Subunit vaccination of mice against new world cutaneous leishmaniasis: comparison of three proteins expressed in amastigotes and six adjuvants

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.

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.

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.

Mechanisms of immune evasion in leishmaniasis

Diseases caused by Leishmania present a worldwide problem, and current therapeutic approaches are unable to achieve a sterile cure. Leishmania is able to persist in host cells by evading or exploiting host immune mechanisms. A thorough understanding of these mechanisms could lead to better strategies for effective management of Leishmania infections. Current research has focused on parasite modification of host cell signaling pathways, entry into phagocytic cells, and modulation of cytokine and chemokine profiles that alter immune cell activation and trafficking to sites of infection. Immuno-therapeutic approaches that target these mechanisms of immune evasion by Leishmania offer promising areas for preclinical and clinical research.

Short-term protection conferred by Leishvacin® against experimental Leishmania amazonensis infection in C57BL/6 mice

To date, there is no vaccine available against human leishmaniasis. Although some vaccination protocols can induce immunity in murine models, they fail to induce protection in humans. The reasons for that remain unclear. The aim of the present study was to characterize the changes in the pattern of the immune response during subcutaneous vaccination with Leishvacin® in mice. We also investigated whether IFN-γ and nitric oxide synthase are indispensable for the protection elicited by the vaccine. C57BL/6 WT vaccinated mice showed smaller lesions and fewer numbers of parasites in footpads until 8 weeks post-infection. Up to this time, they produced higher levels of IFN-γ, IL-2, IL-4, IL-17A and IL-10 and higher specific antibody response than control non-vaccinated mice. Moreover, we showed that IFN-γ, most likely by induction of iNOS expression, is essential for immunity. However, after 12 weeks of infection, we observed loss of difference in lesion size and parasite burden between the groups. Loss of resistance was associated with the disappearance of differences in cytokine patterns between vaccinated and control mice, but not of antibody response, which remained different until a later time of infection. The reversal of resistance to L. amazonensis could not be explained by upregulation of regulatory cytokines. Our data point to a subversion of the host immune response by L. amazonensis even when a protective response was previously induced.

Adjuvants for Leishmania vaccines: from models to clinical application

Two million new cases of leishmaniasis occur every year, with the cutaneous leishmaniasis (CL) presentation accounting for approximately two-thirds of all cases. Despite the high incidence rates and geographic expansion of the disease, CL remains a neglected tropical disease without effective intervention strategies. Efforts to address this deficit have given rise to the experimental murine model of CL. By virtue of its simplicity and pliability, the CL model has been used to provide substantial information regarding cellular immunity, as well as in the discovery and evaluation of various vaccine adjuvants. The CL model has facilitated in vivo studies of the mechanism of action of many adjuvants, including the TLR4 agonist monophosphoryl lipid A, the TLR7/8 agonist imiquimod, the TLR9 agonist CpG, adenoviral vectors, and the immunostimulatory complexes. Together, these studies have helped to unveil the requirement for certain types of immune responses at specific stages of CL disease and provide a basis to aid the design of effective second-generation vaccines for human CL. This review focuses on adjuvants that have been tested in experimental CL, outlining how they have helped advance our understanding of the disease and ultimately, how they have performed when applied within clinical trials against human CL.

Single dose novel Salmonella vaccine enhances resistance against visceralizing L. major and L. donovani infection in susceptible BALB/c mice

Visceral leishmaniasis is a major neglected tropical disease, with an estimated 500,000 new cases and more than 50,000 deaths attributable to this disease every year. Drug therapy is available but costly and resistance against several drug classes has evolved. Despite all efforts, no commercial, let alone affordable, vaccine is available to date. Thus, the development of cost effective, needle-independent vaccines is a high priority. Here, we have continued efforts to develop live vaccine carriers based on recombinant Salmonella. We used an in silico approach to select novel Leishmania parasite antigens from proteomic data sets, with selection criteria based on protein abundance, conservation across Leishmania species and low homology to host species. Five chosen antigens were differentially expressed on the surface or in the cytosol of Salmonella typhimurium SL3261. A two-step procedure was developed to select optimal Salmonella vaccine strains for each antigen, based on bacterial fitness and antigen expression levels. We show that vaccine strains of Salmonella expressing the novel Leishmania antigens LinJ08.1190 and LinJ23.0410 significantly reduced visceralisation of L. major and enhanced systemic resistance against L. donovani in susceptible BALB/c mice. The results show that Salmonella are valid vaccine carriers for inducing resistance against visceral leishmaniasis but that their use may not be suitable for all antigens.

Visceral leishmaniasis: immunology and prospects for a vaccine

Human visceral leishmaniasis (HVL) is the most severe clinical form of a spectrum of neglected tropical diseases caused by protozoan parasites of the genus Leishmania. Caused mainly by L. donovani and L. infantum/chagasi, HVL accounts for more than 50 000 deaths every year. Drug therapy is available but costly, and resistance against several drug classes has evolved. Here, we review our current understanding of the immunology of HVL and approaches to and the status of vaccine development against this disease.

Comparison of BCG, MPL and cationic liposome adjuvant systems in leishmanial antigen vaccine formulations against murine visceral leishmaniasis

Background

The development of an effective vaccine against visceral leishmaniasis (VL) caused by Leishmania donovani is an essential aim for controlling the disease. Use of the right adjuvant is of fundamental importance in vaccine formulations for generation of effective cell-mediated immune response. Earlier we reported the protective efficacy of cationic liposome-associated L. donovani promastigote antigens (LAg) against experimental VL. The aim of the present study was to compare the effectiveness of two very promising adjuvants, Bacille Calmette-Guerin (BCG) and Monophosphoryl lipid A (MPL) plus trehalose dicorynomycolate (TDM) with cationic liposomes, in combination with LAg, to confer protection against murine VL.

Results

All the three formulations afforded significant protection against L. donovani in both the visceral organs, liver and spleen. Although comparable level of protection was observed in BCG+LAg and MPL-TDM+LAg immunized mice, highest level of protection was exhibited by the liposomal LAg immunized group. Significant increase in anti-LAg IgG levels were detected in both MPL-TDM+LAg and liposomal LAg immunized animals with higher levels of IgG2a than IgG1. But BCG+LAg failed to induce any antibody response. As an index of cell-mediated immunity DTH responses were measured and significant response was observed in mice vaccinated with all the three different formulations. However, highest responses were observed with liposomal vaccine immunization. Comparative evaluation of IFN-γ and IL-4 responses in immunized mice revealed that MPL-TDM+LAg group produced the highest level of IFN-γ but lowest IL-4 level, while BCG+LAg demonstrated generation of suboptimum levels of both IFN-γ and IL-4 response. Elicitation of moderate levels of prechallenge IFN-γ along with optimum IL-4 corresponds with successful vaccination with liposomal LAg.

Conclusion

This comparative study reveals greater effectiveness of the liposomal vaccine for protection against progressive VL in BALB/c. Again, evaluation of the immune responses by vaccination emphasizes the need of stimulation of potent cellular immunity based on both Th1 and Th2 cell responses to confer protection against VL.

A review of adjuvants for Leishmania vaccine candidates

Over the last decade, there has been a flurry of research on adjuvants for vaccines, and several novel adjuvants are now licensed products or in late stage clinical development. The success of adjuvants in enhancing the immune response to antigens has led many researchers to re-focus their vaccine development programs. Although several vaccine candidates have been tested against leishmaniasis, there is yet no effective vaccine against this parasitic disease. Recent research has documented that efforts to develop effective Leishmania vaccine have been limited due to lack of an appropriate adjuvant. In view of this, this review paper outlines some of the adjuvants that have been used in Leishmania vaccine candidates and cites a few of the responses obtained from these studies. The aim of the present review is to consolidate these findings to facilitate the application of these adjuvants in general and experimental vaccinology.

The role of liposome charge on immune response generated in BALB/c mice immunized with recombinant major surface glycoprotein of Leishmania (rgp63)

Liposomes as a lipid-based system have been shown to be an effective adjuvant formulation. In this study, the role of liposome charge in induction of a Th1 type of immune response and protection against leishmaniasis in BALB/c mice was studied. Liposomes containing rgp63 were prepared by Dehydration-Rehydration Vesicle (DRV) method. Neutral liposomes consisted of dipalmitoylphosphatidylcholine and cholesterol. Positively and negatively charged liposomes were prepared by adding dimethyldioctadecylammonium bromide (DDAB) or dicetyl phosphate (DCP) to the neutral liposome formulation, respectively. Female BALB/c mice were immunized subcutaneously with negatively, positively charged or neutral liposomes encapsulated with rgp63, rgp63 in soluble form or PBS, three times in 3week intervals. The extent of protection and type of immune response generated were studied in different groups of mice. The group of mice immunized with rgp63 encapsulated in neutral liposomes showed a significantly (P<0.01) smaller footpad swelling upon challenge with Leishmania major compared with positively or negatively charged liposomes. The mice immunized with neutral liposomes also showed a significantly (P<0.01) the lowest splenic parasite burden, the highest IgG2a/IgG1 ratio and IFN-gamma production and the lowest IL-4 level compared to the other groups. The results indicated that a Th1 type of immune response was induced in mice immunized with neutral liposomes more efficiently than positively charged liposomes and conversely negatively charged liposomes induced a Th2 type of immune response.

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.

Transgenic, fluorescent Leishmania mexicana allow direct analysis of the proteome of intracellular amastigotes

Investigating the proteome of intracellular pathogens is often hampered by inadequate methodologies to purify the pathogen free of host cell material. This has also precluded direct proteome analysis of the intracellular, amastigote form of Leishmania spp., protozoan parasites that cause a spectrum of diseases that affect some 12 million patients worldwide. Here a method is presented that combines classic, isopycnic density centrifugation with fluorescent particle sorting for purification by exploiting transgenic, fluorescent parasites to allow direct proteome analysis of the purified organisms. By this approach the proteome of intracellular Leishmania mexicana amastigotes was compared with that of extracellular promastigotes that are transmitted by insect vectors. In total, 509 different proteins were identified by mass spectrometry and database search. This number corresponds to approximately 6% of gene products predicted from the reference genome of Leishmania major. Intracellular amastigotes synthesized significantly more proteins with basic pI and showed a greater abundance of enzymes of fatty acid catabolism, which may reflect their living in acidic habitats and metabolic adaptation to nutrient availability, respectively. Bioinformatics analyses of the genes corresponding to the protein data sets produced clear evidence for skewed codon usage and translational bias in these organisms. Moreover analysis of the subset of genes whose products were more abundant in amastigotes revealed characteristic sequence motifs in 3'-untranslated regions that have been linked to translational control elements. This suggests that proteome data sets may be used to identify regulatory elements in mRNAs. Last but not least, at 6% coverage the proteome identified all vaccine antigens tested to date. Thus, the present data set provides a valuable resource for selection of candidate vaccine antigens.

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.

Leishmania major: Immune response in BALB/c mice immunized with stress-inducible protein 1 encapsulated in liposomes

Protection against leishmaniasis is depending upon generation of a Th1 type of immune response. Field trials of first generation Leishmania vaccine showed a limited efficacy even with multiple doses mainly due to lack of an appropriate adjuvant. In this study, susceptible BALB/c mice were immunized with rLmSTI1 encapsulated in liposomes to explore the extent of protection induced by Leishmania antigen encapsulated in the liposomes against challenge with Leishmania major. The results showed that s.c. immunization of BALB/c mice with liposomal rLmSTI1 induced a significant protection against challenge and a significant lower parasite burden in spleen up to 14 weeks after challenge. The protected animals showed a significantly smaller footpad thickness after challenge, and a higher level of anti-SLA IgG antibodies before and after challenge with a predominant IgG2a titer. The data supports the possibility of using liposomal Leishmania antigens as a vaccine.

Mimetic membrane system to carry multiple antigenic proteins from Leishmania amazonensis

Liposomes have long been used as models for lipid membranes and for the reconstitution of a single or multiple proteins. Also, liposomes have adjuvant activity in vaccines against several protozoan or bacterial organisms. Thus, the main objective of the present study was to obtain a crude extract of detergent-solubilized proteins of Leishmania amazonensis amastigotes and reconstitute them into liposomes. Neutral and zwiterionic detergents were less efficient than an ionic detergent. In order to obtain efficient solubilization using only sodium dodecyl sulfate (SDS), the effects of detergent and protein concentration and incubation time were studied. The maximum of solubilized proteins was obtained instantaneously using a ratio of 0.5 mg/ml of protein to 0.1% (w/v) detergent at 4 degrees C. Dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylserine (DPPS) and cholesterol in a weight ratio of 5:1:4 were used for protein reconstitution into liposomes using the cosolubilization method, yielding 60% of incorporation. The incorporation of multiple parasite proteins results in a vesicular diameter of proteoliposomes of about 140 nm, presenting a final lipid weight ratio for DPPC, DPPS and cholesterol of 1:1:5, with high stability. The detergent-solubilized proteins of L. amazonensis amastigotes present in the proteoliposome, when analyzed by SDS-polyacrylamide gel electrophoresis, include a wide range of parasite-incorporated proteins. BALB/c mice inoculated with these proteoliposomes were able to produce antibodies against the proteins reconstituted in DPPC:DPPS:cholesterol liposomes and were partially resistant to infection with L. amazonensis promastigotes. These results indicate that this system can be used as a possible vaccine against L. amazonensis.

Recent advances in vaccines for leishmaniasis

The observation that recovery from infection with Leishmania confers immunity to reinfection suggests that control of leishmaniasis by vaccination may be possible. However, there are no vaccines available at present to control any form of leishmaniasis, despite considerable efforts. Studies of the immunopathogenesis and mechanisms of protective immunity, mainly derived from animal models of experimental leishmaniasis, have defined a number of features that should be met by an effective vaccine. In addition, several antigens have been identified that may be potential vaccine candidates, and molecular biological techniques have made them available as recombinant proteins for second-generation vaccines. Furthermore, molecules present in the saliva of Leishmania-transmitting vectors have been demonstrated as valuable candidates for the development of anti-Leishmania vaccines. This review concentrates on the most promising vaccine candidates and highlights new approaches for the development of vaccines. Finally, based on present knowledge, the future prospects for developing an effective vaccine against the different clinical forms of leishmaniasis are discussed.

Adjuvant guided polarization of the immune humoral response against a protective multicomponent antigenic protein (Q) from Leishmania infantum. A CpG + Q mix protects Balb/c mice from infection

It has been shown that vaccination with three doses of the Leishmania infantum poly-protein Q containing five genetically fused antigenic determinants from the Lip2a, Lip2b, H2A and P0 proteins, mixed with BCG induces clearance of parasites in 9 out of 10 Leishmania infantum-infected Beagle dogs, in addition to clinical protection. In the present paper we analysed the immunogenic potential of the poly-protein Q and the specificity and polarization of the response against the antigenic determinants of Q when mixed with various adjuvants. The data showed that the Q protein had high intrinsic immunogenic potential and that it was able to induce a long-lasting IgG response. The IgM immunogenic potential of the poly-protein was mainly due to the LiP2a and LiP2b determinants, whereas the IgG immunogenic potential was mainly due to the LiP2a component. It was observed that the protein itself elicited a mixed IgG2a/IgG1 response and that the determinants of Q were endowed with different IgG2a/IgG1 potential. It was also observed that the adjuvants did not influence the intensity or specificity of the IgM response but that they modulated the intensity, the specificity and the polarization of the IgG response against the determinants of Q. CpG-ODN motifs or double-stranded DNA plasmids containing CpG motifs when mixed with Q induced a predominant IgG2a response mainly observed at early stages post-immunization. The data showed that a CpG + Q mix induced significant protection against L. infantum infection in Balb/c mice.

Challenges facing adjuvants for cancer immunotherapy

An adjuvant is defined as a product that increases or modulates the immune response against an antigen (Ag). Based on this general definition many authors have postulated that the ideal adjuvant should increase the potency of the immune response, while being non-toxic and safe. Although dozens of different adjuvants have been shown to be effective in preclinical and clinical studies, only aluminium-based salts (Alum) and squalene-oil-water emulsion (MF59) have been approved for human use. However, for the development of therapeutic vaccines to treat cancer patients, the prerequisites for an ideal cancer adjuvant differ from conventional adjuvants for many reasons. First, the patients that will receive the vaccines are immuno-compromised because of, for example, impaired mechanisms of antigen presentation, non-responsiveness of activated T cells and enhanced inhibition of self-reactivity by regulatory T cells. Second, the tumour Ag are usually self-derived and are, therefore, poorly immunogenic. Third, tumours develop escape mechanisms to avoid the immune system, such as tumour editing, low or non-expression of MHC class I molecules or secretion of suppressive cytokines. Thus, adjuvants for cancer vaccines need to be more potent than for prophylactic vaccines and consequently may be more toxic and may even induce autoimmune reactions. In summary, the ideal cancer adjuvant should rescue and increase the immune response against tumours in immuno-compromised patients, with acceptable profiles of toxicity and safety. The present review discusses the role of cancer adjuvants at the different phases of the generation of antitumour immunity following vaccination.

Does the Leishmania major paradigm of pathogenesis and protection hold for New World cutaneous leishmaniases or the visceral disease?

Parasitic protozoa of the genus Leishmania have provided a useful perspective for immunologists in terms of host defense mechanisms critical for the resolution of infection caused by intracellular pathogens. These organisms, which normally reside in a late endosomal, major histocompatibility complex (MHC) class II(+) compartment within host macrophages cells, require CD4(+) T-cell responses for the control of disease. The paradigm for the CD4(+) T-helper 1 (Th1)/Th2 dichotomy is largely based on the curing/non-curing responses, respectively, to Leishmania major infection. However, this genus of parasitic protozoa is evolutionarily diverse, with the cutaneous disease-causing organisms of the Old World (L. major) and New World (Leishmania mexicana/ Leishmania amazonensis) having diverged 40-80 million years ago. Further adaptations to survive within the visceral organs (for Leishmania donovani, Leishmania chagasi, and Leishmania infantum) must have been required. Consequently, significant differences in host-parasite interactions have evolved. Different virulence factors have been identified for distinct Leishmania species, and there are profound differences in the immune mechanisms that mediate susceptibility/resistance to infection and in the pathology associated with disease. These variations not only point to interesting features of the host-pathogen interaction and immunobiology of this genus of parasitic protozoa, but also have important implications for immunotherapy and vaccine development.

Identification and molecular characterization of a gene encoding a protective Leishmania amazonensis Trp-Asp (WD) protein

Several Leishmania proteins have been identified and characterized in pursuit of understanding pathogenesis and protection in cutaneous leishmaniasis. In the present study, we utilized sera from infected BALB/c mice to screen a Leishmania amazonensis amastigote cDNA expression library and obtained the full-length gene that encodes a novel Trp-Asp (WD) protein designated LAWD (for Leishmania antigenic WD protein). The WD family of proteins mediates protein-protein interactions and coordinates the formation of protein complexes. The single-copy LAWD gene is transcribed as a approximately 3.1-kb mRNA in both promastigotes and amastigotes, with homologues being detected in several other Leishmania species. Immunoelectron microscopy revealed a predominant localization of the LAWD protein in the flagellar pocket. Analyses of sera from human patients with cutaneous and mucocutaneous leishmaniasis indicated that these individuals mounted significant humoral responses against LAWD. Given that recombinant LAWD protein elicited the production of high levels of gamma interferon, but no detectable levels of interleukin-10 (IL-10), in CD4(+) cells of L. amazonensis-infected mice, we further examined whether LAWD could elicit protective immunity. DNA vaccination with the LAWD and IL-12 genes significantly delayed lesion development, which correlated with a dramatic reduction in parasite burdens. Thus, we have successfully identified a promising vaccine candidate and antigenic vehicle to aid in the dissection of the complicated pathogenic immune response of L. amazonensis.

DNA immunization with the gene encoding P4 nuclease of Leishmania amazonensis protects mice against cutaneous Leishmaniasis

Infection with the protozoan parasite Leishmania amazonensis can cause diverse clinical forms of leishmaniasis. Immunization with purified P4 nuclease protein has been shown to elicit a protective response in mice challenged with L. amazonensis and L. pifanoi. To explore the potential of a DNA-based vaccine, we tested the L. amazonensis gene encoding P4 nuclease as well as adjuvant constructs encoding murine interleukin-12 (IL-12) and L. amazonensis HSP70. Susceptible BALB/c mice were immunized with the DNA encoding P4 alone, P4/IL-12, or P4/HSP70 prior to challenge with L. amazonensis promastigotes. Mice given P4/IL-12 exhibited no lesion development and had a 3- to 4-log reduction in tissue parasite burdens compared to controls. This protection corresponded to significant increases in gamma interferon and tumor necrosis factor alpha production and a reduction in parasite-specific immunoglobulin G1, suggesting an enhancement in Th1 responses. Moreover, we immunized mice with the L. amazonensis vaccines to determine if this vaccine regimen could provide cross-protection against a genetically diverse species, L. major. While the P4/HSP70 vaccine led to self-healing lesions, the P4/IL-12 vaccine provided negligible protection against L. major infection. This is the first report of successful use of a DNA vaccine to induce protection against L. amazonensis infection. Additionally, our results indicate that different vaccine combinations, including DNA encoding P4, HSP70, or IL-12, can provide significant protection against both Old World and New World cutaneous leishmaniasis.

Development of a leishmaniasis vaccine: the importance of MPL

The parasite Leishmania has been used for pioneering work to define T-cell subsets and cytokine patterns mediating susceptibility or resistance to infectious pathogens. This understanding has been essential for the development of a new generation of candidate vaccines for major diseases, such as leishmaniases themselves, tuberculosis and others. It is clear that effective vaccines can be developed through a combination of both antigen and adjuvant selection. Until recently, no adjuvants acceptable for use in human T-cell vaccines were available. However, one such adjuvant, monophosphoryl lipid A, has been shown to be safe and effective. Just as the understanding of T-cell responses has been necessary for the development of a new generation of vaccines, an understanding of signaling by antigen-presenting cells has been essential for adjuvant selection. A combination of antigens and an adjuvant that is effective at promoting durable T-helper 1 responses and is safe for human use comprise a promising vaccine candidate, Leish-111f. This vaccine has potential application in both the prevention and treatment of leishmaniasis.

Leishmania model for microbial virulence: the relevance of parasite multiplication and pathoantigenicity

Leishmanial mechanisms of virulence have been proposed previously to involve two different groups of parasite molecules. One group consists of largely surface and secretory products, and the second group includes intracellular molecules, referred to as 'pathoantigens'. In the first group are invasive/evasive determinants, which protect not only parasites themselves, but also infected host cells from premature cytolysis. These determinants help intracellular amastigotes maintain continuous infection by growing at a slow rate in the parasitophorous vacuoles of host macrophages. This is illustrated in closed in vitro systems, e.g. Leishmania amazonensis in macrophage cell lines. Although individual macrophages may become heavily parasitized at times, massive destruction of macrophages has not been observed to result from uncontrolled parasite replication. This is thus unlikely to be the direct cause of virulence manifested as the clinical symptoms seen in human leishmaniasis. Of relevance is likely the second group of immunopathology-causing parasite 'pathoantigens'. These are highly conserved cytoplasmic proteins, which have been found to contain Leishmania-unique epitopes immunologically active in leishmaniasis. How these intracellular parasite antigens become exposed to the host immune system is accounted for by periodic cytolysis of the parasites during natural infection. This event is notable with a small number of parasites, even as they grow in an infected culture. The cytolysis of these parasites to release 'pathoantigens' may be inadvertent or medicated by specific mechanisms. Information on the pathoantigenic epitopes is limited. T-cell epitopes have long been recognized, albeit ill-defined, as important in eliciting CD4+ cell development along either the Th1 or Th2 pathway. Their operational mechanisms in suppressing or exacerbating cutaneous disease are still under intensive investigation. However, immune response to B-cell epitopes of such 'pathoantigens' is clearly futile and counterproductive. Their intracellular location within the parasites renders them inaccessible to the specific antibodies generated. One example is the Leishmania K39 epitope, against which antibodies are produced in exceedingly high titers, especially in Indian kala-azar. Here, we consider the hypothetical emergence of this pathoantigenicity and its potential contributions to the virulent phenotype in the form of immunopathology. Microbial virulence may be similarly explained in other emerging and re-emerging infectious diseases. Attenuation of microbial virulence may be achieved by genetic elimination of pathoantigenicity, thereby providing mutants potentially useful as avirulent live vaccines for immunoprophylasis of infectious diseases.

The Leishmania mexicana cysteine protease, CPB2.8, induces potent Th2 responses

We have previously identified that Leishmania mexicana cysteine proteases (CPs) are virulence factors. We have now produced a recombinant L. mexicana CP, CPB2.8, which has similar enzymatic activity to native enzyme. Inoculation of CPB2.8 (< or =5 microg) into the footpads of BALB/c mice not only up-regulated mRNA transcripts for IL-4 and IL-4 production in the draining popliteal lymph nodes, but also polarized splenocyte anti-CD3 stimulated responses toward a Th2 bias as measured by increased IL-5 production compared with controls. In agreement with promoting a Th2 response, CPB2.8 also induced strong specific IgE responses in treated mice as well as increasing whole IgE levels. Inhibition of the enzyme activity of CPB2.8 by treatment with E-64 ablated the enzyme's ability to induce IgE. Significantly, infection of mice with CPB-deficient parasites failed to stimulate production of IgE, unlike infection with wild-type parasites. Furthermore, enzymatically active (<0.1 U/ml) but not E-64-inactivated CPB2.8 was able to proteolytically cleave CD23 and CD25, although not B220 or CD4 from murine lymphocytes. These properties are similar to those demonstrated by the house dust mite allergen Der p I and provide an explanation for the immunomodulatory activity of the CPB2.8 virulence factor. Vaccination with CPB2.8 enhanced L. mexicana lesion growth compared with control animals. Nevertheless, vaccination with IL-12 and CPB2.8 resulted in a degree of protection associated with inhibition of lesion growth and a Th1 response. Thus, CPB2.8 is a potent Th2-inducing molecule capable of significant vaccine potential if administered with a suitable adjuvant.

Induction of antigen-specific CTL responses using antigens conjugated to short peptide vectors

Linear peptides (SynB vectors) with specific sequence motifs have been identified that are capable of enhancing the transport of a wide range of molecules into cells. These peptide vectors have been used to deliver exogenous peptides and protein Ags across the cell membrane and into the cytoplasm of cells. Specifically, in vitro analysis indicated that these SynB peptides enhanced the uptake of two 9-mer peptide Ags, NP(147-155) and Mtb(250-258) (T cell epitopes of influenza nucleoprotein and Mycobacterium tuberculosis, respectively) and the M. tuberculosis Ag Mtb8.4 protein, into K562 cells when covalently linked to the respective Ags. Furthermore, selected SynB vectors, when conjugated to these same Ags and used as immunogens, resulted in considerably enhanced Ag-specific CTL responses. Several SynB vectors were tested and resulted in varying levels of cellular uptake. The efficiency of uptake correlated with the ability of the SynB construct to deliver each epitope in vivo and induce specific CTL responses in mice. These data suggest that peptide vectors, such as SynB that transport target Ags across the cell membrane in a highly efficient manner, have significant potential for vaccine delivery.

Vaccination against cutaneous leishmaniasis: current status

The different cutaneous leishmaniases are distinct in their etiology, epidemiology, transmission, and geographical distribution. In most instances cutaneous leishmaniasis is limited to one or a few skin ulcers that develop at the site where the parasites were deposited during the bite of the sandfly vector. Lesions typically heal spontaneously after several months but some lesions can be large and follow a chronic, more severe course. Protective immunity is usually acquired following cutaneous infection with Leishmania spp., so prevention of disease through prophylactic immunization appears to be feasible. Since vaccination with live, virulent parasites is associated with an unacceptable rate of adverse events, attention has turned to the use of killed or attenuated parasite vaccines and defined subunit vaccines. Whole parasite vaccines have the advantage of delivering multiple antigenic epitopes that may be necessary for initiation of a broad-based immune response. Persistent or repeated immune-stimulation by parasite antigens and/or sustained expression of interleukin-12 appear to be critical elements in the development of durable immunity. A number of purified or recombinant antigens, when co-administered with a vaccine adjuvant, appear promising as vaccine candidates against cutaneous leishmaniasis. The sustained expression of recombinant Leishmania antigens by vaccination with DNA is an attractive approach because it mimics the persistent antigenic stimulation of subclinical infection. Effective vaccine-induced immunity must generate an antigen-specific memory T cell population that, upon exposure to the infecting parasite, rapidly produces a type 1 effector T cell response that leads to interferon-gamma-mediated activation of infected macrophages to kill the intracellular parasites. This parasite-directed recall response must be prompt and of sufficient magnitude to overcome the subversive effect that the intracellular infection has on macrophage effector function. It is unlikely that vaccination against cutaneous leishmaniasis would induce sterile immunity, but a small number of parasites are likely to persist subclinically.

Saponins, IL12 and BCG adjuvant in the FML-vaccine formulation against murine visceral leishmaniasis

The FML antigen of Leishmania donovani, in combination with either Riedel de Haën (R), QuilA, QS21 saponins, IL12 or BCG, was used in vaccination of an outbred murine model against visceral leishmaniasis (VL). Significant and specific increases in anti-FML IgG and IgM responses were detected for all adjuvants, and in anti-FML IgG1, IgG2a and IgG2b and delayed type of hypersensitivity to L. donovani lysate (DTH), only for all saponins and IL12. The QS21-FML and QuilA-FML groups achieved the highest IgG2a response. QuilA-FML developed the strongest DTH and QS21-FML animals showed the highest serum IFN-gamma concentrations. The reduction of parasitic load in the liver in response to each FML-vaccine formulation was: 52% (P<0.025) for BCG-FML, 73% (P<0.005) for R-FML, 93% (P<0.005) for QuilA-FML and 79.2% (P<0.025) for QS21-FML treated animals, respectively. Protection was specific for R-FML and QS21-FML while the QuilA saponin treatment itself induced 69% of LDU reduction. The FML-saponin vaccines promote significant, specific and strong protective effects against murine visceral leishmaniasis. BCG-FML induced minor and non-specific protection while IL12-FML, although enhancing the specific antibody and IDR response, failed to reduce the parasitic load of infected animals.

Immunization with a polyprotein vaccine consisting of the T-Cell antigens thiol-specific antioxidant, Leishmania major stress-inducible protein 1, and Leishmania elongation initiation factor protects against leishmaniasis

Development of an effective vaccine against Leishmania infection is a priority of tropical disease research. We have recently demonstrated protection against Leishmania major in the murine and nonhuman primate models with individual or combinations of purified leishmanial recombinant antigens delivered as plasmid DNA constructs or formulated with recombinant interleukin-12 (IL-12) as adjuvant. In the present study, we immunized BALB/c mice with a recombinant polyprotein comprising a tandem fusion of the leishmanial antigens thiol-specific antioxidant, L. major stress-inducible protein 1 (LmSTI1), and Leishmania elongation initiation factor (LeIF) delivered with adjuvants suitable for human use. Aspects of the safety, immunogenicity, and vaccine efficacy of formulations with each individual component, as well as the polyprotein referred to as Leish-111f, were assessed by using the L. major challenge model with BALB/c mice. No adverse reactions were observed when three subcutaneous injections of the Leish-111f polyprotein formulated with either MPL-squalene (SE) or Ribi 529-SE were given to BALB/c mice. A predominant Th1 immune response characterized by in vitro lymphocyte proliferation, gamma interferon production, and immunoglobulin G2A antibodies was observed with little, if any, IL-4. Moreover, Leish-111f formulated with MPL-SE conferred immunity to leishmaniasis for at least 3 months. These data demonstrate success at designing and developing a prophylactic leishmaniasis vaccine that proved effective in a preclinical model using multiple leishmanial antigens produced as a single protein delivered with a powerful Th1 adjuvant suitable for human use.

Recombinant cysteine proteinases-based vaccines against Leishmania major in BALB/c mice: the partial protection relies on interferon gamma producing CD8(+) T lymphocyte activation

Together with poloxamer 407 as adjuvant the recombinant type I (rCPB) or type II (rCPA) cysteine proteinases of Leishmania major were screened as potential vaccines against L. major in a mouse model. The vaccines were delivered subcutaneously twice at 3 weeks intervals. Three weeks after booster injection, 5x10(5) stationary phase L. major promastigotes were inoculated subcutaneously in one footpad. Using the footpad thickness increase to monitor the clinical outcome/cutaneous lesion at site of L. major delivery, it was possible to document that rCPB but not rCPA allowed BALB/c mice to mount a partial protective response: indeed over the period under study (weeks 1-12) a clear delay was noticed after the immunization with rCPB. This partial protective effect was no more detectable if CD8 depleting antibody was given intravenously to rCPB-immunized mice, at the time of parasite challenge. Seven weeks after challenge, the draining lymph nodes were monitored for their frequencies of IFN-gamma positive CD4(+) and CD8(+) T lymphocytes using PMA and ionomycin as re-activating signals: interestingly the partial protection achieved in BALB/c mice immunized with rCPB together with poloxamer was correlated only to one immunological parameter, namely the higher frequency of IFN-gamma producing CD8(+) T lymphocytes. Of note also, in the lymph node draining the L. major-loaded footpad of C57BL/6 mice otherwise known to develop a transient lesion, the frequency of IFN-gamma producing CD8(+) T lymphocytes reach similar value 7 weeks after challenge and in absence of any prior immunization. Taken together, it was shown that the induced partial protection was mainly dependent on IFN-gamma producing CD8(+) T cells.

Reduced infectivity of a Leishmania donovani biopterin transporter genetic mutant and its use as an attenuated strain for vaccination

Pterins are essential for the growth of Leishmania species, and recent work has led to the isolation of the biopterin transporter BT1. In this study, we inactivated the Leishmania donovani biopterin transporter BT1 by gene disruption mediated by homologous recombination. No transport of biopterin was detected in this mutant. The L. donovani BT1 null mutant showed a much lesser capacity for inducing infection in mice than wild-type parasites and could elicit protective immunity in mice susceptible to infection against a L. donovani challenge. Splenocytes isolated from mice immunized with the BT1 null mutant parasites produced significant amounts of interferon gamma following stimulation with L. donovani promastigotes as measured by enzyme-linked immunosorbent assay and enzyme-linked immunospot assays. Overall, these results show that by genetically manipulating the pterin transport in L. donovani, it is possible to generate an attenuated organism that could be part of a vaccination strategy.

A vaccination strategy incorporating DNA priming and mucosal boosting using tetanus toxin fragment C (TetC)

Intramuscular (i.m.) immunisation of BALB/c mice with a DNA vaccine, pcDNA3/tetC, encoding fragment C (TetC) from tetanus toxin, stimulated production of TetC specific IgG2a antibodies in the serum and release of IFN-gamma from TetC stimulated splenocytes. A similar pattern of immune response was detected if pcDNA3/tetC primed mice were boosted i.m. with purified TetC protein or TetC and cholera toxin (included as an adjuvant). In contrast, control mice primed with the empty DNA vector pcDNA3 and boosted i.m. with TetC or TetC and CT, generated a dominant IgG1 specific anti-TetC response in the sera and low or undetectable levels of IFN-gamma from stimulated splenocytes. Thus, i.m. priming with a DNA vaccine modulated the subsequent immune response to the same antigen administered as a protein boost. Similar observations were made when DNA primed mice were boosted using the intranasal mucosal route of immunisation. Interestingly, although mice immunised with pcDNA3/tetC and boosted mucosally with TetC and CT produced anti-TetC IgA in mucosal secretions, the titres were reproducibly lower than those detected in mice immunised with the pcDNA3 vector alone. The immunomodulatory effect of pcDNA3/tetC appeared to be antigen specific as mucosal boosting with an unrelated antigen (pertactin) revealed no significant modulation in terms of the anti-pertactin immune response.

An update on antileishmanial vaccine candidates and prospects for a canine Leishmania vaccine

Dogs are the domestic reservoir for Leishmania infantum, the parasite causing zoonotic visceral leishmaniasis (VL) in both the Old and New Worlds. Since the available methods for canine leishmaniasis treatment and control have limited efficacy, the development of a canine Leishmania vaccine is highly desirable. Mechanisms of antileishmanial immune responses in murine, human, and canine infections are briefly presented. Vaccine candidates, including live or killed parasites, Leishmania purified fractions, defined recombinant parasite antigens, live recombinant bacteria expressing Leishmania antigens and antigen-encoding DNA plasmids, are reviewed. Finally, some practical requirements for the evaluation of vaccine candidates in dogs are indicated.

Protection against cutaneous leishmaniasis induced by recombinant antigens in murine and nonhuman primate models of the human disease

Leishmaniasis affects approximately 2 million people each year throughout the world. This high incidence is due in part to the lack of an efficacious vaccine. We present evidence that the recombinant leishmanial antigens LmSTI1 and TSA, which we identified and characterized previously, induce excellent protection in both murine and nonhuman primate (rhesus monkey) models of human cutaneous leishmaniasis. The remarkable protection induced by LmSTI1 and TSA in an animal model that is evolutionarily close to humans qualifies this antigen combination as a promising candidate subunit vaccine against human leishmaniasis.

Vaccination of Balb/c mice against experimental visceral leishmaniasis with the GP36 glycoprotein antigen of Leishmania donovani

Leishmania donovani GP36 glycoprotein is the main antigen of the FML Fucose Mannose Ligand (FML) complex specifically recognized by sera of kala-azar human patients. The GP36 was isolated by chemical elution + sonication and used for Balb/c mouse vaccination in combination with saponin, by the s.c. route, inducing a strong and specific protective effect against experimental visceral leishmaniasis shown by the increase of: specific IgG antibodies (82.6%), mainly IgG2a, the delayed type of hypersensitivity to promastigote lysate (37.8%, P < 0.001), the in vitro cellular proliferative response to GP36 of ganglia lymphocytes (53.5%, P < 0.005) and the decrease of liver parasite burden (68.1%, P < 0.025). Saponin treated controls reacted significantly differently from GP36 vaccinated animals at all the assayed variables (P < 0.05). GP36 induced significant protection against murine visceral leishmaniasis at concentrations commonly used for vaccination with recombinant antigens.

Leishmaniasis: current status of vaccine development

Leishmaniae are obligatory intracellular protozoa in mononuclear phagocytes. They cause a spectrum of diseases, ranging in severity from spontaneously healing skin lesions to fatal visceral disease. Worldwide, there are 2 million new cases each year and 1/10 of the world's population is at risk of infection. To date, there are no vaccines against leishmaniasis and control measures rely on chemotherapy to alleviate disease and on vector control to reduce transmission. However, a major vaccine development program aimed initially at cutaneous leishmaniasis is under way. Studies in animal models and humans are evaluating the potential of genetically modified live attenuated vaccines, as well as a variety of recombinant antigens or the DNA encoding them. The program also focuses on new adjuvants, including cytokines, and delivery systems to target the T helper type 1 immune responses required for the elimination of this intracellular organism. The availability, in the near future, of the DNA sequences of the human and Leishmania genomes will extend the vaccine program. New vaccine candidates such as parasite virulence factors will be identified. Host susceptibility genes will be mapped to allow the vaccine to be targeted to the population most in need of protection.

Distribution of GPI-anchored proteins in the protozoan parasite Leishmania, based on an improved ultrastructural description using high-pressure frozen cells

The cellular distribution of two glycosyl-phosphatidylinositol (GPI)-anchored proteins and a trans-membrane protein and the compartments involved in their trafficking were investigated in the insect stage of Leishmania mexicana, which belongs to the phylogenetically old protozoan family Trypanosomatidae. Electron microscopy of sections from high-pressure frozen and freeze-substituted cells allowed a detailed description of exo- and endocytic structures located in the vesicle-rich, densely packed anterior part of the spindle-shaped cell. A complex of tubular clusters/translucent vesicles is the prominent structure between the trans-side of the single Golgi apparatus and the flagellar pocket, the only site of endo- and exocytosis. A tubulovesicular compartment lined by one or two distinct microtubules and extending along the length of the cell is proposed to be a post-Golgi and probably late endosomal/lysosomal compartment. Using biotinylation experiments, FACS analysis and quantitative immunoelectron microscopy it was found that, at comparable expression levels, 73–75% of the two GPI-anchored proteins but only 13% of the trans-membrane protein are located on the cell surface. The tubulovesicular compartment contains 46%, the ER 5%, the Golgi complex 1.9% and the tubular cluster/translucent vesicle complex 3.6% of the intracellular fraction of the GPI-anchored protease, GP63. The density of GP63 was found to be 23-fold higher on the plasma/flagellar pocket membrane than on the ER and about tenfold higher than on membranes of the Golgi complex or of endo- or exocytic vesicles. These results indicate that there is a considerable concentration gradient of GPI-anchored proteins between the plasma/flagellar pocket membrane and the ER as well as structures involved in exo- or endocytosis. Possible mechanisms how this concentration gradient is established are discussed.