IrFC - An Ixodes ricinus injury-responsive molecule related to Limulus Factor C

From the fat body to the hemolymph: Profiling tick immune and storage proteins through transcriptomics and proteomics

Ticks are blood-feeding arachnids that are known to transmit various pathogenic microorganisms to their hosts. During blood feeding, ticks activate their metabolism and immune system to efficiently utilise nutrients from the host's blood and complete the feeding process. In contrast to insects, in which the fat body is known to be a central organ that controls essential metabolic processes and immune defense mechanisms, the function of the fat body in tick physiology is still relatively unexplored. To fill this gap, we sought to uncover the repertoire of genes expressed in the fat body associated with trachea (FB/Tr) by analyzing the transcriptome of individual, partially fed (previtellogenic) Ixodes ricinus females. The resulting catalog of individual mRNA sequences reveals a broad repertoire of transcripts encoding proteins involved in nutrient storage and distribution, as well as components of the tick immune system. To gain a detailed insight into the secretory products of FB/Tr specifically involved in inter-tissue transport and humoral immunity, the transcriptomic data were complemented with the proteome of soluble proteins in the hemolymph of partially fed female ticks. Among these proteins, the hemolipoglyco-carrier proteins were predominant. When comparing immune peptides and proteins from the fat body with those produced by hemocytes, we found that the fat body serves as a unique producer of certain immune components. Finally, time-resolved transcriptional regulation of selected immune transcripts from the FB/Tr was examined in response to experimental challenges with model microbes and analyzed by RT-qPCR. Overall, our data show that the fat body of ticks, similar to insects, is an important metabolic tissue that also plays a remarkable role in immune defense against invading microbes. These findings improve our understanding of tick biology and its impact on the transmission of tick-borne pathogens.

New insights into the hemolymph coagulation cascade of horseshoe crabs initiated by autocatalytic activation of a lipopolysaccharide-sensitive zymogen

The concept of a chain reaction of proteolytic activation of multiple protease zymogens was first proposed to explain the blood clotting system in mammals as an enzyme cascade. In multicellular organisms, similar enzyme cascades are widely present in signal transduction and amplification systems. The initiation step of the blood coagulation cascade often consists of autocatalytic activation of the corresponding zymogens located on the surfaces of host- or foreign-derived substances at injured sites. However, the molecular mechanism underlying the concept of autocatalytic activation remains speculative. In this review, we will focus on the autocatalytic activation of prochelicerase C on the surface of lipopolysaccharide as a potential initiator of hemolymph coagulation in horseshoe crabs. Prochelicerase C is presumed to have evolved from a common complement factor in Chelicerata; thus, evolutionary insights into the hemolymph coagulation and complement systems in horseshoe crabs will also be discussed.

How Dopamine Influences Survival and Cellular Immune Response of Rhipicephalus microplus Inoculated with Metarhizium anisopliae

Dopamine (DA) is a biogenic monoamine reported to modulate insect hemocytes. Although the immune functions of DA are known in insects, there is a lack of knowledge of DA’s role in the immune system of ticks. The use of Metarhizium anisopliae has been considered for tick control, driving studies on the immune response of these arthropods challenged with fungi. The present study evaluated the effect of DA on the cellular immune response and survival of Rhipicephalus microplus inoculated with M. anisopliae blastospores. Exogenous DA increased both ticks’ survival 72 h after M. anisopliae inoculation and the number of circulating hemocytes compared to the control group, 24 h after the treatment. The phagocytic index of tick hemocytes challenged with M. anisopliae did not change upon injection of exogenous DA. Phenoloxidase activity in the hemolymph of ticks injected with DA and the fungus or exclusively with DA was higher than in untreated ticks or ticks inoculated with the fungus alone, 72 h after treatment. DA was detected in the hemocytes of fungus-treated and untreated ticks. Unveiling the cellular immune response in ticks challenged with entomopathogenic fungi is important to improve strategies for the biological control of these ectoparasites.

Mialostatin, a Novel Midgut Cystatin from Ixodes ricinus Ticks: Crystal Structure and Regulation of Host Blood Digestion

The hard tick Ixodes ricinus is a vector of Lyme disease and tick-borne encephalitis. Host blood protein digestion, essential for tick development and reproduction, occurs in tick midgut digestive cells driven by cathepsin proteases. Little is known about the regulation of the digestive proteolytic machinery of I. ricinus. Here we characterize a novel cystatin-type protease inhibitor, mialostatin, from the I. ricinus midgut. Blood feeding rapidly induced mialostatin expression in the gut, which continued after tick detachment. Recombinant mialostatin inhibited a number of I. ricinus digestive cysteine cathepsins, with the greatest potency observed against cathepsin L isoforms, with which it co-localized in midgut digestive cells. The crystal structure of mialostatin was determined at 1.55 Å to explain its unique inhibitory specificity. Finally, mialostatin effectively blocked in vitro proteolysis of blood proteins by midgut cysteine cathepsins. Mialostatin is likely to be involved in the regulation of gut-associated proteolytic pathways, making midgut cystatins promising targets for tick control strategies.

Tick Immune System: What Is Known, the Interconnections, the Gaps, and the Challenges

Ticks are ectoparasitic arthropods that necessarily feed on the blood of their vertebrate hosts. The success of blood acquisition depends on the pharmacological properties of tick saliva, which is injected into the host during tick feeding. Saliva is also used as a vehicle by several types of pathogens to be transmitted to the host, making ticks versatile vectors of several diseases for humans and other animals. When a tick feeds on an infected host, the pathogen reaches the gut of the tick and must migrate to its salivary glands via hemolymph to be successfully transmitted to a subsequent host during the next stage of feeding. In addition, some pathogens can colonize the ovaries of the tick and be transovarially transmitted to progeny. The tick immune system, as well as the immune system of other invertebrates, is more rudimentary than the immune system of vertebrates, presenting only innate immune responses. Although simpler, the large number of tick species evidences the efficiency of their immune system. The factors of their immune system act in each tick organ that interacts with pathogens; therefore, these factors are potential targets for the development of new strategies for the control of ticks and tick-borne diseases. The objective of this review is to present the prevailing knowledge on the tick immune system and to discuss the challenges of studying tick immunity, especially regarding the gaps and interconnections. To this end, we use a comparative approach of the tick immune system with the immune system of other invertebrates, focusing on various components of humoral and cellular immunity, such as signaling pathways, antimicrobial peptides, redox metabolism, complement-like molecules and regulated cell death. In addition, the role of tick microbiota in vector competence is also discussed.

Analysis of Tick Surface Decontamination Methods

Various microbial pathogens have been found in ticks such as Ixodes ricinus. However, most studies assessed tick microbiomes without prior decontamination of the tick surface, which may alter the results and mislead conclusions regarding the composition of the tick-borne microbiome. The aim of this study was to test four different decontamination methods, namely (i.) 70% ethanol, (ii.) DNA Away, (iii.) 5% sodium hypochlorite and (iv.) Reactive Skin Decontamination Lotion (RSDL), which have been previously reported for tick surface and animal or human skin decontamination. To test the efficiency of decontamination, we contaminated each tick with a defined mixture of Escherichia coli, Micrococcus luteus, Pseudomonas fluorescens, dog saliva and human sweat. No contamination was used as a negative control, and for a positive control, a no decontamination strategy was carried out. After nucleic acid extraction, the recovery rate of contaminants was determined for RNA and DNA samples by qPCR and tick-borne microbiome analyses by bacterial 16S rRNA and 16S rRNA gene amplicon sequencing. Ticks treated with 5% sodium hypochlorite revealed the lowest number of contaminants followed by DNA Away, RSDL and 70% ethanol. Moreover, tick microbiomes after 5% sodium hypochlorite decontamination clustered with negative controls. Therefore, the efficiency of decontamination was optimal with 5% sodium hypochlorite and is recommended for upcoming studies to address the unbiased detection of tick-borne pathogens.

Intermolecular autocatalytic activation of serine protease zymogen factor C through an active transition state responding to lipopolysaccharide

Horseshoe crab hemolymph coagulation is believed to be triggered by the autocatalytic activation of serine protease zymogen factor C to the active form, α-factor C, belonging to the trypsin family, through an active transition state of factor C responding to bacterial lipopolysaccharide (LPS), designated factor C*. However, the existence of factor C* is only speculative, and its proteolytic activity has not been validated. In addition, it remains unclear whether the proteolytic cleavage of the Phe⁷³⁷-Ile⁷³⁸ bond (Phe⁷³⁷ site) of factor C required for the conversion to α-factor C occurs intramolecularly or intermolecularly between the factor C molecules. Here we show that the Phe⁷³⁷ site of a catalytic Ser-deficient mutant of factor C is LPS-dependently hydrolyzed by a Phe⁷³⁷ site-uncleavable mutant, clearly indicating the existence of the active transition state of factor C without cleavage of the Phe⁷³⁷ site. Moreover, we found the following facts using several mutants of factor C: the autocatalytic cleavage of factor C occurs intermolecularly between factor C* molecules on the LPS surface; factor C* does not exhibit intrinsic chymotryptic activity against the Phe⁷³⁷ site, but it may recognize a three-dimensional structure around the cleavage site; and LPS is required not only to complete the substrate-binding site and oxyanion hole of factor C* by interacting with the N-terminal region but also to allow the Phe⁷³⁷ site to be cleaved by inducing a conformational change around the Phe⁷³⁷ site or by acting as a scaffold to induce specific protein-protein interactions between factor C* molecules.

IrC2/Bf - A yeast and Borrelia responsive component of the complement system from the hard tick Ixodes ricinus

Ticks possess components of a primordial complement system that presumably play a role in the interaction of the tick immune system with tick-borne pathogens and affect their transmission. Here we characterized a novel complement component, tagged as IrC2/Bf, from the hard tick Ixodes ricinus, the principal vector of Lyme disease in Europe. IrC2/Bf is a multi-domain molecule composed of 5-7 CCP modules, varied by alternative splicing, followed by a von Willebrand factor A domain and a C-terminal trypsin-like domain. The primary structure and molecular architecture of IrC2/Bf displays the closest homology to the C3-complement component convertases described in horseshoe crabs. The irc2/bf gene is mainly expressed in the tick fat body associated with the trachea and, as determined by western blotting, the protein is present in low amounts in tick hemolymph. Expression of irc2/bf mRNA was significantly up-regulated in response to the intra-hemocoelic injection of the yeast Candida albicans and all tested Borrelia sp. strains (B. burgdorferi NE5264, B. burgdorferi CB26, B. garinii MSLB, B. afzelii CB43), but was not affected by injection of model Gram-negative and Gram-positive bacteria or the aseptic injection control. In-line with these results, RNAi-mediated silencing of irc2/bf inhibited phagocytosis of B. afzelii and C. albicans but not the other bacteria. Tissue expression profiles, specific responses to microbial challenges, and patterns of phagocytic phenotypes upon RNAi silencing observed for IrC2/Bf match well with the previously reported characteristics of I. ricinus C3-related molecule 1 (IrC3-1). Therefore we presume that IrC2/Bf functions as a convertase in the same complement activation pathway protecting ticks against yeast and Borrelia infection.

Characterization of Ixodes ricinus Fibrinogen-Related Proteins (Ixoderins) Discloses Their Function in the Tick Innate Immunity

Ticks are important vectors of serious human and animal disease-causing organisms, but their innate immunity can fight invading pathogens and may have the ability to reduce or block transmission to mammalian hosts. Lectins, sugar-binding proteins, can distinguish between self and non-self-oligosaccharide motifs on pathogen surfaces. Although tick hemolymph possesses strong lectin activity, and several lectins have already been isolated and characterized, little is known about the implementation of these molecules in tick immunity. Here, we have described and functionally characterized fibrinogen-related protein (FReP) lectins in Ixodes ticks. We have shown that the FReP family contains at least 27 genes (ixoderins, ixo) that could, based on phylogenetic and expression analyses, be divided into three groups with differing degrees of expansion. By using RNA interference-mediated gene silencing (RNAi) we demonstrated that IXO-A was the main lectin in tick hemolymph. Further, we found that ixoderins were important for phagocytosis of Gram-negative bacteria and yeasts by tick hemocytes and that their expression was upregulated upon injection of microbes, wounding, or after blood feeding. However, although the tick hemocytes could swiftly phagocytose Borrelia afzelii spirochetes, their transmission and burst of infection in mice was not altered. Our results demonstrate that tick ixoderins are crucial immune proteins that work as opsonins in the tick hemolymph, targeting microbes for phagocytosis or lysis.

Tick Thioester-Containing Proteins and Phagocytosis Do Not Affect Transmission of Borrelia afzelii from the Competent Vector Ixodes ricinus

The present concept of the transmission of Lyme disease from Borrelia-infected Ixodes sp. ticks to the naïve host assumes that a low number of spirochetes that manage to penetrate the midgut epithelium migrate through the hemocoel to the salivary glands and subsequently infect the host with the aid of immunomodulatory compounds present in tick saliva. Therefore, humoral and/or cellular immune reactions within the tick hemocoel may play an important role in tick competence to act as a vector for borreliosis. To test this hypothesis we have examined complement-like reactions in the hemolymph of the hard tick Ixodes ricinus against Borrelia afzelii (the most common vector and causative agent of Lyme disease in Europe). We demonstrate that I. ricinus hemolymph does not exhibit borreliacidal effects comparable to complement-mediated lysis of bovine sera. However, after injection of B. afzelii into the tick hemocoel, the spirochetes were efficiently phagocytosed by tick hemocytes and this cellular defense was completely eliminated by pre-injection of latex beads. As tick thioester-containing proteins (T-TEPs) are components of the tick complement system, we performed RNAi-mediated silencing of all nine genes encoding individual T-TEPs followed by in vitro phagocytosis assays. Silencing of two molecules related to the C3 complement component (IrC3-2 and IrC3-3) significantly suppressed phagocytosis of B. afzelii, while knockdown of IrTep (insect type TEP) led to its stimulation. However, RNAi-mediated silencing of T-TEPs or elimination of phagocytosis by injection of latex beads in B. afzelii-infected I. ricinus nymphs had no obvious impact on the transmission of spirochetes to naïve mice, as determined by B. afzelii infection of murine tissues following tick infestation. This result supports the concept that Borrelia spirochetes are capable of avoiding complement-related reactions within the hemocoel of ticks competent to transmit Lyme disease.

Proteases of haematophagous arthropod vectors are involved in blood-feeding, yolk formation and immunity - a review

Ticks, triatomines, mosquitoes and sand flies comprise a large number of haematophagous arthropods considered vectors of human infectious diseases. While consuming blood to obtain the nutrients necessary to carry on life functions, these insects can transmit pathogenic microorganisms to the vertebrate host. Among the molecules related to the blood-feeding habit, proteases play an essential role. In this review, we provide a panorama of proteases from arthropod vectors involved in haematophagy, in digestion, in egg development and in immunity. As these molecules act in central biological processes, proteases from haematophagous vectors of infectious diseases may influence vector competence to transmit pathogens to their prey, and thus could be valuable targets for vectorial control.

Transcription Factors as a Target for Vaccination Against Ticks and Mites

Ticks and mites are well-known ectoparasites as potential vectors for numerous bacteria, viruses, and parasites. Many being blood feeders add to physiological deterioration, morbidity, and mortality of their vertebrate hosts. To control them, transcription factors have been identified and studied in their role to sustain such arthropod pests. This paper summarizes some of the work done on those factors involved during blood feeding, reproduction, or when interacting with their pathogens and symbiont populations. Any transcription factor supporting the equilibrium developed by the ticks/mites could become a potential target for new control methods to prevent some of their key physiological functions.

Substrate prediction of Ixodes ricinus salivary lipocalins differentially expressed during Borrelia afzelii infection

Evolution has provided ticks with an arsenal of bioactive saliva molecules that counteract host defense mechanisms. This salivary pharmacopoeia enables blood-feeding while enabling pathogen transmission. High-throughput sequencing of tick salivary glands has thus become a major focus, revealing large expansion within protein encoding gene families. Among these are lipocalins, ubiquitous barrel-shaped proteins that sequester small, typically hydrophobic molecules. This study was initiated by mining the Ixodes ricinus salivary gland transcriptome for specific, uncharacterized lipocalins: three were identified. Differential expression of these I. ricinus lipocalins during feeding at distinct developmental stages and in response to Borrelia afzelii infection suggests a role in transmission of this Lyme disease spirochete. A phylogenetic analysis using 803 sequences places the three I. ricinus lipocalins with tick lipocalins that sequester monoamines, leukotrienes and fatty acids. Both structural analysis and biophysical simulations generated robust predictions showing these I. ricinus lipocalins have the potential to bind monoamines similar to other tick species previously reported. The multidisciplinary approach employed in this study characterized unique lipocalins that play a role in tick blood-feeding and transmission of the most important tick-borne pathogen in North America and Eurasia.

Acquisition of exogenous haem is essential for tick reproduction

Haem and iron homeostasis in most eukaryotic cells is based on a balanced flux between haem biosynthesis and haem oxygenase-mediated degradation. Unlike most eukaryotes, ticks possess an incomplete haem biosynthetic pathway and, together with other (non-haematophagous) mites, lack a gene encoding haem oxygenase. We demonstrated, by membrane feeding, that ticks do not acquire bioavailable iron from haemoglobin-derived haem. However, ticks require dietary haemoglobin as an exogenous source of haem since, feeding with haemoglobin-depleted serum led to aborted embryogenesis. Supplementation of serum with haemoglobin fully restored egg fertility. Surprisingly, haemoglobin could be completely substituted by serum proteins for the provision of amino-acids in vitellogenesis. Acquired haem is distributed by haemolymph carrier protein(s) and sequestered by vitellins in the developing oocytes. This work extends, substantially, current knowledge of haem auxotrophy in ticks and underscores the importance of haem and iron metabolism as rational targets for anti-tick interventions.

DOI:http://dx.doi.org/10.7554/eLife.12318.001

Ticks are small blood-feeding parasites that transmit a range of diseases through their bites, including Lyme disease and encephalitis in humans. Like other blood-feeders, ticks acquire essential nutrients from their host in order to develop and reproduce.

Iron and haem (the iron-containing part of haemoglobin) are essential for the metabolism of every breathing animal on Earth. Most organisms obtain iron by degrading haem and, reciprocally, most of the iron in cells is used to make haem. However, an initial search of existing genome databases revealed that ticks lack the genes required to make the proteins that make and degrade haem.

Perner et al. wanted to find out if ticks can steal haem from the host and use it for their own development. To achieve this, Perner et al. exploited a method of tick membrane feeding that simulates natural feeding on a host by using a silicone imitation of a skin and cow smell extracts (“l´odeur de vache”). Ticks were fed either a haemoglobin-rich (whole blood) or a haemoglobin-poor (serum) diet. This experiment revealed that ticks can develop normally without haemoglobin, but female ticks fed a haemoglobin-poor diet lay sterile eggs out of which no offspring can hatch.

Further investigation showed that haemoglobin is vitally important as a source of haem but not as a source of the amino acids needed to produce the vitellin proteins that nourish embryos. As ticks are not armed with the ability to degrade haem, they do not acquire iron from the host haem but rather from a serum transferrin, a major iron transporter protein found in mammalian blood. Further experiments revealed that ticks have evolved proteins that can transport and store haem and so make the obtained haem available across the whole tick body.

Overall, Perner et al.’s findings suggest that targeting the mechanisms by which ticks metabolise haem and iron could lead to the design of new “anti-tick” strategies.

DOI:http://dx.doi.org/10.7554/eLife.12318.002

Tick iron and heme metabolism - New target for an anti-tick intervention

Ticks are blood-feeding parasites and vectors of serious human and animal diseases. Ixodes ricinus is a common tick in Europe, transmitting tick-borne encephalitis, Lyme borreliosis, anaplasmosis, or babesiosis. Immunization of hosts with recombinant tick proteins has, in theory, the potential to interfere with tick feeding and block transmission of pathogens from the tick to the host. However, the efficacy of tick antigens has, to date, not been fully sufficient to achieve this. We have focused on 11 in silico identified genes encoding proteins potentially involved in tick iron and heme metabolism. Quantitative real-time PCR (qRT-PCR) expression profiling was carried out to preferentially target proteins that are up-regulated during the blood meal. RNA interference (RNAi) was then used to score the relative importance of these genes in tick physiology. Finally, we performed vaccination screens to test the suitability of these proteins as vaccine candidates. These newly identified tick antigens have the potential to improve the available anti-tick vaccines.

Multienzyme degradation of host serum albumin in ticks

Host blood proteins, represented mainly by hemoglobin and serum albumin, serve as the ultimate source of amino acids needed for de novo protein synthesis during tick development and reproduction. While uptake and processing of hemoglobin by tick gut cells have been studied in detail, molecular mechanisms of host serum albumin degradation remain unknown. In this work, we have used artificial membrane feeding of Ixodes ricinus females on a hemoglobin-free diet in order to characterize the proteolytic machinery involved in albuminolysis. Morphological comparisons of ticks fed on whole blood (BF) and serum (SF) at microscopic and ultrastructural levels showed that albumin and hemoglobin have different trafficking routes in tick gut cells. Analysis in vitro with selective inhibitors demonstrated that albumin is degraded at an acidic pH by a network of cysteine and aspartic peptidases with predominant involvement of cysteine cathepsins having endo- and exopeptidase activities. The cleavage map of albumin and the roles of individual peptidases in albumin degradation were determined. These results indicate that the albuminolytic pathway is controlled by the same proteolytic system that is responsible for hemoglobinolysis. This was further supported by the overall similarity of gut peptidase profiles in SF and BF ticks at the transcriptional and enzymatic activity levels. In conclusion, our work provides evidence that although hemoglobin and albumin are transported differentially during heterophagy they are digested by a common multienzyme proteolytic network. This central digestive system, critical for successful blood feeding in tick females, thus represents a valuable target for novel anti-tick interventions.

Deep Sequencing Analysis of the Ixodes ricinus Haemocytome

BACKGROUND

Ixodes ricinus is the main tick vector of the microbes that cause Lyme disease and tick-borne encephalitis in Europe. Pathogens transmitted by ticks have to overcome innate immunity barriers present in tick tissues, including midgut, salivary glands epithelia and the hemocoel. Molecularly, invertebrate immunity is initiated when pathogen recognition molecules trigger serum or cellular signalling cascades leading to the production of antimicrobials, pathogen opsonization and phagocytosis. We presently aimed at identifying hemocyte transcripts from semi-engorged female I. ricinus ticks by mass sequencing a hemocyte cDNA library and annotating immune-related transcripts based on their hemocyte abundance as well as their ubiquitous distribution.

METHODOLOGY/PRINCIPAL FINDINGS

De novo assembly of 926,596 pyrosequence reads plus 49,328,982 Illumina reads (148 nt length) from a hemocyte library, together with over 189 million Illumina reads from salivary gland and midgut libraries, generated 15,716 extracted coding sequences (CDS); these are displayed in an annotated hyperlinked spreadsheet format. Read mapping allowed the identification and annotation of tissue-enriched transcripts. A total of 327 transcripts were found significantly over expressed in the hemocyte libraries, including those coding for scavenger receptors, antimicrobial peptides, pathogen recognition proteins, proteases and protease inhibitors. Vitellogenin and lipid metabolism transcription enrichment suggests fat body components. We additionally annotated ubiquitously distributed transcripts associated with immune function, including immune-associated signal transduction proteins and transcription factors, including the STAT transcription factor.

CONCLUSIONS/SIGNIFICANCE

This is the first systems biology approach to describe the genes expressed in the haemocytes of this neglected disease vector. A total of 2,860 coding sequences were deposited to GenBank, increasing to 27,547 the number so far deposited by our previous transcriptome studies that serves as a discovery platform for studies with I. ricinus biochemistry and physiology.

Evolution of the complement system in protostomes revealed by de novo transcriptome analysis of six species of Arthropoda

To elucidate the evolutionary history of the complement system in Arthropoda, de novo transcriptome analysis was performed with six species among the Chelicerata, Myriapoda, and Crustacea, and complement genes were identified based on their characteristic domain structures. Complement C3 and factor B (FB) were identified from a sea spider, a jumping spider, and a centipede, but not from a sea firefly or two millipede species. No additional complement components identifiable by their characteristic domain structures were found from any of these six species. These results together with genome sequence information for several species of the Hexapoda suggest that the common ancestor of the Arthropoda possessed a simple complement system comprising C3 and FB, and thus resembled the alternative pathway of the mammalian complement system. It was lost at least twice independently during the evolution of Arthropoda in the millipede lineage and in the common ancestor of Crustacea and Hexapoda.

Thioester-containing proteins of the tick Ixodes ricinus: gene expression, response to microbial challenge and their role in phagocytosis of the yeast Candida albicans

The ability of ticks to act as vectors for a wide range of serious human and animal infectious diseases is apparently linked to the insufficiency of the tick immune system to effectively eliminate pathogens they transmit. At the tick-pathogen interface, an important role is presumably played by components of an ancient complement system that includes a repertoire of thioester-containing proteins (TEPs), which in Ixodes sp. comprises three α2-macroglobulins (A2M), three C3 complement component-related molecules (C3), two macroglobulin complement-related (Mcr) and one insect-type TEPs (Tep). In order to assess the function of TEPs in tick immunity, a quantitative real-time PCR expression analysis of tick TEPs was performed at various developmental stages of Ixodes ricinus, and in tissues dissected from adult females. Expression of TEP genes was mostly tissue specific; IrA2M1, IrC3-1, IrC3-3 were found to be expressed in cells of tick fat body adjacent to the tracheal trunks, IrA2M2 in hemocytes, IrTep in ovaries, IrMcr1 in salivary glands and only IrA2M3, IrC3-2 and IrMcr2 mRNAs were present in multiple organs. Expression of tick TEPs was further examined in response to injection of model microbes representing Gram-negative, Gram-positive bacteria and yeast. The greatest expression induction was observed for IrA2M1 and IrC3-1 after challenge with the yeast Candida albicans. Phagocytosis of the yeast was strongly dependent on an active thioester bond and the subsequent silencing of individual tick TEPs by RNA interference demonstrated the involvement of IrC3-1 and IrMcr2. This result suggests the existence of a distinct complement-like pathway, different from that leading to phagocytosis of Gram-negative bacteria. Understanding of the tick immune response against model microbes should provide new concepts for investigating interactions between ticks and relevant tick-borne pathogens.