Peptidoglycan recognition proteins in insect immunity

Microalgae functional feed additives strengthen immunity and increase longevity in honey bees

Honey bees (Apis mellifera) are faced with a myriad of immunological threats, often worsened by poor nutrition. The use of functional feed additives offers a promising strategy to address colony nutritional deficiencies while helping strengthen bee immune responses and mitigate stress. Microalgae have gained recognition as beneficial diet ingredients for livestock due to their abundance of essential nutrients and immunomodulatory properties. Here, we tested the effects of microalgae-containing feed on honey bee immunity, lifespan, and nutrient assimilation. Caged bees were fed a commercial artificial diet or the same diet with added pollen, spirulina (Arthrospira platensis) or Chlorella (Chlorella vulgaris). Immune-related gene expression, longevity, and body weight were measured following six days of ad libitum feeding. All diets resulted in similar body weights, indicating adequate nutrient assimilation. While bees fed the pollen-containing diet lived the longest (median lifespan = 51 days), bees fed spirulina- and Chlorella-containing diets lived significantly longer (median lifespan = 48 and 46 days, respectively) than those fed the base diet (median lifespan = 40 days). Spirulina-fed bees exhibited significantly higher expression of several antimicrobial peptide (AMP) genes relative to the base diet and had superior bacterial clearing ability after injection with live E. coli cells. We propose that this increased immunocompetence is at least partially due to elevated AMP levels. Our findings suggest that the tested microalgae can improve honey bee longevity and immune functions with negligible health costs relative to a commonly used artificial diet. Determining the effects of microalgae feed additives in field-relevant contexts and in the face of diverse pathogen challenges should be the focus of future research efforts.

Pathogen growth and virulence dynamics drive the host evolution against coinfections

The occurrence of coinfections, where hosts are simultaneously infected by multiple pathogens, is widespread in nature and has significant negative impacts on global health. In humans, over one-sixth of the world's population is affected by coinfections, contributing to several diseases. However, despite the broad ecological relevance and impact on global health, most biomedical research has focused on understanding interactions between a single host and a single pathogen. The extent to which coinfections could impact host adaptation and immune system evolution, particularly in comparison to infections by single pathogens, thus remains largely unknown. Also, what roles do individual pathogen species play in this evolutionary process? To address these questions, in this study, we combined theoretical modeling and experimental validation in a model insect Tribolium castaneum evolving against two coinfecting bacterial pathogens with contrasting growth (e.g., fast- vs slow-growing) and virulence (fast- vs slow-killing) dynamics. Our findings show that fast-growing pathogens causing rapid mortality surges (i.e., fast-acting) can effectively limit the host's adaptive success against coinfections. While hosts rapidly evolved better survival against slow-growing bacteria causing long-lasting infections, adaptation against coinfections was significantly delayed and resembled the slow rate of adaptation against fast-acting pathogens. Finally, RNAseq analyses revealed that the observed delay in adaptation was associated with the limited scopes for suitable immune modulations against fast-acting pathogens. They might also be costly and pleiotropic (e.g., phenoloxidase activity), posing challenges for further immunomodulation and slowing adaptation. Our study thus highlights how individual pathogens' growth and virulence dynamics critically regulate adaptive responses against coinfections.

Structure, Function, and Regulation of LytA: The N-Acetylmuramoyl-l-alanine Amidase Driving the "Suicidal Tendencies" of Streptococcus pneumoniae-A Review

Streptococcus pneumoniae (pneumococcus) is a significant human pathogen responsible for a range of diseases from mild infections to invasive pneumococcal diseases, particularly affecting children, the elderly, and immunocompromised individuals. Despite pneumococcal conjugate vaccines having reduced disease incidence, challenges persist due to serotype diversity, vaccine coverage gaps, and antibiotic resistance. This review highlights the role of LytA, a key autolysin (N-acetylmuramoyl-l-alanine amidase), in pneumococcal biology. LytA regulates autolysis, contributes to inflammation, and biofilm formation, and impairs bacterial clearance. It also modulates complement activation, aiding immune evasion. LytA expression is influenced by environmental signals and genetic regulation and is tied to competence for genetic transformation, which is an important virulence trait, particularly in meningitis. With the increase in antibiotic resistance, LytA has emerged as a potential therapeutic target. Current research explores its use in bacteriolytic therapies, vaccine development, and synergistic antibiotic strategies. Various compounds, including synthetic peptides, plant extracts, and small molecules, have been investigated for their ability to trigger LytA-mediated bacterial lysis. Future directions include the development of novel anti-pneumococcal interventions leveraging LytA's properties while overcoming vaccine efficacy and resistance-related challenges. Human challenge models and animal studies continue to deepen our understanding of pneumococcal pathogenesis and potential treatment strategies.

Modulation of peptidoglycan recognition protein expression alters begomovirus vectoring efficiency and fitness of Bemisia tabaci

Peptidoglycan recognition proteins (PGRPs) are evolutionarily conserved molecules. Their role in the immune response to invading pathogens makes them a natural target for viral defence study in a wide range of organisms. Silverleaf whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is one of the invasive insect pests and transmits begomoviruses in a circulative and persistent manner to vegetables, legumes, fibres and ornamentals. The virus entry, retention, circulation, and release process involve interactions with several proteins in B. tabaci and evade innate immunity to avoid the antiviral mechanisms. The present study investigated the role of BtPGRP in chilli leaf curl virus (ChiLCV, Begomovirus capsica) transmission by B. tabaci. Silencing of BtPGRP using double-stranded (ds) RNA led to the loss of innate immunity to ChiLCV resulting in increased virus titre in B. tabaci. DsBtPGRP was orally administered to adults of B. tabaci at a concentration of 1, 3, and 5 μg/mL. The expression of BtPGRP was downregulated up to 4.67-fold. The virus titre in B. tabaci increased 90.05 times post-exposure to dsBtPGRP at 5 μg/mL. The test plants inoculated with ChiLCV by dsBtPGRP-exposed B. tabaci expressed severe curling symptoms with a higher virus load and transmission ratio than the control. Besides, the silencing of BtPGRP also induced up to 56.67% mortality in treated B. tabaci. The active site pocket of BtPGRP was found to interact directly with the ChiLCV-CP in computational analyses. Key residues of BtPGRP, including Tyr45, Asp84, His86, Trp87, and Asn119 exhibited critical interactions with the ChiLCV-CP. To our knowledge, this is the first report on the effect of PGRP silencing on ChiLCV acquisition and transmission efficiency and fitness of B. tabaci Asia II I.

A Review on the Extraction, Structural Characterization, Function, and Applications of Peptidoglycan

Peptidoglycan (PGN) is the primary component of bacterial cell walls, consisting of linear glycan chains formed by alternating linkages of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) through glycosidic bonds. It exhibits biological activity in various aspects, making it a biologically significant macromolecule with extensive industrial application. This review aims to explore the latest research advancements in the extraction techniques, structural characterization, functions, and applications of PGN. The review compares the advantages and limitations of traditional chemical lysis methods with modern mechanical-assisted and bio-assisted extraction techniques, discusses chemical composition analysis techniques and structural characterization methods of PGN. The review emphasizes the potential of PGN in immune modulation, specific recognition, and adsorption functions. Furthermore, the review examines potential applications of PGN in vaccine development, the livestock industry, the removal of harmful substances, and protein bioprocessing. In the end, based on the current development trend, future research directions for PGN are proposed, including in-depth studies on the mechanisms of PGN in different hosts and its immunomodulatory effects in various disease models. It is expected that a comprehensive reference framework for the research and application of PGN will be provided through this review, offering ideas and directions for further development and utilization.

Distinct biological control agents differentially modulate the immune system of the sugarcane borer larvae (Diatraea saccharalis)

The humoral response plays a crucial role in insect defense against parasites and pathogens, typically producing antimicrobial peptides through the Toll, IMD, and Jak-STAT signaling pathways, as well as melanization via phenoloxidases. However, many studies use nonpathogenic or opportunistic organisms and often infect insects in nonnatural ways, such as piercing or injecting the pathogen into the hemocoel. The objective of this study was to examine the modulation of the main humoral pathway genes involved in the interaction between the nonmodel organism Diatraea saccharalis (the sugarcane borer) and different biological control agents. We identified and evaluated the expression of DsDorsal (Toll pathway), DsRelish (IMD pathway), DsSTAT (JAK/STAT pathway), DsPPO1, and DsPPO2 (PO pathway) in larvae and pupae of D. saccharalis exposed or not to different biological control agents. The biocontrol agents used were: (i) the bacterium Bacillus thuringiensis var. aizawai GC-91, which is pathogenic to D. saccharalis; (ii) the fungus Metarhizium anisopliae ESALQE9 strain, which is employed to control the froghoppers of the genus Mahanarva in sugarcane fields, though it exhibits low virulence to D. saccharalis; and (iii) the generalist parasitoid Tetrastichus howardi. Our results demonstrate that B. thuringiensis at LC30 induced the expression of DsRelish at 24 h and DsSTAT at 48 h after treatment initiation. In contrast, treatment with the M. anisopliae ESALQE9 strain reduced the levels of DsDorsal and DsSTAT 24 h post-infection compared to the control group. In larvae, DsDorsal, DsSTAT, DsPPO1, and DsPPO2 were induced in response to T. howardi, whereas no induction was observed in pupae. Notably, no immune-related genes were modulated during the pupae-parasitoid interaction. Additionally, we provide an explanation for why T. howardi shows superior parasitism success in D. saccharalis pupae compared with larvae. The data presented here introduce novel perspectives for enhancing pest management through the utilization of biocontrol agents.

Functional characterization and regulatory pattern of Neoseiulus barkeri peptidoglycan recognition protein (PGRP)

Compared with traditional biological control, the co-use of entomopathogenic fungi and multiple enemies has made great progress in biocontrol technology. However, the risk posed by entomopathogenic fungi to their host has not been fully evaluated. Further, the interaction between them has not described adequately. In this study, we investigated the crucial role of peptidoglycan recognition protein (PGRP), an important pattern recognition receptor, involved in the resistance of Neoseiulus barkeri against the entomopathogenic fungus, Beauveria bassiana. Results showed that N. barkeri possessed one NbPGRP gene, which was mainly enriched in the ventral cuticle, and was significantly upregulated after B. bassiana induction. Knocking down NbPGRP resulted in decreased survival rate of mites and increased B. bassiana load. Also, recombinant NbPGRP inhibited the proliferation of B. bassiana, but also attached to spores by binding to surface PAMPs. Furthermore, the transcriptomics after NbPGRP silencing indicates that NbPGRP has a potential regulatory role in mite physiologies. These results showed that NbPGRP has a dual function of inhibiting the proliferation of entomopathogenic fungi and regulating mite resistance upon recognition of entomopathogens. This enhances understanding of the interaction mechanism between entomopathogens and their insect hosts.

The Role of AaPGRP-LB in the Immune Response of Aedes albopictus Against Bacteria Infection

The initial phase of an insect's innate immune response to foreign pathogens is triggered by the identification of exogenous invaders, a mechanism facilitated by pattern recognition receptors. Among these receptors, peptidoglycan recognition proteins (PGRPs), abundant in insects, are essential components of the innate immune system. The roles of PGRPs have been extensively elucidated in Drosophila melanogaster; however, the mechanism underlying the immune response of Aedes albopictus to pathogens is unclear. Herein, we successfully cloned the full-length cDNA of a PGRP gene from Ae. albopictus, designated as the AaPGRP-LB gene. The open reading frame of AaPGRP-LB encodes 203 amino acids, including a secretion signal peptide and a canonical PGRP conserved domain. Multisequence alignment revealed that AaPGRP-LB possesses the amino acid residues essential for zinc binding and amidase activity. Molecular docking studies demonstrated that AaPGRP-LB exhibits a strong binding affinity for DAP-type and LYS-type peptidoglycan. The mRNA expression level of the AaPGRP-LB gene significantly increased after oral infection with Escherichia coli or Staphylococcus aureus. The purified recombinant AaPGRP-LB (rAaPGRP-LB) exhibited strong agglutination properties and demonstrated significant antimicrobial efficacy against E. coli and S. aureus in the presence of zinc ions. This study highlights the critical role of AaPGRP-LB in the immune response of Ae. albopictus. These findings provide a foundation for future research on mosquito immune pathways for innovative vector control and disease prevention strategies.

Systematic analysis of innate immune-related genes in the silkworm: Application to antiviral research

The silkworm, a crucial model organism of the Lepidoptera, offers an excellent platform for investigating the molecular mechanisms underlying the innate immune response of insects toward pathogens. Over the years, researchers worldwide have identified numerous immune-related genes in silkworms. However, these identified silkworm immune genes are not well classified and not well known to the scientific community. With the availability of the latest genome data of silkworms and the extensive research on silkworm immunity, it has become imperative to systematically categorize the immune genes of silkworms with different database IDs. In this study, we present a meticulous organization of prevalent immune-related genes in the domestic silkworm, using the SilkDB 3.0 database as a reliable source for updated gene information. Furthermore, utilizing the available data, we classify the collected immune genes into distinct categories: pattern recognition receptors, classical immune pathways, effector genes and others. In-depth data analysis has enabled us to predict some potential antiviral genes. Subsequently, we performed antiviral experiments on selected genes, exploring their impact on Bombyx mori nucleopolyhedrovirus replication. The outcomes of this research furnish novel insights into the immune genes of the silkworm, consequently fostering advancements in the field of silkworm immunity research by establishing a comprehensive classification and functional understanding of immune-related genes in the silkworm. This study contributes to the broader understanding of insect immune responses and opens up new avenues for future investigations in the domain of host-pathogen interactions.

The Calicophoron daubneyi genome provides new insight into mechanisms of feeding, eggshell synthesis and parasite-microbe interactions

BACKGROUND

The rumen fluke, Calicophoron daubneyi, is the major paramphistome species infecting ruminants within Europe. Adult flukes reside within the rumen where they are in direct contact with a unique collection of microorganisms. Here, we report a 1.76-Gb draft genome for C. daubneyi, the first for any paramphistome species.

RESULTS

Several gene families have undergone specific expansion in C. daubneyi, including the peptidoglycan-recognition proteins (PGRPs) and DM9 domain-containing proteins, which function as pattern-recognition receptors, as well as the saposin-like proteins with putative antibacterial properties, and are upregulated upon arrival of the fluke in the microbe-rich rumen. We describe the first characterisation of a helminth PGRP and show that a recombinant C. daubneyi PGRP binds to the surface of bacteria, including obligate anaerobes from the rumen, via specific interaction with cell wall peptidoglycan. We reveal that C. daubneyi eggshell proteins lack L-DOPA typically required for eggshell crosslinking in trematodes and propose that C. daubneyi employs atypical eggshell crosslinking chemistry that produces eggs with greater stability. Finally, although extracellular digestion of rumen ciliates occurs within the C. daubneyi gut, unique ultrastructural and biochemical adaptations of the gastrodermal cells suggest that adult flukes also acquire nutrients via uptake of volatile fatty acids from rumen fluid.

CONCLUSIONS

Our findings suggest that unique selective pressures, associated with inhabiting a host environment so rich in microbial diversity, have driven the evolution of molecular and morphological adaptations that enable C. daubneyi to defend itself against microorganisms, feed and reproduce within the rumen.

The synergistic potential of polyethylene glycol 400 for the control of Hyphantria cunea larvae by Beauveria bassiana

The efficacy of entomopathogenic fungi as pest control agents is constrained by both their physiological state and external environmental factors. This study identified synergists capable of enhancing the insecticidal activity of Beauveria bassiana (Bb) and investigated the underlying synergistic mechanisms. Our results found that among 6 potential synergists, polyethylene glycol 400 (PEG) and trehalose significantly improved Bb's lethality against Hyphantria cunea larvae, with PEG demonstrating the most pronounced effect. PEG treatment markedly increased Bb spore adhesion and germination rates, while spore hydrophobicity and growth rates remained unaffected. Moreover, PEG-treated spores exhibited higher thermal tolerance compared to untreated ones. In the Bb + PEG treatment group, the hemocyte count, encapsulation and melanization activities, and the expression of related regulatory genes were significantly lower than those in the Bb treatment group. Additionally, pathogen recognition, signal transduction, and humoral immunity effector genes expression were markedly suppressed in the Bb + PEG group. A significant reduction in the content of total amino acids, free fatty acids, glucose, and trehalose, alongside decreased expression of key regulatory genes in the tricarboxylic acid cycle and glycolysis pathways, was observed in the Bb + PEG treatment group. Furthermore, PEG enhanced Bb-induced apoptosis in H. cunea larvae, as evidenced by the upregulation of apoptosis-related genes. Notably, PEG alone did not significantly impact the innate immunity, energy metabolism, or apoptosis in H. cunea larvae. Overall, PEG exhibits considerable potential in amplifying Bb's insecticidal activity by directly optimizing spore performance and indirectly modulating the larvae's innate immunity, energy metabolism, and apoptosis.

Characterization of a novel short-type peptidoglycan recognition protein from the sea cucumber Apostichopus japonicus

Peptidoglycan recognition proteins (PGRPs) represent a key component of the family of pattern recognition receptors (PRRs). The functional mechanisms of PGRPs in innate immunity are poorly understood. In this study, we identified a novel short-type PGRP, AjPGRP-S2, from the sea cucumber Apostichopus japonicus. Our data showed that AjPGRP-S2 encoded an extracellular protein that possessed a signal peptide, a complete zinc (Zn2+) binding site, and a disulfide bond. A recombinant AjPGRP-S2 (rAjPGRP-S2) lacking the signal peptide was generated and demonstrated to exhibit amidase activity. Tissue expression analysis revealed that AjPGRP-S2 was highly expressed in coelomocytes and tube feet. Immune-responsive analysis indicated that AjPGRP-S2 was able to bind to various pathogen-associated molecular patterns (PAMPs) from bacteria and fungi, as well as to Gram-positive and -negative bacteria, and was majorly induced by DAP-PGN challenge. Basing on RNA-Seq and Pearson's correlation testing, RNA interference, and pull-down analysis, AjPGRP-S2 was found to be involved in transducing immune signals to the complement system and other PRRs, such as fibrinogen, by protein interactions to further recognize and kill pathogens. To respond comprehensively against pathogenic invasion, AjPGRP-S2 may also have the potential in transducing immune signals to key processes, such as cell adhesion, nerve conduction, apoptosis, and transcription by complex pathways that have yet to be elucidated. Our findings not only promote our understanding of the immune-related function and mechanisms of the PGRP family in A. japonicus, but also provide important data that will facilitate the identification of key evolutionary characteristics associated with invertebrate PGRPs.

The proteomic content of Varroa destructor gut varies according to the developmental stage of its host

The nutritional physiology of parasites is often overlooked although it is at the basis of host-parasite interactions. In the case of Varroa destructor, one of the major pests of the Western honey bee Apis mellifera, the nature of molecules and tissues ingested by the parasite is still not completely understood. Here, the V. destructor feeding biology was explored through artificial feeding, dissection of the mite's gut and proteomic analyses. More specifically, the proteome of guts extracted from starved mites and honey bee-fed mites was compared to highlight both the parasite proteins likely involved in food processing and the honey bee proteins actually ingested by the mite. We could identify 25 V. destructor candidate proteins likely involved in the parasite digestion. As the host developmental stages infested by the mite are diverse, we also focused on the identity and on the origin of honey bee proteins ingested by the mite when it feeds on larvae, pupae or adults. We highlighted profiles of consumed honey bee proteins and their variations throughout the V. destructor life cycle. These variations matched the ones observed in the honey bee hemolymph, showing that this tissue is an important part of the mite's diet. Based on the variations of abundance of the most consumed honey bee proteins and on their functions, the potential implication of these key candidate nutrients in V. destructor reproduction is also discussed.

Identification and functional characterization of a long-type peptidoglycan recognition protein, PGRP-L in amphibian Xenopus laevis

Peptidoglycan recognition proteins (PGRPs) are a family of multifunctional proteins playing vital roles in PGN metabolism and antibacterial defense, and their functions have been well-characterized in mammals, bony fishes, and insects. However, the information about the functions of amphibian long-type PGRP is rather limited. Here, we identified and cloned a long-type PGRP gene (named Xl-PGRP-L) from African clawed frog, Xenopus laevis. Xl-PGRP-L gene was detected in all orangs/tissues examined, and was rapidly induced in intestine, liver, and lung following the stimulation of PGN. Sequence analysis showed that Xl-PGRP-L possesses four Zn2+-binding residues (His358, Tyr395, His470, and Cys478) required for amidase activity of catalytic PGRPs, and assays for amidase activity revealed that recombinant Xl-PGRP-L cloud degrade PGN in a Zn2+-dependent manner, indicating that Xl-PGRP-L is belonging to catalytic PGRPs. In addition, Xl-PGRP-L have antibacterial activity against Gram-negative bacteria Edwardsiella tarda and Gram-positive bacteria Streptococcus agalactiae. The present investigation represents the first characterization regarding the biological activities of amphibian long-type PGRPs, thus contributes to a better understanding of the functions of tetrapod PGRPs and the molecular mechanisms of amphibian antibacterial defense.

Differential gene expression in Chorthippus parallelus (Zetterstedt, 1821) (Orthoptera: Acrididae: Gomphocerinae) induced by Wolbachia infection

Distinct lineages of the grasshopper Chorthippus parallelus (Orthoptera: Acrididae) form well-known hybrid zones (HZs) both in the Pyrenees and the Alps mountain ranges in South Europe. These HZs represent unique experimental systems to identify "key genes" that maintain genetic boundaries between emerging species. The Iberian endemism C. p. erythropus (Cpe) and the subspecies C. p. parallelus (Cpp), widely distributed throughout the rest of Europe, overlap and form the Pyrenean HZ. Both subspecies differ morphologically, as well as in behavioral, mitochondrial, nuclear, and chromosomal traits, and in the strains of the maternally transmitted bacterial endosymbiont Wolbachia infecting them. This results in either unidirectional and bidirectional cytoplasmic incompatibility between both grasshopper subspecies, pointing out that Wolbachia clearly affects gene expression in the infected individuals. Here we explore how Wolbachia may modify the expression of some major genes involved in relevant pathways in Cpp in the Pyrenean HZ. We have analyzed, through molecular biomarkers, the physiological responses in C. parallelus individuals infected by Wolbachia, with particular attention to the energy metabolism, the immune system response, and the reproduction. qPCR was used to evaluate the expression of selected genes in the gonads of infected and uninfected adults of both sexes, since this tissue constitutes the main target of Wolbachia infection. Transcriptional analyses also showed differential sex-dependent responses in most of the analyzed biomarkers in infected and noninfected individuals. We identified for the first time new sensitive biomarkers that might be involved in the reproductive barrier induced by Wolbachia in the hybrid zone.

Comparative Analysis of PGRP Family in Polymorphic Worker Castes of Solenopsis invicta

Peptidoglycan recognition proteins (PGRPs) are a class of pattern recognition receptors (PRRs) that activate the innate immune system in response to microbial infection by detection of peptidoglycan, a distinct component of bacterial cell walls. Bioinformatic studies have revealed four PGRPs in the red imported fire ant Solenopsis invicta; nonetheless, the mechanism of the immune response of S. invicta induced by pathogens is still poorly understood. The peptidoglycan recognition protein full-length cDNA (designated as SiPGRP-S1/S2/S3/L) from S. invicta was used in this investigation. According to the sequencing analysis, there was a significant degree of homology between the anticipated amino acid sequence of SiPGRPs and other members of the PGRPs superfamily. Molecular docking studies demonstrated that SiPGRPs show strong binding affinity for a variety of PGN substrates. Additionally, tissue distribution analysis indicated that SiPGRPs are primarily expressed in several tissues of naïve larvae, including fat body, hemocytes, head, and thorax, as detected by quantitative real-time PCR (RT-qPCR). Microbial challenges resulted in variable changes in mRNA levels across different tissues. Furthermore, the antibacterial effects of antimicrobial peptides (AMPs) produced by major ants infected with Metarhizium anisopliae were assessed. These AMPs demonstrated inhibitory effects against M. anisopliae, Staphylococcus aureus, and Escherichia coli, with the most pronounced effect observed against E. coli. In conclusion, SiPGRPs act as pattern recognition receptors (PRRs) that identify pathogens and initiate the expression of AMPs in S. invicta, this mechanism contributes to the development of biopesticides designed for the targeted control of invasive agricultural pests.

The influence of inactivated entomopathogenic bacterium Bacillus thuringiensis on the immune responses of the Colorado potato beetle

Background

Invasion of microorganisms into the gut of insects triggers a cascade of immune reactions accompanied by increased synthesis of effectors (such as antimicrobial peptides, cytokines, and amino acids), leading to changes in the physiological state of the host. We hypothesized that even an inactivated bacterium can induce an immune response in an insect. The aim of this study was to compare the roles of reactive oxygen species (ROS) formation and of the response of detoxification and antioxidant systems in a Colorado potato beetle (CPB) larval model in the first hours after invasion by either an inactivated or live bacterium.

Methods

The influence of per os inoculation with inactivated entomopathogenic bacterium Bacillus thuringiensis var. tenebrionis (Bt) on the survival and physiological and biochemical parameters of CPB larvae was assessed as changes in the total hemocyte count (THC), activity of phenoloxidases (POs), glutathione-S-transferases (GSTs), nonspecific esterases (ESTs), catalase, peroxidases, superoxide dismutases (SODs) and formation of reactive oxygen species (ROS).

Results

A series of changes occurred within the hemolymph and the midgut of CPBs inoculated with inactivated Bt at 12 h after inoculation. These physiological and biochemical alterations serve to mediate generalized resistance to pathogens. The changes were associated with an increase in the THC and a 1.4-2.2-fold enhancement of detoxification enzymatic activities (such as GST and EST) as well as increased levels of antioxidants (especially peroxidases) in hemolymph in comparison to the control group. Suppressed EST activity and reduced ROS formation were simultaneously detectable in the larval midgut. Inoculation of beetle larvae with active Bt cells yielded similar results (elevated THC and suppressed PO activity). A fundamental difference in the immune activation processes between larvae that ingested the inactivated bacterium and larvae that had consumed the active bacterium was that the inactivated bacterium did not influence ROS formation in the hemolymph but did reduce their formation in the midgut. At 24 h postinfection with active Bt, ROS levels went up in both the hemolymph and the midgut. This was accompanied by a significant 5.7-fold enhancement of SOD activity and a 5.3-fold suppression of peroxidase activity. The observed alterations may be due to within-gut toxicity caused by early-stage bacteriosis. The imbalance in the antioxidant system and the accumulation of products toxic to the "putative" pathogen can activate detoxification mechanisms, including those of an enzymatic nature (EST and GST). The activation of detoxification processes and of innate immune responses is probably due to the recognition of the "putative" pathogen by gut epithelial cells and is similar in many respects to the immune response at early stages of bacteriosis.

Linc20486 promotes BmCPV replication through inhibiting the transcription of AGO2 and Dicers

The silkworm holds pivotal economic importance, serving not only as a primary source of silk but also as a prominent model organism in scientific research. Nonetheless, silkworm farming remains vulnerable to diverse factors, with viral infections posing the gravest threat to the sericulture industry. Among these, the Bombyx mori cytoplasmic polyhedrosis virus (BmCPV), a member of the Reoviridae family and the cytoplasmic polyhedrosis virus genus, emerges as a significant pathogen in silkworm production. BmCPV infection primarily induces midgut sepsis in silkworms, spreads rapidly, and can inflict substantial economic losses on sericulture production. Presently, effective strategies for preventing and treating BmCPV infections are lacking. Long non-coding RNA (lncRNA) constitutes a class of RNA molecules with transcripts exceeding 200 nt, playing a crucial role in mediating the interplay between pathogens and host cells. Investigation through high-throughput technology has unveiled that BmCPV infection markedly upregulates the expression of Linc20486. This observation suggests potential involvement of Linc20486 in regulating virus replication. Indeed, as anticipated, knockdown of Linc20486 in cells profoundly impedes BmCPV replication, whereas overexpression significantly enhances virus propagation. To probe into the mechanism underlying Linc20486's impact on virus replication, its effects on autophagy, innate immunity, and RNAi-related pathways were scrutinized. The findings revealed that Linc20486 exerts significant influence on the expression of RNAi pathway-related genes, such as Dicer1, Dicer2 and AGO2. This discovery holds promise for unveiling novel avenues to comprehend and combat BmCPV infections in silkworms.

Uncovering hidden dangers: The combined toxicity of abamectin and lambda-cyhalothrin on honey bees

Studying the toxic effects of pesticides on bees has consistently been a prominent area of interest for researchers. Nonetheless, existing research has predominantly concentrated on individual toxicity assessments, leaving a gap in our understanding of mixed toxicity. This study delves into the individual and combined toxic effects of abamectin (ABA) and lambda-cyhalothrin (LCY) on honey bees (Apis mellifera) in laboratory settings. We discovered that ABA (96 h-LC50 value of 0.079 mg/L) exhibited greater acute toxicity to honey bees compared to LCY (96 h-LC50 value of 9.177 mg/L). Moreover, the mixture of ABA and LCY presented an acute antagonistic effect on honey bees. Additionally, our results indicated that exposure to LCY, at medium concentration, led to a reduction in the abundance of gut core bacterium Snodgrassella. However, an increase in the abundance of Bifidobacterium was noted when exposed to a medium concentration of LCY and its mixture with ABA. Transcriptomic analysis revealed significant regulation of certain genes in the medium concentration of all three treatments compared to the control group, primarily enriching in metabolism and immune-related pathways. Following chronic exposure to field-relevant concentrations of ABA, LCY, and their mixture, there were significant alterations in the activities of immunity-related enzyme polyphenol oxidase (PPO) and detoxification enzymes glutathione S-transferase (GST) and carboxylesterase (CarE). Additionally, the expression of four genes (abaecin, cyp9e2, cyp302a1, and GstD1) associated with immune and detoxification metabolism was significantly altered. These findings suggest a potential health risk posed by the insecticides ABA and LCY to honey bees. Despite exhibiting acute antagonistic effect, mixed exposure still induced damage to bees at all levels. This study advances our knowledge of the potential adverse effects of individual or combined exposure to these two pesticides on non-target pollinators and offers crucial guidance for the use of insecticides in agricultural production.

Ontogeny of immunity and natural viral infection in Apis mellifera drones and workers

The most common viral diseases affecting honey bees (Apis mellifera) in Israel include deformed wing viruses (DWV-A and DWV-B) and acute paralysis viruses (ABPV and IAPV). These viruses are transmitted within and between colonies, both horizontally and vertically. All members of the colony contribute to this transmission, on the other hand individual and social immunity, particularly hygienic behaviour, may affect the outcome of the process. In this study, we evaluated the ontogeny of natural infections of DWV-A, DWV-B, ABPV and IAPV, their prevalence and loads, in workers and drones from high (H) and low (L) hygienic colonies. In parallel, we evaluated the expression of two immune genes: peptidoglycan recognition protein S2(PGRP-S2) and hymenoptaecin. The prevalence of DWV-B and IAPV increased with age and was higher in workers than in drones. ABPV was not detected in drones. The expression of both immune genes was significantly affected by age and sex. Drones from H colonies had higher expression of these genes. The increased expression of immune genes with drones' age, particularly in hygienic colonies, suggest additional value of honey bee breeding for hygienic behaviour for sustainable beekeeping.

A novel prolixicin identified in common bed bugs with activity against both bacteria and parasites

The hematophagous common bed bug, Cimex lectularius, is not known to transmit human pathogens outside laboratory settings, having evolved various immune defense mechanisms including the expression of antimicrobial peptides (AMPs). We unveil three novel prolixicin AMPs in bed bugs, exhibiting strong homology to the prolixicin of kissing bugs, Rhodnius prolixus, and to diptericin/attacin AMPs. We demonstrate for the first time sex-specific and immune mode-specific upregulation of these prolixicins in immune organs, the midgut and rest of body, following injection and ingestion of Gr+ (Bacillus subtilis) and Gr- (Escherichia coli) bacteria. Synthetic CL-prolixicin2 significantly inhibited growth of E. coli strains and killed or impeded Trypanosoma cruzi, the Chagas disease agent. Our findings suggest that prolixicins are regulated by both IMD and Toll immune pathways, supporting cross-talk and blurred functional differentiation between major immune pathways. The efficacy of CL-prolixicin2 against T. cruzi underscores the potential of AMPs in Chagas disease management.

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.

Comparative Transcriptomics of Fat Bodies between Symbiotic and Quasi-Aposymbiotic Adult Females of Blattella germanica with Emphasis on the Metabolic Integration with Its Endosymbiont Blattabacterium and Its Immune System

We explored the metabolic integration of Blattella germanica and its obligate endosymbiont Blattabacterium cuenoti by the transcriptomic analysis of the fat body of quasi-aposymbiotic cockroaches, where the endosymbionts were almost entirely removed with rifampicin. Fat bodies from quasi-aposymbiotic insects displayed large differences in gene expression compared to controls. In quasi-aposymbionts, the metabolism of phenylalanine and tyrosine involved in cuticle sclerotization and pigmentation increased drastically to compensate for the deficiency in the biosynthesis of these amino acids by the endosymbionts. On the other hand, the uricolytic pathway and the biosynthesis of uric acid were severely decreased, probably because the reduced population of endosymbionts was unable to metabolize urea to ammonia. Metabolite transporters that could be involved in the endosymbiosis process were identified. Immune system and antimicrobial peptide (AMP) gene expression was also reduced in quasi-aposymbionts, genes encoding peptidoglycan-recognition proteins, which may provide clues for the maintenance of the symbiotic relationship, as well as three AMP genes whose involvement in the symbiotic relationship will require additional analysis. Finally, a search for AMP-like factors that could be involved in controlling the endosymbiont identified two orphan genes encoding proteins smaller than 200 amino acids underexpressed in quasi-aposymbionts, suggesting a role in the host-endosymbiont relationship.

Ligands of HMG-like dorsal switch protein 1 of Spodoptera exigua leads to mortality in diamondback moth, Plutellaxylostella

HMG-like dorsal switch protein 1 (DSP1) is the insect homolog of the high mobility group box 1 (HMGB1) protein of the vertebrates. Previous studies confirmed DSP1 in Spodoptera exigua, Tenebrio molitor, and Aedes albopictus, and were analyzed for their immune roles, survivability, and binding affinity with entomopathogenic bacterial metabolites. The present study aimed to predict, and confirm DSP1 in diamondback moth, Plutella xylostella along with the effect of Spodoptera exigua DSP1 ligands in the survivability of this insect. DSP1 of Plutella xylostella (Px-DSP1) consists of 465 amino acids (AA). Phylogeny analysis showed that Px-DSP1 clustered with other Lepidopteran insects where each insect order clustered separately. Domain analysis showed that like other insects, Px-DSP1 contains two HMG boxes (Box A and Box B), one coiled-coil (CC), five Q-rich low complexity (LC), and an acidic tail (AT). Px-DSP1 was expressed in each developmental stage and tissue. The highest expression was in L4 larvae and fat body tissues. Thermal shift assay (TSA) showed the binding affinity of 3-Ethoxy-4-Methoxyphenol (EMP), Phthalimide (PM), and o-Cyanobenzoic acid (CBA) to rDSP1 of Spodoptera exigua. Mortality bioassay showed that all these metabolites were toxic against P. xylostella larvae. Among these, EMP was more toxic providing more than 65% mortality at 500 ppm concentration. However, PM and CBA also showed more than 60 and 50% mortality, respectively at 500 ppm concentration. We assume that like Se-DSP1, these compounds also bind with Px-DSP1 which leads to the inhibition of DSP1-mediated immunity and impose the mortality of Plutella xylostella larvae.

The microglial innate immune protein PGLYRP1 mediates neuroinflammation and consequent behavioral changes

Peptidoglycan recognition protein 1 (PGLYRP1) is a pattern-recognition protein that mediates antibacterial actions and innate immune responses. Its expression and role in neuroinflammatory conditions remain unclear. We observed the upregulation of PGLYRP1 in inflamed human and mouse spinal cord and brain, with microglia being the primary cellular source. Experiments using a recombinant PGLYRP1 protein show that PGLYRP1 potentiates reactive gliosis, neuroinflammation, and consequent behavioral changes in multiple animal models of neuroinflammation. Furthermore, shRNA-mediated knockdown of Pglyrp1 gene expression attenuates this inflammatory response. In addition, we identify triggering receptor expressed on myeloid cell-1 (TREM1) as an interaction partner of PGLYRP1 and demonstrate that PGLYRP1 promotes neuroinflammation through the TREM1-Syk-Erk1/2-Stat3 axis in cultured glial cells. Taken together, our results reveal a role for microglial PGLYRP1 as a neuroinflammation mediator. Finally, we propose that PGLYRP1 is a potential biomarker and therapeutic target in various neuroinflammatory diseases.

BmPGPR-L4 is a negative regulator of the humoral immune response in the silkworm Bombyx mori

Toll, immune deficiency and prophenoloxidase cascade represent vital immune signaling pathways in insects. Peptidoglycan recognition proteins (PGRPs) are innate immune receptors that activate and regulate the immune signaling pathways. Previously, we reported that BmPGPR-L4 was induced in the silkworm Bombyx mori larvae by bacteria and peptidoglycan challenges. Here, we focused on the function of BmPGRP-L4 in regulating the expression of antimicrobial peptides (AMPs). The hemolymph from BmPGRP-L4-silenced larvae exhibited an enhanced inhibitory effect on the growth of Escherichia coli, either by growth curve or inhibitory zone experiments. Coincidentally, most of the AMP genes were upregulated after RNAi of BmPGRP-L4. Oral administration of heat-inactivated E. coli and Staphylococcus aureus after RNAi of BmPGRP-L4 resulted in the increased expression of BmPGRP-L4 in different tissues of the silkworm larvae, revealing an auto-regulatory mechanism. By contrast, the expression of most AMP genes was downregulated by oral bacterial administration after RNAi of BmPGRP-L4. The above results demonstrate that BmPGRP-L4 recognizes bacterial pathogen-associated molecular patterns and negatively regulates AMP expression to achieve immunological homeostasis. As a negative regulator, BmPGPR-L4 is proposed to be involved in the feedback regulation of the immune signaling pathways of the silkworm to prevent excessive activation of the immune response.

Variation in Bombyx mori immune response against fungal pathogen Beauveria bassiana with variability in cell wall β-1,3-glucan

Entomopathogenic fungi are protected by a cell wall with dynamic structure for adapting to various environmental conditions. β-1,3-Glucan recognition proteins activate the innate immune system of insects by recognizing surface molecules of fungi. However, the associations between pathogenicity and the different components of entomopathogenic fungal cell walls remain unclear. Three Beauveria bassiana strains were selected that have significantly differing virulence against Bombyx mori. The molecular mechanisms underlying the immune response in B. mori were investigated using RNA sequencing, which revealed differences in the immune response to different B. bassiana strains at 12 h post-infection. Immunofluorescence assays revealed that β-1,3-glucan content had an opposite trend to that of fungal virulence. β-1,3-Glucan injection upregulated BmβGRP4 expression and significantly reduced the virulence of the high-virulence strain but not that of the medium-virulence or low-virulence strains. BmβGRP4 silencing in B. mori with RNA interference resulted in the opposite virulence pattern, indicating that the virulence of B. bassiana was affected by the cell walls' content of β-1,3-glucan, which could be recognized by BmβGRP4. Furthermore, interference with the gene CnA (calcineurin catalytic A subunit) involved in β-1,3-glucan synthesis eliminated differences in virulence between B. bassiana strains. These results indicate that strains of a single species of pathogenic fungi that have differing cell wall components are recognized differently by the innate immune system of B. mori.

MicroRNA Targets PAP1 to Mediate Melanization in Plutella xylostella (Linnaeus) Infected by Metarhizium anisopliae

MicroRNAs (miRNAs) play a pivotal role in important biological processes by regulating post-transcriptional gene expression and exhibit differential expression patterns during development, immune responses, and stress challenges. The diamondback moth causes significant economic damage to crops worldwide. Despite substantial advancements in understanding the molecular biology of this pest, our knowledge regarding the role of miRNAs in regulating key immunity-related genes remains limited. In this study, we leveraged whole transcriptome resequencing data from Plutella xylostella infected with Metarhizium anisopliae to identify specific miRNAs targeting the prophenoloxidase-activating protease1 (PAP1) gene and regulate phenoloxidase (PO) cascade during melanization. Seven miRNAs (pxy-miR-375-5p, pxy-miR-4448-3p, pxy-miR-279a-3p, pxy-miR-3286-3p, pxy-miR-965-5p, pxy-miR-8799-3p, and pxy-miR-14b-5p) were screened. Luciferase reporter assays confirmed that pxy-miR-279a-3p binds to the open reading frame (ORF) and pxy-miR-965-5p to the 3' untranslated region (3' UTR) of PAP1. Our experiments demonstrated that a pxy-miR-965-5p mimic significantly reduced PAP1 expression in P. xylostella larvae, suppressed PO activity, and increased larval mortality rate. Conversely, the injection of pxy-miR-965-5p inhibitor could increase PAP1 expression and PO activity while decreasing larval mortality rate. Furthermore, we identified four LncRNAs (MSTRG.32910.1, MSTRG.7100.1, MSTRG.6802.1, and MSTRG.22113.1) that potentially interact with pxy-miR-965-5p. Interference assays using antisense oligonucleotides (ASOs) revealed that silencing MSTRG.7100.1 and MSTRG.22113.1 increased the expression of pxy-miR-965-5p. These findings shed light on the potential role of pxy-miR-965-5p in the immune response of P. xylostella to M. anisopliae infection and provide a theoretical basis for biological control strategies targeting the immune system of this pest.

Histopathological observations and comparative transcriptome analysis of Ophiocordyceps sinensis infection of Hepialus xiaojinensis in the early stage

Hepialus xiaojinensis is a Lepidopteran insect and one of the hosts for the artificial cultivation of Cordyceps. Ophiocordyceps sinensis can infect and coexist with H. xiaojinensis larvae for a long time. Little studies focused on the interaction process through its early infection stage. In this research, we particularly study the interaction of infected and uninfected larvae in the 3rd (OS-3, CK-3) and 4th (OS-4, CK-4) instars. O. sinensis was distributed within the larvae and accompanied by pathological changes in some tissue structures. In response to O. sinensis infection, OS-3 enhanced the antioxidant defense ability, while OS-4 decreased. The transcriptome analysis showed that OS-3 resisted the invasion of O. sinensis by the immune and nervous systems. Correspondingly, OS-4 reduced immune response and utilized more energy for growth and development. This study provides a comprehensive resource for analyzing the mechanism of H. xiaojinensis and O. sinensis interaction.

The immune response of the whitefly Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) when parasitized by Eretmocerus eremicus (Hymenoptera: Aphelinidae)

In insects, the innate immune system is subdivided into cellular and humoral defenses. When parasitoids attack insects, both reactions can be activated and notably, the phenoloxidase (PO) cascade and lytic activity are part of both cellular and humoral defenses. However, to our knowledge, no study has characterized any immune response of the whitefly Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) to the attack of Eretmocerus eremicus (Hymenoptera: Aphelinidae). Therefore, the first objective of the present study was to determine whether whitefly nymphs recently parasitized by E. eremicus exhibit any immune response. For this, we estimate the level of prophenoloxidase (proPO), phenoloxidase (PO), and lytic activity by colorimetric assays. A second objective was to assess whether the observed whitefly immune response could be related to a previously reported preference of the predator Geocoris punctipes (Hemiptera: Lygaeidae) for non-parasitized nymphs. We therefore offered non-parasitized and recently parasitized nymphs to the predator. Our results show that parasitism of whitefly nymphs by E. eremicus induced a highly estimated level of proPO and PO, and a lower level of lytic activity. In addition, we found that G. punctipes did not show a preference for non-parasitized over recently parasitized nymphs. The nymphs of T. vaporariorum activated the PO pathway against E. eremicus; however, the increase in proPO and PO levels was traded-off with decreased lytic activity. In addition, the previously reported preference for non-parasitized nymphs was not seen in our experiments, indicating that the induced immune response did not affect predator behavior by G. punctipes.

A gut commensal bacterium promotes black soldier fly larval growth and development partly via modulation of intestinal protein metabolism

IMPORTANCE

Black solider fly larvae and the gut microbiota can recycle nutrients from various organic wastes into valuable insect biomass. We found that Citrobacter amalonaticus, a gut commensal bacterium of the insect, exerts beneficial effects on larval growth and development and that the expression of many metabolic larval genes was significantly impacted by the symbiont. To identify the larval genes involved in the host-symbiont interaction, we engineered the symbiont to produce double-strand RNA and enabled the strain to silence host genes in the larval gut environment where the interaction takes place. With this approach, we confirmed that two intestinal protease families are involved in the interaction and provided further evidence that intestinal protein metabolism plays a role in the interaction. This work expands the genetic toolkits available to study the insect functional genomics and host-symbiont interaction and provide the prospective for the future application of gut microbiota on the large-scale bioconversion.

Transcriptome analysis of immune-related genes of Asian corn borer (Ostrinia furnacalis [Guenée]) after oral bacterial infection

The Asian corn borer (Ostrinia furnacalis) is an important agricultural pest causing serious damage to economic crops, such as corn and sorghum. The gut is the first line of defense against pathogens that enter through the mouth. Staphylococcus aureus was used to infect the O. furnacalis midgut to understand the midgut immune mechanism against exogenous pathogens to provide new ideas and methods for the prevention and control of O. furnacalis. A sequencing platform was used for genome assembly and gene expression. The unigene sequences were annotated and functionally classified by Gene Ontology and Kyoto Encyclopedia of Genes and Genomes. Significant differences were found in the induced expression profiles before and after infection. Some differentially expressed genes have important relations with lipid metabolism and immune mechanism, suggesting that they play an important role in the innate immune response of O. furnacalis. Furthermore, quantitative real-time polymerase chain reaction assay was used to identify the key genes involved in the signaling pathway, and the expression patterns of these key genes were confirmed. The results could help study the innate immune system of lepidopteran insects and provide theoretical support for the control of related pests and the protection of beneficial insects.

Partner of neuropeptide bursicon homodimer pburs mediates a novel antimicrobial peptide Ten3LP via Dif/Dorsal2 in Tribolium castaneum

Bursicon is a cystine knot family neuropeptide, composed of two subunits, bursicon (burs) and partner of burs (pburs). The subunits can form heterodimers to regulate cuticle tanning and wing maturation and homodimers to signal different biological functions in innate immunity, midgut stem cell proliferation and energy homeostasis, and reproductive physiology in the model insects Drosophila melanogaster or Tribolium castaneum. Here, we report on the role of the pburs homodimer in signaling innate immunity in T. castaneum larvae. Through transcriptome analysis we identified a set of immune-related genes that respond to pburs RNAi. Treating larvae with recombinant-pburs protein led to up-regulation of antimicrobial peptide (AMP) genes in vivo and in vitro. The upregulation of most AMP genes was dependent on the NF-κB transcription factor Relish. Most importantly, we identified a novel AMP, Tenecin 3-like peptide (Ten3LP), regulated by pburs via NF-κB transcription factor Dorsal-related immunity factor (Dif)/Dorsal2, but not Relish. We conducted Ten3LP RNAi, synthesized recombinant Ten3LP protein for microbial inhibition assays and functionally characterized Ten3LP as an AMP specific for fungi and Gram-positive bacteria. We demonstrate that expression of Ten3LP is activated by pburs via the Toll pathway. These findings identify new molecular targets for development of potential antibiotics for treating microbial infections and perhaps for RNAi based pest management technology.

Transfer of Cd along the food chain: The susceptibility of Hyphantria cunea larvae to Beauveria bassiana under Cd stress

Heavy metal can affect the bio-control efficiency of entomopathogenic fungi on pests, but this has not been studied in the food chain. Here, the food chain of soil-Fraxinus mandshurica-Hyphantria cunea was constructed to investigate the effect of cadmium (Cd) exposure on the susceptibility of H. cunea larvae to Beauveria bassiana (Bb) and to analyze the corresponding mechanism through larval innate immunity and energy metabolism. Cd through the food chain synergistically enhanced the susceptibility of H. cunea larvae to Bb. Cellular immunity-related parameters decreased when the Cd treatment group was compared with the control group and when the combined treatment group of Cd and Bb was compared with the Bb treatment group. Cd exposure induced hormesis on pathogen recognition and signal transduction genes of humoral immunity, but reduced the expression of effector genes. The expression of the 13 humoral immunity-related genes in the combined treatment group was lower than in the Bb treatment group. Cd exposure decreased the energy storage of H. cunea larvae before Bb infection and aggravated the disorder level of energy metabolism after Bb infection. Taken together, disturbance of innate immunity and energy metabolism improves the susceptibility of H. cunea larvae to Bb in the Cd-polluted food chain.

Mechanisms and roles of the first stage of nodule formation in lepidopteran insects

Nodule formation is a process of cellular immunity in insects and other arthropods with open circulatory systems. Based on histological observations, nodule formation occurs in 2 stages. The first stage occurs immediately after microbial inoculation and includes aggregate formation by granulocytes. The second stage occurs approximately 2-6 h later and involves the attachment of plasmatocytes to melanized aggregates produced during the first stage. The first stage response is thought to play a major role in the rapid capture of invading microorganisms. However, little is known regarding how granulocytes in the hemolymph form aggregates, or how the first stage of the immunological response protects against invading microorganisms. Since the late 1990s, our understanding of the molecules and immune pathways that contribute to nodule formation has improved. The first stage of nodule formation involves a hemocyte-induced response that is triggered by pathogen-associated molecular pattern (PAMP) recognition proteins in the hemolymph regulated by a serine proteinase cascade and cytokine (Spätzle) and Toll signaling pathways. Hemocyte agglutination proceeds through stepwise release of biogenic amine, 5-HT, and eicosanoids that act downstream of the Toll pathway. The first stage of nodule formation is closely linked to melanization and antimicrobial peptide (AMP) production, which is critical for insect humoral immunity. Nodule formation in response to artificial inoculation with millions of microorganisms has long been studied. It has recently been suggested that this system is the original natural immune system, and enables insects to respond to a single invading microorganism in the hemocoel.

Immune functions of pattern recognition receptors in Lepidoptera

Pattern recognition receptors (PRRs), as the "sensors" in the immune response, play a prominent role in recognizing pathogen-associated molecular patterns (PAMPs) and initiating an effective defense response to pathogens in Lepidoptera. It is becoming increasingly clear that damage-associated molecular patterns (DAMPs) normally play a physiological role within cells; however, when exposed to extracellular, they may become "part-time" critical signals of the immune response. Based on research in recent years, we review herein typical PRRs of Lepidoptera, including peptidoglycan recognition protein (PGRP), gram-negative binding protein (GNBP), β-1,3-glucan recognition protein (βGRP), C-type lectin (CTL), and scavenger receptor (SR). We also outline the ways in which DAMPs participate in the immune response and the correlation between PRRs and immune escape. Taken together, these findings suggest that the role of PRRs in insect innate immunity may be much greater than expected and that it is possible to recognize a broader range of signaling molecules.

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.

Effects of the pyrE deletion mutant from Bacillus thuringiensis on gut microbiota and immune response of Spodoptera exigua

The gut microbiota is essential for the growth and development of insects, and the intestinal immune system plays a critical role in regulating the homeostasis of intestinal microorganisms and their interactions with pathogenic bacteria. Infection with Bacillus thuringiensis (Bt) can disrupt the gut microbiota of insects, but the regulatory factors governing the interaction between Bt and gut bacteria are not well understood. Uracil secreted by exogenous pathogenic bacteria can activate DUOX-mediated reactive oxygen species (ROS) production, which helps maintain intestinal microbial homeostasis and immune balance. To elucidate the regulatory genes involved in the interaction between Bt and gut microbiota, we investigate the effects of uracil derived from Bt on gut microbiota, and host immunity using a uracil deficient Bt strain (Bt GS57△pyrE) obtained by homologous recombination. We analyze the biological characteristics of the uracil deficient strain and found that the deletion of uracil in Bt GS57 strain changed the diversity of gut bacteria in Spodoptera exigua, as investigated using Illumina HiSeq sequencing. Furthermore, qRT-PCR analysis showed that compared with Bt GS57 (control), the expression of the SeDuox gene and the level of ROS were significantly decreased after feeding with Bt GS57△pyrE. Adding uracil to Bt GS57△pyrE restored the expression level of DUOX and ROS to a higher level. Additionally, we observed that PGRP-SA, attacin, defensin and ceropin genes were significant different in the midgut of S. exigua infected by Bt GS57 and Bt GS57△pyrE, with a trend of increasing first and then decreasing. These results suggest that uracil regulates and activates the DUOX-ROS system, affects the expression of antimicrobial peptide genes, and disturb intestinal microbial homeostasis. We preliminarily speculate that uracil is a key factor in the interaction between Bt and gut microbiota, and these findings provide a theoretical basis for clarifying the interaction between Bt, host, and intestinal microorganisms, as well as for gaining new insights into the insecticidal mechanism of B. thuringiensis in insects.

Peptidoglycan Recognition Protein S5 of Bombyx mori Facilitates the Proliferation of Bombyx mori Cypovirus 1

Bombyx mori cypovirus 1 (BmCPV1), a primary pathogen of the silkworm, is a typical dsRNA virus belonging to the Reoviridae family. In this study, a total of 2520 differentially expressed genes (DEGs) were identified by RNA-seq analysis of the silkworm midgut after BmCPV1 infection and Gene Ontology (GO) functional annotation showed that the DEGs predominantly functioned in binding (molecular function), cell (cellular component), and cellular processes (biological process). Additionally, the Kyoto Encyclopedia of Genes and Genomes (KEGG) functional annotation revealed that the DEGs were mainly distributed in global and overview metabolism maps, translation, and signal transduction. Among the identified DEGs, BmPGRP-S5 belongs to the peptidoglycan recognition protein (PGRP) family. Previous studies have revealed that PGRPs were involved in the interactions between silkworm and BmCPV1. Here, we explored the effect of BmPGRP-S5 on BmCPV1 replication and demonstrated that BmPGRP-S5 promotes the proliferation of BmCPV1 in BmN cells through overexpression or knockdown experiments. Knocking down of BmPGRP-S5 in silkworm larvae similarly promoted the proliferation of BmCPV1. Through experimental validation, we therefore determined that BmPGRP-S5 acts as a proviral host factor for BmCPV1 infection. This study clarifies the proliferation mechanism of BmCPV1 and provides new insights into the functional role of BmPGRP-S5.

Preliminary Analysis of Transcriptome Response of Dioryctria sylvestrella (Lepidoptera: Pyralidae) Larvae Infected with Beauveria bassiana under Short-Term Starvation

The Dioryctria genus contains several destructive borer pests that are found in coniferous forests in the Northern Hemisphere. Beauveria bassiana spore powder was tested as a new method of pest control. In this study, Dioryctria sylvestrella (Lepidoptera: Pyralidae) was used as the object. A transcriptome analysis was performed on a freshly caught group, a fasting treatment control group, and a treatment group inoculated with a wild B. bassiana strain, SBM-03. Under the conditions of 72-h fasting and a low temperature of 16 ± 1 °C, (i) in the control group, 13,135 of 16,969 genes were downregulated. However, in the treatment group, 14,558 of 16,665 genes were upregulated. (ii) In the control group, the expression of most genes in the upstream and midstream of the Toll and IMD pathways was downregulated, but 13 of the 21 antimicrobial peptides were still upregulated. In the treatment group, the gene expression of almost all antimicrobial peptides was increased. Several AMPs, including cecropin, gloverin, and gallerimycin, may have a specific inhibitory effect on B. bassiana. (iii) In the treatment group, one gene in the glutathione S-transferase system and four genes in the cytochrome P450 enzyme family were upregulated, with a sharp rise in those that were upregulated significantly. In addition, most genes of the peroxidase and catalase families, but none of the superoxide dismutase family were upregulated significantly. Through innovative fasting and lower temperature control, we have a certain understanding of the specific defense mechanism by which D. sylvestrella larvae may resist B. bassiana in the pre-wintering period. This study paves the way for improving the toxicity of B. bassiana to Dioryctria spp.

The Virtuous Galleria mellonella Model for Scientific Experimentation

The first research on the insect Galleria mellonella was published 85 years ago, and the larva is now widely used as a model to study infections caused by bacterial and fungal pathogens, for screening new antimicrobials, to study the adjacent immune response in co-infections or in host-pathogen interaction, as well as in a toxicity model. The immune system of the G. mellonella model shows remarkable similarities with mammals. Furthermore, results from G. mellonella correlate positively with mammalian models and with other invertebrate models. Unlike other invertebrate models, G. mellonella can withstand temperatures of 37 °C, and its handling and experimental procedures are simpler. Despite having some disadvantages, G. mellonella is a virtuous in vivo model to be used in preclinical studies, as an intermediate model between in vitro and mammalian in vivo studies, and is a great example on how to apply the bioethics principle of the 3Rs (Replacement, Reduction, and Refinement) in animal experimentation. This review aims to discuss the progress of the G. mellonella model, highlighting the key aspects of its use, including experimental design considerations and the necessity to standardize them. A different score in the "cocoon" category included in the G. mellonella Health Index Scoring System is also proposed.

Identification and characterization of two long-type peptidoglycan recognition proteins, PGRP-L1 and PGRP-L2, in the orange-spotted grouper, Epinephelus coioides

Peptidoglycan recognition proteins (PGRPs) play an important role in innate immunity by recognizing components of pathogenic bacteria (such as peptidoglycan, PGN) and are evolutionarily conserved pattern recognition receptors (PRRs) in both invertebrates and vertebrates. In the present study, two long-type PGRPs (designed as Eco-PGRP-L1 and Eco-PGRP-L2) were identified in orange-spotted grouper (Epinephelus coioides), which is a major economic species cultured in Asia. The predicted protein sequences of both Eco-PGRP-L1 and Eco-PGRP-L2 contain a typical PGRP domain. Eco-PGRP-L1 and Eco-PGRP-L2 exhibited organ/tissue-specific expression patterns. An abundant expression of Eco-PGRP-L1 was observed in pyloric caecum, stomach and gill, whereas a highest expression level of Eco-PGRP-L2 was found in head kidney, spleen, skin and heart. In addition, Eco-PGRP-L1 is distributed in the cytoplasm and nucleus, while Eco-PGRP-L2 is mainly localized in cytoplasm. Both Eco-PGRP-L1 and Eco-PGRP-L2 were induced following the stimulation of PGN and have PGN binding activity. In addition, functional analysis revealed that Eco-PGRP-L1 and Eco-PGRP-L2 possess antibacterial activity against Edwardsiella tarda. These results may contribute to understand the innate immune system of orange-spotted grouper.

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.

Silencing of the immune gene BmPGRP-L4 in the midgut affects the growth of silkworm (Bombyx mori) larvae

Peptidoglycan recognition proteins (PGRPs) are one of the receptors in insects' immune pathways, essential for insects to recognize the exogenous pathogens in order to activate the Toll and immune deficiency (IMD) pathway. In the silkworm Bombyx mori, previous studies focused on the short PGRPs and less is known about the long PGRPs. In this study, a long PGRP in silkworm BmPGRP-L4 was cloned and its expression and function were analysed. The results showed that BmPGRP-L4 contains a transmembrane region, a conserved PGRP domain, and an amidase-2 domain. The expression profile demonstrated that BmPGRP-L4 existed in diverse tissues including epidermis, fat body, midgut, and silk glands, with remarkably high expression in the midgut in the 5th instar. Oral infection with Escherichia coli and Staphylococcus aureus significantly induced BmPGRP-L4 in the midgut and epidermis, as well as in the fat body and silk glands. Peptidoglycan also induced the expression of BmPGRP-L4 in midgut tissue ex vivo and BmN4 cells in vitro. RNAi of BmPGRP-L4 was effective in the midgut and epidermis, while the efficiency in the fat body was transient. RNAi-mediated knock-down of BmPGRP-L4 reduced the weight and growth of the silkworm, possibly due to its participation in the immune response and the regulation of the microbiota in the midgut lumen of the silkworm larvae.

Endoplasmic reticulum-associated protein degradation contributes to Toll innate immune defense in Drosophila melanogaster

In Drosophila, the endoplasmic reticulum-associated protein degradation (ERAD) is engaged in regulating pleiotropic biological processes, with regard to retinal degeneration, intestinal homeostasis, and organismal development. The extent to which it functions in controlling the fly innate immune defense, however, remains largely unknown. Here, we show that blockade of the ERAD in fat bodies antagonizes the Toll but not the IMD innate immune defense in Drosophila. Genetic approaches further suggest a functional role of Me31B in the ERAD-mediated fly innate immunity. Moreover, we provide evidence that silence of Xbp1 other than PERK or Atf6 partially rescues the immune defects by the dysregulated ERAD in fat bodies. Collectively, our study uncovers an essential function of the ERAD in mediating the Toll innate immune reaction in Drosophila.

PvML1 suppresses bacterial infection by recognizing LPS and regulating AMP expression in shrimp

Toll and Toll-like receptors (TLRs) play essential roles in the innate immunity of Drosophila and mammals. Recent studies have revealed the presence of Toll-mediated immune signaling pathways in shrimp. However, the recognition and activation mechanism of Toll signaling pathways in crustaceans remain poorly understood due to the absence of key recognition molecules, such as peptidoglycan recognition proteins. Here, a novel MD2-related lipid-recognition (ML) member named PvML1 was characterized in Penaeus vannamei. We found that PvML1 shared a similar 3D structure with human MD2 that could specifically recognize lipopolysaccharides (LPS) participating in LPS-mediated TLR4 signaling. PvML1 was highly expressed in hemocytes and remarkably upregulated after Vibrio parahemolyticus challenge. Furthermore, the binding and agglutinating assays showed that PvML1 possessed strong binding activities to LPS and its key portion lipid A as well as Vibrio cells, and the binding of PvML1 with bacterial cells led to the agglutination of bacteria, suggesting PvML1 may act as a potential pathogen recognition protein upon interaction with LPS. Besides, coating V. parahemolyticus with recombinant PvML1 promoted bacterial clearance in vivo and increased the survival rate of bacterium-challenged shrimp. This result was further confirmed by RNAi experiments. The knockdown of PvML1 remarkably suppressed the clearance of bacteria in hemolymph and decreased the survival rate of infected shrimp. Meanwhile, the silencing of PvML1 severely impaired the expression of a few antimicrobial peptides (AMPs). These results demonstrated the significant correlation of bacterial clearance mediated by PvML1 with the AMP expression. Interestingly, we found that PvML1 interacted with the extracellular region of PvToll2, which had been previously shown to participate in bacterial clearance by regulating AMP expression. Taken together, the proposed antibacterial model mediated by PvML1 might be described as follows. PvML1 acted as a potential recognition receptor for Gram-negative bacteria by binding to LPS, and then it activated PvToll2-mediated signaling pathway by interacting with PvToll2 to eliminate invading bacteria through producing specific AMPs. This study provided new insights into the recognition and activation mechanism of Toll signaling pathways of invertebrates and the defense functions of ML members.

Molecular characterization of peptidoglycan recognition proteins from Mytilus coruscus

Mytilus shows great immune resistance to various bacteria from the living waters, indicating a complex immune recognition mechanism against various microbes. Peptidoglycan recognition proteins (PGRPs) play an important role in the defense against invading microbes via the recognition of the immunogenic substance peptidoglycan (PGN). Therefore, eight PGRPs were identified from the gill transcriptome of Mytilus coruscus. The sequence features, expression pattern in various organs and larval development stages, and microbes induced expression profiles of these Mytilus PGRPs were determined. Our data revealed the constitutive expression of PGRPs in various organs with relative higher expression level in immune-related organs. The expression of PGRPs is developmentally regulated, and most PGRPs are undetectable in larvae stages. The expression level of most PGRPs was significantly increased with in vivo microbial challenges, showing strong response to Gram-positive strain in gill and digestive gland, strong response to Gram-negative strain in hemocytes, and relative weaker response to fungus in the three tested organs. In addition, the function analysis of the representative recombinant expressed PGRP (rMcPGRP-2) confirmed the antimicrobial and agglutination activities, showing the immune-related importance of PGRP in Mytilus. Our work suggests that Mytilus PGRPs can act as pattern recognition receptors to recognize the invading microorganisms and the antimicrobial effectors during the innate immune response of Mytilus.

Characterization of New Defensin Antimicrobial Peptides and Their Expression in Bed Bugs in Response to Bacterial Ingestion and Injection

Common bed bugs, Cimex lectularius, can carry, but do not transmit, pathogens to the vertebrate hosts on which they feed. Some components of the innate immune system of bed bugs, such as antimicrobial peptides (AMPs), eliminate the pathogens. Here, we determined the molecular characteristics, structural properties, and phylogenetic relatedness of two new defensins (CL-defensin1 (XP_024085718.1), CL-defensin2 (XP_014240919.1)), and two new defensin isoforms (CL-defensin3a (XP_014240918.1), CL-defensin3b (XP_024083729.1)). The complete amino acid sequences of CL-defensin1, CL-defensin2, CL-defensin3a, and CL-defensin3b are strongly conserved, with only minor differences in their signal and pro-peptide regions. We used a combination of comparative transcriptomics and real-time quantitative PCR to evaluate the expression of these defensins in the midguts and the rest of the body of insects that had been injected with bacteria or had ingested blood containing the Gram-positive (Gr+) bacterium Bacillus subtilis and the Gram-negative (Gr–) bacterium Escherichia coli. We demonstrate, for the first time, sex-specific and immunization mode-specific upregulation of bed bug defensins in response to injection or ingestion of Gr+ or Gr– bacteria. Understanding the components, such as these defensins, of the bed bugs’ innate immune systems in response to pathogens may help unravel why bed bugs do not transmit pathogens to vertebrates.

Identification of PGRP2 and its three splice variants in grass carp Ctenopharyngodon idella

In this study, peptidoglycan recognition protein 2 (PGRP2) gene was cloned in grass carp Ctenopharyngodon idella, with the open reading frame (ORF) of PGRP2 being 1452 bp, encoding a protein of 483 amino acids. Three splice variants, PGRP2a, PGRP2b, and PGRP2c, were found also in grass carp with the absence of entire exon two and partial exon two of the PGRP2, and were predicted to have 124, 371 and 311 amino acids. But, they all have PGRP domain and signal peptide, except PGRP2a. The PGRP2 and its variants were expressed in all organs/tissues examined, and stimulated following PGN injection. It is further detected that the expression of gcPGRP2 and its variants was up-regulated after the single transfection of each of gcPGRP2 and its variant expression plasmids in CO cells. It is considered that the cloning of PGRP2 in grass carp provides a compositional completeness of PGRP members in this fish with the inclusion of previously reported PGRP5 and PGRP6.

Differences in microbiome composition and transcriptome profiles between male and female Paederus fuscipes harbouring pederin-producing bacteria

Pederin, a group of antitumor compounds, is produced by an endosymbiotic bacterium of Paederus fuscipes. Pederin content differed between male and female P. fuscipes, but the reason why these differences are maintained remains unexplored. Here, the pederin-producing bacteria (PPB) infection rate in P. fuscipes was investigated. Furthermore, we assessed the microbiota structure differences in male and female P. fuscipes harbouring PPB and sequenced the transcriptome of both sexes to shed light on genes of interest. Of the 625 analysed beetles (275 females, 350 males), 96.36% of females and 31.14% of males were positive for PPB infection. PPB accounted for 54.36%-82.70% of the bacterial population in females but showed a much lower abundance in males (0.92%-3.87%). Reproductive organs possessed the highest PPB abundance compared with other parts of females, but no such relationships existed in males. Moreover, we provide the first transcriptome analysis of male and female P. fuscipes harbouring PPB and identified 8893 differentially expressed unigenes. Our results indicated that the pederin content difference between males and females might be caused by the PPB density difference in hosts. The biosequence data would be helpful for illustrating the mechanism that regulates PPB density in P. fuscipes.