Humoral immune response of Galleria mellonella after repeated infection with Bacillus thuringiensis

Identification of function modules in the co-expression protein-protein interaction network of Bombyx mori in response to Beauveria bassiana infection

Beauveria bassiana (B. bassiana) is a common fungal disease in sericulture. Previous research has primarily focused on investigating genes involved in innate immunity. However, the response of Bombyx mori (B. mori) to B. bassiana requires the coordination of other biological processes in addition to the immune system. We measured protein expression profile of B. mori after inoculating B. bassiana using iTRAQ technology in previous. Here we constructed a co-expression protein-protein interaction network of B. mori in response to B. bassiana infection. Subnetworks and modules were analyzed, and the functions of these modules were annotated. The results revealed the identification of numerous proteins associated with cellular immunity, including those involved in phagosomes, lysosomes, mTOR signaling, sugar metabolism, and the ubiquitin-proteasome pathway. Meanwhile, we observed that the pathways involved in protein synthesis were activated, including pyruvate and purine metabolism, RNA transport, ribosome, protein processing in endoplasmic reticulum, and protein export pathways, during B. bassiana infection. Based on this analysis, we selected six candidate genes (shock protein, ribosome, translocon, actin muscle-type A2, peptidoglycan recognition protein, and collagenase) that were found to be related to the response to B. bassiana. Further verification experiments demonstrated significant changes in their expression levels after inoculation with B. bassiana. These research findings provide new insights into the molecular mechanism of insect immune response to fungal infection.

Development of a Galleria mellonella Infection Model to Evaluate the Efficacy of Antibiotic-Loaded Polymethyl Methacrylate (PMMA) Bone Cement

Prosthetic joint infections (PJIs) can have disastrous consequences for patient health, including removal of the device, and placement of cemented implants is often required during surgery to eradicate PJIs. In translational research, in vivo models are widely used to assess the biocompatibility and antimicrobial efficacy of antimicrobial coatings and compounds. Here, we aim to utilize Galleria mellonella implant infection models to assess the antimicrobial activity of antibiotic-loaded bone cement (ALBC) implants. Therefore, we used commercially available bone cement loaded with either gentamicin alone (PALACOS R+G) or with a combination of gentamicin and vancomycin (COPAL G+V), compared to bone cement without antibiotics (PALACOS R). Firstly, the in vitro antimicrobial activity of ALBC was determined against Staphylococcus aureus. Next, the efficacy of ALBC implants was analyzed in both the G. mellonella hematogenous and early-stage biofilm implant infection model, by monitoring the survival of larvae over time. After 24 h, the number of bacteria on the implant surface and in the tissue was determined. Larvae receiving dual-loaded COPAL G+V implants showed higher survival rates compared to implants loaded with only gentamicin (PALACOS R+G) and the control implants without antibiotics (PALACOS R). In conclusion, G. mellonella larvae infection models with antibiotic-loaded bone cements are an excellent option to study (novel) antimicrobial approaches.

The plasticity of immune memory in invertebrates

Whether specific immune protection after initial pathogen exposure (immune memory) occurs in invertebrates has long been uncertain. The absence of antibodies, B-cells and T-cells, and the short lifespans of invertebrates led to the hypothesis that immune memory does not occur in these organisms. However, research in the past two decades has supported the existence of immune memory in several invertebrate groups, including Ctenophora, Cnidaria, Nematoda, Mollusca and Arthropoda. Interestingly, some studies have demonstrated immune memory that is specific to the parasite strain. Nonetheless, other work does not provide support for immune memory in invertebrates or offers only partial support. Moreover, the expected biphasic immune response, a characteristic of adaptive immune memory in vertebrates, varies within and between invertebrate species. This variation may be attributed to the influence of biotic or abiotic factors, particularly parasites, on the outcome of immune memory. Despite its critical importance for survival, the role of phenotypic plasticity in immune memory has not been systematically examined in the past two decades. Additionally, the features of immune responses occurring in diverse environments have yet to be fully characterized.

Immune priming modulates Galleria mellonella and Pseudomonas entomophila interaction. Antimicrobial properties of Kazal peptide Pr13a

Galleria mellonella larvae repeatedly infected with Pseudomonas entomophila bacteria re-induced their immune response. Its parameters, i.e. the defence activities of cell-free hemolymph, the presence and activity of antimicrobial peptides, and the expression of immune-relevant genes were modulated after the re-challenge in comparison to non-primed infected larvae, resulting in better protection. No enhanced resistance was observed when the larvae were initially infected with other microorganisms, and larvae pre-infected with P. entomophila were not more resistant to further infection with other pathogens. Then, the peptide profiles of hemolymph from primed- and non-primed larvae infected with P. entomophila were compared by quantitative RP-HPLC (Reverse Phase - High Performance Liquid Chromatography). The level of carbonic anhydrase, anionic peptide-1, proline peptide-2, and finally, unknown so far, putative Kazal peptide Pr13a was higher in the primed infected animals than in the larvae infected with P. entomophila for the first time. The expression of the Pr13a gene increased two-fold after the infection, but only in the primed animals. To check whether the enhanced level of Pr13a could have physiological significance, the peptide was purified to homogeneity and checked for its defence properties. In fact, it had antibacterial activity: at the concentration of 15 µM and 7.5 µM it reduced the number of P. entomophila and Bacillus thuringiensis CFU, respectively, to about 40%. The antibacterial activity of Pr13a was correlated with changes observed on the surface of the peptide-treated bacteria, e.g. surface roughness and adhesion force. The presented results bring us closer to finding hemolymph constituents responsible for the effect of priming on the immune response in re-infected insects.

Defence response of Galleria mellonella larvae to oral and intrahemocelic infection with Pseudomonasentomophila

We report differences in the course of infection of G. mellonella larvae with P. entomophila via intrahemocelic and oral routes. Survival curves, larval morphology, histology, and induction of defence response were investigated. Larvae injected with 10 and 50 cells of P. entomophila activated a dose-dependent immune response, which was manifested by induction of immune-related genes and dose-dependent defence activity in larval hemolymph. In contrast, after the oral application of the pathogen, antimicrobial activity was detected in whole hemolymph of larvae infected with the 103 but not 105 dose in spite of the induction of immune response manifested as immune-relevant gene expression and defence activity of electrophoretically separated low-molecular hemolymph components. Among known proteins induced after the P. entomophila infection, we identified proline-rich peptide 1 and 2, cecropin D-like peptide, galiomycin, lysozyme, anionic peptide 1, defensin-like peptide, and a 27 kDa hemolymph protein. The expression of the lysozyme gene and the amount of protein in the hemolymph were correlated with inactivity of hemolymph in insects orally infected with a higher dose of P. entomophila, pointing to its role in the host-pathogen interaction.

Involvement of an Enhanced Immunity Mechanism in the Resistance to Bacillus thuringiensis in Lepidopteran Pests

Bacillus thuringiensis (Bt) is the safest, economically successful entomopathogen to date. It is extensively produced in transgenic crops or used in spray formulations to control Lepidopteran pests. The most serious threat to the sustainable usage of Bt is insect resistance. The resistance mechanisms to Bt toxins depend not only on alterations in insect receptors, but also on the enhancement of insect immune responses. In this work, we review the current knowledge of the immune response and resistance of insects to Bt formulations and Bt proteins, mainly in Lepidopteran pests. We discuss the pattern recognition proteins for recognizing Bt, antimicrobial peptides (AMPs) and their synthetic signaling pathways, the prophenoloxidase system, reactive oxygen species (ROS) generation, nodulation, encapsulation, phagocytosis, and cell-free aggregates, which are involved in immune response reactions or resistance to Bt. This review also analyzes immune priming, which contributes to the evolution of insect resistance to Bt, and puts forward strategies to improve the insecticidal activity of Bt formulations and manage insect resistance, targeting the insect immune responses and resistance.

Entomopathogenic potential of bacteria associated with soil-borne nematodes and insect immune responses to their infection

Soil-borne nematodes establish close associations with several bacterial species. Whether they confer benefits to their hosts has been investigated in only a few nematode-bacteria systems. Their ecological function, therefore, remains poorly understood. In this study, we isolated several bacterial species from rhabditid nematodes, molecularly identified them, evaluated their entomopathogenic potential on Galleria mellonella larvae, and measured immune responses of G. mellonella larvae to their infection. Bacteria were isolated from Acrobeloides sp., A. bodenheimeri, Heterorhabditis bacteriophora, Oscheius tipulae, and Pristionchus maupasi nematodes. They were identified as Acinetobacter sp., Alcaligenes sp., Bacillus cereus, Enterobacter sp., Kaistia sp., Lysinibacillus fusiformis, Morganella morganii subsp. morganii, Klebsiella quasipneumoniae subsp. quasipneumoniae, and Pseudomonas aeruginosa. All bacterial strains were found to be highly entomopathogenic as they killed at least 53.33% G. mellonella larvae within 72h post-infection, at a dose of 106 CFU/larvae. Among them, Lysinibacillus fusiformis, Enterobacter sp., Acinetobacter sp., and K. quasipneumoniae subsp. quasipneumoniae were the most entomopathogenic bacteria. Insects strongly responded to bacterial infection. However, their responses were apparently little effective to counteract bacterial infection. Our study, therefore, shows that bacteria associated with soil-borne nematodes have entomopathogenic capacities. From an applied perspective, our study motivates more research to determine the potential of these bacterial strains as biocontrol agents in environmentally friendly and sustainable agriculture.

Characterization and functional analysis of a myeloid differentiation factor 88 in Ostrinia furnacalis Guenée larvae infected by Bacillus thuringiensis

Myeloid differentiation factor 88 (MyD88) is a pivotal adapter protein involved in activating nuclear factor NF-κB of the Toll pathway in insect innate immunity. MyD88 has been extensively studied in vertebrates and Drosophila. However, the information ascribed to MyD88 in Lepidoptera is scarce. In the present study, an Ostrinia furnacalis MyD88 (OfMyD88) cDNA was cloned and functionally characterized (GenBank accession no. MN906311). The complete cDNA sequence of OfMyD88 is 804 bp, and contains a 630 bp open reading frame encoding 209 amino acid residues. OfMyD88 has the death domain (DD), an intermediate domain, and the Toll/interleukin 1 receptor (TIR) domain. OfMyD88 was widely expressed in immune-related tissues such as hemocytes, fat body, midgut, and integument, with the highest expression level in hemocytes, and the lowest expression level in integument. To clarify the immune function of MyD88, O. furnacalis larvae were challenged with Bacillus thuringiensis (Bt) through feeding. Bt oral infection had significantly up-regulated the expression of OfMyD88 and immune genes, including PPO2 (prophenoloxidase 2), Attacin, Gloverin, Cecropin, Moricin, GRP3 (β-1, 3-Glucan recognition protein 3), and Lysozyme, and increased the activities of PO and lysozyme in hemolymph of O. furnacalis larvae. Knockdown of OfMyD88 by RNA interference suppressed the expression levels of immune related genes, but not PPO2 in the larvae orally infected with Bt, suggesting that OfMyD88 is involved in defending against Bt invasion through the Toll signaling pathway, but does not affect the PPO expression in O. furnacalis larvae.

Pre-exposure to Candida albicans induce trans-generational immune priming and gene expression of Musca domestica

Insects have the phenomenon of immune priming by which they can have enhanced protection against reinfection with the same pathogen, and this immune protection can be passed on to their offspring, which is defined as “trans-generational immune priming (TGIP).” But whether housefly possesses TGIP is still unclear. Therefore, we used the housefly as the insect model and Candida albicans as the pathogen to explore whether the housefly is capable of eliciting TGIP, and RNA sequencing (RNA-seq) was performed to explore the molecular mechanism of TGIP of the housefly. We found that the housefly possesses TGIP, and adults pre-exposed to heat-killed C. albicans could confer protection to itself and its offspring upon reinfection with a lethal dose of C. albicans. RNA-seq results showed that 30 and 154 genes were differentially expressed after adults were primed with heat-killed C. albicans (CA-A) and after offspring larvae were challenged with a lethal dose of C. albicans (CA-CA-G), respectively. Among the differentially expressed genes (DEGs), there were 23 immune genes, including 6 pattern recognition receptors (PRRs), 7 immune effectors, and 10 immunoregulatory molecules. More importantly, multiple DEGs were involved in the Toll signaling pathway and phagosome signaling pathway, suggesting that the Toll signaling pathway and phagocytosis might play important roles in the process of TGIP of housefly to C. albicans. Our results expanded on previous studies and provided parameters for exploring the mechanism of TGIP.

Efficient Oral Priming of Tenebrio molitor Larvae Using Heat-Inactivated Microorganisms

Microbial resistance is a global health problem that will increase over time. Advances in insect antimicrobial peptides (AMPs) offer a powerful new approach to combat antimicrobial resistance. Invertebrates represent a rich group of animals for the discovery of new antimicrobial agents due to their high diversity and the presence of adaptive immunity or “immune priming”. Here, we report a priming approach for Tenebrio molitor that simulates natural infection via the oral route. This oral administration has the advantage of minimizing the stress caused by conventional priming techniques and could be a viable method for mealworm immunity studies. When using inactivated microorganisms for oral priming, our results showed an increased survival of T. molitor larvae after exposure to various pathogens. This finding was consistent with the induction of antimicrobial activity in the hemolymph of primed larvae. Interestingly, the hemolymph of larvae orally primed with Escherichia coli showed constitutive activity against Staphylococcus aureus and heterologous activity for other Gram-negative bacteria, such as Salmonella enterica. The priming of T. molitor is generally performed via injection of the microorganism. To our knowledge, this is the first report describing the oral administration of heat-inactivated microorganisms for priming mealworms. This technique has the advantage of reducing the stress that occurs with the conventional methods for priming vertebrates.

Innate immune memory in invertebrates: Concept and potential mechanisms

Invertebrates are the protagonists of a recent paradigm shift because they now show that vertebrates are not the only group with immune memory. This review discusses the concept of immune priming, its characteristics, and differences with trained immunity and immune enhancement. We include an update of the current status of immune priming within generations in different groups of invertebrates which now include work in 5 Phyla: Ctenophora, Cnidaria, Mollusca, Nematoda, and Arthropoda. Clearly, few Phyla have been studied. We also resume and discuss the effector mechanism related to immune memory, including integrating viral elements into the genome, endoreplication, and epigenetics. The roles of other elements are incorporated, such as hemocytes, immune pathways, and metabolisms. We conclude that taking care of the experimental procedure will discern if results provide or do not support the invertebrates' immune memory and that regarding mechanisms, indeed, there are no studies on the immune memory mechanisms, this is how specificity is reached, and how and where the immune memory is stored and how is recall upon subsequent encounters. Finally, we discuss the possibility of having more than one mechanism working in different groups of invertebrates depending on the environmental conditions.

Why and how do protective symbionts impact immune priming with pathogens in invertebrates?

Growing evidence demonstrates that invertebrates display adaptive-like immune abilities, commonly known as "immune priming". Immune priming is a process by which a host improves its immune defences following an initial pathogenic exposure, leading to better protection after a subsequent infection with the same - or different - pathogens. Nevertheless, beneficial symbionts can enhance similar immune priming processes in hosts, such as when they face repeated infections with pathogens. This "symbiotic immune priming" protects the host against pathogenic viruses, bacteria, fungi, or eukaryotic parasites. In this review, we explore the extent to which protective symbionts interfere and impact immune priming against pathogens from both a mechanical (proximal) and an evolutionary (ultimate) point of view. We highlight that the immune priming of invertebrates is the cornerstone of the tripartite interaction of hosts/symbionts/pathogens. The main shared mechanism of immune priming (induced by symbionts or pathogens) is the sustained immune response at the beginning of host-microbial interactions. However, the evolutionary outcome of immune priming leads to a specific discrimination, which provides enhanced tolerance or resistance depending on the type of microbe. Based on several studies testing immune priming against pathogens in the presence or absence of protective symbionts, we observed that both types of immune priming could overlap and affect each other inside the same hosts. As protective symbionts could be an evolutionary force that influences immune priming, they may help us to better understand the heterogeneity of pathogenic immune priming across invertebrate populations and species.

Establishing an invertebrate Galleria mellonella greater wax moth larval model of Neisseria gonorrhoeae infection

Neisseria gonorrhoeae (gonococcus) causes the human sexually transmitted disease gonorrhea. Studying gonococcal pathogenesis and developing new vaccines and therapies to combat the increasing prevalence of multi-antibiotic resistant bacteria has made use of many ex vivo models based on human cells and tissues, and in vivo vertebrate models, for example, rodent, pig and human. The focus of the current study was to examine the utility of the invertebrate greater wax moth Galleria mellonella as an in vivo model of gonococcal infection. We observed that a threshold of ~106 - 107 gonococci/larva was required to kill >50% of larvae (P < 0.05), and increased toxicity correlated with reduced health index scores and pronounced histopathological changes such as increases in the total lesion grade, melanized nodules, hemocyte reaction, and multifocal adipose body degeneration. Larval death was independent of the expression of pilus or Opa protein or LOS sialylation within a single gonococcal species studied, but the model could demonstrate relative toxicity of different isolates. N. meningitidis, N. lacatamica and gonococci all killed larvae equally, but were significantly less toxic (P > 0.05) than Pseudomonas aeruginosa. Larvae primed with nontoxic doses of gonococci were more susceptible to subsequent challenge with homologous and heterologous bacteria, and larval survival was significantly reduced (P < 0.05) in infected larvae after depletion of their hemocytes with clodronate-liposomes. The model was used to test the anti-gonococcal properties of antibiotics and novel antimicrobials. Ceftriaxone (P < 0.05) protected larvae from infection with different gonococcal isolates, but not azithromycin or monocaprin or ligand-coated silver nanoclusters (P > 0.05).

The Heat Shock Protein 60 and Pap1 Participate in the Sporothrixschenckii-Host Interaction

Sporothrixschenckii is one of the etiological agents of sporotrichosis, a worldwide-distributed subcutaneous mycosis. Its cell wall contains a glycoconjugate composed of rhamnose, mannose, glucuronic acid, and proteins, named peptidorhamnomannan, which harbors important Sporothrix-specific immunogenic epitopes. Although the peptidorhamnomannan carbohydrate moiety has been extensively studied, thus far, little is known about the protein core. Here, using LC-MS/MS, we analyzed the S.schenckii peptidorhamnomannan peptide fraction and generated mass signals of 325 proteins, most of them likely to be moonlighting proteins. Among the identified proteins, chaperonin GroEL/Hsp60 and the uncharacterized protein Pap1 were selected for further analysis. Both proteins were heterologously expressed in bacteria, and they showed adhesive properties to the extracellular matrix proteins laminin, elastin, fibrinogen, and fibronectin, although Pap1 also was bound to type-I and type-II collagen. The inoculation of concentrations higher than 40 μg of these proteins, separately, increased immune effectors in the hemolymph of Galleriamellonella larvae and protected animals from an S.schenckii lethal challenge. These observations were confirmed when yeast-like cells, pre-incubated with anti-rHsp60 or anti-rPap1 antibodies were used to inoculate larvae. The animals inoculated with pretreated cells showed increased survival rates when compared to the control groups. In conclusion, we report that Hsp60 and Pap1 are part of the cell wall peptidorhamnomannan, can bind extracellular matrix components, and contribute to the S.schenckii virulence. To our knowledge, this is the first report about moonlighting protein in the S.schenckii cell wall with an important role during the pathogen-host interaction.

Current understanding of immune priming phenomena in insects

It may seem that the most important issues related to insect immunity have already been described. However, novel phenomena observed in recent years shed new light on the understanding of the immune response in insects.The adaptive abilities of insects helped them to populate all ecological land niches.One important adaptive ability of insects that facilitates their success is the plasticity of their immune system. Although they only have innate immune mechanisms, insects can increase their resistance after the first encounter with the pathogen. In recent years, this phenomenon,namedimmunepriming, has become a "hot topic" in immunobiology.Priming can occur within or across generations. In the first case, the resistance of a given individual can increase after surviving a previous infection. Transstadial immune priming occurs when infection takes place at one of the initial developmental stages and increased resistance is observed at the pupal or imago stages. Priming across generations (transgenerationalimmune priming, TGIP) relies on the increased resistance of the offspring when one or both parents are infected during their lifetime.Despite the attention that immune priming has received, basic questions remain to be answered, such as regulation of immune priming at the molecular level. Research indicates that pathogen recognition receptors (PRRs) can be involved in the priming phenomenon. Recent studies have highlighted the special role of microRNAs and epigenetics, which can influence expression of genes that can be transmitted through generations although they are not encoded in the nucleotide sequence. Considerable amounts of research are required to fully understand the mechanisms that regulate priming phenomena. The aim of our work is to analyse thoroughly the most important information on immune priming in insects and help raise pertinent questions such that a greater understanding of this phenomenon can be obtained in the future.

Response Mechanisms of Invertebrates to Bacillus thuringiensis and Its Pesticidal Proteins

Extensive use of chemical insecticides adversely affects both environment and human health. One of the most popular biological pest control alternatives is bioinsecticides based on Bacillus thuringiensis This entomopathogenic bacterium produces different protein types which are toxic to several insect, mite, and nematode species. Currently, insecticidal proteins belonging to the Cry and Vip3 groups are widely used to control insect pests both in formulated sprays and in transgenic crops. However, the benefits of B. thuringiensis-based products are threatened by insect resistance evolution. Numerous studies have highlighted that mutations in genes coding for surrogate receptors are responsible for conferring resistance to B. thuringiensis Nevertheless, other mechanisms may also contribute to the reduction of the effectiveness of B. thuringiensis-based products for managing insect pests and even to the acquisition of resistance. Here, we review the relevant literature reporting how invertebrates (mainly insects and Caenorhabditis elegans) respond to exposure to B. thuringiensis as either whole bacteria, spores, and/or its pesticidal proteins.

The greater wax moth Galleria mellonella: biology and use in immune studies

The greater wax moth Galleria mellonella is an invertebrate that is increasingly being used in scientific research. Its ease of reproduction, numerous offspring, short development cycle, and finally, its known genome and immune-related transcriptome provide a convenient research model for investigation of insect immunity at biochemical and molecular levels. Galleria immunity, consisting of only innate mechanisms, shows adaptive plasticity, which has recently become the subject of intensive scientific research. This insect serves as a mini host in studies of the pathogenicity of microorganisms and in vivo tests of the effectiveness of single virulence factors as well as new antimicrobial compounds. Certainly, the Galleria mellonella species deserves our attention and appreciation for its contribution to the development of research on innate immune mechanisms. In this review article, we describe the biology of the greater wax moth, summarise the main advantages of using it as a model organism and present some of the main techniques facilitating work with this insect.

Host-pathogen interaction after infection of Galleria mellonella with the filamentous fungus Beauveria bassiana

The filamentous fungus Beauveria bassiana is a natural pathogen of the greater wax moth Galleria mellonella. Infection with this fungus triggered systemic immune response in G. mellonella; nevertheless, the infection was lethal if spores entered the insect hemocel. We observed melanin deposition in the insect cuticle and walls of air bags, while the invading fungus interrupted tissue continuity. We have shown colonization of muscles, air bags, and finally colonization and complete destruction of the fat body-the main organ responsible for the synthesis of defense molecules in response to infection. This destruction was probably not caused by simple fungal growth, because the fat body was not destroyed during colonization with a human opportunistic pathogen Candida albicans. This may mean that the infecting fungus is able to destroy actively the insect's fat body as part of its virulence mechanism. Finally, we were unable to reduce the extremely high virulence of B. bassiana against G. mellonella by priming of larvae with thermally inactivated fungal spores.

Immune responses to Bacillus thuringiensis in the midgut of the diamondback moth, Plutella xylostella

The diamondback moth, Plutella xylostella, is the first insect to develop resistance to Bacillus thuringiensis (Bt) in the field. To date, little is known about the molecular mechanism of the interaction between Bt and midgut immunity in P. xylostella. Here, we report immune responses in the P. xylostella midgut to Bt strain Bt8010 using a combined approach of transcriptomics and quantitative proteomics. Many genes in the Toll, IMD, JNK and JAK-STAT pathways and antimicrobial peptide genes were activated at 18 h post-infection. In the prophenoloxidase (PPO) cascade, four serpin genes were activated, and the PPO1 gene was suppressed by Bt8010. Inhibition of the two PPO proteins was observed at 18 h post-infection. Feeding Bt8010-infected larvae recombinant PPOs enhanced their survival. These results revealed that the Toll, IMD, JNK and JAK-STAT pathways were triggered and participated in the immune defence of the midgut against Bt8010, while the PPO cascade was inhibited and played an important role in this process.

Antifungal Activity of Anionic Defense Peptides: Insight into the Action of Galleria mellonella Anionic Peptide 2

Anionic antimicrobial peptides constitute an integral component of animal innate immunity, however the mechanisms of their antifungal activity are still poorly understood. The action of a unique Galleria mellonella anionic peptide 2 (AP2) against fungal pathogen Candida albicans was examined using different microscopic techniques and Fourier transform infrared (FTIR) spectroscopy. Although the exposure to AP2 decreased the survival rate of C. albicans cells, the viability of protoplasts was not affected, suggesting an important role of the fungal cell wall in the peptide action. Atomic force microscopy showed that the AP2-treated cells became decorated with numerous small clods and exhibited increased adhesion forces. Intensified lomasome formation, vacuolization, and partial distortion of the cell wall was also observed. FTIR spectroscopy suggested AP2 interactions with the cell surface proteins, leading to destabilization of protein secondary structures. Regardless of the anionic character of the whole AP2 molecule, bioinformatics analyses revealed the presence of amphipathic α-helices with exposed positively charged lysine residues. High content of the α-helical structure was confirmed after deconvolution of the IR absorption spectrum and during circular dichroism measurements. Our results indicated that the antimicrobial properties of G. mellonella AP2 rely on the same general characteristics found in cationic defense peptides.

Immune response of Galleria mellonella after injection with non-lethal and lethal dosages of Candida albicans

The immune response of Galleria mellonella to injection with non-lethal and lethal dosages of Candida albicans was compared. Larvae infected with the non-lethal dosage (2 × 10⁴ cells/larva) did not show significant morphological changes, while those infected with the lethal dosage (2 × 10⁵ cells/larva) showed inhibition of motility and cocoon formation and became darker around the area of injection after 24 h. While the administration of the lower dosage caused approx. 5- and 20-fold induction of genes for gallerimycin and galiomycin, respectively, the injection with the higher dosage induced approx. 25 and 120-fold expression of the respective genes. Similar differences were obtained for the insect metalloproteinase inhibitor (IMPI) and hemolin gene transcripts. The relatively low level of immune gene expression was confirmed by an assay of hemolymph antifungal activity, which was detected only in larvae infected with lethal dosage of C. albicans. Furthermore, greater amounts of immune-inducible peptides were detected in the hemolymph extracts in the same group of larvae. The stronger humoral immune response was not correlated with survival. Phenol oxidase (PO) activity was induced only in the hemolymph of larvae infected with the non-lethal dose; injection of the lethal dose resulted in strong inhibition of this enzyme after 24 h. We showed that PO is susceptible to regulation by immune priming with the non-lethal dose of C. albicans. The activity of this enzyme was enhanced in primed larvae at the time of re-injection. When both primed and non-primed larvae received 2 × 10⁵ cells, the inhibition of PO was stronger in the primed group. G. mellonella infected with the lethal dose of C. albicans died despite the strong induction of humoral defence mechanisms. The priming-enhanced activity of PO was correlated with increased resistance to subsequent infection.

Insect Defense Proteins and Peptides

The composition of insect hemolymph can change depending on many factors, e.g. access to nutrients, stress conditions, and current needs of the insect. In this chapter, insect immune-related polypeptides, which can be permanently or occasionally present in the hemolymph, are described. Their division into peptides or low-molecular weight proteins is not always determined by the length or secondary structure of a given molecule but also depends on the mode of action in insect immunity and, therefore, it is rather arbitrary. Antimicrobial peptides (AMPs) with their role in immunity, modes of action, and classification are presented in the chapter, followed by a short description of some examples: cecropins, moricins, defensins, proline- and glycine-rich peptides. Further, we will describe selected immune-related proteins that may participate in immune recognition, may possess direct antimicrobial properties, or can be involved in the modulation of insect immunity by both abiotic and biotic factors. We briefly cover Fibrinogen-Related Proteins (FREPs), Down Syndrome Cell Adhesion Molecules (Dscam), Hemolin, Lipophorins, Lysozyme, Insect Metalloproteinase Inhibitor (IMPI), and Heat Shock Proteins. The reader will obtain a partial picture presenting molecules participating in one of the most efficient immune strategies found in the animal world, which allow insects to inhabit all ecological land niches in the world.

New solvent options for in vivo assays in the Galleria mellonella larvae model

Experimentation in mammals is a long and expensive process in which ethical aspects must be considered, which has led the scientific community to develop alternative models such as that of Galleria mellonella. This model is a cost and time effective option to act as a filter in the drug discovery process. The main limitation of this model is the lack of variety in the solvents used to administer compounds, which limits the compounds that can be studied using this model. Five aqueous (DMSO, MeOH, acetic acid, HCl and NaOH) and four non-aqueous (olive oil, isopropyl myristate, benzyl benzoate and ethyl oleate) solvents was assessed to be used as vehicles for toxicity and antimicrobial activity in vivo assays. All the tested solvents were innocuous at the tested concentrations except for NaOH, which can be used at a maximum concentration of 0.5 M. The toxicity of two additional compounds, 5-aminosalicylic acid and DDT, was also assessed. The results obtained allow for the testing of a broader range of compounds using wax moth larvae. This model appears as an alternative to mammal models, by acting as a filter in the drug development process and reducing costs and time invested in new drugs.

Host-pathogen interactions upon the first and subsequent infection of Galleria mellonella with Candida albicans

Insects are able to develop enhanced resistance in response to repeated infection. This phenomenon is called immune priming. In this work, so-called "primed" Galleria mellonella larvae were re-infected with a lethal dose of Candida albicans 48 h after injection of a non-lethal dose, while "non-primed" larvae were infected only with a lethal dose. The increased resistance of the primed larvae correlated with a slower rate of body colonisation by the fungus. Changes in the protein profiles were detected in the whole hemolymph of the primed insects. The analysis of low-molecular weight proteins and peptides obtained with the use of three different organic solvents and comparative quantitative HPLC analysis thereof showed that the primed larvae did not have higher amounts of any infection-inducible polypeptides than the non-primed larvae. Moreover, electrophoresis of low-molecular weight polypeptides revealed an even lower level of immune-induced peptides in the primed larvae than in the non-primed ones. Furthermore, the defence activity of larval hemolymph, i.e. the antifungal, antibacterial, and lysozyme-type activity, was up-regulated in the primed larvae at the time of re-infection and, consequently, at the early time points after the infection with the lethal dose. Twenty four hours after the infection, these parameters were equally high in the non-primed and primed larvae. Accordingly, at the time of the injection of the lethal dose, certain immune-inducible genes were up-regulated. However, 24 h after the infection with the lethal dose, their expression in both groups was incomparably higher than at the time of the infection and, in most cases, it was as high in the primed larvae as in the non-primed ones. We found that only anti yeast-like activity was enhanced 24 h after the re-infection. This correlated with results obtained by testing the priming effect in heterologous systems: the primed animals did not exhibit higher resistance to the other pathogens tested.

Two sporulated Bacillus enhance immunity in Galleria mellonella protecting against Candida albicans

The aim of this study was to evaluate the effects of Bacillus subtilis and Bacillus atrophaeus on Galleria mellonella immunity challenged by Candida albicans. Firstly, we analyzed the susceptibility of G. mellonella to bacilli (vegetative and sporulating forms). It was found that both vegetative and sporulating forms were not pathogenic to G. mellonella at a concentration of 1 × 10⁴ cells/larva. Next, larvae were pretreated with two species of Bacillus, in the vegetative and sporulating forms, and then challenged with C. albicans. In addition, the gene expression of antimicrobial peptides (AMPs) such as Gallerimycin, Gloverin, Cecropin-D and Galiomicin was investigated. Survival rates increased in the Bacillus treated larvae compared with control larvae inoculated with C. albicans only. Cells and spores of Bacillus spp. upregulated Gloverin, Galiomicin and Gallerimycin genes in relation to the control group (PBS + PBS). When these larvae were infected with C. albicans, the group pretreated with spores of B. atrophaeus and B. subtilis showed a greater increase in expression of Galiomycin (49.08-fold and 13.50-fold) and Gallerimycin (27.88-fold and 68.15-fold), respectively, compared to the group infected with C. albicans only (p = 0.0001). After that, we investigated the effects of B. subtilis and B. atrophaeus on immune system of G. mellonella evaluating the number of hemocytes, quantification of melanization, cocoon formation and colony forming units (CFU) count. Hemocyte count increased in response to stimulation by Bacillus, and a higher increase was achieved when larvae were inoculated with B. subtilis spores (p = 0.0011). In the melanization assay, all groups tested demonstrated lower production of melanin compared to that in the phosphate-buffered saline (PBS) group. In addition, full cocoon formation was observed in all groups analyzed, which corresponded to a healthier wax worm. Hemolymph culture revealed higher growth of B. atrophaeus and B. subtilis in the groups inoculated with spores. We concluded that spores and cells of B. atrophaeus and B. subtilis stimulated the immune system of G. mellonella larvae and protected them of C. albicans infection.

Analysis of some immunogenic properties of the recombinant Sporothrix schenckii Gp70 expressed in Escherichia coli

AIM

Sporothrix schenckii is the causative agent of sporotrichosis. A 70-kDa glycoprotein, Gp70, is a candidate for the development of prophylactic alternatives to control the disease, and its gene (GP70) is predicted to encode for a protein of 43 kDa, contrasting with the molecular weight of the native protein.

MATERIALS & METHODS

The GP70 was expressed in bacteria, the recombinant protein purified, used in immunoassays and injected to Galleria mellonella.

RESULTS & CONCLUSION

The recombinant protein was detected by anti-Gp70 antibodies, confirming that the Gp70 backbone is a 43-kDa peptide. This protein showed enzyme activity of cyclase and was recognized by sera of patients with sporotrichosis. Although it was not useful for serodiagnosis of sporotrichosis, it conferred protection to animals against experimental sporotrichosis.

Recent Advances in the Use of Galleria mellonella Model to Study Immune Responses against Human Pathogens

The use of invertebrates for in vivo studies in microbiology is well established in the scientific community. Larvae of Galleria mellonella are a widely used model for studying pathogenesis, the efficacy of new antimicrobial compounds, and immune responses. The immune system of G. mellonella larvae is structurally and functionally similar to the innate immune response of mammals, which makes this model suitable for such studies. In this review, cellular responses (hemocytes activity: phagocytosis, nodulation, and encapsulation) and humoral responses (reactions or soluble molecules released in the hemolymph as antimicrobial peptides, melanization, clotting, free radical production, and primary immunization) are discussed, highlighting the use of G. mellonella as a model of immune response to different human pathogenic microorganisms.