Pathogen infection routes and host innate immunity: Lessons from insects

Morphological and Chemical Changes in the Hemolymph of the Wax Moth Galleria mellonella Infected by the Entomopathogenic Fungus Conidiobolus coronatus

Hemolymph enables communication between organs in insects and ensures necessary coordination and homeostasis. Its composition can provide important information about the physiological state of an insect and can have diagnostic significance, which might be particularly important in the case of harmful insects subjected to biological control. Galleria mellonella Linnaeus 1758 (Lepidoptera: Pyralidae) is a global pest to honey bee colonies. The hemolymph of its larvae was examined after infection with the soil fungus Conidiobolus coronatus (Constantin) Batko 1964 (Entomophthorales). It was found that after one hour of contact with the fungus, the volume of the hemolymph increased while its total protein content decreased. In larvae with a high pathogen load, just before death, hemolymph volume decreased to nearly initial levels, while total protein content and synthesis (incorporation of 35S-labeled methionine) increased. The hemolymph polypeptide profile (SDS-PAGE followed by autoradiography) of infected insects was significantly different from that of healthy larvae. Hemocytes of infected larvae did not surround the fungal hyphae, although they encapsulated small foreign bodies (phase contrast microscopy). Infection had a negative effect on hemocytes, causing oenocyte and spherulocyte deformation, granulocyte degranulation, plasmatocyte vacuolization, and hemocyte disintegration. GC-MS analysis revealed the presence of 21 compounds in the hemolymph of control insects. C. coronatus infection caused the appearance of 5 fatty acids absent in healthy larvae (heptanoic, decanoic, adipic, suberic, tridecanoic), the disappearance of 4 compounds (monopalmitoylglycerol, monooleoylglycerol, monostearin, and cholesterol), and changes in the concentrations of 8 compounds. It remains an open question whether substances appearing in the hemolymph of infected insects are a product of the fungus or if they are released from the insect tissues damaged by the growing hyphae.

Global climate change and its impact on the distribution and efficacy of Bacillus thuringiensis as a biopesticide

This study is the first modeling exercise to assess the impacts of climate change on the current and future global distribution of Bacillus thuringiensis (Bt). Bt is a common Gram-positive, rod-shaped bacterium widely distributed in various environments, including soil and water. It is widely recognized as a source of effective and safe agricultural biopesticides for pest management in various climatic regions globally. In the present work, ensemble species distribution models were developed for Bt based on the generalized linear model (GLM), generalized boosting model (GBM), random forest (RF), and maximum entropy (MaxEnt) under two distinct scenarios, SSP2-4.5 (optimistic) and SSP5-8.5 (pessimistic) for the year of 2050, 2070, and 2090. The performance of our models was evaluated based on true skill statistics (TSS) and the area under the receiver-operator curve (AUC) indices. Both AUC and TSS values were observed in an acceptable range, with AUC at 0.84 and TSS at 0.512, respectively. Results indicate that most of the areas currently suitable for Bt will likely remain stable in the future, particularly Central America, Central and South Africa, South Asia, and parts of Oceania. Norway, Peru, and the UK will have notable habitat gains by 2090 based on SSP2-4.5 and SSP5-8.5 scenarios. On the contrary, Serbia, Guinea, Poland, Croatia, Spain and Romania showed notable losses under both scenarios. Our results underscore Bt potential to improve pest control, crop yields, and environmental sustainability, especially in regions where agriculture is predominant. Our research highlights the need to understand ecological dynamics for future conservation and agricultural planning in the face of climate change.

Research progress of aphid immunity system: Potential effective target for green pest management

Due to the absence of acquired immunity, insects primarily rely on their innate immune system to resist pathogenic microorganisms and parasitoids in natural habitats. This innate immune system can be classified into cellular immunity and humoral immunity. Cellular immunity is mediated by hemocytes, which perform phagocytosis, aggregation, and encapsulation to fight against invaders, whereas the humoral immunity primarily activates the immune signaling pathways and induces the generation of immune effectors. Existing studies have revealed that the hemipteran aphids lack some crucial immune genes compared to other insect species, indicating the different immune mechanisms in aphids. The current review summarizes the adverse impacts of pathogenic microorganisms and parasitoids on aphids, introduces the cellular and humoral immune systems in insects, and analyzes the differences between aphids and other insect species. Furthermore, our review also discussed the existing prospects and challenges in aphid immunity research, and proposed the potential application of immune genes in green pest management.

Uncovering the immune evasion mechanism of microbial pathogens: lessons from anti-encapsulation strategies adopted by a fungus

Overview of Hemocytin-Mediated Cellular Encapsulation and Melanization Responses in Insects Against Fungal Pathogens.

The humoral immune response of the lepidopteran model insect, silkworm Bombyx mori L., to microbial pathogens

Insects are valuable models for studying innate immunity and its role in combating infections. The silkworm Bombyx mori L., a well-studied insect model, is susceptible to a range of pathogens, including bacteria, fungi, viruses, and microsporidia. Their susceptibility makes it a suitable model for investigating host-pathogen interactions and immune responses against infections and diseases. This review focuses on the humoral immune response and the production of antimicrobial peptides (AMPs), the phenoloxidase (PO) system, and other soluble factors that constitute the primary defense of silkworms against microbial pathogens. The innate immune system of silkworms relies on pattern recognition receptors (PRRs) to recognize pathogen-associated molecular patterns (PAMPs), which then activate various immune pathways including Imd, Toll, JAK/STAT, and RNA interference (RNAi). Their activation triggers the secretion of AMPs, enzymatic defenses (lysozyme and PO), and the generation of reactive oxygen species (ROS). Collectively, these pathways work together to neutralize and eliminate pathogens, thereby contributing to the defense mechanism of silkworms. Understanding the innate immunity of silkworms can uncover conserved molecular pathways and key immune components shared between insects and vertebrates. Additionally, it can provide valuable insights for improving sericulture practices, developing strategies to control diseases affecting silk production, and providing a theoretical foundation for developing pest control measures.

Sgabd-2 plays specific role in immune response against biopesticide Metarhizium anisopliae in Aphis citricola

Metarhizium anisopliae is an effective biopesticide for controlling Aphis citricola, which has developed resistance to many chemical pesticides. However, the powerful immune system of A. citricola has limited the insecticidal efficacy of M. anisopliae. The co-evolution between insects and entomogenous fungi has led to emergence of new antifungal immune genes, which remain incompletely understood. In this study, an important immune gene Sgabd-2 was identified from A. citricola through transcriptome analysis. Sgabd-2 gene showed high expression in the 4th instar nymph and adult stages, and was mainly distributed in the abdominal region of A. citricola. The recombinant protein (rSgabd-2) exhibited no antifungal activity but demonstrated clear agglutination activity towards the conidia of M. anisopliae. RNA interference of Sgabd-2 by dsRNA feeding resulted in decreased phenoloxidase (PO) activity and weakened defense for A. citricola against M. anisopliae. Simultaneous silence of GNBP-1 and Sgabd-2 effectively reduced the immunity of A. citricola against M. anisopliae more than the individual RNAi of GNBP-1 or Sgabd-2. Furthermore, a genetically engineered M. anisopliae expressing double-stranded RNA (dsSgabd-2) targeting Sgabd-2 in A. citricola successfully suppressed the expression of Sgabd-2 and demonstrated increased virulence against A. citricola. Our findings elucidated Sgabd-2 as a critical new antifungal immune gene and proposed a genetic engineering strategy to enhance the insecticidal virulence of entomogenous fungi through RNAi-mediated inhibition of pest immune genes.

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

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

Immune Reactions of Vector Insects to Parasites and Pathogens

This overview initially describes insect immune reactions and then brings together present knowledge of the interactions of vector insects with their invading parasites and pathogens. It is a way of introducing this Special Issue with subsequent papers presenting the latest details of these interactions in each particular group of vectors. Hopefully, this paper will fill a void in the literature since brief descriptions of vector immunity have now been brought together in one publication and could form a starting point for those interested and new to this important area. Descriptions are given on the immune reactions of mosquitoes, blackflies, sandflies, tsetse flies, lice, fleas and triatomine bugs. Cellular and humoral defences are described separately but emphasis is made on the co-operation of these processes in the completed immune response. The paper also emphasises the need for great care in extracting haemocytes for subsequent study as appreciation of their fragile nature is often overlooked with the non-sterile media, smearing techniques and excessive centrifugation sometimes used. The potential vital role of eicosanoids in the instigation of many of the immune reactions described is also discussed. Finally, the priming of the immune system, mainly in mosquitoes, is considered and one possible mechanism is presented.

Odorant binding protein 18 increases the pathogen resistance of the imported willow leaf beetle, Plagiodera versicolora

Background

Insect odorant-binding proteins (OBPs) are a class of small molecular weight soluble proteins. In the past few years, OBPs had been found to work as carriers of ligands and play a crucial role in olfaction and various other physiological processes, like immunity. A subset of insect OBPs had been found to be expressed differently and play a function in immunity of fungal infection. However, there are few studies on the role of OBPs in immunity of bacterial infection.

Methods

To identify the immune-related OBPs of Plagiodera versicolora after infected by Pseudomonas aeruginosa, we determined the mortality of P. versicolora to P. aeruginosa and selected the time point of 50% mortality of larvae to collect samples for RNA-seq. RNAi technology was used to investigate the function of immune-related OBPs after P. aeruginosa infection.

Results

RNA-seq data shows that PverOBP18 gene significantly up-regulated by 1.8-fold and further RT-qPCR affirmed its expression. Developmental expression profile showed that the expression of PverOBP18 was highest in the pupae, followed by the female adults, and lower in the 1st-3rd larvae and male adults with lowest in eggs. Tissue expression profiling showed that PverOBP18 was dominantly expressed in the epidermis. RNAi knockdown of PverOBP18 significantly reduced the expression of bacterial recognition receptor gene PGRP and antibacterial peptide gene Attacin and reduced the resistance of P. versicolora to P. aeruginosa infection.

Conclusion

Our results indicated that PverOBP18 gene increased the pathogen resistance of P. versicolora by cooperating with the immune genes and provided valuable insights into using OBPs as targets to design novel strategies for management of P. versicolora.

A gut-isolated Enterococcus strain (HcM7) triggers the expression of antimicrobial peptides that aid resistance to nucleopolyhedrovirus infection of Hyphantria cunea larvae

BACKGROUND

Commensal microorganisms are widely distributed in insect gut tissues and play important roles in host nutrition, metabolism, reproductive regulation, and especially immune functioning and tolerance to pathogens. Consequently, gut microbiota represent a promising resource for the development of microbial-based products for pest control and management. However, the interactions among host immunity, entomopathogen infections, and gut microbiota remain poorly understood for many arthropod pests.

RESULTS

We previously isolated an Enterococcus strain (HcM7) from Hyphantria cunea larvae guts that increased the survival rates of larvae challenged with nucleopolyhedrovirus (NPV). Here, we further investigated whether this Enterococcus strain stimulates a protective immune response against NPV proliferation. Infection bioassays demonstrated that re-introduction of the HcM7 strain to germfree larvae preactivated the expression of several antimicrobial peptides (particularly H. cunea gloverin 1, HcGlv1), resulting in the significant repression of virus replication in host guts and hemolymph, and consequently improved host survivorship after NPV infection. Furthermore, silencing of the HcGlv1 gene by RNA interference markedly enhanced the deleterious effects of NPV infection, revealing a role of this gut symbiont-induced gene in host defenses against pathogenic infections.

CONCLUSION

These results show that some gut microorganisms can stimulate host immune systems, thereby contributing to resistance to entomopathogens. Furthermore, HcM7, as a functional symbiotic bacteria of H. cunea larvae, may be a potential target for increasing the effectiveness of biocontrol agents against this devastating pest. © 2023 Society of Chemical Industry.

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.

Insect-pathogen crosstalk and the cellular-molecular mechanisms of insect immunity: uncovering the underlying signaling pathways and immune regulatory function of non-coding RNAs

Multicellular organisms are constantly subjected to pathogens that might be harmful. Although insects lack an adaptive immune system, they possess highly effective anti-infective mechanisms. Bacterial phagocytosis and parasite encapsulation are some forms of cellular responses. Insects often defend themselves against infections through a humoral response. This phenomenon includes the secretion of antimicrobial peptides into the hemolymph. Specific receptors for detecting infection are required for the recognition of foreign pathogens such as the proteins that recognize glucans and peptidoglycans, together referred to as PGRPs and βGRPs. Activation of these receptors leads to the stimulation of signaling pathways which further activates the genes encoding for antimicrobial peptides. Some instances of such pathways are the JAK-STAT, Imd, and Toll. The host immune response that frequently accompanies infections has, however, been circumvented by diseases, which may have assisted insects evolve their own complicated immune systems. The role of ncRNAs in insect immunology has been discussed in several notable studies and reviews. This paper examines the most recent research on the immune regulatory function of ncRNAs during insect-pathogen crosstalk, including insect- and pathogen-encoded miRNAs and lncRNAs, and provides an overview of the important insect signaling pathways and effector mechanisms activated by diverse pathogen invaders.

The susceptibility of Hyphantria cunea larvae to microbial pesticides Bacillus thuringiensis and Mamestra brassicae nuclear polyhedrosis virus under Cd stress

Heavy metal pollution is an abiotic factor that can affect the efficiency of pest control. In this study, two microbial pesticides, Bacillus thuringiensis and Mamestra brassicae nuclear polyhedrosis virus (MbNPV), were used to treat Hyphantria cunea larvae with Cd pre-exposure, and the humoral and cellular immunity of H. cunea larvae with Cd exposure were evaluated. The results showed that Cd exposure increased the susceptibility of H. cunea larvae to microbial pesticides B. thuringiensis and MbNPV, and the lethal effect of Cd exposure and microbial pesticides on H. cunea larvae was synergistic. Cd exposure significantly decreased the expression of pathogen recognition genes (GNBP1 and GNBP3), signal transduction genes (Relish, Myd88, Tube, and Imd), and antimicrobial peptide gene (Lebocin) in the humoral immunity of H. cunea larvae compared with the untreated larvae. Parameters of cellular immunity, including the number of hemocytes, phagocytic activity, melanization activity, encapsulation activity, and the expression of three phagocytic regulatory genes (HEM1, GALE1, GALE2), were also found to decrease significantly in Cd-treated larvae. TOPSIS analysis showed that humoral immunity, cellular immunity, and total immunity levels of H. cunea larvae with Cd exposure were weaker than those in untreated larvae. Correlation analysis showed that the mortality of two microbial pesticides investigated in H. cunea larvae was negatively correlated with the humoral and cellular immunity of larvae. Taken togther, Cd exposure results in immunotoxic effects on H. cunea larvae and the use of microbial pesticides are an effective strategy for pest control in heavy metal-polluted areas.

Functional role of thioester-containing proteins in the Drosophila anti-pathogen immune response

Thioester-containing proteins (TEPs) are present in many animal species ranging from deuterostomes to protostomes, which emphasizes their evolutionary conservation and importance in animal physiology. Phylogenetically, insect TEPs share sequence similarity with mammalian α2-macroglobulin. Drosophila melanogaster is specifically considered a superb model for teasing apart innate immune processes. Here we review recent discoveries on the involvement of Drosophila TEPs in the immune response against bacterial pathogens, nematode parasites, and parasitoid wasps. This information generates novel insights into the role of TEPs as regulators of homeostasis in Drosophila and supports the complexity of immune recognition and specificity in insects and more generally in invertebrates. These developments together with recent advances in gene editing and multi-omics will enable the fly immunity community to appreciate the molecular and mechanistic contributions of TEPs to the modulation of the host defense against infectious disease and possibly to translate this information into tangible therapeutic benefits.