Lundep, a sand fly salivary endonuclease increases Leishmania parasite survival in neutrophils and inhibits XIIa contact activation in human plasma

Neutrophil extracellular traps formation: effect of Leishmania major promastigotes and salivary gland homogenates of Phlebotomus papatasi in human neutrophil culture

BACKGROUND

Leishmaniasis as a neglected tropical disease (NTD) is caused by the inoculation of Leishmania parasites via the bite of phlebotomine sand flies. After an infected bite, a series of innate and adaptive immune responses occurs, among which neutrophils can be mentioned as the initiators. Among the multiple functions of these fighting cells, neutrophil extracellular traps (NETs) were studied in the presence of Leishmania major promastigotes and salivary gland homogenates (SGH) of Phlebotomus papatasi alone, and in combination to mimic natural conditions of transmission.

MATERIAL & METHODS

The effect of L. major and SGH on NETs formation was studied in three different groups: neutrophils + SGH (NS), neutrophils + L. major (NL), neutrophils + L. major + SGH (NLS) along with negative and positive controls in 2, 4 and 6 h post-incubation. Different microscopic methods were used to visualize NETs comprising: fluorescence microscopy by Acridine Orange/ Ethidium Bromide staining, optical microscopy by Giemsa staining and scanning electron microscopy. In addition, the expression level of three different genes NE, MPO and MMP9 was evaluated by Real-Time PCR.

RESULTS

All three microscopical methods revealed similar results, as in NS group, chromatin extrusion as a sign of NETosis, was not very evident in each three time points; but, in NL and especially NLS group, more NETosis was observed and the interaction between neutrophils and promastigotes in NL and also with saliva in NLS group, gradually increased over times. Real-time reveals that, the expression of MPO, NE and MMP9 genes increased during 2 and 4 h after exposure, and then decreased at 6 h in most groups.

CONCLUSION

Hence, it was determined that the simultaneous presence of parasite and saliva in NLS group has a greater impact on the formation of NETs compared to NL and NS groups.

Exploring the leishmanicidal potential of terpenoids: a comprehensive review on mechanisms of cell death

Leishmaniasis is a neglected tropical disease with a wide spectrum of clinical manifestations, ranging from visceral to cutaneous, with millions of new cases and thousands of deaths reported each year. The species of Leishmania and the immune response of the host determine the severity of the disease. Leishmaniasis remains challenging to diagnose and treat, and there is no vaccine available. Several studies have been conducted on the use of herbal medicines for the treatment of leishmaniasis. Natural products can provide an inexhaustible source of chemical diversity with therapeutic potential. Terpenes are a class of natural products derived from a single isoprene unit, a five-carbon compound that forms the basic structure of isoprenoids. This review focuses on the most important and recent advances in the treatment of parasites of the genus Leishmania with different subclasses of terpenes. Several mechanisms have been proposed in the literature, including increased oxidative stress, immunomodulatory role, and induction of different types of parasite cell death. However, this information needs to be brought together to provide an overview of how these compounds can be used as therapeutic tools for drug development and as a successful adjuvant strategy against Leishmania sp.

First evidence of experimental genetic hybridization between cutaneous and visceral strains of Leishmania donovani within its natural vector Phlebotomus argentipes

Leishmaniasis is a neglected tropical disease caused by protozoan parasites of genus Leishmania, and transmitted by different species of Phlebotomine sand flies. More than 20 species of Leishmania are known to cause disease in humans and other animals. Leishmania donovani species complex is known to have a vast diversity of clinical manifestations in humans, but underlying mechanisms for such diversity are yet unknown. Long believed to be strictly asexual, Leishmania have been shown to undergo a cryptic sexual cycle inside its sandfly vector. Natural populations of hybrid parasites have been associated with the rise of atypical clinical outcomes in the Indian subcontinent (ISC). However, formal demonstration of genetic crossing in the major endemic sandfly species in the ISC remain unexplored. Here, we investigated the ability of two distinct variants of L. donovani associated with strikingly different forms of the disease to undergo genetic exchange inside its natural vector, Phlebotomus argentipes. Clinical isolates of L. donovani either from a Sri Lankan cutaneous leishmaniasis (CL) patient or an Indian visceral leishmaniasis (VL) patient were genetically engineered to express different fluorescent proteins and drug-resistance markers and subsequently used as parental strains in experimental sandfly co-infection. After 8 days of infection, sand flies were dissected and midgut promastigotes were transferred into double drug-selective media. Two double drug-resistant, dual fluorescent hybrid cell lines were recovered, which after cloning and whole genome sequencing, were shown to be full genomic hybrids. This study provides the first evidence of L. donovani hybridization within its natural vector Ph. argentipes.

Balancing the functions of DNA extracellular traps in intracellular parasite infections: implications for host defense, disease pathology and therapy

The release of DNA to the extracellular milieu is a biological process referred to as etosis, which is involved in both physiological and pathological functions. Although the release of DNA extracellular traps (ETs) was initially attributed to innate immune cells such as neutrophils, eosinophils, and macrophages, recent studies have shown that T cells, as well as non-immune cells, are capable of releasing ETs. These structures were described primarily for their potential to trap and kill pathogens, presenting an important strategy of host defense. Intriguingly, these functions have been associated with intracellular pathogens such as the parasites Leishmania sp. and Trypanosoma cruzi, causative agents of leishmaniasis and Chagas disease, respectively. These are two devastating tropical diseases that lead to thousands of deaths every year. In an apparent contradiction, ETs can also induce and amplify inflammation, which may lead to worsening disease pathology. This has prompted the concept of targeting ETs' release as a means of controlling tissue destruction to treat human diseases. What is the best approach to prevent disease severity: inducing ETs to kill pathogens or preventing their release? In this Perspective article, we will discuss the importance of understanding ETs released by different cell types and the need to balance their potentially complementary functions. In addition, we will explore other functions of ETs and their translational applications to benefit individuals infected with intracellular parasites and other pathogens. Ultimately, a better understanding of the role of ETs in disease pathogenesis will provide valuable insights into developing novel therapies for human diseases.

Leishmania infantum Axenic Amastigotes Induce Human Neutrophil Extracellular Traps and Resist NET-Mediated Killing

Neutrophils are multifaceted cells that, upon activation, release meshes of chromatin associated with different proteins, known as neutrophil extracellular traps (NETs). Leishmania amazonensis promastigotes and amastigotes induce NET release, and we have identified the signaling pathways involved in NET extrusion activated by promastigotes. Amastigotes maintain the infection in vertebrate hosts, and we have shown the association of NETs with amastigotes in human biopsies of cutaneous leishmaniasis. However, the interaction of amastigotes and neutrophils remains poorly understood. Our study aimed to characterize the pathways involved in the formation of NETs induced by axenic amastigotes from L. infantum, the causal agent of visceral leishmaniasis. Human neutrophils pretreated with signaling pathway inhibitors were incubated with amastigotes, and NET release was quantified in the culture supernatant. Amastigote viability was checked after incubation with NETs. We found that the release of NETs by neutrophils stimulated with these amastigotes requires the participation of elastase and peptidyl arginine deaminase and the involvement of PI3K, ROS, and calcium. Moreover, amastigotes are not susceptible to NET-mediated killing. Altogether, these findings improve our comprehension of the signaling pathways implicated in the interaction between amastigotes and human neutrophils.

Is There Any Difference in the In Situ Immune Response in Active Localized Cutaneous Leishmaniasis That Respond Well or Poorly to Meglumine Antimoniate Treatment or Spontaneously Heal?

Localized cutaneous leishmaniasis caused by Leishmania braziliensis can either respond well or poorly to the treatment or heal spontaneously; It seems to be dependent on the parasite and/or host factors, but the mechanisms are not fully understood. We evaluated the in situ immune response in eighty-two active lesions from fifty-eight patients prior to treatment classified as early spontaneous regression (SRL-n = 14); treatment responders (GRL-n = 20); and non-responders (before first treatment/relapse, PRL1/PRL2-n = 24 each). Immunohistochemistry was used to identify cell/functional markers which were correlated with the clinical characteristics. PRL showed significant differences in lesion number/size, clinical evolution, and positive parasitological examinations when compared with the other groups. SRL presented a more efficient immune response than GRL and PRL, with higher IFN-γ/NOS2 and a lower percentage of macrophages, neutrophils, NK, B cells, and Ki-67+ cells. Compared to SRL, PRL had fewer CD4+ Tcells and more CD163+ macrophages. PRL1 had more CD68+ macrophages and Ki-67+ cells but less IFN-γ than GRL. PRL present a less efficient immune profile, which could explain the poor treatment response, while SRL had a more balanced immune response profile for lesion healing. Altogether, these evaluations suggest a differentiated profile of the organization of the inflammatory process for lesions of different tegumentary leishmaniasis evolution.

An insight into the female and male Sabethes cyaneus mosquito salivary glands transcriptome

Mosquitoes are responsible for the death and debilitation of millions of people every year due to the pathogens they can transmit while blood feeding. While a handful of mosquitoes, namely those in the Aedes, Anopheles, and Culex genus, are the dominant vectors, many other species belonging to different genus are also involved in various pathogen cycles. Sabethes cyaneus is one of the many poorly understood mosquito species involved in the sylvatic cycle of Yellow Fever Virus. Here, we report the expression profile differences between male and female of Sa.cyaneus salivary glands (SGs). We find that female Sa.cyaneus SGs have 165 up-regulated and 18 down-regulated genes compared to male SGs. Most of the up-regulated genes have unknown functions, however, odorant binding proteins, such as those in the D7 protein family, and mucins were among the top 30 genes. We also performed various in vitro activity assays of female SGs. In the activity analysis we found that female SG extracts inhibit coagulation by blocking factor Xa and has endonuclease activity. Knowledge about mosquitoes and their physiology are important for understanding how different species differ in their ability to feed on and transmits pathogens to humans. These results provide us with an insight into the Sabethes SG activity and gene expression that expands our understanding of mosquito salivary glands.

Self-Hybridization in Leishmania major

Genetic exchange between different Leishmania strains in the sand fly vector has been experimentally demonstrated and is supported by population genetic studies. In nature, opportunities for Leishmania interstrain mating are restricted to flies biting multiply infected hosts or through multiple bites of different hosts. In contrast, self-mating could occur in any infected sand fly. By crossing two recombinant lines derived from the same Leishmania major strain, each expressing a different drug-resistance marker, self-hybridization in L. major was confirmed in a natural sand fly vector, Phlebotomus duboscqi, and in frequencies comparable to interstrain crosses. We provide the first high resolution, whole-genome sequencing analysis of large numbers of selfing progeny, their parents, and parental subclones. Genetic exchange consistent with classical meiosis is supported by the biallelic inheritance of the rare homozygous single nucleotide polymorphisms (SNPs) that arose by mutation during the generation of the parental clones. In contrast, heterozygous SNPs largely failed to be transmitted in Mendelian ratios for reasons not understood. SNPs that were heterozygous in both parents, however, recombined to produce homozygous alleles in some hybrids. For trisomic chromosomes present in both parents, transmittal to the progeny was only altered by self-hybridization, involving a gain or loss of somy in frequencies predicted by a meiotic process. Whole-genome polyploidization was also observed in the selfing progeny. Thus, self-hybridization in Leishmania, with its potential to occur in any infected sand fly, may be an important source of karyotype variation, loss of heterozygosity, and functional diversity. IMPORTANCE Leishmania are parasitic protozoa that cause a wide spectrum of diseases collectively known as the leishmaniases. Sexual reproduction in Leishmania has been proposed as an important source of genetic diversity and has been formally demonstrated to occur inside the sand fly vector midgut. Nevertheless, in the wild, opportunities for genetic exchange between different Leishmania species or strains are restricted by the capacity of different Leishmania strains to colonize the same sand fly. In this work, we report the first high resolution, whole-genome sequence analysis of intraclonal genetic exchange as a type of self-mating in Leishmania. Our data reveal that self-hybridization can occur with comparable frequency as interstrain mating under experimental lab conditions, leading to important genomic alterations that can potentially take place within every naturally infected sand fly.

Humoral response in Leishmaniasis

Leishmaniasis presents different types of clinical manifestations that can be divided into cutaneous leishmaniasis and visceral leishmaniasis. The host's immune system, associated with genetic and nutritional factors, is strongly involved in the evolution of the disease or parasite escape. Humoral immunity is characterized by the production of antibodies capable of promoting neutralization, opsonization, and activation of the complement system. In this scenario, B lymphocytes produce antibodies that play an important role in Leishmania infection although neglected for a long time. Thus, relevant aspects in the establishment of Leishmania infection will be addressed, highlighting the importance of humoral immunity during the entire process of Leishmania infection.

An overview of the sand fly salivary proteins in vaccine development against leishmaniases

Leishmaniases are a group of vector-borne parasitic diseases transmitted through the infected sand flies. Leishmania parasites are inoculated into the host skin along with sand fly saliva. The sand fly saliva consists of biologically active molecules with anticoagulant, anti-inflammatory, and immunomodulatory properties. Such properties help the parasite circumvent the host's immune responses. The salivary compounds support the survival and multiplication of the parasite and facilitate the disease progression. It is documented that frequent exposure to uninfected sand fly bites produces neutralizing antibodies against specific salivary proteins and further activates the cellular mechanisms to prevent the establishment of the disease. The immune responses due to sand fly saliva are highly specific and depend on the composition of the salivary molecules. Hence, thorough knowledge of these compounds in different sand fly species and information about their antigenicity are paramount to designing an effective vaccine. Herein, we review the composition of the sand fly saliva, immunomodulatory properties of some of its components, immune responses to its proteins, and potential vaccine candidates against leishmaniases.

Laboratory diagnostics for human Leishmania infections: a polymerase chain reaction-focussed review of detection and identification methods

Leishmania infections span a range of clinical syndromes and impact humans from many geographic foci, but primarily the world's poorest regions. Transmitted by the bite of a female sand fly, Leishmania infections are increasing with human movement (due to international travel and war) as well as with shifts in vector habitat (due to climate change). Accurate diagnosis of the 20 or so species of Leishmania that infect humans can lead to the successful treatment of infections and, importantly, their prevention through modelling and intervention programs. A multitude of laboratory techniques for the detection of Leishmania have been developed over the past few decades, and although many have drawbacks, several of them show promise, particularly molecular methods like polymerase chain reaction. This review provides an overview of the methods available to diagnostic laboratories, from traditional techniques to the now-preferred molecular techniques, with an emphasis on polymerase chain reaction-based detection and typing methods.

Understanding pathogen survival and transmission by arthropod vectors to prevent human disease

Many endemic poverty-associated diseases, such as malaria and leishmaniasis, are transmitted by arthropod vectors. Pathogens must interact with specific molecules in the vector gut, the microbiota, and the vector immune system to survive and be transmitted. The vertebrate host, in turn, is infected when the pathogen and vector-derived factors, such as salivary proteins, are delivered into the skin by a vector bite. Here, we review recent progress in our understanding of the biology of pathogen transmission from the human to the vector and back, from the vector to the host. We also highlight recent advances in the biology of vector-borne disease transmission, which have translated into additional strategies to prevent human disease by either reducing vector populations or by disrupting their ability to transmit pathogens.

Unweaving the NET: Microbial strategies for neutrophil extracellular trap evasion

Circa 20 years ago, a new type of defense mechanism was described in neutrophils. At the time, this mechanism corresponded to the extrusion of DNA, associated with histones, granular and cytosolic proteins from the cell and it was produced in response to exposure to pathogens or interleukins. The resulting NET-like structure was described as to entrap and/or kill microbes. However, shortly after the discovery the so-called Neutrophil Extracellular Traps, it was soon noticed and often mentioned in the literature that certain microbes are able to evade NET-mediated entrapment and/or death, to the point where its antimicrobial capacities were questioned, depending on the infection context. In this review, we summarize the diversity of strategies published thus far that viruses, fungi, bacteria and protists employ as to prevent or endure NETs. Moreover, we point to a few perspectives on the matter and a few evolutionary speculations on NETs evasion.

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.

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.

Host–Pathogen Interaction in Leishmaniasis: Immune Response and Vaccination Strategies

Leishmaniasis is a zoonotic and vector-borne infectious disease that is caused by the genus Leishmania belonging to the trypanosomatid family. The protozoan parasite has a digenetic life cycle involving a mammalian host and an insect vector. Leishmaniasisis is a worldwide public health problem falling under the neglected tropical disease category, with over 90 endemic countries, and approximately 1 million new cases and 20,000 deaths annually. Leishmania infection can progress toward the development of species–specific pathologic disorders, ranging in severity from self-healing cutaneous lesions to disseminating muco-cutaneous and fatal visceral manifestations. The severity and the outcome of leishmaniasis is determined by the parasite’s antigenic epitope characteristics, the vector physiology, and most importantly, the immune response and immune status of the host. This review examines the nature of host–pathogen interaction in leishmaniasis, innate and adaptive immune responses, and various strategies that have been employed for vaccine development.

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.

Neutrophil-dendritic cell interaction plays an important role in live attenuated Leishmania vaccine induced immunity

Background

Neutrophils are involved in the initial host responses to pathogens. Neutrophils can activate T cell responses either independently or through indirect involvement of Dendritic cells (DCs). Recently we have demonstrated direct neutrophil-T cell interactions that initiate adaptive immune responses following immunization with live attenuated Leishmania donovani centrin deleted parasite vaccine (LdCen^-/-). However, neutrophil-DC interactions in T cell priming in vaccine immunity in general are not known. In this study we evaluated the interaction between neutrophils and DCs during LdCen^-/- infection and compared with wild type parasite (LdWT) both in vitro and in vivo.

Methodology/findings

LdCen^-/- parasite induced increased expression of CCL3 in neutrophils caused higher recruitment of DCs capable of inducing a strong proinflammatory response and elevated co-stimulatory molecule expression compared to LdWT infection. To further illustrate neutrophil-DCs interactions in vivo, we infected LYS-eGFP mice with red fluorescent LdWT/LdCen^-/- parasites and sort selected DCs that engulfed the neutrophil containing parasites or DCs that acquired the parasites directly in the ear draining lymph nodes (dLN) 5d post infection. The DCs predominantly acquired the parasites by phagocytosing infected neutrophils. Specifically, DCs containing LdCen^-/- parasitized neutrophils exhibited a proinflammatory phenotype, increased expression of costimulatory molecules and initiated higher CD4⁺T cell priming ex-vivo. Notably, potent DC activation occurred when LdCen^-/- parasites were acquired indirectly via engulfment of parasitized neutrophils compared to direct engulfment of LdCen^-/- parasites by DCs. Neutrophil depletion in LdCen^-/- infected mice significantly abrogated expression of CCL3 resulting in decreased DC recruitment in ear dLN. This event led to poor CD4⁺Th1 cell priming ex vivo that correlated with attenuated Tbet expression in ear dLN derived CD4⁺ T cells in vivo.

Conclusions

Collectively, LdCen^-/- containing neutrophils phagocytized by DC markedly influence the phenotype and antigen presenting capacity of DCs early on and thus play an immune-regulatory role in shaping vaccine induced host protective response.

Visceral Leishmaniasis (VL), caused by the protozoan parasites of the genus Leishmania is a neglected tropical disease. Leishmania donovani is the principal causative agent of VL in East Africa and the Indian subcontinent whereas in Europe, North Africa, and Latin America VL is mainly caused by Leishmania infantum. No licensed vaccine exists against VL. We have reported previously that live attenuated centrin gene-deleted L. donovani (LdCen^-/-) parasite vaccine induced strong innate immunity which leads to a protective Th1 response in animal models. We recently demonstrated that neutrophils play an indispensable role following immunization with LdCen^-/- parasites in inducing protective Th1 immune response. However, neutrophils also secrete chemokines that attract other innate cells such as dendritic cells and regulate their activities. In the current study we analyzed the interplay between neutrophils and DCs, and its effects on T cell activation during LdCen^-/- infection and compared with wild type parasite (LdWT) infection. We observed that higher recruitment of DCs occurred in LdCen^-/- infected mice ear draining lymph nodes compared to LdWT. This recruitment is facilitated by increased secretion of the chemokine CCL3 by neutrophils. A markedly decreased DC recruitment was observed in LdCen^-/- infected mice following CCL3 neutralization indicating the key role of neutrophils in DC recruitment. Further, we demonstrated that DCs that ingest LdCen^-/- infected neutrophils are better activated than those that acquire the parasites independent of neutrophils. Notably neutrophil depletion in LdCen^-/- infected mice also attenuated activation of DCs in the ear dLN that resulted in poor CD4⁺T cell priming. Our results reveal that interaction between neutrophils and DCs play an important role in shaping proinflammatory immune response induced by a live attenuated Leishmania vaccine.

Stress conditions promote Leishmania hybridization in vitro marked by expression of the ancestral gamete fusogen HAP2 as revealed by single-cell RNA-seq

Leishmania are protozoan parasites transmitted by the bite of sand fly vectors producing a wide spectrum of diseases in their mammalian hosts. These diverse clinical outcomes are directly associated with parasite strain and species diversity. Although Leishmania reproduction is mainly clonal, a cryptic sexual cycle capable of producing hybrid genotypes has been inferred from population genetic studies and directly demonstrated by laboratory crosses. Experimentally, mating competence has been largely confined to promastigotes developing in the sand fly midgut. The ability to hybridize culture promastigotes in vitro has been limited so far to low-efficiency crosses between two Leishmania tropica strains, L747 and MA37, that mate with high efficiency in flies. Here, we show that exposure of promastigote cultures to DNA damage stress produces a remarkably enhanced efficiency of in vitro hybridization of the L. tropica strains and extends to other species, including Leishmania donovani, Leishmania infantum, and Leishmania braziliensis, a capacity to generate intra- and interspecific hybrids. Whole-genome sequencing and total DNA content analyses indicate that the hybrids are in each case full genome, mostly tetraploid hybrids. Single-cell RNA sequencing of the L747 and MA37 parental lines highlights the transcriptome heterogeneity of culture promastigotes and reveals discrete clusters that emerge post-irradiation in which genes potentially involved in genetic exchange are expressed, including the ancestral gamete fusogen HAP2. By generating reporter constructs for HAP2, we could select for promastigotes that could either hybridize or not in vitro. Overall, this work reveals that there are specific populations involved in Leishmania hybridization associated with a discernible transcriptomic signature, and that stress facilitated in vitro hybridization can be a transformative approach to generate large numbers of hybrid genotypes between diverse species and strains.

Ixodes ricinus Salivary Serpin Iripin-8 Inhibits the Intrinsic Pathway of Coagulation and Complement

Tick saliva is a rich source of antihemostatic, anti-inflammatory, and immunomodulatory molecules that actively help the tick to finish its blood meal. Moreover, these molecules facilitate the transmission of tick-borne pathogens. Here we present the functional and structural characterization of Iripin-8, a salivary serpin from the tick Ixodes ricinus, a European vector of tick-borne encephalitis and Lyme disease. Iripin-8 displayed blood-meal-induced mRNA expression that peaked in nymphs and the salivary glands of adult females. Iripin-8 inhibited multiple proteases involved in blood coagulation and blocked the intrinsic and common pathways of the coagulation cascade in vitro. Moreover, Iripin-8 inhibited erythrocyte lysis by complement, and Iripin-8 knockdown by RNA interference in tick nymphs delayed the feeding time. Finally, we resolved the crystal structure of Iripin-8 at 1.89 Å resolution to reveal an unusually long and rigid reactive center loop that is conserved in several tick species. The P1 Arg residue is held in place distant from the serpin body by a conserved poly-Pro element on the P′ side. Several PEG molecules bind to Iripin-8, including one in a deep cavity, perhaps indicating the presence of a small-molecule binding site. This is the first crystal structure of a tick serpin in the native state, and Iripin-8 is a tick serpin with a conserved reactive center loop that possesses antihemostatic activity that may mediate interference with host innate immunity.

Revisiting the Mechanisms of Immune Evasion Employed by Human Parasites

For the establishment of a successful infection, i.e., long-term parasitism and a complete life cycle, parasites use various diverse mechanisms and factors, which they may be inherently bestowed with, or may acquire from the natural vector biting the host at the infection prelude, or may take over from the infecting host, to outmaneuver, evade, overcome, and/or suppress the host immunity, both innately and adaptively. This narrative review summarizes the up-to-date strategies exploited by a number of representative human parasites (protozoa and helminths) to counteract the target host immune defense. The revisited information should be useful for designing diagnostics and therapeutics as well as vaccines against the respective parasitic infections.

Insect vectors' saliva and gut microbiota as a blessing in disguise: probability versus possibility

Drawing of host blood is a natural phenomenon during the bite of blood-probing insect vectors. Along with the blood meal, the vectors introduce salivary components and a trail of microbiota. In the case of infected vectors, the related pathogen accompanies the aforementioned biological components. In addition to Anopheles gambiae or Anopheles stephensi, the bites of other nonmalarial vectors cannot be ignored in malaria-endemic regions. Similarly, the bite incidence of Phlebotomus papatasi cannot be ignored in visceral leishmaniasis-endemic regions. Even the chances of getting bitten by uninfected vectors are higher than the infected vectors. We have discussed the probability or possibility of uninfected, infected, and/or nonvector's saliva and gut microbiota as a therapeutic option leading to the initial deterrent to pathogen establishment.

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.

Lutzomyia longipalpis: an update on this sand fly vector

Lutzomyia longipalpis is the most important vector of Leishmania infantum, the etiological agent of visceral leishmaniasis (VL) in the New World. It is a permissive vector susceptible to infection with several Leishmania species. One of the advantages that favors the study of this sand fly is the possibility of colonization in the laboratory. For this reason, several researchers around the world use this species as a model for different subjects including biology, insecticides testing, host-parasite interaction, physiology, genetics, proteomics, molecular biology, and saliva among others. In 2003, we published our first review (Soares & Turco 2003) on this vector covering several aspects of Lu. longipalpis. This current review summarizes what has been published between 2003-2020. During this period, modern approaches were incorporated following the development of more advanced and sensitive techniques to assess this sand fly.

Neutrophils and Visceral Leishmaniasis: Impact on innate immune response and cross-talks with macrophages and dendritic cells

Neutrophils with their array of microbicidal activities are the first innate immune cells to guard against infection. They are also most crucial for the host's initial defense against Leishmania parasites which cause clinically diverse diseases ranging from self-healing cutaneous leishmaniasis (CL) to a more severe visceral form, visceral leishmaniasis (VL). Neutrophils are recruited in large numbers at the infection site after bite of sandfly, which is the vector for the disease. The initial interaction of neutrophils with the parasites may modulate the subsequent innate and adaptive immune responses and hence affect the disease outcome. The purpose of this review is to comprehensively appraise the role of neutrophils during the early stages of Leishmania infection with a focus on the visceral form of the disease. In the past decade, new insights regarding the role of neutrophils in VL have surfaced which have been extensively elaborated in the present review. In addition, since much of the information regarding neutrophil-Leishmania early interaction has accumulated through studies on mouse models of CL, these studies are also revisited. We begin by reviewing the factors which drive the recruitment of neutrophils at the site of injection by the sandfly. We then discuss the studies delineating the molecular mechanisms involved in the uptake of the Leishmania parasite by neutrophils and how the parasite subverts their microbicidal functions. In the end, the interaction of infected neutrophils with macrophages and dendritic cells is summarized.

Essential Role of Neutrophils in the Protective Immune Response Induced by a Live Attenuated Leishmania Vaccine

No licensed vaccine exists against visceral leishmaniasis (VL), a disease caused by the Leishmania donovani parasite. We have previously reported both macrophages and dendritic cells play important role in the protection induced by a live attenuated centrin gene-deleted L. donovani (LdCen^-/- ) parasite vaccine. The role of neutrophils in orchestrating the initial innate response to pathogens is widely recognized. To investigate the early interaction of LdCen^-/- with neutrophils, we immunized mice intradermally in the ear pinna with LdCen^-/- Compared with LdWT infection, LdCen^-/- parasites induced higher recruitment of neutrophils to the ear dermis and ear draining lymph nodes (dLN) as early as 6-18 h after immunization, which were predominantly proinflammatory in nature. Neutrophils from ear dLN of LdCen^-/- -immunized mice exhibited heightened expression of costimulatory molecules and attenuated expression of coinhibitory molecules necessary for higher T cell activation. Further phenotypic characterization revealed heterogeneous neutrophil populations containing Nα and Nβ subtypes in the ear dLN. Of the two, the parasitized Nα subset from LdCen^-/- -immunized mice exhibited much stronger Ag-specific CD4⁺ T cell proliferation ex vivo. Adoptive transfer of neutrophils bearing LdCen^-/- parasites induced an increased Th1 response in naive mice. Importantly, neutrophil depletion significantly abrogated Ag-specific CD4⁺ T cell proliferation in LdCen^-/- -immunized mice and impaired protection against virulent challenge. Conversely, replenishing of neutrophils significantly restored the LdCen^-/- -induced host-protective response. These results suggest that neutrophils are indispensable for protective immunity induced by LdCen^-/- parasite vaccine.

Cytokine saga in visceral leishmaniasis

In humans, infection with Leishmania manifests into a spectrum of diseases. The manifestation of the diseases depend on the resultant evasion of the parasite to immune responses namely by macrophages, which is an exclusive host of Leishmania. The B cells valiantly mount antibody responses, however, to no avail as the Leishmania parasites occupy the intracellular niches of the macrophages and subvert the immune response. Extensive studies have been documented on the role of cell-mediated immunity (CMI) in protection and counter survival strategies of the parasites leading to downregulation of CMI. The present review attempts to discuss the cytokines in progression or resolution of visceral form of leishmaniasis or kala-azar, predominantly affecting the Indian subcontinent. The components/cytokine(s) responsible for the regulation of the critical balance of T helper cells and their subsets have been discussed in the perspective. Therefore, any strategy involving the treatment of visceral leishmania (VL) needs to consider the balance and regulation of T cell function.

Amine-binding properties of salivary yellow-related proteins in phlebotomine sand flies

The amine-binding properties of sand fly salivary yellow-related proteins (YRPs) were described only in Lutzomyia longipalpis sand flies. Here, we experimentally confirmed the kratagonist function of YRPs in the genus Phlebotomus. We utilized microscale thermophoresis technique to determine the amine-binding properties of YRPs in saliva of Phlebotomus perniciosus and P. orientalis, the Old-World vectors of visceral leishmaniases causative agents. Expressed and purified YRPs from three different sand fly species were tested for their interactions with various biogenic amines, including serotonin, histamine and catecholamines. Using the L. longipalpis YRP LJM11 as a control, we have demonstrated the comparability of the microscale thermophoresis method with conventional isothermal titration calorimetry described previously. By homology in silico modeling, we predicted the surface charge and both amino acids and hydrogen bonds of the amine-binding motifs to influence the binding affinities between closely related YRPs. All YRPs tested bound at least two biogenic amines, while the affinities differ both among and within species. Low affinity was observed for histamine. The salivary recombinant proteins rSP03B (P. perniciosus) and rPorASP4 (P. orientalis) showed high-affinity binding of serotonin, suggesting their capability to facilitate inhibition of the blood vessel contraction and platelet aggregation.

Vaccines for leishmaniasis and the implications of their development for American tegumentary leishmaniasis

The leishmaniases are a collection of vector-borne parasitic diseases caused by a number of different Leishmania species that are distributed worldwide. Clinical and laboratory research have together revealed several important immune components that control Leishmania infection and indicate the potential of immunization to prevent leishmaniasis. In this review we introduce previous and ongoing experimental research efforts to develop vaccines against Leishmania species. First, second and third generation vaccine strategies that have been proposed to counter cutaneous and visceral leishmaniasis (CL and VL, respectively) are summarized. One of the major bottlenecks in development is the transition from results in animal model studies to humans, and we highlight that although American tegumentary leishmaniasis (ATL; New World CL) can progress to destructive and disfiguring mucosal lesions, most research has been conducted using mouse models and Old World Leishmania species. We conclude that assessment of vaccine candidates in ATL settings therefore appears merited.

Leishmania Spp-Host Interaction: There Is Always an Onset, but Is There an End?

For a long time Leishmaniasis had been considered as a neglected tropical disease. Recently, it has become a priority in public health all over the world for different aspects such as geographic spread, number of population living at risk of infection as well as the potential lethality and/or the development of disfiguring lesions in the, respectively, visceral and tegumentary forms of the disease. As a result, several groups have been bending over this issue and many valuable data have been published. Nevertheless, parasite-host interactions are still not fully known and, consequently, we do not entirely understand the infection dynamics and parasite persistence. This knowledge may point targets for modulation or blockage, being very useful in the development of measures to interfere in the course of infection/ disease and to minimize the risks and morbidity. In the present review we will discuss some aspects of the Leishmania spp-mammalian host interaction in the onset of infection and after the clinical cure of the lesions. We will also examine the information already available concerning the parasite strategy to evade immune response mainly at the beginning of the infection, as well as during the parasite persistence. This knowledge can improve the conditions of treatment, follow-up and cure control of patients, minimizing the potential damages this protozoosis can cause to infected individuals.

An insight into the sialome, mialome and virome of the horn fly, Haematobia irritans

BACKGROUND

The horn fly (Haematobia irritans) is an obligate blood feeder that causes considerable economic losses in livestock industries worldwide. The control of this cattle pest is mainly based on insecticides; unfortunately, in many regions, horn flies have developed resistance. Vaccines or biological control have been proposed as alternative control methods, but the available information about the biology or physiology of this parasite is rather scarce.

RESULTS

We present a comprehensive description of the salivary and midgut transcriptomes of the horn fly (Haematobia irritans), using deep sequencing achieved by the Illumina protocol, as well as exploring the virome of this fly. Comparison of the two transcriptomes allow for identification of uniquely salivary or uniquely midgut transcripts, as identified by statistically differential transcript expression at a level of 16 x or more. In addition, we provide genomic highlights and phylogenetic insights of Haematobia irritans Nora virus and present evidence of a novel densovirus, both associated to midgut libraries of H. irritans.

CONCLUSIONS

We provide a catalog of protein sequences associated with the salivary glands and midgut of the horn fly that will be useful for vaccine design. Additionally, we discover two midgut-associated viruses that infect these flies in nature. Future studies should address the prevalence, biological effects and life cycles of these viruses, which could eventually lead to translational work oriented to the control of this economically important cattle pest.

Aedes aegypti NeSt1 Protein Enhances Zika Virus Pathogenesis by Activating Neutrophils

Saliva from the mosquito vector of flaviviruses is capable of changing the local immune environment, leading to an increase in flavivirus-susceptible cells at the infected bite site. In addition, an antibody response to specific salivary gland (SG) components changes the pathogenesis of flaviviruses in human populations. To investigate whether antigenic SG proteins are capable of enhancing infection with Zika virus (ZIKV), a reemerging flavivirus primarily transmitted by the Aedes aegypti mosquito, we screened for antigenic SG proteins using a yeast display library and demonstrate that a previously undescribed SG protein we term neutrophil stimulating factor 1 (NeSt1) activates primary mouse neutrophils ex vivo Passive immunization against NeSt1 decreases pro-interleukin-1β and CXCL2 expression, prevents macrophages from infiltrating the bite site, protects susceptible IFNAR-/- IFNGR-/- (AG129) mice from early ZIKV replication, and ameliorates virus-induced pathogenesis. These findings indicate that NeSt1 stimulates neutrophils at the mosquito bite site to change the immune microenvironment, allowing a higher level of early viral replication and enhancing ZIKV pathogenesis.IMPORTANCE When a Zika virus-infected mosquito bites a person, mosquito saliva is injected into the skin along with the virus. Molecules in this saliva can make virus infection more severe by changing the immune system to make the skin a better place for the virus to replicate. We identified a molecule that activates immune cells, called neutrophils, to recruit other immune cells, called macrophages, that the virus can infect. We named this molecule neutrophil-stimulating factor 1 (NeSt1). When we used antibodies to block NeSt1 in mice and then allowed Zika virus-infected mosquitoes to feed on these mice, they survived much better than mice that do not have antibodies against NeSt1. These findings give us more information about how mosquito saliva enhances virus infection, and it is possible that a vaccine against NeSt1 might protect people against severe Zika virus infection.

Neutrophil properties in healthy and Leishmania infantum-naturally infected dogs

Visceral leishmaniasis is a chronic disease that affects humans and dogs as well. Dogs, the domestic reservoir of Leishmania, play a central role in the transmission of visceral leishmaniasis, the most severe form of this disease. Neutrophils are the most abundant leukocytes in blood and interact with the parasite after infection. Here, we evaluate the effector properties of neutrophils from healthy and naturally Leishmania infantum-infected dogs. Our results showed that the parasite induced neutrophil extracellular trap (NET) release from neutrophils in both groups. Additionally, phagocytosis and NETs contributed differently to parasite killing by neutrophils from healthy and infected animals, and IFN-γ, IL-8, IL-4 and TNF-α production by neutrophils from both groups were differentially modulated by the parasite. Our results contribute to a better understanding of the complex role played by neutrophils in canine visceral leishmaniasis, which may favor the development of more effective therapies.

How to master the host immune system? Leishmania parasites have the solutions!

Infection by protozoan parasites of the genus Leishmania results in the development of leishmaniasis, an increasingly prevalent group of diseases affecting over 12 million people worldwide. Leishmaniasis can have very different outcomes ranging from cutaneous lesions, mucosal lesions to visceralization depending on the species of the infecting parasite and on the immune response developed by the host. As an obligate intracellular parasite, residing within macrophages, Leishmania evolved in strict contact with the host immune system, developing different mechanisms to evade or modulate the immune response. Various types of immune responses are observed during different Leishmania spp. infections, resulting in parasite clearance but also contributing to the pathogenesis, thus increasing the complexity of the course of the disease. Interestingly, depending on the type of leishmaniasis developed, opposite treatment strategies, which either boost or inhibit the inflammatory response, have shown efficacy. In this review, we summarize the contribution of different immune cell types to the development of the anti-leishmanial immune response and the parasite strategies to evade and modulate host immunity. Further, we discuss the involvement of co-infecting pathogens in the determination of the outcome of leishmaniasis and on the effectiveness of treatment and the implication of the immune response for treatment and vaccine development.

Tinkering with targeting nucleotide signaling for control of intracellular Leishmania parasites

Nucleotides are one of the most primitive extracellular signalling molecules across all phyla and regulate a multitude of responses. The biological effects of extracellular nucleotides/sides are mediated via the specific purinergic receptors present on the cell surface. In mammalian system, adenine nucleotides are the predominant nucleotides found in the extracellular milieu and mediate a constellation of physiological functions. In the context of host-pathogen interaction, extracellular ATP is recognized as a danger signal and potentiates the release of pro-inflammatory mediators from activated immune cells, on the other hand, its breakdown product adenosine exerts potential anti-inflammatory and immunosuppressive actions. Therefore, it is increasingly apparent that the interplay between extracellular ATP/adenosine ratios has a significant role in coordinating the regulation of the immune system in health and diseases. Several pathogens express ectonucleotidases on their surface and exploit the purinergic signalling as one of the mechanisms to modulate the host immune response. Leishmania pathogens are one of the most successful intracellular pathogens which survive within host macrophages and manipulate protective Th1 response into disease promoting Th2 response. In this review, we discuss the regulation of extracellular ATP and adenosine levels, the role of ATP/adenosine counter signalling in regulating the inflammation and immune responses during infection and how Leishmania parasites exploit the purinergic signalling to manipulate host response. We also discuss the challenges and opportunities in targeting purinergic signalling and the future prospects.

Cloning, expression and purification of 3'-nucleotidase/nuclease, an enzyme responsible for the Leishmania escape from neutrophil extracellular traps

Leishmaniasis is one of the most significant of the neglected tropical diseases, with 350 million people in 98 countries worldwide living at risk of developing one of the many forms of the disease. During the transmission of the parasite from its vector to the vertebrate host, neutrophils are rapidly recruited to the site of the sandfly bite. Using different strategies, neutrophils can often kill a large number of parasites. However, some parasites can resist neutrophil-killing mechanisms and survive until macrophage arrival at the infection site. One of the strategies for neutrophil-mediated killing is the production of neutrophil extracellular traps (NETs). Because of its ecto-localized nuclease activity, the enzyme 3'-nucleotidase/nuclease (3'NT/NU), present in different Leishmania species, was recently identified as part of a possible parasite escape mechanism from NET-mediated death. Previous studies showed that 3'NT/NU also plays an important role in the establishment of Leishmania infection by generating extracellular adenosine that favors the parasite and macrophage interaction. This study aims to deepen the knowledge about 3'NT/NU, mainly with respect to its nuclease activity that is little studied in the current literature. For this, we cloned, expressed and purified the recombinant La3'NT/NU and have confirmed its contribution to the parasite escape from NET-mediated killing.

Comparative Evolution of Sand Fly Salivary Protein Families and Implications for Biomarkers of Vector Exposure and Salivary Vaccine Candidates

Sand fly salivary proteins that produce a specific antibody response in humans and animal reservoirs have been shown to be promising biomarkers of sand fly exposure. Furthermore, immunity to sand fly salivary proteins were shown to protect rodents and non-human primates against Leishmania infection. We are missing critical information regarding the divergence amongst sand fly salivary proteins from different sand fly vectors, a knowledge that will support the search of broad or specific salivary biomarkers of vector exposure and those for vaccines components against leishmaniasis. Here, we compare the molecular evolution of the salivary protein families in New World and Old World sand flies from 14 different sand fly vectors. We found that the protein families unique to OW sand flies are more conserved than those unique to NW sand flies regarding both sequence polymorphisms and copy number variation. In addition, the protein families unique to OW sand flies do not display as many conserved cysteine residues as the one unique to the NW group (28.5% in OW vs. 62.5% in NW). Moreover, the expression of specific protein families is restricted to the salivary glands of unique sand fly taxon. For instance, the ParSP15 family is unique to the Larroussius subgenus whereas phospholipase A2 is only expressed in member of Larroussius and Adlerius subgenera. The SP2.5-like family is only expressed in members of the Phlebotomus and Paraphlebotomus subgenera. The sequences shared between OW and NW sand flies have diverged at similar rates (38.7 and 45.3% amino acid divergence, respectively), yet differences in gene copy number were evident across protein families and sand fly species. Overall, this comparative analysis sheds light on the different modes of sand fly salivary protein family divergence. Also, it informs which protein families are unique and conserved within taxon for the choice of taxon-specific biomarkers of vector exposure, as well as those families more conserved across taxa to be used as pan-specific vaccines for leishmaniasis.

De novo assembly and annotation of Hyalomma dromedarii tick (Acari: Ixodidae) sialotranscriptome with regard to gender differences in gene expression

BACKGROUND

Hard ticks are hematophagous ectoparasites characterized by their long-term feeding. The saliva that they secrete during their blood meal is their crucial weapon against host-defense systems including hemostasis, inflammation and immunity. The anti-hemostatic, anti-inflammatory and immune-modulatory activities carried out by tick saliva molecules warrant their pharmacological investigation. The Hyalomma dromedarii Koch, 1844 tick is a common parasite of camels and probably the best adapted to deserts of all hard ticks. Like other hard ticks, the salivary glands of this tick may provide a rich source of many compounds whose biological activities interact directly with host system pathways. Female H. dromedarii ticks feed longer than males, thereby taking in more blood. To investigate the differences in feeding behavior as reflected in salivary compounds, we performed de novo assembly and annotation of H. dromedarii sialotranscriptome paying particular attention to variations in gender gene expression.

RESULTS

The quality-filtered Illumina sequencing reads deriving from a cDNA library of salivary glands led to the assembly of 15,342 transcripts. We deduced that the secreted proteins included: metalloproteases, glycine-rich proteins, mucins, anticoagulants of the mandanin family and lipocalins, among others. Expression analysis revealed differences in the expression of transcripts between male and female H. dromedarii that might explain the blood-feeding strategies employed by both genders.

CONCLUSIONS

The annotated sialome of H. dromedarii helps understand the interaction of tick-host molecules during blood-feeding and can lead to the discovery of new pharmacologically active proteins of ticks of the genus Hyalomma.

Leishmania Hijacks Myeloid Cells for Immune Escape

Protozoan parasites of the Leishmania genus are the causative agents of leishmaniasis, a group of neglected tropical diseases whose clinical manifestations vary depending on the infectious Leishmania species but also on host factors. Recognition of the parasite by host myeloid immune cells is a key to trigger an effective Leishmania-specific immunity. However, the parasite is able to persist in host myeloid cells by evading, delaying and manipulating host immunity in order to escape host resistance and ensure its transmission. Neutrophils are first in infiltrating infection sites and could act either favoring or protecting against infection, depending on factors such as the genetic background of the host or the parasite species. Macrophages are the main host cells where the parasites grow and divide. However, macrophages are also the main effector population involved in parasite clearance. Parasite elimination by macrophages requires the priming and development of an effector Th1 adaptive immunity driven by specific subtypes of dendritic cells. Herein, we will provide a comprehensive outline of how myeloid cells regulate innate and adaptive immunity against Leishmania, and the mechanisms used by the parasites to promote their evasion and sabotage. Understanding the interactions between Leishmania and the host myeloid cells may lead to the development of new therapeutic approaches and improved vaccination to leishmaniases, an important worldwide health problem in which current therapeutic or preventive approaches are limited.

Immunity to LuloHya and Lundep, the salivary spreading factors from Lutzomyia longipalpis, protects against Leishmania major infection

Salivary components from disease vectors help arthropods to acquire blood and have been shown to enhance pathogen transmission in different model systems. Here we show that two salivary enzymes from Lutzomyia longipalpis have a synergist effect that facilitates a more efficient blood meal intake and diffusion of other sialome components. We have previously shown that Lundep, a highly active endonuclease, enhances parasite infection and prevent blood clotting by inhibiting the intrinsic pathway of coagulation. To investigate the physiological role of a salivary hyaluronidase in blood feeding we cloned and expressed a recombinant hyaluronidase from Lu. longipalpis. Recombinant hyaluronidase (LuloHya) was expressed in mammalian cells and biochemically characterized in vitro. Our study showed that expression of neutrophil CXC chemokines and colony stimulating factors were upregulated in HMVEC cells after incubation with LuloHya and Lundep. These results were confirmed by the acute hemorrhage, edema and inflammation in a dermal necrosis (dermonecrotic) assay involving a massive infiltration of leukocytes, especially neutrophils, in mice co-injected with hemorrhagic factor and these two salivary proteins. Moreover, flow cytometry results showed that LuloHya and Lundep promote neutrophil recruitment to the bite site that may serve as a vehicle for establishment of Leishmania infection. A vaccination experiment demonstrated that LuloHya and Lundep confer protective immunity against cutaneous leishmaniasis using the Lu. longipalpis-Leishmania major combination as a model. Animals (C57BL/6) immunized with LuloHya or Lundep showed minimal skin damage while lesions in control animals remained ulcerated. This protective immunity was abrogated when B-cell-deficient mice were used indicating that antibodies against both proteins play a significant role for disease protection. Rabbit-raised anti-LuloHya antibodies completely abrogated hyaluronidase activity in vitro. Moreover, in vivo experiments demonstrated that blocking LuloHya with specific antibodies interferes with sand fly blood feeding. This work highlights the relevance of vector salivary components in blood feeding and parasite transmission and further suggests the inclusion of these salivary proteins as components for an anti-Leishmania vaccine.

A deep insight into the male and female sialotranscriptome of adult Culex tarsalis mosquitoes

Previously, a Sanger-based sialotranscriptome analysis of adult female Culex tarsalis was published based on ∼2000 ESTs. During the elapsed 7.5 years, pyrosequencing has been discontinued and Illumina sequences have increased considerable in size and decreased in price. We here report an Illumina-based sialotranscriptome that allowed finding the missing apyrase from the salivary transcriptome of C. tarsalis, to determine several full-length members of the 34-62 kDa family, when a single EST has been found previously, in addition to identifying many salivary families with lower expression levels that were not detected previously. The use of multiple libraries including salivary glands and carcasses from male and female organisms allowed for an unprecedented insight into the tissue specificity of transcripts, and in this particular case permitting identification of transcripts putatively associated with blood feeding, when exclusive of female salivary glands, or associated with sugar feeding, when transcripts are found upregulated in both male and female glands.

Live Attenuated Leishmania donovani Centrin Gene-Deleted Parasites Induce IL-23-Dependent IL-17-Protective Immune Response against Visceral Leishmaniasis in a Murine Model

No vaccine exists against visceral leishmaniasis. To develop effective vaccines, we have previously reported protective role of live attenuated centrin gene-deleted Leishmania donovani (LdCen^-/- ) parasites through induction of Th1 type immune response in mice, hamsters, and dogs. In this study, we specifically explored the role of Th17 cells in LdCen^-/- -induced host protection in mice. Our results showed that compared with wild-type L. donovani infection, LdCen^-/- parasites induce significantly higher expression of Th17 differentiation cytokines in splenic dendritic cells. There was also induction of IL-17 and its promoting cytokines in total splenocytes and in both CD4 and CD8 T cells following immunization with LdCen^-/- Upon challenge with wild-type parasites, IL-17 and its differentiating cytokines were significantly higher in LdCen^-/- -immunized mice compared with nonimmunized mice that resulted in parasite control. Alongside IL-17 induction, we observed induction of IFN-γ-producing Th1 cells as reported earlier. However, Th17 cells are generated before Th1 cells. Neutralization of either IL-17 or IFN-γ abrogated LdCen^-/- -induced host protection further confirming the essential role of Th17 along with Th1 cytokines in host protection. Treatment with recombinant IL-23, which is required for stabilization and maintenance of IL-17, heightened Th17, and Tc17 responses in immunized mice splenocytes. In contrast, Th17 response was absent in immunized IL-23R^-/- mice that failed to induce protection upon virulent Leishmania challenge suggesting that IL-23 plays an essential role in IL-17-mediated protection by LdCen^-/- parasites. This study unveiled the role of IL-23-dependent IL-17 induction in LdCen^-/- parasite-induced immunity and subsequent protection against visceral leishmaniasis.

Survival Mechanisms Used by Some Leishmania Species to Escape Neutrophil Killing

Neutrophils are the most abundant leukocytes in human blood. Upon microbial infection, they are massively and rapidly recruited from the circulation to sites of infection where they efficiently kill pathogens. To this end, neutrophils possess a variety of weapons that can be mobilized and become effective within hours following infection. However, several microbes including some Leishmania spp. have evolved a variety of mechanisms to escape neutrophil killing using these cells as a basis to better invade the host. In addition, neutrophils are also present in unhealing cutaneous lesions where their role remains to be defined. Here, we will review recent progress in the field and discuss the different strategies applied by some Leishmania parasites to escape from being killed by neutrophils and as recently described for Leishmania mexicana, even replicate within these cells. Subversion of neutrophil killing functions by Leishmania is a strategy that allows parasite spreading in the host with a consequent deleterious impact, transforming the primary protective role of neutrophils into a deleterious one.

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.

The Sand Fly Salivary Protein Lufaxin Inhibits the Early Steps of the Alternative Pathway of Complement by Direct Binding to the Proconvertase C3b-B

Saliva of the blood feeding sand fly Lutzomyia longipalpis was previously shown to inhibit the alternative pathway (AP) of the complement system. Here, we have identified Lufaxin, a protein component in saliva, as the inhibitor of the AP. Lufaxin inhibited the deposition of C3b, Bb, Properdin, C5b, and C9b on agarose-coated plates in a dose-dependent manner. It also inhibited the activation of factor B in normal serum, but had no effect on the components of the membrane attack complex. Surface plasmon resonance (SPR) experiments demonstrated that Lufaxin stabilizes the C3b-B proconvertase complex when passed over a C3b surface in combination with factor B. Lufaxin was also shown to inhibit the activation of factor B by factor D in a reconstituted C3b-B, but did not inhibit the activation of C3 by reconstituted C3b-Bb. Proconvertase stabilization does not require the presence of divalent cations, but addition of Ni²⁺ increases the stability of complexes formed on SPR surfaces. Stabilization of the C3b-B complex to prevent C3 convertase formation (C3b-Bb formation) is a novel mechanism that differs from previously described strategies used by other organisms to inhibit the AP of the host complement system.

G Protein-Coupled Kinin Receptors and Immunity Against Pathogens

For decades, immunologists have considered the complement system as a paradigm of a proteolytic cascade that, acting cooperatively with the immune system, enhances host defense against infectious organisms. In recent years, advances made in thrombosis research disclosed a functional link between activated neutrophils, monocytes, and platelet-driven thrombogenesis. Forging a physical barrier, the fibrin scaffolds generated by synergism between the extrinsic and intrinsic (contact) pathways of coagulation entrap microbes within microvessels, limiting the systemic spread of infection while enhancing the clearance of pathogens by activated leukocytes. Insight from mice models of thrombosis linked fibrin formation via the intrinsic pathway to the autoactivation of factor XII (FXII) by negatively charged "contact" substances, such as platelet-derived polyphosphates and DNA from neutrophil extracellular traps. Following cleavage by FXIIa, activated plasma kallikrein (PK) initiates inflammation by liberating the nonapeptide bradykinin (BK) from an internal domain of high molecular weight kininogen (HK). Acting as a paracrine mediator, BK induces vasodilation and increases microvascular permeability via activation of endothelial B2R, a constitutively expressed subtype of kinin receptor. During infection, neutrophil-driven extravasation of plasma fuels inflammation via extravascular activation of the kallikrein-kinin system (KKS). Whether liberated by plasma-borne PK, tissue kallikrein, and/or microbial-derived proteases, the short-lived kinins activate immature dendritic cells via B2R, thus linking the infection-associated innate immunity/inflammation to the adaptive arm of immunity. As inflammation persists, a GPI-linked carboxypeptidase M removes the C-terminal arginine from the primary kinin, converting the B2R agonist into a high-affinity ligand for B1R, a GPCR subtype that is transcriptionally upregulated in injured/inflamed tissues. As reviewed here, lessons taken from studies of kinin receptor function in experimental infections have shed light on the complex proteolytic circuits that, acting at the endothelial interface, reciprocally couple immunity to the proinflammatory KKS.

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.

Evasion of Neutrophil Extracellular Traps by Respiratory Pathogens

The release of neutrophil extracellular traps (NETs) is a major immune mechanism intended to capture pathogens. These histone- and protease-coated DNA structures are released by neutrophils in response to a variety of stimuli, including respiratory pathogens, and have been identified in the airways of patients with respiratory infection, cystic fibrosis, acute lung injury, primary graft dysfunction, and chronic obstructive pulmonary disease. NET production has been demonstrated in the lungs of mice infected with Staphylococcus aureus, Klebsiella pneumoniae, and Aspergillus fumigatus. Since the discovery of NETs over a decade ago, evidence that "NET evasion" might act as an immune protection strategy among respiratory pathogens, including group A Streptococcus, Bordetella pertussis, and Haemophilus influenzae, has been growing, with the majority of these studies being published in the past 2 years. Evasion strategies fall into three main categories: inhibition of NET release by down-regulating host inflammatory responses; degradation of NETs using pathogen-derived DNases; and resistance to the microbicidal components of NETs, which involves a variety of mechanisms, including encapsulation. Hence, the evasion of NETs appears to be a widespread strategy to allow pathogen proliferation and dissemination, and is currently a topic of intense research interest. This article outlines the evidence supporting the three main strategies of NET evasion-inhibition, degradation, and resistance-with particular reference to common respiratory pathogens.

Neutrophil Extracellular Traps Reprogram IL-4/GM-CSF-Induced Monocyte Differentiation to Anti-inflammatory Macrophages

Monocyte-derived dendritic cells (mo-DCs) are essential for the development of a Th1 protective immune response against Leishmania parasites. It is well known that IL-4 and GM-CSF drive differentiation of human monocytes to dendritic cells (DCs). Here, we investigate if neutrophil extracellular traps (NETs) disrupt this process. NETs-enriched supernatants, generated after human neutrophil activation by Leishmania promastigotes, were added to monocytes and differentiation monitored by expression of molecules associated with macrophage and DCs phenotypes, cytokine production, and parasite killing. We found that NETs addition to IL-4/GM-CSF-treated monocytes prevented then to fully differentiate into DCs. No effect was observed if NETs were treated with DNase or by filtering the traps. Moreover, NETs closely interact with monocytes and downregulate the expression of the IL-4 receptor, which in turn disrupts fully differentiation of monocytes into DCs. Neutrophil elastase inhibition rescues the monocytes to DCs differentiation. Monocytes cultured with IL-4/GM-CSF and NETs differentiated into macrophages, as observed by the increased expression of CD68, CD32, and CD163, and decreased expression of CD80. Moreover, NET addition to IL-4/GM-CSF-treated monocytes rendered these cells less efficient to kill Leishmania parasites. Altogether, our results show that NETs interfere with IL-4/GM-CSF driven differentiation, reprogramming the generation of mo-DCs to an anti-inflammatory macrophage.