Immune responsiveness in vector insects

Activation of immune pathways in common bed bugs, Cimex lectularius, in response to bacterial immune challenges - a transcriptomics analysis

The common bed bug, Cimex lectularius, is an urban pest of global health significance, severely affecting the physical and mental health of humans. In contrast to most other blood-feeding arthropods, bed bugs are not major vectors of pathogens, but the underlying mechanisms for this phenomenon are largely unexplored. Here, we present the first transcriptomics study of bed bugs in response to immune challenges. To study transcriptional variations in bed bugs following ingestion of bacteria, we extracted and processed mRNA from body tissues of adult male bed bugs after ingestion of sterile blood or blood containing the Gram-positive (Gr+) bacterium Bacillus subtilis or the Gram-negative (Gr-) bacterium Escherichia coli. We analyzed mRNA from the bed bugs' midgut (the primary tissue involved in blood ingestion) and from the rest of their bodies (RoB; body minus head and midgut tissues). We show that the midgut exhibits a stronger immune response to ingestion of bacteria than the RoB, as indicated by the expression of genes encoding antimicrobial peptides (AMPs). Both the Toll and Imd signaling pathways, associated with immune responses, were highly activated by the ingestion of bacteria. Bacterial infection in bed bugs further provides evidence for metabolic reconfiguration and resource allocation in the bed bugs' midgut and RoB to promote production of AMPs. Our data suggest that infection with particular pathogens in bed bugs may be associated with altered metabolic pathways within the midgut and RoB that favors immune responses. We further show that multiple established cellular immune responses are preserved and are activated by the presence of specific pathogens. Our study provides a greater understanding of nuances in the immune responses of bed bugs towards pathogens that ultimately might contribute to novel bed bug control tactics.

Japanese Encephalitis Virus Interaction with Mosquitoes: A Review of Vector Competence, Vector Capacity and Mosquito Immunity

Japanese encephalitis virus (JEV) is a mosquito-borne zoonotic flavivirus and a major cause of human viral encephalitis in Asia. We provide an overview of the knowledge on vector competence, vector capacity, and immunity of mosquitoes in relation to JEV. JEV has so far been detected in more than 30 mosquito species. This does not necessarily mean that these species contribute to JEV transmission under field conditions. Therefore, vector capacity, which considers vector competence, as well as environmental, behavioral, cellular, and biochemical variables, needs to be taken into account. Currently, 17 species can be considered as confirmed vectors for JEV and 10 other species as potential vectors. Culex tritaeniorhynchus and Culex annulirostris are considered primary JEV vectors in endemic regions. Culex pipiens and Aedes japonicus could be considered as potentially important vectors in the case of JEV introduction in new regions. Vector competence is determined by various factors, including vector immunity. The available knowledge on physical and physiological barriers, molecular pathways, antimicrobial peptides, and microbiome is discussed in detail. This review highlights that much remains to be studied about vector immunity against JEV in order to identify novel strategies to reduce JEV transmission by mosquitoes.

Identification, structural characterisation and expression analysis of a defensin gene from the tiger beetle Calomera littoralis (Coleoptera: Cicindelidae)

In this study, a defensin gene (Clit-Def) has been characterised in the tiger beetle Calomera littoralis for the first time. Bioinformatic analysis showed that the gene has an open reading frame of 246bp that contains a 46 amino acid mature peptide. The phylogenetic analysis showed a high variability in the coleopteran defensins analysed. The Clit-Def mature peptide has the features to be involved in the antimicrobial function: a predicted cationic isoelectric point of 8.94, six cysteine residues that form three disulfide bonds, and the typical cysteine-stabilized α-helix β-sheet (CSαβ) structural fold. Real time quantitative PCR analysis showed that Clit-Def was upregulated in the different body parts analysed after infection with lipopolysaccharides of Escherichia coli, and also indicated that has an expression peak at 12h post infection. The expression patterns of Clit-Def suggest that this gene plays important roles in the humoral system in the adephagan beetle Calomera littoralis.

Purification and characterization of a novel defensin from the salivary glands of the black fly, Simulium bannaense

Background

Black flies (Diptera: Simuliidae) are haematophagous insects that can cause allergic reactions and act as vectors of pathogens. Although their saliva has been thought to contain a diverse array of physiologically active molecules, little information is available on antimicrobial factors in black fly salivary glands, especially no defensins have been reported so far.

Methods

A novel cationic defensin designated SibaDef was purified using reverse phase high-performance liquid chromatography (RP-HPLC) from the salivary glands of the black fly Simulium bannaense. The amino acid sequence of SibaDef was determined by a combination method of automated Edman degradation and cDNA sequencing. The morphologic changes of Gram-positive bacteria Staphylococcus aureus or Bacillus subtilis treated with SibaDef were assessed by scanning electron microscopy (SEM). Quantitative PCR (qPCR) was performed to analyze the expression of SibaDef mRNA in whole bodies of insects after oral infection with the bacteria S. aureus or B. subtilis.

Results

Surprisingly, the phylogenetic analysis of defensin-related amino acid sequences demonstrated that SibaDef is most closely related to defensins from the human body louse Pediculus humanus corporis (Anoplura: Pediculidae), rather than to other dipteran defensins. SibaDef showed potent antimicrobial activities against Gram-positive bacteria with minimal inhibitory concentrations (MICs) ranging from 0.83 μM to 2.29 μM. SEM analysis indicated that SibaDef killed microorganisms through the disruption of cell membrane integrity. The transcript levels of SibaDef in the bacteria-immunized flies increased with the time course, reaching maximum at 36 h and then slowly decreased from that time point.

Conclusions

Our results indicate that SibaDef is involved in the innate humoral response of the black fly S. bannaense, and it might play a significant role in the defence against microorganisms in both sugar and blood meals.

The effect of bacterial challenge on ferritin regulation in the yellow fever mosquito, Aedes aegypti

Secreted ferritin is the major iron storage and transport protein in insects. Here, we characterize the message and protein expression profiles of yellow fever mosquito (Aedes aegypti) ferritin heavy chain homologue (HCH) and light chain homologue (LCH) subunits in response to iron and bacterial challenge. In vivo experiments demonstrated tissue-specific regulation of HCH and LCH expression over time post-blood meal (PBM). Transcriptional regulation of HCH and LCH was treatment specific, with differences in regulation for naïve versus mosquitoes challenged with heat-killed bacteria (HKB). Translational regulation by iron regulatory protein (IRP) binding activity for the iron-responsive element (IRE) was tissue-specific and time-dependent PBM. However, mosquitoes challenged with HKB showed little change in IRP/IRE binding activity compared to naïve animals. The changes in ferritin regulation and expression in vivo were confirmed with in vitro studies. We challenged mosquitoes with HKB followed by a blood meal to determine the effects on ferritin expression, and demonstrate a synergistic, time-dependent regulation of expression for HCH and LCH.

Insect transferrins: multifunctional proteins

BACKGROUND

Many studies have been done evaluating transferrin in insects. Genomic analyses indicate that insects could have more than one transferrin. However, the most commonly studied insect transferrin, Tsf1, shows greatest homology to mammalian blood transferrin.

SCOPE OF REVIEW

Aspects of insect transferrin structure compared to mammalian transferrin and the roles transferrin serves in insects are discussed in this review.

MAJOR CONCLUSIONS

Insect transferrin can have one or two lobes, and can bind iron in one or both. The iron binding ligands identified for the lobes of mammalian blood transferrin are generally conserved in the lobes of insect transferrins that have an iron binding site. Available information supports that the form of dietary iron consumed influences the regulation of insect transferrin. Although message is expressed in several tissues in many insects, fat body is the likely source of hemolymph transferrin. Insect transferrin is a vitellogenic protein that is down-regulated by Juvenile Hormone. It serves a role in transporting iron to eggs in some insects, and transferrin found in eggs appears to be endowed from the female. In addition to the roles of transferrin in iron delivery, this protein also functions to reduce oxidative stress and to enhance survival of infection.

GENERAL SIGNIFICANCE

Future studies in Tsf1 as well as the other insect transferrins that bind iron are warranted because of the roles of transferrin in preventing oxidative stress, enhancing survival to infections and delivering iron to eggs for development. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Identification of a cysteine-rich antimicrobial peptide from salivary glands of the tick Rhipicephalus haemaphysaloides

The presence of an effective immune response in the hemocoel of ticks is crucial for survival, as it prevents the invasion of pathogens throughout the animal's body. Antimicrobial peptides (AMPs) play an important role in this response by rapidly killing invading microorganisms. In this study, a subtraction hybridization cDNA library was constructed from the salivary glands of the unfed and fed female tick Rhipicephalus haemaphysaloides, and a novel cysteine-rich AMP designated Rhamp (R. haemaphysaloides antimicrobial peptide) was isolated and identified. The Rhamp was encoded by a gene with an open reading frame of 303 bp which encoded a mature peptide with 8 kDa molecular weight. No identity was found by BLAST search to any database entries. The sequence encoding the Rhamp was subcloned into the pGEX-4T vector and expressed in Escherichia coli. The recombinant protein of Rhamp showed chymotrypsin and elastase-inhibitory activity and markedly inhibited the growth of gram-negative bacteria, including Pseudomonas aeruginosa, Salmonella typhimurium, and E. coli. Moreover, the recombinant protein also exerted low hemolytic activity. These results indicate the Rhamp is a novel antimicrobial peptide with proteinase activity from the tick R. haemaphysaloides.

Molecular characterization and expression analysis of BmNOX in two strains of Bombyx mori with contrasting viral resistance phenotype

We recently documented the identification of a 26.5 kDa protein named BmNox in the gut fluid of Nistari strain of Bombyx mori, which possessed antiviral activity against BmNPV in vitro. In this report, we report the characterization of the full-length gene encoding BmNOX and the levels of expression of this gene in select tissues of silkworm larvae from a BmNPV-susceptible and a BmNPV-resistant strain to the defense capability in Bombyx mori larvae challenged with BmNPV. We also evaluated the BmNox expression in various stages of larval life of a resistant and a susceptible strain of Bombyx mori selected from among a panel of strains of silkworm. Nistari, a multivoltine strain of silkworm, expressed BmNOX during all five larval stages, and were highly resistant to BmNPV infection. In sharp contrast, CSR(2), a bivoltine strain, showed weaker expression of BmNOX in the anterior midgut in larval life and was highly susceptible to BmNPV infection. BmNOX is a secretory protein with dual expression in gut fluid and mid gut tissue. BmNOX is expressed heavily in the posterior mid gut, with weaker expression in the fore- and mid-gut regions.

Knock-down of REL2, but not defensin A, augments Aedes aegypti susceptibility to Bacillus subtilis and Escherichia coli

Some components of the Toll and Imd immune signaling pathways have remained conserved between Drosophila and mosquitoes, however, important differences in the way invading microorganisms activate these pathways in these organisms have started to be revealed. In the present study, we have attempted to silence the Aedes aegypti NF-kappaB-like factor REL2, which is analogous to Drosophila Relish, and analyze the effects on mosquito mortality upon infection with a Gram-negative and a Gram-positive bacterium, both containing a DAP-type peptidoglycan, and effects on embryo development. Moreover, we have silenced one of the antimicrobial peptides (AMPs) controlled by REL2, defensin A, a major AMP in A. aegypti, and compared the results on mosquito mortality upon bacterial infection to those obtained with REL2 silencing. Results show that REL2 is crucial for A. aegypti immunity upon infection with Escherichia coli and Bacillus subtilis, corroborating with previous studies on that REL2/Imd in mosquitoes is involved in a generalized antibacterial defense, differently than its analogous in Drosophila. However, defensin A silencing did not cause a significant increase in mortality in infected mosquitoes, indicating that this peptide is not essential in mosquito protection against the two bacteria and that other immune factors controlled by REL2 are playing this role. In regard to embryo development, REL2 knock-down did not cause any significant effect on: the number of laid eggs, number of developed pupae, percent of emerged adults, and ratio between emerged females versus males. A slight decrease in the number of hatched eggs (percent eclosion) was observed in REL2 knock-down mosquitoes, but these observations were inconclusive.

Two novel defensin-encoding genes of the Chagas disease vector Triatoma brasiliensis (Reduviidae, Triatominae): gene expression and peptide-structure modeling

Defensins are cysteine-rich peptides involved in the innate immunity of insects and many other organisms. In the present study, two novel defensin-encoding cDNAs and the respective genomic DNAs (def3 and def4) of Triatoma brasiliensis were identified and their tissue-specific and temporal expression was characterized. Both of the deduced mature peptides consisted of 43 amino acid residues and were highly similar to previously identified triatomine defensins (81.4-100%). Semi-quantitative RT-PCR data showed that def3 was constitutively expressed in the fat body and was induced in salivary glands and the small intestine at 5 and 3 days after feeding (daf), respectively. The def4 mRNA level was highly up-regulated in the stomach and fat-body tissues at 5 and 3 daf, respectively. The three-dimensional structures of these defensins were predicted using a homology modeling approach with Def-AAA, the defensin from Anopheles gambiae, as template (62-74% identity). A map of the electrostatic potential of these models revealed that, despite their similar folding patterns, mature Def2 and Def4 have a more cationic structure than is the case for Def1 and Def3. Such differences may orient the antimicrobial profile of these defensins against distinct targets in different organs of the insect.

Detection of genes encoding antimicrobial peptides in Mexican strains of Trichoplusia ni (Hübner) exposed to Bacillus thuringiensis

The systemic immune response of Trichoplusia ni after Bacillus thuringiensis (Bt) exposure was evaluated by comparing the expression of genes encoding antimicrobial peptides (AMPs) in Bt-susceptible and -resistant T. ni strains that were either exposed or not to XenTari (Bt-XT). AMP genes were detected by RT-PCR using primers for attacin, gloverin, lebocin, lysozyme, and peptidoglycan recognition peptide (PGRP). In general, AMP genes were detected more frequently in Mexican field strains previously exposed to Bt (SALX and GTOX) than in a Mexican laboratory strain (NL), but expression was similar to the AMP expression in USA laboratory strains (US and USX). Among the AMPs, transcripts for lebocin were the least detected (11.7%) and those for lysozyme were the most detected (84.8%) in all samples. Lebocin was detected only in 2nd instar and pupa. All untreated controls expressed attacin. Attacin and gloverin were not detected in any midgut sample, and their highest detection was in pupa. Lysozyme was rarely detected in 2nd instar larvae from any strain or treatment but was detected in almost all midgut and hemolymph samples. Overall, AMPs were found more in T. ni strains previously exposed to Bt-XT, especially lebocin and globerin (1.8-fold increase) and PGRP (3.8-fold increase). The data suggest that the expression of AMPs in T. ni correlates to previous Bt exposure.

Babesial vector tick defensin against Babesia sp. parasites

Antimicrobial peptides are major components of host innate immunity, a well-conserved, evolutionarily ancient defensive mechanism. Infectious disease-bearing vector ticks are thought to possess specific defense molecules against the transmitted pathogens that have been acquired during their evolution. We found in the tick Haemaphysalis longicornis a novel parasiticidal peptide named longicin that may have evolved from a common ancestral peptide resembling spider and scorpion toxins. H. longicornis is the primary vector for Babesia sp. parasites in Japan. Longicin also displayed bactericidal and fungicidal properties that resemble those of defensin homologues from invertebrates and vertebrates. Longicin showed a remarkable ability to inhibit the proliferation of merozoites, an erythrocyte blood stage of equine Babesia equi, by killing the parasites. Longicin was localized at the surface of the Babesia sp. parasites, as demonstrated by confocal microscopic analysis. In an in vivo experiment, longicin induced significant reduction of parasitemia in animals infected with the zoonotic and murine B. microti. Moreover, RNA interference data demonstrated that endogenous longicin is able to directly kill the canine B. gibsoni, thus indicating that it may play a role in regulating the vectorial capacity in the vector tick H. longicornis. Theoretically, longicin may serve as a model for the development of chemotherapeutic compounds against tick-borne disease organisms.

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.

Exploration of mosquito immunity using cells in culture

The propagation of immune-responsive cells in vitro has provided the basis for substantial contributions to our understanding of many aspects of the mammalian immune response. In contrast, the potential for exploring the innate immune response of insects using cultured cells is only beginning to be developed, particularly with various mosquito cell lines from the genera Aedes and Anopheles. Immune-reactive mosquito cell lines express various defensive factors, including transferrin, lysozyme, cecropin, defensin, and prophenoloxidase activities. In this review, we discuss insect immunity in the context of key concepts that have emerged in the study of the mammalian immune system, with emphasis on the properties of the cells that participate in the immune response. The nature of established cell lines and their contributions to our understanding of immune functions in humans and insects is described, with emphasis on our own work with the C7-10 and Aag-2 mosquito cell lines from Aedes albopictus and Aedes aegypti, respectively. Finally, we offer some speculation on further advances in insect immunology that may be facilitated by work with cells in culture.

Wolbachia neither induces nor suppresses transcripts encoding antimicrobial peptides

Wolbachia are intracellular maternally inherited microorganisms that are associated with reproductive abnormalities such as cytoplasmic incompatibility (CI), feminization and parthenogenesis in the various arthropod species they infect. Surveys indicate that these bacteria infect more than 16% of all insect species as well as isopods, mites and nematodes, making Wolbachia one of the most ubiquitous parasites yet described. However, nothing is known about the interactions of this bacterium with the host's immune system. We studied the expression of inducible antimicrobial markers in the adults of two Wolbachia infected insect species, Drosophila simulans and Aedes albopictus. The lack of available immune markers in the mosquito species led us to clone part of the defensin gene from this species, which was found to be very similar to the other mosquito defensins cloned from Anopheles gambiae and Aedes aegypti. Comparisons of the expression pattern of the antibacterial markers between Wolbachia-infected and cured lines, and also between bacteria-challenged and unchallenged adults indicated that Wolbachia does not either constitutively induce or suppress the transcription of these antibacterial genes. In addition, no difference in the transcription of these genes was found between double and single Wolbachia-infected strains or between strains in which Wolbachia has different tissue tropisms.

Peptide sequence of an antibiotic cecropin from the vector mosquito, Aedes albopictus

We have identified a 35-amino acid antibiotic cecropin secreted by an established mosquito cell line. C7-10 cells from the vector mosquito, Aedes albopictus, were incubated with heat-killed Escherichia coli, and materials secreted into the cell culture supernatant were recovered by acid precipitation. Following batch elution from Sep-Pak C18 cartridges and further purification by reverse phase high performance liquid chromatography (RP-HPLC) a predominant peak of antibacterial activity was characterized by mass spectrometry, amino acid composition analysis, and Edman degradation, yielding the sequence GGLKKLGKKLEGVGKRVFKASEKALPVAVGIKALG. Unlike other cecropins, the peptide was not amidated at the C-terminus. Aedes albopictus Cecropin A (AalCecA) is the first cecropin to be described from a mosquito vector of human disease. Consistent with the classification of mosquitoes among the Dipteran suborder Nematocera, AalCecA shares only 36% amino acid identity with cecropins from Drosophila melanogaster and other Cyclorrhaphid flies, whose mature cecropins share 80% to 100% amino acid identity.