The immune response to sand fly salivary proteins and its influence on leishmania immunity

An integrated bioinformatic analysis of microarray datasets to identify biomarkers and miRNA-based regulatory networks in leishmaniasis

Micro RNAs (miRNAs, miRs) and relevant networks might exert crucial functions during differential host cell infection by the different Leishmania species. Thus, a bioinformatic analysis of microarray datasets was developed to identify pivotal shared biomarkers and miRNA-based regulatory networks for Leishmaniasis. A transcriptomic analysis by employing a comprehensive set of gene expression profiling microarrays was conducted to identify the key genes and miRNAs relevant for Leishmania spp. infections. Accordingly, the gene expression profiles of healthy human controls were compared with those of individuals infected with Leishmania mexicana, L. major, L. donovani, and L. braziliensis. The enrichment analysis for datasets was conducted by utilizing EnrichR database, and Protein-Protein Interaction (PPI) network to identify the hub genes. The prognostic value of hub genes was assessed by using receiver operating characteristic (ROC) curves. Finally, the miRNAs that interact with the hub genes were identified using miRTarBase, miRWalk, TargetScan, and miRNet. Differentially expressed genes were identified between the groups compared in this study. These genes were significantly enriched in inflammatory responses, cytokine-mediated signaling pathways and granulocyte and neutrophil chemotaxis responses. The identification of hub genes of recruited datasets suggested that TNF, SOCS3, JUN, TNFAIP3, and CXCL9 may serve as potential infection biomarkers and could deserve value as prognostic biomarkers for leishmaniasis. Additionally, inferred data from miRWalk revealed a significant degree of interaction of a number of miRNAs (hsa-miR-8085, hsa-miR-4673, hsa-miR-4743-3p, hsa-miR-892c-3p, hsa-miR-4644, hsa-miR-671-5p, hsa-miR-7106-5p, hsa-miR-4267, hsa-miR-5196-5p, and hsa-miR-4252) with the majority of the hub genes, suggesting such miRNAs play a crucial role afterwards parasite infection. The hub genes and hub miRNAs identified in this study could be potentially suggested as therapeutic targets or biomarkers for the management of leishmaniasis.

Immune response profiles from humans experimentally exposed to Phlebotomus duboscqi bites

Introduction

Cutaneous leishmaniasis is a neglected vector-borne parasitic disease prevalent in 92 countries with approximately one million new infections annually. Interactions between vector saliva and the human host alter the response to infection and outcome of disease.

Methods

To characterize the human immunological responses developed against saliva of Phlebotomus duboscqi, a Leishmania major (L. major) vector, we repeatedly exposed the arms of 14 healthy U.S volunteers to uninfected P. duboscqi bites. Blood was collected a week after each exposure and used to assess total IgG antibodies against the proteins of P. duboscqi salivary gland homogenate (SGH) and the levels of IFN-gamma and IL-10 from peripheral blood mononuclear cells (PBMCs) stimulated with SGH or recombinant sand fly proteins. We analyzed skin punch biopsies of the human volunteer arms from the insect bite site and control skin site after multiple P. duboscqi exposures (four volunteers) using immunohistochemical staining.

Results

A variety of immediate insect bite skin reactions were observed. Late skin reactions to insect bites were characterized by macular hyperpigmentation and/or erythematous papules. Hematoxylin and eosin staining showed moderate mononuclear skin infiltrate with eosinophils in those challenged recently (within 2 months), eosinophils were not seen in biopsies with recall challenge (6 month post bites). An increase in plasma antigen-specific IgG responses to SGH was observed over time. Western Blot results showed strong plasma reactivity to five P. duboscqi salivary proteins. Importantly, volunteers developed a cellular immunity characterized by the secretion of IFN-gamma upon PBMC stimulation with P. duboscqi SGH and recombinant antigens.

Discussion

Our results demonstrate that humans mounted a local and systemic immune response against P. duboscqi salivary proteins. Specifically, PduM02/SP15-like and PduM73/adenosine deaminase recombinant salivary proteins triggered a Th1 type immune response that might be considered in future development of a potential Leishmania vaccine.

Mosquito Salivary Proteins and Arbovirus Infection: From Viral Enhancers to Potential Targets for Vaccines

Arthropod-borne viruses present important public health challenges worldwide. Viruses such as DENV, ZIKV, and WNV are of current concern due to an increasing incidence and an expanding geographic range, generating explosive outbreaks even in non-endemic areas. The clinical signs associated with infection from these arboviruses are often inapparent, mild, or nonspecific, but occasionally develop into serious complications marked by rapid onset, tremors, paralysis, hemorrhagic fever, neurological alterations, or death. They are predominately transmitted to humans through mosquito bite, during which saliva is inoculated into the skin to facilitate blood feeding. A new approach to prevent arboviral diseases has been proposed by the observation that arthropod saliva facilitates transmission of pathogens. Viruses released within mosquito saliva may more easily initiate host invasion by taking advantage of the host's innate and adaptive immune responses to saliva. This provides a rationale for creating vaccines against mosquito salivary proteins, especially because of the lack of licensed vaccines against most of these viruses. This review aims to provide an overview of the effects on the host immune response by the mosquito salivary proteins and how these phenomena alter the infection outcome for different arboviruses, recent attempts to generate mosquito salivary-based vaccines against flavivirus including DENV, ZIKV, and WNV, and the potential benefits and pitfalls that this strategy involves.

Structural analysis of PpSP15 and PsSP9 sand fly salivary proteins designed with a self-cleavable linker as a live vaccine candidate against cutaneous leishmaniasis

Background

Leishmania parasites are deposited in the host through sand fly bites along with sand fly saliva. Therefore, salivary proteins are promising vaccine candidates for controlling leishmaniasis. Herein, two immunogenic salivary proteins, PpSP15 from Phlebotomus papatasi and PsSP9 from Phlebotomus sergenti, were selected as vaccine candidates to be delivered by live Leishmania tarentolae as vector. The stepwise in silico protocol advantaged in this study for multi-protein design in L. tarentolae is then described in detail.

Methods

All possible combinations of two salivary proteins, PpSP15 and PsSP9, with or without T2A peptide were designed at the mRNA and protein levels. Then, the best combination for the vaccine candidate was selected based on mRNA and protein stability along with peptide analysis.

Results

At the mRNA level, the most favored secondary structure was PpSP15-T2A-PsSP9. At the protein level, the refined three-dimensional models of all combinations were structurally valid; however, local quality estimation showed that the PpSp15-T2A-PsSP9 fusion had higher stability for each amino acid position, with low root-mean-square deviation (RMSD), compared with the original proteins. In silico evaluation confirmed the PpSP15-T2A-PsSP9 combination as a good Th1-polarizing candidate in terms of high IFN-γ production and low IL-10/TGF-β ratio in response to three consecutive immunizations. Potential protein expression was then confirmed by Western blotting.

Conclusions

The approach presented herein is among the first studies to have privileged protein homology modeling along with mRNA analysis for logical live vaccine design-coding multi-proteins.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05437-x.

Different inoculum of Leishmania braziliensis concentrations influence immunopathogenesis and clinical evolution in the ear dermis hamster model of cutaneous leishmaniasis

The golden hamster (Mesocricetus auratus) is commonly used as a promising model for Leishmania braziliensis infection developing skin-ulcerated lesions. However, different protocols using high concentration of parasites inoculated in the footpad result in severe clinical disease. Here, we further investigate the outcome of the site of infection and concentration of L. braziliensis parasites inoculated on the immunopathogenesis and clinical evolution. Initially, hamsters were infected in the ear dermis or hind footpad with a concentration of 1x10⁵ parasites. Animals infected in the ear dermis developed a disease, with an increased parasite load that more closely resembled human CL lesions comparing to the group infected in the footpad. Next, we evaluated if different parasite concentrations (10⁴ , 10⁵ and 10⁶ ) inoculated in the ear dermis would impact the course and clinical aspects of infection. Hamsters infected with 10⁴ and 10⁵ parasites developed mild lesions compared to the group infected with 10⁶ that presented severe and persistent lesions. The parasite load varied between the different parasite concentrations. The inflammatory response was more intense when infection was initiated with 10⁶ parasites accompanied by an increased initial expression of IL-4, IL-10 and arginase in the lymph node followed by expression of both pro-and anti-inflammatory cytokines comparing to groups infected with 10⁴ and 10⁵ parasites. In conclusion, the number of parasites inoculated, and the initial site of infection could influence the inflammatory response, and clinical presentation. Our results suggest that the ear dermis infection model induces a chronic disease that relate to immunopathological aspects of CL natural infection.

Healed Lesions of Human Cutaneous Leishmaniasis Caused By Leishmania major Do Not Shelter Persistent Residual Parasites

In human cutaneous leishmaniasis (HCL) caused by Leishmania (L.) major, the cutaneous lesions heal spontaneously and induce a Th1-type immunity that confers solid protection against reinfection. The same holds true for the experimental leishmaniasis induced by L. major in C57BL/6 mice where residual parasites persist after spontaneous clinical cure and induce sustainable memory immune responses and resistance to reinfection. Whether residual parasites also persist in scars of cured HCL caused by L. major is still unknown. Cutaneous scars from 53 volunteers with healed HCL caused by L. major were biopsied and the tissue sample homogenates were analyzed for residual parasites by four methods: i) microscope detection of amastigotes, ii) parasite culture by inoculation on biphasic medium, iii) inoculation of tissue exctracts to the footpad of BALB/c mice, an inbred strain highly susceptible to L. major, and iv) amplification of parasite kDNA by a highly sensitive real-time PCR (RT-PCR). Our results show that the scars of healed lesions of HCL caused by L. major do not contain detectable residual parasites, suggesting that this form likely induces a sterile cure at least within the scars. This feature contrasts with other Leishmania species causing chronic, diffuse, or recidivating forms of leishmaniasis where parasites do persist in healed lesions. The possibility that alternative mechanisms to parasite persistence are needed to boost and maintain long-term immunity to L. major, should be taken into consideration in vaccine development against L. major infection.

Ayaconin, a novel inhibitor of the plasma contact system from the sand fly Lutzomyia ayacuchensis, a vector of Andean-type cutaneous leishmaniasis

Transcriptome analysis of the salivary gland cDNA library from a phlebotomine sand fly, Lutzomyia ayacuchensis, identified a transcript coding for the PpSP15/SL1 family protein as the second most abundant salivary component. In the present study, a recombinant protein of the PpSP15/SL1 family protein, designated ayaconin, was expressed in Escherichia coli, and its biological activity was characterized. The recombinant ayaconin purified from the soluble fraction of E. coli lysate efficiently inhibited the intrinsic but not extrinsic blood coagulation pathway. When the target of ayaconin was evaluated using fluorescent substrates of coagulation factors, ayaconin inhibited factor XIIa (FXIIa) activity more efficiently in a dose-dependent manner, suggesting that FXII is the primary target of ayaconin. In addition, incubation of ayaconin with FXII prior to activation effectively inhibited FXIIa activity, whereas such inhibition was not observed when ayaconin was mixed after the production of FXIIa, indicating that ayaconin inhibits the activation process of FXII to produce FXIIa, but not the enzymatic activity of FXIIa. Moreover, ayaconin was shown to bind to FXII, suggesting that the binding of ayaconin to FXII is involved in the inhibitory mechanism against FXII activation. These results suggest that ayaconin plays an important role in the blood-sucking of Lu. ayacuchensis.

Leishmania tarentolae as Potential Live Vaccine Co-Expressing Distinct Salivary Gland Proteins Against Experimental Cutaneous Leishmaniasis in BALB/c Mice Model

Leishmaniasis is a neglected vector-borne disease caused by Leishmania parasites transmitted through the infected sand flies bite. Current treatments are limited, partly due to their high cost and significant adverse effects, and no human vaccine is yet available. Sand flies saliva has been examined for their potential application as an anti-Leishmania vaccine. The salivary protein, PpSP15, was the first protective vaccine candidate against L. major. Additionally, PsSP9 was already introduced as a highly immunogenic salivary protein against L. tropica. Herein, we aimed to develop an effective multivalent live vaccine to control Cutaneous Leishmaniasis induced by two main species, L. major and L. tropica. Hence, the two above-mentioned salivary proteins using T2A linker were incorporated inside the L. tarentolae genome as a safe live vector. Then, the immunogenicity and protective effects of recombinant L. tarentolae co-expressing PpSP15 and PsSP9 were evaluated in pre-treated BALB/c mice with CpG against L. major and L. tropica. Following the cytokine assays, parasite burden and antibody assessment at different time-points at pre and post-infection, promising protective Th1 immunity was obtained in vaccinated mice with recombinant L. tarentolae co-expressing PpSP15 and PsSP9. This is the first study demonstrating the potency of a safe live vaccine based on the combination of different salivary proteins against the infectious challenge with two different species of Leishmania.

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.

Effects of multiple feedings on sensitized rabbits on the fitness of Phlebotomus papatasi (Diptera: Psychodidae)

In the present study, we evaluated the effects of antibodies anti-sandfly saliva on the fecundity of Phlebotomus papatasi, vector of zoonotic cutaneous leishmaniasis in the Old World. Rabbits were repeatedly exposed to sandfly bites. Immune sera showed increased levels of anti-sandfly saliva antibody compared to the pre-exposition period. The analysis of biological parameters revealed no decline on the feeding success of females P. papatasi fed on rabbits repeatedly exposed to sandfly bites. Our results showed that anti-sandfly saliva antibodies of rabbits are not detrimental to the fitness of females P. papatasi. Thus, rabbits did not acquire resistance to sandflies following repeated exposures, and that contribute in maintaining a high density of P. papatasi. To control sandfly infestations and Leishmania transmission, more studies are needed for a better understanding of the mechanisms governing the resistance of hosts to bites of sandflies.

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.

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

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

A comparative approach on the activation of the three complement system pathways in different hosts of Visceral Leishmaniasis after stimulation with Leishmania infantum

Visceral Leishmaniasis is an infectious disease that affects mainly humans and dogs, with the latter being important reservoirs of the parasite. Conversely, cats are naturally resistant. The immune system can offer important explanation to this problematic as there is no evidence on the role that the complement system plays in cats. In this context, effect of the complement system from human, dog and cat sera on Leishmania infantum was evaluated. Activation of the classical, alternative and lectin pathways was assessed through hemolytic and ELISA assays. Lytic activity of the complement on the parasite's viability was investigated by Transmission Electron Microscopy and Flow Cytometry. Complement proteins were more consumed in dog serum on the classical and alternative pathways, leading to less hemolytic activity, and only in cat serum they were consumed on the lectin pathway when incubated with L. infantum. Lytic activity on the parasite's surface was more accentuated in human serum, and varied throughout the parasite's developmental stages.

Cloning, high-level gene expression and bioinformatics analysis of SP15 and LeIF from Leishmania major and Iranian Phlebotomus papatasi saliva as single and novel fusion proteins: a potential vaccine candidate against leishmaniasis

BACKGROUND

Early exacerbation of cutaneous leishmaniasis is mainly affected by both the salivary and Leishmania parasite components. Little is known of the vaccine combination made by immunogenic proteins of sandfly saliva (SP15) with Leishmania parasites (LeIF) as a single prophylactic vaccine, namely SaLeish. Also, there are no data available to determine the species-specific sequence of SP15 isolated from the Iranian Phlebotomus papatasi.

METHODS

Integrated bioinformatics and genetic engineering methods were employed to design, optimize and obtain a vector-parasite-based vaccine formulation in a whole-length fusion form of LeIF-SP15 against leishmaniasis. Holistic gene optimization was initially performed to obtain a high yield of pure 'whole-SaLeish' expression using bioinformatics analyses. Genomic and salivary gland RNAs of wild-caught P. papatasi were extracted and their complementary DNA was amplified and cloned into pJET vector.

RESULTS

The new chimeric protein of whole-SaLeish and randomly selected transcripts of native PpIRSP15 (GenBank accession nos. MT025054 and MN938854, MN938855 and MN938856) were successfully expressed, purified and validated by immunoblotting assay. Furthermore, despite the single amino acid polymorphisms of PpIRSP15 found at positions Y23 and E73 within the population of wild Iranian sandflies, antigenicity and conservancy of PpIRSP15 epitopes remained constant to activate T cells.

CONCLUSIONS

The SaLeish vaccine strategy takes advantage of a plethora of vector-parasite immunogenic proteins with potential protective efficacy to stimulate both the innate and specific cellular immune responses against Leishmania parasites.

The human immune response to saliva of Phlebotomus alexandri, the vector of visceral leishmaniasis in Iraq, and its relationship to sand fly exposure and infection

Background

Sand fly saliva exposure plays an important role in immunity against leishmaniasis where it has mostly been associated with protection. Phlebotomus (Ph.) alexandri transmits Leishmania (L.) infantum, the causative agent of visceral leishmaniasis (VL), in Iraq. Our group recently demonstrated that 20% of Operation Iraqi Freedom (OIF) deployers had asymptomatic VL (AVL) indicative of prior infection by the parasite L. infantum. Little is known about Ph. alexandri saliva, and the human immune response to it has never been investigated. Here, we characterize the humoral and cellular immune response to vector saliva in OIF deployers naturally exposed to bites of Ph. alexandri and characterize their immunological profiles in association to AVL.

Methodology/Principal findings

The humoral response to Ph. alexandri salivary gland homogenate (SGH) showed that 64% of 200 OIF deployers developed an antibody response. To assess the cellular immune response to saliva, we selected a subcohort of subjects based on their post-travel (median 4 months; range 1–22 months) antibody response (SGH Antibody [Ab] positive or negative) as well as their AVL status; ten never-traveled controls were also included. Banked peripheral blood mononuclear cells (PBMC), collected ~10 years after end of deployment, were stimulated with SGH for 96 hours. The levels of IFN- γ, IL-6, IL-10, IL-13 and IL-17 were determined by ELISA. Our findings indicate that OIF deployers mounted a cellular response to SGH where the anti-SGH+ asymptomatic subjects developed the highest cytokine levels. Further, stimulation with SGH produced a mixture of pro-inflammatory and anti-inflammatory cytokines. Contrary to our hypothesis, we observed no correlation between the cellular immune response to Ph. alexandri SGH and prevention from asymptomatic infection with L. infantum.

Conclusions/Significance

As we found, although all infected deployers demonstrated persistent disease control years after deployment, this did not correlate with anti-saliva systemic cellular response. More exposure to this vector may facilitate transmission of the L. infantum parasite. Since exposure to saliva of Ph. alexandri may alter the human immune response to bites of this vector, this parameter should be taken into consideration when considering the VL risk.

This is the first report of human immune responses to Phlebotomus alexandri. Phlebotomus alexandri is a sand fly found in Southwest Asia and is the vector for transmission of the parasite Leishmania infantum, agent of visceral leishmaniasis. During the bite of this sand fly, a small amount of saliva is injected into the skin. In this study, we report the human immune response to Phlebotomus alexandri saliva. We found that 64% of people who traveled to endemic Iraq developed antibodies directed toward this sand fly’s saliva. This suggests that saliva is very immunogenic and that anti-saliva immune responses could be a good indicator of vector exposure. Additionally, we studied the cellular immune responses in saliva-stimulated white blood cells and found a Th2 biased cytokine profile.

Species diversity and spatial distribution of CL/VL vectors: assessing bioclimatic effect on expression plasticity of genes possessing vaccine properties isolated from wild-collected sand flies in endemic areas of Iran

Background

Leishmaniasis is one of the ten most important neglected tropical diseases worldwide. Understanding the distribution of vectors of visceral and cutaneous leishmaniasis (VL/CL) is one of the significant strategic frameworks to control leishmaniasis. In this study, the extent of the bioclimatic variability was investigated to recognize a rigorous cartographic of the spatial distribution of VL/CL vectors as risk-maps using ArcGIS modeling system. Moreover, the effect of bioclimatic diversity on the fold change expression of genes possessing vaccine traits (SP15 and LeIF) was evaluated in each bioclimatic region using real-time PCR analysis.

Methods

The Inverse Distance Weighting interpolation method was used to obtain accurate geography map in closely-related distances. Bioclimatic indices were computed and vectors spatial distribution was analyzed in ArcGIS10.3.1 system. Species biodiversity was calculated based on Shannon diversity index using Rv.3.5.3. Expression fold change of SP15 and LeIF genes was evaluated using cDNA synthesis and RT-qPCR analysis.

Results

Frequency of Phlebotomus papatasi was predominant in plains areas of Mountainous bioclimate covering the CL hot spots. Mediterranean region was recognized as an important bioclimate harboring prevalent patterns of VL vectors. Semi-arid bioclimate was identified as a major contributing factor to up-regulate salivary-SP15 gene expression (P = 0.0050, P < 0.05). Also, Mediterranean bioclimate had considerable effect on up-regulation of Leishmania-LeIF gene in gravid and semi-gravid P. papatasi population (P = 0.0109, P < 0.05).

Conclusions

The diversity and spatial distribution of CL/VL vectors associated with bioclimatic regionalization obtained in our research provide epidemiological risk maps and establish more effectively control measures against leishmaniasis. Oscillations in gene expression indicate that each gene has its own features, which are profoundly affected by bioclimatic characteristics and physiological status of sand flies. Given the efficacy of species-specific antigens for vaccine production, it is essential to consider bioclimatic factors that have a fundamental role in affecting the regulatory regions of environmentally responsive loci for genes used in vaccine design.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12879-021-06129-0.

Understanding the immune responses involved in mediating protection or immunopathology during leishmaniasis

Leishmaniasis is a vector-borne Neglected Tropical Disease (NTD) transmitted by the sand fly and is a major public health problem worldwide. Infections caused by Leishmania clinically manifest as a wide range of diseases, such as cutaneous (CL), diffuse cutaneous (DCL), mucosal (MCL) and visceral leishmaniasis (VL). The host innate and adaptative immune responses play critical roles in the defense against leishmaniasis. However, Leishmania parasites also manipulate the host immune response for their survival and replication. In addition, other factors such as sand fly salivary proteins and microbiota also promote disease susceptibility and parasite spread by modulating local immune response. Thus, a complex interplay between parasite, sand fly and the host immunity governs disease severity and outcome. In this review, we discuss the host immune response during Leishmania infection and highlight the factors associated with resistance or susceptibility.

Leishmaniasis

Leishmaniasis is one of the chronic debilitating vector-borne diseases caused by obligate intracellular protozoa. The global burden of disease although not increasing but potential risk of spread is there. At least 20 species of Leishmania are pathogenic to human beings. The transmission is from female sandfly through a blood meal. The disease pathogenesis is dependent on parasite and host mechanism-primarily cell-mediated immunity. The three common forms are visceral, cutaneous, and mucocutaneous. The diagnostic tests are mainly based on aspiration from the spleen or bone marrow. The use of K39 antibodies is the best serodiagnostic test. Antimonial, amphotericin B, miltefosine, and paromomycin are the drugs used to treat leishmaniasis. Amphotericin therapy shows the response within 7 to 10 days in most subjects, and 2 weeks of therapy is sufficient. However, those going into relapse need new treatment regimes. There is a definite benefit of combination therapy. However, there is still no breakthrough on a vaccine for prophylaxis. How to cite this article: Daga MK, Rohatgi I, Mishra R. Leishmaniasis. Indian J Crit Care Med 2021;25(Suppl 2):S166-S170.

Engineering a vector-based pan-Leishmania vaccine for humans: proof of principle

Leishmaniasis is a spectrum of diseases transmitted by sand fly vectors that deposit Leishmania spp. parasites in the host skin during blood feeding. Currently, available treatment options are limited, associated with high toxicity and emerging resistance. Even though a vaccine for human leishmaniasis is considered an achievable goal, to date we still do not have one available, a consequence (amongst other factors) of a lack of pre-clinical to clinical translatability. Pre-exposure to uninfected sand fly bites or immunization with defined sand fly salivary proteins was shown to negatively impact infection. Still, cross-protection reports are rare and dependent on the phylogenetic proximity of the sand fly species, meaning that the applicability of a sand fly saliva-based vaccine will be limited to a defined geography, one parasite species and one form of leishmaniasis. As a proof of principle of a future vector saliva-based pan-Leishmania vaccine, we engineered through a reverse vaccinology approach that maximizes translation to humans, a fusion protein consisting of immunogenic portions of PdSP15 and LJL143, sand fly salivary proteins demonstrated as potential vaccine candidates against cutaneous and visceral leishmaniasis, respectively. The in silico analysis was validated ex vivo, through T cell proliferation experiments, proving that the fusion protein (administered as a DNA vaccine) maintained the immunogenicity of both PdSP15 and LJL143. Additionally, while no significant effect was detected in the context of L. major transmission by P. duboscqi, this DNA vaccine was defined as partially protective, in the context of L. major transmission by L. longipalpis sand flies. Importantly, a high IFNγ response alone was not enough to confer protection, that mainly correlated with low T cell mediated Leishmania-specific IL-4 and IL-10 responses, and consequently with high pro/anti-inflammatory cytokine ratios. Overall our immunogenicity data suggests that to design a potentially safe vector-based pan-Leishmania vaccine, without geographic restrictions and against all forms of leishmaniasis is an achievable goal. This is why we propose our approach as a proof-of principle, perhaps not only applicable to the anti-Leishmania vector-based vaccines’ field, but also to other branches of knowledge that require the design of multi-epitope T cell vaccines with a higher potential for translation.

A scalable and reproducible manufacturing process for Phlebotomus papatasi salivary protein PpSP15, a vaccine candidate for leishmaniasis

Cutaneous leishmaniasis is a parasitic and neglected tropical disease transmitted by the bites of sandflies. The emergence of cutaneous leishmaniasis in areas of war, conflict, political instability, and climate change has prompted efforts to develop a preventive vaccine. One vaccine candidate antigen is PpSP15, a 15 kDa salivary antigen from the sandfly Phlebotomus papatasi that facilitates the infection of the Leishmania parasite and has been shown to induce parasite-specific cell-mediated immunity. Previously, we developed a fermentation process for producing recombinant PpSP15 in Pichia pastoris and a two-chromatographic-step purification process at 100 mL scale. Here we expand the process design to the 10 L scale and examine its reproducibility by performing three identical process runs, an essential transition step towards technology transfer for pilot manufacture. The process was able to reproducibly recover 81% of PpSP15 recombinant protein with a yield of 0.75 g/L of fermentation supernatant, a purity level of 97% and with low variance among runs. Additionally, a freeze-thaw stability study indicated that the PpSP15 recombinant protein remains stable after undergoing three freeze-thaw cycles, and an accelerated stability study confirmed its stability at 37 °C for at least one month. A research cell bank for the expression of PpSP15 was generated and fully characterized. Collectively, the cell bank and the production process are ready for technology transfer for future cGMP pilot manufacturing.

Th1 concomitant immune response mediated by IFN-γ protects against sand fly delivered Leishmania infection: Implications for vaccine design

Leishmaniasis is an unresolved global health problem with a high socio-economic impact. Data generated in mouse models has revealed that the Th1 response, with IL-12, IFN-γ, TNF-α, and IL-2 as prominent cytokines, predominantly controls the disease progression. Premised on these findings, all examined vaccine formulations have been aimed at generating a long-lived memory Th1 response. However, all vaccine formulations with the exception of live Leishmania inoculation (leishmanization) have failed to sufficiently protect against sand fly delivered infection. It has been recently unraveled that sand fly dependent factors may compromise pre-existing Th1 memory. Further scrutinizing the immune response after leishmanization has uncovered the prominent role of early (within hours) and robust IFN-γ production (Th1 concomitant immunity) in controlling the sand fly delivered secondary infection. The response is dependent upon parasite persistence and subclinical ongoing primary infection. The immune correlates of concomitant immunity (Resident Memory T cells and Effector T subsets) mitigate the early effects of sand fly delivered infection and help to control the disease. In this review, we have described the early events after sand fly challenge and the role of Th1 concomitant immunity in the protective immune response in leishmanized resistant mouse model, although leishmanization is under debate for human use. Undoubtedly, the lessons we learn from leishmanization must be further implemented in alternative vaccine approaches.

Interleukin-27 Functional Duality Balances Leishmania Infectivity and Pathogenesis

IL-27 is a cytokine that exerts diverse effects on the cells of innate and adaptive immune systems. Chiefly expressed in macrophages and dendritic cells during the early phase of Leishmania infection, IL-27 contributes to the protection against L. major infection but suppresses the protective Th1 response against L. donovani, L. infantum, L. amazonensis and L. braziliensis infections, suggesting its functional duality. During the late stage of Leishmania infection, IL-27 limits the immunopathogenic reactions and tissue damages. Herein, we analyze the mechanism of the functional duality of IL-27 in the resistance or susceptibility to Leishmania infection, prompting IL-27 for anti-Leishmanial therapy.

Insights into the salivary N-glycome of Lutzomyia longipalpis, vector of visceral leishmaniasis

During Leishmania transmission sand flies inoculate parasites and saliva into the skin of vertebrates. Saliva has anti-haemostatic and anti-inflammatory activities that evolved to facilitate bloodfeeding, but also modulate the host's immune responses. Sand fly salivary proteins have been extensively studied, but the nature and biological roles of protein-linked glycans remain overlooked. Here, we characterised the profile of N-glycans from the salivary glycoproteins of Lutzomyia longipalpis, vector of visceral leishmaniasis in the Americas. In silico predictions suggest half of Lu. longipalpis salivary proteins may be N-glycosylated. SDS-PAGE coupled to LC-MS analysis of sand fly saliva, before and after enzymatic deglycosylation, revealed several candidate glycoproteins. To determine the diversity of N-glycan structures in sand fly saliva, enzymatically released sugars were fluorescently tagged and analysed by HPLC, combined with highly sensitive LC-MS/MS, MALDI-TOF-MS, and exoglycosidase treatments. We found that the N-glycan composition of Lu. longipalpis saliva mostly consists of oligomannose sugars, with Man5GlcNAc2 being the most abundant, and a few hybrid-type species. Interestingly, some glycans appear modified with a group of 144 Da, whose identity has yet to be confirmed. Our work presents the first detailed structural analysis of sand fly salivary glycans.

Immunity to vector saliva is compromised by short sand fly seasons in endemic regions with temperate climates

Individuals exposed to sand fly bites develop humoral and cellular immune responses to sand fly salivary proteins. Moreover, cellular immunity to saliva or distinct salivary proteins protects against leishmaniasis in various animal models. In Tbilisi, Georgia, an endemic area for visceral leishmaniasis (VL), sand flies are abundant for a short period of ≤3 months. Here, we demonstrate that humans and dogs residing in Tbilisi have little immunological memory to saliva of P. kandelakii, the principal vector of VL. Only 30% of humans and 50% of dogs displayed a weak antibody response to saliva after the end of the sand fly season. Likewise, their peripheral blood mononuclear cells mounted a negligible cellular immune response after stimulation with saliva. RNA seq analysis of wild-caught P. kandelakii salivary glands established the presence of a typical salivary repertoire that included proteins commonly found in other sand fly species such as the yellow, SP15 and apyrase protein families. This indicates that the absence of immunity to P. kandelakii saliva in humans and dogs from Tbilisi is probably caused by insufficient exposure to sand fly bites. This absence of immunity to vector saliva will influence the dynamics of VL transmission in Tbilisi and other endemic areas with brief sand fly seasons.

Exploring the relationship between susceptibility to canine leishmaniosis and anti-Phlebotomus perniciosus saliva antibodies in Ibizan hounds and dogs of other breeds in Mallorca, Spain

Background

Canine leishmaniosis caused by Leishmania infantum is a neglected zoonosis transmitted by sand flies like Phlebotomus perniciosus. Clinical signs and disease susceptibility vary according to various factors, including host immune response and breed. In particular, Ibizan hounds appear more resistant. This immunocompetence could be attributed to a more frequent exposure to uninfected sand flies, eliciting a stronger anti-sand fly saliva antibody response.

Methods

This study aimed to investigate the prevalence of anti-P. perniciosus saliva antibodies in Ibizan hounds and dogs of other breeds in the Leishmania-endemic area of Mallorca, Spain, and to correlate these antibody levels with clinical, immunological and parasitological parameters. Anti-sand fly saliva IgG was examined in 47 Ibizan hounds and 45 dogs of other breeds using three methods: P. perniciosus whole salivary gland homogenate (SGH) ELISA; recombinant protein rSP03B ELISA; and rSP03B rapid tests (RT). Additionally, diagnostic performance was evaluated between methods.

Results

Results indicate significantly higher anti-SGH antibodies (P = 0.0061) and a trend for more positive SGH ELISA and RT results in Ibizan hounds compared to other breeds. General linear model analysis also found breed to be a significant factor in SGH ELISA units and a marginally significant factor in RT result. Although infection rates were similar between groups, Ibizan hounds included significantly more IFN-γ producers (P = 0.0122) and papular dermatitis cases (P < 0.0001). Older age and L. infantum seropositivity were also considered significant factors in sand fly saliva antibody levels according to at least one test. Fair agreement was found between all three tests, with the highest value between SGH and rSP03B RT.

Conclusions

To our knowledge, this is the first study elaborating the relationship between anti-P. perniciosus saliva antibodies and extensive clinical data in dogs in an endemic area. Our results suggest that Ibizan hounds experience a higher frequency of exposure to sand flies and have a stronger cellular immune response to L. infantum infection than other breed dogs. Additional sampling is needed to confirm results, but anti-P. perniciosus saliva antibodies appear to negatively correlate with susceptibility to L. infantum infection and could possibly contribute to the resistance observed in Ibizan hounds.

Functional genomics in sand fly-derived Leishmania promastigotes

Background

Leishmania development in the sand fly gut leads to highly infective forms called metacyclic promastigotes. This process can be routinely mimicked in culture. Gene expression–profiling studies by transcriptome analysis have been performed with the aim of studying promastigote forms in the sand fly gut, as well as differences between sand fly–and culture-derived promastigotes.

Findings

Transcriptome analysis has revealed the crucial role of the microenvironment in parasite development within the sand fly gut because substantial differences and moderate correlation between the transcriptomes of cultured and sand fly–derived promastigotes have been found. Sand fly–derived metacyclics are more infective than metacyclics in culture. Therefore, some caution should be exercised when using cultured promastigotes, depending on the experimental design. The most remarkable examples are the hydrophilic acidic surface protein/small endoplasmic reticulum protein (HASP/SHERP) cluster, the glycoprotein 63 (gp63), and autophagy genes, which are up-regulated in sand fly–derived promastigotes compared with cultured promastigotes. Because HASP/SHERP genes are up-regulated in nectomonad and metacyclic promastigotes in the sand fly, the encoded proteins are not metacyclic specific. Metacyclic promastigotes are distinguished by morphology and high infectivity. Isolating them from the sand fly gut is not exempt from technical difficulty, because other promastigote forms remain in the gut even 15 days after infection. Leishmania major procyclic promastigotes within the sand fly gut up-regulate genes involved in cell cycle regulation and glucose catabolism, whereas metacyclics increase transcript levels of fatty acid biosynthesis and ATP-coupled proton transport genes. Most parasite's signal transduction pathways remain uncharacterized. Future elucidation may improve understanding of parasite development, particularly signaling molecule-encoding genes in sand fly versus culture and between promastigote forms in the sand fly gut.

Conclusions

Transcriptome analysis has been demonstrated to be technically efficacious to study differential gene expression in sand fly gut promastigote forms. Transcript and protein levels are not well correlated in these organisms (approximately 25% quantitative coincidences), especially under stress situations and at differentiation processes. However, transcript and protein levels behave similarly in approximately 60% of cases from a qualitative point of view (increase, decrease, or no variation). Changes in translational efficiency observed in other trypanosomatids strongly suggest that the differences are due to translational regulation and regulation of the steady-state protein levels. The lack of low-input sample strategies does not allow translatome and proteome analysis of sand fly–derived promastigotes so far.

Exploring sand fly salivary proteins to design multiepitope subunit vaccine to fight against visceral leishmaniasis

Visceral leishmaniasis (VL) is caused by the parasites of Leishmania donovani complex, leads to the death of 20 000 to 40 000 people from 56 affected countries, worldwide. Till date, there is not a single available vaccine candidate to prevent the VL infection, and treatment only relies upon expensive and toxic chemotherapeutic options. Consequently, immunoinformatics approach was applied to design a multiepitope-based subunit vaccine to enhance the humoral as well as cell-mediated immunity. Constructed vaccine candidate was further subjected to evaluation on allergenicity and antigenicity and physiochemical parameters. Later on, disulfide engineering was performed to increase the stability of vaccine construct. Also, molecular docking and molecular dynamics simulation study were performed to check the binding affinity and stability of toll-like receptor-4 to vaccine construct complex. Finally, codon optimization and in silico cloning were performed to ensure the expression of proposed vaccine construct in a microbial expression system.

Quantification of Leishmania Parasites in Murine Models of Visceral Infection

Visceral leishmaniasis (VL) is mainly caused by Leishmania donovani (India and East Africa), and Leishmania infantum (Mediterranean Basin and South America) infections. Although murine models of visceral infection lack the clinicopathological aspects of VL in humans, they have been proven useful at advancing our knowledge in the Leishmania field. Indeed, these models have been used not only to better understand the pathophysiology of the infection but also in drug and vaccine development. This chapter focuses on the protocols used to experimentally infect mice and to quantify parasite burdens in mice infected with L. infantum using limiting dilution methodology of target organs and whole-mouse in vivo imaging.

DNA plasmid coding for Phlebotomus sergenti salivary protein PsSP9, a member of the SP15 family of proteins, protects against Leishmania tropica

BACKGROUND

The vector-borne disease leishmaniasis is transmitted to humans by infected female sand flies, which transmits Leishmania parasites together with saliva during blood feeding. In Iran, cutaneous leishmaniasis (CL) is caused by Leishmania (L.) major and L. tropica, and their main vectors are Phlebotomus (Ph.) papatasi and Ph. sergenti, respectively. Previous studies have demonstrated that mice immunized with the salivary gland homogenate (SGH) of Ph. papatasi or subjected to bites from uninfected sand flies are protected against L. major infection.

METHODS AND RESULTS

In this work we tested the immune response in BALB/c mice to 14 different plasmids coding for the most abundant salivary proteins of Ph. sergenti. The plasmid coding for the salivary protein PsSP9 induced a DTH response in the presence of a significant increase of IFN-γ expression in draining lymph nodes (dLN) as compared to control plasmid and no detectable PsSP9 antibody response. Animals immunized with whole Ph. sergenti SGH developed only a saliva-specific antibody response and no DTH response. Mice immunized with whole Ph. sergenti saliva and challenged intradermally with L. tropica plus Ph. sergenti SGH in their ears, exhibited no protective effect. In contrast, PsSP9-immunized mice showed protection against L. tropica infection resulting in a reduction in nodule size, disease burden and parasite burden compared to controls. Two months post infection, protection was associated with a significant increase in the ratio of IFN-γ to IL-5 expression in the dLN compared to controls.

CONCLUSION

This study demonstrates that while immunity to the whole Ph. sergenti saliva does not induce a protective response against cutaneous leishmaniasis in BALB/c mice, PsSP9, a member of the PpSP15 family of Ph. sergenti salivary proteins, provides protection against L. tropica infection. These results suggest that this family of proteins in Ph. sergenti, Ph. duboscqi and Ph. papatasi may have similar immunogenic and protective properties against different Leishmania species. Indeed, this anti-saliva immunity may act as an adjuvant to accelerate the cell-mediated immune response to co-administered Leishmania antigens, or even cause the activation of infected macrophages to remove parasites more efficiently. These findings highlight the idea of applying arthropod saliva components in vaccination approaches for diseases caused by vector-borne pathogens.

The Binomial Parasite-Host Immunity in the Healing Process and in Reactivation of Human Tegumentary Leishmaniasis

Leishmaniasis is a vector-borne infectious disease caused by different species of protozoa from the Leishmania genus. Classically, the disease can be classified into two main clinical forms: Visceral (VL) and Tegumentary (TL) leishmaniasis. TL is a skin/mucosal granulomatous disease that manifests mainly as cutaneous localized or disseminated ulcers, papules diffusely distributed, mucosal lesions or atypical lesions. Once the etiology of the infection is confirmed, treatment can take place, and different drugs can be administered. It has already been shown that, even when the scar is clinically evident, inflammation is still present in the native tissue, and the decrease of the inflammatory process occurs slowly during the 1st years after clinical healing. The maintenance of residual parasites in the scar tissue is also well documented. Therefore, it is no longer a surprise that, under some circumstances, therapeutic failure and/or lesion reactivation occurs. All over the years, an impressive amount of data on relapses, treatment resistance and lesion reactivation after healing has been collected, and several factors have been pointed out as having a role in the process. Different factors such as Leishmania species, parasite variability, Leishmania RNA virus 1, parasite load, parasite persistence, age, nutritional status, gender, co-morbidities, co-infection, pregnancy, immunosuppression, lesion duration, number and localization of lesions, drug metabolism, irregular treatment and individual host cellular immune response were described and discussed in the present review. Unfortunately, despite this amount of information, a conclusive understanding remains under construction. In addition, multifactorial influence cannot be discarded. In this context, knowing why leishmaniasis has been difficult to treat and control can help the development of new approaches, such as drugs and immunotherapy in order to improve healing maintenance. In this sense, we would like to highlight some of the findings that may influence the course of Leishmania infection and the therapeutic response, with an emphasis on TL.

Salivary Gland Extract Modulates the Infection of Two Leishmania enriettii Strains by Interfering With Macrophage Differentiation in the Model of Cavia porcellus

The subgenus Mundinia includes several Leishmania species that have human and veterinary importance. One of those members, Leishmania Mundinia enriettii was isolated from the guinea pig Cavia porcellus in the 1940s. Several histopathological studies have already been performed in this species in the absence of salivary gland extract (SGE), which are determinant and the early and future events of the infection. Our main hypothesis is that SGE could differentially modulate the course of the lesion and macrophage differentiation caused by avirulent and virulent L. enriettii strains. Here, the C. porcellus nasal region was infected using needles with two strains of L. enriettii (L88 and Cobaia) in the presence/absence of SGE and followed for 12 weeks. Those strains vary in terms of virulence, and their histopathological development was characterized. Some L88-infected animals could develop ulcerated/nodular lesions, whereas Cobaia strain developed non-ulcerated nodular lesions. Animals experimentally inoculated developed a protuberance and/or lesion after the 4th and 5th weeks of infection. Macroscopically, the size of lesion in L88-infected animals was smaller in the presence of SGE. Remarkable differences were detected microscopically in the presence of SGE for both strains. After the 6th and 7th weeks, L88-infected animals were heavily parasitized with an intense inflammatory profile bearing amastigotes and pro-inflammatory cells compared to those infected by Cobaia strain. Morphometry analysis revealed that L1+ macrophages were abundant in the L88 infection, but not in the Cobaia infection. In the presence of SGE, an increased CD163+ macrophage infiltrate by both strains was detected. Interestingly, this effect was more pronounced in Cobaia-infected animals. This study showed the role of SGE during the course of L. enriettii (strains L88 and Cobaia) infection and its role in modulating macrophage attraction to the lesion site. SGE decreased L1+ macrophages and this may favor an escaping mechanism for L88 parasites. On the other hand, in the presence of SGE, an increase in CD163+ cells during Cobaia infection may be important for its control. Although both strains healed at the end of the infection, the role of SGE was determinant for the kinetics of the immunopathological events in this dermotropic species.

Mosquito Saliva Reshapes Alphavirus Infection and Immunopathogenesis

Alphaviruses are transmitted to humans via bites of infected mosquitoes. Although alphaviruses have caused a wide range of outbreaks and crippling disease, the availability of licensed vaccines or antiviral therapies remains limited. Mosquito vectors such as Aedes and Culex are the main culprits in the transmission of alphaviruses. This review explores how mosquito saliva may promote alphavirus infection. Identifying the roles of mosquito-derived factors in alphavirus pathogenesis will generate novel tools to circumvent and control mosquito-borne alphavirus infections in humans.

Leishmania proteophosphoglycans regurgitated from infected sand flies accelerate dermal wound repair and exacerbate leishmaniasis via insulin-like growth factor 1-dependent signalling

Leishmania parasites are transmitted to vertebrate hosts by female phlebotomine sand flies as they bloodfeed by lacerating the upper capillaries of the dermis with their barbed mouthparts. In the sand fly midgut secreted proteophosphoglycans from Leishmania form a biological plug known as the promastigote secretory gel (PSG), which blocks the gut and facilitates the regurgitation of infective parasites. The interaction between the wound created by the sand fly bite and PSG is not known. Here we nanoinjected a sand fly egested dose of PSG into BALB/c mouse skin that lead to the differential expression of 7,907 transcripts. These transcripts were transiently up-regulated during the first 6 hours post-wound and enriched for pathways involved in inflammation, cell proliferation, fibrosis, epithelial cell differentiation and wound remodelling. We found that PSG significantly accelerated wound healing in vitro and in mice; which was associated with an early up-regulation of transcripts involved in inflammation (IL-1β, IL-6, IL-10, TNFα) and inflammatory cell recruitment (CCL2, CCL3, CCL4, CXCL2), followed 6 days later by enhanced expression of transcripts associated with epithelial cell proliferation, fibroplasia and fibrosis (FGFR2, EGF, EGFR, IGF1). Dermal expression of IGF1 was enhanced following an infected sand fly bite and was acutely responsive to the deposition of PSG but not the inoculation of parasites or sand fly saliva. Antibody blockade of IGF1 ablated the gel’s ability to promote wound closure in mouse ears and significantly reduced the virulence of Leishmania mexicana infection delivered by an individual sand fly bite. Dermal macrophages recruited to air-pouches on the backs of mice revealed that IGF1 was pivotal to the PSG’s ability to promote macrophage alternative activation and Leishmania infection. Our data demonstrate that through the regurgitation of PSG Leishmania exploit the wound healing response of the host to the vector bite by promoting the action of IGF1 to drive the alternative activation of macrophages.

Female phlebotomine sand flies efficiently transmit Leishmania parasites, yet the underlying mechanisms remain poorly understood. During transmission, promastigote secretory gel (PSG) regurgitated from the blocked sand fly gut promote Leishmania infection and exacerbates disease. Here we investigate mouse ear-skin response to PSG and find that a significant proportion of up-regulated transcripts are involved in wound healing. Following a wound to skin preconditioned with PSG, key transcripts associated with inflammation and cell recruitment were enhanced by 4 hours and those associated with epithelial cell differentiation, proliferation and fibrosis were enhanced in the late phase of wound healing; including insulin growth factor-1 (IGF1) and its receptor. The presence of PSG significantly accelerated wound closure in vitro and in vivo, and this was directed by the presence of IGF1. IGF1 was essential to both the wound healing and disease-exacerbating properties of PSG, arising from its ability to alternatively activate macrophages. IGF1-blockade efficiently reduced the virulence of Leishmania infection from sand fly bite transmission indicating that it is essential to the function of PSG during natural infection. Our findings open the dual possibility of exploiting PSG, or defined components thereof, as a novel wound healing therapy and provide new targets for anti-leishmanial therapeutic design.

Antibody response to sand fly saliva is a marker of transmission intensity but not disease progression in dogs naturally infected with Leishmania infantum

BACKGROUND

Antibody responses to sand fly saliva have been suggested to be a useful marker of exposure to sand fly bites and Leishmania infection and a potential tool to monitor the effectiveness of entomological interventions. Exposure to sand fly bites before infection has also been suggested to modulate the severity of the infection. Here, we test these hypotheses by quantifying the anti-saliva IgG response in a cohort study of dogs exposed to natural infection with Leishmania infantum in Brazil.

METHODS

IgG responses to crude salivary antigens of the sand fly Lutzomyia longipalpis were measured by ELISA in longitudinal serum samples from 47 previously unexposed sentinel dogs and 11 initially uninfected resident dogs for up to 2 years. Antibody responses were compared to the intensity of transmission, assessed by variation in the incidence of infection between seasons and between dogs. Antibody responses before patent infection were then compared with the severity of infection, assessed using tissue parasite loads and clinical symptoms.

RESULTS

Previously unexposed dogs acquired anti-saliva antibody responses within 2 months, and the rate of acquisition increased with the intensity of seasonal transmission. Over the following 2 years, antibody responses varied with seasonal transmission and sand fly numbers, declining rapidly in periods of low transmission. Antibody responses varied greatly between dogs and correlated with the intensity of transmission experienced by individual dogs, measured by the number of days in the field before patent infection. After infection, anti-saliva antibody responses were positively correlated with anti-parasite antibody responses. However, there was no evidence that the degree of exposure to sand fly bites before infection affected the severity of the infection.

CONCLUSIONS

Anti-saliva antibody responses are a marker of current transmission intensity in dogs exposed to natural infection with Leishmania infantum, but are not associated with the outcome of infection.

Inhibition of the complement system by saliva of Anopheles (Nyssorhynchus) aquasalis

Anopheline mosquitoes are vectors of malaria parasites. Their saliva contains anti-hemostatic and immune-modulator molecules that favor blood feeding and parasite transmission. In this study, we describe the inhibition of the alternative pathway of the complement system (AP) by Anopheles aquasalis salivary gland extracts (SGE). According to our results, the inhibitor present in SGE acts on the initial step of the AP blocking deposition of C3b on the activation surfaces. Properdin, which is a positive regulatory molecule of the AP, binds to SGE. When SGE was treated with an excess of properdin, it was unable to inhibit the AP. Through SDS-PAGE analysis, A. aquasalis presented a salivary protein with the same molecular weight as recombinant complement inhibitors belonging to the SG7 family described in the saliva of other anopheline species. At least some SG7 proteins bind to properdin and are AP inhibitors. Searching for SG7 proteins in the A. aquasalis genome, we retrieved a salivary protein that shared an 85% identity with albicin, which is the salivary alternative pathway inhibitor from A. albimanus. This A. aquasalis sequence was also very similar (81% ID) to the SG7 protein from A. darlingi, which is also an AP inhibitor. Our results suggest that the salivary complement inhibitor from A. aquasalis is an SG7 protein that can inhibit the AP by binding to properdin and abrogating its stabilizing activity. Albicin, which is the SG7 from A. albimanus, can directly inhibit AP convertase. Given the high similarity of SG7 proteins, the SG7 from A. aquasalis may also directly inhibit AP convertase in the absence of properdin.

Gut Microbes Egested during Bites of Infected Sand Flies Augment Severity of Leishmaniasis via Inflammasome-Derived IL-1β

Leishmania donovani parasites are the cause of visceral leishmaniasis and are transmitted by bites from phlebotomine sand flies. A prominent feature of vector-transmitted Leishmania is the persistence of neutrophils at bite sites, where they protect captured parasites, leading to enhanced disease. Here, we demonstrate that gut microbes from the sand fly are egested into host skin alongside Leishmania parasites. The egested microbes trigger the inflammasome, leading to a rapid production of interleukin-1β (IL-1β), which sustains neutrophil infiltration. Reducing midgut microbiota by pretreatment of Leishmania-infected sand flies with antibiotics or neutralizing the effect of IL-1β in bitten mice abrogates neutrophil recruitment. These early events are associated with impairment of parasite visceralization, indicating that both gut microbiota and IL-1β are important for the establishment of Leishmania infections. Considering that arthropods harbor a rich microbiota, its potential egestion after bites may be a shared mechanism that contributes to severity of vector-borne disease.

Combining epidemiology with basic biology of sand flies, parasites, and hosts to inform leishmaniasis transmission dynamics and control

Quantitation of the nonlinear heterogeneities in Leishmania parasites, sand fly vectors, and mammalian host relationships provides insights to better understand leishmanial transmission epidemiology towards improving its control. The parasite manipulates the sand fly via production of promastigote secretory gel (PSG), leading to the “blocked sand fly” phenotype, persistent feeding attempts, and feeding on multiple hosts. PSG is injected into the mammalian host with the parasite and promotes the establishment of infection. Animal models demonstrate that sand flies with the highest parasite loads and percent metacyclic promastigotes transmit more parasites with greater frequency, resulting in higher load infections that are more likely to be both symptomatic and efficient reservoirs. The existence of mammalian and sand fly “super-spreaders” provides a biological basis for the spatial and temporal clustering of clinical leishmanial disease. Sand fly blood-feeding behavior will determine the efficacies of indoor residual spraying, topical insecticides, and bed nets. Interventions need to have sufficient coverage to include transmission hot spots, especially in the absence of field tools to assess infectiousness. Interventions that reduce sand fly densities in the absence of elimination could have negative consequences, for example, by interfering with partial immunity conferred by exposure to sand fly saliva. A deeper understanding of both sand fly and host biology and behavior is essential to ensuring effectiveness of vector interventions.

We review recent research that sheds light on the quantitative biology of leishmanial transmission between sand flies and mammalian hosts and use these insights to better understand transmission, the observed epidemiology of the disease, and their implications in choice of control strategy. Using animal models, we show how the parasite-induced processes manipulate sand fly blood-feeding behavior and the infectious metacyclic dose to promote host infection and to differentially regulate the onward transmission potential of individual vectors and hosts. The existence of subpopulations of mammalian and sand fly “super-spreaders” provides a biological basis for the spatial and temporal clustering of clinical leishmanial disease. While tools are unavailable to distinguish these individuals in mixed populations, blanket interventions will be necessary to ensure inclusion of transmission hot spots. Interventions that reduce sand fly densities without elimination could interfere with vector—host dynamics and conferred partial immunity to host populations.

Human cellular and humoral immune responses to Phlebotomus papatasi salivary gland antigens in endemic areas differing in prevalence of Leishmania major infection

Background

Sand fly saliva compounds are able to elicit specific immune responses that have a significant role in Leishmania parasite establishment and disease outcome. Characterizing anti-saliva immune responses in individuals living in well defined leishmaniasis endemic areas would provide valuable insights regarding their effect on parasite transmission and establishment in humans.

Methodology/Principal findings

We explored the cellular and humoral immune responses to Phlebotomus (P.) papatasi salivary gland extracts (SGE) in individuals living in cutaneous leishmaniasis (CL) old or emerging foci (OF, EF). OF was characterized by a higher infection prevalence as assessed by higher proportions of leishmanin skin test (LST) positive individuals compared to EF. Subjects were further subdivided into healed, asymptomatic or naïve groups. We showed anti-SGE proliferation in less than 30% of the individuals, regardless of the immune status, in both foci. IFN-γ production was higher in OF and only observed in immune individuals from OF and naïve subjects from EF. Although IL-10 was not detected, addition of anti-human IL-10 antibodies revealed an increase in proliferation and IFN-γ production only in individuals from OF. The percentage of seropositive individuals was similar in immune and naïves groups but was significantly higher in OF. No correlation was observed between anti-saliva immune responses and LST response. High anti-SGE-IgG responses were associated with an increased risk of developing ZCL. No differences were observed for anti-SGE humoral or cellular responses among naïve individuals who converted or not their LST response or developed or not ZCL after the transmission season.

Conclusions/Significance

These data suggest that individuals living in an old focus characterized by a frequent exposure to sand fly bites and a high prevalence of infection, develop higher anti-saliva IgG responses and IFN-γ levels and a skew towards a Th2-type cellular response, probably in favor of parasite establishment, compared to those living in an emerging focus.

During murine experimental leishmaniasis sand fly saliva components modulate the host immune response and facilitate infection while pre-exposition to uninfected sand fly bites is associated with a protective cellular response against subsequent infection. Human anti-saliva immune responses are not well defined in leishmaniasis endemic areas. Here, we report an analysis of anti P. papatasi saliva cellular and humoral responses in individuals residing in endemic foci showing different prevalence rates of L. major infection. Individuals were further subdivided based on LST response and presence of typical CL scars. We showed higher anti-saliva cellular and humoral responses and a skew towards a Th2 response in the old focus characterized by the highest prevalence of infection. No correlation was observed between LST and anti-saliva cellular or humoral response. We showed that high anti-saliva IgG responses constituted a risk factor for the development of CL. Our findings suggest that the anti-P. papatasi saliva cellular and humoral response profiles vary with the level of sand fly exposure and the prevalence of infection in CL endemic areas. Such studies in humans from highly endemic areas could contribute to a better understanding of the immune response to sand fly saliva and its role in leishmaniasis outcome.

Epidemiological consequences of immune sensitisation by pre-exposure to vector saliva

Blood-feeding arthropods—like mosquitoes, sand flies, and ticks—transmit many diseases that impose serious public health and economic burdens. When a blood-feeding arthropod bites a mammal, it injects saliva containing immunogenic compounds that facilitate feeding. Evidence from Leishmania, Plasmodium and arboviral infections suggests that the immune responses elicited by pre-exposure to arthropod saliva can alter disease progression if the host later becomes infected. Such pre-sensitisation of host immunity has been reported to both exacerbate and limit infection symptoms, depending on the system in question, with potential implications for recovery. To explore if and how immune pre-sensitisation alters the effects of vector control, we develop a general model of vector-borne disease. We show that the abundance of pre-sensitised infected hosts should increase when control efforts moderately increase vector mortality rates. If immune pre-sensitisation leads to more rapid clearance of infection, increasing vector mortality rates may achieve greater than expected disease control. However, when immune pre-sensitisation prolongs the duration of infection, e.g., through mildly symptomatic cases for which treatment is unlikely to be sought, vector control can actually increase the total number of infected hosts. The rising infections may go unnoticed unless active surveillance methods are used to detect such sub-clinical individuals, who could provide long-lasting reservoirs for transmission and suffer long-term health consequences of those sub-clinical infections. Sensitivity analysis suggests that these negative consequences could be mitigated through integrated vector management. While the effect of saliva pre-exposure on acute symptoms is well-studied for leishmaniasis, the immunological and clinical consequences are largely uncharted for other vector-parasite-host combinations. We find a large range of plausible epidemiological outcomes, positive and negative for public health, underscoring the need to quantify how immune pre-sensitisation modulates recovery and transmission rates in vector-borne diseases.

Many diseases of health and economic importance are transmitted by arthropod vectors, like mosquitoes, sand flies, and ticks. When a blood-feeding arthropod bites a mammal, it injects saliva containing compounds that facilitate feeding. The immune responses elicited by previous exposure to vector saliva can alter disease severity if the host later becomes infected. Such pre-sensitisation of host immunity has been linked to either exacerbation or mitigation of symptoms in a number of disease systems. We develop a general model of vector-borne disease to examine how vector control efforts alter the frequency of immune pre-sensitisation and thus change the epidemiological impact of control. We show that the abundance of pre-sensitised infected hosts should increase when control efforts moderately increase vector mortality rates. When immune pre-sensitisation leads to longer infections—by generating sub-clinical cases for which treatment is not rapidly sought—killing vectors can lead to unexpected increases in the number of infected hosts. The rising case burden may go unnoticed unless sub-clinical individuals are tested for infection. Conversely, if immune pre-sensitisation leads to more rapid clearance of infection, increasing vector mortality rates may achieve greater than expected disease control. Our findings highlight the need to quantify how immune pre-sensitisation modulates clinical outcomes and parasite transmission in humans.

Insights into the sand fly saliva: Blood-feeding and immune interactions between sand flies, hosts, and Leishmania

Background

Leishmaniases are parasitic diseases present worldwide that are transmitted to the vertebrate host by the bite of an infected sand fly during a blood feeding. Phlebotomine sand flies inoculate into the mammalian host Leishmania parasites embedded in promastigote secretory gel (PSG) with saliva, which is composed of a diverse group of molecules with pharmacological and immunomodulatory properties.

Methods and findings

In this review, we focus on 3 main aspects of sand fly salivary molecules: (1) structure and composition of salivary glands, including the properties of salivary molecules related to hemostasis and blood feeding, (2) immunomodulatory properties of salivary molecules and the diverse impacts of these molecules on leishmaniasis, ranging from disease exacerbation to vaccine development, and (3) use of salivary molecules for field applications, including monitoring host exposure to sand flies and the risk of Leishmania transmission. Studies showed interesting differences between salivary proteins of Phlebotomus and Lutzomyia species, however, no data were ever published on salivary proteins of Sergentomyia species.

Conclusions

In the last 15 years, numerous studies have characterized sand fly salivary proteins and, in parallel, have addressed the impact of such molecules on the biology of the host–sand fly–parasite interaction. The results obtained shall pave the way for the development of field-application tools that could contribute to the management of leishmaniasis in endemic areas.

Seroconversion to Lutzomyia intermedia LinB-13 as a biomarker for developing cutaneous leishmaniasis

Sand flies inject saliva while feeding in the vertebrate host and anti-saliva antibodies can be used as biomarkers of exposure to Leishmania vectors. We expressed recombinant salivary proteins from Lutzomyia intermedia, a vector of Leishmania braziliensis, and evaluated the seroreactivity in exposed individuals in search for exposure markers. We found a strong correlation among positive serology to recombinant proteins LinB-13, 26, 15, 21 and to salivary proteins: rLinB-13 was the top performing molecule; IgG4 was the most predominant antibody subclass and antibodies to rLinB-13 did not cross react with Lu. longipalpis salivary proteins. By evaluating a cohort of contacts of CL patients, we confirmed that rLinB-13, an antigen 5-related protein, is a marker of exposure to Lu. intermedia with high degree of accuracy. In a 5-year follow up, we determined that individuals who developed CL presented higher anti-rLinB13 IgG responses, before the appearance of clinical symptoms. They also presented a lower frequency of cellular responses to the parasite (DTH). Our results show that seroconversion to a salivary molecule, rLinB-13, is a marker of risk for CL development caused by Leishmania braziliensis. This highlight the possibility of developing tools based on vector molecules to manage the disease in endemic areas.

Mosquito Biting Modulates Skin Response to Virus Infection

Mosquito-borne infections are increasing in number and are spreading to new regions at an unprecedented rate. In particular, mosquito-transmitted viruses, such as those that cause Zika, dengue, West Nile encephalitis, and chikungunya, have become endemic or have caused dramatic epidemics in many parts of the world. Aedes and Culex mosquitoes are the main culprits, spreading infection when they bite. Importantly, mosquitoes do not act as simple conduits that passively transfer virus from one individual to another. Instead, host responses to mosquito-derived factors have an important influence on infection and disease, aiding replication and dissemination within the host. Here, we discuss the latest research developments regarding this fascinating interplay between mosquito, virus, and the mammalian host.

Immunity to Lutzomyia whitmani Saliva Protects against Experimental Leishmania braziliensis Infection

Background

Previous works showed that immunization with saliva from Lutzomyia intermedia, a vector of Leishmania braziliensis, does not protect against experimental infection. However, L. braziliensis is also transmitted by Lutzomyia whitmani, a sand fly species closely related to Lu. intermedia. Herein we describe the immune response following immunization with Lu. whitmani saliva and the outcome of this response after L. braziliensis infection.

Methods and findings

BALB/c mice immunized with Lu. whitmani saliva developed robust humoral and cellular immune responses, the latter characterized by an intense cellular infiltrate and production of IFN-γ and IL-10, by both CD4⁺ and CD8⁺ cells. Mice immunized as above and challenged with L. braziliensis plus Lu. whitmani saliva displayed significantly smaller lesions and parasite load at the challenge site. This protection was associated with a higher (p<0.05) IFN-γ production in response to SLA stimulation. Long-term persisting immunity was also detected in mice immunized with Lu. whitmani saliva. Furthermore, individuals residing in an endemic area for cutaneous leishmaniasis (CL) presented antibody responses to Lu. whitmani saliva. However CL patients, with active lesions, displayed a lower humoral response to Lu. whitmani saliva compared to individuals with subclinical Leishmania infection.

Conclusion

Pre-exposure to Lu. whitmani saliva induces protection against L. braziliensis in a murine model. We also show that Lu. whitmani salivary proteins are immunogenic in naturally exposed individuals. Our results reinforce the importance of investigating the immunomodulatory effect of saliva from different species of closely related sand flies.

The saliva from sand flies contains biologically active proteins that permit the insect to obtain a blood meal. When vertebrates are continuously exposed to these molecules, through insect biting, for example, they induce an immune response (antibody and cell-mediated immunity) in the vertebrate host. Previously, we showed that immunity to salivary proteins from Lutzomyia intermedia a vector of Leishmania braziliensis, the main species that causes cutaneous leishmaniasis (CL) in Brazil, did not protect but exacerbated CL. In the present work, we investigated if immunity to Lutzomyia whitmani, another vector of L. braziliensis, induced a similar effect or not. We observed that mice immunized with Lu. whitmani saliva develop immunity to the salivary components and that this immunity protected the mice against CL development. We further observed that people residing in areas where Lu. whitmani occurs also develop antibodies to saliva and that CL patients have lower levels of these antibodies. These evidences point to differences in the protein repertoire present in the saliva of different sand flies and highlight the concept that salivary proteins should be considered as additional components of a vaccine for leishmaniasis.

Differential expression profiles of the salivary proteins SP15 and SP44 from Phlebotomus papatasi

Background

Sand fly saliva has been shown to help parasite establishment and to induce immune responses in vertebrate hosts. In the current study, we investigated the pattern of expression of two Phlebotomus papatasi salivary transcripts in specific physiological and seasonal conditions at a hyperendemic area of zoonotic cutaneous leishmaniasis (ZCL) in Iran.

Methods

Sand flies were collected during 2012–2013, and grouped according to physiological stages such as unfed, fed, semi-gravid, gravid, parous, nulliparous, infected or non-infected with Leishmania major and also based on the season in which they were collected. Quantitative Real-Time PCR was applied for assessment of the expression of two relevant salivary transcripts, PpSP15 and PpSP44, associated to protection from and exacerbation of ZCL, respectively.

Results

The expression of PpSP15 and PpSP44 transcripts was significantly up-regulated (1.74 and 1.4 folds, respectively) in blood fed compared to unfed flies. Among four groups of fed, unfed, semi-gravid and gravid flies, the lowest levels of PpSP15 and PpSP44 expression were observed in gravid flies. Additionally, the expression levels of both PpSP15 and PpSP44 transcripts in P. papatasi collected during summer were significantly up-regulated (3.7 and 4.4 folds, respectively) compared to spring collections. In addition, the PpSP15 transcript exhibited a significant up-regulation (P < 0.05) in non-infected flies compared to those infected with L. major.

Conclusions

This study contributes to our knowledge of the differential expression of salivary genes among different groups within a P. papatasi population under natural field conditions. Cutaneous and visceral leishmaniasis are of public health importance in many parts of Iran and neighbouring countries where P. papatasi is the proven and dominant sand fly vector for ZCL, the most prevalent and endemic form of the disease in Iran. Therefore, the current study could be helpful in understanding the influence of salivary genes on Leishmania transmission by phlebotomine sand flies. Our findings demonstrate the differential expression of salivary transcripts under various physiological conditions potentially influencing the sand fly capacity for parasite transmission as well as the outcome of disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1633-z) contains supplementary material, which is available to authorized users.

Mosquito Saliva Increases Endothelial Permeability in the Skin, Immune Cell Migration, and Dengue Pathogenesis during Antibody-Dependent Enhancement

Dengue remains the most prevalent arthropod-borne viral disease in humans. While probing for blood vessels, Aedes aegypti and Ae. albopictus mosquitoes transmit the four serotypes of dengue virus (DENV1-4) by injecting virus-containing saliva into the skin. Even though arthropod saliva is known to facilitate transmission and modulate host responses to other pathogens, the full impact of mosquito saliva on dengue pathogenesis is still not well understood. Inoculating mice lacking the interferon-α/β receptor intradermally with DENV revealed that mosquito salivary gland extract (SGE) exacerbates dengue pathogenesis specifically in the presence of enhancing serotype-cross-reactive antibodies—when individuals already carry an increased risk for severe disease. We further establish that SGE increases viral titers in the skin, boosts antibody-enhanced DENV infection of dendritic cells and macrophages in the dermis, and amplifies dendritic cell migration to skin-draining lymph nodes. We demonstrate that SGE directly disrupts endothelial barrier function in vitro and induces endothelial permeability in vivo in the skin. Finally, we show that surgically removing the site of DENV transmission in the skin after 4 hours rescued mice from disease in the absence of SGE, but no longer prevented lethal antibody-enhanced disease when SGE was present. These results indicate that SGE accelerates the dynamics of dengue pathogenesis after virus transmission in the skin and induces severe antibody-enhanced disease systemically. Our study reveals novel aspects of dengue pathogenesis and suggests that animal models of dengue and pre-clinical testing of dengue vaccines should consider mosquito-derived factors as well as enhancing antibodies.

Mosquitoes inject saliva into the skin while probing for blood vessels. Saliva facilitates blood feeding and can contain pathogens when the mosquito is infected. In tropical regions, Aedes mosquitoes transmit the four serotypes of dengue virus (DENV1-4) and infect almost 400 million humans every year. DENV causes severe disease especially in people who have already been exposed to a different serotype. During antibody-dependent enhancement, antibodies that were generated during the first infection bind, but do not neutralize, DENV, and instead enhance infection of immune cells. We injected mouse ears with DENV alone or with extracts from mosquito salivary glands to study the impact on disease. We found that saliva induced severe disease and death only during antibody-enhanced infection. Saliva increased DENV infection in the dermis, immune cell migration to skin and lymph nodes, and permeability of endothelial cells that line blood vessels. Removing the site of DENV inoculation in the skin rescued mice from severe disease, but this protective effect was lost when saliva was present. Our study reveals that mosquito saliva affects dendritic cell migration, increases endothelial permeability, and augments dengue disease severity. Mosquito saliva and enhancing antibodies thus need to be considered when developing vaccines and drugs against dengue.

A sand fly salivary protein vaccine shows efficacy against vector-transmitted cutaneous leishmaniasis in nonhuman primates

Currently, there are no commercially available human vaccines against leishmaniasis. In rodents, cellular immunity to salivary proteins of sand fly vectors is associated to protection against leishmaniasis, making them worthy targets for further exploration as vaccines. We demonstrate that nonhuman primates (NHP) exposed to Phlebotomus duboscqi uninfected sand fly bites or immunized with salivary protein PdSP15 are protected against cutaneous leishmaniasis initiated by infected bites. Uninfected sand fly-exposed and 7 of 10 PdSP15-immunized rhesus macaques displayed a significant reduction in disease and parasite burden compared to controls. Protection correlated to the early appearance of Leishmania-specific CD4(+)IFN-γ(+) lymphocytes, suggesting that immunity to saliva or PdSP15 augments the host immune response to the parasites while maintaining minimal pathology. Notably, the 30% unprotected PdSP15-immunized NHP developed neither immunity to PdSP15 nor an accelerated Leishmania-specific immunity. Sera and peripheral blood mononuclear cells from individuals naturally exposed to P. duboscqi bites recognized PdSP15, demonstrating its immunogenicity in humans. PdSP15 sequence and structure show no homology to mammalian proteins, further demonstrating its potential as a component of a vaccine for human leishmaniasis.

Leishmania vaccine development: exploiting the host-vector-parasite interface

Visceral leishmaniasis (VL) is a disease transmitted by phlebotomine sand flies, fatal if untreated, and with no available human vaccine. In rodents, cellular immunity to Leishmania parasite proteins as well as salivary proteins of the sand fly is associated with protection, making them worthy targets for further exploration as vaccines. This review discusses the notion that a combination vaccine including Leishmania and vector salivary antigens may improve vaccine efficacy by targeting the parasite at its most vulnerable stage just after transmission. Furthermore, we put forward the notion that better modeling of natural transmission is needed to test efficacy of vaccines. For example, the fact that individuals living in endemic areas are exposed to sand fly bites and will mount an immune response to salivary proteins should be considered in pre-clinical and clinical evaluation of leishmaniasis vaccines. Nevertheless, despite remaining obstacles there is good reason to be optimistic that safe and effective vaccines against leishmaniasis can be developed.