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.

Sequential blood meals promote Leishmania replication and reverse metacyclogenesis augmenting vector infectivity

Sand flies, similar to most vectors, take multiple blood meals during their lifetime^1-4. The effect of subsequent blood meals on pathogens developing in the vector and their impact on disease transmission have never been examined. Here, we show that ingestion of a second uninfected blood meal by Leishmania-infected sand flies triggers dedifferentiation of metacyclic promastigotes, considered a terminally differentiated stage inside the vector ⁵ , to a leptomonad-like stage, the retroleptomonad promastigote. Reverse metacyclogenesis occurs after every subsequent blood meal where retroleptomonad promastigotes rapidly multiply and differentiate to metacyclic promastigotes enhancing sand fly infectiousness. Importantly, a subsequent blood meal amplifies the few Leishmania parasites acquired by feeding on infected hosts by 125-fold, and increases lesion frequency by fourfold, in twice-fed compared with single-fed flies. These findings place readily available blood sources as a critical element in transmission and propagation of vector-borne pathogens.

A second generation leishmanization vaccine with a markerless attenuated Leishmania major strain using CRISPR gene editing

Leishmaniasis is a neglected tropical disease caused by Leishmania protozoa transmitted by infected sand flies. Vaccination through leishmanization with live Leishmania major has been used successfully but is no longer practiced because it resulted in occasional skin lesions. A second generation leishmanization is described here using a CRISPR genome edited L. major strain (LmCen-/-). Notably, LmCen-/- is a genetically engineered centrin gene knock-out mutant strain that is antibiotic resistant marker free and does not have detectable off-target mutations. Mice immunized with LmCen-/- have no visible lesions following challenge with L. major-infected sand flies, while non-immunized animals develop large and progressive lesions with a 2-log fold higher parasite burden. LmCen-/- immunization results in protection and an immune response comparable to leishmanization. LmCen-/- is safe since it is unable to cause disease in immunocompromised mice, induces robust host protection against vector sand fly challenge and because it is marker free, can be advanced to human vaccine trials.

Leishmania amazonensis impairs DC function by inhibiting CD40 expression via A2B adenosine receptor activation

Dendritic cells (DCs) play an essential role in the modulation of immune responses and several studies have evaluated the interactions between Leishmania parasites and DCs. While extracellular ATP exhibits proinflammatory properties, adenosine is an important anti-inflammatory mediator. Here we investigated the effects of Leishmania infection on DC responses and the participation of purinergic signalling in this process. Bone marrow-derived dendritic cells (BMDCs) from C57BL/6J mice infected with Leishmania amazonensis, Leishmania braziliensis or Leishmania major metacyclic promastigotes showed decreased major histocompatibility complex (MHC) class II and CD86 expression and increased ectonucleotidase expression as compared with uninfected cells. In addition, L. amazonensis-infected DCs, which had lower CD40 expression, exhibited a decreased ability to induce T-cell proliferation. The presence of MRS1754, a highly selective A(2B) adenosine receptor antagonist at the time of infection increased MHC class II, CD86 and CD40 expression in L. amazonensis-infected DCs and restored the ability of the infected DCs to induce T-cell proliferation. Similar results were obtained through the inhibition of extracellular ATP hydrolysis using suramin. In conclusion, we propose that A(2B) receptor activation may be used by L. amazonensis to inhibit DC function and evade the immune response.

Ecto-nucleotidase activities of promastigotes from Leishmania (Viannia) braziliensis relates to parasite infectivity and disease clinical outcome

Background

Leishmania (Viannia) braziliensis has been associated with a broad range of clinical manifestations ranging from a simple cutaneous ulcer to destructive mucosal lesions. Factors leading to this diversity of clinical presentations are not clear, but parasite factors have lately been recognized as important in determining disease progression. Given the fact that the activity of ecto-nucleotidases correlates with parasitism and the development of infection, we evaluated the activity of these enzymes in promastigotes from 23 L. braziliensis isolates as a possible parasite-related factor that could influence the clinical outcome of the disease.

Methodology/Principal Findings

Our results show that the isolates differ in their ability to hydrolyze adenine nucleotides. Furthermore, we observed a positive correlation between the time for peak of lesion development in C57BL/6J mice and enzymatic activity and clinical manifestation of the isolate. In addition, we found that L. (V.) braziliensis isolates obtained from mucosal lesions hydrolyze higher amounts of adenine nucleotides than isolates obtained from skin lesions. One isolate with high (PPS6m) and another with low (SSF) ecto-nucleotidase activity were chosen for further studies. Mice inoculated with PPS6m show delayed lesion development and present larger parasite loads than animals inoculated with the SSF isolate. In addition, PPS6m modulates the host immune response by inhibiting dendritic cell activation and NO production by activated J774 macrophages. Finally, we observed that the amastigote forms from PPS6m and SSF isolates present low enzymatic activity that does not interfere with NO production and parasite survival in macrophages.

Conclusions/Significance

Our data suggest that ecto-nucleotidases present on the promastigote forms of the parasite may interfere with the establishment of the immune response with consequent impaired ability to control parasite dissemination and this may be an important factor in determining the clinical outcome of leishmaniasis.

Cutaneous leishmaniasis is a widespread tropical disease caused by different species of Leishmania protozoa that are transmitted by infected sandflies. Clinical presentations are extremely diverse and dependent on a variety of parasite and host factors that are poorly understood. Leishmania (V.) braziliensis infection may result in a devastating disease manifestation characterized by the development of destructive lesions in the oral, nasal, and pharyngeal mucosal. Ecto-nucleotidases are enzymes that are involved in the hydrolysis of extracellular nucleotides. These enzymes have been shown to correlate with virulence of Leishmania parasites. In this work, we evaluated the ecto-nucleotidase activity of promastigotes from the twenty three different L. braziliensis isolates. We demonstrated that isolates obtained from mucosal lesions present higher levels of ecto-nucleotidase activity than those from cutaneous lesions. In addition, we show that in the murine model of cutaneous leishmaniasis, promastigote forms of parasite with higher activity induce a delayed/decreased immune response that may correlate with spreading of the parasites throughout the body. Thus, we propose that the level of ecto-nucleotidase activity of promastigotes may be a marker for the development of severe clinical forms of cutaneous leishmaniasis and also a possible target for future therapeutic intervention.

A sand fly salivary protein acts as a neutrophil chemoattractant

Apart from bacterial formyl peptides or viral chemokine mimicry, a non-vertebrate or insect protein that directly attracts mammalian innate cells such as neutrophils has not been molecularly characterized. Here, we show that members of sand fly yellow salivary proteins induce in vitro chemotaxis of mouse, canine and human neutrophils in transwell migration or EZ-TAXIScan assays. We demonstrate murine neutrophil recruitment in vivo using flow cytometry and two-photon intravital microscopy in Lysozyme-M-eGFP transgenic mice. We establish that the structure of this ~ 45 kDa neutrophil chemotactic protein does not resemble that of known chemokines. This chemoattractant acts through a G-protein-coupled receptor and is dependent on calcium influx. Of significance, this chemoattractant protein enhances lesion pathology (P < 0.0001) and increases parasite burden (P < 0.001) in mice upon co-injection with Leishmania parasites, underlining the impact of the sand fly salivary yellow proteins on disease outcome. These findings show that some arthropod vector-derived factors, such as this chemotactic salivary protein, activate rather than inhibit the host innate immune response, and that pathogens take advantage of these inflammatory responses to establish in the host.

Immune mimicry has been shown in chemokine like moieties from bacteria and viruses. Here, the authors characterise a sand fly salivary protein that induces neutrophil chemotaxis and explore its impact in a model of parasitic infection.

Heme Oxygenase-1 Induction by Blood-Feeding Arthropods Controls Skin Inflammation and Promotes Disease Tolerance

Hematophagous vectors lacerate host skin and capillaries to acquire a blood meal, resulting in leakage of red blood cells (RBCs) and inflammation. Here, we show that heme oxygenase-1 (HO-1), a pleiotropic cytoprotective isoenzyme that mitigates heme-mediated tissue damage, is induced after bites of sand flies, mosquitoes, and ticks. Further, we demonstrate that erythrophagocytosis by macrophages, including a skin-residing CD163⁺CD91⁺ professional iron-recycling subpopulation, produces HO-1 after bites. Importantly, we establish that global deletion or transient inhibition of HO-1 in mice increases inflammation and pathology following Leishmania-infected sand fly bites without affecting parasite number, whereas CO, an end product of the HO-1 enzymatic reaction, suppresses skin inflammation. This indicates that HO-1 induction by blood-feeding sand flies promotes tolerance to Leishmania infection. Collectively, our data demonstrate that HO-1 induction through erythrophagocytosis is a universal mechanism that regulates skin inflammation following blood feeding by arthropods, thus promoting early-stage disease tolerance to vector-borne pathogens.

Distinct gene expression patterns in vector-residing Leishmania infantum identify parasite stage-enriched markers

BACKGROUND

Leishmaniasis is a vector-borne neglected disease. Inside the natural sand fly vector, the promastigote forms of Leishmania undergo a series of extracellular developmental stages to reach the infectious stage, the metacyclic promastigote. There is limited information regarding the expression profile of L. infantum developmental stages inside the sand fly vector, and molecular markers that can distinguish the different parasite stages are lacking.

METHODOLOGY/PRINCIPAL FINDINGS

We performed RNAseq on unaltered midguts of the sand fly Lutzomyia longipalpis after infection with L. infantum parasites. RNAseq was carried out at various time points throughout parasite development. Principal component analysis separated the transcripts corresponding to the different Leishmania promastigote stages, the procyclic, nectomonad, leptomonad and metacyclics. Importantly, there were a significant number of differentially expressed genes when comparing the sequential development of the various Leishmania stages in the sand fly. There were 836 differentially expressed (DE) genes between procyclic and long nectomonad promastigotes; 113 DE genes between nectomonad and leptomonad promastigotes; and 302 DE genes between leptomonad and metacyclic promastigotes. Most of the DE genes do not overlap across stages, highlighting the uniqueness of each Leishmania stage. Furthermore, the different stages of Leishmania parasites exhibited specific transcriptional enrichment across chromosomes. Using the transcriptional signatures exhibited by distinct Leishmania stages during their development in the sand fly midgut, we determined the genes predominantly enriched in each stage, identifying multiple potential stage-specific markers for L. infantum.

CONCLUSIONS

Overall, these findings demonstrate the transcriptional plasticity of the Leishmania parasite inside the sand fly vector and provide a repertoire of potential stage-specific markers for further development as molecular tools for epidemiological studies.

Towards a Sustainable Vector-Control Strategy in the Post Kala-Azar Elimination Era

Visceral leishmaniasis (VL) is a potentially deadly parasitic disease. In the Indian sub-continent, VL is caused by Leishmania donovani and transmitted via the bite of an infected Phlebotomus argentipes female sand fly, the only competent vector species in the region. The highest disease burden is in the northern part of the Indian sub-continent, especially in the state of Bihar. India, Bangladesh, and Nepal embarked on an initiative, coordinated by World Health Organization, to eliminate VL as a public health problem by the year 2020. The main goal is to reduce VL incidence below one case per 10,000 people through early case-detection, prompt diagnosis and treatment, and reduction of transmission using vector control measures. Indoor residual spraying, a major pillar of the elimination program, is the only vector control strategy used by the government of India. Though India is close to its VL elimination target, important aspects of vector bionomics and sand fly transmission dynamics are yet to be determined. To achieve sustained elimination and to prevent a resurgence of VL, knowledge gaps in vector biology and behavior, and the constraints they may pose to current vector control methods, need to be addressed. Herein, we discuss the successes and failures of previous and current vector-control strategies implemented to combat kala-azar in Bihar, India, and identify gaps in our understanding of vector transmission towards development of innovative tools to ensure sustained vector control in the post-elimination period.

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.

Patchy Parasitized Skin Governs Leishmania donovani Transmission to Sand Flies

Doehl et al. have combined empirical data with computer simulation to demonstrate that RAG-2 mice intravenously infected with Leishmania donovani form heterogeneous skin parasite patches that govern infectiousness to sand flies. This model provides a much-needed tool to explore the relevance of asymptomatic and symptomatic visceral leishmaniasis patients as infection reservoirs.

A Composite Recombinant Salivary Proteins Biomarker for Phlebotomus argentipes Provides a Surveillance Tool Postelimination of Visceral Leishmaniasis in India

Incidence of visceral leishmaniasis (VL) in the Indian subcontinent (ISC) has declined by more than 95% since initiation of the elimination program in 2005. As the ISC transitions to the postelimination surveillance phase, an accurate measurement of human-vector contact is needed to assure long-term success. To develop this tool, we identified PagSP02 and PagSP06 from saliva of Phlebotomus argentipes, the vector of Leishmania donovani in the ISC, as immunodominant proteins in humans. We also established the absence of cross-reactivity with Phlebotomus papatasi saliva, the only other human-biting sand fly in the ISC. Importantly, by combining recombinant rPagSP02 and rPagSP06 we achieved greater antibody recognition and specificity than single salivary proteins. The receiver operating characteristics curve for rPagSP02 + rPagSP06 predicts exposure to Ph. argentipes bites with 90% specificity and 87% sensitivity compared to negative control sera (P >.0001). Overall, rPagSP02 + rPagSP06 provides an effective surveillance tool for monitoring vector control efforts after VL elimination.

Leishmania: A Maestro in Epigenetic Manipulation of Macrophage Inflammasomes

Epigenetic manipulation of host cells by intracellular pathogens has become increasingly evident. Lecoeur et al. show us how Leishmania amazonensis inhibits macrophage inflammasomes by modifying histone H3 activation marks on NF-κB-associated gene promoters that increase the expression of inhibitors and downmodulates activators of this pathway.

The impact of blood on vector-borne diseases with emphasis on mosquitoes and sand flies

The impact of blood and its factors on vector-borne diseases is significant and multifaceted yet understudied. While blood is expected to play a central role in transmission, pathogen development, vector behavior, and vector competence, in experimental settings, most studies are developed in the frame of a single, infected blood meal. To effectively combat vector-borne diseases, we need to determine what is the influence of insect blood-feeding behavior on transmission and development of pathogens, toward translation to natural field settings. This review summarizes current findings, highlights key gaps, and outlines future research directions to enhance our understanding of the role of blood in vector-borne disease transmission.

Leishmania genetic exchange is mediated by IgM natural antibodies

Host factors that mediate Leishmania genetic exchange are not well defined. Here we demonstrate that natural IgM (IgMn)1-4 antibodies mediate parasite genetic exchange by inducing the transient formation of a spherical parasite clump that promotes parasite fusion and hybrid formation. We establish that IgMn from Leishmania-free animals binds to the surface of Leishmania parasites to induce significant changes in the expression of parasite transcripts and proteins. Leishmania binding to IgMn is partially lost after glycosidase treatment, although parasite surface phosphoglycans, including lipophosphoglycan, are not required for IgMn-induced parasite clumping. Notably, the transient formation of parasite clumps is essential for Leishmania hybridization in vitro. In vivo, we observed a 12-fold increase in hybrid formation in sand flies provided a second blood meal containing IgMn compared with controls. Furthermore, the generation of recombinant progeny from mating hybrids and parental lines were only observed in sand flies provided with IgMn. Both in vitro and in vivo IgM-induced Leishmania crosses resulted in full genome hybrids that show equal patterns of biparental contribution. Leishmania co-option of a host natural antibody to facilitate mating in the insect vector establishes a new paradigm of parasite-host-vector interdependence that contributes to parasite diversity and fitness by promoting genetic exchange.

Leishmania sand fly-transmission is disrupted by Delftia tsuruhatensis TC1 bacteria

Most human pathogenic Leishmania species are zoonotic agents; therefore, sand fly-based control strategies are essential to prevent parasite circulation. Here, we used the Delftia tsuruhatensis TC1 strain, that inhibits the development of Plasmodium in mosquitoes, but in the context of Leishmania-infected sand flies. We show that D. tsuruhatensis TC1 colonizes the midgut of Phlebotomus duboscqi sand flies and impacts the development of L. major parasites, independently of the colonization timing. This phenotype is likely an indirect consequence of D. tsuruhatensis colonization, related with the induction of sand fly gut dysbiosis. Importantly, Leishmania-infected, D. tsuruhatensis-fed sand flies are less able to transmit L. major parasites and cause disease in mice. Modelling supports the disruption of disease endemicity in the field, highlighting D. tsuruhatensis as a promising agent for the control of leishmaniasis.