An arthropod defensin expressed by the hemocytes of the American dog tick, Dermacentor variabilis (Acari: Ixodidae)

Myticofensin, a novel antimicrobial peptide family identified from Mytilus coruscus

In this study, seven transcripts representing a novel antimicrobial peptide (AMP) family with structural features similar to those of arthropod defensins were identified from Mytilus coruscus. These novel defensins from the Mytilus AMP family were named myticofensins. To explore the possible immune-related functions of these myticofensins, we examined their expression profiles in different tissues and larval stages, as well as in three immune-related tissues under the threat of different microbes. Our data revealed that the seven myticofensins had relatively high expression levels in immune-related tissues. Most myticofensins were undetectable, or had low expression levels, in different larval mussel stages. Additionally, in vivo microbial challenges significantly increased the expression levels of myticofensins in M. coruscus hemocytes, gills, and digestive glands, showing different immune response patterns under challenges from different microbes. Our data indicates that different myticofensins may have different immune functions in different tissues. Furthermore, peptide sequences corresponding to the beta-hairpin, alpha-helix, and N-terminal loop of myticofensin were synthesized and the antimicrobial activities of these peptide fragments were tested. Our data confirms the diversity of defensins in Mytilus and reports the complex regulation of these defensins in the mussel immune response to different microbes in immune-related tissues. The immune system of Mytilus has been studied for years as they are a species with strong environmental adaptations. Our data can be regarded as a step forward in the study of the adaptation of Mytilus spp. to an evolving microbial world.

Defensins as a promising class of tick antimicrobial peptides: a scoping review

Background

Ticks are hematophagous parasites that transmit an extensive range of pathogens to their vertebrate hosts. Ticks can destroy invading microorganisms or alleviate infection via their rudimentary but orchestrated innate immune system. Antimicrobial peptides (AMPs) are important components of tick innate immunity. Among these humoral effector molecules, defensins are well-studied and widely identified in various species of Ixodidae (hard ticks) and Argasidae (soft ticks). This review was aimed at presenting the characterization of tick defensins from structure-based taxonomic status to antimicrobial function.

Main text

All published papers written in English from 2001 to May 2022 were searched through PubMed and Web of Science databases with the combination of relevant terms on tick defensins. Reports on identification and characterization of tick defensins were included. Of the 329 entries retrieved, 57 articles were finally eligible for our scoping review.

Tick defensins mainly belong to the antibacterial ancient invertebrate-type defensins of the cis-defensins superfamily. They are generally small, cationic, and amphipathic, with six cysteine residues forming three intra-molecular disulfide bonds. Tick defensins primarily target membranes of a variety of pathogens, including Gram-positive and Gram-negative bacteria, fungi, viruses, and protozoa. Since tick defensins have a high degree of variability, we summarize their common biological properties and enumerate representative peptides. Along with the various and potent antimicrobial activities, the role of tick defensins in determining vector competence is discussed.

Conclusions

Due to their broad-spectrum antimicrobial activities, tick defensins are considered novel candidates or targets for controlling infectious diseases.

Graphical Abstract

A novel defensin-like peptide contributing to antimicrobial and antioxidant capacity of the tick Dermacentor silvarum (Acari: Ixodidae)

Defensins are the most diverse groups of antimicrobial peptides in invertebrate animals. In ticks, defensins show great potential as targets for tick control, and display future prospect for therapeutic drug development. In the present study, a novel defensin-like gene (Ds-defensin) contributing to the antimicrobial and antioxidant capacity of the tick Dermacentor silvarum was characterized. The full-length of the Ds-defensin gene was 382 bp, which displayed tissue-specific expression and was highly abundant in the salivary glands and carcasses of the adults. It encodes a 71-amino acid defensin-like protein, and the protein precursor is characterized by a 22-amino acid signal peptide and a 34-amino acid mature peptide. The peptide displayed potent activity against most of the tested gram-positive bacteria, including Staphylococcus aureus, S. carnosus and Nocardia asteroides, and one tested gram-negative bacterium, Psychrobacter faecalis. Scanning electron microscopy revealed that the cell wall and surface of treated bacteria became rough and gradually formed pores after a 30-min exposure to the Ds-defensin peptide. Additionally, the peptide also showed significant antioxidant capacity. The above results implied that the defensin-like peptide may play an important role in tick defense and the interaction with microorganisms.

Antibacterial and antifungal activity of defensins from the Australian paralysis tick, Ixodes holocyclus

Tick innate immunity involves humoral and cellular responses. Among the humoral effector molecules in ticks are the defensins which are a family of small peptides with a conserved γ-core motif that is crucial for their antimicrobial activity. Defensin families have been identified in several hard and soft tick species. However, little is known about the presence and antimicrobial activity of defensins from the Australian paralysis tick Ixodes holocyclus. In this study the I. holocyclus transcriptome was searched for the presence of defensins. Unique and non-redundant defensin sequences were identified and designated as holosins 1 - 5. The antimicrobial activity of holosins 2 and 3 and of the predicted γ-cores of holosins 1-4 (HoloTickCores 1-4), was assessed using Gram-negative and Gram-positive bacteria as well as the fungus Fusarium graminearum and the yeast Candida albicans. All holosins had molecular features that are conserved in other tick defensins. Furthermore holosins 2 and 3 were very active against the Gram-positive bacteria Staphylococcus aureus and Listeria grayi. Holosins 2 and 3 were also active against F. graminearum and C. albicans and 5 μM of peptide abrogate the growth of these microorganisms. The activity of the synthetic γ-cores was lower than that of the mature defensins apart from HoloTickCore 2 which had activity comparable to mature holosin 2 against the Gram-negative bacterium Escherichia coli. This study reveals the presence of a multigene defensin family in I. holocyclus with wide antimicrobial activity.

Functional characterization of two defensins, HlDFS1 and HlDFS2, from the hard tick Haemaphysalis longicornis

Background

Ticks are second to mosquitoes as vectors of human arthropod-borne diseases. Ticks rely heavily on antimicrobial peptides (AMPs) to defend against microbes and defensins are major components of innate immunity in ticks.

Results

Two novel defensin genes, named HlDFS1 and HlDFS2, were identified from a cDNA library of the hard tick Haemaphysalis longicornis collected in southeast China. The peptides encoded by both genes shares typical features of type-2 arthropod defensin superfamily. The expressions of both genes increased in ticks during blood-feeding. The synthetic minimum functional peptides HlDFS1 and HlDFS2 showed broad spectrum antimicrobial activity against various Gram-positive and Gram-negative bacteria. Moreover, HlDFS1 and HlDFS2 exhibit bactericidal activity to some drug resistant bacteria. HlDFS1, but not HlDFS2, showed inhibitory activity against fungus Candida albicans. HlDFS1 and HlDFS2 had no significant hemolysis effect on human erythrocytes at low concentrations and did not impair mammalian cell survival. Finally, HlDFS1 and HlDFS2 significantly protected mice against lethal infection by Staphylococcus aureus and Micrococcus luteus.

Conclusions

HlDFS1 and HlDFS2 are two novel functional defensins from the hard tick Haemaphysalis longicornis. They showed bactericidal activity against various Gram-positive and Gram-negative bacteria and significantly protect mice against lethal bacterial infection. Thus, HlDFS1 and HlDFS2 can be introduced to the medical field as new drug candidates with antibacterial activity.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2397-9) contains supplementary material, which is available to authorized users.

Tick Humoral Responses: Marching to the Beat of a Different Drummer

Ticks transmit a variety of human pathogens, including Borrelia burgdorferi, the etiological agent of Lyme disease. Multiple pathogens that are transmitted simultaneously, termed “coinfections,” are of increasing importance and can affect disease outcome in a host. Arthropod immunity is central to pathogen acquisition and transmission by the tick. Pattern recognition receptors recognize pathogen-associated molecular patterns and induce humoral responses through the Toll and Immune Deficiency (IMD) pathways. Comparative analyses between insects and ticks reveal that while the Toll pathway is conserved, the IMD network exhibits a high degree of variability. This indicates that major differences in humoral immunity exist between insects and ticks. While many variables can affect immunity, one of the major forces that shape immune outcomes is the microbiota. In light of this, we discuss how the presence of commensal bacteria, symbionts and/or coinfections can lead to altered immune responses in the tick that impact pathogen persistence and subsequent transmission. By investigating non-insect arthropod immunity, we will not only better comprehend tick biology, but also unravel the intricate effects that pathogen coinfections have on vector competence and tick-borne disease transmission.

Convergent evolution of defensin sequence, structure and function

Defensins are a well-characterised group of small, disulphide-rich, cationic peptides that are produced by essentially all eukaryotes and are highly diverse in their sequences and structures. Most display broad range antimicrobial activity at low micromolar concentrations, whereas others have other diverse roles, including cell signalling (e.g. immune cell recruitment, self/non-self-recognition), ion channel perturbation, toxic functions, and enzyme inhibition. The defensins consist of two superfamilies, each derived from an independent evolutionary origin, which have subsequently undergone extensive divergent evolution in their sequence, structure and function. Referred to as the cis- and trans-defensin superfamilies, they are classified based on their secondary structure orientation, cysteine motifs and disulphide bond connectivities, tertiary structure similarities and precursor gene sequence. The utility of displaying loops on a stable, compact, disulphide-rich core has been exploited by evolution on multiple occasions. The defensin superfamilies represent a case where the ensuing convergent evolution of sequence, structure and function has been particularly extreme. Here, we discuss the extent, causes and significance of these convergent features, drawing examples from across the eukaryotes.

Understanding the biology and control of the poultry red mite Dermanyssus gallinae: a review

Dermanyssus gallinae, the poultry red mite (PRM), is a blood-feeding ectoparasite capable of causing pathology in birds, amongst other animals. It is an increasingly important pathogen in egg layers and is responsible for substantial economic losses to the poultry industry worldwide. Even though PRM poses a serious problem, very little is known about the basic biology of the mite. Here we review the current body of literature describing red mite biology and discuss how this has been, or could be, used to develop methods to control PRM infestations. We focus primarily on the PRM digestive system, salivary glands, nervous system and exoskeleton and also explore areas of PRM biology which have to date received little or no study but have the potential to offer new control targets.

Cloning and characterization of a male-specific defensin-like antimicrobial peptide from the tick Haemaphysalis longicornis

A novel gene was identified from a cDNA library of the male accessory glands of Haemaphysalis longicornis. The full-length cDNA of the gene was 349bp, encoding a 79-amino acid defensin-like protein and therefore the protein was named HlMS-defensin. Reverse transcriptase-PCR results suggested that this gene was expressed exclusively in male ticks. The tissue expression pattern and the mRNA levels of HlMS-defensin during blood feeding were determined using real-time PCR. HlMS-defensin was expressed predominately in the male accessory gland and was up-regulated during blood feeding. The antimicrobial activity of a synthetic peptide based on the predicted mature portion of HlMS-defensin was examined against a variety of Gram-positive and Gram-negative bacteria and fungi. It appears that ticks use the antimicrobial peptide to protect their reproductive tracts from microbial infections. The protective role of HlMS-defensin during mating was also discussed.

Fibrinogen-related proteins in ixodid ticks

BACKGROUND

Fibrinogen-related proteins with lectin activity are believed to be part of the tick innate immune system. Several fibrinogen-related proteins have been described and characterised mainly on the basis of their cDNA sequences while direct biochemical evidence is missing. One of them, the haemolymph lectin Dorin M from the tick Ornithodoros moubata was isolated and characterised in more depth.

RESULTS

Several fibrinogen-related proteins were detected in the haemolymph of ixodid ticks Dermacentor marginatus, Rhipicephalus appendiculatus, R. pulchellus, and R. sanguineus. These proteins were recognised by sera directed against the tick lectin Dorin M and the haemagglutination activity of the ticks R. appendiculatus and D. marginatus. Cross-reactivity of the identified proteins with antibodies against the fibrinogen domain of the human ficolin was also shown. The carbohydrate-binding ability of tick haemolymph was confirmed by haemagglutination activity assays, and this activity was shown to be inhibited by neuraminic acid and sialylated glycoproteins as well as by N-acetylated hexosamines. The fibrinogen-related proteins were shown to be glycosylated and they were localised in salivary glands, midguts, and haemocytes of D. marginatus. Hemelipoglycoprotein was also recognised by sera directed against the fibrinogen-related proteins in all three Rhipicephalus species as well as in D. marginatus. However, this protein does not contain the fibrinogen domain and thus, the binding possibly results from the structure similarity between hemelipoglycoprotein and the fibrinogen domain.

CONCLUSIONS

The presence of fibrinogen-related proteins was shown in the haemolymph of four tick species in high abundance. Reactivity of antibodies directed against ficolin or fibrinogen-related proteins with proteins which do not contain the fibrinogen domain points out the importance of sequence analysis of the identified proteins in further studies. Previously observed expression of fibrinogen-related proteins in haemocytes together with the results of this study suggest involvement of fibrinogen-related proteins in tick immunity processes. Thus, they have potential as targets for anti-tick vaccines and as antimicrobial proteins in pharmacology. Research on fibrinogen-related proteins could reveal further details of tick innate immunity processes.

Functional characterization of two defensin isoforms of the hard tick Ixodes ricinus

BACKGROUND

The immune system of ticks is stimulated to produce many pharmacologically active molecules during feeding and especially during pathogen invasion. The family of cationic peptides - defensins - represents a specific group of antimicrobial compounds with six conserved cysteine residues in a molecule.

RESULTS

Two isoforms of the defensin gene (def1 and def2) were identified in the European tick Ixodes ricinus. Expression of both genes was induced in different tick organs by a blood feeding or pathogen injection. We have tested the ability of synthetic peptides def1 and def2 to inhibit the growth or directly kill several pathogens. The antimicrobial activities (expressed as minimal inhibition concentration and minimal bactericidal concentration values) against Gram positive bacteria were confirmed, while Gram negative bacteria, yeast, Tick Borne Encephalitis and West Nile Viruses were shown to be insensitive. In addition to antimicrobial activities, the hemolysis effect of def1 and def2 on human erythrocytes was also established.

CONCLUSIONS

Although there is nothing known about the realistic concentration of defensins in I. ricinus tick body, these results suggest that defensins play an important role in defence against different pathogens. Moreover this is a first report of a one amino acid substitution in a defensins molecule and its impact on antimicrobial activity.

A Kunitz protease inhibitor from Dermacentor variabilis, a vector for spotted fever group rickettsiae, limits Rickettsia montanensis invasion

A defining facet of tick-Rickettsia symbioses is the molecular strategy employed by each partner to ensure its own survival. Ticks must control rickettsial colonization to avoid immediate death. In the current study, we show that rickettsial abundance in the tick midgut increases once the expression of a Kunitz-type serine protease inhibitor from the American dog tick (Dermacentor variabilis) (DvKPI) is suppressed by small interfering RNA (siRNA). A series of in vitro invasion assays suggested that DvKPI limits rickettsial colonization during host cell entry. Interestingly, we observed that DvKPI associates with rickettsiae in vitro as well as in the tick midgut. Collectively, our data demonstrate that DvKPI limits host cell invasion by Rickettsia montanensis, possibly through an association with the bacterium.

Pyrosequencing and characterization of immune response genes from the American dog tick, Dermacentor variabilis (L.)

Ticks continue to be a threat to animal and human health, and new and novel control strategies are needed for ticks and tick-borne pathogens. The characterization of the tick-pathogen interface and the tick immune response to microbial infections is fundamental toward the formulation of new control strategies for ticks and the pathogens they transmit. Our overall hypothesis for this research is that the tick immune system manages the maintenance of pathogens. Therefore, discovery of tick immune response genes may provide targets for novel control strategies directed toward reducing vector competency and pathogen transmission. In these studies, 454 pyrosequencing, a high-throughput genomic sequencing method was used to discover tick genes expressed in response to bacterial and fungal infections. Expressed sequence tags (ESTs) were analysed from Dermacentor variabilis ticks that had been injected with bacteria (Escherichia coli, Bacillus subtilis, Micrococcus luteus) or fungi (Saccharomyces cerevisiae and Candida albicans) and ticks that were naturally infected with the intracellular bacterium, Anaplasma marginale. By this approach, ESTs were assembled into 5995 contigs. Contigs fell into the five main functional categories of metabolism, genetic information processing, environmental information processing, cellular processes and human diseases. We identified more than 30 genes that are likely to encode for proteins involved in tick immune function. We further analysed by reverse transcriptase PCR (RT-PCR) the expression of 22 of these genes in each of our bacterial or fungal treatment groups and found that seven were up-regulated. Up-regulation of these seven genes was confirmed for bacterial, but not fungal treatment by quantitative PCR (qPCR). One of these products was novel, encoding a new tick defensin. Our results clearly demonstrate the complexities of the tick immune system and mark new directions for further study and characterization of proteins that modulate microbial infections in the American dog tick.

A novel defensin-like peptide from salivary glands of the hard tick, Haemaphysalis longicornis

A novel defensin-like antimicrobial peptide named longicornsin was isolated from the salivary glands of the hard tick, Haemaphysalis longicornis, using a 10-kDa cut-off Centriprep filter and reversed-phase high-performance liquid chromatography (RP-HPLC). Its amino acid sequence was determined as DFGCGQGMIFMCQRRCMRLYPGSTGFCRGFRCMCDTHIPLRPPFMVG by Edman degradation. The cDNA encoding longicornsin was cloned by cDNA library screening. The predicted protein from the cDNA sequence was composed of 78 amino acids including a mature longicornsin. It showed similarity with defensin-like peptides from other ticks by BLAST search. Different from most other tick defensin-like peptides, longicornsin had a C-terminal extension. Purified longicornsin exerted potent antimicrobial activities against bacteria and fungi. Interestingly, it even showed strong antimicrobial ability against drug-resistant microorganisms and Helicobacter pylori. The results of this study indicated that longicornsin is a potential candidate for novel antimicrobial drug design.

Purification and characterization of a novel salivary antimicrobial peptide from the tick, Ixodes scapularis

A novel antimicrobial peptide was isolated from the saliva of the Lyme disease tick vector, Ixodes scapularis, henceforth designated as ISAMP (I. scapularis Antimicrobial Peptide). ISAMP was purified using a sequential method including ultra filtration, gel filtration and reverse-phase high performance liquid chromatography. The purified peak had a molecular weight of 5.3kDa by MALDI/TOF-MS and its amino acid sequence, determined by Edman degradation was PDxGxPxxVKAGRxPxxSI. A BLASTP search revealed that the protein is a putative 5.3kDa secreted protein (AAM93656) from I. scapularis. The predicted protein is composed of 69 amino acids with no conserved domain motifs. Purified ISAMP was found to have antimicrobial activities against bacteria. Gene expression studies were carried out to observe ISAMP expression in different tick tissues. RT-PCR results indicated that the gene was expressed in hemocytes, fat body and salivary gland but virtually no expression was observed in the midgut. ISAMP is only similar to other Ixodid tick proteins, thus it is a member of a unique family.

New defensins from hard and soft ticks: similarities, differences, and phylogenetic analyses

Despite the importance of ticks as vectors of disease very little is known about their immune system. Antimicrobial peptides, including defensins (phylogenetically ancient antibacterial peptides) are major components of innate immunity in ticks that have been shown to provide protection against gram-negative and gram-positive bacteria, fungi, viruses and protozoan parasites. With the aim of studying the evolution of the genes involved in tick defense, we identified the preprodefensin genes from four Ornithodoros tick species (O. papillipes: isoforms A, B, and D; O. tartakovskyi and O. puertoricensis: isoforms A and B; O. rostratus: isoform A) and from two Dermacentor tick species (D. reticulatus and D. marginatus: one isoform) not previously described. Phylogenetic analyses revealed that Ornithodoros defensin isoforms (A, B, C, and D) form 4 separate clades, while hard tick defensins are divided into several branches based on particular tick species.

Identification and characterization of antimicrobial peptide, defensin, in the taiga tick, Ixodes persulcatus

Ixodes persulcatus is the primary vector for human tick-borne diseases in Japan. A cDNA library was constructed from whole body homogenates of fed nymphs of I. persulcatus. From this library, one cDNA encoding defensin-like antimicrobial peptide was identified. The amino-acid sequence showed high similarity to those of the defensins of other ticks and arthropods. I. persulcatus defensin mRNA transcripts were detected at all life cycle stages of fed ticks and found to be predominantly expressed in the midguts of adult female ticks, but not in the salivary glands, a finding corroborated by Western blotting analysis. To investigate the function of I. persulcatus defensin, we examined its antibacterial activity by evaluation of growth of several bacterial strains in the presence of the synthetic peptide. The defensin from I. persulcatus markedly inhibited the growth of Gram-positive bacteria including Staphylococcus aureus, Bacillus subtilis and Corynebacterium renale, but not Gram-negative bacteria except Escherichia coli O157. In conclusion, these results suggest that I. persulcatus defensin may be playing a significant role in the defence against microbes from bloodmeals.

The role of saliva in tick feeding

When attempting to feed on their hosts, ticks face the problem of host hemostasis (the vertebrate mechanisms that prevent blood loss), inflammation (that can produce itching or pain and thus initiate defensive behavior on their hosts) and adaptive immunity (by way of both cellular and humoral responses). Against these barriers, ticks evolved a complex and sophisticated pharmacological armamentarium, consisting of bioactive lipids and proteins, to assist blood feeding. Recent progress in transcriptome research has uncovered that hard ticks have hundreds of different proteins expressed in their salivary glands, the majority of which have no known function, and include many novel protein families (e.g., their primary structure is unique to ticks). This review will address the vertebrate mechanisms of these barriers as a guide to identify the possible targets of these large numbers of known salivary proteins with unknown function. We additionally provide a supplemental Table that catalogues over 3,500 putative salivary proteins from various tick species, which might assist the scientific community in the process of functional identification of these unique proteins. This supplemental file is accessble fromhttp://exon.niaid.nih.gov/transcriptome/tick_review/Sup-Table-1.xls.gz.

Silencing expression of the defensin, varisin, in male Dermacentor variabilis by RNA interference results in reduced Anaplasma marginale infections

Antimicrobial peptides, including defensins, are components of the innate immune system in ticks that have been shown to provide protection against both gram-negative and gram-positive bacteria. Varisin, one of the defensins identified in Dermacentor variabilis, was shown to be produced primarily in hemocytes but transcript levels were also expressed in midguts and other tick cells. In this research, we studied the role of varisin in the immunity of ticks to the gram-negative cattle pathogen, Anaplasma marginale. Expression of the varisin gene was silenced by RNA interference (RNAi) in which male ticks were injected with varisin dsRNA and then allowed to feed and acquire A. marginale infection on an experimentally-infected calf. Silencing expression of varisin in hemocytes, midguts and salivary glands was confirmed by real time RT-PCR. We expected that silencing of varisin would increase A. marginale infections in ticks, but the results demonstrated that bacterial numbers, as determined by an A. marginale msp4 quantitative PCR, were significantly reduced in the varisin-silenced ticks. Furthermore, colonies of A. marginale in ticks used for RNAi were morphologically abnormal from those seen in elution buffer injected control ticks. The colony shape was irregular and in some cases the A. marginale appeared to be free in the cytoplasm of midgut cells. Some ticks were found to be systemically infected with a microbe that may have been related to the silencing of varisin. This appears to be the first report of the silencing of expression of a defensin in ticks by RNAi that resulted in reduced A. marginale infections.

Using RNA interference to determine the role of varisin in the innate immune system of the hard tick Dermacentor variabilis (Acari: Ixodidae)

Defensins are an important component of the innate immune system of ticks. These small peptides are produced by various genera of ticks, and expressed in various tissues. In this study we used RNA interference to silence the expression of the defensin varisin produced by the hemocytes of the American dog tick, Dermacentor variabilis. Ticks were injected with double stranded varisin RNA prior to being placed on a rabbit. After feeding, the ticks were removed, bled, and the hemolymph plasma and hemocytes separated. Hemocytes were screened for the presence (or absence) of both varisin transcript and peptide. Varisin peptide was below detectable levels and the transcript showed a greater than 99% knockdown. The antimicrobial activity of the hemolymph plasma was reduced 2-4 fold compared to that of control injected ticks indicating varisin accounts for a large portion of the antimicrobial activity of the hemolymph.

An insight into the sialome of the blood-sucking bug Triatoma infestans, a vector of Chagas' disease

Triatoma infestans is a hemiptera, vector of Chagas' disease that feeds exclusively on vertebrate blood in all life stages. Hematophagous insects' salivary glands (SG) produce potent pharmacological compounds that counteract host hemostasis, including anticlotting, antiplatelet, and vasodilatory molecules. To obtain a further insight into the salivary biochemical and pharmacological complexity of this insect, a cDNA library from its SG was randomly sequenced. Also, salivary proteins were submitted to two-dimensional gel (2D-gel) electrophoresis followed by MS analysis. We present the analysis of a set of 1534 (SG) cDNA sequences, 645 of which coded for proteins of a putative secretory nature. Most salivary proteins described as lipocalins matched peptide sequences obtained from proteomic results.

Gene organization of a novel defensin of Ixodes ricinus: first annotation of an intron/exon structure in a hard tick defensin gene and first evidence of the occurrence of two isoforms of one member of the arthropod defensin family

Antimicrobial peptides (defensins) are effectors of the immune system. Herein, we describe a novel Ixodes ricinus defensin gene(s), analyse its structure and compare it with other known antimicrobial peptides from different tick species. For the first time, an intron/exon structure is discovered in a hard-tick defensin gene. The intron/exon genomic organization of the gene is similar to the organization in Ornithodoros moubata, but not to that of the intronless defensins of Dermacentor variabilis and Ixodes scapularis. The analysis of the deduced amino acid sequences of different recombinants from the I. ricinus cDNA library reveals the presence of two defensin isoforms with three amino acid substitutions. Whether or not these substitutions affect the biological properties of the peptides is currently unknown. The expression of the defensin gene is strongly induced in the tick midgut after infection with Borrelia burgdorferi.

Sequence characterization and expression patterns of two defensin-like antimicrobial peptides from the tick Haemaphysalis longicornis

Two cDNAs encoding defensin-like antimicrobial peptides were cloned and sequenced from the tick Haemaphysalis longicornis. The full-length cDNA of Hlgut-defensin (H. longicornis midgut defensin) is 333bp, encoding an expected protein with 73 amino acids. The full-length cDNA of Hlsal-defensin (H. longicornis salivary gland defensin) is 382bp, encoding an expected protein with 81 amino acids. The antibacterial activities of the synthetic peptides based on the Hlgut-defensin and Hlsal-defensin sequences were tested against a variety of Gram-positive and Gram-negative bacteria. Using real-time PCR, the tissue-specific expression of two defensin-like peptides were determined and it was also found that the gene transcripts of Hlgut-defensin and Hlsal-defensin were significantly induced by a lipopolysaccharide (LPS) injection.

Tissue and life-stage distribution of a defensin gene in the Lone Star tick, Amblyomma americanum

The transcript sequence of the Amblyomma americanum Linnaeus (Acari: Ixodidae) defensin, termed amercin (amn), was ascertained and a 219-bp amn coding region identified. The gene encodes a 72-amino acid prepropeptide with a putative 37-amino acid mature peptide. This gene shows little similarity to either of the defensins from Amblyomma hebraeum Koch, the only other Amblyomma species for which a defensin has been described. Sequence comparisons with other tick defensins reveal amn to be shorter (6 bp or 2 amino acids) than the Ixodes scapularis Linnaeus (Acari: Ixodidae) and Dermacentor variabilis (Say) (Acari: Ixodidae) defensin sequences. The amercin prepropeptide has 60.8% and 59.5% similarity with the I. scapularis and D. variabilis prepropeptides, respectively, whereas the mature amercin peptide has 73.7% and 71.1% similarity with the mature peptides of these ticks. Similarity with other tick defensins ranges from 42% to 71%. In A. americanum, defensin transcript was found in the midgut, fat body and salivary gland tissues, as well as in the haemocytes. Defensin transcript was also present in early-stage eggs (less than 48 h old), late-stage eggs (approximately 2 weeks old), larvae and nymphs of A. americanum and I. scapularis, both of which are vector-competent for Borrelia spirochetes.

New tick defensin isoform and antimicrobial gene expression in response to Rickettsia montanensis challenge

Recent studies aimed at elucidating the rickettsia-tick interaction have discovered that the spotted fever group rickettsia Rickettsia montanensis, a relative of R. rickettsii, the etiologic agent of Rocky Mountain spotted fever, induces differential gene expression patterns in the ovaries of the hard tick Dermacentor variabilis. Here we describe a new defensin isoform, defensin-2, and the expression patterns of genes for three antimicrobials, defensin-1 (vsnA1), defensin-2, and lysozyme, in the midguts and fat bodies of D. variabilis ticks that were challenged with R. montanensis. Bioinformatic and phylogenetic analyses of the primary structure of defensin-2 support its role as an antimicrobial. The tissue distributions of the three antimicrobials, especially the two D. variabilis defensin isoforms, are markedly different, illustrating the immunocompetence of the many tissues that R. montanensis presumably invades once acquired by the tick. Antimicrobial gene expression patterns in R. montanensis-challenged ticks suggest that antimicrobial genes play a role during the acquisition-invasion stages in the tick.

Babesiaparasites develop and are transmitted by the non-vector soft tickOrnithodoros moubata(Acari: Argasidae)

Ornithodoros moubataticks were fed on blood infected withBabesia equi. However, the parasites were quickly cleared as evidenced by the disappearance ofB. equi-specific ribosomal RNA from the ticks. We hypothesized that if theBabesiaparasite can escape midgut-associated barriers a non-vector tick can become infected withBabesia. To test this hypothesis,B. equiparasite-infected blood fromin vitroculture was injected into the haemocoel of ticks.B. equi-specific rRNA was surprisingly detected 45 days after injection even in the eggs.Babesia-free dogs were infested withO. moubataticks that were infected by inoculation withB. gibsoni-infected red blood cells. Parasitaemia and antibody production against Bg-TRAP ofB. gibsoniincreased gradually. These results indicate thatO. moubatamay be a useful vector model forBabesiaparasites and also a very important tool for studies on tick immunity againstBabesiaparasites and tick-Babesiainteractions.

Differential expression of two glutathione S-transferases identified from the American dog tick, Dermacentor variabilis

Reciprocal signalling and gene expression play a cardinal role during pathogen-host molecular interactions and are prerequisite to the maintenance of balanced homeostasis. Gene expression repertoire changes during rickettsial infection and glutathione-S-transferases (GSTs) were among the genes found up-regulated in Rickettsia-infected Dermacentor variabilis. GSTs are well known to play an important part in cellular stress responses in the host. We have cloned two full-length GSTs from D. variabilis (DvGST1 and DvGST2). Comparison of these two DvGST molecules with those of other species indicate that DvGST1 is related to the mammalian class theta and insect class delta GSTs, while DvGST2 does not seem to fall in the same family. Northern blotting analyses revealed differential expression patterns, where DvGST1 and DvGST2 transcripts are found in the tick gut, with DvGST2 transcripts also present in the ovaries. Both DvGST transcripts are up-regulated upon tick feeding. Challenge of fed adult ticks with Escherichia coli injection showed decreased transcript amounts compared with ticks injected with phosphate-buffered saline (sham) and naïve ticks.

A defensin-like gene expressed in the black-legged tick, Ixodes scapularis

The black-legged tick Ixodes scapularis Linnaeus (Acari: Ixodidae) is an important vector of microbial pathogens. Knowledge of the tick's innate immune response, particularly defensin and other antimicrobial peptides, is important for understanding how microbes survive in this tick. A defensin gene (slnA) from I. scapularis was obtained by reverse transcription-polymerase chain reaction (RT-PCR) using mRNA extracted from tissues of female ticks. RT-PCR indicated the gene was expressed in the midgut, haemocytes, and fat-body, although no evidence of a peptide was found. Sequencing a cloned cDNA fragment revealed a 225 bp open reading frame encoding a 74 amino acid pre-prodefensin, including the putative 38 amino acid mature peptide. Similarity between the defensin amino acid sequences of I. scapularis and Dermacentor variabilis (Say) (Acari: Ixodidae) was 62.2% for the pre-prodefensin region; for the mature defensins from these two species the similarity was 78.9%, with the six cysteine residues being located in the same relative position. PCR amplification and sequencing of chromosomal DNA suggests that slnA, along with vsnA, the defensin gene from D. variabilis, does not contain any introns. This is in contrast to the defensins described for the soft tick, Ornithodoros moubata (sensu Walton) (Acari: Argasidae). The role of defensin in the innate immune response of I. scapularis following microbial invasions is discussed.

Capillary tube feeding system for studying tick-pathogen interactions of Dermacentor variabilis (Acari: Ixodidae) and Anaplasma marginale (Rickettsiales: Anaplasmataceae)

A capillary tube feeding (CTF) system was adapted for studying the interaction between Dermacentor variabilis (Say) and the rickettsial cattle pathogen Anaplasma marginale Theiler. A. marginale undergoes a complex developmental cycle in ticks that begins in midguts and ends by transmission from salivary glands. In this CTF system, male D. variabilis were fed A. marginale-infected blood or cultured tick cells. Ticks that fed on highly rickettsemic calves developed midgut and salivary gland infections as detected by PCR, whereas ticks that were fed from capillary tubes on the same blood developed only midgut infections. An unexpected result of capillary tube feeding was that antibodies against the A. marginale adhesin, major surface protein la, enhanced midgut infections and caused cell culture-derived A. marginale to infect midguts. Another unexpected result was the infection of the midguts of the nonvector tick Amblyomma americanum (L.), after capillary tube feeding on infected bovine blood. The gut cell response of ticks to A. marginale, as determined from SDS-polyacrylamide gel electrophoresis protein profiles, did not differ when ticks were fed infected or uninfected cells from capillary tubes. Selected protein bands, as identified by tryptic digestion-mass spectrometry, contained mostly proteins of bovine origin, including bovine albumin, undigested alpha- and beta-chain hemoglobin and hemoglobin fragments. Although infection of ticks by A. marginale CTF system was not the same as infection by feeding on cattle, the results obtained demonstrated the potential use of this system for identifying aspects of pathogen-vector interactions that are not readily recognized in naturally feeding ticks.

Differential expression of Ixodes ricinus tick genes induced by blood feeding or Borrelia burgdorferi infection

Ixodes ricinus L. is the principal European vector of Borrelia burgdorferi sensu lato, the causative agent of Lyme borreliosis. Subtractive hybridization was used to isolate tick genes that were induced in whole ticks after blood meals on uninfected and B. burgdorferi-infected guinea pigs. Novel cDNA clones with similarity to cytochrome c oxidase, salivary secreted protein, actin, and a cysteine protease propeptide were induced after a blood meal. Novel cDNA clones with similarity to thioredoxin peroxidases, dolichyl-phosphate beta-glucosyltransferase, glutathione S-transferase, defensin, ML domain-containing protein, and von Willebrand factor were induced after B. burgdorferi infection. Virtual Northern analysis was used to verify that these genes were differentially expressed in ticks after a pathogen-infected blood meal and to detect their tissues of expression. The characterization of genes that are induced after an infected blood meal is essential for gaining an understanding of the molecular mechanisms that underlie vector-pathogen interactions.

Host blood proteins and peptides in the midgut of the tick Dermacentor variabilis contribute to bacterial control

Antimicrobial midgut proteins and peptides that result from blood digestion in feeding American dog ticks Dermacentor variabilis (Say) were identified. Midgut extracts from these ticks showed antimicrobial activity against Micrococcus luteus, regardless of whether they were challenged with peptidoglycan, blood meal components, rabbit blood, Bacillus subtilis, Escherischia coli or Borrelia burgdorferi. However, no peptide band co-migrating with defensin was found in midgut extracts from the challenged ticks. Partial purification of the midgut extracts using C(18) Sep Paks and gel electrophoresis showed the presence of 4 distinct bands with rMW 4.1, 5.3, 5.7 and 8.0 kDa identified by tryptic digestion-mass fingerprinting as digestive fragments of rabbit alpha-, beta-, gamma-chain hemoglobin, and rabbit ubiquitin. No evidence of varisin, a defensin previously identified in the hemolymph of D. variabilis, was found in the tryptic digest, although varisin was found in a hemocyte lysate using the same methods. However, varisin transcript was detected in midgut cell lysates. Also present in all midgut samples was a cluster of 3 overlapping bands with rMW 13.0, 14.1 and 14.7 kDa which were identified by tryptic-digestion LC-MS and MALDI-TOF as rabbit alpha- and beta-chain hemoglobin (undigested) and transtherytin. Lysozyme transcript was detected in midgut cell extracts but the peptide was not. Studies done on other tick species demonstrated that hemoglobin digestion resulted in antimicrobial fragments. Antimicrobial hemoglobin fragments (including fragments larger than any reported previously) also were found in D. variabilis, as well as ubiquitin, a peptide known to occur as part of an antimicrobial complex in vertebrate leukocytes. In addition, we noted that Borrelia burgdorferi spirochetes were not lysed in the midgut lumen, which would be expected if defensin and lysozyme were active in this location. In this respect, the midgut's response to microbial challenge differs from that of the hemolymph. In summary, the midgut's antimicrobial activity appears to be primarily a byproduct of hemoglobin digestion rather than expression of immune peptides and proteins.

Immune-responsive lysozymes from hemocytes of the American dog tick, Dermacentor variabilis and an embryonic cell line of the Rocky Mountain wood tick, D. andersoni

Immune-responsive lysozyme encoding cDNAs were identified from two medically important tick species by an expressed sequence tag approach of D. variabilis hemocytes (Dv Lys) and a D. andersoni embryonic derived cell line, DAE100. Comparative sequence analyses indicated the Dermacentor molecules to be products of orthologous genes and to be most similar to arthropod c-type lysozymes. Northern blotting analyses demonstrated that Dv Lys expression levels were most abundant in tick hemocytes and to a much lesser degree in the midgut while barely detectable in ovary, salivary gland, and Malpighian tubule tissues. Involvement of the Dermacentor c-type lysozymes in innate immunity was demonstrated by Escherichia coli challenges of D. variabilis ticks by injection resulting in a temporal profile of significantly elevated transcript abundances above those of naive controls that was similarly observed of the D. andersoni cells co-cultured with E. coli. In contrast to that reported of the digestive gut lysozyme of the soft tick Ornithodoros moubata, Dv Lys levels were not statistically differentially regulated by blood meal digestion. Additionally, given the differences in tissue distribution, sequence characteristics and phylogenetic placements between the Dermacentor and Ornithodoros lysozymes demonstrates that ticks possess differently adapted c-type lysozymes that are spatially and temporally differentially expressed.

Fate of GFP-expressing Escherichia coli in the midgut and response to ingestion in a tick, Ornithodoros moubata (Acari: Argasidae)

Ticks are well-known vectors of various pathogens but migration of the pathogens in the tick midgut is not fully understood. In the present study, the fate of microbes in the midgut of Ornithodoros moubata was observed using green fluorescent protein (GFP)-expressing Escherichia coli. Fluctuations in the percentage of hemocytes in the hemolymph (Hc) and expression of an antimicrobial peptide, defensin, in the midgut was also investigated. Most E. coli gradually disappeared in the midgut after ingestion fluctuations in Hc coincided with the changes. Expression of defensin was also confirmed and slightly up-regulated after E. coli ingestion. Moreover, it was demonstrated that E. coli can not pass through the tick midgut epithelium after ingestion by the hemolymph cultures. It is known that various pathogens and host immunoglobulins ingested with a blood meal can enter into the hemocoel, which suggests the presence of unique and complex passage mechanisms for each molecule and organism. The results obtained here help to clarify that digestion enzymes is an important function of the tick midgut to protect against invading molecules and organisms.