Insect immune activation by recombinant Galleria mellonella apolipophorin III(1)

An ELISA-based method for Galleria mellonella apolipophorin-III quantification

Mammalian models, such as murine, are used widely in pathophysiological studies because they have a high degree of similarity in body temperature, metabolism, and immune response with humans. However, non-vertebrate animal models have emerged as alternative models to study the host-pathogen interaction with minimal ethical concerns. Galleria mellonella is an alternative model that has proved useful in studying the interaction of the host with either bacteria or fungi, performing drug testing, and assessing the immunological response to different microorganisms. The G. mellonella immune response includes cellular and humoral components with structural and functional similarities to the immune effectors found in higher vertebrates, such as humans. An important humoral effector stimulated during infections is apolipophorin III (apoLp-III), an opsonin characterized by its lipid and carbohydrate-binding properties that participate in lipid transport, as well as immunomodulatory activity. Despite some parameters, such as the measurement of phenoloxidase activity, melanin production, hemocytes counting, and expression of antimicrobial peptides genes are already used to assess the G. mellonella immune response to pathogens with different virulence degrees, the apoLp-III quantification remains to be a parameter to assess the immune response in this invertebrate. Here, we propose an immunological tool based on an enzyme-linked immunosorbent assay that allows apoLp-III quantification in the hemolymph of larvae challenged with pathogenic agents. We tested the system with hemolymph coming from larvae infected with Escherichia coli, Candida albicans, Sporothrix schenckii, Sporothrix globosa, and Sporothrix brasiliensis. The results revealed significantly higher concentrations of apoLp-III when each microbial species was inoculated, in comparison with untouched larvae, or inoculated with phosphate-buffered saline. We also demonstrated that the apoLp-III levels correlated with the strains' virulence, which was already reported. To our knowledge, this is one of the first attempts to quantify apoLp-III, using a quick and easy-to-use serological technique.

Changes in the apolipophorin III in Galleria mellonella larvae treated with Pseudomonas aeruginosa exotoxin A

In the present study, we have demonstrated a correlation in time between changes in the amount of apolipophorin III (apoLp-III) in the fat body and hemocytes of Galleria mellonella larvae challenged with Pseudomonas aeruginosa exotoxin A (exoA). An increase in the amount of apoLp-III was detected 1-8 h after the challenge; then, a temporary decrease was observed after 15 h followed by an increase in the level of apoLp-III, however to a different extent. The profile of apoLp-III forms in the hemolymph, hemocytes, and fat body of the exoA-challenged larvae was analyzed using two-dimensional electrophoresis (IEF/SDS-PAGE) and immunoblotting with anti-apoLp-III antibodies. Two apoLp-III forms differing in isoelectric point values estimated at ∼ 6.5 and ∼ 6.1 in the hemolymph and ∼ 6.5 and ∼ 5.9 in the hemocytes as well as one isoform with pI ∼ 6.5 in the fat body with an additional apoLp-III-derived polypeptide with estimated pI ∼ 6.9 were detected in the control insects. The injection of exoA caused a significant decrease in the abundance of both apoLp-III isoforms in the insect hemolymph. In the hemocytes, a decrease in the amount of the pI ∼ 5.9 isoform was detected, while the major apoLp-III isoform (pI ∼ 6.5) remained unchanged. In addition, appearance of an additional apoLp-III-derived polypeptide with an estimated pI ∼ 5.2 was observed. Interestingly, there were no statistically significant differences in the amount of the main isoform in the fat body between the control and exoA-challenged insects, but the polypeptide with pI ∼ 6.9 disappeared completely. It should be noted that the decrease in the amount of apoLp-III and other proteins was especially noticeable at the time points when exoA was detected in the studied tissues.

Lipids as a key element of insect defense systems

The relationship between insect pathogenic fungi and their insect hosts is a classic example of a co-evolutionary arms race between pathogen and target host: parasites evolve towards mechanisms that increase their advantage over the host, and the host increasingly strengthens its defenses. The present review summarizes the literature data describing the direct and indirect role of lipids as an important defense mechanism during fungal infection. Insect defense mechanisms comprise anatomical and physiological barriers, and cellular and humoral response mechanisms. The entomopathogenic fungi have the unique ability to digest the insect cuticle by producing hydrolytic enzymes with chitin-, lipo- and proteolytic activity; besides the oral tract, cuticle pays the way for fungal entry within the host. The key factor in insect resistance to fungal infection is the presence of certain types of lipids (free fatty acids, waxes or hydrocarbons) which can promote or inhibit fungal attachment to cuticle, and might also have antifungal activity. Lipids are considered as an important source of energy, and as triglycerides are stored in the fat body, a structure analogous to the liver and adipose tissue in vertebrates. In addition, the fat body plays a key role in innate humoral immunity by producing a range of bactericidal proteins and polypeptides, one of which is lysozyme. Energy derived from lipid metabolism is used by hemocytes to migrate to the site of fungal infection, and for phagocytosis, nodulation and encapsulation. One polyunsaturated fatty acid, arachidonic acid, is used in the synthesis of eicosanoids, which play several crucial roles in insect physiology and immunology. Apolipoprotein III is important compound with antifungal activity, which can modulate insect cellular response and is considered as important signal molecule.

The physical interactome between Peregrinus maidis proteins and the maize mosaic virus glycoprotein provides insights into the cellular biology of a rhabdovirus in the insect vector

Rhabdovirus glycoproteins (G) serve multifunctional roles in virus entry, assembly, and exit from animal cells. We hypothesize that maize mosaic virus (MMV) G is required for invasion, infection, and spread in Peregrinus maidis, the planthopper vector. Using a membrane-based yeast two-hybrid assay, we identified 107 P. maidis proteins that physically interacted with MMV G, of which approximately 53% matched proteins with known functions including endocytosis, vesicle-mediated transport, protein synthesis and turnover, nuclear export, metabolism and host defense. Physical interaction networks among conserved proteins indicated a possible cellular coordination of processes associated with MMV G translation, protein folding and trafficking. Non-annotated proteins contained predicted functional sites, including a diverse array of ligand binding sites. Cyclophilin A and apolipophorin III co-immunoprecipitated with MMV G, and each showed different patterns of localization with G in insect cells. This study describes the first protein interactome for a rhabdovirus spike protein and insect vector.

Neurological aspects of SARS-CoV-2 infection: lipoproteins and exosomes as Trojan horses

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) primarily targets lipid-producing cells for viral tropism. In this review, we connect systemic lipid couriers, particularly high-density lipoproteins (HDLs) and exosomes, with the neurological facets of SARS-CoV-2 infection. We discuss how SARS-CoV-2 preferentially targets lipid-secreting cells and usurps host cell lipid metabolism for efficient replication and systemic spreading. Besides providing natural veils for viral materials against host immunity, the inherent properties of some of these endogenous lipid particles to traverse the blood-brain barrier (BBB) also offer alternative routes for SARS-CoV-2 neurotropism. Importantly, virus-driven neurological aberrations mediated by HDLs and exosomes are fueled by lipid rafts, which are implicated in the production and transmigration of these lipid particles across the BBB. Finally, we discuss how repurposing existing drugs targeting lipid rafts and cholesterol homeostasis may be beneficial toward alleviating the global coronavirus disease 2019 (COVID-19) disease burden.

Proteomic analysis of summer and winter Apis mellifera workers shows reduced protein abundance in winter samples

Apis mellifera workers display two stages; short lived summer bees that engage in nursing, hive maintenance and foraging, and long lived winter bees (diutinus bees) which remain within the hive and are essential for thermoregulation and rearing the next generation of bees in spring before dying. Label free quantitative proteomic analysis was conducted on A. mellifera workers sampled in June and December to compare the proteomes of summer and winter bees. Proteomic analysis was performed on head, abdominal and venom sac samples and revealed an elevated level of protein abundance in summer bees. Head and abdominal samples displayed an increased abundance in cuticular proteins in summer samples whereas an increase in xenobiotic proteins was observed in winter samples. Several carbohydrate metabolism pathways which have been linked to energy production and longevity in insects were increased in abundance in winter samples in comparison to summer samples. Proteomic analysis of the venom sacs of summer samples showed an increased abundance of bee venom associated proteins in comparison to winter workers. These data provides an insight into the adaptions of A. mellifera workers in summer and winter and may aid in future treatment and disease studies on honeybee colonies. Data are available via ProteomeXchange with identifier PXD030483.

Galleria mellonella as a Suitable Model of Bacterial Infection: Past, Present and Future

The increasing interest for Galleria mellonella larvae as an infection model is evidenced by the number of papers reporting its use, which increases exponentially since the early 2010s. This popularity was initially linked to limitation of conventional animal models due to financial, technical and ethical aspects. In comparison, alternative models (e.g. models using Caenorhabditis elegans, Drosophila melanogaster or G. mellonella) were cheap, simple to use and not limited by ethical regulation. Since then, similar results have been established with G. mellonella model comparatively to vertebrates, and it is more and more often used as a robust model per se, not only as an alternative to the murine model. This review attempts to summarize the current knowledge supporting the development of this model, both on immunological and microbiological aspects. For that, we focus on investigation of virulence and new therapies for the most important pathogenic bacteria. We also discuss points out directions for standardization, as well as recent advances and new perspectives for monitoring host-pathogen interactions.

Integration of transcriptomics and network analysis reveals co-expressed genes in Frankliniella occidentalis larval guts that respond to tomato spotted wilt virus infection

Background

The gut is the first barrier to infection by viruses that are internally borne and transmitted persistently by arthropod vectors to plant and animal hosts. Tomato spotted wilt virus (TSWV), a plant-pathogenic virus, is transmitted exclusively by thrips vectors in a circulative-propagative manner. Frankliniella occidentalis (western flower thrips), the principal thrips vector of TSWV, is transmission-competent only if the virus is acquired by young larvae. To begin to understand the larval gut response to TSWV infection and accumulation, a genome-assisted, transcriptomic analysis of F. occidentalis gut tissues of first (early L1) and second (early L2 and late L2) instar larvae was conducted using RNA-Seq to identify differentially-expressed transcripts (DETs) in response to TSWV compared to non-exposed cohorts.

Results

The larval gut responded in a developmental stage-dependent manner, with the majority of DETs (71%) associated with the early L1 stage at a time when virus infection is limited to the midgut epithelium. Provisional annotations of these DETs inferred roles in digestion and absorption, insect innate immunity, and detoxification. Weighted gene co-expression network analysis using all assembled transcripts of the gut transcriptome revealed eight gene modules that distinguish larval development. Intra-module interaction network analysis of the three most DET-enriched modules revealed ten central hub genes. Droplet digital PCR-expression analyses of select network hub and connecting genes revealed temporal changes in gut expression during and post exposure to TSWV.

Conclusions

These findings expand our understanding of the developmentally-mediated interaction between thrips vectors and orthotospoviruses, and provide opportunities for probing pathways for biomarkers of thrips vector competence.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08100-4.

Proteomic profiling of bacterial and fungal induced immune priming in Galleria mellonella larvae

Some insects display immunological priming as a result of elevated humoral and cellular responses which give enhanced survival against subsequent infection. The humoral immune response of Galleria mellonella larvae following pre-exposure to heat killed Staphylococcus aureus or Candida albicans cells was determined by quantitative mass spectrometry in order to assess the relationship between the humoral immune response and resistance to subsequent bacterial or fungal infection. Larvae pre-exposed to heat killed S. aureus showed increased resistance to subsequent bacterial and fungal infection. Larvae displayed an increased hemocyte density (14.08 ± 2.14 × 10⁶ larva^-1 (p < 0.05) compared to the PBS injected control [10.41 ± 1.67 × 10⁶ larva^-1]) and increased abundance of antimicrobial proteins (cecropin-D-like peptide (+22.23 fold), hdd11 (+12.61 fold) and prophenol oxidase activating enzyme 3 (+5.96 fold) in response to heat killed S. aureus. Larvae pre-exposed to heat killed C. albicans cells were resistant to subsequent fungal infection but not bacterial infection and showed a reduced hemocyte density (6.01 ± 1.63 × 10⁶ larva^-1 (p < 0.01) and increased abundance of hdd11 (+32.73 fold) and moricin-like peptide C1 (+16.76 fold). While immune priming is well recognised in G. mellonella larvae the results presented here indicate distinct differences in the response of larvae following exposure to heat killed bacterial and fungal cells.

Identification and characterization of Staphylococcus spp. and their susceptibility to insect apolipophorin III

Aim: This study investigated the effect of an insect antimicrobial protein, apolipophorin III (apoLp-III), against two newly isolated, identified and characterized clinical strains of Staphylococcus spp. Materials & methods: Both strains were identified by 16S rRNA sequencing and metabolic and phenotypic profiling. The antibacterial activity of apoLp-III was tested using a colony counting assay. ApoLp-III interaction with bacterial cell surface was analyzed by Fourier transform IR spectroscopy. Results: Staphylococcus epidermidis and Staphylococcus capitis were identified. ApoLp-III exerted a dose-dependent bactericidal effect on the tested strains. The differences in the Staphylococcus spp. surface components may contribute to the various sensitivities of these strains to apoLp-III. Conclusion: ApoLp-III may provide a baseline for development of antibacterial preparations against Staphylococcus spp. involved in dermatological problems.

High-Density Lipoproteins Are Bug Scavengers

Lipoproteins were initially defined according to their composition (lipids and proteins) and classified according to their density (from very low- to high-density lipoproteins—HDLs). Whereas their capacity to transport hydrophobic lipids in a hydrophilic environment (plasma) is not questionable, their primitive function of cholesterol transporter could be challenged. All lipoproteins are reported to bind and potentially neutralize bacterial lipopolysaccharides (LPS); this is particularly true for HDL particles. In addition, HDL levels are drastically decreased under infectious conditions such as sepsis, suggesting a potential role in the clearance of bacterial material and, particularly, LPS. Moreover, "omics" technologies have unveiled significant changes in HDL composition in different inflammatory states, ranging from acute inflammation occurring during septic shock to low-grade inflammation associated with moderate endotoxemia such as periodontal disease or obesity. In this review, we will discuss HDL modifications associated with exposure to pathogens including bacteria, viruses and parasites, with a special focus on sepsis and the potential of HDL therapy in this context. Low-grade inflammation associated with atherosclerosis, periodontitis or metabolic syndrome may also highlight the protective role of HDLs in theses pathologies by other mechanisms than the reverse transport of cholesterol.

Candida albicans increases the pathogenicity of Staphylococcus aureus during polymicrobial infection of Galleria mellonella larvae

This study detailed the responses of Galleria mellonella larvae to disseminated infection caused by co-infection with Candida albicans and Staphylococcus aureus. Doses of C. albicans (1×10⁵ larva^-1) and S. aureus (1×10⁴ larva^-1) were non-lethal in mono-infection but when combined significantly (P<0.05) reduced larval survival at 24, 48 and 72 h relative to larvae receiving S. aureus (2×10⁴ larva^-1) alone. Co-infected larvae displayed a significantly higher density of S. aureus larva^-1 compared to larvae infected solely with S. aureus. Co-infection resulted in dissemination throughout the host and the appearance of large nodules. Co-infection of larvae with C. albicans and S. aureus (2×10⁴ larva^-1) resulted in an increase in the density of circulating haemocytes compared to that in larvae infected with only S. aureus. Proteomic analysis of co-infected larval haemolymph revealed increased abundance of proteins associated with immune responses to bacterial and fungal infection such as cecropin-A (+45.4-fold), recognition proteins [e.g. peptidoglycan-recognition protein LB (+14-fold)] and proteins associated with nodule formation [e.g. Hdd11 (+33.3-fold)]. A range of proteins were also decreased in abundance following co-infection, including apolipophorin (-62.4-fold), alpha-esterase 45 (-7.7-fold) and serine proteinase (-6.2-fold). Co-infection of larvae resulted in enhanced proliferation of S. aureus compared to mono-infection and an immune response showing many similarities to the innate immune response of mammals to infection. The utility of G. mellonella larvae for studying polymicrobial infection is highlighted.

Lipid-bound apoLp-III is less effective in binding to lipopolysaccharides and phosphatidylglycerol vesicles compared to the lipid-free protein

Apolipophorin III (apoLp-III) is an insect apolipoprotein that is predominantly present in a lipid-free state in the hemolymph. ApoLp-III from Galleria mellonella is able to interact with membrane components of Gram-negative bacteria, as part of an innate immune response to infection. The protein also exists in a lipoprotein-associated state when large amounts of lipids are mobilized. Therefore, lipid-bound apoLp-III was generated to analyze the binding interaction with lipopolysaccharides and phosphatidylglycerol, both abundantly present in membranes of Gram-negative bacteria. G. mellonella apoLp-III was lipidated with palmitoyl-2-oleoyl-glycero-3-phosphocholine to form lipid-protein complexes. The particle shape was discoidal with a 16.4 nm diameter, a molecular mass of 460 kDa, and contained 4 apoLp-III molecules. These discoidal lipoproteins were used to compare the lipopolysaccharide and phosphatidylglycerol binding activity with lipid-free apoLp-III. Lipopolysaccharide binding interaction was analyzed by non-denaturing PAGE, showing reduced ability of the lipid-bound protein to form lipopolysaccharide-protein complexes and to disaggregate lipopolysaccharide micelles. The apoLp-III-induced release of calcein from phosphatidylglycerol vesicles was decreased approximately fivefold when the protein was in the lipid-bound form, indicating reduced binding interaction with the phosphatidylglycerol membrane surface. These results show that when apoLp-III adopts a lipid-bound conformation, it is markedly less effective in interacting with lipopolysaccharides and phosphatidylglycerol vesicles. Thus, in order to be an effective antimicrobial protein, apoLp-III needs to be in a lipid-free state.

Wheat streak mosaic virus alters the transcriptome of its vector, wheat curl mite (Aceria tosichella Keifer), to enhance mite development and population expansion

Wheat streak mosaic virus (WSMV; genus Tritimovirus; family Potyviridae) is an economically important wheat virus that is transmitted by the wheat curl mite (WCM; Aceria tosichella Keifer) in a persistent manner. Virus-vector coevolution may potentially influence vector gene expression to prolong viral association and thus increase virus transmission efficiency and spread. To understand the transcriptomic responses of WCM to WSMV, RNA sequencing was performed to assemble and analyse transcriptomes of WSMV viruliferous and aviruliferous mites. Among 7291 de novo-assembled unigenes, 1020 were differentially expressed between viruliferous and aviruliferous WCMs using edgeR at a false discovery rate ≤0.05. Differentially expressed unigenes were enriched for 108 gene ontology terms, with the majority of the unigenes showing downregulation in viruliferous mites in comparison to only a few unigenes that were upregulated. Protein family and metabolic pathway enrichment analyses revealed that most downregulated unigenes encoded enzymes and proteins linked to stress response, immunity and development. Mechanistically, these predicted changes in mite physiology induced by viral association could be suggestive of pathways needed for promoting virus-vector interactions. Overall, our data suggest that transcriptional changes in viruliferous mites facilitate prolonged viral association and alter WCM development to expedite population expansion, both of which could enhance viral transmission.

Nematobacterial Complexes and Insect Hosts: Different Weapons for the Same War

Entomopathogenic nematodes (EPNs) are widely used as biological control agents against insect pests, the efficacy of these organisms strongly depends on the balance between the parasitic strategies and the immune response of the host. This review summarizes roles and relationships between insect hosts and two well-known EPN species, Steinernema feltiae and Steinernema carpocapsae and outlines the main mechanisms of immune recognition and defense of insects. Analyzing information and findings about these EPNs, it is clear that these two species use shared immunosuppression strategies, mainly mediated by their symbiotic bacteria, but there are differences in both the mechanism of evasion and interference of the two nematodes with the insect host immune pathways. Based on published data, S. feltiae takes advantage of the cross reaction between its body surface and some host functional proteins, to inhibit defensive processes; otherwise, secretion/excretion products from S. carpocapsae seem to be the main nematode components responsible for the host immunosuppression.

Studies on localization and protein ligands of Galleria mellonella apolipophorin III during immune response against different pathogens

A lipid-binding protein apolipophorin III (apoLp-III), an exchangeable component of lipophorin particles, is involved in lipid transport and immune response in insects. In Galleria mellonella, apoLp-III binding to high-density lipophorins and formation of low-density lipophorin complexes upon immune challenge was reported. However, an unanswered question remains whether apoLp-III could form different complexes in a pathogen-dependent manner. Here we report on pathogen- and time-dependent alterations in the level of apoLp-III free and lipophorin-bound form that occur in the hemolymph and hemocytes shortly after immunization of G. mellonella larvae with different pathogens, i.e. Gram-negative bacterium Escherichia coli, Gram-positive bacterium Micrococcus luteus, yeast-like fungus Candida albicans, and filamentous fungus Fusarium oxysporum. These changes were accompanied by differently persistent re-localization of apoLp-III in the hemocytes. The apoLp-III-interacting proteins were recovered from immune hemolymph by affinity chromatography on a Sepharose bed with immobilized anti-apoLp-III antibodies. ApoLp-I, apoLp-II, hexamerin, and arylphorin were identified as main components that bound to apoLp-III; the N-terminal amino acid sequences of G. mellonella apoLp-I and apoLp-II were determined for the first time. In the recovered complexes, the pathogen-dependent differences in the content of individual apolipophorins were detected. Apolipophorins may thus be postulated as signaling molecules responding in an immunogen-dependent manner in early steps of G. mellonella immune response.

Expression of the C-terminal domain of human apolipoprotein A-I using a chimeric apolipoprotein

Human apolipoprotein A-I (apoA-I) is the most abundant protein in high-density lipoprotein, an anti-atherogenic lipid-protein complex responsible for reverse cholesterol transport. The protein is composed of an N-terminal helix bundle domain, and a small C-terminal (CT) domain. To facilitate study of CT-apoA-I, a novel strategy was employed to produce this small domain in a bacterial expression system. A protein construct was designed of insect apolipophorin III (apoLp-III) and residues 179-243 of apoA-I, with a unique methionine residue positioned between the two proteins and an N-terminal His-tag to facilitate purification. The chimera was expressed in E. coli, purified by Ni-affinity chromatography, and cleaved by cyanogen bromide. SDS-PAGE revealed the presence of three proteins with masses of 7 kDa (CT-apoA-I), 18 kDa (apoLp-III), and a minor 26 kDa band of uncleaved chimera. The digest was reloaded on the Ni-affinity column to bind apoLp-III and uncleaved chimera, while CT-apoA-I was washed from the column and collected. Alternatively, CT-apoA-I was isolated from the digest by reversed-phase HPLC. CT-apoA-I was α-helical, highly effective in solubilizing phospholipid vesicles and disaggregating LPS micelles. However, CT-apoA-I was less active compared to full-length apoA-I in protecting lipolyzed low density lipoproteins from aggregating, and disrupting phosphatidylglycerol bilayer vesicles. Thus the novel expression system produced mg quantities of functional CT-apoA-I, facilitating structural and functional studies of this critical domain of apoA-I.

Immunity of the greater wax moth Galleria mellonella

Investigation of insect immune mechanisms provides important information concerning innate immunity, which in many aspects is conserved in animals. This is one of the reasons why insects serve as model organisms to study virulence mechanisms of human pathogens. From the evolutionary point of view, we also learn a lot about host-pathogen interaction and adaptation of organisms to conditions of life. Additionally, insect-derived antibacterial and antifungal peptides and proteins are considered for their potential to be applied as alternatives to antibiotics. While Drosophila melanogaster is used to study the genetic aspect of insect immunity, Galleria mellonella serves as a good model for biochemical research. Given the size of the insect, it is possible to obtain easily hemolymph and other tissues as a source of many immune-relevant polypeptides. This review article summarizes our knowledge concerning G. mellonella immunity. The best-characterized immune-related proteins and peptides are recalled and their short characteristic is given. Some other proteins identified at the mRNA level are also mentioned. The infectious routes used by Galleria natural pathogens such as Bacillus thuringiensis and Beauveria bassiana are also described in the context of host-pathogen interaction. Finally, the plasticity of G. mellonella immune response influenced by abiotic and biotic factors is described.

The lipid composition of Legionella dumoffii membrane modulates the interaction with Galleria mellonella apolipophorin III

Apolipophorin III (apoLp-III), an insect homologue of human apolipoprotein E (apoE), is a widely used model protein in studies on protein-lipid interactions, and anti-Legionella activity of Galleria mellonella apoLp-III has been documented. Interestingly, exogenous choline-cultured Legionella dumoffii cells are considerably more susceptible to apoLp-III than non-supplemented bacteria. In order to explain these differences, we performed, for the first time, a detailed analysis of L. dumoffii lipids and a comparative lipidomic analysis of membranes of bacteria grown without and in the presence of exogenous choline. (31)P NMR analysis of L. dumoffii phospholipids (PLs) revealed a considerable increase in the phosphatidylcholine (PC) content in bacteria cultured on choline medium and a decrease in the phosphatidylethanolamine (PE) content in approximately the same range. The interactions of G. mellonella apoLp-III with lipid bilayer membranes prepared from PLs extracted from non- and choline-supplemented L. dumoffii cells were examined in detail by means of attenuated total reflection- and linear dichroism-Fourier transform infrared spectroscopy. Furthermore, the kinetics of apoLp-III binding to liposomes formed from L. dumoffii PLs was analysed by fluorescence correlation spectroscopy and fluorescence lifetime imaging microscopy using fluorescently labelled G. mellonella apoLp-III. Our results indicated enhanced binding of apoLp-III to and deeper penetration into lipid membranes formed from PLs extracted from the choline-supplemented bacteria, i.e. characterized by an increased PC/PE ratio. This could explain, at least in part, the higher susceptibility of choline-cultured L. dumoffii to G. mellonella apoLp-III.

Backbone and side chain chemical shift assignments of apolipophorin III from Galleria mellonella

Apolipophorin III, a 163 residue monomeric protein from the greater wax moth Galleria mellonella (abbreviated as apoLp-IIIGM), has roles in upregulating expression of antimicrobial proteins as well as binding and deforming bacterial membranes. Due to its similarity to vertebrate apolipoproteins there is interest in performing atomic resolution analysis of apoLp-IIIGM as part of an effort to better understand its mechanism of action in innate immunity. In the first step towards structural characterization of apoLp-IIIGM, 99 % of backbone and 88 % of side chain (1)H, (13)C and (15)N chemical shifts were assigned. TALOS+ analysis of the backbone resonances has predicted that the protein is composed of five long helices, which is consistent with the reported structures of apolipophorins from other insect species. The next stage in the characterization of apoLp-III from G. mellonella will be to utilize these resonance assignments in solving the solution structure of this protein.

Entomopathogenic Fungi: New Insights into Host-Pathogen Interactions

Although many insects successfully live in dangerous environments exposed to diverse communities of microbes, they are often exploited and killed by specialist pathogens. Studies of host-pathogen interactions (HPI) provide valuable insights into the dynamics of the highly aggressive coevolutionary arms race between entomopathogenic fungi (EPF) and their arthropod hosts. The host defenses are designed to exclude the pathogen or mitigate the damage inflicted while the pathogen responds with immune evasion and utilization of host resources. EPF neutralize their immediate surroundings on the insect integument and benefit from the physiochemical properties of the cuticle and its compounds that exclude competing microbes. EPF also exhibit adaptations aimed at minimizing trauma that can be deleterious to both host and pathogen (eg, melanization of hemolymph), form narrow penetration pegs that alleviate host dehydration and produce blastospores that lack immunogenic sugars/enzymes but facilitate rapid assimilation of hemolymph nutrients. In response, insects deploy an extensive armory of hemocytes and macromolecules, such as lectins and phenoloxidase, that repel, immobilize, and kill EPF. New evidence suggests that immune bioactives work synergistically (eg, lysozyme with antimicrobial peptides) to combat infections. Some proteins, including transferrin and apolipophorin III, also demonstrate multifunctional properties, participating in metabolism, homeostasis, and pathogen recognition. This review discusses the molecular intricacies of these HPI, highlighting the interplay between immunity, stress management, and metabolism. Increased knowledge in this area could enhance the efficacy of EPF, ensuring their future in integrated pest management programs.

Galleria mellonella infection models for the study of bacterial diseases and for antimicrobial drug testing

Galleria mellonella (greater wax moth or honeycomb moth) has been introduced as an alternative model to study microbial infections. G. mellonella larvae can be easily and inexpensively obtained in large numbers and are simple to use as they don't require special lab equipment. There are no ethical constraints and their short life cycle makes them ideal for large-scale studies. Although insects lack an adaptive immune response, their innate immune response shows remarkable similarities with the immune response in vertebrates. This review gives a current update of what is known about the immune system of G. mellonella and provides an extensive overview of how G. mellonella is used to study the virulence of Gram-positive and Gram-negative bacteria. In addition, the use of G. mellonella to evaluate the efficacy of antimicrobial agents and experimental phage therapy are also discussed. The review concludes with a critical assessment of the current limitatons of G. mellonella infection models.

Different forms of apolipophorin III in Galleria mellonella larvae challenged with bacteria and fungi

Apolipophorin III (apoLp-III), a lipid-binding protein and an insect homolog of human apolipoprotein E, plays an important role in lipid transport and immune response in insects. In the present study, we have demonstrated a correlation in time between changes in the apoLp-III abundance occurring in the hemolymph, hemocytes, and fat body after immunization of Galleria mellonella larvae with Gram-negative bacteria Escherichia coli, Gram-positive bacteria Micrococcus luteus, yeast Candida albicans, and a filamentous fungus Fusarium oxysporum. Using two-dimensional electrophoresis (IEF/SDS-PAGE) and immunoblotting with anti-apoLp-III antibodies, the profile of apoLp-III forms in G. mellonella larvae challenged with the bacteria and fungi has been analyzed. Besides the major apoLp-III protein (pI=6.5), one and three additional apoLp-III forms differing in the pI value have been detected, respectively, in the hemolymph, hemocytes, and fat body of non-immunized insects. Also, evidence has been provided that particular apoLp-III-derived polypeptides appear after the immune challenge and are present mainly in the hemolymph and hemocytes. The time of their appearance and persistence in the hemolymph was dependent on the pathogen used. At least two of the apoLp-III forms detected in hemolymph bound to the microbial cell surface. The increasing number of hemolymph apoLp-III polypeptides and differences in their profiles observed in time after the challenge with different immunogens confirmed the important role of apoLp-III in discriminating between pathogens by the insect defense system and in antibacterial as well as antifungal immune response.

Molecular characterization of an Apolipophorin-III gene from the Chinese oak silkworm, Antheraea pernyi (Lepidoptera: Saturniidae)

Apolipophorin-III (ApoLp-III) acts in lipid transport, lipoprotein metabolism, and innate immunity in insects. In this study, an ApoLp-III gene of Antheraea pernyi pupae (Ap-ApoLp-III) was isolated and characterized. The full-length cDNA of Ap-ApoLp-III is 687 bp, including a 5'-untranslated region (UTR) of 40 bp, 3'-UTR of 86 bp and an open reading frame of 561 bp encoding a polypeptide of 186 amino acids that contains an Apolipophorin-III precursor domain (PF07464). The deduced Ap-apoLp-III protein sequence has 68, 59, and 23% identity with its orthologs of Manduca sexta, Bombyx mori, and Aedes aegypti, respectively. Phylogenetic analysis showed that the Ap-apoLp-III was close to that of Bombycoidea. qPCR analysis revealed that Ap-ApoLp-III expressed during the four developmental stages and in integument, fat body, and ovaries. After six types of microorganism infections, expression levels of the Ap-ApoLp-III gene were upregulated significantly at different time points compared with control. RNA interference (RNAi) of Ap-ApoLp-III showed that the expression of Ap-ApoLp-III was significantly downregulated using qPCR after injection of E. coli. We infer that the Ap-ApoLp-III gene acts in the innate immunity of A. pernyi.

Comprehensive identification of novel proteins and N-glycosylation sites in royal jelly

Background

Royal jelly (RJ) is a proteinaceous secretion produced from the hypopharyngeal and mandibular glands of nurse bees. It plays vital roles in honeybee biology and in the improvement of human health. However, some proteins remain unknown in RJ, and mapping N-glycosylation modification sites on RJ proteins demands further investigation. We used two different liquid chromatography-tandem mass spectrometry techniques, complementary N-glycopeptide enrichment strategies, and bioinformatic approaches to gain a better understanding of novel and glycosylated proteins in RJ.

Results

A total of 25 N-glycosylated proteins, carrying 53 N-glycosylation sites, were identified in RJ proteins, of which 42 N-linked glycosylation sites were mapped as novel on RJ proteins. Most of the glycosylated proteins were related to metabolic activities and health improvement. The 13 newly identified proteins were also mainly associated with metabolic processes and health improvement activities.

Conclusion

Our in-depth, large-scale mapping of novel glycosylation sites represents a crucial step toward systematically revealing the functionality of N-glycosylated RJ proteins, and is potentially useful for producing a protein with desirable pharmacokinetic and biological activity using a genetic engineering approach. The newly-identified proteins significantly extend the proteome coverage of RJ. These findings contribute vital and new knowledge to our understanding of the innate biochemical nature of RJ at both the proteome and glycoproteome levels.

Genomic organization, sequence characterization and expression analysis of Tenebrio molitor apolipophorin-III in response to an intracellular pathogen, Listeria monocytogenes

Apolipophorin III (apoLp-III) is a well-known hemolymph protein having a functional role in lipid transport and immune response of insects. We cloned full-length cDNA encoding putative apoLp-III from larvae of the coleopteran beetle, Tenebrio molitor (TmapoLp-III), by identification of clones corresponding to the partial sequence of TmapoLp-III, subsequently followed with full length sequencing by a clone-by-clone primer walking method. The complete cDNA consists of 890 nucleotides, including an ORF encoding 196 amino acid residues. Excluding a putative signal peptide of the first 20 amino acid residues, the 176-residue mature apoLp-III has a calculated molecular mass of 19,146Da. Genomic sequence analysis with respect to its cDNA showed that TmapoLp-III was organized into four exons interrupted by three introns. Several immune-related transcription factor binding sites were discovered in the putative 5'-flanking region. BLAST and phylogenetic analyses reveal that TmapoLp-III has high sequence identity (88%) with Tribolium castaneum apoLp-III but shares little sequence homologies (<26%) with other apoLp-IIIs. Homology modeling of Tm apoLp-III shows a bundle of five amphipathic alpha helices, including a short helix 3'. The 'helix-short helix-helix' motif was predicted to be implicated in lipid binding interactions, through reversible conformational changes and accommodating the hydrophobic residues to the exterior for stability. Highest level of TmapoLp-III mRNA was detected at late pupal stages, albeit it is expressed in the larval and adult stages at lower levels. The tissue specific expression of the transcripts showed significantly higher numbers in larval fat body and adult integument. In addition, TmapoLp-III mRNA was found to be highly upregulated in late stages of L. monocytogenes or E. coli challenge. These results indicate that TmapoLp-III may play an important role in innate immune responses against bacterial pathogens in T. molitor.

Proteomic and properties analysis of botanical insecticide rhodojaponin III-induced response of the diamondback moth, Plutella xyllostella (L.)

BACKGROUND

Rhodojaponin III, as a botanical insecticide, affects a wide variety of biological processes in insects, including reduction of feeding, suspension of development, and oviposition deterring of adults in a dose-dependent manner. However, the mode of these actions remains obscure.

PRINCIPAL FINDINGS

In this study, a comparative proteomic approach was adopted to examine the effect of rhodojaponin III on the Plutella xyllostella (L.). Following treating 48 hours, newly emergence moths were collected and protein samples were prepared. The proteins were separated by 2-DE, and total 31 proteins were significantly affected by rhodojaponin III compared to the control identified by MALDI-TOF/TOF-MS/MS. These differentially expressed proteins act in the nervous transduction, odorant degradation and metabolic change pathways. Further, gene expression patterns in treated and untreated moths were confirmed by qRT-PCR and western blot analysis. RNAi of the chemosensory protein (PxCSP) gene resulted in oviposition significantly increased on cabbage plants treated with rhodojaponin III.

CONCLUSIONS

These rhodojaponin III-induced proteins and gene properties analysis would be essential for a better understanding of the potential molecular mechanism of the response to rhodojaponin III from moths of P. xylostella.

Characterization of the apoLp-III/LPS complex: insight into the mode of binding interaction

Apolipoproteins are able to associate with lipopolysaccharides (LPS), potentially providing protection against septic shock. To gain insight into the molecular details of this binding interaction, apolipophorin III (apoLp-III) from Galleria mellonella was used as a model. The binding of apoLp-III to LPS was optimal around 37-40 °C, close to the LPS phase transition temperature. ApoLp-III formed complexes with LPS from E. coli (serotype O55:B5) with a diameter of ~20 nm and a molecular weight of ~390 kDa, containing four molecules of apoLp-III and 24 molecules of LPS. The LPS-bound form of the protein was substantially more resistant to guanidine-induced denaturation compared to unbound protein. The denaturation profile displayed a multiphase character with a steep drop in secondary structure between 0 and 1 M guanidine-HCl and a slower decrease above 1 M guanidine-HCl. In contrast, apoLp-III bound to detoxified LPS was only slightly more resistant to guanidine-HCl induced denaturation compared to unbound protein. Analysis of size-exclusion FPLC elution profiles of mixtures of apoLp-III with LPS or detoxified LPS indicated a much weaker binding interaction with detoxified LPS compared to intact LPS. These results indicate that apoLp-III initially interacts with exposed carbohydrate regions, but that the lipid A region is required for a more stable LPS binding interaction.

SERINE proteinase like activity in apolipophorin III from the hemolymph of desert locust, Schistocerca gregaria

Apolipophorin III (apoLp-III) has been known as a lipid transport protein of insects. Recent studies indicated the involvement of apoLp-III in immune reactions and in the control of cell destruction, but no enzymatic activity has so far been detected. In the present study, a protease from the hemolymph of Schistocerca gregaria was purified to homogeneity and its enzymatic activity was examined. Identity as chymotrypsin-like proteinase was established by its high affinity toward bulky aromatic substrates and its catalytic specificity for amide or ester bonds on the synthetic substrates, Suc-Ala-Ala-Pro-Xaa-AMC (where Xaa was Phe, Tyr, Trp, and Lys, and AMC is 7-amino-4-methyl-coumarin) and thiolbenzyl ester substrate Suc-Ala-Ala-Pro-Phe-SBzl. The sensitivity for serine protease and chymotrypsin-specific covalent inhibitors, PMSF, TPCK, and noncovalent inhibitors SGCI, showed that it is a chymotrypsin-like proteinase. It showed its maximum activity at pH 8.0 and 55°C for the hydrolysis of Suc-Ala-Ala-Pro-Tyr-AMC. According to similarities in the amino terminal sequence, molar mass (19 kDa) and retention on reversed-phase analytical high-performance liquid chromatography (HPLC) column, this protein is S. gregaria homologue of Locusta migratoria apoLp-III. Our data suggest that apoLp-III also has an inherent proteolytic activity. Results indicated that S. gregaria apoLp-III is a good catalyst and could be used as a biotechnological tool in food processing and in agricultural biotechnology.

Molecular characterization and gene expression of apolipophorin III from the ghost moth, Thitarodes pui (Lepidoptera, Hepialidae)

Apolipophorin III (apoLp-III) functions in lipid transport and immune activation in insects. We cloned a cDNA encoding putative apoLp-III from larvae of Thitarodes pui, a host species of Ophiocordyceps sinensis, with great economic importance in the Tibetan Plateau. Excluding a putative signal peptide of the first 20 amino acid residues, the 171-residue mature apoLp-III has a calculated molecular mass of 18,606 Da. T. pui apoLp-III shares little sequence homologies (<36%) with other apoLp-IIIs. Phylogenetic analysis reveals that T. pui apoLp-III belongs to a distinct, early diverging lineage of lepidopteran apoLp-IIIs. Homology modeling of T. pui apoLp-III shows a bundle of five amphipathic α-helices, including a short helix 3'. T. pui apoLp-III was constitutively expressed in larval fat body at lower levels than pupal and adult fat body. Significant induction of apoLp-III expression, associated with strongest nodulation response, was observed in both sixth and eighth instar larvae challenged with Beauveria bassiana conidia at 1 hr after inoculation, compared with saline-injected controls. The inoculation experiment as well as previous field studies revealed the relative susceptibility of the sixth instar to the entomopathogenic fungus. ApoLp-III transcripts in the infected sixth and eighth instars were found to be induced highest 2- and 14.7-fold, respectively, during the first 12 hr. In late-stage infection, the infected susceptible sixth instar showed decrease in apoLp-III expression followed by production of B. bassiana hyphal bodies, whereas the infected eighth instar showed longer lasting increase in the expression. These results suggest that apoLp-III might contribute to T. pui immune response against fungal pathogens.

Effect of Pseudomonas aeruginosa elastase B on level and activity of immune proteins/peptides of Galleria mellonella hemolymph

Abstract Susceptibility of proteins and peptides present in immune hemolymph of Galleria mellonella Fabricius (Lepidoptera: Pyralidae) larvae to proteolytic degradation by purified elastase B of Pseudomonas aeruginosa was studied. Results showed that apoLp-III protein was gradually digested by elastase B in vitro. Additionally, polipeptides with molecular mass 6.5 and 4 kDa were degraded after treatment with the studied enzyme. The lack of these peptides and the decrease in anti-Escherichia coli activity could indicate that inducible antimicrobial peptides were digested by elastase B. On the contrary, no change in the lysosome activity level was observed in immune hemolymph incubated with elastase B. Thus, elastase B might contribute to the pathogenesis of P. aeruginosa.

The effect of Galleria mellonella apolipophorin III on yeasts and filamentous fungi

Galleria mellonella apolipophorin III (apoLp-III) has been implicated in the innate immune response against bacterial infections. The protein binds components of bacterial cell wall and inhibits growth of selected Gram-positive and Gram-negative bacteria. Interaction of apoLp-III with fungal β-1,3-glucan suggests antifungal properties of the protein. In the present study, the effect of apoLp-III on the growth, metabolic activity and cell surface characteristics of selected yeasts and filamentous fungi was investigated using light, confocal and atomic force microscopy. ApoLp-III bound to the cell surface of different yeasts and filamentous fungi as confirmed by immunoblotting with anti-apoLp-III antibodies. Incubation of the fungi in the presence of apoLp-III induced alterations in growth morphology. Candida albicans underwent transition from yeast-like to hyphal growth with formation of true hyphae, whereas Fusarium oxysporum hyphae exhibited decreased metabolic activity, increased vacuolization and appearance of numerous monophialids with microconidia. Atomic force microscopy imaging demonstrated evident alterations in the fungal cell surface after incubation with apoLp-III, suggesting that the protein affected the cell wall components.

RNA interference mediated knockdown of apolipophorin-III leads to knockdown of manganese superoxide dismutase in Hyphantria cunea

Apolipophorin-III (apoLp-III), a hemolymph protein that facilitates lipid transport in aqueous media in insects was recently shown to play a role in insect immune activation. Here, we report another novel possible function of apoLp-III in insects. To identify genes affected by apoLp-III expression in larvae, we decreased endogenous apoLp-III mRNA in Hyphantria cunea (Hc) through RNA interference; subsequently, we observed lower levels of antioxidant enzymes, including manganese superoxide dismutase (MnSOD), glutathione S-transferase, and immune proteins. Knockdown of Hc apoLp-III led to decreased MnSOD expression in fat body tissues and elevated superoxide anion levels in Hc fat body cells, suggesting that Hc apoLp-III is involved in the action and/or expression of antioxidant enzymes, especially MnSOD.

Apolipophorin-III mediates antiplasmodial epithelial responses in Anopheles gambiae (G3) mosquitoes

Background

Apolipophorin-III (ApoLp-III) is known to play an important role in lipid transport and innate immunity in lepidopteran insects. However, there is no evidence of involvement of ApoLp-IIIs in the immune responses of dipteran insects such as Drosophila and mosquitoes.

Methodology/Principal Findings

We report the molecular and functional characterization of An. gambiae apolipophorin-III (AgApoLp-III). Mosquito ApoLp-IIIs have diverged extensively from those of lepidopteran insects; however, the predicted tertiary structure of AgApoLp-III is similar to that of Manduca sexta (tobacco hornworm). We found that AgApoLp-III mRNA expression is strongly induced in the midgut of An. gambiae (G3 strain) mosquitoes in response to Plasmodium berghei infection. Furthermore, immunofluorescence stainings revealed that high levels of AgApoLp-III protein accumulate in the cytoplasm of Plasmodium-invaded cells and AgApoLp-III silencing increases the intensity of P. berghei infection by five fold.

Conclusion

There are broad differences in the midgut epithelial responses to Plasmodium invasion between An. gambiae strains. In the G3 strain of An. gambiae AgApoLp-III participates in midgut epithelial defense responses that limit Plasmodium infection.

Immunosuppression induced by entomopathogens is rescued by addition of apolipophorin III in the diamondback moth, Plutella xylostella

Apolipophorin III (ApoLpIII) has been known to play critical roles in lipid transport and immune activation in insects. This study reports a partial ApoLpIII gene cloned from the diamondback moth, Plutella xylostella. It showed that the gene was expressed in all developmental stages of P. xylostella. In larval stage, it was expressed in all tested tissues of hemocyte, fat body, gut, and epidermis. In response to bacterial challenge, the larvae showed an enhanced level of ApoLpIII expression by a quantitative real-time RT-PCR. RNA interference of ApoLpIII by its specific double stranded RNA (dsRNA) caused significant knockdown of its expression level and resulted in significant suppression in hemocyte nodule formation in response to bacterial challenge. However, larvae treated with the dsRNA exhibited a significant recovery in the cellular immune response by addition of a recombinant ApoLpIII. Parasitization by an endoparasitoid wasp, Cotesia plutellae, suppressed expression of ApoLpIII and resulted in a significant suppression in the hemocyte nodule formation. The addition of the recombinant ApoLpIII to the parasitized larvae significantly restored the hemocyte activity. Infection of an entomopathogenic bacterium, Xenorhabdus nematophila, caused potent pathogenicity of P. xylostella. However, the addition of the recombinant ApoLpIII to the infected larvae significantly prevented the lethal pathogenicity. This study suggests that ApoLpIII limits pathogenicity induced by parasitization or bacterial infection in P. xylostella.

Role of adhesion in arthropod immune recognition

The recognition and inactivation of toxins and pathogens are mediated by a combination of cell-free and cellular mechanisms. A number of soluble and membrane-bound pattern recognition molecules interact with elicitors to become involved in both cell-free inactivation as well as cellular uptake reactions. Here we describe the possible recognition and effector function of key arthropod immune proteins, such as peroxinectin, hemolin, and hemomucin, as an outcome of changes in adhesiveness, which drive self-assembly reactions leading to cell-free coagulation and cellular uptake reactions. The fact that some of these proteins are essential for immune and developmental functions in some species, but are not found in closely related species, may point to the existence of multiprotein assemblies, which are conserved at the mechanistic level and can function with more than one combination of protein constituents.

Illness-induced anorexia and its possible function in the caterpillar, Manduca sexta

Although many animals exhibit illness-induced anorexia when immune-challenged, the adaptive significance of this behavior remains unclear. Injecting Manduca sexta larvae (caterpillars) with live bacteria (Serratia marcescens), heat-killed bacteria or bacterial lipopolysaccharides resulted in a decline in feeding, demonstrating illness-induced anorexia in this species. We used M. sexta to test four commonly suggested adaptive functions for illness-induced anorexia. (1) Food deprivation did not reduce the iron content of the hemolymph. (2) Immune-challenged M. sexta were not more likely to move to a different part of the plant. Therefore, the decline in feeding is unlikely to be an adaptive response allowing the animal to move away from a patch of contaminated food. (3) M. sexta force-fed S. marcescens bacteria were not more susceptible to a S. marcescens systemic infection than were M. sexta force-fed nutrient broth. (4) Force-feeding infected M. sexta during illness-induced anorexia did not increase mortality and short-term food deprivation did not enhance survival. However, force-feeding M. sexta with a high lipid diet (linseed oil and water) resulted in an increase in mortality when challenged with S. marcescens. Force-feeding sucrose or water did not reduce resistance. Force-feeding a high lipid diet into healthy animals did not reduce weight gain, suggesting that it was not toxic. We hypothesize that there is a conflict between lipid metabolism and immune function, although whether this conflict has played a role in the evolution of illness-induced anorexia remains unknown. The adaptive function of illness-induced anorexia requires further study in both vertebrates and invertebrates.

Hemocytes ultrastructure of Aedes aegypti (Diptera: Culicidae)

Mosquitoes have an efficient defence system against infection. Insect blood cells (hemocytes) play an essential role in defense against parasites and other pathogenic organisms that infect insects. We have identified by light and transmission electron microscopy six hemocytes cell types from the hemolymph of Aedes aegypti. They were: prohemocytes (20%), adipohemocytes (29%), granulocytes (16%), plasmatocytes (27%), oenocytoids (7%) and thrombocytoids (0.9%). The prohemocytes were the smallest hemocytes found in the hemolymph. Its cytoplasm occupies only a narrow area around the nucleus. The adipohemocytes were the most abundant cell type presented. These hemocytes exhibited a large lipid like vesicle and mitochondria. In electron micrographs, the granulocytes showed cytoplasm containing dilated rough endoplasmic reticulum (RER) and a round or elongated mitochondria. Electron-dense granules with a proteinaceous material were also present. The plasmatocytes were polymorphic and exhibited plasma membrane with irregular processes, philopodia and pseudopodia. Ultrastructural investigation revealed that the reticular cytoplasm showed a well-developed RER, a Golgi and vacuoles. Oenocytoids showed homogeneous cytoplasm with many mitochondria and ribosomes are scattered throughout the cytoplasm, abundant RER and a small smooth endoplasmic reticulum (SER) present at the cell poles. Thrombocytoids were very fragile and few in number. Similar characteristics were found in oenocytoids, possessing a homogeneous cytoplasm with poorly developed organelles, few mitochondria and granules.

Apolipophorin III: lipopolysaccharide binding requires helix bundle opening

Apolipophorin III (apoLp-III) is a prototypical apolipoprotein used for structure-function studies. Besides its crucial role in lipid transport, apoLp-III is able to associate with fungal and bacterial membranes and stimulate cellular immune responses. We recently demonstrated binding interaction of apoLp-III of the greater wax moth, Galleria mellonella, with lipopolysaccharides (LPS). In the present study, the requirement of helix bundle opening for LPS binding interaction was investigated. Using site-directed mutagenesis, two cysteine residues were introduced in close spatial proximity (P5C/A135C). When the helix bundle was locked by disulfide bond formation, the tethered helix bundle failed to associate with LPS. In contrast, the mutant protein regained its ability to bind upon reduction with dithiothreitol. Thus, helix bundle opening is a critical event in apoLp-III binding interaction with LPS. This mechanism implies that the hydrophobic interior of the protein interacts directly with LPS, analogous to that observed for lipid interaction.

Tyrosine fluorescence analysis of apolipophorin III–lipopolysaccharide interaction

Apolipophorin III (apoLp-III) is an exchangeable apolipoprotein that binds to lipopolysaccharides (LPS). Polyacrylamide gel electrophoresis analysis demonstrated that apoLp-III from Galleria mellonella associated with various truncated LPS variants, including lipid A. Subsequent binding studies were performed employing the intrinsic tyrosine fluorescence properties of apoLp-III, which is highly quenched in the unbound state. A marked increase in tyrosine fluorescence intensity was observed upon binding to LPS or detoxified LPS, indicating a new microenvironment for Tyr-142. This also implies that the LPS carbohydrate region is involved in LPS binding. Dissociation constants (Kd) measured by apoLp-III titration were estimated at approximately 1 microM. Increasing the ionic strength did not decrease the Kd, neither did LPS phosphate removal. In addition, truncation apoLp-III mutants, lacking two complete helices, were still able to associate with LPS. This indicates that the association of apoLp-III with LPS may not be governed by charge but by hydrophobic interactions.

Down-regulation of antibacterial peptide synthesis in an insect model induced by the body-surface of an entomoparasite (Steinernema feltiae)

This study focuses on the interaction between an entomopathogenic nematode and an insect model to further investigate the role of parasite body-surface and its compounds (particularly epicuticular lipids) in the immunosuppression of host defences. Our goal was to ascertain the potential interference of the parasite epicuticular lipids with the antimicrobial response mounted by Gram-negative bacteria-challenged hosts. Since the parasite model used in this study (Steinernema feltiae) releases symbiontic bacteria in the host hemocoel during the late phase of infection, the inhibition of the antimicrobial response could be needed to avoid the activity of host factors potentially harmful for the microorganisms. After bacterial challenge, when insect hosts were infected with purified parasite cuticles, we always observed lack of bacterial clearance concurrently with the absence of hemolymph low molecular weight components. The observed effects seem to be related to the interaction of parasite cuticular lipids (PCLs) with specific components of the host hemolymph; these host interacting proteins (HIP17, HIP26 and HIP35) were removed by the parasite, and their absence (or reduction) apparently prevented antimicrobial peptide synthesis. The inhibitory properties were lost when cuticles were pre-treated with compounds (such as lipase or methanol-chloroform) affecting their lipidic moiety. Moreover, the key role of epicuticular lipids was also confirmed by the inhibitory properties of methanol-chloroform extracted lipids, which were comparable to those of parasite whole cuticles. Finally, the involvement of HIPs was assessed by their partial purification followed by injection into the host. When HIPs were co-injected with bacteria into cuticle-inhibited larvae, the antimicrobial activity was completely restored.

Immune recognition of fungal β-glucans

The recognition of conserved microbial structures is a key aspect of metazoan immunity, and beta-glucans are emerging as a major target for the recognition of fungal pathogens. A number of receptors for these carbohydrates have been identified, which upon recognition, trigger a variety of immune responses. In contrast to many other systems, there is little apparent conservation in these mechanisms between vertebrates and invertebrates. In this review, we will highlight all the known receptors for beta-glucans and will discuss the various immune responses they can initiate, with reference to fungal infection, in both vertebrates and invertebrates.

Apolipophorin III is a substrate for protease IV fromPseudomonas aeruginosa

Our results demonstrated that Pseudomonas aeruginosa serine protease IV degraded apolipophorin III from the haemolymph of Galleria mellonella larvae. ApoLp-III protein was degraded in a stepwise manner. Four intermediate forms of 15, 13.3, 11.9 and 9.5 kDa were detected after 30 min digestion while only one of 5.6 kDa was released after 1-h incubation time. N-terminal amino acid sequence analysis of 5.6 kDa peptide revealed that it was released from apoLp-III after cleavage between lysine 70 and 71. ApoLp-III degradation by protease IV was inhibited by 1 mM TLCK but not 1 mM EDTA, additionally demonstrating that digestion was catalysed by a serine protease. Our data also indicated apoLp-III degradation in vivo during P. aeruginosa infection of G. mellonella larvae.

Effects of two hemolymph proteins on humoral defense reactions in the wax moth, Galleria mellonella

Two hemolymph proteins were isolated from the wax moth, Galleria mellonella, larvae by a two-step procedure consisting of acid extraction and reversed phase (RP)-HPLC. One was an apolipophorin III (apoLp-III) previously characterized as a lipopolysaccharide (LPS) binding protein in the hemolymph of G. mellonella. The other was confirmed to be a new protein with a molecular mass of 23,768.69 Da, referred to as Gm protein-24. The full-length cDNA of Gm protein-24 was cloned from the fat body. The cDNA structure showed that it is a 219-residues protein derived from the precursor of 236 amino acids. The effects of apoLp-III and Gm protein-24 have been tested on the insect humoral immunity. ApoLp-III enhanced the activity of antibacterial peptide such as cecropin but Gm protein-24 had no effect on cecropin activity. On the other hand, Gm protein-24 and apoLp-III were both involved in the activation of prophenoloxidase (PPO) cascade, which has been regarded as a critical immune reaction in insect hemolymph. Of note, the Gm protein-24 was a significantly stronger activator of PPO cascade than apoLp-III.