C-type lectin interacting with β-integrin enhances hemocytic encapsulation in the cotton bollworm, Helicoverpa armigera

Sublethal effects of chlorantraniliprole on immunity in Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae): Promote encapsulation by upregulating a heat shock protein 70 family gene SfHSP68.1

As an agricultural pest, the fall armyworm (FAW), Spodoptera frugiperda, poses a severe threat to agriculture in China. Chlorantraniliprole has been widely used to control this pest. In our previous studies, we discovered that LD10, LD20, and LD30 chlorantraniliprole promoted encapsulation in the 4th instar larvae of the FAW, with LD30 chlorantraniliprole having the most significant effect. To further investigate the molecular mechanism underlying the sublethal effects of chlorantraniliprole on encapsulation in the FAW, this study conducted the effects of encapsulation in 4th instar larvae of the FAW exposed to LD30 chlorantraniliprole. Then, we analyzed the transcriptome of the FAW hemolymph treated with LD30 chlorantraniliprole and identified genes related to encapsulation using RNAi. Our results showed that the encapsulation in the FAW was enhanced at 6, 12, 18, 24, and 48 h after exposure to LD30 chlorantraniliprole. Additionally, LD30 chlorantraniliprole significantly affected the expression of certain immune-related genes, with the heat shock protein 70 family gene SfHSP68.1 showing the most significant upregulation. Subsequent interference with SfHSP68.1 resulted in a significant inhibition of encapsulation in FAW. These findings suggested that LD30 chlorantraniliprole can promote encapsulation in the FAW by upregulating SfHSP68.1 expression. This study provides valuable insights into the sublethal effects of chlorantraniliprole on encapsulation in the FAW and the interaction between encapsulation and heat shock proteins (HSPs).

A C-type lectin (CTL2) mediated both humoral and cellular immunity against bacterial infection in Tribolium castaneum

C-type lectins (CTLs) play essential roles in humoral and cellular immune responses of invertebrates. Previous studies have demonstrated the involvement of CTLs in the humoral immunity of Tribolium castaneum, a worldwide pest in stored products. However, the function of CTLs in cellular immunity remains unclear. Here, we identified a CTL gene located on chromosome X and designated it as CTL2 (TcCTL2) from T. castaneum. It encodes a protein of 305 amino acids with a secretion signal peptide and a carbohydrate-recognition domain. TcCTL2 was mainly expressed in the early pupae and primarily distributed in the hemocytes in the late larvae. It was significantly upregulated after larvae were infected with Escherichia coli or Staphylococcus aureus, while knockdown of TcCTL2 exacerbates larval mortality and bacterial colonization after infection. The purified recombinant TcCTL2 (rTcCTL2) can bind to pathogen-associated molecular patterns and microbes and promote hemocyte-mediated encapsulation, melanization and phagocytosis in vitro. rTcCTL2 also induced bacterial agglutination in a Ca2+-dependent manner. Knockdown of TcCTL2 drastically suppressed encapsulation, melanization, and phagocytosis. Furthermore, silencing of TcCTL2 followed by bacterial infection significantly decreased the expression of transcription factors in Toll and IMD pathways, antimicrobial peptides, and prophenoloxidases and phenoloxidase activity. These results unveiled that TcCTL2 mediates both humoral and cellular immunity to promote bacterial clearance and protect T. castaneum from infectious microbes, which will deepen the understanding of the interaction between CTLs and innate immunity in T. castaneum and permit the optimization of pest control strategies by a combination of RNAi technology and bacterial infection.

SfMBP: A novel microbial binding protein and pattern recognition receptor in the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae)

The fall armyworm, Spodoptera frugiperda, poses a significant threat as a highly destructive agricultural pest in many countries. Understanding the complex interplay between the insect immune system and entomopathogens is critical for optimizing biopesticide efficacy. In this study, we identified a novel microbial binding protein, SfMBP, in S. frugiperda. However, the specific role of SfMBP in the immune response of S. frugiperda remains elusive. Encoded by the LOC118269163 gene, SfMBP shows significant induction in S. frugiperda larvae infected with the entomopathogen Beauveria bassiana. Consisting of 115 amino acids with a signal peptide, an N-terminal flexible region and a C-terminal β-sheet, SfMBP lacks any known functional domains. It is expressed predominantly during early larval stages and in the larval epidermis. Notably, SfMBP is significantly induced in larvae infected with bacteria and fungi and in SF9 cells stimulated by peptidoglycan. While recombinant SfMBP (rSfMBP) does not inhibit bacterial growth, it demonstrates binding capabilities to bacteria, fungal spores, peptidoglycan, lipopolysaccharides, and polysaccharides. This binding is inhibited by monosaccharides and EDTA. Molecular docking reveals potential Zn2+-interacting residues and three cavities. Furthermore, rSfMBP induces bacterial agglutination in the presence of Zn2+. It also binds to insect hemocytes and SF9 cells, enhancing phagocytosis and agglutination responses. Injection of rSfMBP increased the survival of S. frugiperda larvae infected with B. bassiana, whereas blocking SfMBP with the antibody decreased survival. These results suggest that SfMBP acts as a pattern recognition receptor that enhances pathogen recognition and cellular immune responses. Consequently, this study provides valuable insights for the development of pest control measures.

A pattern recognition receptor C-type lectin TcCTL14 contributes to immune response and development in the red flour beetle, Tribolium castaneum

Evidence is accumulating that pattern recognition receptor (PRR) C-type lectins (CTL) play essential roles in recognition of pathogens. TcCTL14 (accession no. TC00871) contains the most domains among all CTL of Tribolium castaneum. Yet the biological function of TcCTL14 remains unclear. In this study, TcCTL14 exhibiting typical motif and domain of CTL was cloned from T. castaneum. The expression pattern analysis showed that TcCTL14 was highly expressed in late pupae and central nervous system, and was upregulated after treatment with Escherichia coli and Staphylococcus aureus, respectively. Analysis of binding affinity revealed that recombinant TcCTL14 not only could bind to lipopolysaccharide and peptidoglycan in a dose-dependent fashion, but possibly could bind to and agglutinate different bacteria in a Ca2+ -dependent fashion. Knockdown of TcCTL14 before injection with bacteria led to the downregulation of nuclear factor-κB transcription factors of Toll/IMD and 4 antimicrobial peptides. Knockdown of TcCTL14 also caused suppressed metamorphosis, reduced fecundity, and delayed embryogenesis of T. castaneum. Further observation discovered that knockdown of TcCTL14 inhibited the development of ovaries and embryos. The detection of signaling pathways revealed that TcCTL14 may be involved in metamorphosis and fecundity by impacting 20-hydroxyecdysone and vitellogenin, respectively. Overall, these results indicate that TcCTL14 may contribute to immune response by agglutination or regulating the expression of antimicrobial peptides by the Toll/IMD pathway, and is required for T. castaneum development including metamorphosis, fecundity, and embryogenesis. These findings will improve the functional cognition of PRR CTL in insects and provide the new strategy for pest control.

Identification and Expression of Integrins during Testicular Fusion in Spodoptera litura

Integrin members are cell adhesion receptors that bind to extracellular matrix (ECM) proteins to regulate cell-cell adhesion and cell-ECM adhesion. This process is essential for tissue development and organogenesis. The fusion of two testes is a physiological phenomenon that is required for sperm production and effective reproduction in many Lepidoptera. However, the molecular mechanism of testicular fusion is unclear. In Spodoptera litura, two separated testes fuse into a single testis during the larva-to-pupa transformation. We identified five α and five β integrin subunits that were closely associated with testicular fusion. Integrin α1 and α2 belong to the position-specific 1 (PS1) and PS2 groups, respectively. Integrin α3, αPS1/αPS2, and αPS3 were clustered into the PS3 group. Integrin β1 belonged to the insect β group, and β2, β3, and β5 were clustered in the βν group. Among these integrins, α1, α2, α3, αPS1/PS2, αPS3, β1, and β4 subunits were highly expressed when the testes fused. However, their expression levels were much lower before and after the fusion of the testis. The qRT-PCR and immunohistochemistry analyses indicated that integrin β1 mRNA and the protein were highly expressed in the peritoneal sheath of the testis, particularly when the testes fused. These results indicate that integrins might participate in S. litura testicular fusion.

Functional Analysis of a CTL-X-Type Lectin CTL16 in Development and Innate Immunity of Tribolium castaneum

C-type lectins (CTLs) are a class of proteins containing carbohydrate recognition domains (CRDs), which are characteristic modules that recognize various glycoconjugates and function primarily in immunity. CTLs have been reported to affect growth and development and positively regulate innate immunity in Tribolium castaneum. However, the regulatory mechanisms of TcCTL16 proteins are still unclear. Here, spatiotemporal analyses displayed that TcCTL16 was highly expressed in late pupae and early adults. TcCTL16 RNA interference in early larvae shortened their body length and narrowed their body width, leading to the death of 98% of the larvae in the pupal stage. Further analysis found that the expression level of muscle-regulation-related genes, including cut, vestigial, erect wing, apterous, and spalt major, and muscle-composition-related genes, including Myosin heavy chain and Myosin light chain, were obviously down-regulated after TcCTL16 silencing in T. castaneum. In addition, the transcription of TcCTL16 was mainly distributed in the hemolymph. TcCTL16 was significantly upregulated after challenges with lipopolysaccharides, peptidoglycans, Escherichia coli, and Staphylococcus aureus. Recombinant CRDs of TcCTL16 bind directly to the tested bacteria (except Bacillus subtilis); they also induce extensive bacterial agglutination in the presence of Ca²⁺. On the contrary, after TcCTL16 silencing in the late larval stage, T. castaneum were able to develop normally. Moreover, the transcript levels of seven antimicrobial peptide genes (attacin2, defensins1, defensins2, coleoptericin1, coleoptericin2, cecropins2, and cecropins3) and one transcription factor gene (relish) were significantly increased under E. coli challenge and led to an increased survival rate of T. castaneum when infected with S. aureus or E. coli, suggesting that TcCTL16 deficiency could be compensated for by increasing AMP expression via the IMD pathways in T. castaneum. In conclusion, this study found that TcCTL16 could be involved in developmental regulation in early larvae and compensate for the loss of CTL function by regulating the expression of AMPs in late larvae, thus laying a solid foundation for further studies on T. castaneum CTLs.

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.

Deacetylation of K481 and K484 on Penaeid Shrimp Hemocyanin Is Critical for Antibacterial Activity

Although invertebrates' innate immunity relies on several immune-like molecules, the diversity of these molecules and their immune response mechanisms are not well understood. Here, we show that Penaeus vannamei hemocyanin (PvHMC) undergoes specific deacetylation under Vibrio parahaemolyticus and LPS challenge. In vitro deacetylation of PvHMC increases its binding capacity with LPS and antibacterial activity against Gram-negative bacteria. Lysine residues K481 and K484 on the Ig-like domain of PvHMC are the main acetylation sites modulated by the acetyltransferase TIP60 and deacetylase HDAC3. Deacetylation of PvHMC on K481 and K484 allows PvHMC to form a positively charged binding pocket that interacts directly with LPS, whereas acetylation abrogates the positive charge to decrease PvHMC-LPS attraction. Besides, V. parahaemolyticus and LPS challenge increases the expression of Pvhdac3 to induce PvHMC deacetylation. This work indicates that, during bacterial infections, deacetylation of hemocyanin is crucial for binding with LPS to clear Gram-negative bacteria in crustaceans.

Immulectin-2 from the ghost moth, Thitarodes xiaojinensis (Lepidoptera: Hepialidae), modulates cellular and humoral responses against fungal infection

C type-lectins constitute a large family of pattern recognition receptors, playing important roles in insect immune defenses. Thitarodes xiaojinensis larvae showed distinct immune features after Ophiocordyceps sinensis, Cordyceps militaris, or Beauveria bassiana infection. Based on transcriptome and immunoblot analysis, we found that immulectin-2 (IML2) was induced after T. xiaojinensis larvae were infected by C. militaris or B. bassiana but maintained at a low level after larvae injected with O. sinensis or Ringer's buffer. Recombinant IML2 (rIML2) could promote melanization, encapsulation, phagocytosis, and hemocyte aggregation in vitro. RNA interference with IML2 induced a significant reduction in the transcript levels of various antimicrobial peptides. Importantly, we found that the abundance of O. sinensis blastospores coated with rIML2 dramatically decreased in the host hemolymph. Overall, this study demonstrated that T. xiaojinensis IML2 modulates cellular and humoral responses to entomopathogenic fungi, broadening our view of the immune interaction between O. sinensis and its host.

Infection of the entomopathogenic fungus Metarhizium rileyi suppresses cellular immunity and activates humoral antibacterial immunity of the host Spodoptera frugiperda

BACKGROUND

Metarhizium rileyi is an entomopathogenic fungus with promising potential for controlling agricultural pests, including Spodoptera frugiperda. Following penetration of the host through the cuticle, M. rileyi cells transform into in vivo blastospores or hyphal bodies, propagating within the hemocoel. However, the strategies and molecular mechanisms by which M. rileyi survives upon exposure to the powerful insect immune system remain unclear.

RESULTS

We determined the pathogenicity of M. rileyi and found that either conidial immersion or blastospore injection significantly decreased S. frugiperda survival in a dose-dependent manner. Injection of M. rileyi blastospores decreased the number of S. frugiperda hemocytes and impaired host cellular reactions such as nodulation, encapsulation and phagocytosis. Blastospore injection led to increased antibacterial activity in plasma at 48 h post-injection (hpi). RNA-sequencing analyses identified a large number of antimicrobial peptide genes upregulated in the fat body of M. rileyi-infected larvae at 48 hpi, which may be attributable to the activation of Toll and IMD signaling pathway.

CONCLUSION

This study demonstrates that the compromised cellular immunity of the insect host is due to the marked decrease in hemocytes and impaired cellular cytoskeletons, which may facilitate early infection by M. rileyi. Late in the course of infection, the enhanced antibacterial activity of plasma, which may be in response to intestinal evading bacteria, cannot inhibit hyphal growth in hemolymph. Our data provide a comprehensive resource for exploring the molecular mechanism employed by M. rileyi to overcome S. frugiperda immunity. © 2022 Society of Chemical Industry.

Integrins in the Immunity of Insects: A Review

Integrins are a large group of cell-surface proteins that are classified as transmembrane proteins. Integrins are classified into different types based on sequence variations, leading to structural and functional diversity. They are broadly distributed in animals and have a wide range of biological functions such as cell-to-cell communication, intracellular cytoskeleton organization, cellular signaling, immune responses, etc. Integrins are among the most abundant cell surface proteins in insects, exhibiting their indispensability in insect physiology. Because of their critical biological involvement in physiological processes, they appear to be a novel target for designing effective pest control strategies. In the current literature review, we first discuss the discovery and expression responses of integrins against various types of pathogens. Secondly, we examine the specific biological roles of integrins in controlling microbial pathogens, such as phagocytosis, encapsulation, nodulation, immune signaling, and so on. Finally, we describe the possible uses of integrins to control agricultural insect pests.

Expression of Modified Snowdrop Lectin (Galanthus nivalis Agglutinin) Protein Confers Aphids and Plutella xylostella Resistance in Arabidopsis and Cotton

Cotton is a major fiber crop in the world that can be severely infested by pests in agricultural fields. Identifying new insect-resistance genes and increasing the expression of known insect-resistance genes are imperative in cultivated cotton. Galanthus nivalis agglutinin (GNA), a lectin that is toxic to both chewing and sucking pests, is mainly expressed in monocotyledons. It is necessary to improve the expression of the GNA protein and to test whether the lectin confers insect resistance to dicotyledons plants. We report a modified GNA gene (ASGNA) via codon optimization, its insertion into Arabidopsis thaliana, and transient expression in cotton to test its efficacy as an insect-resistance gene against cotton aphids and Plutella xylostella. The amount of ASGNA in transgenic plants reached approximately 6.5 μg/g of fresh weight. A feeding bioassay showed that the survival rate of aphids feeding on the leaves of ASGNA transgenic plants was lower than those of aphids feeding on the leaves of non-optimized GNA (NOGNA) transgenic plants and wild-type plants. Meanwhile, the fertility rate was 36% when fed on the ASGNA transgenic plants, while the fertility was 70% and 95% in NOGNA transgenic plants and wild-type plants. Correspondingly, the highest mortality of 55% was found in ASGNA transgenic lines, while only 35% and 20% mortality was observed in NOGNA transgenic plants and wild-type plants, respectively. Similar results were recorded for aphids feeding on cotton cotyledons with transient expression of ASGNA. Taken together, the results show that ASGNA exhibited high insecticidal activity towards sap-sucking insects and thus is a promising candidate gene for improving insect resistance in cotton and other dicotyledonous plants.

The Dual Functions of a Bracovirus C-Type Lectin in Caterpillar Immune Response Manipulation

Parasitoids are widespread in natural ecosystems and normally equipped with diverse viral factors to defeat host immune responses. On the other hand, parasitoids can enhance the antibacterial abilities and improve the hypoimmunity traits of parasitized hosts that may encounter pathogenic infections. These adaptive strategies guarantee the survival of parasitoid offspring, yet their underlying mechanisms are poorly understood. Here, we focused on Cotesia vestalis, an endoparasitoid of the diamondback moth Plutella xylostella, and found that C. vestalis parasitization decreases the number of host hemocytes, leading to disruption of the encapsulation reaction. We further found that one bracovirus C-type lectin gene, CvBV_28-1, is highly expressed in the hemocytes of parasitized hosts and participates in suppressing the proliferation rate of host hemocytes, which in turn reduces their population and represses the process of encapsulation. Moreover, CvBV_28-1 presents a classical bacterial clearance ability via the agglutination response in a Ca²⁺-dependent manner in response to gram-positive bacteria. Our study provides insights into the innovative strategy of a parasitoid-derived viral gene that has dual functions to manipulate host immunity for a successful parasitism.

Calcium and integrin-binding protein 1-like interacting with an integrin α-cytoplasmic domain facilitates cellular immunity in Helicoverpa armigera

Integrins are transmembrane receptor heterodimers composed of α and β subunits. They are known to mediate extracellular signals to promote cell adhesion and spreading, and are therefore essential for cellular immunity. However, proteins that bind to integrin cytoplasmic domains and mediate intracellular signaling to promote cell adhesion require identification. Calcium and integrin-binding protein 1 (CIB1) that binds to the integrin α-cytoplasmic domain has rarely been examined in insects. In this study, we found that 20-hydroxyecdysone promoted cell phagocytosis and spreading in Helicoverpa armigera. Transcriptomic analyses of hemocytes identified an integrin α gene (HaINTα-PS1) whose expression could be induced by either 20-hydroxyecdysone injection or bead challenge. Isothermal titration calorimetry assays showed that H. armigera CIB1-like (HaCIB1-like) weakly bound to the cytoplasmic domain of HaINTα-PS1 in the presence of calcium. HaINTα-PS1 or HaCIB1-like knockdown inhibited hemocytic encapsulation and phagocytosis, and plasmatocyte spreading. Moreover, HaCIB1-like overexpression in a H. armigera epidermal cell line overexpanded cells and impaired cell phagocytosis. Thus, insect CIB1-like potentially interacted with integrin α-cytoplasmic domain and facilitated cell adhesion. This study enriches our understanding of the molecular mechanism underlying integrin-mediated cellular immunity in insects.

Functional analysis of TcCTL12 in innate immunity and development in Tribolium castaneum

C-type lectins (CTLs) play vital roles in invertebrates' innate immunity. Six CTL-X type lectins are identified in Tribolium castaneum. However, their functions and regulating mechanisms remain elusive. Here, TcCTL12, one CTL-X, was identified and cloned from T. castaneum. Spatiotemporal expression profiling revealed that TcCTL12 highly expressed in late pupa and early adult of T. castaneum in comparison with other developmental stages, and exhibited the highest expression level in the haemolymph and central nervous system (CNS). Then, the expression of TcCTL12 was remarkably induced by the stimulation of Escherichia coli and Staphylococcus aureus. Moreover, the recombinant protein TcCTL12 could bind pathogen-associated molecular patterns (PAMPs) including LPS and PGN, and displayed agglutinative activity to both Gram-positive and Gram-negative bacteria in a calcium-dependent manner in vitro. Furthermore, RNAi of TcCTL12 caused T. castaneum pupation and eclosion defected. The abnormal pupa thinned their epidermal, and appeared the abnormal development of muscle cell compared with the control group. Additionally, depletion of TcCTL12 resulted in reducing fertility of offspring and affected their fecundity. In sum, these results indicated that TcCTL12 had extensive functions in the regulation of development in T. castaneum, in addition to the immune response. It further expanded insights into CTL functions in insects.

An integrin alpha 4 (ChIntα 4) from oyster Crassostrea hongkongensis mediates the hemocytes phagocytosis towards Vibrio alginolyticus

Integrins, a family of cell adhesion transmembrane receptors, mediate cell adhesion, migration, proliferation, apoptosis, and phagocytosis. In the present study, an integrin ChIntα 4 from Crassostrea hongkongensis was characterized to investigate its role in defensing against pathogenic bacterium Vibrio alginolyticus. The full-length cDNA sequence of ChIntα 4 was 3572 bp with an open reading frame (ORF) of 3168 bp, which encoded a polypeptide with 1055 amino acids. The mRNA expression of ChIntα 4 in the hemocytes was significantly up-regulated at 6 h and 24 h post V. alginolyticus stimulation (p < 0.01). The recombinant ChIntα 4 protein could agglutinate the rabbit red blood cells and Gram-negative bacteria V. alginolyticus and Escherichia coli. Moreover, the phagocytic activity of the hemocytes was significantly down-regulated from 46.9% to 32.7% when blocked with anti-ChIntα 4 antibody, and it was significantly up-regulated from 42.7% to 59.5% post transfection with pCI-neo-ChIntα 4 plasmid (p < 0.05). In conclusion, these findings demonstrated that ChIntα 4 might be involved in resisting V. alginolyticus infection and regulating phagocytosis as a cell adhesion receptor in C. hongkongensis.

CTL10 has multiple functions in the innate immune responses of the silkworm, Bombyx mori

Insect C-type lectins (CTLs) play crucial roles in modulating the humoral and cellular immune responses. In the domesticated silkworm Bombyx mori L., BmCTL10 gene encodes an immulectin containing two carbohydrate recognition domains (CRDs). The phylogenetic analysis showed that BmCTL10 didn't cluster with other immulectin homologs in B. mori. BmCTL10 was mainly expressed in second to fifth instar larvae, wandering stage larvae, prepupa, and adults. In naïve fifth instar larvae, BmCTL10 was predominantly expressed in the fat body and epidermis. In second instar larvae, the topical application of Beauveria bassiana by immersion caused down-regulation of BmCTL10. The intra-hemocoel injection of E. coli, S. aureus, B. bassiana, and 20-hydroxyecdysone in fifth instar larvae caused tissue and time-specific inductions. The recombinant protein (rBmCTL10) can bind to larval hemocytes and various pathogen-associated molecular patterns to enhance hemocyte-mediated nodulation, phagocytosis, and encapsulation. rBmCTL10 caused significant upregulation of most antimicrobial peptides and nitric oxide synthase 1 in hemocytes in vivo. Yeast two-hybrid demonstrated that integrin β3 and β4 subunits can interact with BmCTL10. Furthermore, only CRD2 can interact with the β3, while both CRD1 and CRD2 can interact with the β4. Taken together, this study showed that BmCTL10 has multiple functions in the innate immune responses of B. mori and two integrin β subunits are their potential receptors.

Genome-Wide Analysis of Gene Families of Pattern Recognition Receptors in Fig Wasps (Hymenoptera, Chalcidoidea)

Pattern recognition receptors (PRRs) play important roles in detecting pathogens and initiating the innate immune response. Different evolutionary histories of pollinators and non-pollinators may result in different immune recognition systems. A previous study had reported that there were significant differences in peptidoglycan recognition proteins (PGRPs) between pollinators and non-pollinators in gene number and lineage of specific genes. In this study, based on the genomic data of 12 fig wasp species, with seven pollinators and five non-pollinators, we investigated the evolution patterns of PRRs, such as Gram-negative bacteria-binding proteins (GNBPs), C-type lectins (CTLs), scavenger receptors class B (SCRBs), fibrinogen-related proteins (FREPs), galectins, and thioester-containing proteins (TEPs). Our results showed that pollinators had no GNBP, but non-pollinators all had two gene members, which were clustered into two different clades in the phylogenetic tree, with each clade having specific domain and motif characteristics. The analysis of CTL and SCRB gene families also showed that there were lineage-specific genes and specific expansion in non-pollinators. Our results showed that there were significant differences in immune recognition between pollinators and non-pollinators, and we concluded that they had undergone flexible adaptive evolution in different environments. Our study can provide more molecular evidence for future functional studies on the immune system of fig wasps.

Diversity of insect antimicrobial peptides and proteins - A functional perspective: A review

The innate immune response of insects provides a robust line of defense against pathogenic microbes and eukaryotic parasites. It consists of two types of overlapping immune responses, named humoral and cellular, which share protective molecules and regulatory mechanisms that closely coordinate to prevent the spread and replication of pathogens within the compromised insect hemocoel. The major feature of the humoral part of the insect immune system involves the production and secretion of antimicrobial peptides from the fat body, which is considered analogous to adipose tissue and liver in vertebrates. Previous research has identified and characterized the nature of antimicrobial peptides that are directed against various targets during the different stages of infection. Here we review this information focusing mostly on the diversity and mode of action of these host defense components, and their critical contribution to maintaining host homeostasis. Extending this knowledge is paramount for understanding the evolution of innate immune function and the physiological balance required to provide sufficient protection to the host against external enemies while avoiding overactivation signaling events that would severely undermine physiological stability.

Steroid hormone 20-hydroxyecdysone promotes CTL1-mediated cellular immunity in Helicoverpa armigera

Mermithid nematodes, such as Ovomermis sinensis, are used as biological control agents against many insect pests, including cotton bollworm (Helicoverpa armigera). However, given the host's robust immune system, the infection rate of O. sinensis is low, thus restricting its widespread use. To understand the host defense mechanisms against mermithid nematodes, we identified and characterized a protein involved in the recognition of O. sinensis, the potential O. sinensis-binding protein C-type lectin 1 (HaCTL1a and/or HaCTL1b), which was eluted from the surface of O. sinensis after incubation with H. armigera plasma. HaCTL1b is homologous to the previously reported HaCTL1a protein. HaCTL1 was predominantly expressed in hemocytes and was induced by the steroid hormone 20-hydroxyecdysone through ecdysone receptor (HaEcR) or ultraspiracle (HaUSP), or both. Binding assays confirmed the interactions of the HaCTL1 proteins with O. sinensis but not with Romanomermis wuchangensis, a parasitic nematode of mosquito. Moreover, the HaCTL1 proteins were secreted into the hemocoel and promoted hemocyte-mediated encapsulation and phagocytosis. A knockdown of HaEcR and/or HaUSP resulted in compromised encapsulation and phagocytosis. Thus, HaCTL1 appears to modulate cellular immunity in the defense against parasitic nematodes, and the 20-hydroxyecdysone-HaEcR-HaUSP complex is involved in regulating the process.

Can Plant Lectins Help to Elucidate Insect Lectin-Mediated Immune Response?

Simple Summary

Lectins are proteins that can recognize and selectively bind specific sugar structures. These proteins are present in all kingdoms of life, including plants, animals, fungi and microorganisms and play a role in a broad range of processes. The interactions between lectins and their target carbohydrates play a primordial role in plant and animal immune systems. Despite being the largest and most diverse taxa on earth, the study of lectins and their functions in insects is lagging behind. To study the role of insect lectins in the immune response, plant lectins could provide an interesting tool. Plant lectins have been well characterized and many of them possess immunomodulatory properties in vertebrate cells. The increasing knowledge on the immunomodulatory effects of plant lectins could complement the missing knowledge on the endogenous insect lectins and contribute to understanding the processes and mechanisms by which lectins participate in insect immunity. This review summarizes existing studies of immune responses stimulated by endogenous or exogenous lectins.

Abstract

Lectins are carbohydrate-binding proteins that recognize and selectively bind to specific sugar structures. This group of proteins is widespread in plants, animals, and microorganisms, and exerts a broad range of functions. Many plant lectins were identified as exogenous stimuli of vertebrate immunity. Despite being the largest and most diverse taxon on earth, the study of lectins and their functions in insects is lagging behind. In insects, research on lectins and their biological importance has mainly focused on the C-type lectin (CTL) family, limiting our global understanding of the function of insect lectins and their role in insect immunity. In contrast, plant lectins have been well characterized and the immunomodulatory effects of several plant lectins have been documented extensively in vertebrates. This information could complement the missing knowledge on endogenous insect lectins and contribute to understanding of the processes and mechanisms by which lectins participate in insect immunity. This review summarizes existing studies of immune responses stimulated by endogenous or exogenous lectins. Understanding how lectins modulate insect immune responses can provide insight which, in turn, can help to elaborate novel ideas applicable for the protection of beneficial insects and the development of novel pest control strategies.

Regulators and signalling in insect antimicrobial innate immunity: Functional molecules and cellular pathways

Insects possess an immune system that protects them from attacks by various pathogenic microorganisms that would otherwise threaten their survival. Immune mechanisms may deal directly with the pathogens by eliminating them from the host organism or disarm them by suppressing the synthesis of toxins and virulence factors that promote the invasion and destructive action of the intruder within the host. Insects have been established as outstanding models for studying immune system regulation because innate immunity can be explored as an integrated system at the level of the whole organism. Innate immunity in insects consists of basal immunity that controls the constitutive synthesis of effector molecules such as antimicrobial peptides, and inducible immunity that is activated after detection of a microbe or its product(s). Activation and coordination of innate immune defenses in insects involve evolutionary conserved immune factors. Previous research in insects has led to the identification and characterization of distinct immune signalling pathways that modulate the response to microbial infections. This work has not only advanced the field of insect immunology, but it has also rekindled interest in the innate immune system of mammals. Here we review the current knowledge on key molecular components of insect immunity and discuss the opportunities they present for confronting infectious diseases in humans.

A Pattern Recognition Receptor C-type Lectin-S6 (CTL-S6) is Involved in the Immune Response in the Silkworm (Lepidoptera: Bombycidae)

Insect innate immunity is initiated by the special recognition and binding of the foreign pathogens, which is accomplished by the pattern recognition receptors (PRRs). As an important type of PRRs, C-type lectins (CTLs) play various roles in insect innate immunity, including pathogen recognition, stimulation of prophenoloxidase, regulation of cellular immunity and so on. In this study, we have cloned the full-length cDNA of a CTL gene named CTL-S6 from the silkworm, Bombyx mori. The open reading frame (ORF) of B. mori CTL-S6 encodes 378 amino acids, which contain a secretion signal peptide. The mRNA of CTL-S6 exhibited the highest transcriptional level in the midgut. Its transcriptional level increased dramatically in fat body and hemocytes upon Escherichia coli or Micrococcus luteus challenge. Purified recombinant CTL-S6 could bind to bacterial cell wall components, including peptidoglycan (PGN, from Bacillus subtilis) and lipopolysaccharide (LPS, from E. coli 0111:B4), and recombinant CTL-S6 was involved in the encapsulation and melanization of hemocytes. Furthermore, the addition of recombinant CTL-S6 to the hemolymph of silkworm resulted in a significant increase in phenoloxidase activity. Overall, our results indicated that B. mori CTL-S6 may serve as a PRR for the recognition of foreign pathogens, prophenoloxidase pathway stimulation and involvement in the innate immunity.

Comparative genomic analysis of C-type lectin-domain genes in seven holometabolous insect species

C-type lectins (CTLs) recognize various glycoconjugates through carbohydrate recognition domains (CRDs) and they play important roles in immune responses. In this study, comparative genomic analysis of CTLs were performed in 7 holometabolous species. CTL-S1 to S8 and CTL-X1 to X4 orthologous groups existed in the 7 species, while CTL-X5 group with dual-CRD, CTL-S11 group with triple-CRD, CTL-S9 group with a long C-terminus and Lepidopteran specific CTL-S10 group were not conserved. SliCTL-S12 to S14 cluster was only present in Spodoptera litura, and CTL-S genes were expanded on chromosomes 2 L and 2 R in Drosophila melanogaster. Most IMLs were clustered into three groups and the numbers of IMLs vary among species due to gene duplications. D. melanogaster specific CTLs and Lepidopteran IMLs within each of the three groups evolved more rapidly with higher dN/dS ratios. Two CRDs in IMLs clustered into two clades, with conserved Cys4-Cys5 and Cys1-Cys2 bonds in the first and second CRDs, respectively. The CTL-S and CTL-X family members in S. litura were mainly expressed in the fat body of 5th but not 6th instar larvae, and responded differently to S. litura nucleopolyhedrovirus (SpltNPV) and Nomuraea rileyi infection. The transcription levels of SliCTLs that expressed in fat body but not highly expressed in hemocytes were decreased at the middle and late stages of SpltNPV infection, and the mRNA levels of SliCTLs highly or specifically expressed in hemocytes were mainly decreased by SpltlNPV, N. rileyi and Bacillus thuringiensis infection. These results provide valuable information for further exploration of CTL functions in host-pathogen interaction.

C-type lectin-mediated microbial homeostasis is critical for Helicoverpa armigera larval growth and development

The immune system of a host functions critically in shaping the composition of the microbiota, and some microbes are involved in regulating host endocrine system and development. However, whether the immune system acts on endocrine and development by shaping the composition of the microbiota remains unclear, and few molecular players or microbes involved in this process have been identified. In the current study, we found that RNA interference of a C-type lectin (HaCTL3) in the cotton bollworm Helicoverpa armigera suppresses ecdysone and juvenile hormone signaling, thus reducing larval body size and delaying pupation. Depletion of HaCTL3 also results in an increased abundance of Enterocuccus mundtii in the hemolymph, which may escape from the gut. Furthermore, HaCTL3 and its controlled antimicrobial peptides (attacin, lebocin, and gloverin) are involved in the clearance of E. mundtii from the hemolymph via phagocytosis or direct bactericidal activity. Injection of E. mundtii into larval hemocoel mimics HaCTL3-depleted phenotypes and suppresses ecdysone and juvenile hormone signaling. Taken together, we conclude that HaCTL3 maintains normal larval growth and development of H. armigera via suppressing the abundance of E. mundtii in the hemolymph. Our results provide the first evidence of an immune system acting on an endocrine system to modulate development via shaping the composition of microbiota in insect hemolymph. Thus, this study will deepen our understanding of the interaction between immunity and development.

Considering that a large number of hemocytes and multiple soluble effectors are present in insect hemolymph, it is conventionally believed that healthy insect hemolymph is a hostile environment for bacteria and is, therefore, sterile. However, increasing evidences disprove this opinion, although the interactive mechanism between hemolymph microbiota and insect host, as well as the function of hemolymph microbiota, remain unclear. C-type lectin (CTL), as pattern recognition receptor (PRR), plays important roles in defending against various pathogens. Here we found that various bacteria colonized the hemolymph of the cotton bollworm Helicoverpa armigera. We first reported that an H. armigera CTL (HaCTL3) was involved in modulating larval growth and development. Further study indicated that HaCTL3-mediated homeostasis of Enterocuccus mundtii in the hemolymph is critical for normal larval growth and development. Our study demonstrated that this PRR modulated insect development through shaping hemolymph microbiota, which may represent a novel mechanism of immune system regulation during insect development.

Pattern recognition receptors from lepidopteran insects and their biological functions

Lepidopteran insects have potent innate immunity to fight against the invading pathogens. As the initiation step, pattern recognition receptors (PRRs) recognize and bind microbial surface configurations known as pathogen-associated molecular patterns (PAMPs). Aftermath, they initiate both cellular and humoral immune responses, including phagocytosis, agglutination, nodulation, encapsulation, prophenoloxidase activation, and synthesis of antimicrobial peptides. In this review, we summarize the recent findings concerning PRRs in lepidoptaeran insects, mostly agriculture pests including Helicoverpa armigera, Plutella xylostella, and Spodoptera exigua. We mainly focus on the function and phylogeny of C-type lectins (CTLs), peptidoglycan recognition proteins (PGRPs), β-1,3-glucan recognition proteins (βGRPs), and galectins (GALEs). It enriches our understanding of the immune system of lepidopteran insects and provides directions in the future research.

iTRAQ-Based Comparative Proteomic Analysis of Larval Midgut From the Beet Armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) Challenged With the Entomopathogenic Bacteria Serratia marcescens

Entomopathogenic bacteria Serratia marcescens is widely used as an environmentally friendly biocontrol agent against various pests, including Spodoptera exigua. Understanding the immune defense mechanism of S. exigua through comparative proteomic analysis can identify the key proteins expressed in response to the microbial infection. Here, we employed the as isobaric tags for relative and absolute quantification (iTRAQ) technique to investigate the effects of S. marcescens on the proteomic expression of S. exigua. Based on the molecular functional analysis, the differentially expressed proteins (DEPs) were mainly involved in the binding process and catalytic activities. Further bioinformatics analysis revealed important DEPs that played a crucial role in innate immunity of S. exigua with recognition (C-type lectin), melanization (propanol oxidase 3, serine protease, Serine-type carboxypeptidase activity, clip domain serine protease 4), antimicrobial activity (lysozyme, lysozyme-like, gloverin, cecropin B), detoxification (acetyl-CoA C-acetyltransferase, 3-dehydroecdysone 3-alpha-reductase, glucuronosyltransferase, glutathione S-transferase) and others. The Quantitative real-time PCR (qRT-PCR) results further indicated the significant upregulation of the immune-related genes in Spodoptera exigua following S. marcescens infection. To the best of our knowledge, this is the first iTRAQ based study to characterize S. marcescens mediated proteomic changes in S. exigua and identified important immune-related DEPs. The results of this study will provide an essential resource for understanding the host-pathogen interactions and the development of novel microbial biopesticides against various pests.

Identification of a Conserved Prophenoloxidase Activation Pathway in Cotton Bollworm Helicoverpa armigera

Melanization is a prominent insect humoral response for encapsulation of and killing invading pathogens. It is mediated by a protease cascade composed of a modular serine protease (SP), and clip domain SPs (cSPs), which converts prophenoloxidase (PPO) into active phenoloxidase (PO). To date, melanization pathway in cotton bollworm Helicoverpa armigera, an important agricultural pest, remains largely unclear. To biochemically reconstitute the pathway in vitro, the putative proteases along with modified proteases containing the factor Xa cleavage site were expressed by Drosophila S2 cell expression system. Purified recombinant proteins were used to examine their role in activating PPO. It is revealed that cascade is initiated by a modular SP-SP41, followed by cSP1 and cSP6. The three-step SP41/cSP1/cSP6 cascade could further activate PPO, and the PO activity was significantly enhanced in the presence of two cSP homologs (cSPHs), cSPH11 and cSPH50, suggesting the latter are cofactors for PPO activation. Moreover, baculovirus infection was efficiently blocked by the reconstituted PPO activation cascade, and the effect was boosted by cSPH11 and cSPH50. Taken together, we unraveled a conserved PPO activation cascade in H. armigera, which is similar to that exists in lepidopteran biochemical model Manduca sexta and highlighted its role in antagonizing viral infection.

The C-type lectin IML-10 promotes hemocytic encapsulation by enhancing aggregation of hemocytes in the Asian corn borer Ostrinia furnacalis

C-type lectins participate in hemocytic encapsulation as pattern recognition receptors; however, the molecular mechanisms underlying their function remain unknown. In this study, we determined that the encapsulation-promoting function of a C-type lectin, IML-10, may be related to its interaction with hemocytes in the agricultural pest Ostrinia furnacalis. IML-10 possesses two carbohydrate-recognition domains (CRDs) containing EPN and QPD motifs with 4 and 6 conserved cysteine residues, respectively. IML-10 was found to mainly be secreted by the fat body into the larval plasma, and its expression was induced by Sephadex A-25 beads. Anti-IML-10 antibodies inhibited encapsulation-promoting function of IML-10 in the larval plasma. The encapsulation rate of Sephadex A-25 beads decreased from approximately 90%-30% when expression of IML-10 in O. furnacalis larvae was inhibited by RNAi. Moreover, the Sephadex bead-encapsulating ability of hemocytes decreased to almost zero in O. furnacalis larvae with IML-10 knocked out by CRISPR/Cas9, with IML-10 expression clearly decreasing compared to that of the control. Similar to the larval plasma, recombinant IML-10 promoted Sephadex bead encapsulation by hemocytes. Immunohistochemistry analysis showed that IML-10 was able to bind to the surface of both granulocytes and plasmatocytes but not to Sephadex beads as foreign objects. Furthermore, recombinant IML-10 promoted hemocyte aggregation but not adhesion. Therefore, we speculate that IML-10 binds to the surface of hemocytes to promote their aggregation and further improve their encapsulation capacity. These results contribute to clarifying the function of insect C-type lectins in encapsulation.

Immune signaling pathways in the endoparasitoid, Pteromalus puparum

Parasitoids serve as effective biocontrol agents for agricultural pests. However, they face constant challenges from host immune defense and numerous pathogens and must develop potent immune defense against these threats. Despite the recent advances in innate immunity, little is known about the immunological mechanisms of parasitoids. Here, we identified and characterized potential immune-related genes of the endoparasitoid, Pteromalus puparum, which act in regulating populations of some members of the Pieridae. We identified 216 immune-related genes based on interrogating the P. puparum genome and transcriptome databases. We categorized the cognate gene products into recognition molecules, signal moieties and effector proteins operating in four pathways, Toll, IMD, JAK/STAT, and JNK. Comparative analyses of immune-related genes from seven insect species indicate that recognition molecules and effector proteins are more expanded and diversified than signaling genes in these signal pathways. There are common 1:1 orthologs between the endoparasitoid P. puparum and its relative, the ectoparasitoid Nasonia vitripennis. The developmental expression profiles of immune genes randomly selected from the transcriptome analysis were verified by a quantitative polymerase chain reaction. Our work provides comprehensive analyses of P. puparum immune genes, some of which may be exploited in advancing parasitoid-based biocontrol technologies.

Insect C-Type Lectins in Microbial Infections

C-type lectins (CTLs) are a family of carbohydrate-recognition domain (CRD)-containing proteins that bind to ligands in a calcium-dependent manner. CTLs act as important components of insect innate immune responses, such as pattern recognition, agglutination, encapsulation, melanization, phagocytosis and prophenoloxidase activation, as well as gut microbiome homeostasis maintenance, to defend against pathogens. Besides, some insect CTLs can facilitate pathogen infection and colonization. In this review, we describe the properties of insect CTLs and focus on explaining their role in viral, bacterial, parasitic and fungal infections.

20-Hydroxyecdysone enhances Immulectin-1 mediated immune response against entomogenous fungus in Locusta migratoria

BACKGROUND

Entomogenous fungi are important factors in biological control, but innate immunity of insects restricts the efficiency of fungus infection. 20-hydroxyecdysone (20E) is involved in regulating the immune response of insects. Our previous studies have revealed that 20E enhances the expression of antibacterial peptides in the worldwide pest Locusta migratoria. However, the mechanism by which 20E controls innate immunity against entomogenous fungi is still unknown.

RESULTS

In the present study, based on the transcriptome of L. migratoria fat bodies challenged by 20E, immulectin-1 (LmIML-1) was screened and identified to be involved in modulating antifungal immunity. Spatio-temporal expression analysis showed LmIML-1 was highly expressed in the fifth instar nymph stage, and mainly distributed in the fat bodies and hemolymph. Both exogenous and endogenous 20E could increase the transcription of LmIML-1. In contrast, transcription of LmIML-1 did not increase when the 20E signal was blocked by RNAi of LmEcR (ecdysone receptor). The expressed recombinant protein rLmIML-1 possessed agglutination activity and promoted the encapsulation. RNA interference of LmIML-1 reduced the encapsulation of hemocytes, decreased the antifungal activity of plasma against Metarhizium anisopliae and accelerated the death of nymphs under the stress of entomogenous fungus. Meanwhile, 20E did not increase the antifungal activity with silence of LmIML-1 in L. migratoria.

CONCLUSION

20E enhances antifungal immunity by activating immulectin-1 in L. migratoria. Our findings indicate a potential mechanism of 20E systematically regulating innate immune response to resist pathogens and provide a well-defined molecular target for improving biological control. © 2019 Society of Chemical Industry.

Functional conservation and division of two single-carbohydrate-recognition domain C-type lectins from the nipa palm hispid beetle Octodonta nipae (Maulik)

As an invasive pest, the complete and effective innate immune system is crucial for the nipa palm hispid beetle Octodonta nipae (Maulik) to adjust to new environments. C-type lectins (CTLs) are large families of carbohydrate-binding proteins that possess one or more characteristic carbohydrate-recognition domains (CRD) and function as pattern-recognition receptors, which play important roles in mediating humoral and cellular immunity. In the present study, for the first time, we report two CTL-Ss (single-CRD CTLs) from O. nipae (Maulik) (designated OnCTL1 and OnCTL2). The two CTL-Ss share high identity at conserved amino acids associated with conserved carbohydrate binding sites Gln-Pro-Asp (QPD) motifs and clearly show a 1:1 orthologous relationship in insects, which endow them with functional conservation and diversification. mRNA abundance analysis showed that OnCTL1 was upregulated upon Staphylococcus aureus and Escherichia coli challenge at 6 and 12 h, while OnCTL2 underwent no changes upon E. coli challenge and was even downregulated after S. aureus infection. Knockdown of OnCTL1 significantly decreased the transcripts of two key serine proteases (prophenoloxidase activating factors), OnPPAF1 and OnPPAF3, followed by the reduction of haemolymph phenoloxidase activity; it also increased the expression of Defensin2B. In contrast, silencing of OnCTL2 significantly decreased the expression of Defensin2B and Attacin3C, the encapsulation index, and the phagocytosis rate compared to the dsEGFP group. The spreading results showed that more irregularly shaped plasmatocytes and lower levels of aggregation were found in OnCTL2-silenced pupae than in the dsOnCTL1 and dsEGFP groups. We can infer from the results of this study that the two OnCTLs play important roles in the immune system and generate a functional division: OnCTL1 seems to function more in humoral immunity including mediating bacterial recognition and activating the phenoloxidase cascade, and OnCTL2 plays a greater role in enhancing cellular immunity. These observations could replenish information on the functional diversification of insect CTLs, and also provide valuable information to unravel the immunity in O. nipae.

Identification and functional characterization of a C-type lectin gene from Litopenaeus vannamei that is associated with ER-stress response

C-type lectins (CTLs), which bind carbohydrates in a Ca²⁺-dependent manner, are involved in many cellular activities, especially immunity. CTLs play important roles in both the antibacterial and the antiviral immune response and are also associated with autoimmunity. Several CTLs have been investigated in crustaceans, primarily with respect to their function in the immune response. In this study, we cloned a novel CTL gene (LvCTLU) from Litopenaeus vannamei. LvCTLU is involved in microbe agglutination and phagocytosis. Downregulating LvCTLU increased the cumulative mortality of L. vannamei after Vibrio parahemolyticus infection. Similar to other reported CTLs, LvCTLU also had antiviral properties. Downregulation of LvCTLU also increased the cumulative mortality of L. vannamei after infection with white spot syndrome virus. More importantly, LvCTLU expression was induced by the unfolded protein response (UPR), which is the key pathway in the endoplasmic reticulum (ER)-stress response of eukaryotic organism. Our results suggested that this protein might be involved in the shrimp ER-stress response. Reporter gene assay indicated that LvCTLU was regulated by X-box-binding protein 1, which is the key transcription factor in the UPR. Our study thus revealed that LvCTLU plays vital roles in both the anti-pathogen immune response and the ER-stress response.

Spiroplasma eriocheiris Invasion Into Macrobrachium rosenbergii Hemocytes Is Mediated by Pathogen Enolase and Host Lipopolysaccharide and β-1, 3-Glucan Binding Protein

Spiroplasma eriocheiris is a crustacean pathogen, without a cell wall, that causes enormous economic loss. Macrobrachium rosenbergii hemocytes are the major targets during S. eriocheiris infection. As wall-less bacteria, S. eriocheiris, its membrane protein should interact with host membrane protein directly and firstly when invaded in host cell. In this investigation, six potential hemocyte receptor proteins were identified firstly that mediate interaction between S. eriocheiris and M. rosenbergii. Among these proteins, lipopolysaccharide and β-1, 3-glucan binding protein (MrLGBP) demonstrated to bind to S. eriocheiris using bacterial binding assays and confocal microscopy. Four spiroplasma ligand proteins for MrLGBP were isolated and identified. But, competitive assessment demonstrated that only enolase of S. eriocheiris (SeEnolase) could be a candidate ligand for MrLGBP. Subsequently, the interaction between MrLGBP and SeEnolase was confirmed by co-immunoprecipitation and co-localization in vitro. After the interaction between MrLGBP and SeEnolase was inhibited by antibody neutralization test, the virulence ability of S. eriocheiris was effectively reduced. The quantity of S. eriocheiris decreased in Drosophila S2 cells after overexpression of MrLGBP, compared with the controls. In addition, RNA interference (RNAi) knockdown of MrLGBP made M. rosenbergii more sensitive to S. eriocheiris infection. Further studies found that the immune genes, including MrLGBP and prophenoloxidase (MrproPO), MrRab7A, and Mrintegrin α1 were significantly up-regulated by SeEnolase stimulation. After SeEnolase pre-stimulation, the ability of M. rosenbergii resistance to S. eriocheiris was significantly improved. Collectively, this investigation demonstrated that MrLGBP and pathogen SeEnolase involved in mediating S. eriocheiris invasion into M. rosenbergii hemocytes.

The diversity of pattern recognition receptors (PRRs) involved with insect defense against pathogens

Through evolution, selection pressures cause both insects and the pathogens attacking them to adapt so that they will both survive and this has been called the co-evolutionary 'arms race'. Insects expand their repertoire of pattern recognition receptors (PRRs), a fundamental and core component of their immune systems, to adapt to the constantly changing and unpredictable diversity of pathogens. In this review, we discuss the diversity of PRRs based on studies conducted in recent years. The strategies associated with PRR diversity summarized here are genetic evolution, isoform diversity based on alternative splicing, 'part-time' PRRs, PRRs with opsonin function, and regulation of complex signaling pathways. Taken together, these data indicate that the function of PRRs in insect immunity is far more complex and possesses more features than originally thought.

Molecular characterization of immune responses of Helicoverpa armigera to infection with the mermithid nematode Ovomermis sinensis

Background

Mermithid nematodes, such as Ovomermis sinensis, display a broad host range including some lepidopteran pests. Infective juveniles penetrate their host through the cuticle, complete their growth within the hemocoel and eventually kill the host upon their emergence. Hence, mermithid nematodes are considered potential biological control agents of insect pests. Our previous data indicate that the infection rate of O. sinensis on cotton bollworm (Helicoverpa armigera) is low, which may be largely due to the strong immune system of the host. However, current knowledge on the interactions of mermithid nematodes with their hosts and the mechanisms employed by hosts to defend themselves against mermithid nematodes is limited.

Results

Here, we investigated the response of H. armigera to O. sinensis infection. Parasitism by O. sinensis caused a sharp decline in the survival rate of H. armigera. The hemocytic phagocytosis ability, antibacterial activity, and phenoloxidase (PO) activity in plasma of H. armigera increased at 1 d post parasitism (dpp) but decreased at 3 dpp. Further, we investigated gene expression in the fat body of parasitized and non-parasitized H. armigera larvae at 1, 3, and 5 dpp using a digital gene expression system. In total, 41, 60 and 68 immune-related differentially expressed genes were identified at 1, 3, and 5 dpp, respectively. These genes encoded pattern recognition receptors (PRRs), antimicrobial peptides (AMPs), serine proteases (SPs), SP inhibitors, mucins and other immune-related proteins. The expression of most PRRs, AMPs, SPs, and mucins was upregulated in the fat body of larvae at 1 dpp, downregulated at 3 dpp, and then again upregulated at 5 dpp by O. sinensis. The increased expression of SP inhibitors may contribute to the inhibited PO activity at 5 dpp.

Conclusions

This study demonstrates that parasitism by O. sinensis modulates the immune reaction of the host H. armigera by altering the expression of immune-related genes. Our data provide a basis for future investigation of the molecular mechanisms employed by the mermithid nematode O. sinensis to modulate the immunity of the host H. armigera. These data will also likely facilitate the improvement of success in parasitism of H. armigera by O. sinensis.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5544-1) contains supplementary material, which is available to authorized users.

Cellular immunity in the insect Galleria mellonella against insect non-parasitic nematodes

Immunity to microbial infections is well understood; however, information regarding the immunity to parasitic multicellular organisms remains lacking. To understand innate host cellular immunity to nematodes, we compared the cellular response of the greater wax moth (Galleria mellonella) larvae against the non-parasitic, bacterial-feeding nematode Caenorhabditis elegans and pathogenic nematode Heterorhabditis bacteriophora. When intact first-instar or dauer larvae of C. elegans were injected into a G. mellonella larva, most of the nematodes were alive and not confined by the surrounding reaction by insect haemocytes (encapsulation), similarly as the pathogenic nematode, whereas most of the heat-killed nematodes of both species were severely encapsulated by 24 h after inoculation. Other non-parasitic nematodes were also not encapsulated. Surprisingly, C. elegans injected into the insect haemocoel grew and propagated in the live insect, resulting in death of the host insect. Our results suggest that C. elegans has some basic mechanisms to evade immunity of G. mellonenlla and grow in the haemocoel.

20-Hydroxyecdysone promotes release of GBP-binding protein from oenocytoids to suppress hemocytic encapsulation

Growth-blocking peptide (GBP) is an insect cytokine that stimulates plasmatocyte adhesion, thereby playing a critical role in encapsulation reaction. It has been previously demonstrated that GBP-binding protein (GBPB) is released upon oenocytoid lysis in response to GBP and is responsible for subsequent clearance of GBP from hemolymph. However, current knowledge about GBPB is limited and the mechanism by which insects increase GBPB levels to inactivate GBP remains largely unexplored. Here, we have identified one GBP precursor (HaGBP precursor) gene and two GBPB (namely HaGBPB1 and HaGBPB2) genes from the cotton bollworm, Helicoverpa armigera. The HaGBP precursor was found to be predominantly expressed in fat body, whereas HaGBPB1 and HaGBPB2 were mainly expressed in hemocytes. Immunological analyses indicated that both HaGBPB1 and HaGBPB2 are released from hemocytes into the plasma during the wandering stage. Additionally, 20-hydroxyecdysone (20E) treatment or bead challenge could promote the release of HaGBPB1 and HaGBPB2 at least partly from oenocytoids into the plasma. Furthermore, we demonstrate that the N-terminus of HaGBPB1 is responsible for binding to HaGBP and suppresses HaGBP-induced plasmatocyte spreading and encapsulation. Overall, this study helps to enrich our understanding of the molecular mechanism underlying 20E mediated regulation of plasmatocyte adhesion and encapsulation via GBP-GBPB interaction.