Failure of a multi-subunit recombinant leishmanial vaccine (MML) to protect dogs from Leishmania infantum infection and to prevent disease progression in infected animals

The development and clinical evaluation of second-generation leishmaniasis vaccines

Infection with Leishmania parasites results in a range of clinical manifestations and outcomes. Control of Leishmania parasite transmission is extremely difficult due to the large number of vectors and potential reservoirs, and none of the current treatments are ideal. Vaccination could be an effective strategy to provide sustained control. In this review, the current global situation with regard to leishmaniasis, the immunology of Leishmania infection and various efforts to identify second generation vaccine candidates are briefly discussed. The variety of clinical trials conducted using the only current second generation vaccine approved for clinical use, LEISH-F1+MPL-SE, are described. Given that epidemiological evidence suggests that reducing the canine reservoir also positively impacts human incidence, efforts at providing a vaccine for leishmaniasis in dogs are highlighted. Finally, potential refinements and surrogate markers that could expedite the introduction of a vaccine that can limit the severity and incidence of leishmaniasis are discussed.

Leishmaniasis

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

Development of Vaccines against Visceral Leishmaniasis

Leishmaniasis is a neglected disease resulting in a global morbidity of 2,090 thousand Disability-Adjusted Life Years and a mortality rate of approximately 60,000 per year. Among the three clinical forms of leishmaniasis (cutaneous, mucosal, and visceral), visceral leishmaniasis (VL) accounts for the majority of mortality, as if left untreated VL is almost always fatal. Caused by infection with Leishmania donovani or L. infantum, VL represents a serious public health problem in endemic regions and is rapidly emerging as an opportunistic infection in HIV patients. To date, no vaccine exists for VL or any other form of leishmaniasis. In endemic areas, the majority of those infected do not develop clinical symptoms and past infection leads to robust immunity against reinfection. Thus the development of vaccine for Leishmania is a realistic public health goal, and this paper summarizes advances in vaccination strategies against VL.

Leishmaniasis Vaccine: Where are We Today?

Leishmaniasis is a disease that ranges in severity from skin lesions to serious disfigurement and fatal systemic infection. WHO has classified the disease as emerging and uncontrolled and estimates that the infection results in two million new cases a year. There are 12 million people currently infected worldwide, and leishmaniasis threatens 350 million people in 88 countries. Current treatment is based on chemotherapy, which relies on a handful of drugs with serious limitations such as high cost, toxicity, difficult route of administration and lack of efficacy in endemic areas. Vaccination remains the best hope for control of all forms of the disease, and the development of a safe, effective and affordable antileishmanial vaccine is a critical global public-health priority. Extensive evidence from studies in animal models indicates that solid protection can be achieved by immunization with defined subunit vaccines or live-attenuated strains of Leishmania. However, to date, no such vaccine is available despite substantial efforts by many laboratories. The major impediment in vaccine design is the translation of data from animal models to human disease, and the transition from the laboratory to the field. Furthermore, a thorough understanding of protective immune responses and generation and maintenance of the immunological memory, the most important and least-studied aspect of antiparasitic vaccine development, during Leishmania infection is needed. This review focuses on recent findings in antileishmania vaccine field and highlights current difficulties facing vaccine development and implementation.

KSAC, the first defined polyprotein vaccine candidate for visceral leishmaniasis

A subunit vaccine using a defined antigen(s) may be one effective solution for controlling leishmaniasis. Because of genetic diversity in target populations, including both dogs and humans, a multiple-antigen vaccine will likely be essential. However, the cost of a vaccine to be used in developing countries must be considered. We describe herein a multiantigen vaccine candidate comprised of antigens known to be protective in animal models, including dogs, and to be recognized by humans immune to visceral leishmaniasis. The polyprotein (KSAC) formulated with monophosphoryl lipid A, a widely used adjuvant in human vaccines, was found to be immunogenic and capable of inducing protection against Leishmania infantum, responsible for human and canine visceral leishmaniasis, and against L. major, responsible for cutaneous leishmaniasis. The results demonstrate the feasibility of producing a practical, cost-effective leishmaniasis vaccine capable of protecting both humans and dogs against multiple Leishmania species.

Efficacy of Leishmania donovani ribosomal P1 gene as DNA vaccine in experimental visceral leishmaniasis

The acidic ribosomal proteins of the protozoan parasites have been described as prominent antigens during human disease. We present here data showing the molecular cloning and protective efficacy of P1 gene of Leishmania donovani as DNA vaccine. The PCR amplified complete ORF cloned in either pQE or pVAX vector was used either as peptide or DNA vaccine against experimentally induced visceral leishmaniasis in hamsters. The recombinant protein rLdP1 was given along with Freund's adjuvant and the plasmid DNA vaccine, pVAX-P1 was used alone either as single dose or double dose (prime and boost) in different groups of hamsters which were subsequently challenged with a virulent dose of 1×10(7) L. donovani (MHOM/IN/DD8/1968 strain) promastigotes by intra-cardiac route. While the recombinant protein rLdP1 or DNA vaccine pVAX-P1 in single dose format were not found to be protective, DNA vaccine in a prime-boost mode was able to induce protection with reduced mortality, a significant (75.68%) decrease in splenic parasite burden and increased expression of Th1 type cytokines in immunized hamsters. Histopathology of livers and spleens from these animals showed formation of mature granulomas with compact arrangement of lymphocytes and histiocytes, indicating its protective potential as vaccine candidate.

Gentamicin-attenuated Leishmania infantum: cellular immunity production and protection of dogs against experimental canine leishmaniasis

An attenuated line of Leishmania infantum (L. infantum H-line) has been established by culturing promastigotes in vitro under gentamicin pressure. Here, we show that L. infantum H-line induced significantly higher levels of IFN-γ and lower levels of IL-10 compared with those in dogs infected with L. infantum wild type (WT). Anti-Leishmania-specific total IgG, IgG1, and IgG2 antibodies were present in the serum of all infected dogs, with levels of IgG2 subclass highest in the sera of dogs inoculated with L. infantum H-line. Relatively high levels of IgG1 were found in the sera of dogs infected with L. infantum WT. Six of seven dogs immunized intradermally (i.d.) with the attenuated line later showed a positive skin test to leishmanin, whereas the dogs infected with L. infantum WT did not. No clinical abnormalities were observed, and no parasites found in the visceral organs of the dogs inoculated intravenously (i.v.) with L. infantum H-line over 24 months post-inoculation. Dogs which had been immunized with L. infantum H-line i.d. 12 months previously were protected against challenge with L. infantum WT. These data suggest that the L. infantum H-line was safe and induced a protection which is correlated with cellular immunity in dogs.

Immunity to Leishmania and the rational search for vaccines against canine leishmaniasis

The control of infection by Leishmania infantum (syn. Leishmania chagasi) in dogs is essential to stop the current spread of zoonotic visceral leishmaniasis. The past few years have seen significant advances in achieving efficient immunization of dogs and, more than ever before, an effective vaccine against canine leishmaniasis can now be considered a feasible goal. This article summarizes experimental data gathered from recent dog trials aimed at identifying immunological mechanisms implicated in protection against canine infection to discuss their potential to serve as quantitative surrogate markers of immunization and, more importantly, its usefulness to evaluate whether the immunity induced by the vaccine candidate is strong enough to protect against canine leishmaniasis.

Treatment of canine visceral leishmaniasis by the vaccine Leish-111f+MPL-SE

Immunotherapy of canine visceral leishmaniasis (CVL) may provide an alternative to both marginally effective chemotherapy and undesired euthanasia of infected dogs and could have a great impact not only on animal welfare, but also on control of human disease. Therefore, we examined the potential immunotherapeutic efficacy of the subunit vaccine Leish-111f+MPL-SE, which has undergone rigorous preclinical testing and been demonstrated safe in human clinical trials. Two separate trials were performed in Salvador, Brazil, to evaluate the vaccine for therapeutic efficacy against CVL caused by natural infection: an Open Trial and a Blinded Trial. In the Open Trial 59 dogs with clinically active CVL were sequentially allocated to four groups: group 1 received Leish-111f+MPL-SE; group 2 was treated with Glucantime; group 3 received a combination of the vaccine and Glucantime; and group 4 was given no treatment. At the 6-month assessment, the 13 non-treated dogs had either died or showed no clinical improvement. In contrast, most dogs in groups 1-3 showed initial improvement (100%, 80%, and 92%, respectively). Upon evaluation for a mean of 36 months after therapy, the following cure rates were observed: 75% for group 1 dogs (exact 95% confidence interval [CI] 43-95%), 64% for group 2 dogs (exact 95% CI 31-89%), and 50% for group 3 dogs (exact 95% CI 19-81%). Therapeutic efficacy of the Leish-111f+MPL-SE vaccine was reconfirmed in a subsequent Blinded Trial. The vaccine was effective for mild cases of CVL and was compromised in dogs with severe disease. Although further studies are required to understand mechanisms of action, the Leish-111f+MPL-SE vaccine is a promising tool to control VL in both dogs and humans.

Intranasal immunization with Leish-111f induces IFN-gamma production and protects mice from Leishmania major infection

The mucosal vaccination is a non-invasive alternative approach for not only mucosal pathogens but also parenteral pathogens, since it induces both mucosal and systemic immunoreactions. The purpose of this study was to evaluate the application of intranasal (i.n.) immunization with a recombinant leishmanial protein against Leishmania infection. BALB/c mice were i.n. administered 1-3 times with Leish-111f plus cholera toxin (CT) adjuvant (Leish-111f/CT). Splenocytes from i.n. immunized mice produced high level of IFN-gamma but not IL-4 in response to Leish-111f. When infected with 1x10(6) of Leishmania major promastigotes 2 weeks after the final administration, lesion development was completely controlled in all mice i.n. administered with Leish-111f/CT. Mice i.n. administered with Leish-111f alone showed neither cytokine productions nor lesion control even after 6 administrations, suggesting the importance of CT adjuvant. This report demonstrated for the first time that i.n. administration of a recombinant leishmanial protein induces Th1 type immunity and protects mice from Leishmania infection.

Comparison of liposome based antigen delivery systems for protection against Leishmania donovani

Liposomes have been widely exploited as antigen delivery systems for a variety of diseases including leishmaniasis. These vesicles can be prepared in various ways which may affect the immunogenicity of the encapsulated antigens. In this study we compared the vaccine potentiality of three cationic formulations with Leishmania donovani promastigote membrane antigens (LAg) and the best vesicle was evaluated for long-term protection against experimental visceral leishmaniasis. We immunized mice with LAg encapsulated in multilamellar vesicles (MLV), dehydration-rehydration vesicles (DRV) and reverse-phase evaporation vesicles (REV) and challenged them with parasites ten days after vaccination. LAg in MLV or DRV induced almost complete protection, while LAg alone or entrapped in REV exhibited partial resistance. Protection observed with antigen incorporated MLV or DRV was predominantly Th1 as evidenced by elicitation of significantly high DTH, IgG2a antibodies and IFN-gamma. MLV encapsulated LAg demonstrated durable cell-mediated immunity and mice challenged ten weeks after vaccination could also resist experimental challenge strongly. Field trials of L. donovani vaccine were unsatisfactory mainly due to lack of an appropriate adjuvant. Cationic MLV when used as adjuvant with protein antigens induced sustained Th1 immunity. Adjuvant potential of cationic MLV can be utilized to design subunit vaccines.

Immunotherapy with the saponin enriched-Leishmune vaccine versus immunochemotherapy in dogs with natural canine visceral leishmaniasis

Leishmune, the first licensed vaccine for prophylaxis against canine visceral leishmaniasis (CVL) and is also immunotherapeutic when used with double saponin adjuvant concentration. The Leishmune therapeutic vaccine was assessed for immunotherapy (IT) in 31 infected dogs and for immunochemotherapy (ICT) in combination with allopurinol or amphotericinB/allopurinol, in 35 dogs. Compared to infected untreated control dogs, at month 3, both treatments increased the proportion of dogs showing intradermal response to Leishmania antigen to a similar extent (from 8 to 67%, in the IT and to 76%, in the ICT groups), and conversely reduced from 100 to 38% (IT) and to 18% (ICT) the proportion of symptomatic cases, from 54 to 12% (IT) and to 15% (ICT) the proportion of parasite evidence in lymph nodes and from 48 to 19% (IT) and 12% (ICT) the proportion of deaths, indicating that the immunotherapy with enriched-Leishmune vaccine promotes the control of the clinical and parasitological signs of CVL rendering most dogs asymptomatic although PCR positive. By month 8, negative lymph node PCR results were obtained in 80% of the ICT-treated dogs, but only in 33% of the IT group (p=0.0253), suggesting that the combination of additional chemotherapy with Leishmune-enriched saponin vaccination abolished, not only the symptoms but also the latent infection condition, curing the dogs. The animals were followed up until 4.5 years after the beginning of the experiment and, compared to the untreated control group at month 3 (12/25 dogs; 48%), a decrease in the rate of CVL deaths was only seen after ICT treatment (7/35 dogs; 20%; 0.0273) but not after IT treatment (10/31 dogs; 32%; p=0.278), pointing out an additional advantage of the ICT treatment with the enriched-Leishmune in the control and cure of CVL.

Optimized subunit vaccine protects against experimental leishmaniasis

Development of a protective subunit vaccine against Leishmania spp. depends on antigens and adjuvants that induce appropriate immune responses. We evaluated a second generation polyprotein antigen (Leish-110f) in different adjuvant formulations for immunogenicity and protective efficacy against Leishmania spp. challenges. Vaccine-induced protection was associated with antibody and T cell responses to Leish-110f. CD4 T cells were the source of IFN-gamma, TNF, and IL-2 double- and triple-positive populations. This study establishes the immunogenicity and protective efficacy of the improved Leish-110f subunit vaccine antigen adjuvanted with natural (MPL-SE) or synthetic (EM005) Toll-like receptor 4 agonists.

The chimerical multi-component Q protein from Leishmania in the absence of adjuvant protects dogs against an experimental Leishmania infantum infection

The protective potential against Leishmania infection of the Leishmania chimerical Q protein administered as a single (Q) or double dose (Q+Q) without adjuvant was analyzed in a double-blind placebo controlled experiment in dogs. During vaccination the protein induced an intense early anti-Q response but no reactivity against total Leishmania infantum proteins was detected. Several end-points were taken into consideration. In the vaccinated animals the amount and intensity of clinical symptoms was lower than in the control group. Pathological signs of disease were observed in liver, kidney and spleen of all dogs from the control group in contrast to the normal appearance of the organs of the vaccinated animals. Vaccination was able to induce parasite clearance in most dogs. Only 1/7 dog was parasite DNA positive in skin in the Q group in contrast to 6/7 dogs in control and 4/7 in Q+Q. Significant anti-SLA clearance was observed in the vaccinated animals at the end of the study. Differences between control and vaccinated animals were also observed at the biochemical level, DTH and nitrite oxide production.

Decrease of the incidence of human and canine visceral leishmaniasis after dog vaccination with Leishmune in Brazilian endemic areas

Leishmune, the first prophylactic vaccine licensed against canine visceral leishmaniasis (CVL), has been used in Brazil since 2004, where seropositive dogs are sacrificed in order to control human visceral leishmaniasis (VL). We demonstrate here that vaccination with Leishmune does not interfere with the serological control campaign (110,000 dogs). Only 1.3% of positivity (76 among 5860) was detected among Leishmune uninfected vaccinees. We also analyzed the possible additive effect of Leishmune vaccination over dog culling, on the decrease of the incidence of CVL and VL in two Brazilian endemic areas, from 2004 to 2006. In Araçatuba, a 25% of decline was seen in CVL with a 61% decline in human cases, indicating the additive effect of Leishmune vaccination of 5.7% of the healthy dogs (1419 dogs), on regular dog culling. In Belo Horizonte (BH), rising curves of canine and human incidence were observed in the districts of Barreiro, Venda Nova and Noroeste, while the canine and human incidence of Centro Sul, Leste, Nordeste, Norte, Pampulha and Oeste, started to decrease or maintained a stabilized plateau after Leishmune vaccination. Among the districts showing a percent decrease of human incidence (-36.5%), Centro Sul and Pampulha showed the highest dog vaccination percents (63.27% and 27.27%, respectively) and the lowest dog incidence (-3.36% and 1.89%, respectively). They were followed by Oeste, that vaccinated 25.30% of the animals and experienced an increase of only 12.86% of dog incidence and by Leste and Nordeste, with lower proportions of vaccinees (11.72% and 10.76%, respectively) and probably because of that, slightly higher canine incidences (42.77% and 35.73%). The only exception was found in Norte district where the reduced human and canine incidence were not correlated to Leishmune vaccination. Much lower proportions of dogs were vaccinated in Venda Nova (4.35%), Noroeste (10.27%) and Barreiro (0.09%) districts, which according to that exhibited very increased canine incidences (24.48%, 21.85% and 328.57%, respectively), and pronounced increases in human incidence (14%, 4% and 17%, respectively). The decrease of canine (p=-0.008) and human incidences (p=-0.048) is directly correlated to the increase of the number of vaccinated dogs, confirming the additive control effect of Leishmune vaccination over dog culling, reducing the parasite reservoir, protecting dogs and, in this way, reducing the risk of transmission of VL to humans and becoming a new effective control tool.

New strategies for the control of arthropod vectors of disease in dogs and cats

Arthropod-borne diseases (ABDs) in cats and dogs have a major impact on animal health and welfare and, in many cases, also on human health. Many ABDs are expected to increase in prevalence as a result of changing social habits, habitat modifications, introductions of exotic vectors and climate change. Control has, historically, focused on the use of insecticides and chemotherapy. We review alternative, emerging approaches to ABDs that currently offer promise, particularly modelling and molecular techniques and the development of novel vaccines that target molecules produced by arthropods during the bloodmeal. We argue that there is an urgent need to establish effective surveillance systems for most ABDs across various countries in order to facilitate a detailed risk analysis, which should include evaluation of potential spread to new areas and the possible introduction of new exotic species or disease agents. This will require clear and exhaustive knowledge on the distribution of ABDs in different areas, understanding of the diagnostic limitations pertaining to ABDs and standardization of techniques among reference laboratories in different countries. Continuous monitoring of insecticide resistance and the development of management strategies to minimize its onset are also essential. Ultimately, it is probable that approaches which attempt to reduce vector abundance or treat hosts with chemotherapy alone are unlikely to be effective in the long term. More suitable approaches may include greater use of a range of mutually compatible options in integrated management programmes.

Recent developments in leishmaniasis vaccine delivery systems

BACKGROUND

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

OBJECTIVE

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

METHODS

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

CONCLUSION

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

Cutaneous leishmaniasis: progress towards a vaccine

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

Immunogenicity assay of the Leishmune vaccine against canine visceral leishmaniasis in Brazil

Leishmune is the industrialized version of the FML-saponin vaccine which has been shown to develop 92-95% protection in vaccinated dogs and 76-80% vaccine efficacy against field canine visceral leishmaniasis (CVL) in Brazil. Leishmune has been proven to be safe and tolerable and a transmission-blocking vaccine which renders vaccinated dogs non-infectious to sand fly vectors. In the present investigation, 550 healthy seronegative dogs of endemic and epidemic areas of Brazil were monitored for Leishmune-induced immunogenicity during a 2-year trial. Another group of 588 untreated exposed dogs was also studied in parallel. Both groups were seronegative on day 0. The strong immunogenicity induced by Leishmune vaccine was demonstrated by the 98% of FML-seroconversion, increase in absorbencies, the 82.7% DTH positive reactions and increase in skin test size diameters, the average increase in CD8+ total lymphocytes population in blood (27.1%), expected for QS21 saponin-containing vaccine, the sustained proportions of CD4+ T cells, and the average increased proportions of CD21+ B lymphocytes (42.3%). The Leishmune-induced protection against CVL is demonstrated by the results: 98.8% asymptomatic dogs (at the end of first year) and 99% healthy survivors (at the end of the second year) among vaccinated dogs, compared to the 79.4% asymptomatic and 61% survivor dogs (p<0.001) monitored in the untreated exposed cohort. In spite of the low vaccine coverage, it was possible to detect a 66.1% (p<0.005) reduction in Belo Horizonte and an 80.2% (p<0.005) reduction in Araçatuba of the incidence of CVL among vaccinated dogs, when compared to the global incidence of CVL of each town, respectively. Our preliminary results support the potential use of Leishmune to prevent CVL epidemics.

FML vaccine against canine visceral leishmaniasis: from second-generation to synthetic vaccine

The Leishmania donovani glycoprotein fraction, known as FML, successfully underwent preclinical and clinical (Phase I-III) vaccine trials against canine visceral leishmaniasis (92-95% of protection and 76-80% of vaccine efficacy) when formulated with a QS21 saponin-containing adjuvant. It became the licensed Leishmune vaccine for canine prophylaxis in Brazil. The immune response raised by the vaccine is long lasting, immunotherapeutic and reduces dog infectivity blocking the transmission of the disease, as revealed by an in vivo assay. The preliminary epidemiological control data of vaccinated areas in Brazil indicate that, in spite of the still low vaccine coverage, there was a significant decrease in the incidence of the human and canine disease. A 36-kDa glycoprotein, in the FML complex, is the human marker of the disease, which was protective in mice as native recombinant protein or DNA vaccine. The DNA vaccine is now being tested against the canine disease. This review resumes the development of the second-generation FML-saponin-Leishmune vaccine, its adjuvant and of the NH36 DNA vaccine, toward the identification of its major epitopes that might be included in a possible future synthetic vaccine.

Canine leishmaniosis--new concepts and insights on an expanding zoonosis: part two

Canine leishmaniosis is a widely spread zoonosis that is potentially fatal to humans and dogs. Infection with Leishmania infantum is considerably more prevalent than clinical disease, and infected dogs with no signs of disease might, potentially, transmit infection. Diagnosis of asymptomatic infection by serology is insufficient and PCR markedly increases its sensitivity. A new therapy exclusively for canine leishmaniosis is needed because current drugs do not reliably eliminate infection and might provoke resistance. Protection against sand-fly bites by topical insecticides is effective in reducing infection, and recent development of vaccines has indicated that prevention by vaccination is feasible. Integrated prevention with topical insecticides reducing the feeding of vectors and vaccination blocking early infection would be the basis of successful control programs.

Associations among immunological, parasitological and clinical parameters in canine visceral leishmaniasis: Emaciation, spleen parasitism, specific antibodies and leishmanin skin test reaction

Associations among parameters commonly used as markers of infection by Leishmania sp., or of susceptibility to visceral leishmaniasis, were investigated in 325 stray dogs from an area where this disease is endemic. Evidence of infection (presence of Leishmania in splenic cultures, positive leishmanin skin test (LST) or detection of anti-Leishmania antibody activity in the serum) was found in 57% of the animals. Both evidence of weight loss (chi(2)-test, P=0.0005) and presence of specific antibody activity in the serum (chi(2)-test, P<0.0001) were directly associated with positive splenic culture. The frequencies of animals with positive splenic culture were directly correlated with the intensities of antibody activity in the serum as measured by ELISA (relative risk of 3.4 for animals with moderate antibody levels and relative risk of 8.43 for animals with high-antibody levels). A negative association was observed between positive leishmanin skin test results and emaciation (chi(2), P=0.0089). Furthermore, animals with positive splenic cultures and negative leishmanin skin test results had higher levels of total serum IgG (Kruskal-Wallis test, P=0.001) and IgG2 (Kruskal-Wallis test, P=0.05) than animals with negative splenic cultures, and were more emaciated than animals with negative LST results and positive splenic cultures. The data presented herein suggest that associating these common parameters may improve their performance in predicting susceptibility to canine visceral leishmaniasis.

Immunogenicity of HSP-70, KMP-11 and PFR-2 leishmanial antigens in the experimental model of canine visceral leishmaniasis

Zoonotic visceral leishmaniasis (ZVL) is a parasitic disease caused by Leishmania infantum/L. chagasi that is emerging as an important medical and veterinary problem. Dogs are the domestic reservoir for this parasite and, therefore, the main target for controlling the transmission to humans. In the present work, we have evaluated the immunogenicity of the Leishmania infantum heat shock protein (HSP)-70, paraflagellar rod protein (PFR)-2 and kinetoplastida membrane protein (KMP)-11 recombinant proteins in dogs experimentally infected with the parasite. We have shown that peripheral blood mononuclear cells (PBMC) from experimentally infected dogs proliferated in response to these recombinant antigens and against the soluble leishmanial antigen (SLA). We have also quantified the mRNA expression level of the cytokines induced in PBMC upon stimulation with the HSP-70, PFR-2 and KMP-11 proteins. These recombinant proteins induced an up-regulation of IFN-gamma. HSP-70 and PFR-2 also produced an increase of the TNF-alpha transcripts abundance. No measurable induction of IL-10 was observed and low levels of IL-4 mRNA were produced in response to the three mentioned recombinant antigens. Serum levels of specific antibodies against HSP-70, PFR-2 and KMP-11 recombinant proteins were also determined in these animals. Our study showed that HSP-70, KMP-11 and PFR-2 proteins are recognized by infected canines. Furthermore, these antigens produce a Th1-type immune response, suggesting that they may be involved in protection. The identification as vaccine candidates of Leishmania antigens that elicit appropriate immune responses in the canine model is a key step in the rational approach to generate a vaccine for canine visceral leishmaniasis.

Evaluation of an immunochemotherapeutic protocol constituted of N-methyl meglumine antimoniate (Glucantime) and the recombinant Leish-110f + MPL-SE vaccine to treat canine visceral leishmaniasis

The evaluation of the efficacy of an immunochemotherapy protocol to treat symptomatic dogs naturally infected with Leishmania chagasi was studied. This clinical trial had the purpose to test the combination of N-methyl meglumine antimoniate (Glucantime^®) and the second generation recombinant vaccine Leish-110f^® plus the adjuvant MPL-SE^® to treat the canine leishmaniasis (CanL). Thirty symptomatic naturally infected mongrel dogs were divided into five groups. Animals received standard treatment with Glucantime^® or treatment with Glucantime^®/Leish-110f^® + MPL-SE^® as immunochemotherapy protocol. Additional groups received Leish-110f^® + MPL-SE^® only, MPL-SE^® only, or placebo. Evaluation of haematological, biochemical (renal and hepatic function) and plasmatic proteins, immunological (humoral and cellular immune response) and the parasitological test revealed improvement of the clinical parameters and parasitological cure in dogs in both chemotherapy alone and immunochemotherapy cohorts. However, the immunotherapy and immunochemotherapy cohorts had reduced number of deaths, higher survival probability, and specific cellular reactivity to leishmanial antigens, in comparison with chemotherapy cohort only and control groups (adjuvant alone and placebo).

These results support the notion of using well-characterized recombinant vaccine as an adjunct to improve the current chemotherapy of CanL.

Vaccines for leishmaniasis in the fore coming 25 years

Human vaccination against leishmaniasis using live Leishmania was used in Middle East and Russia (1941-1980). First-generation vaccines, composed by killed parasites induce low efficacies (54%) and were tested in humans and dogs Phase III trials in Asia and South America since 1940. Second-generation vaccines using live genetically modified parasites, or bacteria or viruses containing Leishmania genes, recombinant or native fractions are known since the 1990s. Due to the loss of PAMPs, the use of adjuvants increased vaccine efficacies of the purified antigens to 82%, in Phase III dog trials. Recombinant second-generation vaccines and third-generation DNA vaccines showed average values of parasite load reduction of 68% and 59% in laboratory animal models, respectively, but their success in field trials had not yet been reported. This review is focused on vaccine candidates that show any efficacy against leishmaniasis and that are already in different phase trials. A lot of interest though was generated in recent years, by the studies going on in experimental models. The promising candidates may find a place in the forth coming years. Among them most probably are the multiple-gene DNA vaccines that are stable and do not require cold-chain transportation. In the mean time, second-generation vaccines with native antigens and effective adjuvants are likely to be licensed and used in Public Health control programs in the fore coming 25 years. To date, only three vaccines have been licensed for use: one live vaccine for humans in Uzbekistan, one killed vaccine for human immunotherapy in Brazil and a second-generation vaccine for dog prophylaxis in Brazil.

Coencapsulation of CpG oligodeoxynucleotides with recombinant Leishmania major stress-inducible protein 1 in liposome enhances immune response and protection against leishmaniasis in immunized BALB/c mice

CpG oligodeoxynucleotides (CpG ODN) have been shown to have potent adjuvant activity for a wide range of antigens. The purpose of this study was to determine the potential benefit of using liposomes as a delivery vehicle to enhance the adjuvant activity of CpG ODN with Leishmania major stress-inducible protein 1 (LmSTI1) antigen in induction of the Th1 response in a murine model of leishmaniasis. BALB/c mice were immunized subcutaneously three times in 3-week intervals with liposomal recombinant LmSTI1 (Lip-rLmSTI1), rLmSTI1 coencapsulated with CpG ODN in a liposome (Lip-rLmSTI1-CpG ODN), rLmSTI1 plus CpG ODN in phosphate-buffered saline (PBS), rLmSTI1 plus non-CpG ODN in PBS, rLmSTI1 in PBS, empty liposome, or PBS. The intensity of infection induced by L. major promastigote challenge was measured by footpad swelling. A significant (P < 0.001) inhibition of infection in mice immunized with Lip-rLmSTI1-CpG ODN was shown compared to the other groups, and no parasite was detected in the spleens of this group 14 weeks after challenge. The highest immunoglobulin G2a (IgG2a) titer and the highest IgG2a/IgG1 ratio were also shown in the sera of mice immunized with Lip-rLmSTI1-CpG ODN before and 14 weeks after challenge. The results indicated the superiority of CpG ODN in its liposomal form over its soluble form to induce the Th1 response when used in association with rLmSTI1 antigen. It seems that using a liposome delivery system carrying CpG ODN as an adjuvant coencapsulated with Leishmania antigen plays an important role in vaccine development strategies against leishmaniasis.

Heterologous prime-boost vaccination with a non-replicative vaccinia recombinant vector expressing LACK confers protection against canine visceral leishmaniasis with a predominant Th1-specific immune response

Leishmaniasis caused by Leishmania infantum is a severe endemic disease in the Mediterranean basin, being domestic dogs the main reservoir of the disease that plays a key role in the transmission to humans. Studies on vaccines against canine leishmaniasis, aimed to modify the T cell repertoire, have advanced in recent years. LACK vaccination assays, using protein or DNA vectors, show protection against cutaneous L. major infections by redirecting the early IL-4 responses to a protective Th1 response. The aim of this study was to define the effectiveness and type of immune response in a canine visceral leishmaniasis model of two poxvirus vectors (Western reserve strain, WR and modified vaccinia virus Ankara, MVA) expressing the LACK protein of L. infantum in prime/boost vaccination protocols. The results obtained showed that dog vaccination priming with DNA-LACK followed by a booster with MVA-LACK or rVV-LACK triggered a Th1 type of immune response, leading to protection against canine visceral leishmaniasis. This protection correlated with absence of visceral leishmaniasis symptoms, lower Leishmania-specific antibodies, higher degree of T cell activation in Leishmania-target organs and higher synthesis of Th1 cytokines. In addition, we found that dogs boosted with the non-replicative virus show less VL symptoms and higher degree of T cell activation, providing evidences for a clear advantage of MVA-LACK as a vaccination vector against canine visceral leishmaniasis.

Vaccination with plasmid DNA encoding KMPII, TRYP, LACK and GP63 does not protect dogs against Leishmania infantum experimental challenge

Vaccination of dogs, the domestic reservoir of Leishmania infantum, is the best method for controlling zoonotic visceral leishmaniasis. This strategy would reduce the incidence of disease in both the canine and, indirectly, the human population. Different vaccination approaches have been investigated against canine leishmaniasis (CaL) but to date there is only one licensed vaccine against this disease in dogs, in Brazil. DNA immunization is a promising method for inducing both humoral and cellular immune responses against this parasitic disease. Here, we report the results of a multiantigenic plasmid DNA vaccine encoding KMPII, TRYP, LACK and GP63 L. infantum antigens against experimentally induced CaL. Twelve dogs were randomly assigned to two groups receiving, at a 15 days interval, either four doses of plasmid DNA or similar injections of PBS. After vaccination, dogs were intravenously challenged with 5 x 10(7) promastigotes of L. infantum. The vaccine showed to be safe and well-tolerated. Neither cellular immune response nor antibodies directed against whole Leishmania antigen were detected after immunization in vaccinated dogs, although anti-LACK-specific antibodies were sporadically detected in two vaccinated dogs before challenge, thus suggesting that antigens were indeed expressed. A delay in the development of detectable specific immune response and parasite multiplication in vaccinated dogs was observed after challenge. Nevertheless, the multiantigenic Leishmania DNA vaccine was unable to induce protection against parasite dissemination or disease. This study emphasizes the need to strengthen DNA vaccines in order to obtain effective immune responses in models other than the murine.

Immunization with H1, HASPB1 and MML Leishmania proteins in a vaccine trial against experimental canine leishmaniasis

The protective capabilities of three Leishmania recombinant proteins – histone 1 (H1) and hydrophilic acylated surface protein B1 (HASPB1) immunized singly, or together as a protein cocktail vaccine with Montanide™, and the polyprotein MML immunized with MPL^®-SE adjuvant – were assessed in beagle dogs. Clinical examination of the dogs was carried out periodically under blinded conditions and the condition of the dogs defined as asymptomatic or symptomatic. At the end of the trial, we were able to confirm that following infection with L. infantum promastigotes, five out of eight dogs immunized with H1 Montanide™, and four out of eight dogs immunized with either the combination of HASPB1 with Montanide™ or the combination of H1 + HASPB1 with Montanide™, remained free of clinical signs, compared with two out of seven dogs immunized with the polyprotein MML and adjuvant MPL^®-SE, and two out of eight dogs in the control group. The results demonstrate that HASPB1 and H1 antigens in combination with Montanide™ were able to induce partial protection against canine leishmaniasis, even under extreme experimental challenge conditions.

Leishmania spp.: proficiency of drug-resistant parasites

Leishmaniasis is a disease caused by at least 17 different species of protozoan Leishmania parasites and currently affects around 12 million people living mostly in tropical and subtropical areas. Failure to treat leishmaniasis successfully is often due to drug resistance. However, there are no cellular and molecular markers of chemoresistance against leishmanicidal drugs and the only reliable method for monitoring resistance of individual isolates is the in vitro amastigote/macrophage model. It is thus necessary to find cellular and molecular markers that can be used systematically to identify the drug-resistant phenotype of the infecting parasites. Until now, whether drug resistance in Leishmania compromises parasite proficiency, e.g. in terms of infectivity or metabolism, has not been systematically evaluated. Therefore, here we examine whether the physiological changes expressed by drug-resistant Leishmania reflect a modification of parasite vitality in drug-resistant compared with drug-sensitive parasites. Finally, the clinical implications of drug resistance in Leishmania are also discussed.

Immunogenicity of the P-8 amastigote antigen in the experimental model of canine visceral leishmaniasis

The P-8 proteoglycolipid complex (P-8 PGLC), an amastigote antigen of Leishmania pifanoi, has been demonstrated to induce protection in mouse models, as well as to induce Tc1/Th1-like cellular responses in American cutaneous leishmaniasis patients. Because the immunization with P-8 PGLC in the murine model does not appear to be genetically restricted, we have studied the reactivity of the P-8 PGLC in Leishmania infantum infected dogs. In this study, it is shown that PBMC from experimentally infected dogs (asymptomatic, oligosymptomatic) significantly proliferated in response to soluble leishmanial antigen (SLA) or the P-8 PGLC. Further, quantification of the gene expression induced by the stimulation with P-8 in asymptomatically infected dogs showed an up-regulation of IFN-gamma and TNF-alpha, which were three to 4-fold higher than that induced by soluble Leishmania antigen (SLA). While no measurable induction of IL-10 was observed, low levels of IL-4 mRNA were observed in response to both P-8 and SLA antigens. Thus, our studies establish that P-8 is recognized by infected canines and elicits a potentially curative/protective Th1-like immune response. The identification of Leishmania antigens that elicit appropriate immune responses across different host species (humans, canine) and disease manifestations (cutaneous or visceral) could be an advantage in generating a general vaccine for leishmaniasis.

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.

Efficacy of a combination of 10% imidacloprid/50% permethrin for the prevention of leishmaniasis in kennelled dogs in an endemic area

The efficacy of imidacloprid 10%and permethrin 50% (Advantix; Bayer AG, Germany) in a spot-on formulation was evaluated in the field as a control measure to prevent canine leishmaniasis (CanL) in dogs in an endemic area of southern Italy. In February 2005, out of 845 dogs initially tested for CanL, 631 dogs which tested negative (315 from a kennel in Bari (KB) and 316 from a kennel in Ginosa (KG)) in a serological and a parasitological examination were allocated to one of three groups: Group A-treated with imidacloprid 10% and permethrin 50% once a month; Group B-treated every 2 weeks; and Group C-untreated control animals. All the dogs were examined serologically and parasitologically for CanL prior to the start of the study, in November 2005 (end of the sandfly season) and in March 2006 (end of the study). An initial CanL seroprevalence of 24.7% (209 dogs) was detected in KB and KG. In KB Leishmania infection, inferred by positivity in at least one of the three tests performed at the interim or final follow-up, was found in one animal from Group A and in nine from Group C. No positive animals were detected in Group B, thus giving a final protection efficacy of 88.9% in Group A and 100% in Group B. In KG Leishmania infection was identified in one animal from Groups A and B, respectively, and 11 from Group C (protection efficacy of 90.36% in Group A and 90.73% in Group B). The incidence density rates (IDRs) of infection in both Groups A and B at each kennel were statistically significantly lower than that registered in Group C (KB p<0.05 and KG p<0.01). The results clearly show that a combination of imidacloprid 10% and permethrin 50%, by virtue of its repellent activity against sandflies, is effective under both application regimes in preventing CanL in the field in endemic areas.

Immunology of canine leishmaniasis

The role of dogs as the main reservoir of visceral leishmaniasis has led to an increased interest in the immune responses and in Leishmania antigens implicated in protective cellular immunity in canine visceral leishmaniasis. The primary goal is to control the prevalence of human disease. Immune responses in canine visceral leishmaniasis are reviewed. Cellular immune responses toward a Th1 subset mediated by IFN-gamma and TNF-alpha predominate in asymptomatic dogs exhibiting apparent resistance to visceral leishmaniasis. On the other hand, while the role of Th2 cytokines, such as IL-4 and IL-10, in symptomatic animals is still controversial, there is increasing evidence for a correlation of these cytokines with progressive disease. CD8+ cytotoxic T cells seem also likely to be involved in resistance to visceral leishmaniasis. Several Leishmania antigens implicated in protective immune responses are described and some pivotal points for development of an effective vaccine against canine visceral leishmaniasis are discussed.