disease control management is the introduction of an effective vaccine, particularly against its cutaneous form. Advances in understanding underlying immune mechanisms are feasibale using a variety of candidate antigens, including attenuated live parasites, crude antigens, pure or recombinant Leishmania proteins, Leishmania genes encoding protective proteins, as well as immune system activators from the saliva of parasite vectors. However, there is still no vaccine against different types of human leishmaniasis. In this study, we review the works conducted or being performed in this field.

Acid Stripping after Infection Improves the Detection of Viral HLA Class I Natural Ligands Identified by Mass Spectrometry

Identification of a natural human leukocyte antigen (HLA) ligandome is a key element to understand the cellular immune response. Advanced high throughput mass spectrometry analyses identify a relevant, but not complete, fraction of the many tens of thousands of self-peptides generated by antigen processing in live cells. In infected cells, in addition to this complex HLA ligandome, a minority of peptides from degradation of the few proteins encoded by the viral genome are also bound to HLA class I molecules. In this study, the standard immunopeptidomics strategy was modified to include the classical acid stripping treatment after virus infection to enrich the HLA ligandome in virus ligands. Complexes of HLA-B*27:05-bound peptide pools were isolated from vaccinia virus (VACV)-infected cells treated with acid stripping after virus infection. The HLA class I ligandome was identified using high throughput mass spectrometry analyses, yielding 37 and 51 natural peptides processed and presented untreated and after acid stripping treatment VACV-infected human cells, respectively. Most of these virus ligands were identified in both conditions, but exclusive VACV ligands detected by mass spectrometry detected on acid stripping treatment doubled the number of those identified in the untreated VACV-infected condition. Theoretical binding affinity prediction of the VACV HLA-B*27:05 ligands and acute antiviral T cell response characterization in the HLA transgenic mice model showed no differences between HLA ligands identified under the two conditions: untreated and under acid stripping condition. These findings indicated that acid stripping treatment could be useful to identify HLA class I ligands from virus-infected cells.

IL-12 DNA Displays Efficient Adjuvant Effects Improving Immunogenicity of Ag85A in DNA Prime/MVA Boost Immunizations

Mycobacterium tuberculosis (Mtb) infection is one of the leading causes of death worldwide. The Modified Vaccinia Ankara (MVA) vaccine vector expressing the mycobacterial antigen 85A (MVA85A) was demonstrated to be safe, although it did not improve BCG efficacy, denoting the need to search for improved tuberculosis vaccines. In this work, we investigated the effect of IL-12 DNA -as an adjuvant- on an Ag85A DNA prime/MVA85A boost vaccination regimen. We evaluated the immune response profile elicited in mice and the protection conferred against intratracheal Mtb H37Rv challenge. We observed that the immunization scheme including DNA-A85A+DNA-IL-12/MVA85A induced a strong IFN-γ production to Ag85A in vitro, with a significant expansion of IFN-γ⁺CD4⁺ and IFN-γ⁺CD8⁺ anti-Ag85A lymphocytes. Furthermore, we also detected a significant increase in the proportion of specific CD8⁺CD107⁺ T cells against Ag85A. Additionally, inclusion of IL-12 DNA in the DNA-A85A/MVA85A vaccine scheme induced a marked augment in anti-Ag85A IgG levels. Interestingly, after 30 days of infection with Mtb H37Rv, DNA-A85A+DNA-IL-12/MVA85A vaccinated mice displayed a significant reduction in lung bacterial burden. Together, our findings suggest that IL-12 DNA might be useful as a molecular adjuvant in an Ag85A DNA/MVA prime-boost vaccine against Mtb infection.

Immunoproteomic analysis of a Chikungunya poxvirus-based vaccine reveals high HLA class II immunoprevalence

BACKGROUND

Efficient adaptive antiviral cellular and humoral immune responses require previous recognition of viral antigenic peptides bound to human leukocyte antigen (HLA) class I and II molecules, which are exposed on the surface of infected and antigen presenting cells, respectively. The HLA-restricted immune response to Chikungunya virus (CHIKV), a mosquito-borne Alphavirus of the Togaviridae family responsible for severe chronic polyarthralgia and polyarthritis, is largely unknown.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, a high-throughput mass spectrometry analysis of complex HLA-bound peptide pools isolated from large amounts of human cells infected with a vaccinia virus (VACV) recombinant expressing CHIKV structural proteins was carried out. Twelve viral ligands from the CHIKV polyprotein naturally presented by different HLA-A, -B, and -C class I, and HLA-DR and -DP class II molecules were identified.

CONCLUSIONS/SIGNIFICANCE

The immunoprevalence of the HLA class II but not the HLA class I-restricted cellular immune response against the CHIKV structural polyprotein was greater than that against the VACV vector itself. In addition, most of the CHIKV HLA class I and II ligands detected by mass spectrometry are not conserved compared to its closely related O'nyong-nyong virus. These findings have clear implications for analysis of both cytotoxic and helper immune responses against CHIKV as well as for the future studies focused in the exacerbated T helper response linked to chronic musculoskeletal disorders in CHIKV patients.

NYVAC vector modified by C7L viral gene insertion improves T cell immune responses and effectiveness against leishmaniasis

The NYVAC poxvirus vector is used as vaccine candidate for HIV and other diseases, although there is only limited experimental information on its immunogenicity and effectiveness for use against human pathogens. Here we defined the selective advantage of NYVAC vectors in a mouse model by comparing the immune responses and protection induced by vectors that express the LACK (Leishmania-activated C-kinase antigen), alone or with insertion of the viral host range gene C7L that allows the virus to replicate in human cells. Using DNA prime/virus boost protocols, we show that replication-competent NYVAC-LACK that expresses C7L (NYVAC-LACK-C7L) induced higher-magnitude polyfunctional CD8(+) and CD4(+) primary adaptive and effector memory T cell responses (IFNγ, TNFα, IL-2, CD107a) to LACK antigen than non-replicating NYVAC-LACK. Compared to NYVAC-LACK, the NYVAC-LACK-C7L-induced CD8(+) T cell population also showed higher proliferation when stimulated with LACK antigen. After a challenge by subcutaneous Leishmania major metacyclic promastigotes, NYVAC-LACK-C7L-vaccinated mouse groups showed greater protection than the NYVAC-LACK-vaccinated group. Our results indicate that the type and potency of immune responses induced by LACK-expressing NYVAC vectors is improved by insertion of the C7L gene, and that a replication-competent vector as a vaccine renders greater protection against a human pathogen than a non-replicating vector.

Expression analysis of activated protein kinase C gene (LACK1) in antimony sensitive and resistant Leishmania tropica clinical isolates using real-time RT-PCR

BACKGROUND

Resistance to pentavalent antimonial drugs has become a serious problem in the treatment of cutaneous leishmaniasis in some endemic areas. Investigations on molecular markers involved in drug resistance are essential for monitoring of the disease. Leishmania-activated C kinase gene (LACK1) is involved in multiple central processes such as signal transduction. According to the probable role of the LACK1 gene in antimony resistance, we used real-time reverse transcription-polymerase chain reaction (PCR) to investigate the expression of this gene in clinical L. tropica strains, which were resistant or sensitive to meglumine antimoniate.

METHODS

We analyzed the expression level of LACK in 18 sensitive and 14 resistant L. tropica isolates collected from patients with anthroponotic cutaneous leishmaniasis. After cDNA synthesis, gene expression analysis was performed by quantitative real-time PCR using SYBR Green. In addition, the full length of the LACK gene from six reference strains was cloned and sequenced then deposited in the NCBI database to confirm our strains.

RESULTS

Real-time reverse transcription-PCR revealed that the average RNA expression level of LACK in isolates from unresponsive and responsive patients were 0.479 and 4.583, respectively, and expression of LACK was significantly downregulated (9.56-fold) in resistant isolates compared to sensitive ones.

CONCLUSION

Results of the present study suggest the probable role of the LACK gene in antimony resistance. Moreover, it can be considered as a potential marker for monitoring antimony resistance in clinical isolates. However, further studies are required to exploit the biological functions of it in antimony resistance.

Enhancing poxvirus vectors vaccine immunogenicity

Attenuated recombinant poxvirus vectors expressing heterologous antigens from pathogens are currently at various stages in clinical trials with the aim to establish their efficacy. This is because these vectors have shown excellent safety profiles, significant immunogenicity against foreign expressed antigens and are able to induce protective immune responses. In view of the limited efficacy triggered by some poxvirus strains used in clinical trials (i.e, ALVAC in the RV144 phase III clinical trial for HIV), and of the restrictive replication capacity of the highly attenuated vectors like MVA and NYVAC, there is a consensus that further improvements of these vectors should be pursuit. In this review we considered several strategies that are currently being implemented, as well as new approaches, to improve the immunogenicity of the poxvirus vectors. This includes heterologous prime/boost protocols, use of co-stimulatory molecules, deletion of viral immunomodulatory genes still present in the poxvirus genome, enhancing virus promoter strength, enhancing vector replication capacity, optimizing expression of foreign heterologous sequences, and the combined use of adjuvants. An optimized poxvirus vector triggering long-lasting immunity with a high protective efficacy against a selective disease should be sought.

The viral transcription group determines the HLA class I cellular immune response against human respiratory syncytial virus

The cytotoxic T-lymphocyte-mediated killing of virus-infected cells requires previous recognition of short viral antigenic peptides bound to human leukocyte antigen class I molecules that are exposed on the surface of infected cells. The cytotoxic T-lymphocyte response is critical for the clearance of human respiratory syncytial virus infection. In this study, naturally processed viral human leukocyte antigen class I ligands were identified with mass spectrometry analysis of complex human leukocyte antigen-bound peptide pools isolated from large amounts of human respiratory syncytial virus-infected cells. Acute antiviral T-cell response characterization showed that viral transcription determines both the immunoprevalence and immunodominance of the human leukocyte antigen class I response to human respiratory syncytial virus. These findings have clear implications for antiviral vaccine design.

Evaluation of protective effect of IL-22 and IL-12 on cutaneous leishmaniasis in BALB/c mice

OBJECTIVE

To investigate the protective effect of IL-22 and IL-12 on cutaneous leishmaniasisin BALB/c mice.

METHODS

The protective effect of IL-22 and IL-12 on cutaneous leishmanias in BALB/c mice was evaluated by measurement of IL-4, INF-γ, total IgG, IgG1 and IgG2a after challenge with Leishamania major. Clinical evaluations were performed by measurement of lesion diameter, and survival rate of the mice.

RESULTS

In week 27 post infection, the mortality rates for control groups were 100%. While the survival rates for the IL-12, IL-12 + IL-22, and IL-22(5 ng/g) groups were 100%. The size of lesions decreased in the presence IL-22 (5 ng/g) of mice weight, which was statistically significant in comparison with other groups (P<0.05). Mean of total IgG, IgG1 and IgG2a for IL-22 (5 ng/g) group was more than other groups. In IL-22 group (5 ng/g), INF-γ production was significantly higher than other groups and IL-4 was significantly lower than other groups.

CONCLUSIONS

The results obtained indicate the effectiveness of IL-22 and its effect on IL-12 in protection of cutaneous leishmaniasis.

Identifying vaccine targets for anti-leishmanial vaccine development

Leishmaniasis is a neglected tropical disease spread by an arthropod vector. It remains a significant health problem with an incidence of 0.2–0.4 million visceral leishmaniasis and 0.7–1.2 million cutaneous leishmaniasis cases each year. There are limitations associated with the current therapeutic regimens for leishmaniasis and the fact that after recovery from infection the host becomes immune to subsequent infection therefore, these factors force the feasibility of a vaccine for leishmaniasis. Publication of the genome sequence of Leishmania has paved a new way to understand the pathogenesis and host immunological status therefore providing a deep insight in the field of vaccine research. This review is an effort to study the antigenic targets in Leishmania to develop an anti-leishmanial vaccine.

Novel mucosal DNA-MVA HIV vaccination in which DNA-IL-12 plus cholera toxin B subunit (CTB) cooperates to enhance cellular systemic and mucosal genital tract immunity

Induction of local antiviral immune responses at the mucosal portal surfaces where HIV-1 and other viral pathogens are usually first encountered remains a primary goal for most vaccines against mucosally acquired viral infections. Exploring mucosal immunization regimes in order to find optimal vector combinations and also appropriate mucosal adjuvants in the HIV vaccine development is decisive. In this study we analyzed the interaction of DNA-IL-12 and cholera toxin B subunit (CTB) after their mucosal administration in DNA prime/MVA boost intranasal regimes, defining the cooperation of both adjuvants to enhance immune responses against the HIV-1 Env antigen. Our results demonstrated that nasal mucosal DNA/MVA immunization schemes can be effectively improved by the co-delivery of DNA-IL-12 plus CTB inducing elevated HIV-specific CD8 responses in spleen and more importantly in genital tract and genito-rectal draining lymph nodes. Remarkably, these CTL responses were of superior quality showing higher avidity, polyfunctionality and a broader cytokine profile. After IL-12+CTB co-delivery, the cellular responses induced showed an enhanced breadth recognizing with higher efficiency Env peptides from different subtypes. Even more, an in vivo CTL cytolytic assay demonstrated the higher specific CD8 T-cell performance after the IL-12+CTB immunization showing in an indirect manner its potential protective capacity. Improvements observed were maintained during the memory phase where we found higher proportions of specific central memory and T memory stem-like cells T-cell subpopulations. Together, our data show that DNA-IL-12 plus CTB can be effectively employed acting as mucosal adjuvants during DNA prime/MVA boost intranasal vaccinations, enhancing magnitude and quality of HIV-specific systemic and mucosal immune responses.

A comprehensive analysis of LACK (Leishmania homologue of receptors for activated C kinase) in the context of Visceral Leishmaniasis

The Leishmania homologue of activated C kinase (LACK) a known T cell epitope from soluble Leishmania antigens (SLA) that confers protection against Leishmania challenge. This antigen has been found to be highly conserved among Leishmania strains. LACK has been shown to be protective against L. donovani challenge. A comprehensive analysis of several LACK sequences was completed. The analysis shows a high level of conservation, lower variability and higher antigenicity in specific portions of the LACK protein. This information provides insights for the potential consideration of LACK as a putative candidate in the context of visceral Leishmaniasis vaccine target.

Effect of IL-22 on DNA vaccine encoding LACK gene of Leishmania major in BALB/c mice

In the present study, the effect of IL-22 together with the plasmid encoding LACK (Leishmania homolog of receptors for activated C-kinase) gene of Leishmania major on the trend of leishmaniasis in BALB/c mice was evaluated. Evaluation of the cellular and humoral immunity was performed by measurement of IL-4 and IFN-γ, culture of splenocytes and MTT assay, and measurement of total IgG, IgG1, and IgG2a in the control and immunized groups. Clinical evaluations were also carried out by measurement of the lesion size, survival rate, and body weight of mice. Comparison of the mean size of lesions in the LACK and LACK+IL-22 groups demonstrated that the mean size of lesions of the two groups was significantly different from week four (p<0.05). The survival rate at day 170 after challenge for the PBS, pcDNA3 (empty plasmid), pcLACK (pcDNA3 containing LACK gene), and pcLACK+IL-22 groups were 20%, 40%, 60%, and 80%, respectively. According to the results of IFN-γ, IL-4, total IgG, IgG1, and IgG2a measurement and the MTT assay, IL-22 obviously caused an increase in IFN-γ production and a decrease in IL-4 production before and after the challenge (p<0.05). The results showed the effectiveness of IL-22 in DNA vaccine. It showed that IL-22 brought about Th1 cytokine responses and high survival rate of mice.

Vaccination with L. infantum chagasi nucleosomal histones confers protection against new world cutaneous leishmaniasis caused by Leishmania braziliensis

Background

Nucleosomal histones are intracellular proteins that are highly conserved among Leishmania species. After parasite destruction or spontaneous lysis, exposure to these proteins elicits a strong host immune response. In the present study, we analyzed the protective capability of Leishmania infantum chagasi nucleosomal histones against L. braziliensis infection using different immunization strategies.

Methodology/Principal Findings

BALB/c mice were immunized with either a plasmid DNA cocktail (DNA) containing four Leishmania nucleosomal histones or with the DNA cocktail followed by the corresponding recombinant proteins plus CpG (DNA/Protein). Mice were later challenged with L. braziliensis, in the presence of sand fly saliva. Lesion development, parasite load and the cellular immune response were analyzed five weeks after challenge. Immunization with either DNA alone or with DNA/Protein was able to inhibit lesion development. This finding was highlighted by the absence of infected macrophages in tissue sections. Further, parasite load at the infection site and in the draining lymph nodes was also significantly lower in vaccinated animals. This outcome was associated with increased expression of IFN-γ and down regulation of IL-4 at the infection site.

Conclusion

The data presented here demonstrate the potential use of L. infantum chagasi nucleosomal histones as targets for the development of vaccines against infection with L. braziliensis, as shown by the significant inhibition of disease development following a live challenge.

DNA Vaccines against Protozoan Parasites: Advances and Challenges

Over the past 15 years, DNA vaccines have gone from a scientific curiosity to one of the most dynamic research field and may offer new alternatives for the control of parasitic diseases such as leishmaniasis and Chagas disease. We review here some of the advances and challenges for the development of DNA vaccines against these diseases. Many studies have validated the concept of using DNA vaccines for both protection and therapy against these protozoan parasites in a variety of mouse models. The challenge now is to translate what has been achieved in these models into veterinary or human vaccines of comparable efficacy. Also, genome-mining and new antigen discovery strategies may provide new tools for a more rational search of novel vaccine candidates.

Oral immunization using live Lactococcus lactis co-expressing LACK and IL-12 protects BALB/c mice against Leishmania major infection

Leishmaniasis is a parasitic disease affecting over 12 million individuals worldwide. Current treatments are laborious, expensive, cause severe side effects, and emerging drug resistance has been reported. While vaccination is the most cost-effective means to control infectious diseases there is no human vaccine currently available against Leishmania infections. Lactococcus lactis is a non-pathogenic, non-colonizing Gram-positive lactic acid bacterium commonly used in the dairy industry. Recently, L. lactis was used for the expression and delivery of biologically active molecules, such as antigens and cytokines, in mice and humans. In this study, we report the generation of L. lactis(alr-) strains solely expressing the protective Leishmania antigen, LACK, in the cytoplasm, secreted or anchored to the bacterial cell wall or co-expressing mouse IL-12. We show that oral immunization using live L. lactis, secreting both LACK and IL-12 was the only regimen that partially protected BALB/c mice against subsequent Leishmania major challenge. This highlights the importance of temporal and physical proximity of the delivered antigen and adjuvant for optimal immune priming by oral immunization since co-administration of L. lactis strains independently expressing secLACK and secIL-12 did not induce protective immunity. Protected animals displayed a delay in footpad swelling, which correlated with a significant reduction of parasite burden. Immunization with the L. lactis strain secreting both LACK and IL-12 induced an antigen-specific mucosal immune response and a LACK-specific T(H)1 immune response in splenocytes and mesenteric lymph node cells. Further, protection in immunized animals correlated with a strong Leishmania-specific T(H)1 immune response post-challenge, detectable in splenocytes and lymph node cells draining the site of infection. This report demonstrates the use of L. lactis as an oral live vaccine against L. major infection in susceptible BALB/c mice. The vaccine strains generated in this study provide the basis for the development of an inexpensive and safe oral live vaccine against the human parasite Leishmania.

Sorzano CO, Esteban M. High quality long-term CD4+ and CD8+ effector memory populations stimulated by DNA-LACK/MVA-LACK regimen in Leishmania major BALB/c model of infection

Heterologous vaccination based on priming with a plasmid DNA vector and boosting with an attenuated vaccinia virus MVA recombinant, with both vectors expressing the Leishmania infantum LACK antigen (DNA-LACK and MVA-LACK), has shown efficacy conferring protection in murine and canine models against cutaneus and visceral leishmaniasis, but the immune parameters of protection remain ill defined. Here we performed by flow cytometry an in depth analysis of the T cell populations induced in BALB/c mice during the vaccination protocol DNA-LACK/MVA-LACK, as well as after challenge with L. major parasites. In the adaptive response, there is a polyfunctional CD4⁺ and CD8⁺ T cell activation against LACK antigen. At the memory phase the heterologous vaccination induces high quality LACK-specific long-term CD4⁺ and CD8⁺ effector memory cells. After parasite challenge, there is a moderate boosting of LACK-specific CD4⁺ and CD8⁺ T cells. Anti-vector responses were largely CD8⁺-mediated. The immune parameters induced against LACK and triggered by the combined vaccination DNA/MVA protocol, like polyfunctionality of CD4⁺ and CD8⁺ T cells with an effector phenotype, could be relevant in protection against leishmaniasis.

IL-12 and GM-CSF in DNA/MVA immunizations against HIV-1 CRF12_BF Nef induced T-cell responses with an enhanced magnitude, breadth and quality

In Argentina, the HIV epidemic is characterized by the co-circulation of subtype B and BF recombinant viral variants. Nef is an HIV protein highly variable among subtypes, making it a good tool to study the impact of HIV variability in the vaccine design setting. We have previously reported a specific cellular response against NefBF with low cross-reactivity to NefB in mice. The aim of this work was to analyze whether the co-administration of IL-12 and GM-CSF, using DNA and MVA vaccine vectors, could improve the final cellular response induced. Mice received three DNA priming doses of a plasmid that express NefBF plus DNAs expressing IL-12 and/or GM-CSF. Afterwards, all the groups were boosted with a MVAnefBF dose. The highest increase in the magnitude of the NefBF response, compared to that induced in the control was found in the IL-12 group. Importantly, a response with higher breadth was detected in groups which received IL-12 or GM-CSF, evidenced as an increased frequency of recognition of homologous (BF) and heterologous (B) Nef peptides, as well as a higher number of other Nef peptide pools representing different viral subtypes. However, these improvements were lost when both DNA cytokines were simultaneously administered, as the response was focused against the immunodominant peptide with a detrimental response towards subdominant epitopes. The pattern of cytokines secreted and the specific-T-cell proliferative capacity were improved in IL-12 and IL-12+GM-CSF groups. Importantly IL-12 generated a significant higher T-cell avidity against a B heterologous peptide.

This study indicates that the incorporation of DNA expressing IL-12 in DNA/MVA schemes produced the best results in terms of improvements of T-cell-response key properties such as breadth, cross-reactivity and quality (avidity and pattern of cytokines secreted). These relevant results contribute to the design of strategies aimed to induce T-cell responses against HIV antigens with higher quality.

Towards development of novel immunization strategies against leishmaniasis using PLGA nanoparticles loaded with kinetoplastid membrane protein-11

BACKGROUND

Vaccine development has been a priority in the fight against leishmaniases, which are vector-borne diseases caused by Leishmania protozoa. Among the different immunization strategies employed to date is inoculation of plasmid DNA coding for parasite antigens, which has a demonstrated ability to induce humoral and cellular immune responses. In this sense, inoculation of plasmid DNA encoding Leishmania kinetoplasmid membrane protein-11 (KMP-11) was able to confer protection against visceral leishmaniasis. However, recently the use of antigen delivery systems such as poly(lactic-co-glycolic acid) (PLGA) nanoparticles has also proven effective for eliciting protective immune responses.

METHODS

In the present work, we tested two immunization strategies with the goal of obtaining protection, in terms of lesion development and parasite load, against cutaneous leishmaniasis caused by L. braziliensis. One strategy involved immunization with plasmid DNA encoding L. infantum chagasi KMP-11. Alternatively, mice were primed with PLGA nanoparticles loaded with the recombinant plasmid DNA and boosted using PLGA nanoparticles loaded with recombinant KMP-11.

RESULTS

Both immunization strategies elicited detectable cellular immune responses with the presence of both proinflammatory and anti-inflammatory cytokines; mice receiving the recombinant PLGA nanoparticle formulations also demonstrated anti-KMP-11 IgG1 and IgG2a. Mice were then challenged with L. braziliensis, in the presence of sand fly saliva. Lesion development was not inhibited following either immunization strategy. However, immunization with PLGA nanoparticles resulted in a more prominent reduction in parasite load at the infection site when compared with immunization using plasmid DNA alone. This effect was associated with a local increase in interferon-gamma and in tumor necrosis factor-alpha. Both immunization strategies also resulted in a lower parasite load in the draining lymph nodes, albeit not significantly.

CONCLUSION

Our results encourage the pursuit of immunization strategies employing nanobased delivery systems for vaccine development against cutaneous leishmaniasis caused by L. braziliensis infection.

Immunization against Leishmania major infection using LACK- and IL-12-expressing Lactococcus lactis induces delay in footpad swelling

BACKGROUND

Leishmania is a mammalian parasite affecting over 12 million individuals worldwide. Current treatments are expensive, cause severe side effects, and emerging drug resistance has been reported. Vaccination is the most cost-effective means to control infectious disease but currently there is no vaccine available against Leishmaniasis. Lactococcus lactis is a non-pathogenic, non-colonizing Gram-positive lactic acid bacterium commonly used in the dairy industry. Recently, L. lactis was used to express biologically active molecules including vaccine antigens and cytokines.

METHODOLOGY/PRINCIPAL FINDINGS

We report the generation of L. lactis strains expressing the protective Leishmania antigen, LACK, in the cytoplasm, secreted or anchored to the bacterial cell wall. L. lactis was also engineered to secrete biologically active single chain mouse IL-12. Subcutaneous immunization with live L. lactis expressing LACK anchored to the cell wall and L. lactis secreting IL-12 significantly delayed footpad swelling in Leishmania major infected BALB/c mice. The delay in footpad swelling correlated with a significant reduction of parasite burden in immunized animals compared to control groups. Immunization with these two L. lactis strains induced antigen-specific multifunctional T(H)1 CD4(+) and CD8(+) T cells and a systemic LACK-specific T(H)1 immune response. Further, protection in immunized animals correlated with a Leishmania-specific T(H)1 immune response post-challenge. L. lactis secreting mouse IL-12 was essential for directing immune responses to LACK towards a protective T(H)1 response.

CONCLUSIONS/SIGNIFICANCE

This report demonstrates the use of L. lactis as a live vaccine against L. major infection in BALB/c mice. The strains generated in this study provide the basis for the development of an inexpensive and safe vaccine against the human parasite Leishmania.

Leishmaniasis

Leishmaniasis is a disease that ranges in severity from skin lesions to serious disfigurement and fatal systemic infection. WHO has classified the disease as emerging and uncontrolled and estimates that the infection results in two million new cases a year. There are 12 million people currently infected worldwide, and leishmaniasis threatens 350 million people in 88 countries. Vaccination remains the best hope for control of all forms of the disease, and the development of a safe, effective and affordable antileishmanial vaccine is a critical global public-health priority. However, to date, no such vaccine is available despite substantial efforts by many laboratories. Main obstacle in vaccine design is the transition from the laboratory to the field and extrapolation of data from animal models to humans. This review discusses recent findings in the antileishmania vaccine field and current difficulties hampering vaccine implementation.

The utility of rhesus monkey (Macaca mulatta) and other non-human primate models for preclinical testing of Leishmania candidate vaccines

Leishmaniasis causes significant morbidity and mortality, constituting an important global health problem for which there are few effective drugs. Given the urgent need to identify a safe and effective Leishmania vaccine to help prevent the two million new cases of human leishmaniasis worldwide each year, all reasonable efforts to achieve this goal should be made. This includes the use of animal models that are as close to leishmanial infection in humans as is practical and feasible. Old world monkey species (macaques, baboons, mandrills etc.) have the closest evolutionary relatedness to humans among the approachable animal models. The Asian rhesus macaques (Macaca mulatta) are quite susceptible to leishmanial infection, develop a human-like disease, exhibit antibodies to Leishmania and parasite-specific T-cell mediated immune responses both in vivo and in vitro, and can be protected effectively by vaccination. Results from macaque vaccine studies could also prove useful in guiding the design of human vaccine trials. This review summarizes our current knowledge on this topic and proposes potential approaches that may result in the more effective use of the macaque model to maximize its potential to help the development of an effective vaccine for human leishmaniasis.

Recent developments in leishmaniasis vaccine delivery systems

BACKGROUND

The observation that recovery from infection with Leishmania confers immunity to reinfection suggests that control of leishmaniasis by vaccination may be possible. New generation vaccines, particularly those based on recombinant proteins and DNA, are found to be less immunogenic.

OBJECTIVE

There is an urgent need for the development of new and improved vaccine adjuvants.

METHODS

Based on their principal mechanisms of action, adjuvants can be broadly separated into two classes: immunostimulatory adjuvants and vaccine delivery systems. Vaccine delivery systems can carry both antigen and adjuvant for effective delivery to the antigen-presenting cells (APCs). In this article, we review the adjuvants, the delivery systems and their combinations used in the search of an effective vaccine against leishmaniasis.

CONCLUSION

Based on current knowledge, cationic liposomes appear to have better prospects as effective delivery systems for developing a vaccine for leishmaniasis.

Cutaneous leishmaniasis: progress towards a vaccine

Leishmaniases are vector-borne diseases due to the protozoan parasite Leishmania . Since no prevention method is available and as current therapy is costly, often poorly tolerated and not always efficacious, the development of alternative therapies, including vaccines, constitutes the priority in the fight of Leishmania infection. This review focuses on recent advances in the development of vaccines against leishmaniasis, with emphasis on the cutaneous form. Indeed, the fact that recovery from leishmaniasis is associated with immunity against new infection provides a rational basis for the development of vaccination strategy against infection with Leishmania . Evidence from animal studies demonstrate that protection can be achieved following infection with live-attenuated Leishmania as well as through immunization with purified proteins or DNA vaccines. In addition, recent results have shown that immunization against the saliva of the insect vector could have synergistic effects with conventional vaccination. Finally, vaccination using dendritic cells was recently demonstrated as a possible tool for Leishmania vaccination.

cDNA expression library screening and identification of two novel antigens: ubiquitin and receptor for activated C kinase (RACK) homologue, of the fish parasite Trypanosoma carassii

Trypanosoma carassii is a kinetoplastid parasite infecting cyprinid fish with a high prevalence in nature. Antibodies have been shown to play a protective role in the immune response against this parasite in common carp, Cyprinus carpio. To identify immunogenic and putative protective T. carassii antigens we constructed a lambdaTriplEx2 expression library of the parasite and screened this with pooled carp immune serum collected 6 weeks post-infection. Screening of the library not only revealed ribosomal proteins but identified ubiquitin and a homologue of the receptor for activated C kinase (RACK) as immunogenic proteins. Equivalents of all these proteins have been identified as immunogenic in expression library screenings of other Trypanosomatida, suggesting an evolutionary conservation of their immunogenicity. The possibility that ubiquitin and/or the homologue of RACK could represent protective antigens and be targets for the design of novel therapies is discussed.

Intradermal NKT cell activation during DNA priming in heterologous prime-boost vaccination enhances T cell responses and protection against Leishmania

Heterologous prime-boost vaccination employing DNA-vaccinia virus (VACV) modality using the Leishmania homologue of receptors for activated C kinase (LACK) (p36) antigen has been shown to elicit protective immunity against both murine cutaneous and visceral leishmaniasis. However, DNA priming is known to have limited efficacy; therefore in the current study the effect of NKT cell activation using alpha-galactosyl-ceramide (alphaGalCer) during intradermal DNAp36 priming was examined. Vaccinated mice receiving alphaGalCer + DNAp36 followed by a boost with VVp36 appeared to be resolving their lesions and had at ten- to 20-fold higher reductions in parasite burdens. NKT cell activation during alphaGalCer + DNAp36 priming resulted in higher numbers of antigen-reactive effector CD4(+) and CD8(+) T cells producing granzyme and IFN-gamma, with lower levels of IL-10. Although immunodepletion studies indicate that both CD4 and CD8 T cells provide protection in the vaccinated mice, the contribution of CD4(+) T cells was significantly increased in mice primed with DNAp36 together with alphaGalCer. Notably 5 months after boosting, mice vaccinated with DNAp36 + alphaGalCer continued to show sustained and heightened T cell immune responses. Thus, heterologous prime-boost vaccination using alphaGalCer during priming is highly protective against murine cutaneous leishmaniasis, resulting in the heightened activation and development of CD4 and CD8 T cells (effector and memory T cells).

Monocyte cytokine and costimulatory molecule expression in patients infected with Leishmania mexicana

Leishmania mexicana causes localized and diffuse cutaneous leishmaniasis. Patients with localized cutaneous leishmaniasis (LCL) develop a benign disease, whereas patients with diffuse cutaneous leishmaniasis (DCL) suffer from a progressive disease associated with anergy of the cellular response towards Leishmania antigens. We evaluated the production of the interleukins (IL) IL-12, IL-15, IL-18 and tumour necrosis factor-alpha (TNF-alpha) and the expression of the costimulatory molecules CD40, B7-1 and B7-2 in monocytes from LCL and DCL patients, stimulated in vitro with Leishmania mexicana lipophosphoglycan (LPG) for 18 h. LCL monocytes significantly increased TNF-alpha, IL-15 and IL-18 production, and this increase was associated with reduced amounts of IL-12. DCL monocytes produced no IL-15 or IL-18 and showed a decreasing tendency of TNF-alpha and IL-12 production as the severity of the disease increased. No difference was observed in the expression of CD40 and B7-1 between both groups of patients, yet B7-2 expression was significantly augmented in DCL patients. It remains to be established if this elevated B7-2 expression in DCL patients is cause or consequence of the Th2-type immune response that characterizes these patients. These data suggest that the diminished ability of the monocytes from DCL patients to produce cell-activating innate proinflammatory cytokines when stimulated with LPG is a possible cause for disease progression.

Mucosal HIV-1 Pox Virus Prime-Boost Immunization Induces High-Avidity CD8+ T Cells with Regime-Dependent Cytokine/Granzyme B Profiles

The quality of virus-specific CD8(+) CTL immune responses generated by mucosal and systemic poxvirus prime-boost vaccines were evaluated in terms of T cell avidity and single-cell analysis of effector gene expression. Intranasal (I.N.) immunization regimes generated higher avidity CTL responses specific for HIV K(d)Gag(197-205) (amino acid sequence AMQMLKETI; H-2K(d) binding) compared with i.m. immunization regime. Single-cell RT-PCR of K(d)Gag(197-205)-specific mucosal and systemic CTL revealed that the cytokine and granzyme B expression profiles were dependent on both the route and time after immunization. The I.N./i.m.-immunized group elicited elevated number of CTL-expressing granzyme B mRNA from the genitomucosal sites compared with the i.m./i.m. regime. Interestingly, CTL generated after both I.N. or i.m. immunization demonstrated expression of Th2 cytokine IL-4 mRNA that was constitutively expressed over time, although lower numbers were observed after I.N./I.N. immunization. Results suggest that after immunization, Ag-specific CTL expression of IL-4 may be an inherent property of the highly evolved poxvirus vectors. Current observations indicate that the quality of CTL immunity generated after immunization can be influenced by the inherent property of vaccine vectors and route of vaccine delivery. A greater understanding of these factors will be crucial for the development of effective vaccines in the future.

Leishmaniavaccines: progress and problems

Leishmaniaare protozoan parasites spread by a sandfly insect vector and causing a spectrum of diseases collectively known as leishmaniasis. The disease is a significant health problem in many parts of the world resulting in an estimated 12 million new cases each year. Current treatment is based on chemotherapy, which is difficult to administer, expensive and becoming ineffective due to the emergence of drug resistance. Leishmaniasis is considered one of a few parasitic diseases likely to be controllable by vaccination. The relatively uncomplicated leishmanial life cycle and the fact that recovery from infection renders the host resistant to subsequent infection indicate that a successful vaccine is feasible. Extensive evidence from studies in animal models indicates that solid protection can be achieved by immunisation with protein or DNA vaccines. However, to date no such vaccine is available despite substantial efforts by many laboratories. Advances in our understanding ofLeishmaniapathogenesis and generation of host protective immunity, together with the completedLeishmaniagenome sequence open new avenues for vaccine research. The major remaining challenges are the translation of data from animal models to human disease and the transition from the laboratory to the field. This review focuses on advances in anti-leishmania vaccine development over the recent years and examines current problems hampering vaccine development and implementation.

Virus-like particles: designing an effective AIDS vaccine

Viruses that infect eukaryotic organisms have the unique characteristic of self-assembling into particles. The mammalian immune system is highly attuned to recognizing and attacking these viral particles following infection. The use of particle-based immunogens, often delivered as live-attenuated viruses, has been an effective vaccination strategy for a variety of viruses. The development of an effective vaccine against the human immunodeficiency virus (HIV) has proven to be a challenge, since HIV infects cells of the immune system causing severe immunodeficiency resulting in the syndrome known as AIDS. In addition, the ability of the virus to adapt to immune pressure and reside in an integrated form in host cells presents hurdles for vaccinologists to overcome. A particle-based vaccine strategy has promise for eliciting high titer, long-lived, immune responses to a diverse number of viral epitopes against different HIV antigens. Live-attenuated viruses are effective at generating both cellular and humoral immune responses. However, while these vaccines stimulate immunity, challenged animals rarely clear the viral infection and the degree of attenuation directly correlates with protection from disease. Further, a live-attenuated vaccine has the potential to revert to a pathogenic form. Alternatively, virus-like particles (VLPs) mimic the viral particle without causing an immunodeficiency disease. VLPs are self-assembling, non-replicating, non-pathogenic particles that are similar in size and conformation to intact virions. A variety of VLPs for lentiviruses are currently in preclinical and clinical trials. This review focuses on our current status of VLP-based AIDS vaccines, regarding issues of purification and immune design for animal and clinical trials.

Elicitation of T cell responses to histologically unrelated tumors by immunization with the novel cancer-testis antigen, brother of the regulator of imprinted sites

Brother of the regulator of imprinted sites (BORIS) was previously described as a transcription factor for epigenetic reprogramming the expression of which is strictly confined to germ cells of adult testes but is aberrantly activated in the vast majority of neoplastic cells. Considering the critical role of BORIS in cancerogenesis and the fact that its expression pattern may preclude thymic tolerance, we generated DNA- and protein-based mouse BORIS antitumor vaccines using a non-DNA-binding version of the BORIS molecule. Clinical use of BORIS as a vaccine Ag would require that certain safety concerns be met. Specifically, administration of the functional BORIS protein would hypothetically pose a risk of BORIS accelerating the progression of cancer. To alleviate such safety concerns, we have developed vaccines based on the BORIS molecule lacking the DNA-binding zinc fingers domain. To enhance anti-BORIS cellular immune responses, we used a standard molecular adjuvant approach. It consisted of plasmids encoding murine IL-12 and IL-18 for a DNA-based vaccine and conventional Th1 type adjuvant, Quil A, for a protein-based vaccine. Both DNA- and protein-based vaccines induced Ag-specific CD4(+) T cell proliferation with Th1 and Th2 cytokine profiles, respectively. Protein-based, but not DNA-based, BORIS vaccine induced a significant level of Ab production in immunized animals. Importantly, potent anticancer CD8(+)-cytotoxic lymphocytes were generated after immunization with the DNA-based, but not protein-based, BORIS vaccine. These cytolytic responses were observed across a wide range of different mouse cancers including mammary adenocarcinoma, glioma, leukemia, and mastocytoma.

Immunological responses and the pattern of disease in mice infected with transfected Leishmania major constitutively expressing active IL-1alpha

Immunity against leishmaniasis is mediated mainly by CD4+ T lymphocytes, which function by secreting cytokines, which in turn activate various effector mechanisms. Interleukin 1 (IL-1) represents one of the most pleiotropic proinflammatory cytokines and is required for normal regulation of Th1/Th2 responses. The aim of this study was to induce the expression of the inflammatory cytokine IL-1alpha by Leishmania parasites and to determine its effect on the parasite development. Leishmania constitutively producing IL-1alpha was engineered, using the pX63Hyg-IL-1alpha vector. IL-1alpha was produced by both promastigotes and amastigotes, and remained unchanged after transformation and development in mice. The protection against the disease achieved in BALB/c mice by the transfected parasites was superior to that obtained with the wild type. One month after infection a nodule was demonstrated in 22% and 60% of the mice inoculated with transfected parasites and the wild type, respectively. This tendency continued for an additional 2.5 months, after which the rate of infection increased to 90% and 100% in these two groups, respectively. The present study suggests that, during initial infection, the pathway of IL-1alpha production and its accessibility to the immunological cells might be important in the outcome of leishmanial infection.

MVA-LACK as a safe and efficient vector for vaccination against leishmaniasis

An optimal vaccine against leishmaniasis should elicit parasite specific CD4+ and cytotoxic CD8+ T cells. In this investigation, we described a prime/boost immunization approach based on DNA and on poxvirus vectors (Western Reserve, WR, and the highly attenuated modified vaccinia virus Ankara, MVA), both expressing the LACK antigen of Leishmania infantum, that triggers different levels of specific CD8+ T cell responses and protection (reduction in lesion size and parasitemia) against L. major infection in mice. A prime/boost vaccination with DNA-LACK/MVA-LACK elicits higher CD8+ T cell responses than a similar protocol with the replication competent VV-LACK. Both CD4+ and CD8+ T cells were induced by DNA-LACK/MVA-LACK immunization. The levels of IFN-gamma and TNF-alpha secreting CD8+ T cells were higher in splenocytes from DNA-LACK/MVA-LACK than in DNA-LACK/VV-LACK immunized animals. Moreover, protection against L. major was significantly higher in DNA-LACK/MVA-LACK than in DNA-LACK/VV-LACK immunized animals when boosted with the same virus dose, and correlated with high levels of IFN-gamma and TNF-alpha secreting CD8+ T cells. In DNA-LACK/MVA-LACK vaccinated animals, the extent of lesion size reduction ranged from 65 to 92% and this protection was maintained for at least 17 weeks after challenge with the parasite. These findings demonstrate that in heterologous prime/boost immunization approaches, the protocol DNA-LACK/MVA-LACK is superior to DNA-LACK/VV-LACK in triggering specific CD8+ T cell immune responses and in conferring protection against cutaneous leishmaniasis. Thus, MVA-LACK is a safe and efficient vector for vaccination against leishmaniasis.

Heterologous prime-boost vaccination with the LACK antigen protects against murine visceral leishmaniasis

This study reports the efficacy of a heterologous prime-boost vaccination using DNA and vaccinia viruses (Western Reserve [WR] virus and modified [attenuated] vaccinia virus Ankara [MVA]) expressing the LACK antigen (Leishmania homologue of receptors for activated C kinase) and an intradermal murine infection model employing Leishmania infantum. At 1 month postinfection, vaccinated mice showed high levels of protection in the draining lymph node (240-fold reduction in parasite burden) coupled with significant levels of gamma interferon (20 to 200 ng/ml) and tumor necrosis factor alpha/lymphotoxin (8 to 134 pg/ml). Significant but lower levels of protection (6- to 30-fold) were observed in the spleen and liver. Comparable levels of protection were found for mice boosted with either LACK-WR or LACK-MVA, supporting the use of an attenuated vaccinia virus-based vaccine against human visceral leishmaniasis.

Fasciola hepatica: humoral and cytokine responses to a member of the saposin-like protein family following delivery as a DNA vaccine in mice

The humoral and cellular responses to DNA vaccination of BALB/c mice with a novel antigen from the Fasciola hepatica saposin-like protein family (FhSAP-2) have been investigated. Two constructs were produced containing the FhSAP-2 DNA sequence, one intended for extracellular secretion of FhSAP-2 protein, and one expressing FhSAP-2 in the cytoplasm of a transfected cell. The constructs were tested in HEK 293T cells, with the secretory construct producing less detectable FhSAP-2 relative to cytoplasmic construct when observed by fluorescence. The size of expressed protein was confirmed by Western blot of cell lysate, but FhSAP-2 was undetectable in cell supernatants. Both, secretory and cytoplasmic constructs as well as FhSAP-2 recombinant protein were tested in mice. The antibody response elicited in mice vaccinated with the rFhSAP-2 induced high levels of IgG(1), IgG(2), and IgE as well as high levels of IL-10 and IFNgamma indicating a mixed Th1/Th2 response. Vaccination of mice intramuscularly with the cytoplasmic FhSAP-2 construct resulted in a dominant IgG(2a) isotype antibody as well as a dominant IFNgamma cytokine, with significant IgE, IgG(1), and IL-10 responses also present, suggesting a mixed Th1/Th2 profile. Isotype and cytokine profiles elicited by the FhSAP-2 secretory construct were similar to those obtained with the cytoplasmic construct but at levels that were significantly lower. The results demonstrate that FhSAP-2 can be delivered as a DNA vaccine construct and induces a stronger Th1 response than the recombinant protein alone. This could result in an improvement in the immunoprophylactic potential of this candidate vaccine against F. hepatica.

The role of cytokine DNAs as vaccine adjuvants for optimizing cellular immune responses

Cytokines represent a diverse group of immunologic effector and regulatory proteins that are critical components of the host response to invading pathogens. They have also been utilized as adjuvants to enhance immune responses to vaccines. In particular, plasmid cytokines have been studied extensively as candidate adjuvants for DNA vaccines in preclinical models and are now entering early-phase clinical trials. Here, we review recent advances in our understanding of cytokine biology, T-lymphocyte differentiation, and potential applications of plasmid cytokines in the rational design of improved vaccines.

DNA vaccines: designing strategies against parasitic infections

The complexity of parasitic infections requires novel approaches to vaccine design. The versatility of DNA vaccination provides new perspectives. This review discusses the use of prime-boost immunizations, genetic adjuvants, multivalent vaccines and codon optimization for optimal DNA vaccine design against parasites.

Vaccination with Plasmids Encoding the Leishmania Major GP63 Glycoprotein and CD40L Results in a Partial Suppression of the Inflammatory Reaction after Experimental Infection

The development of an effective vaccine against leishmaniasis is the aim of an intensive research effort, to bring relief to thousands of people worldwide. DNA vaccination is a promising approach in this direction, since it is able to generate a strong cellular immune response. We tested whether the co-administration of a plasmid encoding a truncated gp63 gene, that allows extracellular secretion of the encoded protein, and a plasmid encoding for CD40L could induce a protective response in genetically susceptible BALB/c mice and reduce the local inflammatory swelling after infection with Leishmania major. We document that vaccination with the combination of plasmids for gp63 and CD40L reduced inflammatory swelling, while vaccination with the truncated gp63-encoding plasmid resulted in an exacerbation of the local inflammatory reaction. These preliminary data indicate that the CD40L expression plasmid is consequently an efficient adjuvant for the induction of protective responses in the context of a DNA vaccination against leishmaniasis.

Leishmania major LACK antigen is required for efficient vertebrate parasitization

The Leishmania major LACK antigen is a key target of the immune response in susceptible BALB/c mice and remains a viable vaccine candidate for human leishmaniasis. We describe the genomic organization of the four lack genes in the L. major diploid genome together with results of selected lack gene targeting. Parasites containing a single lack gene in either the upstream or downstream locus grew comparably to wild-type promastigotes in vitro, but failed to parasitize BALB/c mice efficiently, even in a T cell-deficient environment. The replication of single copy lack mutants as amastigotes was attenuated in macrophages in vitro, and parasites failed to increase in numbers in immunodeficient mice, despite their persistence over months. Complementation with an additional lack copy was sufficient to induce robust lesion development, which also occurred using parasites with two lack genes. Conversely, attempts to generate lack-null parasites failed, suggesting that LACK is required for parasite viability. These data suggest that LACK is critical for effective mammalian parasitization and thus represents a potential drug target for leishmaniasis.

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.