Optimized subunit vaccine protects against experimental leishmaniasis

Leishmania vaccine development: A comprehensive review

Infectious diseases like leishmaniasis, malaria, HIV, tuberculosis, leprosy and filariasis are responsible for an immense burden on public health systems. Among these, leishmaniasis is under the category I diseases as it is selected by WHO (World Health Organization) on the ground of diversity and complexity. High cost, resistance and toxic effects of Leishmania traditional drugs entail identification and development of therapeutic alternative. Since the natural infection elicits robust immunity, consistence efforts are going on to develop a successful vaccine. Clinical trials have been conducted on vaccines like Leish-F1, F2, and F3 formulated using specific Leishmania antigen epitopes. Current strategies utilize individual or combined antigens from the parasite or its insect vector's salivary gland extract, with or without adjuvant formulation for enhanced efficacy. Promising animal data supports multiple vaccine candidates (Lmcen-/-, LmexCen-/-), with some already in or heading for clinical trials. The crucial challenge in Leishmania vaccine development is to translate the research knowledge into affordable and accessible control tools that refines the outcome for those who are susceptible to infection. This review focuses on recent findings in Leishmania vaccines and highlights difficulties facing vaccine development and implementation.

Development of a multi-epitope vaccine candidate for leishmanial parasites applying immunoinformatics and in vitro approaches

Leishmaniasis is a neglected tropical disease, and its severity necessitates the development of a potent and efficient vaccine for the disease; however, no human vaccine has yet been approved for clinical use. This study aims to design and evaluate a multi-epitope vaccine against the leishmanial parasite by utilizing helper T-lymphocyte (HTL), cytotoxic T-lymphocyte (CTL), and linear B-lymphocyte (LBL) epitopes from membrane-bound acid phosphatase of Leishmania donovani (LdMAcP). The designed multi-epitope vaccine (LdMAPV) was highly antigenic, non-allergenic, and non-toxic, with suitable physicochemical properties. The three-dimensional structure of LdMAPV was modeled and validated, succeeded by molecular docking and molecular dynamics simulation (MDS) studies that confirmed the high binding affinity and stable interactions between human toll-like receptors and LdMAPV. In silico disulfide engineering provided improved stability to LdMAPV, whereas immune simulation displayed the induction of both immune responses, i.e., antibody and cell-mediated immune responses, with a rise in cytokines. Furthermore, LdMAPV sequence was codon optimized and cloned into the pET-28a vector, followed by its expression in a bacterial host. The recombinant protein was purified using affinity chromatography and subjected to determine its effect on cytotoxicity, cytokines, and nitric oxide generation by mammalian macrophages. Altogether, this report provides a multi-epitope vaccine candidate from a leishmanial protein participating in parasitic virulence that has shown its potency to be a promising vaccine candidate against leishmanial parasites.

Safety and Immunogenicity of the ID93 + GLA-SE Tuberculosis Vaccine in BCG-Vaccinated Healthy Adults: A Randomized, Double-Blind, Placebo-Controlled Phase 2 Trial

INTRODUCTION

This randomized, double-blind, placebo-controlled, phase 2a trial was conducted to evaluate the safety and immunogenicity of the ID93 + glucopyranosyl lipid adjuvant (GLA)-stable emulsion (SE) vaccine in human immunodeficiency virus (HIV)-negative, previously Bacillus Calmette-Guérin (BCG)-vaccinated, and QuantiFERON-TB-negative healthy adults in South Korea.

METHODS

Adults (n = 107) with no signs or symptoms of tuberculosis were randomly assigned to receive three intramuscular injections of 2 μg ID93 + 5 μg GLA-SE, 10 μg ID93 + 5 μg GLA-SE, or 0.9% normal saline placebo on days 0, 28, and 56. For safety assessment, data on solicited adverse events (AEs), unsolicited AEs, serious AEs (SAEs), and special interest AEs were collected. Antigen-specific antibody responses were measured using serum enzyme-linked immunosorbent assay. T-cell immune responses were measured using enzyme-linked immunospot and intracellular cytokine staining.

RESULTS

No SAEs, deaths, or AEs leading to treatment discontinuation were found. The solicited local and systemic AEs observed were consistent with those previously reported. Compared with adults administered with the placebo, those administered with three intramuscular vaccine injections exhibited significantly higher antigen-specific antibody levels and Type 1 T-helper cellular immune responses.

CONCLUSION

The ID93 + GLA-SE vaccine induced antigen-specific cellular and humoral immune responses, with an acceptable safety profile in previously healthy, BCG-vaccinated, Mycobacterium tuberculosis-uninfected adult healthcare workers.

TRIAL REGISTRATION

This clinical trial was retrospectively registered on 16 January 2019 at Clinicaltrials.gov (NCT03806686).

Anti-leishmanial therapy: Caught between drugs and immune targets

Leishmaniasis is an enigmatic disease that has very restricted options for chemotherapy and none for prophylaxis. As a result, deriving therapeutic principles for curing the disease has been a major objective in Leishmania research for a long time. Leishmania is a protozoan parasite that lives within macrophages by subverting or switching cell signaling to the pathways that ensure its intracellular survival. Therefore, three groups of molecules aimed at blocking or eliminating the parasite, at least, in principle, include blockers of macrophage receptor- Leishmania ligand interaction, macrophage-activating small molecules, peptides and cytokines, and signaling inhibitors or activators. Macrophages also act as an antigen-presenting cell, presenting antigen to the antigen-specific T cells to induce activation and differentiation of the effector T cell subsets that either execute or suppress anti-leishmanial functions. Three groups of therapeutic principles targeting this sphere of Leishmania-macrophage interaction include antibodies that block pro-leishmanial response of T cells, ligands that activate anti-leishmanial T cells and the antigens for therapeutic vaccines. Besides these, prophylactic vaccines have been in clinical trials but none has succeeded so far. Herein, we have attempted to encompass all these principles and compose a comprehensive review to analyze the feasibility and adoptability of different therapeutics for leishmaniasis.

Leishmaniasis Vaccines: Applications of RNA Technology and Targeted Clinical Trial Designs

Leishmania parasites cause a variety of discrete clinical diseases that present in regions where their specific sand fly vectors sustain transmission. Clinical and laboratory research indicate the potential of immunization to prevent leishmaniasis and a wide array of vaccine candidates have been proposed. Unfortunately, multiple factors have precluded advancement of more than a few Leishmania targeting vaccines to clinical trial. The recent maturation of RNA vaccines into licensed products in the context of COVID-19 indicates the likelihood of broader use of the technology. Herein, we discuss the potential benefits provided by RNA technology as an approach to address the bottlenecks encountered for Leishmania vaccines. Further, we outline a variety of strategies that could be used to more efficiently evaluate Leishmania vaccine efficacy, including controlled human infection models and initial use in a therapeutic setting, that could prioritize candidates before evaluation in larger, longer and more complicated field trials.

Design of a Chimeric Multi-Epitope Vaccine (CMEV) against Both Leishmania martiniquensis and Leishmania orientalis Parasites Using Immunoinformatic Approaches

Leishmaniasis is a parasitic disease caused by protozoan flagellates of the genus Leishmania. Recently, Leishmania martiniquensis and Leishmania orientalis, emerging species of Leishmania, were isolated from patients in Thailand. Development of the vaccine is demanded; however, genetic differences between the two species make it difficult to design a vaccine that is effective for both species. In this study, we applied immuno-informatic approaches to design a chimeric multi-epitope vaccine (CMEV) against both L. martiniquensis and L. orientalis. We identified seven helper T lymphocyte (HTL) epitopes, sixteen cytotoxic T lymphocyte (CTL) epitopes, and eleven B-cell epitopes from sixteen conserved antigenic proteins found in both species. All these epitopes were joined together, and to further enhance immunogenicity, protein and peptides adjuvant were also added at the N-terminal of the molecule by using specific linkers. The candidate CMEV was subsequently analyzed from the perspectives of the antigenicity, allergenicity, and physiochemical properties. The interaction of the designed multi-epitope vaccine and immune receptor (TLR4) of the host were evaluated based on molecular dockings of the predicted 3D structures. Finally, in silico cloning was performed to construct the expression vaccine vector. Docking analysis showed that the vaccine/TLR4 complex took a stable form. Based on the predicted immunogenicity, physicochemical, and structural properties in silico, the vaccine candidate was expected to be appropriately expressed in bacterial expression systems and show the potential to induce a host immune response. This study proposes the experimental validation of the efficacy of the candidate vaccine construct against the two Leishmania.

Immunoinformatics Approach to Design a Novel Subunit Vaccine Against Visceral Leishmaniasis

Visceral leishmaniasis (VL) infection is mostly caused by Leishmania donovani and affects countries worldwide. Despite the need for a safe and effective vaccine against leishmaniasis due to the increased drug resistance, however, no vaccine has yet been licensed for clinical use. This study revolves around the immunoinformatics approach to design a multi-epitope vaccine against VL infection. In this case, the proteome of L. donovani has been investigated, and three host non-homologous and antigenic extracellular secretory proteins have been identified as potential vaccine candidates with low transmembrane helices (≤ 1). The multi-epitope subunit vaccine construct consists of T-cell (cytotoxic T-lymphocyte (CTL) and helper T-lymphocyte (HTL)) epitopes accompanied by appropriate adjuvant and linkers. A 372-amino acid vaccine construct has been established with specific characteristics, such as soluble, stable, antigenic, non-allergenic, non-toxic, and non-host homologous. Besides, the tertiary structure of the designed vaccine was modeled and validated. Also, the stability and affinity of the vaccine- TLR4 complex were confirmed by using molecular docking and molecular dynamics (MD) simulation. In addition, in silico immunization assay showed the efficiency of this candidate vaccine to stimulate an effective immune response. Furthermore, the refined vaccine was optimized and cloned in the pET28a (+) vector, and its successful expression was confirmed virtually. However, the experimental validation is required to verify the multi-epitope vaccine efficacy against VL infection.

Lymphatic filariasis and visceral leishmaniasis coinfection: A review on their epidemiology, therapeutic, and immune responses

Coinfection is less commonly observed in individuals around the world, yet it is more common than the single infection. Around 800 million people worldwide are infected with helminths as a result of various diseases. Lymphatic filariasis (LF) and visceral leishmaniasis (VL) are chronic, deadly, crippling, and debilitating neglected tropical diseases (NTDs) that are endemic in tropical and subtropical regions of the world. Due to poor hygienic conditions, poverty, and genetic predisposition, those living in endemic areas are more likely to develop both leishmaniasis and filariasis. One of the key challenges in the management of LF/VL coinfection is the development of an effective therapeutic strategy that not only treats the first episode of VL but also prevents LF. However, there is a scarcity of knowledge and data on the relationship between LF and VL coinfection. While reviewing it was apparent that only a few studies relevant to LF/VL coinfections have been reported from southeastern Spain, Sudan, and the Indian subcontinents, highlighting the need for greater research in the most affected areas. We also looked at LF and VL as a single disease and also as a coinfection. Some features of the immune response evolved in mammalian hosts against LF and VL alone or against coinfection are also discussed, including epidemiology, therapeutic regimens, and vaccines. In addition to being potentially useful in clinical research, our findings imply the need for improved diagnostic methodology and therapeutics, which could accelerate the deployment of more specific and effective diagnosis for treatments to lessen the impact of VL/LF coinfections in the population.

From infection to vaccination: reviewing the global burden, history of vaccine development, and recurring challenges in global leishmaniasis protection

INTRODUCTION

Leishmaniasis is a major public health problem and the second most lethal parasitic disease in the world due to the lack of effective treatments and vaccines. Even when not lethal, leishmaniasis significantly affects individuals and communities through life-long disabilities, psycho-sociological trauma, poverty, and gender disparity in treatment.

AREAS COVERED

This review discusses the most relevant and recent research available on Pubmed and GoogleScholar highlighting leishmaniasis' global impact, pathogenesis, treatment options, and lack of effective control strategies. An effective vaccine is necessary to prevent morbidity and mortality, lower health care costs, and reduce the economic burden of leishmaniasis for endemic low- and middle-income countries. Since there are several forms of leishmaniasis, a pan-Leishmania vaccine without geographical restrictions is needed. This review also focuses on recent advances and common challenges in developing prophylactic strategies against leishmaniasis.

EXPERT OPINION

Despite advances in pre-clinical vaccine research, approval of a human leishmaniasis vaccine still faces major challenges - including manufacturing of candidate vaccines under Good Manufacturing Practices, developing well-designed clinical trials suitable in endemic countries, and defined correlates of protection. In addition, there is a need to explore Challenge Human Infection Model to avoid large trials because of fluctuating incidence and prevalence of leishmanasis.

MPLA and AddaVax® Adjuvants Fail to Promote Intramuscular LaAg Vaccine Protectiveness against Experimental Cutaneous Leishmaniasis

There is so far no vaccine approved for human leishmaniasis, mainly because of the lack of appropriate adjuvants. This study aimed to evaluate in mice the capacity of a mixture of monophosphoryl lipid A (MPLA) and AddaVax^® adjuvants in enhancing the efficacy of a Leishvacin^®-like vaccine comprised of Leishmania amazonensis whole antigens (LaAg). For that, mice were immunized with LaAg plus MPLA/AddaVax^® by the intramuscular route (i.m.) prior to challenge with 2 × 10⁵ and 2 × 10⁶ living parasites. Immunization with LaAg alone reduced the lesion growth of the 2 × 10⁵-challenged mice only in the peak of infection, but that was not accompanied by reduced parasite load, and thus not considered protective. Mice given a 2 × 10⁶ -challenge were not protected by LaAg. The association of LaAg with MPLA/AddaVax^® was able to enhance the cutaneous hypersensitivity response compared with LaAg alone. Despite this, there was no difference in proliferative cell response to antigen ex vivo. Moreover, regardless of the parasite challenge, association of LaAg with MPL/AddaVax^® did not significantly enhance protection in comparison with LaAg alone. This work demonstrated that MPL/AddaVax^® is not effective in improving the efficacy of i.m. LaAg vaccine against cutaneous leishmaniasis.

Immunogenicity of Potential CD4+ and CD8+ T Cell Epitopes Derived From the Proteome of Leishmania braziliensis

Background: A safe and effective vaccine against human leishmaniasis still requires the identification of better antigens for immunization and adequate models to evaluate the immune response. To support vaccine development, this work tested the immunogenicity of 10 different peptides derived from the proteome of Leishmania braziliensis, which were selected by their in silico affinity to MHC complexes.

Research design and Methods: Comparative cell proliferation assays were performed by culturing, in the presence of each peptide, PBMC cells from subclinical subjects (SC), cutaneous leishmaniasis patients with active disease (AD), post-treatment (PT) individuals, and healthy controls. Culture supernatants were then used for Th1, Th2, and Th17 cytokine measurements. Cells from selected PT samples were also used to assess the expression, by T cells, of the T-bet Th1 transcription factor.

Results: A robust cell proliferation was observed for the SC group, for all the tested peptides. The levels of Th1 cytokines were peptide-dependent and had substantial variations between groups, where, for instance, IFN-γ and TNF levels were some of the highest, particularly on PT cultures, when compared to IL-2. On the other hand, Th2 cytokines displayed much less variation. IL-6 was the most abundant among all the evaluated cytokines while IL-4 and IL-10 could be found at much lower concentrations. IL-17 was also detected with variations in SC and AD groups. T-bet was up-regulated in CD4⁺ and CD8⁺ T cells from the PT group after stimulation with all peptides.

Conclusions: The peptide epitopes can differentially stimulate cells from SC, AD, and PT individuals, leading to distinct immune responses.

Allopurinol therapy provides long term clinical improvement, but additional immunotherapy is required for sustained parasite clearance, in L. infantum-infected dogs

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L. infantum-infected dogs were treated with allopurinol alone or plus Leish-F2 + SLA-SE.

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Both treatment regimen generated long term clinical improvement.

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Immunochemotherapy, but not chemotherapy alone, generated sustained parasite control.

There is little evidence that current control strategies for canine leishmaniosis (CanL), the veterinary disease caused by L. infantum infection, are having a positive impact. This is of critical importance because dogs are a primary reservoir for L. infantum and a significant source of parasite transmission to humans. Drugs intended primarily for human use are prohibited for the treatment of CanL because of concerns over the propagation of resistant parasites. Although allopurinol effectively decreases parasite burden in CanL the treatment needs to be maintained for life. We hypothesized that during the allopurinol-induced parasite reduction dogs may become capable of developing a more robust immune response that may permit more effective control of parasites. To test this, we investigated the clinical and parasitological impact of short-term treatment with allopurinol, either alone or in combination with a defined subunit vaccine, on dogs naturally infected with L. infantum. A total of 28 dogs were distributed as follows: untreated; oral allopurinol alone (20 mg/kg, once each day for 90 days); or allopurinol with immunization with the Leish-F2 antigen formulated with the Toll-like receptor (TLR) 4 agonist Second generation Lipid Adjuvant (SLA) in stable emulsion (SE; SLA-SE). Dogs that did not receive treatment had a progressive decline in their clinical condition and an increase in their infection levels, while treatment with allopurinol alone alleviated the clinical symptoms of CanL but did not generate sustained reduction in parasites. Concomitant immunization with Leish-F2 + SLA-SE, however, improved clinical condition while also providing long-term clearance of L. infantum from lymphoid tissues and systemic organs. These results have important implications for both the management of CanL and for limiting L. infantum transmission to humans.

Heterologous Immunization With Defined RNA and Subunit Vaccines Enhances T Cell Responses That Protect Against Leishmania donovani

The rapid generation of strong T cell responses is highly desirable and viral vectors can have potent CD8+ T cell-inducing activity. Immunity to leishmaniasis requires selective T cell responses, with immunization schemes that raise either CD4 or CD8 T cell responses being protective in small animal models. We have defined the leishmaniasis vaccine candidate recombinant fusion antigens, LEISH-F2 and LEISH-F3+, that when formulated in a stable emulsion with a Toll-like receptor (TLR) 4 agonist, induce protective CD4+ T cell responses in animal models as well as providing therapeutic efficacy in canine leishmaniasis and in clinical trials in leishmaniasis patients. We used the genetic sequences of these validated vaccine antigens to design RNA vaccine constructs. Immunization of mice with the RNA replicons induced potent, local innate responses that were surprisingly independent of TLR7 and activated antigen-presenting cells (APC) to prime for extremely potent antigen-specific T helper 1 type responses upon heterologous boosting with either of the subunit vaccines (recombinant antigen with second generation glucopyranosyl lipid A in stable oil-in-water emulsion; SLA-SE). Inclusion of RNA in the immunization schedule also generated MHCI-restricted T cell responses. Immunization with LEISH-F2-expressing RNA vaccine followed later by subunit vaccine afforded protection against challenge with Leishmania donovani. Together, these data indicate the utility of heterologous prime-boost immunization schemes for the induction of potent antigen-specific CD4 and CD8 T cell responses for protection against intracellular pathogens.

Immunoinformatics-aided design of a potential multi-epitope peptide vaccine against Leishmania infantum

Visceral leishmaniasis (VL) or kala-azar, the most severe form of the disease, is endemic in more than eighty countries across the world. To date, there is no approved vaccine against VL in the market. Recent advances in reverse vaccinology could be promising approach in designing the efficient vaccine for VL treatment. In this study, an efficient multi-epitope vaccine against Leishmania infantum, the causative agent of VL, was designed using various computational vaccinology methods. Potential immunodominant epitopes were selected from four antigenic proteins, including histone H1, sterol 24-c-methyltransferase (SMT), Leishmania-specific hypothetical protein (LiHy), and Leishmania-specific antigenic protein (LSAP). To enhance vaccine immunogenicity, two resuscitation-promoting factor of Mycobacterium tuberculosis, RpfE and RpfB, were employed as adjuvants. All the aforesaid segments were joined using proper linkers. Homology modeling, followed by refinement and validation was performed to obtain a high-quality 3D structure of designed vaccine. Docking analyses and molecular dynamics (MD) studies indicated vaccine/TLR4 complex was in the stable form during simulation time. In sum, we expect our designed vaccine is able to induce humoral and cellular immune responses against L. infantum, and may be promising medication for VL, after in vitro and in vivo immunological assays.

Growth arrested live-attenuated Leishmania infantum KHARON1 null mutants display cytokinesis defect and protective immunity in mice

There is no safe and efficacious vaccine against human leishmaniasis available and live attenuated vaccines have been used as a prophylactic alternative against the disease. In order to obtain an attenuated Leishmania parasite for vaccine purposes, we generated L. infantum KHARON1 (KH1) null mutants (ΔLikh1). This gene was previously associated with growth defects in L. mexicana. ΔLikh1 was obtained and confirmed by PCR, qPCR and Southern blot. We also generate a KH1 complemented line with the introduction of episomal copies of KH1. Although ΔLikh1 promastigote forms exhibited a growth pattern similar to the wild-type line, they differ in morphology without affecting parasite viability. L. infantum KH1-deficient amastigotes were unable to sustain experimental infection in macrophages, forming multinucleate cells which was confirmed by in vivo attenuation phenotype. The cell cycle analysis of ΔLikh1 amastigotes showed arrested cells at G₂/M phase. ΔLikh1-immunized mice presented reduced parasite burden upon challenging with virulent L. infantum, when compared to naïve mice. An effect associated with increased Li SLA-specific IgG serum levels and IL-17 production. Thus, ΔLikh1 parasites present an infective-attenuated phenotype due to a cytokinesis defect, whereas it induces immunity against visceral leishmaniasis in mouse model, being a candidate for antileishmanial vaccine purposes.

Peptide Vaccines for Leishmaniasis

Due to an increase in the incidence of leishmaniases worldwide, the development of new strategies such as prophylactic vaccines to prevent infection and decrease the disease have become a high priority. Classic vaccines against leishmaniases were based on live or attenuated parasites or their subunits. Nevertheless, the use of whole parasite or their subunits for vaccine production has numerous disadvantages. Therefore, the use of Leishmania peptides to design more specific vaccines against leishmaniases seems promising. Moreover, peptides have several benefits in comparison with other kinds of antigens, for instance, good stability, absence of potentially damaging materials, antigen low complexity, and low-cost to scale up. By contrast, peptides are poor immunogenic alone, and they need to be delivered correctly. In this context, several approaches described in this review are useful to solve these drawbacks. Approaches, such as, peptides in combination with potent adjuvants, cellular vaccinations, adenovirus, polyepitopes, or DNA vaccines have been used to develop peptide-based vaccines. Recent advancements in peptide vaccine design, chimeric, or polypeptide vaccines and nanovaccines based on particles attached or formulated with antigenic components or peptides have been increasingly employed to drive a specific immune response. In this review, we briefly summarize the old, current, and future stands on peptide-based vaccines, describing the disadvantages and benefits associated with them. We also propose possible approaches to overcome the related weaknesses of synthetic vaccines and suggest future guidelines for their development.

Recent progress in drug targets and inhibitors towards combating leishmaniasis

Lesihmaniasis is one of the major neglected tropical disease caused by the parasite of the genus Leishmania. The disease has more than one clinical forms and the visceral form is considered fatal. With the lack of potential vaccine, chemotherapy is the major treatment source considered for the control of the disease in the infected people. Drugs including amphotericin B and miltefosine are widely used for the treatment, however, development of resistance by the parasite towards the administered drug and high-toxicity of the drug are of major concern. Hence, more attention has been shown on identifying new targets, effective inhibitors, and better drug delivery system against the disease. This review deals with recent studies on drug targets and exploring their essentiality for the survival of Leishmania. Further, new inhibitors for those targets, novel anti-leishmanial peptides and vaccines against leishmaniasis were discussed. We believe that this pool of information will ease the researchers to gain knowledge and help in choosing right targets and design of new inhibitors against Leishmaniasis.

Vaccine candidates against leishmania under current research

INTRODUCTION

The search for vaccines to prevent human leishmaniasis is an active field of investigation aimed to prevent the devastating effects of this family of diseases on human health. The design and commercialization of several vaccines against canine leishmaniasis is a hopeful advance toward the achievement of a human vaccine.

AREAS COVERED

This review includes a summary of the most relevant immunological aspects accompanying leishmaniasis in natural hosts as well as a description of the latest advances in the multiple strategies that are being followed to develop leishmanial prophylactic vaccines. We have combined citations of the latest specialized reviews with research articles presenting the most recent results.

EXPERT COMMENTARY

Achieving safe, effective, durable and low-cost prophylactic vaccines against leishmaniasis is still a major challenge. These vaccines should control not only parasite progression, but also the accompanying pathology, which results from an imbalanced interaction between the infectious agent and the human host immune system. Different strategies for development of vaccines are currently under investigation. They range from the use of live non-pathogenic vectors to the employment of subunit vaccines combined with adjuvants and/or delivery systems inducing cell-mediated immunity.

Analysis of the Antigenic and Prophylactic Properties of the Leishmania Translation Initiation Factors eIF2 and eIF2B in Natural and Experimental Leishmaniasis

Different members of intracellular protein families are recognized by the immune system of the vertebrate host infected by parasites of the genus Leishmania. Here, we have analyzed the antigenic and immunogenic properties of the Leishmania eIF2 and eIF2B translation initiation factors. An in silico search in Leishmania infantum sequence databases allowed the identification of the genes encoding the α, β, and γ subunits and the α, β, and δ subunits of the putative Leishmania orthologs of the eukaryotic initiation factors F2 (LieIF2) or F2B (LieIF2B), respectively. The antigenicity of these factors was analyzed by ELISA using recombinant versions of the different subunits. Antibodies against the different LieIF2 and LieIF2B subunits were found in the sera from human and canine visceral leishmaniasis patients, and also in the sera from hamsters experimentally infected with L. infantum. In L. infantum (BALB/c) and Leishmania major (BALB/c or C57BL/6) challenged mice, a moderate humoral response against these protein factors was detected. Remarkably, these proteins elicited an IL-10 production by splenocytes derived from infected mice independently of the Leishmania species employed for experimental challenge. When DNA vaccines based on the expression of the LieIF2 or LieIF2B subunit encoding genes were administered in mice, an antigen-specific secretion of IFN-γ and IL-10 cytokines was observed. Furthermore, a partial protection against murine CL development due to L. major infection was generated in the vaccinated mice. Also, in this work we show that the LieIF2α subunit and the LieIF2Bβ and δ subunits have the capacity to stimulate IL-10 secretion by spleen cells from naïve mice. B-lymphocytes were identified as the major producers of this anti-inflammatory cytokine. Taking into account the data found in this study, it may be hypothesized that these proteins act as virulence factors implicated in the induction of humoral responses as well as in the production of the down-regulatory IL-10 cytokine, favoring a pathological outcome. Therefore, these proteins might be considered markers of disease.

Mathematical analysis of a model for zoonotic visceral leishmaniasis

Zoonotic visceral leishmaniasis (ZVL), caused by the protozoan parasite Leishmania infantum and transmitted to humans and reservoir hosts by female sandflies, is endemic in many parts of the world (notably in Africa, Asia and the Mediterranean). This study presents a new mathematical model for assessing the transmission dynamics of ZVL in human and non-human animal reservoir populations. The model undergoes the usual phenomenon of backward bifurcation exhibited by similar vector-borne disease transmission models. In the absence of such phenomenon (which is shown to arise due to the disease-induced mortality in the host populations), the nontrivial disease-free equilibrium of the model is shown to be globally-asymptotically stable when the associated reproduction number of the model is less than unity. Using case and demographic data relevant to ZVL dynamics in Arac̣atuba municipality of Brazil, it is shown, for the default case when systemic insecticide-based drugs are not used to treat infected reservoir hosts, that the associated reproduction number of the model ( ℛ 0 ) ranges from 0.3 to 1.4, with a mean of ℛ 0 = 0.85 . Furthermore, when the effect of such drug treatment is explicitly incorporated in the model (i.e., accounting for the additional larval and sandfly mortality, following feeding on the treated reservoirs), the range of ℛ 0 decreases to ℛ 0 ∈ [ 0.1 , 0.6 ] , with a mean of ℛ 0 = 0.35 (this significantly increases the prospect of the effective control or elimination of the disease). Thus, ZVL transmission models (in communities where such treatment strategy is implemented) that do not explicitly incorporate the effect of such treatment may be over-estimating the disease burden (as measured in terms of ℛ 0 ) in the community. It is shown that ℛ 0 is more sensitive to increases in sandfly lifespan than that of the animal reservoir (so, a strategy that focuses on reducing sandflies, rather than the animal reservoir (e.g., via culling), may be more effective in reducing ZVL burden in the community). Further sensitivity analysis of the model ranks the sandfly removal rate (by natural death or by feeding from insecticide-treated reservoir hosts), the biting rate of sandflies on the reservoir hosts and the progression rate of exposed reservoirs to active ZVL as the three parameters with the most effect on the disease dynamics or burden (as measured in terms of the reproduction number ℛ 0 ). Hence, this study identifies the key parameters that play a key role on the disease dynamics, and thereby contributing in the design of effective control strategies (that target the identified parameters).

Evaluation of the adjuvant effect of agonists of toll-like receptor 4 and 7/8 in a vaccine against leishmaniasis in BALB/c mice

There is no effective vaccine against human leishmaniasis. Achieving successful vaccines seems to need powerful adjuvants. Separate or combined use of toll like receptor (TLR) agonists as adjuvant is a promising approach in Leishmania vaccine research. In present study, we evaluated adjuvant effect of separate or combined use of a TLR7/8 agonist, R848 and a TLR4 agonist, monophosphoryl lipid A (MPL) beside soluble Leishmania antigen (SLA) in BALB/c mice. Mice were vaccinated three times by SLA with separate or combined TLR7/8 and TLR4 agonists and were then challenged by Leishmania major. Delay type hypersensitivity, lesion development, parasite load, and cytokines (interferon gamma, and interleukin-10) response were assessed. Results showed: 1) MPL can slightly assist SLA in parasite load reduction, but it is not able to increase SLA ability in evoking DTH and cytokine responses or decreasing lesion diameter. 2) R848 does not affect the DTH response and parasite load of mice vaccinated with SLA, but it decreases/inhibits cytokine responses induced by SLA, leading to increase lesion diameter. 3) MPL neutralized inhibitory effect of R848. In overall, these data emphasize that MPL slightly assists SLA to make a more potent vaccine, but R848 is not a good adjuvant to induce T cell-dependent immune response in BALB/c mice, and therefore combination of these TLR agonists in the current formulation, is not recommended for making a more powerful adjuvant.

F1 Domain of the Leishmania (Leishmania) donovani Nucleoside Hydrolase Promotes a Th1 Response in Leishmania (Leishmania) infantum Cured Patients and in Asymptomatic Individuals Living in an Endemic Area of Leishmaniasis

The Leishmania (Leishmania) donovani nucleoside hydrolase NH36 is the main antigen of the Leishmune® vaccine and one of the promising candidates for vaccination against visceral leishmaniasis. The antigenicity of the N-terminal (F1), the central (F2), or the C-terminal recombinant domain (F3) of NH36 was evaluated using peripheral blood mononuclear cells (PBMC) from individuals infected with L. (L.) infantum from an endemic area of visceral leishmaniasis of Spain. Both NH36 and F1 domains significantly increased the PBMC proliferation stimulation index of cured patients and infected asymptomatic individuals compared to healthy controls. Moreover, F1 induced a 19% higher proliferative response than NH36 in asymptomatic exposed subjects. In addition, in patients cured from visceral leishmaniasis, proliferation in response to NH36 and F1 was accompanied by a significant increase of IFN-γ and TNF-α secretion, which was 42-43% higher, in response to F1 than to NH36. The interleukin 17 (IL-17) secretion was stronger in asymptomatic subjects, in response to F1, as well as in cured cutaneous leishmaniasis after NH36 stimulation. While no IL-10 secretion was determined by F1, a granzyme B increase was detected in supernatants from cured patients after stimulation with either NH36 or F1. These data demonstrate that F1 is the domain of NH36 that induces a recall cellular response in individuals with acquired resistance to the infection by L. (L.) infantum. In addition, F1 and NH36 discriminated the IgG3 humoral response in patients with active visceral leishmaniasis due to L. (L.) donovani (Ethiopia) and L. (L.) infantum (Spain) from that of endemic and non-endemic area controls. NH36 showed higher reactivity with sera from L. (L.) donovani-infected individuals, indicating species specificity. We conclude that the F1 domain, previously characterized as an inducer of the Th1 and Th17 responses in cured/exposed patients infected with L. (L.) infantum chagasi, may also be involved in the generation of a protective response against L. (L.) infantum and represents a potential vaccine candidate for the control of human leishmaniasis alone, or in combination with other HLA epitopes/antigens.

Lipid based delivery and immuno-stimulatory systems: Master tools to combat leishmaniasis

Disease management of leishmaniasis is appalling due to lack of a human vaccine and the toxicity and resistance concerns with limited therapeutic drugs. The challenges in development of a safe vaccine for generation and maintenance of robust antileishmanial protective immunity through a human administrable route of immunization can be addressed through immunomodulation and targeted delivery. The versatility of lipid based particulate system for deliberate delivery of diverse range of molecules including immunomodulators, antigens and drugs have essentially found pivotal role in design of proficient vaccination and therapeutic strategies against leishmaniasis. The prospects of lipid based preventive and curative formulations for leishmaniasis have been highlighted in this review.

Possibilities and challenges for developing a successful vaccine for leishmaniasis

Leishmaniasis is a vector-borne disease caused by different species of protozoan parasites of the genus Leishmania. It is a major health problem yet neglected tropical diseases, with approximately 350 million people worldwide at risk and more than 1.5 million infections occurring each year. Leishmaniasis has different clinical manifestations, including visceral (VL or kala-azar), cutaneous (CL), mucocutaneous (MCL), diffuse cutaneous (DCL) and post kala-azar dermal leishmaniasis (PKDL). Currently, the only mean to treat and control leishmaniasis is by rational medications and vector control. However, the number of available drugs is limited and even these are either exorbitantly priced, have toxic side effects or prove ineffective due to the emergence of resistant strains. On the other hand, the vector control methods are not so efficient. Therefore, there is an urgent need for developing a safe, effective, and affordable vaccine for the prevention of leishmaniasis. Although in recent years a large body of researchers has concentrated their efforts on this issue, yet only three vaccine candidates have gone for clinical trial, until date. These are: (i) killed vaccine in Brazil for human immunotherapy; (ii) live attenuated vaccine for humans in Uzbekistan; and (iii) second-generation vaccine for dog prophylaxis in Brazil. Nevertheless, there are at least half a dozen vaccine candidates in the pipeline. One can expect that, in the near future, the understanding of the whole genome of Leishmania spp. will expand the vaccine discovery and strategies that may provide novel vaccines. The present review focuses on the development and the status of various vaccines and potential vaccine candidates against leishmaniasis.

Status of vaccine research and development of vaccines for leishmaniasis

A number of leishmaniasis vaccine candidates are at various stages of pre-clinical and clinical development. Leishmaniasis is a vector-borne neglected tropical disease (NTD) caused by a protozoan parasite of the genus Leishmania and transmitted to humans by the bite of a sand fly. Visceral leishmaniasis (VL, kala-azar) is a high mortality NTD found mostly in South Asia and East Africa, while cutaneous leishmaniasis (CL) is a disfiguring NTD highly endemic in the Middle East, Central Asia, North Africa, and the Americas. Estimates attribute 50,000 annual deaths and 3.3 million disability-adjusted life years to leishmaniasis. There are only a few approved drug treatments, no prophylactic drug and no vaccine. Ideally, an effective vaccine against leishmaniasis will elicit long-lasting immunity and protect broadly against VL and CL. Vaccines such as Leish-F1, F2 and F3, developed at IDRI and designed based on selected Leishmania antigen epitopes, have been in clinical trials. Other groups, including the Sabin Vaccine Institute in collaboration with the National Institutes of Health are investigating recombinant Leishmania antigens in combination with selected sand fly salivary gland antigens in order to augment host immunity. To date, both VL and CL vaccines have been shown to be cost-effective in economic modeling studies.

Protection and Long-Lived Immunity Induced by the ID93/GLA-SE Vaccine Candidate against a Clinical Mycobacterium tuberculosis Isolate

Mycobacterium tuberculosis HN878 represents a virulent clinical strain from the W-Beijing family, which has been tested in small animal models in order to study its virulence and its induction of host immune responses following infection. This isolate causes death and extensive lung pathology in infected C57BL/6 mice, whereas lab-adapted strains, such as M. tuberculosis H37Rv, do not. The use of this clinically relevant isolate of M. tuberculosis increases the possibilities of assessing the long-lived efficacy of tuberculosis vaccines in a relatively inexpensive small animal model. This model will also allow for the use of knockout mouse strains to critically examine key immunological factors responsible for long-lived, vaccine-induced immunity in addition to vaccine-mediated prevention of pulmonary immunopathology. In this study, we show that the ID93/glucopyranosyl lipid adjuvant (GLA)-stable emulsion (SE) tuberculosis vaccine candidate, currently in human clinical trials, is able to elicit protection against M. tuberculosis HN878 by reducing the bacterial burden in the lung and spleen and by preventing the extensive lung pathology induced by this pathogen in C57BL/6 mice.

Schistosomiasis vaccine candidate Sm14/GLA-SE: Phase 1 safety and immunogenicity clinical trial in healthy, male adults

DESIGN

Safety and immunogenicity of a recombinant 14kDa, fatty acid-binding protein(FABP) from Schistosoma mansoni (rSm14) were evaluated through an open, non-placebo-controlled, dose-standardized trial, performed at a single research site. The vaccine was formulated with glucopyranosyl lipid A (GLA) adjuvant in an oil-in-water emulsion (SE) and investigated in 20 male volunteers from a non-endemic area for schistosomiasis in the state of Rio de Janeiro, Brazil. Fifty microgram rSm14 with 10 μg GLA-SE (rSm14/GLA-SE)/dose were given intramuscularly three times with 30-day intervals. Participants were assessed clinically, biochemically and immunologically for up to 120 days.

METHODS

Participants were screened for inclusion by physical examination, haematology and blood chemistry; then followed to assess adverse events and immunogenicity. Sera were tested for IgG (total and isotypes) and IgE. T cell induction of cytokines IL-2, IL-5, IL-10, IFNγ and TNFα was assessed by Milliplex kit and flow cytometry.

RESULTS

The investigational product showed high tolerability; some self-limited, mild adverse events were observed during and after vaccine administration. Significant increases in Sm14-specific total IgG, IgG1 and IgG3 were observed 30 days after the first vaccination with specific IgG2 and IgG4 after 60 days. An increase in IgE antibodies was not observed at any time point. The IgG response was augmented after the second dose and 88% of all vaccinated subjects had developed high anti-Sm14 IgG titres 90 days after the first injection. From day 60 and onwards, there was an increase in CD4(+) T cells producing single cytokines, particularly TNFα and IL-2, with no significant increase of multi-functional TH1 cells.

CONCLUSION

Clinical trial data on tolerability and specific immune responses after vaccination of adult, male volunteers in a non-endemic area for schistosomiasis with rSm14/GLA-SE, support this product as a safe, strongly immunogenic vaccine against schistosomiasis paving the way for follow-up Phase 2 trials. Study registration ID: NCT01154049 at http://www.clinicaltrials.gov.

Differential Immune Response against Recombinant Leishmania donovani Peroxidoxin 1 and Peroxidoxin 2 Proteins in BALB/c Mice

We assessed the immune response against recombinant proteins of two related, albeit functionally different, peroxidoxins from Leishmania donovani: peroxidoxin 1 (LdPxn1) and peroxidoxin 2 (LdPxn2) in BALB/c mice. We also evaluated the effect of coadministration of TLR agonists (CpG ODN and GLA-SE) on the antigen-specific immune response. Immunization with recombinant LdPxn1 alone induced a predominantly Th2 type immune response that is associated with the production of high level of IgG1 and no IgG2a isotype while rLdPxn2 resulted in a mixed Th1/Th2 response characterized by the production of antigen-specific IgG2a in addition to IgG1 isotype. Antigen-stimulated spleen cells from mice that were immunized with rLdPxn1 produced low level of IL-10 and IL-4 and no IFN-γ, whereas cells from mice immunized with rLdPxn2 secreted high level of IFN-γ, low IL-4, and no IL-10. Coadministration of CpG ODN or GLA-SE with rLdPxn1 skewed the immune response towards a Th 1 type as indicated by robust production of IgG2a isotype. Furthermore, the presence of TLR agonists together with rLdPxn1 antigen enhanced the production of IFN-γ and to a lesser extent of IL-10. TLR agonists also enhanced a more polarized Th 1 type immune response against rLdPxn2.

Phage-fused epitopes fromLeishmania infantumused as immunogenic vaccines confer partial protection againstLeishmania amazonensisinfection

Two mimotopes ofLeishmania infantumidentified by phage display were evaluated as vaccine candidates in BALB/c mice againstLeishmania amazonensisinfection. The epitope-based immunogens, namely B10 and C01, presented as phage-fused peptides; were used without association of a Th1 adjuvant, and they were administered isolated or in combination into animals. Both clones showed a specific production of interferon-gamma (IFN-γ), interleukin-12 (IL-12) and granulocyte/macrophage colony-stimulating factor (GM-CSF) afterin vitrospleen cells stimulation, and they were able to induce a partial protection against infection. Significant reductions of parasite load in the infected footpads, liver, spleen, bone marrow and paws’ draining lymph nodes were observed in the immunized mice, in comparison with the control groups (saline, saponin, wild-type and non-relevant clones). Protection was associated with an IL-12-dependent production of IFN-γ, mediated mainly by CD8+T cells, against parasite proteins. Protected mice also presented low levels of IL-4 and IL-10, as well as increased levels of parasite-specific IgG2a antibodies. The association of both clones resulted in an improved protection in relation to their individual use. More importantly, the absence of adjuvant did not diminish the cross-protective efficacy againstLeishmaniaspp. infection. This study describes for the first time two epitope-based immunogens selected by phage display technology againstL. infantuminfected dogs sera, which induced a partial protection in BALB/c mice infected withL. amazonensis.

From mouse to man: safety, immunogenicity and efficacy of a candidate leishmaniasis vaccine LEISH-F3+GLA-SE

Key antigens of Leishmania species identified in the context of host responses in Leishmania-exposed individuals from disease-endemic areas were prioritized for the development of a subunit vaccine against visceral leishmaniasis (VL), the most deadly form of leishmaniasis. Two Leishmania proteins-nucleoside hydrolase and a sterol 24-c-methyltransferase, each of which are protective in animal models of VL when properly adjuvanted- were produced as a single recombinant fusion protein NS (LEISH-F3) for ease of antigen production and broad coverage of a heterogeneous major histocompatibility complex population. When formulated with glucopyranosyl lipid A-stable oil-in-water nanoemulsion (GLA-SE), a Toll-like receptor 4 TH1 (T helper 1) promoting nanoemulsion adjuvant, the LEISH-F3 polyprotein induced potent protection against both L. donovani and L. infantum in mice, measured as significant reductions in liver parasite burdens. A robust immune response to each component of the vaccine with polyfunctional CD4 TH1 cell responses characterized by production of antigen-specific interferon-γ, tumor necrosis factor and interleukin-2 (IL-2), and low levels of IL-5 and IL-10 was induced in immunized mice. We also demonstrate that CD4 T cells, but not CD8 T cells, are sufficient for protection against L. donovani infection in immunized mice. Based on the sum of preclinical data, we prepared GMP materials and performed a phase 1 clinical study with LEISH-F3+GLA-SE in healthy, uninfected adults in the United States. The vaccine candidate was shown to be safe and induced a strong antigen-specific immune response, as evidenced by cytokine and immunoglobulin subclass data. These data provide a strong rationale for additional trials in Leishmania-endemic countries in populations vulnerable to VL.

Design and preclinical characterization of a novel vaccine adjuvant formulation consisting of a synthetic TLR4 agonist in a thermoreversible squalene emulsion

We describe the development, analytical characterization, stability and preclinical efficacy of AF04, a combination adjuvant comprising the synthetic toll-like receptor 4 (TLR4) agonist, E6020, formulated in AF03, a thermoreversible squalene emulsion. By using AF04 with the recombinant major outer membrane protein of Chlamydia trachomatis (Ct-MOMP) and with the recombinant surface glycoprotein gB from human cytomegalovirus (CMV-gB) as model antigens, we show that AF03 and E6020 can synergize to augment specific antibody and Th-1 cellular immune responses in mice. In terms of formulation, we observe that the method used to incorporate E6020 into AF03 affects its partition between the oil and water phases of the emulsion which in turn has a significant impact on the tolerability (IV pyrogenicity test in rabbits) of this novel adjuvant combination.

Visceral Leishmaniasis: Advancements in Vaccine Development via Classical and Molecular Approaches

Visceral leishmaniasis (VL) or kala-azar, a vector-borne protozoan disease, shows endemicity in larger areas of the tropical, subtropical and the Mediterranean countries. WHO report suggested that an annual incidence of VL is nearly 200,000 to 400,000 cases, resulting in 20,000 to 30,000 deaths per year. Treatment with available anti-leishmanial drugs are not cost effective, with varied efficacies and higher relapse rate, which poses a major challenge to current kala-azar control program in Indian subcontinent. Therefore, a vaccine against VL is imperative and knowing the fact that recovered individuals developed lifelong immunity against re-infection, it is feasible. Vaccine development program, though time taking, has recently gained momentum with the emergence of omic era, i.e., from genomics to immunomics. Classical as well as molecular methodologies have been overtaken with alternative strategies wherein proteomics based knowledge combined with computational techniques (immunoinformatics) speed up the identification and detailed characterization of new antigens for potential vaccine candidates. This may eventually help in the designing of polyvalent synthetic and recombinant chimeric vaccines as an effective intervention measures to control the disease in endemic areas. This review focuses on such newer approaches being utilized for vaccine development against VL.

Leishmania donovani Nucleoside Hydrolase Terminal Domains in Cross-Protective Immunotherapy Against Leishmania amazonensis Murine Infection

Nucleoside hydrolases of the Leishmania genus are vital enzymes for the replication of the DNA and conserved phylogenetic markers of the parasites. Leishmania donovani nucleoside hydrolase (NH36) induced a main CD4(+) T cell driven protective response against L. chagasi infection in mice which is directed against its C-terminal domain. In this study, we used the three recombinant domains of NH36: N-terminal domain (F1, amino acids 1-103), central domain (F2 aminoacids 104-198), and C-terminal domain (F3 amino acids 199-314) in combination with saponin and assayed their immunotherapeutic effect on Balb/c mice previously infected with L. amazonensis. We identified that the F1 and F3 peptides determined strong cross-immunotherapeutic effects, reducing the size of footpad lesions to 48 and 64%, and the parasite load in footpads to 82.6 and 81%, respectively. The F3 peptide induced the strongest anti-NH36 antibody response and intradermal response (IDR) against L. amazonenis and a high secretion of IFN-γ and TNF-α with reduced levels of IL-10. The F1 vaccine, induced similar increases of IgG2b antibodies and IFN-γ and TNF-α levels, but no IDR and no reduction of IL-10. The multiparameter flow cytometry analysis was used to assess the immune response after immunotherapy and disclosed that the degree of the immunotherapeutic effect is predicted by the frequencies of the CD4(+) and CD8(+) T cells producing IL-2 or TNF-α or both. Total frequencies and frequencies of double-cytokine CD4 T cell producers were enhanced by F1 and F3 vaccines. Collectively, our multifunctional analysis disclosed that immunotherapeutic protection improved as the CD4 responses progressed from 1+ to 2+, in the case of the F1 and F3 vaccines, and as the CD8 responses changed qualitatively from 1+ to 3+, mainly in the case of the F1 vaccine, providing new correlates of immunotherapeutic protection against cutaneous leishmaniasis in mice based on T-helper TH1 and CD8(+) mediated immune responses.

Cross-Protective Immunity to Leishmania amazonensis is Mediated by CD4+ and CD8+ Epitopes of Leishmania donovani Nucleoside Hydrolase Terminal Domains

The nucleoside hydrolase (NH) of Leishmania donovani (NH36) is a phylogenetic marker of high homology among Leishmania parasites. In mice and dog vaccination, NH36 induces a CD4+ T cell-driven protective response against Leishmania chagasi infection directed against its C-terminal domain (F3). The C-terminal and N-terminal domain vaccines also decreased the footpad lesion caused by Leishmania amazonensis. We studied the basis of the crossed immune response using recombinant generated peptides covering the whole NH36 sequence and saponin for mice prophylaxis against L. amazonensis. The F1 (amino acids 1-103) and F3 peptide (amino acids 199-314) vaccines enhanced the IgG and IgG2a anti-NH36 antibodies to similar levels. The F3 vaccine induced the strongest DTH response, the highest proportions of NH36-specific CD4+ and CD8+ T cells after challenge and the highest expression of IFN-γ and TNF-α. The F1 vaccine, on the other hand, induced a weaker but significant DTH response and a mild enhancement of IFN-γ and TNF-α levels. The in vivo depletion with anti-CD4 or CD8 monoclonal antibodies disclosed that cross-protection against L. amazonensis infection was mediated by a CD4+ T cell response directed against the C-terminal domain (75% of reduction of the size of footpad lesion) followed by a CD8+ T cell response against the N-terminal domain of NH36 (57% of reduction of footpad lesions). Both vaccines were capable of inducing long-term cross-immunity. The amino acid sequence of NH36 showed 93% identity to the sequence of the NH A34480 of L. amazonensis, which also showed the presence of completely conserved predicted epitopes for CD4+ and CD8+ T cells in F1 domain, and of CD4+ epitopes differing by a single amino acid, in F1 and F3 domains. The identification of the C-terminal and N-terminal domains as the targets of the immune response to NH36 in the model of L. amazonensis infection represents a basis for the rationale development of a bivalent vaccine against leishmaniasis.

Gentamicin-attenuated Leishmania infantum vaccine: protection of dogs against canine visceral leishmaniosis in endemic area of southeast of Iran

An attenuated line of Leishmania infantum (L. infantum H-line) has been established by culturing promastigotes in vitro under gentamicin pressure. A vaccine trial was conducted using 103 naive dogs from a leishmaniosis non-endemic area (55 vaccinated and 48 unvaccinated) brought into an endemic area of southeast Iran. No local and/or general indications of disease were observed in the vaccinated dogs immediately after vaccination. The efficacy of the vaccine was evaluated after 24 months (4 sandfly transmission seasons) by serological, parasitological analyses and clinical examination. In western blot analysis of antibodies to L. infantum antigens, sera from 10 out of 31 (32.2%) unvaccinated dogs, but none of the sera from vaccinated dogs which were seropositive at >100, recognized the 21 kDa antigen of L. infantum wild-type (WT). Nine out of 31 (29%) unvaccinated dogs, but none of vaccinated dogs, were positive for the presence of Leishmania DNA. One out of 46 (2.2%) vaccinated dogs and 9 out of 31 (29%) unvaccinated dogs developed clinical signs of disease. These results suggest that gentamicin-attenuated L. infantum induced a significant and strong protective effect against canine visceral leishmaniosis in the endemic area.

Enhanced protective efficacy of nonpathogenic recombinant leishmania tarentolae expressing cysteine proteinases combined with a sand fly salivary antigen

BACKGROUND

Novel vaccination approaches are needed to prevent leishmaniasis. Live attenuated vaccines are the gold standard for protection against intracellular pathogens such as Leishmania and there have been new developments in this field. The nonpathogenic to humans lizard protozoan parasite, Leishmania (L) tarentolae, has been used effectively as a vaccine platform against visceral leishmaniasis in experimental animal models. Correspondingly, pre-exposure to sand fly saliva or immunization with a salivary protein has been shown to protect mice against cutaneous leishmaniasis.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we tested the efficacy of a novel combination of established protective parasite antigens expressed by L. tarentolae together with a sand fly salivary antigen as a vaccine strategy against L. major infection. The immunogenicity and protective efficacy of different DNA/Live and Live/Live prime-boost vaccination modalities with live recombinant L. tarentolae stably expressing cysteine proteinases (type I and II, CPA/CPB) and PpSP15, an immunogenic salivary protein from Phlebotomus papatasi, a natural vector of L. major, were tested both in susceptible BALB/c and resistant C57BL/6 mice. Both humoral and cellular immune responses were assessed before challenge and at 3 and 10 weeks after Leishmania infection. In both strains of mice, the strongest protective effect was observed when priming with PpSP15 DNA and boosting with PpSP15 DNA and live recombinant L. tarentolae stably expressing cysteine proteinase genes.

CONCLUSION/SIGNIFICANCE

The present study is the first to use a combination of recombinant L. tarentolae with a sand fly salivary antigen (PpSP15) and represents a novel promising vaccination approach against leishmaniasis.

Adjuvants containing natural and synthetic Toll-like receptor 4 ligands

The last decade has seen an increased focus on the development of adjuvants for vaccines, and several novel adjuvants are now in licensed products or in late-stage clinical development. These advancements have been aided by the discovery of receptors and signaling pathways of the innate immune system and an increased understanding of how these innate responses influence the adaptive immune response. Successful vaccine development relies on knowledge of which adjuvants to use and the proper formulation of adjuvants and antigens to achieve safe, stable and immunogenic vaccines. In this review, the authors focus on the current use of natural and synthetic lipopolysaccharide analogues that retain their adjuvant properties with reduced toxicity compared with the parent compound for use in emerging vaccines. The authors review how these compounds initiate signal transduction through Toll-like receptor 4, insights from structure-function studies and how formulation parameters can influence their effectiveness as vaccine adjuvants.

Cross-protective effect of a combined L5 plus L3 Leishmania major ribosomal protein based vaccine combined with a Th1 adjuvant in murine cutaneous and visceral leishmaniasis

Background

Two Leishmania major ribosomal proteins L3 (LmL3) and L5 (LmL5) have been described as protective molecules against cutaneous leishmaniasis due to infection with L. major and Leishmania braziliensis in BALB/c mice when immunized with a Th1 adjuvant (non-methylated CpG-oligodeoxynucleotides; CpG-ODN). In the present study we analyzed the cross-protective efficacy of an LmL3-LmL5-CpG ODN combined vaccine against infection with Leishmania amazonensis and Leishmania chagasi (syn. Leishmania infantum) the etiologic agents of different clinical forms of human leishmaniasis in South America.

Methods

The combined vaccine was administered subcutaneously to BALB/c mice. After immunization the cellular and humoral responses elicited were analyzed. Mice were independently challenged with L. amazonensis and L. chagasi. The size of the cutaneous lesions caused by the infection with the first species, the parasite loads and the immune response in both infection models were analyzed nine weeks after challenge.

Results

Mice vaccinated with the combined vaccine showed a Th1-like response against LmL3 and LmL5. Vaccinated mice were able to delay lesion development due to L. amazonensis infection and to control parasite loads in the site of infection. A reduction of the parasite burden in the lymph nodes draining the site of infection and in the liver and spleen was observed in the vaccinated mice after a subcutaneous infection with L. chagasi. In both models of infection, protection was correlated to parasite antigen-specific production of IFN-γ and down-regulation of parasite-mediated IL-4 and IL-10 responses.

Conclusions

The data presented here demonstrate the potential use of L. major L3 and L5 recombinant ribosomal proteins for the development of vaccines against various Leishmania species.

Key roles of adjuvants in modern vaccines

Vaccines containing novel adjuvant formulations are increasingly reaching advanced development and licensing stages, providing new tools to fill previously unmet clinical needs. However, many adjuvants fail during product development owing to factors such as manufacturability, stability, lack of effectiveness, unacceptable levels of tolerability or safety concerns. This Review outlines the potential benefits of adjuvants in current and future vaccines and describes the importance of formulation and mechanisms of action of adjuvants. Moreover, we emphasize safety considerations and other crucial aspects in the clinical development of effective adjuvants that will help facilitate effective next-generation vaccines against devastating infectious diseases.

Sand-fly saliva-leishmania-man: the trigger trio

Leishmaniases are worldwide diseases transmitted to the vertebrate host by the bite of an infected sand-fly. Sand-fly biting and parasite inoculation are accompanied by the injection of salivary molecules, whose immunomodulatory properties are actively being studied. This mini review focuses on how the interactions between sand-fly saliva and the immune system may shape the outcome of infection, given its immunomodulatory properties, in experimental models and in the endemic area. Additionally, we approach the recent contributions regarding the identification of individual salivary components and how these are currently being considered as additional components of a vaccine against leishmaniasis.

New vaccines for neglected parasitic diseases and dengue

Neglected tropical diseases (NTDs) are a significant source of morbidity and socioeconomic burden among the world's poor. Virtually all of the 2.4 billion people who live on less than $2 per d, more than a third of the world's population, are at risk for these debilitating NTDs. Although chemotherapeutic measures exist for many of these pathogens, they are not sustainable countermeasures on their own because of rates of reinfection, risk of drug resistance, and inconsistent maintenance of drug treatment programs. Preventative and therapeutic NTD vaccines are needed as long-term solutions. Because there is no market in the for-profit sector of vaccine development for these pathogens, much of the effort to develop vaccines is driven by nonprofit entities, mostly through product development partnerships. This review describes the progress of vaccines under development for many of the NTDs, with a specific focus on those about to enter or that are currently in human clinical trials. Specifically, we report on the progress on dengue, hookworm, leishmaniasis, schistosomiasis, Chagas disease, and onchocerciasis vaccines. These products will be some of the first with specific objectives to aid the world's poorest populations.

Targeting Leishmania major Antigens to Dendritic Cells In Vivo Induces Protective Immunity

Efficient vaccination against the parasite Leishmania major, the causative agent of human cutaneous leishmaniasis, requires development of type 1 T-helper (Th1) CD4⁺ T cell immunity. Because of their unique capacity to initiate and modulate immune responses, dendritic cells (DCs) are attractive targets for development of novel vaccines. In this study, for the first time, we investigated the capacity of a DC-targeted vaccine to induce protective responses against L. major. To this end, we genetically engineered the N-terminal portion of the stress-inducible 1 protein of L. major (LmSTI1a) into anti-DEC205/CD205 (DEC) monoclonal antibody (mAb) and thereby delivered the conjugated protein to DEC⁺ DCs in situ in the intact animal. Delivery of LmSTI1a to adjuvant-matured DCs increased the frequency of antigen-specific CD4⁺ T cells producing IFN-γ⁺, IL-2⁺, and TNF-α⁺ in two different strains of mice (C57BL/6 and Balb/c), while such responses were not observed with the same doses of a control Ig-LmSTI1a mAb without receptor affinity or with non-targeted LmSTI1a protein. Using a peptide library for LmSTI1a, we identified at least two distinct CD4⁺ T cell mimetopes in each MHC class II haplotype, consistent with the induction of broad immunity. When we compared T cell immune responses generated after targeting DCs with LmSTI1a or other L. major antigens, including LACK (Leishmania receptor for activated C kinase) and LeIF (Leishmania eukaryotic ribosomal elongation and initiation factor 4a), we found that LmSTI1a was superior for generation of IFN-γ-producing CD4⁺ T cells, which correlated with higher protection of susceptible Balb/c mice to a challenge with L. major. For the first time, this study demonstrates the potential of a DC-targeted vaccine as a novel approach for cutaneous leishmaniasis, an increasing public health concern that has no currently available effective treatment.

Development and in vitro evaluation of a nanoemulsion for transcutaneous delivery

OBJECTIVE

The purpose of this study is to develop a nanoemulsion formulation for its use as a transcutaneous vaccine delivery system.

MATERIALS AND METHODS

With bovine albumin-fluorescein isothiocyanate conjugate (FITC-BSA) as a vaccine model, formulations were selected with the construction of pseudo-ternary phase diagrams and a short-term stability study. The size of the emulsion droplets was furthered optimized with high-pressure homogenization. The optimized formulation was evaluated for its skin permeation efficiency. In vitro skin permeation studies were conducted with shaved BALB/c mice skin samples with a Franz diffusion cell system. Different drug concentrations were compared, and the effect of the nanoemulsion excipients on the permeation of the FITC-BSA was also studied.

RESULTS

The optimum homogenization regime was determined to be five passes at 20 000 psi, with no evidence of protein degradation during processing. With these conditions, the particle diameter was 85.2 nm ± 15.5 nm with a polydispersity index of 0.186 ± 0.026 and viscosity of 14.6 cP ± 1.2 cP. The optimized formulation proved stable for 1 year at 4 °C. In vitro skin diffusion studies show that the optimized formulation improves the permeation of FITC-BSA through skin with an enhancement ratio of 4.2 compared to a neat control solution. Finally, a comparison of the skin permeation of the nanoemulsion versus only the surfactant excipients resulted in a steady state flux of 23.44 μg/cm(2)/h for the nanoemulsion as opposed to 6.10 μg/cm(2)/h for the emulsifiers.

CONCLUSION

A novel nanoemulsion with optimized physical characteristics and superior skin permeation compared to control solution was manufactured. The formulation proposed in this study has the flexibility for the incorporation of a variety of active ingredients and warrants further development as a transcutaneous vaccine delivery vehicle.

Development of novel prime-boost strategies based on a tri-gene fusion recombinant L. tarentolae vaccine against experimental murine visceral leishmaniasis

Visceral leishmaniasis (VL) is a vector-borne disease affecting humans and domestic animals that constitutes a serious public health problem in many countries. Although many antigens have been examined so far as protein- or DNA-based vaccines, none of them conferred complete long-term protection. The use of the lizard non-pathogenic to humans Leishmania (L.) tarentolae species as a live vaccine vector to deliver specific Leishmania antigens is a recent approach that needs to be explored further. In this study, we evaluated the effectiveness of live vaccination in protecting BALB/c mice against L. infantum infection using prime-boost regimens, namely Live/Live and DNA/Live. As a live vaccine, we used recombinant L. tarentolae expressing the L. donovani A2 antigen along with cysteine proteinases (CPA and CPB without its unusual C-terminal extension (CPB^-CTE)) as a tri-fusion gene. For DNA priming, the tri-fusion gene was encoded in pcDNA formulated with cationic solid lipid nanoparticles (cSLN) acting as an adjuvant. At different time points post-challenge, parasite burden and histopathological changes as well as humoral and cellular immune responses were assessed. Our results showed that immunization with both prime-boost A2-CPA-CPB^-CTE-recombinant L. tarentolae protects BALB/c mice against L. infantum challenge. This protective immunity is associated with a Th1-type immune response due to high levels of IFN-γ production prior and after challenge and with lower levels of IL-10 production after challenge, leading to a significantly higher IFN-γ/IL-10 ratio compared to the control groups. Moreover, this immunization elicited high IgG1 and IgG2a humoral immune responses. Protection in mice was also correlated with a high nitric oxide production and low parasite burden. Altogether, these results indicate the promise of the A2-CPA-CPB^-CTE-recombinant L. tarentolae as a safe live vaccine candidate against VL.

Visceral leishmaniasis (VL) is the most severe form of leishmaniasis and has emerged as an opportunistic infection in HIV-1 infected patients in many parts of the world. Drug-resistant forms have developed so emergence and increased the need for advanced preventive strategies. Using live avirulent organisms as a vaccine has been proven to be more effective than other regimens. The lizard protozoan parasite Leishmania tarentolae is considered as nonpathogenic to humans. In our previous work, a recombinant L. tarentolae strain expressing the amastigote-specific L. donovani A2 antigen as a vaccine candidate elicited protection against L. infantum challenge in mice. Furthermore, combinations of CPA/CPB cysteine proteinases were more protective against visceral and cutaneous Leishmania infections than the individual forms. Herein, we used DNA/Live and Live/Live prime-boost vaccination strategies against visceral leishmaniasis in BALB/c mice consisting of the A2-CPA-CPB^-CTE tri-fusion genes formulated with cationic solid lipid nanoparticles and a recombinant L. tarentolae expressing the tri-fusion. Assessments of cytokine production, humoral responses, parasite burden and histopathological studies support that the recombinant L. tarentolae A2-CPA-CPB^-CTE candidate vaccine elicits a protective response against visceral leishmaniasis in mice and represents an important step forward in the development of new vaccine combinations against Leishmania infections.

Full-length Plasmodium falciparum circumsporozoite protein administered with long-chain poly(I·C) or the Toll-like receptor 4 agonist glucopyranosyl lipid adjuvant-stable emulsion elicits potent antibody and CD4+ T cell immunity and protection in mice

The Plasmodium falciparum circumsporozoite (CS) protein (CSP) is a major vaccine target for preventing malaria infection. Thus, developing strong and durable antibody and T cell responses against CSP with novel immunogens and potent adjuvants may improve upon the success of current approaches. Here, we compare four distinct full-length P. falciparum CS proteins expressed in Escherichia coli or Pichia pastoris for their ability to induce immunity and protection in mice when administered with long-chain poly(I · C) [poly(I · C)LC] as an adjuvant. CS proteins expressed in E. coli induced high-titer antibody responses against the NANP repeat region and potent CSP-specific CD4(+) T cell responses. Moreover, E. coli-derived CS proteins in combination with poly(I · C)LC induced potent multifunctional (interleukin 2-positive [IL-2(+)], tumor necrosis factor alpha-positive [TNF-α(+)], gamma interferon-positive [IFN-γ(+)]) CD4(+) effector T cell responses in blood, in spleen, and particularly in liver. Using transgenic Plasmodium berghei expressing the repeat region of P. falciparum CSP [Pb-CS(Pf)], we showed that there was a 1- to 4-log decrease in malaria rRNA in the liver following a high-dose challenge and ~50% sterilizing protection with a low-dose challenge compared to control levels. Protection was directly correlated with high-level antibody titers but not CD4(+) T cell responses. Finally, protective immunity was also induced using the Toll-like receptor 4 agonist glucopyranosyl lipid adjuvant-stable emulsion (GLA-SE) as the adjuvant, which also correlated with high antibody titers yet CD4(+) T cell immunity that was significantly less potent than that with poly(I · C)LC. Overall, these data suggest that full-length CS proteins and poly(I · C)LC or GLA-SE offer a simple vaccine formulation to be used alone or in combination with other vaccines for preventing malaria infection.

Current and novel approaches to vaccine development against tuberculosis

Antibiotics and vaccines are the two most successful medical countermeasures that humans have created against a number of pathogens. However a select few e.g., Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB) have evaded eradication by vaccines and therapeutic approaches. TB is a global public health problem that kills 1.4 million people per year. The past decade has seen significant progress in developing new vaccine candidates, but the most fundamental questions in understanding disease progression and protective host responses that are responsible for controlling Mtb infection still remain poorly resolved. Current TB treatment requires intense chemotherapy with several antimicrobials, while the only approved vaccine is the classical viable whole-cell based Bacille-Calmette-Guerin (BCG) that protects children from severe forms of TB, but fails to protect adults. Taken together, there is a growing need to conduct basic and applied research to develop novel vaccine strategies against TB. This review is focused on the discussion surrounding current strategies and innovations being explored to discover new protective antigens, adjuvants, and delivery systems in the hopes of creating an efficacious TB vaccine.