Role of Sand Fly Saliva in Human and Experimental Leishmaniasis: Current Insights

Within-host bayesian joint modeling of longitudinal and time-to-event data of Leishmania infection

The host immune system plays a significant role in managing and clearing pathogen material during an infection, but this complex process presents numerous challenges from a modeling perspective. There are many mathematical and statistical models for these kinds of processes that take into account a wide range of events that happen within the host. In this work, we present a Bayesian joint model of longitudinal and time-to-event data of Leishmania infection that considers the interplay between key drivers of the disease process: pathogen load, antibody level, and disease. The longitudinal model also considers approximate inflammatory and regulatory immune factors. In addition to measuring antibody levels produced by the immune system, we adapt data from CD4+ and CD8+ T cell proliferation, and expression of interleukin 10, interferon-gamma, and programmed cell death 1 as inflammatory or regulatory factors mediating the disease process. The model is developed using data collected from a cohort of dogs naturally exposed to Leishmania infantum. The cohort was chosen to start with healthy infected animals, and this is the majority of the data. The model also characterizes the relationship features of the longitudinal outcomes and time-to-death due to progressive Leishmania infection. In addition to describing the mechanisms causing disease progression and impacting the risk of death, we also present the model's ability to predict individual trajectories of Canine Leishmaniosis (CanL) progression. The within-host model structure we present here provides a way forward to address vital research questions regarding the understanding of the progression of complex chronic diseases such as Visceral Leishmaniasis, a parasitic disease causing significant morbidity worldwide.

Finding Correlations Between mRNA and Protein Levels in Leishmania Development: Is There a Discrepancy?

Multiple genes and proteins have been identified as differentially expressed in the stages of the Leishmania life cycle. The differentiation processes are implicated in specific transcriptional and proteomic adjustments driven by gene expression regulation mechanisms. Leishmania parasites lack gene-specific transcriptional control, and gene expression regulation mostly depends on posttranscriptional mechanisms. Due to the lack of transcriptional regulation, criticism regarding the relevance of transcript quantification as a possible and efficient prediction of protein levels is recurrent in studies that use transcriptomic information. The advent of high-throughput technologies has improved the analysis of genomes, transcriptomes and proteomes for different organisms under several conditions. Nevertheless, defining the correlation between transcriptional and proteomic profiles requires arduous and expensive work and remains a challenge in Leishmania. In this review, we analyze transcriptomic and proteomic data for several Leishmania species in two different stages of the parasite life cycle: metacyclogenesis and amastigogenesis (amastigote differentiation). We found a correlation between mRNA and protein levels of 60.9% and 69.8% for metacyclogenesis and amastigogenesis, respectively; showing that majority mRNA and protein levels increase or decrease concomitantly. Among the analyzed genes that did not present correlation indicate that transcriptomic data should be carefully interpreted as protein expression. We also discuss possible explanations and mechanisms involved for this lack of correlation.

Sand flies: Basic information on the vectors of leishmaniasis and their interactions with Leishmania parasites

Blood-sucking arthropods transmit a variety of human pathogens acting as disseminators of the so-called vector-borne diseases. Leishmaniasis is a spectrum of diseases caused by different Leishmania species, transmitted quasi worldwide by sand flies. However, whereas many laboratories focus on the disease(s) and etiological agents, considerably less study the respective vectors. In fact, information on sand flies is neither abundant nor easy to find; aspects including basic biology, ecology, and sand-fly-Leishmania interactions are usually reported separately. Here, we compile elemental information on sand flies, in the context of leishmaniasis. We discuss the biology, distribution, and life cycle, the blood-feeding process, and the Leishmania-sand fly interactions that govern parasite transmission. Additionally, we highlight some outstanding questions that need to be answered for the complete understanding of parasite–vector–host interactions in leishmaniasis.

In this review, numerous aspects of sand flies as vectors of Leishmania parasites—from biology to the vector parasite interactions—are discussed.

Immune Responses in Leishmaniases: An Overview

Leishmaniasis is a parasitic, widespread, and neglected disease that affects more than 90 countries in the world. More than 20 Leishmania species cause different forms of leishmaniasis that range in severity from cutaneous lesions to systemic infection. The diversity of leishmaniasis forms is due to the species of parasite, vector, environmental and social factors, genetic background, nutritional status, as well as immunocompetence of the host. Here, we discuss the role of the immune system, its molecules, and responses in the establishment, development, and outcome of Leishmaniasis, focusing on innate immune cells and Leishmania major interactions.

Mast-Cell Response to Leishmania mexicana and Sand-Fly Salivary Proteins Is Modulated by Orchiectomy

Mast cells (MCs) play a crucial role during Leishmania infections, which is transmitted through the bite of an infected sand fly that injects saliva together with the parasite. Sand fly saliva is a complex fluid that modulates the host immune response. In addition, hormonal factors modulate the host immune response and alter susceptibility to infections. Thus, to assess the impact of male sex hormones on the mast-cell (MC) response to Leishmania infections, we orchiectomized male mice, infected them with the parasite in the presence of sand fly salivary proteins, and analyzed the inflammatory response of MCs. Our results showed that the MC response to the parasite and vector salivary proteins differed between orchiectomized and sham-operated mice. In orchiectomized mice, MC showed a retarded activation pattern, associated with slower degranulation and weaker TNF-α, histamine, and tryptase staining in response to the infection with Leishmania mexicana combined with vector-salivary proteins, as compared to sham mice. Furthermore, neutrophil infiltration was slower in orchiectomized mice, and numbers of infected macrophages and lesion sizes were smaller. Our results show that, during Leishmania infection, male sex hormones modulate the mast-cell response against the parasite and salivary proteins of the sand fly vector, inducing an intense inflammatory response. Their absence in orchiectomized mice retards the inflammatory response, enabling better control of the infection and slower disease progression.

Some Good and Some Bad: Sand Fly Salivary Proteins in the Control of Leishmaniasis and in Autoimmunity

Sand flies are hematophagous insects responsible for the transmission of vector-borne diseases to humans. Prominent among these diseases is Leishmaniasis that affects the skin and mucous surfaces and organs such as liver and spleen. Importantly, the function of blood-sucking arthropods goes beyond merely transporting pathogens. The saliva of vectors of disease contains pharmacologically active components that facilitate blood feeding and often pathogen establishment. Transcriptomic and proteomic studies have enumerated the repertoire of sand fly salivary proteins and their potential use for the control of Leishmaniasis, either as biomarkers of vector exposure or as anti-Leishmania vaccines. However, a group of specific sand fly salivary proteins triggers formation of cross-reactive antibodies that bind the ectodomain of human desmoglein 1, a member of the epidermal desmosomal cadherins. These cross-reactive antibodies are associated with skin autoimmune blistering diseases, such as pemphigus, in certain immunogenetically predisposed individuals. In this review, we focus on two different aspects of sand fly salivary proteins in the context of human disease: The good, which refers to salivary proteins functioning as biomarkers of exposure or as anti-Leishmania vaccines, and the bad, which refers to salivary proteins as environmental triggers of autoimmune skin diseases.

Association of Salivary Cholinesterase With Arthropod Vectors of Disease

Acetylcholinesterase (AChE) was previously reported to be present in saliva of the southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini), with proposed potential functions to 1) reduce acetylcholine toxicity during rapid engorgement, 2) modulate host immune responses, and 3) to influence pathogen transmission and establishment in the host. Potential modulation of host immune responses might include participation in salivary-assisted transmission and establishment of pathogens in the host as has been reported for a number of arthropod vector-borne diseases. If the hypothesis that tick salivary AChE may alter host immune responses is correct, we reasoned that similar cholinesterase activities might be present in saliva of additional arthropod vectors. Here, we report the presence of AChE-like activity in the saliva of southern cattle ticks, Rhipicephalus (Boophilus) microplus; the lone star tick, Amblyomma americanum (Linnaeus); Asian tiger mosquitoes, Aedes albopictus (Skuse); sand flies, Phlebotomus papatasi (Scopoli); and biting midges, Culicoides sonorensis Wirth and Jones. Salivary AChE-like activity was not detected for horn flies Haematobia irritans (L.), stable flies Stomoxys calcitrans (L.), and house flies Musca domestica L. Salivary cholinesterase (ChE) activities of arthropod vectors of disease-causing agents exhibited various Michaelis-Menten KM values that were each lower than the KM value of bovine serum AChE. A lower KM value is indicative of higher affinity for substrate and is consistent with a hypothesized role in localized depletion of host tissue acetylcholine potentially modulating host immune responses at the arthropod bite site that may favor ectoparasite blood-feeding and alter host defensive responses against pathogen transmission and establishment.

A prospective cohort study of Cutaneous Leishmaniasis due to Leishmania major: Dynamics of the Leishmanin skin test and its predictive value for protection against infection and disease

BACKGROUND

Leishmanin Skin Test (LST) is considered as a useful indicator of past infection by Leishmania parasites. However, the temporal dynamics of a positive LST under different epidemiologic scenarios and whether it relates to the protection against the recurrence of an overt disease are not fully documented.

METHODOLOGY/PRINCIPAL FINDINGS

We report here on a population based prospective study conducted on 2686 individuals living in two foci located in Central Tunisia, to assess over a one-year epidemiologic season, the incidence of Leishmania (L.) major infection and disease and changes in LST reactivity. The two foci were both endemic for Cutaneous Leishmaniasis (CL) due to L. major, but contrasted in their history for this disease (ie: an old focus versus a recent focus). We found that most infections occurred in the new focus (290/1000; 95% CI: 265-315 person-years) with an incidence rate of CL lesions 2.4 times higher than in the old focus. Likewise, the rates of LST reactivity reversion and loss, in the new focus, were 99/1000[38-116] person-years and 14/1000[8-21] person-years, respectively. Loss of LST reactivity was not noticed in the old focus. Interestingly, the incidence rates of symptomatic infection did not differ significantly according to the LST status at enrolment (negative versus positive) between the combined foci and the new one.

CONCLUSIONS/SIGNIFICANCE

Our findings confirm LST as a good tool for assessing L. major cryptic infection. However, the instability of the LST positivity in new foci should be considered as an important confounder of the outcome of this infection when developing a research protocol for vaccine trial.

Immunization of Experimental Dogs With Salivary Proteins From Lutzomyia longipalpis, Using DNA and Recombinant Canarypox Virus Induces Immune Responses Consistent With Protection Against Leishmania infantum

Metacyclic Leishmania promastigotes are transmitted by sand flies that inject parasites and saliva into the host's skin. Previous studies have demonstrated that DNA plasmids encoding Lutzomyia longipalpis salivary proteins LJM17 and LJL143, when used to immunize dogs, resulted in a systemic and local Th1 cell-mediated immunity that interfered in parasite survival in vitro. Here we evaluated the ability of these same salivary antigens to induce anti-Leishmania immunity and to confer protection by immunizing dogs using a novel vaccination strategy more suitable for use in the field. The strategy consisted of a single dose of plasmid followed by two doses of recombinant Canarypoxvirus (rCanarypoxvirus) expressing L. longipalpis salivary proteins (LJM17 or LJL143). Thirty days after the final immunization, dogs were intradermally challenged with 10⁷Leishmania infantum promastigotes in the presence of L. longipalpis saliva. We followed the experimentally infected dogs for 10 months to characterize clinical, parasitological, and immunological parameters. Upon vaccination, all immunized dogs presented strong and specific humoral responses with increased serum concentrations of IFN-γ, TNF, IL-7, and IL-15. The serum of dogs immunized with LJM17 also exhibited high levels of IL-2, IL-6, and IL-18. L. infantum infection was established in all experimental groups as evidenced by the presence of anti-Leishmania IgG, and by parasite detection in the spleen and skin. Dogs immunized with LJM17-based vaccines presented higher circulating levels of IFN-γ, IL-2, IL-6, IL-7, IL-15, IL-18, TNF, CXCL10, and GM-CSF post-infection when compared with controls. Results demonstrated that relevant Leishmania-specific immune responses were induced following vaccination of dogs with L. longipalpis salivary antigen LJM17 administered in a single priming dose of plasmid DNA, followed by two booster doses of recombinant Canarypox vector. Importantly, a significant increase in pro-inflammatory cytokines and chemokines known to be relevant for protection against leishmaniasis was evidenced after challenging LJM17-vaccinated dogs as compared to controls. Although similar results were observed following immunization with LJL143, the pro-inflammatory response observed after immunization was attenuated following infection. Collectively, these data suggest that the LJM17-based vaccine induced an immune profile consistent with the expected protective immunity against canine leishmaniosis. These results clearly support the need for further evaluation of the LJM17 antigen, using a heterologous prime-boost vaccination strategy against canine visceral leishmaniosis (CVL).

Immunity Against Leishmania major Infection: Parasite-Specific Granzyme B Induction as a Correlate of Protection

Zoonotic cutaneous leishmaniasis (ZCL) caused by Leishmania (L.) major infection is characterized by different clinical presentations which depend in part on the host factors. In attempt to investigate the impact of the host's immune response in the outcome of the disease, we conducted a prospective study of 453 individuals living in endemic foci of L. major transmission in Central Tunisia. Several factors were assessed at the baseline including (i) the presence of typical scars of ZCL, (ii) in vivo hypersensitivity reaction to leishmanin, and (iii) the in vitro release of granzyme B (Grz B) by peripheral blood mononuclear cells (PBMC) in response to stimulation with live L. major promastigotes. After one season of parasite's transmission, repeated clinical examinations allowed us to diagnose the new emerging ZCL cases. Heterogeneity was observed in terms of number of lesions developed by each individual as well as their size and spontaneous outcome, which led us to establish the parameter “severity of the disease.” The efficacy of the presence of typical ZCL scar, the leishmanin skin test (LST) positive reactivity and the high levels of Grz B (≥2 ng/ml), in the protection against the development of ZCL were 29, 15, and 22%, respectively. However, these factors were more efficient against development of intermediate or severe forms of ZCL. Levels of Grz B >2 ng/ml showed the best efficacy of protection (equals to 72.8%) against development of these forms of ZCL. The association of such parameter with the positivity of the LST exhibited a better efficacy (equals to 83.6%). In conclusion, our results support the involvement of Leishmania-specific cytotoxic cellular immune response in host protection against Leishmania-infection. This factor could be of great interest in monitoring the success of vaccination against human leishmaniasis.

Vaccination and immunization strategies to design Aedes aegypti salivary protein based subunit vaccine tackling Flavivirus infection

Flavivirus causes arthropod-borne severe diseases that sometimes lead to the death. The Flavivirus species including Dengue virus, Zika virus and yellow fever virus are transmitted by the bite of Aedes mosquitoes. All these viral species target the people living in their respective endemic zone causing a high mortality rate. Recent studies show that immune factors present in the Ae. aegypti saliva is the hidden culprit promoting blood meal collection, suppressing host immune molecules and promoting disease establishment. This study was designed to develop a subunit vaccine using Aedes mosquito salivary proteins targeting the aforementioned Flaviviruses. Subunit vaccine was designed very precisely by combining the immunogenic B-cell epitope with CTL and HTL epitopes and also suitable adjuvant and linkers. Immunogenicity, allergenicity and physiochemical characterization were also performed for scientific validation. Molecular docking and molecular dynamics simulations studies were carried out to confirm the stable affinity between the vaccine protein (3D) and TLR3 receptor. At last, in silico cloning was executed to get the subunit vaccine restriction clone into pET28a vectro to express it in microbial expression system. Additionally, this study warrants the experimental evaluation for the validation purposes.

Canine leishmaniasis: Genome-wide analysis and antibody response to Lutzomyia longipalpis saliva

The anti-inflammatory properties of sand fly saliva favor the establishment of the Leishmania infantum infection. In contrast, an antibody response against Lutzomyia longipalpis saliva is often associated with a protective cell-mediated response against canine visceral leishmaniasis. Genetic studies may demonstrate to what extent the ability to secrete anti-saliva antibodies depends on genetic or environmental factors. However, the genetic basis of canine antibody response against sand fly saliva has not been assessed. The aim of this study was to identify chromosomal regions associated with the anti-Lu. longipalpis salivary IgG response in 189 dogs resident in endemic areas in order to provide information for prophylactic strategies. Dogs were classified into five groups based on serological and parasitological diagnosis and clinical evaluation. Anti-salivary gland homogenate (SGH) IgG levels were assessed by Enzyme-Linked Immunosorbent Assay (ELISA). Genomic DNA was isolated from blood samples and genotyped using a SNP chip with 173,662 single nucleotide polymorphism (SNP) markers. The following linear regression model was fitted: IgG level = mean + origin + sex + age + use of a repellent collar, and the residuals were assumed as pseudo-phenotypes for the association test between phenotypes and genotypes (GWA). A component of variance model that takes into account polygenic and sample structure effects (EMMAX) was employed for GWA. Phenotypic findings indicated that anti-SGH IgG levels remained higher in exposed and subclinically infected dogs than in severely diseased dogs even in regression model residuals. Five associated markers were identified on chromosomes 2, 20 and 31. The mapped genes included CD180 (RP105) and MITF related to the rapid activation of B lymphocytes and differentiation into antibody-secreting plasma cells. The findings pointed to chromosomal segments useful for functional confirmation studies and a search for adjuvant molecules of the anti-saliva response.

Clinical and immunopathological findings during long term follow-up in Leishmania infantum experimentally infected dogs

Canine Visceral Leishmaniasis (CVL) is caused by Leishmania infantum, which in the New World is transmitted by Lutzomyia longipalpis. While prospective clinical and immunological assessments of dogs experimentally challenged with L. infantum have been previously reported over a relatively short follow-up period, the long-term characterization of infected animals has not been performed to date. We evaluated dogs in a subclinical state for six years following experimental infection with L. infantum and Lu. longipalpis saliva, via an intradermal route, to characterize clinical, parasitological and immunological parameters arising from L. infantum experimental infection. We also assess these parameters in a group of naturally infected animals. The immune profiles of the experimentally and naturally infected animals exhibited increases of IFN-γ, IL-6 and IL-18, and decreases in TNF, IL-2, IL-8 and CXCL1, compared to controls. Our results indicate that over a six-year follow-up post-challenge, subclinically infected dogs presented low CVL clinical scores despite the persistence of Leishmania parasites in the lymph nodes, spleen and skin. Similarities observed among immune profiles in the context of experimental and natural infection seem to suggest that an enduring activation of the host immune response may lead to the control of parasite growth, thereby limiting disease severity.

A defined subunit vaccine that protects against vector-borne visceral leishmaniasis

Vaccine development for vector-borne pathogens may be accelerated through the use of relevant challenge models, as has been the case for malaria. Because of the demonstrated biological importance of vector-derived molecules in establishing natural infections, incorporating natural challenge models into vaccine development strategies may increase the accuracy of predicting efficacy under field conditions. Until recently, however, there was no natural challenge model available for the evaluation of vaccine candidates against visceral leishmaniasis. We previously demonstrated that a candidate vaccine against visceral leishmaniasis containing the antigen LEISH-F3 could provide protection in preclinical models and induce potent T-cell responses in human volunteers. In the present study, we describe a next generation candidate, LEISH-F3+, generated by adding a third antigen to the LEISH-F3 di-fusion protein. The rationale for adding a third component, derived from cysteine protease (CPB), was based on previously demonstrated protection achieved with this antigen, as well as on recognition by human T cells from individuals with latent infection. Prophylactic immunization with LEISH-F3+formulated with glucopyranosyl lipid A adjuvant in stable emulsion significantly reduced both Leishmania infantum and L. donovani burdens in needle challenge mouse models of infection. Importantly, the data obtained in these infection models were validated by the ability of LEISH-F3+/glucopyranosyl lipid A adjuvant in stable emulsion to induce significant protection in hamsters, a model of both infection and disease, following challenge by L. donovani-infected Lutzomyia longipalpis sand flies, a natural vector. This is an important demonstration of vaccine protection against visceral leishmaniasis using a natural challenge model.

Liposomal adjuvant development for leishmaniasis vaccines

Leishmaniasis is a parasitic disease that ranges in severity from skin lesions to fatality. Since long-lasting protection is induced upon recovery from cutaneous leishmaniasis, development of an effective vaccine is promising. However, there is no vaccine for use in humans yet. It seems limited efficacy in leishmaniasis vaccines is due to lack of an appropriate adjuvant or delivery system. Hence, the use of particulate adjuvants such as liposomes for effective delivery to the antigen presenting cells (APCs) is a valuable strategy to enhance leishmaniasis vaccine efficacy. The extraordinary versatility of liposomes because of their unique amphiphilic and biphasic nature allows for using antigens or immunostimulators within the core, on the surface or within the bilayer, and modulates both the magnitude and the T-helper bias of the immune response. In this review article, we attempt to summarize the role of liposomal adjuvants in the development of Leishmania vaccines and describe the main physicochemical properties of liposomes like phospholipid composition, surface charge, and particle size during formulation design. We also suggest potentially useful formulation strategies in order for future experiments to have a chance to succeed as liposomal vaccines against leishmaniasis.

Maxadilan-simile expression in Nyssomyia neivai, a sandfly vector in an endemic region of Brazil, and its immunogenicity in patients with American tegumentary leishmaniasis

BACKGROUND

Maxadilan (Max) is a salivary component in the sandfly Lutzomyia longipalpis (Lutz & Neiva 1912), a vector of visceral leishmaniasis. Max has a powerful vasodilatory effect and is a candidate vaccine that has been tested in experimental leishmaniasis. Nyssomyia neivai (Pinto 1926) is a vector of the pathogen responsible for American tegumentary leishmaniasis (ATL) in Brazil.

OBJECTIVE

We searched for Max expression in Ny. neivai and for antibodies against Max in ATL patients.

METHODS

cDNA and protein were extracted from the cephalic segment, including salivary glands, of Ny. neivai and analysed by polymerase chain reaction, DNA sequencing, and blotting assays. The results were compared with data obtained from Lu. longipalpis samples. We quantified antibodies against Max in serum samples from 41 patients with ATL (31 and 10 with the cutaneous and mucocutaneous forms, respectively) and 63 controls from the endemic northeastern region of São Paulo state, using enzyme-linked immunosorbent assay.

FINDINGS

Recognition of a Max-simile peptide by specific antibodies confirmed expression of a Max sequence in Ny. neivai (GenBank EF601123.1). Compared to controls, patients with ATL presented higher levels of antibodies against Max (p = 0.004); 24.4% of the patients with ATL and 3.2% of the controls presented anti-Max levels above the cutoff index (p = 0.014). The anti-Max levels were not associated with the specific clinical form of ATL, leishmanin skin test response, absence or presence of amastigotes in histopathologic exam, results of indirect immunofluorescence testing for leishmaniasis, or duration of cutaneous form disease.

MAIN CONCLUSION

High serum anti-Max levels did not protect patients against ATL, but confirmed previous natural exposure to Ny. neivai bites in this ATL endemic region.

Human immune response to salivary proteins of wild-caught Phlebotomus papatasi

Phlebotomine sand flies are the known vectors of Leishmania parasites. New approaches in vaccination against Leishmania have investigated the possibility of integrating Phlebotomus papatasi salivary proteins to enhance the immune response and protect against the transmission of the infection. The aim of the present study was to screen human immune responses to wild sand fly saliva and evaluate immunogenic salivary proteins. Blood samples were collected from donors in control and sand fly infested areas. Antibodies specific for sand fly antigens in donor plasma were probed using immunoblotting. In addition, recall proliferation capability of peripheral blood mononuclear cells (PBMC) was tested after sand fly salivary homogenates stimulation. The significant immunogenic salivary proteins (SPs) identified by immunoblotting were SP28, SP32, and SP36. A specific proliferative response of PBMC after stimulation with sand fly salivary homogenates was evident in donors that have antibody responses against sand fly salivary proteins. Individuals with antibody recognition to a higher number of salivary proteins (i.e., 3 or more SP bands) showed lower PBMC proliferative responses after in vitro stimulation with salivary gland homogenates (SGH) only in the sand fly infested, leishmaniasis free area. Interestingly, the presence of a humoral immune response to many SP antigens inversely correlates with a strong cell-mediated immune response (CMI). It was also noticed that some other heavily expressed antigens, in sand fly salivary homogenate, lack or have weak humoral immune reactivity in exposed individuals. Therefore, considering these antigens alone as CMI activators, without including the immunodominant humoral immune response proteins, needs future investigation.

The site of the bite: Leishmania interaction with macrophages, neutrophils and the extracellular matrix in the dermis

Leishmania spp., the causative agents of leishmaniasis, are intracellular parasites, transmitted to humans via the bite of their sand fly vectors. Once inoculated, the promastigotes are exposed to the dermis, which is composed of extracellular matrix (ECM), growth factors and its resident cells. Promastigote forms are phagocytosed by macrophages recruited to the site of the sand fly bite, either directly or after interaction with neutrophils. Since Leishmania is an intracellular parasite, its interaction with the host ECM has been neglected as well as the immediate steps after the sand fly bite. However, promastigotes must overcome the obstacles presented by the dermis ECM in order to establish the infection. Thus, the study of the interaction between Leishmania promastigotes and ECM components as well as the earliest stages of infection are important steps to understand the establishment of the disease, and could contribute in the future to new drug developments towards leishmaniasis.

Does Leishmaniasis disease alter the parenchyma and protein expression in salivary glands?

Leishmaniasis is considered a serious public health problem in several regions in Brazil and worldwide. This research aimed to perform a histopathological and proteomic study of parotid, submandibular and sublingual glands of BALB/c mice infected by Leishmania (L) infantum chagasi using histological, immunohistochemical and epifluorescence techniques. Twelve isogenic BALB/c male mice, around six- to eight-weeks old, were separated into two groups: the animals of the control group were injected with 0.15 ml of NaCl, while those in the experimental group were inoculated with 5 × 10(6) amastigote forms of Leishmania (L) infantum chagasi by the ip route. After 50 days, animals were euthanized and major salivary glands were collected to perform histological, immunohistochemical and epifluorescence techniques using anti-Caspase-2, anti-Ki-67 and anti-β-catenin antibodies, respectively. The histological and morphometric evaluation showed clusters of mononuclear inflammatory cells and a higher area and perimeter of the parotid gland. However, none of the salivary glands had morphophysiological impairment. There was no immunoreactivity to the anti-caspase-2 antibody and Ki67 expression in acinar and ductal cells in both groups. According to the immunofluorescence staining, the β-catenin antibodies did not show nuclear expression, suggesting no uncontrolled proliferation. The data obtained in this study showed population and morphological stability of major salivary glands after 50 days post-infection by Leishmania (L) infantum chagasi.

Intradermal Immunization of Leishmania donovani Centrin Knock-Out Parasites in Combination with Salivary Protein LJM19 from Sand Fly Vector Induces a Durable Protective Immune Response in Hamsters

Background

Visceral leishmaniasis (VL) is a neglected tropical disease and is fatal if untreated. There is no vaccine available against leishmaniasis. The majority of patients with cutaneous leishmaniasis (CL) or VL develop a long-term protective immunity after cure from infection, which indicates that development of an effective vaccine against leishmaniasis is possible. Such protection may also be achieved by immunization with live attenuated parasites that do not cause disease. We have previously reported a protective response in mice, hamsters and dogs with Leishmania donovani centrin gene knock-out parasites (LdCen^-/-), a live attenuated parasite with a cell division specific centrin1 gene deletion. In this study we have explored the effects of salivary protein LJM19 as an adjuvant and intradermal (ID) route of immunization on the efficacy of LdCen^-/- parasites as a vaccine against virulent L. donovani.

Methodology/Principal Findings

To explore the potential of a combination of LdCen^-/- parasites and salivary protein LJM19 as vaccine antigens, LdCen^-/- ID immunization followed by ID challenge with virulent L. donovani were performed in hamsters in a 9-month follow up study. We determined parasite burden (serial dilution), antibody production (ELISA) and cytokine expression (qPCR) in these animals. Compared to controls, animals immunized with LdCen^-/- + LJM19 induced a strong antibody response, a reduction in spleen and liver parasite burden and a higher expression of pro-inflammatory cytokines after immunization and one month post-challenge. Additionally, a low parasite load in lymph nodes, spleen and liver, and a non-inflamed spleen was observed in immunized animals 9 months after the challenge infection.

Conclusions

Our results demonstrate that an ID vaccination using LdCen^-/-parasites in combination with sand fly salivary protein LJM19 has the capability to confer long lasting protection against visceral leishmaniasis that is comparable to intravenous or intracardial immunization.

Leishmaniasis is a disease with a wide spectrum of clinical manifestations caused by different species of protozoa belonging to the Leishmania genus that are transmitted by sand fly vectors. Visceral infections of Leishmania cause significant mortality and morbidity and development of a vaccine to prevent leishmaniasis has become a high priority. We have previously reported that intravenous immunization with a live attenuated parasite vaccine comprised of Leishmania donovani parasites lacking the centrin gene conferred protection in mice, hamsters and dogs. In the current report, we describe the immunological response and associated protection to the ID immunization with attenuated parasites in combination with a sand fly salivary protein (LJM19). We observe that protection against experimental ID challenge with L. donovani resulting from ID immunization with live attenuated parasites in combination with LJM19 is comparable to intracardial immunization and offers improved protective immunity compared to immunization with salivary protein alone and non-immunized hamsters. This study supports the potential use of the genetically attenuated vaccine and a recombinant sand fly salivary protein for control of visceral leishmaniasis.

A proteomic map of the unsequenced kala-azar vector Phlebotomus papatasi using cell line

The debilitating disease kala-azar or visceral leishmaniasis is caused by the kinetoplastid protozoan parasite Leishmania donovani. The parasite is transmitted by the hematophagous sand fly vector of the genus Phlebotomus in the old world and Lutzomyia in the new world. The predominant Phlebotomine species associated with the transmission of kala-azar are Phlebotomus papatasi and Phlebotomus argentipes. Understanding the molecular interaction of the sand fly and Leishmania, during the development of parasite within the sand fly gut is crucial to the understanding of the parasite life cycle. The complete genome sequences of sand flies (Phlebotomus and Lutzomyia) are currently not available and this hinders identification of proteins in the sand fly vector. The current study utilizes a three frame translated transcriptomic data of P. papatasi in the absence of genomic sequences to analyze the mass spectrometry data of P. papatasi cell line using a proteogenomic approach. Additionally, we have carried out the proteogenomic analysis of P. papatasi by comparative homology-based searches using related sequenced dipteran protein data. This study resulted in the identification of 1313 proteins from P. papatasi based on homology. Our study demonstrates the power of proteogenomic approaches in mapping the proteomes of unsequenced organisms.

Differential Gene Expression and Infection Profiles of Cutaneous and Mucosal Leishmania braziliensis Isolates from the Same Patient

Background

Leishmaniasis is a complex disease in which clinical outcome depends on factors such as parasite species, host genetics and immunity and vector species. In Brazil, Leishmania (Viannia) braziliensis is a major etiological agent of cutaneous (CL) and mucosal leishmaniasis (MCL), a disfiguring form of the disease, which occurs in ~10% of L. braziliensis-infected patients. Thus, clinical isolates from patients with CL and MCL may be a relevant source of information to uncover parasite factors contributing to pathogenesis. In this study, we investigated two pairs of L. (V.) braziliensis isolates from mucosal (LbrM) and cutaneous (LbrC) sites of the same patient to identify factors distinguishing parasites that migrate from those that remain at the primary site of infection.

Methodology/Principal Findings

We observed no major genomic divergences among the clinical isolates by molecular karyotype and genomic sequencing. RT-PCR revealed that the isolates lacked Leishmania RNA virus (LRV). However, the isolates exhibited distinct in vivo pathogenesis in BALB/c mice; the LbrC isolates were more virulent than the LbrM isolates. Metabolomic analysis revealed significantly increased levels of 14 metabolites in LbrC parasites and 31 metabolites in LbrM parasites that were mainly related to inflammation and chemotaxis. A proteome comparative analysis revealed the overexpression of LbrPGF2S (prostaglandin f2-alpha synthase) and HSP70 in both LbrC isolates. Overexpression of LbrPGF2S in LbrC and LbrM promastigotes led to an increase in infected macrophages and the number of amastigotes per cell at 24–48 h post-infection (p.i.).

Conclusions/Significance

Despite sharing high similarity at the genome structure and ploidy levels, the parasites exhibited divergent expressed genomes. The proteome and metabolome results indicated differential profiles between the cutaneous and mucosal isolates, primarily related to inflammation and chemotaxis. BALB/c infection revealed that the cutaneous isolates were more virulent than the mucosal parasites. Furthermore, our data suggest that the LbrPGF2S protein is a candidate to contribute to parasite virulence profiles in the mammalian host.

Leishmaniasis is a critical public health problem worldwide. The clinical outcome of leishmaniasis depends on the infecting parasite species, host genetics and immune response and insect species. Leishmania braziliensis is a major etiological agent of cutaneous and mucosal leishmaniasis in Brazil. Fewer than 10% of L. braziliensis-infected patients with CL develop the mucosal form (a severe clinical manifestation). The small number of parasites in the mucosae increases the difficulty of obtaining clinical isolates, and parasite samples are frequently derived from individuals with different genetic backgrounds. Therefore, clinical isolates from cutaneous and mucosal sites from the same patient represent unique tools to understand parasite factors that contribute to disease outcome and pathogenesis. In this study, we investigated parasite factors involved in disease progression using two pairs of L. (V.) braziliensis isolates from mucosal (LbrM) and cutaneous (LbrC) sites of the same patient. In conclusion, the murine infection and proteome and metabolome data suggest that the differences between the cutaneous and mucosal isolates are mainly related to inflammation and chemotaxis. Our data also suggest that the LbrPGF2S protein plays a role in parasite virulence in the mammalian host.

Immunogenicity and Serological Cross-Reactivity of Saliva Proteins among Different Tsetse Species

Tsetse are vectors of pathogenic trypanosomes, agents of human and animal trypanosomiasis in Africa. Components of tsetse saliva (sialome) are introduced into the mammalian host bite site during the blood feeding process and are important for tsetse’s ability to feed efficiently, but can also influence disease transmission and serve as biomarkers for host exposure. We compared the sialome components from four tsetse species in two subgenera: subgenus Morsitans: Glossina morsitans morsitans (Gmm) and Glossina pallidipes (Gpd), and subgenus Palpalis: Glossina palpalis gambiensis (Gpg) and Glossina fuscipes fuscipes (Gff), and evaluated their immunogenicity and serological cross reactivity by an immunoblot approach utilizing antibodies from experimental mice challenged with uninfected flies. The protein and immune profiles of sialome components varied with fly species in the same subgenus displaying greater similarity and cross reactivity. Sera obtained from cattle from disease endemic areas of Africa displayed an immunogenicity profile reflective of tsetse species distribution. We analyzed the sialome fractions of Gmm by LC-MS/MS, and identified TAg5, Tsal1/Tsal2, and Sgp3 as major immunogenic proteins, and the 5'-nucleotidase family as well as four members of the Adenosine Deaminase Growth Factor (ADGF) family as the major non-immunogenic proteins. Within the ADGF family, we identified four closely related proteins (TSGF-1, TSGF-2, ADGF-3 and ADGF-4), all of which are expressed in tsetse salivary glands. We describe the tsetse species-specific expression profiles and genomic localization of these proteins. Using a passive-immunity approach, we evaluated the effects of rec-TSGF (TSGF-1 and TSGF-2) polyclonal antibodies on tsetse fitness parameters. Limited exposure of tsetse to mice with circulating anti-TSGF antibodies resulted in a slight detriment to their blood feeding ability as reflected by compromised digestion, lower weight gain and less total lipid reserves although these results were not statistically significant. Long-term exposure studies of tsetse flies to antibodies corresponding to the ADGF family of proteins are warranted to evaluate the role of this conserved family in fly biology.

Insect saliva contains many proteins that are injected into the mammalian host during the blood feeding process. Saliva proteins enhance the blood feeding ability of insects, but they can also induce mammalian immune responses that inhibit successful feeding, or modulate the bite site to benefit pathogen transmission. Here we studied saliva from four different tsetse species that belong to two distant species groups. We show that the saliva protein profiles of different species groups vary. Experimental mice subjected to fly bites display varying immunological responses against the abundant saliva proteins and the antigenicity of the shared saliva proteins in different tsetse species differs. We show that one member of the ADGF family with adenosine deaminase motifs, TSGF-2, is non-immunogenic in Glossina morsitans in mice, while the same protein from Glossina fuscipes is highly immunogenic. Such species-specific immune responses could be exploited as biomarkers of host exposures in the field. We also show that short-term exposure of G. morsitans to mice passively immunized by anti-TSGF antibodies leads to slight but not statistically significant negative fitness effects. Thus, future investigations with non-antigenic saliva proteins are warranted as they can lead to novel mammalian vaccine targets to reduce tsetse populations in the field.

Expression, purification, crystallization and crystallographic study of Lutzomyia longipalpis LJL143

Leishmaniasis is a neglected vector-borne disease with a global prevalence of over 12 million cases and 59,000 annual deaths. Transmission of the parasite requires salivary proteins, including LJL143 from the New World sandfly Lutzomyia longipalpis. LJL143 is a known marker of sandfly exposure in zoonotic hosts. LJL143 was crystallized from soluble protein expressed using Pichia pastoris. X-ray data were collected to 2.6 Å resolution from orthorhombic crystals belonging to space group P2(1)2(1)2(1), with average unit-cell parameters a = 57.39, b = 70.24, c = 79.58 Å. The crystals are predicted to have a monomer in the asymmetric unit, with an estimated solvent content of 48.5%. LJL143 has negligible homology to any reported structures, so the phases could not be determined by molecular replacement. All attempts at S-SAD failed and future studies include experimental phase determination using heavy-atom derivatives.

Eosinophils and mast cells in leishmaniasis

Leishmania spp. are parasitic protozoa endemic in tropical and subtropical regions and the causative agent of leishmaniasis, a collection of syndromes whose clinical manifestations vary according to host and pathogen factors. Leishmania spp. are inoculated into the mammalian host by the bite of an infected sand fly, whereupon they are taken up by phagocytosis, convert into the replicative amastigote stage within macrophages, reproduce, spread to new macrophages and cause disease manifestations. A curative response against leishmaniasis depends in the classical activation of macrophages and the IL-12-dependent onset of an adaptive type 1 response characterized by the production of IFN-γ. Emerging evidence suggests that neutrophils, dendritic cells and other immune cells can serve as either temporary or stable hosts for Leishmania spp. Furthermore, it is becoming apparent that the initial interactions of the parasite with resident or early recruited immune cells can shape both the macrophage response and the type of adaptive immune response being induced. In this review, we compile a growing number of studies demonstrating how the earliest interactions of Leishmania spp. with eosinophils and mast cells influence the macrophage response to infection and the development of the adaptive immune response, hence, determining the ultimate outcome of infection.

Canine Leishmania vaccines: still a long way to go

Dogs are the main reservoir host for zoonotic visceral leishmaniasis, a sand fly-borne disease caused by Leishmania infantum. In endemic areas, "susceptible" dogs suffer from a severe disease characterized by chronic polymorphic viscerocutaneous signs that manifest several months from the exposure, whereas "resistant" dogs can remain subclinically infected for years or lifelong. The protective immune response to Leishmania is cell-mediated; for visceralizing Leishmania species a mixed T helper (Th)1/Th2 response with a dominant Th1 profile is required for protection. The activation of the adaptive immune system in naturally resistant dogs is revealed by parasite-specific lymphoproliferation, delayed-type hypersensitivity, the production of interferon-γ and tumour necrosis factor-α cytokines, and enhanced macrophage leishmanicidal activity via nitric oxide. Hence, an effective canine Leishmania vaccine should induce strong and long-lasting Th1-dominated immunity to control both infection progression and the parasite transmissibility via the vector. Preclinical research in rodent models has evaluated the efficacy of several categories of Leishmania antigens including killed parasites, cell purified fractions, parasite protein components or subunits, single or multiple chimeric recombinant proteins, plasmid DNA and viral particles encoding parasite virulence factors. Promising antigen(s)/adjuvant combinations from each of the above categories have also been tested in dogs; they mostly resulted in limited or no protection in Phase I-II studies (designed to test vaccine safety, immunogenicity and laboratory-induced protection) in which vaccinated dogs were challenged by the artificial intravenous injection of high-load L. infantum promastigotes. The recombinant A2 antigen plus saponin conferred about 40% protection against infection by this challenge system and has been registered in Brazil as a canine vaccine (LeishTec(®)). An increasing number of efficacy studies have privileged the use of natural challenge consisting in the long-term exposure of vaccinated dogs in endemic settings (Phase III). A 2-year field model including regular assessments by a set of standard diagnostic markers useful for an accurate infection staging has been developed. Again, most of the vaccines tested by this system, which included several antigen categories and adjuvants, failed to protect against infection and disease. Only two vaccines, consisting of parasite purified fractions with saponin derivative adjuvants, showed to confer significant protection against disease and death under natural conditions, and have been registered as canine vaccines: FML-QuilA (Leishmune(®)) in Brazil, and LiESP/QA-21 (CaniLeish(®)) in Europe.

Prostaglandin E2/leukotriene B4 balance induced by Lutzomyia longipalpis saliva favors Leishmania infantum infection

Background

Eicosanoids and sand fly saliva have a critical role in the Leishmania infection. Here, we evaluated the effect of Lutzomyia longipalpis salivary gland sonicate (SGS) on neutrophil and monocyte recruitment and activation of eicosanoid production in a murine model of inflammation.

Methods

C57BL/6 mice were inoculated intraperitonealy with Lutzomyia longipalpis SGS or Leishmania infantum or both, followed by analyses of cell recruitment, parasite load and eicosanoid production.

Results

Intraperitoneal injection of Lutzomyia longipalpis SGS together with Leishmania infantum induced an early increased parasite viability in monocytes and neutrophils. L. longipalpis SGS increased prostaglandin E₂ (PGE₂), but reduced leukotriene B₄ (LTB₄) production ex vivo in peritoneal leukocytes. In addition, the pharmacological inhibition of cyclooxygenase 2 (COX-2) with NS-398 decreased parasite viability inside macrophages during Leishmania infection in the presence of L. longipalpis SGS arguing that PGE₂ production is associated with diminished parasite killing.

Conclusions

These findings indicate that L. longipalpis SGS is a critical factor driving immune evasion of Leishmania through modulation of PGE₂/LTB₄ axis, which may represent an important mechanism on establishment of the infection.

Salivary gland homogenates from wild-caught sand flies Lutzomyia flaviscutellata and Lutzomyia (Psychodopygus) complexus showed inhibitory effects on Leishmania (Leishmania) amazonensis and Leishmania (Viannia) braziliensis infection in BALB/c mice

During the natural transmission of Leishmania parasites, the infected sand fly female regurgitates promastigotes into the host's skin together with its saliva. It has been reported that vector saliva contains immunomodulatory molecules that facilitate the establishment of infection. Thus, the main objective of this study was to evaluate the specificity of Lutzomyia (Lu.) flaviscutellata and Lu. (Psychodopygus) complexus salivas on the infectivity of Leishmania (L.) (Leishmania) amazonensis and L. (Viannia) braziliensis, respectively. BALB/c mice were inoculated into the skin of hind footpad with L. (L.) amazonensis and L. (V.) braziliensis promastigotes in the absence or presence of Lu. flaviscutellata and Lu. (P.) complexus salivary gland homogenates (SGHs). The evolution of the infection was evaluated by lesion size, histopathological analysis and determination of the parasite load in the skin biopsies collected from the site of infection at 4 and 8 weeks PI. The lesion size and the parasite load of both groups of mice infected in the presence of SGHs were smaller than the control groups. The histopathological features showed that the inflammatory reaction was less prominent in the groups of mice infected in the presence of both SGHs when compared to the control group. The results showed that the presence of SGHs of Lu. flaviscutellata and Lu. (P.) complexus led to induction of processes that were disadvantageous to parasite establishment during infection by L. (L.) amazonensis and L. (V.) braziliensis. An inhibitory effect on Leishmania infection could be observed in both groups inoculated with SGHs, especially when the SGH from Lu. (P.) complexus was used.

What's behind a sand fly bite? The profound effect of sand fly saliva on host hemostasis, inflammation and immunity

Sand flies are blood-feeding insects and vectors of the Leishmania parasite. For many years, saliva of these insects has represented a gold mine for the discovery of molecules with anti-hemostatic and immuno-modulatory activities. Furthermore, proteins in sand fly saliva have been shown to be a potential vaccine against leishmaniasis and also markers of vector exposure. A bottleneck to progress in these areas of research has been the identification of molecules responsible for the observed activities and properties of saliva. Over the past decade, rapid advances in transcriptomics and proteomics resulted in the completion of a number of sialomes (salivary gland transcriptomes) and the expression of several recombinant salivary proteins from different species of sand fly vectors. This review will provide readers with a comprehensive update of recent advances in the characterization of these salivary molecules and their biological activities and offer insights pertaining to their protective effect against leishmaniasis and their potential as markers of vector exposure.

Vector saliva in vaccines for visceral leishmaniasis: a brief encounter of high consequence?

Visceral leishmaniasis (VL) is a vector-borne disease transmitted by phlebotomine sand flies and remains the most serious form of the disease with no available human vaccine. Repeatedly, studies have demonstrated the immunogenicity and protective efficacy of a number of sand fly salivary proteins against cutaneous and visceral leishmaniasis. All Leishmania species including agents of VL are co-deposited into the skin together with vector saliva. Generally, the immune response to a protective salivary protein in vaccinated animals is rapid and possibly acts on the parasites soon after delivery into the skin by the bite of an infective sand fly. This is followed by the development of a stronger Leishmania-specific immunity in saliva-vaccinated animals compared to controls. Considering that several of the most efficacious protective molecules were identified from a proven vector of VL, we put forward the notion that a combination vaccine that includes a Leishmania antigen and a vector salivary protein has the potential to improve vaccine efficacy by targeting the parasite at it most vulnerable stage just after transmission.

Characterization of the early inflammatory infiltrate at the feeding site of infected sand flies in mice protected from vector-transmitted Leishmania major by exposure to uninfected bites

Background

Mice exposed to sand fly saliva are protected against vector-transmitted Leishmania major. Although protection has been related to IFN- γ producing T cells, the early inflammatory response orchestrating this outcome has not been defined.

Methodology/Principal findings

Mice exposed to uninfected P. duboscqi bites and naïve mice were challenged with L. major-infected flies to characterize their early immune response at the bite site. Mostly, chemokine and cytokine transcript expression post-infected bites was amplified in exposed compared to naïve mice. In exposed mice, induced chemokines were mostly involved in leukocyte recruitment and T cell and NK cell activation; IL-4 was expressed at 6 h followed by IFN-γ and iNOS2 as well as IL-5 and IL-10 expression. In naïve animals, the transcript expression following Leishmania-infected sand fly bites was suppressed. Expression profiles translated to an earlier and significantly larger recruitment of leukocytes including neutrophils, macrophages, Gr⁺ monocytes, NK cells and CD4⁺ T cells to the bite site of exposed compared to naïve mice post-infected bites. Additionally, up to 48 hours post-infected bites the number of IFN-γ-producing CD4⁺T cells and NK cells arriving at the bite site was significantly higher in exposed compared to naïve mice. Thereafter, NK cells become cytolytic and persist at the bite site up to a week post-bite.

Conclusion/Significance

The quiet environment induced by a Leishmania-infected sand fly bite in naïve mice was significantly altered in animals previously exposed to saliva of uninfected flies. We propose that the enhanced recruitment of Gr⁺ monocytes, NK cells and CD4 Th1 cells observed at the bite site of exposed mice creates an inhospitable environment that counters the establishment of L. major infection.

Sand flies transmit Leishmania parasites during bloodfeeding. Salivary molecules are deposited alongside parasites and can reshape the host's immune response to infection. Exposure to uninfected sand fly bites or immunization with salivary molecules protects the host against Leishmania infection. Here we show that mice exposed to bites of uninfected Phlebotomus duboscqi sand flies are protected against P. duboscqi-transmitted L. major and characterize the formerly unknown early cellular infiltrate at the bite site following L.major vector-transmission. The kinetics and nature of the inflammatory response at the bite site of exposed mice were notably different from those of naïve mice showing an amplified expression of cytokines and chemokines after parasite transmission. The transcripts reflected a faster and more robust infiltrate of immune cells to the bite site of exposed mice composed of neutrophils, macrophages, monocytes, NK cells and CD4+ T cells. In addition, there was an increased influx of activated IFN-γ producing CD4+ T cells and Granzyme B-producing mature NK cells in exposed animals. These findings suggest that the observed robust and persistent proinflammatory response in exposed animals restrict parasite multiplication.

Distinct cellular migration induced by Leishmania infantum chagasi and saliva from Lutzomyia longipalpis in a hemorrhagic pool model

Recruitment of a specific cell population after Leishmania infection can influence the outcome of the disease. Cellular migration in response to Leishmania or vector saliva has been reported in air pouch model, however, cellular migration induced by Leishmania associated with host's blood and vector saliva in this model has not been described. Herein we investigated cellular migration into air pouch of hamster after stimulation with combination of L. chagasi and host's blood and Lutzomyia longipalpis saliva. Migration induced by saliva was 3-fold more than those induced by L. chagasi alone. Additionally, L. chagasi associated with blood and saliva induced significantly even more leukocytes into air pouch than Leishmania alone. L. chagasi recruited a diverse cell population; however, most of these cells seem to have not migrated to the inflammatory exudate, remaining in the pouch lining tissue. These results indicate that L. chagasi can reduce leukocyte accumulation to the initial site of infection, and when associated with vector saliva in the presence of blood components, increase the influx of more neutrophils than macrophages, suggesting that the parasite has developed a strategy to minimize the initial inflammatory response, allowing an unlimited progression within the host. This work reinforces the importance of studies on the salivary components of sand fly vectors of leishmaniasis in the transmission process and the establishment of the infection.

A Listeria monocytogenes-based vaccine that secretes sand fly salivary protein LJM11 confers long-term protection against vector-transmitted Leishmania major

Cutaneous leishmaniasis is a sand fly-transmitted disease characterized by skin ulcers that carry significant scarring and social stigmatization. Over the past years, there has been cumulative evidence that immunity to specific sand fly salivary proteins confers a significant level of protection against leishmaniasis. In this study, we used an attenuated strain of Listeria monocytogenes as a vaccine expression system for LJM11, a sand fly salivary protein identified as a good vaccine candidate. We observed that mice were best protected against an intradermal needle challenge with Leishmania major and sand fly saliva when vaccinated intravenously. However, this protection was short-lived. Importantly, groups of vaccinated mice were protected long term when challenged with infected sand flies. Protection correlated with smaller lesion size, fewer scars, and better parasite control between 2 and 6 weeks postchallenge compared to the control group of mice vaccinated with the parent L. monocytogenes strain not expressing LJM11. Moreover, protection correlated with high numbers of CD4(+), gamma interferon-positive (IFN-γ(+)), tumor necrosis factor alpha-positive/negative (TNF-α(+/-)), interleukin-10-negative (IL-10(-)) cells and low numbers of CD4(+) IFN-γ(+/-) TNF-α(-) IL-10(+) T cells at 2 weeks postchallenge. Overall, our data indicate that delivery of LJM11 by Listeria is a promising vaccination strategy against cutaneous leishmaniasis inducing long-term protection against ulcer formation following a natural challenge with infected sand flies.

Repeated exposure to Lutzomyia intermedia sand fly saliva induces local expression of interferon-inducible genes both at the site of injection in mice and in human blood

During a blood meal, Lutzomyia intermedia sand flies transmit Leishmania braziliensis, a parasite causing tegumentary leishmaniasis. In experimental leishmaniasis, pre-exposure to saliva of most blood-feeding sand flies results in parasite establishment in absence of any skin damages in mice challenged with dermotropic Leishmania species together with saliva. In contrast, pre-immunization with Lu. intermedia salivary gland sonicate (SGS) results in enhanced skin inflammatory exacerbation upon co-inoculation of Lu. intermedia SGS and L. braziliensis. These data highlight potential unique features of both L. braziliensis and Lu. intermedia. In this study, we investigated the genes modulated by Lu. intermedia SGS immunization to understand their potential impact on the subsequent cutaneous immune response following inoculation of both SGS and L. braziliensis. The cellular recruitment and global gene expression profile was analyzed in mice repeatedly inoculated or not with Lu. intermedia. Microarray gene analysis revealed the upregulation of a distinct set of IFN-inducible genes, an immune signature not seen to the same extent in control animals. Of note this INF-inducible gene set was not induced in SGS pre-immunized mice subsequently co-inoculated with SGS and L. braziliensis. These data suggest the parasite prevented the upregulation of this Lu. intermedia saliva-related immune signature. The presence of these IFN-inducible genes was further analyzed in peripheral blood mononuclear cells (PBMCs) sampled from uninfected human individuals living in a L. braziliensis-endemic region of Brazil thus regularly exposed to Lu. intermedia bites. PBMCs were cultured in presence or absence of Lu. intermedia SGS. Using qRT-PCR we established that the IFN-inducible genes induced in the skin of SGS pre-immunized mice, were also upregulated by SGS in PBMCs from human individuals regularly exposed to Lu. intermedia bites, but not in PBMCs of control subjects. These data demonstrate that repeated exposure to Lu. intermedia SGS induces the expression of potentially host-protective IFN-inducible genes.

Leishmaniasis is a vector-borne parasitic disease of serious public health importance. No efficient vaccine is currently available. Parasites are transmitted to mammalian hosts during sand fly bites. During this process, both parasites and sand fly salivary products are delivered into the skin. Immunization with salivary proteins from most sand fly species can protect mice against cutaneous leishmaniasis; however, immunization with sand fly saliva of Lutzomyia intermedia leads to aggravation of leishmaniasis. We investigated the impact of Lutzomyia intermedia saliva exposure on the development of immune response to Leishmania braziliensis, the major causative agent of tegumentary leishmaniasis in Brazil. To this end, we analyzed in mice the gene expression pattern induced by immunization with salivary gland extracts. Among the genes highly induced were the interferon-inducible genes known to contribute to resistance against parasite infections. These genes were also induced in blood cells of human individuals that were naturally pre-exposed to bites of Lutzomyia intermedia sand flies. Interestingly, subsequent infection with Leishmania braziliensis blocked the induction of these genes in mice. These data show that the induction of potentially protective genes by insect saliva can be altered by the infecting parasite. This should be considered when including salivary components in a vaccine.

Phlebotomine sandflies and the spreading of leishmaniases and other diseases of public health concern

Phlebotomine sandflies transmit pathogens that affect humans and animals worldwide. We review the roles of phlebotomines in the spreading of leishmaniases, sandfly fever, summer meningitis, vesicular stomatitis, Chandipura virus encephalitis and Carrión's disease. Among over 800 species of sandfly recorded, 98 are proven or suspected vectors of human leishmaniases; these include 42 Phlebotomus species in the Old World and 56 Lutzomyia species in the New World (all: Diptera: Psychodidae). Based on incrimination criteria, we provide an updated list of proven or suspected vector species by endemic country where data are available. Increases in sandfly diffusion and density resulting from increases in breeding sites and blood sources, and the interruption of vector control activities contribute to the spreading of leishmaniasis in the settings of human migration, deforestation, urbanization and conflict. In addition, climatic changes can be expected to affect the density and dispersion of sandflies. Phlebovirus infections and diseases are present in large areas of the Old World, especially in the Mediterranean subregion, in which virus diversity has proven to be higher than initially suspected. Vesiculovirus diseases are important to livestock and humans in the southeastern U.S.A. and Latin America, and represent emerging human threats in parts of India. Carrión's disease, formerly restricted to regions of elevated altitude in Peru, Ecuador and Colombia, has shown recent expansion to non-endemic areas of the Amazon basin.

Experimental infection of dogs with Leishmania and saliva as a model to study Canine Visceral Leishmaniasis

Background

Canine Visceral Leishmaniasis (CVL) is a zoonotic disease caused by Leishmania infantum, transmitted by the bite of Lutzomyia longipalpis sand flies. Dogs are the main domestic reservoir of the parasite. The establishment of an experimental model that partially reproduces natural infection in dogs is very important to test vaccine candidates, mainly regarding those that use salivary proteins from the vector and new therapeutical approaches.

Methodology/Principal Findings

In this report, we describe an experimental infection in dogs, using intradermal injection of Leishmania infantum plus salivary gland homogenate (SGH) of Lutzomyia longipalpis. Thirty-five dogs were infected with 1×10⁷ parasites combined with five pairs of Lutzomyia longipalpis salivary glands and followed for 450 days after infection and clinical, immunological and parasitological parameters were evaluated. Two hundred and ten days after infection we observed that 31,4% of dogs did not display detectable levels of anti-Leishmania antibodies but all presented different numbers of parasites in the lymph nodes. Animals with a positive xenodiagnosis had at least 3,35×10⁵ parasites in their lymph nodes. An increase of IFN-γ and IL-10 levels was detected during infection. Twenty two percent of dogs developed symptoms of CVL during infection.

Conclusion

The infection model described here shows some degree of similarity when compared with naturally infected dogs opening new perspectives for the study of CVL using an experimental model that employs the combination of parasites and sand fly saliva both present during natural transmission.

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

Background

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

Methodology/Principal Findings

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

Conclusion

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

New Insights on the Inflammatory Role of Lutzomyia longipalpis Saliva in Leishmaniasis

When an haematophagous sand fly vector insect bites a vertebrate host, it introduces its mouthparts into the skin and lacerates blood vessels, forming a hemorrhagic pool which constitutes an intricate environment of cell interactions. In this scenario, the initial performance of host, parasite, and vector “authors” will heavily influence the course of Leishmania infection. Recent advances in vector-parasite-host interaction have elucidated “co-authors” and “new roles” not yet described. We review here the stimulatory role of Lutzomyia longipalpis saliva leading to inflammation and try to connect them in an early context of Leishmania infection.

DNA vaccination with KMP11 and Lutzomyia longipalpis salivary protein protects hamsters against visceral leishmaniasis

It was recently shown that immunization of hamsters with DNA plasmids coding LJM19, a sand fly salivary protein, partially protected against a challenge with Leishmania chagasi, whereas immunization with KMP11 DNA plasmid, a Leishmania antigen, induced protection against L. donovani infection. In the present study, we evaluated the protective effect of immunization with both LJM19 and KMP11 DNA plasmid together. Concerning the protection against an infection by L. chagasi, immunization with DNA plasmids coding LJM19 or KMP11, as well as with both plasmids combined, induced IFN-γ production in draining lymph nodes at 7, 14 and 21 days post-immunization. Immunized hamsters challenged with L. chagasi plus Salivary Gland Sonicate (SGS) from Lutzomyia longipalpis showed an enhancement of IFN-γ/IL-10 and IFN-γ/TGF-β in draining lymph nodes after 7 and 14 days of infection. Two and five months after challenge, immunized animals showed reduced parasite load in the liver and spleen, as well as increased IFN-γ/IL-10 and IFN-γ/TGF-β ratios in the spleen. Furthermore, immunized animals remained with a normal hematological profile even five months after the challenge, whereas L. chagasi in unimmunized hamsters lead to a significant anemia. The protection observed with LJM19 or KMP11 DNA plasmids used alone was very similar to the protection obtained by the combination of both plasmids.

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.

Vaccines to combat the neglected tropical diseases

The neglected tropical diseases (NTDs) represent a group of parasitic and related infectious diseases such as amebiasis, Chagas disease, cysticercosis, echinococcosis, hookworm, leishmaniasis, and schistosomiasis. Together, these conditions are considered the most common infections in low- and middle-income countries, where they produce a level of global disability and human suffering equivalent to better known conditions such as human immunodeficiency virus/acquired immunodeficiency syndrome and malaria. Despite their global public health importance, progress on developing vaccines for NTD pathogens has lagged because of some key technical hurdles and the fact that these infections occur almost exclusively in the world's poorest people living below the World Bank poverty line. In the absence of financial incentives for new products, the multinational pharmaceutical companies have not embarked on substantive research and development programs for the neglected tropical disease vaccines. Here, we review the current status of scientific and technical progress in the development of new neglected tropical disease vaccines, highlighting the successes that have been achieved (cysticercosis and echinococcosis) and identifying the challenges and opportunities for development of new vaccines for NTDs. Also highlighted are the contributions being made by non-profit product development partnerships that are working to overcome some of the economic challenges in vaccine manufacture, clinical testing, and global access.

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.

Lutzomyia longipalpis saliva or salivary protein LJM19 protects against Leishmania braziliensis and the saliva of its vector, Lutzomyia intermedia

Background

Leishmania transmission occurs in the presence of insect saliva. Immunity to Phlebotomus papatasi or Lutzomyia longipalpis saliva or salivary components confers protection against an infection by Leishmania in the presence of the homologous saliva. However, immunization with Lutzomyia intermedia saliva did not protect mice against Leishmania braziliensis plus Lu. intermedia saliva. In the present study, we have studied whether the immunization with Lu. longipalpis saliva or a DNA plasmid coding for LJM19 salivary protein would be protective against L. braziliensis infection in the presence of Lu. intermedia saliva, the natural vector for L. braziliensis.

Methodology/Principal Findings

Immunization with Lu. longipalpis saliva or with LJM19 DNA plasmid induced a Delayed-Type Hypersensitivity (DTH) response against Lu. longipalpis as well as against a Lu. intermedia saliva challenge. Immunized and unimmunized control hamsters were then intradermally infected in the ears with L. braziliensis in the presence of Lu. longipalpis or Lu. intermedia saliva. Animals immunized with Lu. longipalpis saliva exhibited smaller lesion sizes as well as reduced disease burdens both at lesion site and in the draining lymph nodes. These alterations were associated with a significant decrease in the expression levels of IL-10 and TGF-β. Animals immunized with LJM19 DNA plasmid presented similar findings in protection and immune response and additionally increased IFN-γ expression.

Conclusions/Significance

Immunization with Lu. longipalpis saliva or with a DNA plasmid coding LJM19 salivary protein induced protection in hamsters challenged with L. braziliensis plus Lu. intermedia saliva. These findings point out an important role of immune response against saliva components, suggesting the possibility to develop a vaccine using a single component of Lu. longipalpis saliva to generate protection against different species of Leishmania, even those transmitted by a different vector.

Leishmaniasis, caused by parasitic protozoa Leishmania, is transmitted by bites of female sand flies that, during blood-feeding, inject humans with parasites and saliva. Sand fly saliva has been investigated as a potential vaccine candidate. It was previously shown that immunization with Lutzomyia longipalpis saliva or salivary proteins protects against cutaneous and visceral leishmaniasis. In the present study, we evaluated if immunization with Lu. longipalpis saliva or DNA plasmid coding for a specific sand fly salivary protein (LJM19) can protect hamsters against L. braziliensis plus another sand fly saliva. Immunization with saliva or LJM19 DNA plasmid induced a mononuclear cell infiltrate which can be a marker of protection. The immune response induced by immunization with these insect molecules was able to protect animals against L. braziliensis infection as shown by the significant reduction in lesion size, parasite load in the ear and draining lymph node. These data show the important role of immune response against sand fly saliva components, suggesting the possibility to develop vaccines using a single component of saliva against Leishmania transmitted by different vectors.

BluePort: a platform to study the eosinophilic response of mice to the bite of a vector of Leishmania parasites, Lutzomyia longipalpis sand flies

Background

Visceral Leishmaniasis is a serious human disease transmitted, in the New World, by Lutzomyia longipalpis sand flies. Natural resistance to Leishmania transmission in residents of endemic areas has been attributed to the acquisition of immunity to sand fly salivary proteins. One theoretical way to accelerate the acquisition of this immunity is to increase the density of antigen-presenting cells at the sand fly bite site. Here we describe a novel tissue platform that can be used for this purpose.

Methodology/Principal Findings

BluePort is a well-vascularized and macrophage-rich compartment induced in the subcutaneous tissue of mice via injection of agarose beads covered with Cibacron blue. We describe the sequence of inflammatory events leading to its formation and how it can be used to study the dermal response to the bite of L. longipalpis sand flies. Results presented indicate that a shift in the inflammatory response, from neutrophilic to eosinophilic, is the main histopathological feature associated with the immunity acquired through repeated exposure to the bite of sand flies, and that the BluePort tissue compartment could be used to accelerate this process. In addition, changes observed inside the BluePort parenchyma indicate that it could be used to study complex immunobiological processes, and to develop ectopic secondary lymphoid structures.

Conclusions/Significance

Understanding the characteristics of the dermal response to the bite of sand flies is a critical element of strategies to control leishmaniasis using vaccines that target salivary proteins. Finding that dermal eosinophilia is such a prominent component of the anti-salivary immunity induced by repeated exposure to sand fly bites raises one important consideration: how to avoid the immunological conflict derived from a protective Th2-driven immunity directed to sand fly saliva with a protective Th1-driven immunity directed to the parasite. The BluePort platform is an ideal tool to address experimentally this conundrum.

Immunity to Lutzomyia intermedia saliva modulates the inflammatory environment induced by Leishmania braziliensis

Background

During blood feeding, sand flies inject Leishmania parasites in the presence of saliva. The types and functions of cells present at the first host-parasite contact are critical to the outcome on infection and sand fly saliva has been shown to play an important role in this setting. Herein, we investigated the in vivo chemotactic effects of Lutzomyia intermedia saliva, the vector of Leishmania braziliensis, combined or not with the parasite.

Methods and Findings

We tested the initial response induced by Lutzomyia intermedia salivary gland sonicate (SGS) in BALB/c mice employing the air pouch model of inflammation. L. intermedia SGS induced a rapid influx of macrophages and neutrophils. In mice that were pre-sensitized with L. intermedia saliva, injection of SGS was associated with increased neutrophil recruitment and a significant up-regulation of CXCL1, CCL2, CCL4 and TNF-α expression. Surprisingly, in mice that were pre-exposed to SGS, a combination of SGS and L. braziliensis induced a significant migration of neutrophils and an important modulation in cytokine and chemokine expression as shown by decreased CXCL10 expression and increased IL-10 expression.

Conclusion

These results confirm that sand fly saliva modulates the initial host response. More importantly, pre-exposure to L. intermedia saliva significantly modifies the host's response to L. braziliensis, in terms of cellular recruitment and expression of cytokines and chemokines. This particular immune modulation may, in turn, favor parasite multiplication.

Transmission of Leishmania parasites occurs during blood feeding, when infected female sand flies inject humans with parasites and saliva. Chemokines and cytokines are secreted proteins that regulate the initial immune responses and have the potential of attracting and activating cells. Herein, we studied the expression of such molecules and the cellular recruitment induced by salivary proteins of the Lutzomyia intermedia sand fly. Of note, Lutzomyia intermedia is the main vector of Leishmania braziliensis, a parasite species that causes cutaneous leishmaniasis, a disease associated with the development of destructive skin lesions that can be fatal if left untreated. We observed that L. intermedia salivary proteins induce a potent cellular recruitment and modify the expression profile of chemokines and cytokines in mice. More importantly, in mice previously immunized with L. intermedia saliva, the alteration in the initial inflammatory response was even more pronounced, in terms of the number of cells recruited and in terms of gene expression pattern. These findings indicate that an existing immunity to L. intermedia sand fly induces an important modulation in the initial immune response that may, in turn, promote parasite multiplication, leading to the development of cutaneous leishmaniasis.

Imaging of the host/parasite interplay in cutaneous leishmaniasis

An understanding of host–parasite interplay is essential for the development of therapeutics and vaccines. Immunoparasitologists have learned a great deal from ‘conventional’ in vitro and in vivo approaches, but recent developments in imaging technologies have provided us (immunologists and parasitologists) with the ability to ask new and exciting questions about the dynamic nature of the parasite–immune system interface. These studies are providing us with new insights into the mechanisms involved in the initiation of a Leishmania infection and the consequent induction and regulation of the immune response. Here, we review some of the recent developments and discuss how these observations can be further developed to understand the immunology of cutaneous Leishmania infection in vivo.