Characterization of a Novel C-Type Lectin, Bombyx mori Multibinding Protein, from the B. mori Hemolymph: Mechanism of Wide-Range Microorganism Recognition and Role in Immunity

Lectin diversity and their positive roles in WSSV replication through regulation of calreticulin expression and inhibiting ALFs expression

In biological evolution, gene duplication (GD) generates new genes to facilitate new functions. C-type lectins (CTLs) in crayfish have been extended by GD to expand their family members. In this study, four CTL genes generated by GD were identified from Procambarus clarkii (PcLec1-4). Among these four genes, PcLec1 can also generate new isoforms with different numbers of tandem repeats through DNA slip mispairing. PcLec1-4 was widely expressed in multiple tissues. The expression levels of PcLec1-4 were upregulated in the intestine of P. clarkii upon white spot syndrome virus (WSSV) challenge at multiple time points. Further analysis indicated that GATA transcription factor regulated PcLec1-4 expression. RNA interference and recombinant PcLec1-4 protein injection experiments suggested that PcLec1-4 promoted the expression of calreticulin (PcCRT) and negatively regulated the expression of antimicrobial peptides, thereby promoting WSSV replication. This study contributes to the understanding of the function of CTLs produced by GD during WSSV invasion in crustaceans.

Comparative transcriptome analysis of hepatopancreas reveals the potential mechanism of shrimp resistant to Vibrio parahaemolyticus infection

Vibrio parahaemolyticus carrying a pathogenic plasmid (VPAHPND) is one of the main causative agents of acute hepatopancreatic necrosis disease (AHPND) in shrimp aquaculture. Knowledge about the mechanism of shrimp resistant to VPAHPND is very helpful for developing efficient strategy for breeding AHPND resistant shrimp. In order to learn the mechanism of shrimp resistant to AHPND, comparative transcriptome was applied to analyze the different expressions of genes in the hepatopancreas of shrimp from different families with different resistance to VPAHPND. Through comparative analysis on the hepatopancreas of shrimp from VPAHPND resistant family and susceptible family, we found that differentially expressed genes (DEGs) were mainly involved in immune and metabolic processes. Most of the immune-related genes among DEGs were highly expressed in the hepatopancreas of shrimp from resistant family, involved in recognition of pathogen-associated molecular patterns, phagocytosis and elimination of pathogens, maintenance of reactive oxygen species homeostasis and other immune processes etc. However, most metabolic-related genes were highly expressed in the hepatopancreas of shrimp from susceptible family, involved in metabolism of lipid, vitamin, cofactors, glucose, carbohydrate and serine. Interestingly, when we analyzed the expression of above DEGs in the shrimp after VPAHPND infection, we found that the most of identified immune-related genes remained at high expression levels in the hepatopancreas of shrimp from the VPAHPND resistant family, and most of the identified metabolic-related genes were still at high expression levels in the hepatopancreas of shrimp from the VPAHPND susceptible family. The data suggested that the differential expression of these immune-related and metabolic-related genes in hepatopancreas might contribute to the resistance variations of shrimp to VPAHPND. These results provided valuable information for understanding the resistant mechanism of shrimp to VPAHPND.

Hemocytin, the special aggregation factor connecting insect hemolymph immunity, a potential target of insecticidal immunosuppresant

Insects possess an effective innate immunity that enables them to adapt to their intricate living environment and fend off various pathogens (or parasites). This innate immunity comprises both humoral and cellular immunity, which synergistically orchestrate immune responses. Hemocytin, a lectin with a distinctive structure, plays a crucial role in insect hemolymph immunity. Hemocytin is involved in the early immune response, facilitating processes such as coagulation, nodulation, and encapsulation in the hemolymph. It prevents hemolymph overflow and microbial pathogens invasion resulting from epidermal damage, and also aids in the recognition and elimination of invaders. However, the research on hemocytin is still limited. Our previous findings demonstrated that destruxin A effectively inhibits insect hemolymph immunity by interacting with hemocytin, suggesting that hemocytin could be a potential target for insecticides development. Therefore, it is crucial to gain a deeper understanding of hemocytin. This review integrates recent advancements in the study of the structure and function of insect hemocytin and also explores the potential of hemocytin as a target for insecticides. This review aims to enhance our comprehension of insect innate immunity and provide innovative ideas for the development of environmentally friendly pesticides.

The mechanisms and factors that induce trained immunity in arthropods and mollusks

Besides dividing the organism's immune system into adaptive and innate immunity, it has long been thought that only adaptive immunity can establish immune memory. However, many studies have shown that innate immunity can also build immunological memory through epigenetic reprogramming and modifications to resist pathogens' reinfection, known as trained immunity. This paper reviews the role of mitochondrial metabolism and epigenetic modifications and describes the molecular foundation in the trained immunity of arthropods and mollusks. Mitochondrial metabolism and epigenetic modifications complement each other and play a key role in trained immunity.

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.

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.

Serotonin- and eicosanoid-dependent rapid hemocyte aggregation in the hemolymph is the first step in nodule formation in Bombyx mori larvae

Nodule formation is a well-known process in cellular immunity of insects. However, few studies have investigated the role of hemocytes in rapid aggregation before tissue adhesion. In addition, since nodule formation is usually elicited by injecting large quantities of microbes, it remains unclear whether nodule formation is a natural response. The present study addressed these issues. A small number of nodules adhered to the dorsal vessels 1 min after Saccharomyces cerevisiae injection, while numerous aggregates of hemocytes and S. cerevisiae cells were observed in the hemolymph. The aggregate number decreased sharply after 5 min, corresponding to a rapid increase in the number of nodules. This suggests that aggregates formed in the hemolymph in response to S. cerevisiae injection eventually attached to the tissues. Nodules were induced using conditions that do not occur in nature, i.e., injection of nearly 2,000,000 S. cerevisiae cells. However, many aggregates contained only one S. cerevisiae cell, suggesting that aggregate formation can begin with the invasion of a single cell and that nodule formation is not an unnatural response. Biosynthesis inhibitors of serotonin (5-HT) and eicosanoids inhibited aggregate and nodule formation. In addition, injection of 5-HT and prostaglandin E2 induced hemocyte aggregation within 1 min in the hemolymph, along with hemocytin release. This suggested that 5-HT and eicosanoids induce rapid aggregation in response to invading microorganisms.

Nodule formation in Bombyx mori larvae is regulated by BmToll10-3

Nodule formation is a two-step cell-mediated immune response that is elicited by the cytokine spätzle1. Spätzle1 is activated within 30 s of invasion by microorganisms via an extracellular signaling pathway that consists of pathogen-associated molecular pattern recognition receptors, C-type lectins, and serine proteases. Here, we investigated a hemocyte molecule that is involved in eliciting the first step of nodule formation. BmToll10-3 was one of 14 Toll homologs identified in the silkworm Bombyx mori; it is an ortholog of Spodoptera exigua Toll. Previous research suggested that SeToll elicits nodule formation, but no evidence was presented to indicate whether SeToll elicited the first or second step of nodule formation. Reverse transcription-polymerase chain reaction and immunostaining confirmed that BmToll10-3 is expressed in granulocytes. To determine whether BmToll10-3 is involved in eliciting the first step of nodule formation, we tested an antiserum raised against BmToll10-3 in a nodule formation assay. The antiserum strongly inhibited the first step of nodule formation in B. mori larvae. Next, we tried to knock out BmToll10-3 using genome editing. Strains that were heterozygous for a truncated BmToll10-3 allele were generated, but no strain that was homozygous for truncated BmToll10-3 was generated. Nonetheless, several healthy homozygous larvae were identified before pupation, and we used these larvae in a nodule formation assay. The larvae that were homozygous for truncated BmToll10-3 did not form nodules. These results suggest that BmToll10-3 is involved in a cellular immunity, nodule formation.

Transcriptome Analysis Reveals Early Hemocyte Responses upon In Vivo Stimulation with LPS in the Stick Insect Bacillus rossius (Rossi, 1788)

Simple Summary

Non-model insect species such as B. rossius suffer from a profound gap of knowledge regarding the temporal progression of physiological responses following the challenge with bacterial pathogens or cell wall components thereof. The reason for this mostly lies in the lack of genomic/transcriptomic resources, which would provide an unparalleled in-depth capacity in the analysis of molecular, biochemical, and metabolic mechanisms. We present a high-quality transcriptome obtained from high-coverage sequencing of hemocytes harvested from adult stick insect specimens both pre- and post-LPS stimulation. Such a resource served as the basis for a stringent differential gene expression and functional enrichment analyses, the results of which were characterized and discussed in depth. Selected transcripts encoding for C-type lectins and ML-domain containing proteins were further investigated from a phylogenetic perspective. Overall, these findings shed light on the physiological responses driven by a short-term LPS stimulation in the European stick insect.

Abstract

Despite a growing number of non-model insect species is being investigated in recent years, a greater understanding of their physiology is prevented by the lack of genomic resources. This is the case of the common European stick insect Bacillus rossius (Rossi, 1788): in this species, some knowledge is available on hemocyte-related defenses, but little is known about the physiological changes occurring in response to natural or experimental challenges. Here, the transcriptional signatures of adult B. rossius hemocytes were investigated after a short-term (2 h) LPS stimulation in vivo: a total of 2191 differentially expressed genes, mostly involved in proteolysis and carbohydrate and lipid metabolic processes, were identified in the de novo assembled transcriptome and in-depth discussed. Overall, the significant modulation of immune signals—such as C-type lectins, ML domain-containing proteins, serpins, as well as Toll signaling-related molecules—provide novel information on the early progression of LPS-induced responses in B. rossius.

Hemocyte Clusters Defined by scRNA-Seq in Bombyx mori: In Silico Analysis of Predicted Marker Genes and Implications for Potential Functional Roles

Within the hemolymph, insect hemocytes constitute a heterogeneous population of macrophage-like cells that play important roles in innate immunity, homeostasis and development. Classification of hemocytes in different subtypes by size, morphology and biochemical or immunological markers has been difficult and only in Drosophila extensive genetic analysis allowed the construction of a coherent picture of hemocyte differentiation from pro-hemocytes to granulocytes, crystal cells and plasmatocytes. However, the advent of high-throughput single cell technologies, such as single cell RNA sequencing (scRNA-seq), is bound to have a high impact on the study of hemocytes subtypes and their phenotypes in other insects for which a sophisticated genetic toolbox is not available. Instead of averaging gene expression across all cells as occurs in bulk-RNA-seq, scRNA-seq allows high-throughput and specific visualization of the differentiation status of individual cells. With scRNA-seq, interesting cell types can be identified in heterogeneous populations and direct analysis of rare cell types is possible. Next to its ability to profile the transcriptomes of individual cells in tissue samples, scRNA-seq can be used to propose marker genes that are characteristic of different hemocyte subtypes and predict their functions. In this perspective, the identities of the different marker genes that were identified by scRNA-seq analysis to define 13 distinct cell clusters of hemocytes in larvae of the silkworm, Bombyx mori, are discussed in detail. The analysis confirms the broad division of hemocytes in granulocytes, plasmatocytes, oenocytoids and perhaps spherulocytes but also reveals considerable complexity at the molecular level and highly specialized functions. In addition, predicted hemocyte marker genes in Bombyx generally show only limited convergence with the genes that are considered characteristic for hemocyte subtypes in Drosophila.

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.

Evaluation of the collagen-binding properties and virulence of killed Streptococcus mutans in a silkworm model

Streptococcus mutans, a major pathogen of dental caries, is also known as a causative agent of cardiovascular disease. A 120 kDa collagen-binding protein (Cnm) of S. mutans is an important contributor to the pathogenicity of cardiovascular disease. Although dead bacteria have been detected in cardiovascular specimens by molecular biological methods, the pathogenicity of the bacteria remains unknown. Here, we analyzed the pathogenicity of killed S. mutans by focusing on collagen-binding ability and the effects on silkworms. In live S. mutans, Cnm-positive S. mutans had high collagen-binding activity, while Cnm-negative S. mutans had no such activity. After treatment with killed Cnm-positive S. mutans, amoxicillin-treated bacteria still had collagen-binding ability, while lysozyme-treated bacteria lost this ability. When live and amoxicillin-treated S. mutans strains were administered to silkworms, the survival rates of the silkworms were reduced; this reduction was more pronounced in Cnm-positive S. mutans infection than in Cnm-negative S. mutans infection. However, the administration of any of the lysozyme-treated bacteria did not reduce the survival rate of the silkworms. These results suggest that amoxicillin-killed Cnm-positive S. mutans strains maintain collagen-binding properties and pathogenicity in the silkworm model, and are possibly associated with pathogenicity in cardiovascular diseases.

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.

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.

A humoral factor, hemolymph proteinase 8, elicits a cellular defense response of nodule formation in Bombyx mori larvae in association with recognition by C-type lectins

Previously, we found that nodule formation, a cellular defense response in insects, is regulated by humoral factors called C-type lectins in the hemolymph. To elucidate the factors that elicit nodule formation following the recognition of microorganisms by C-type lectins, a reproducible quantitative in vitro assay system was constructed. Then, using this system, the inhibitory activities of antisera raised against hemolymph proteases (HPs), serine protease homologues (SPHs), and pathogen-associated molecular pattern (PAMP)-recognition proteins were assessed. Among the antisera raised against HP and SPH, only that against HP8, a terminal proteinase that activates Spätzle, consistently inhibited in-vitro nodule-like aggregate formation in all three tested microorganisms, Micrococcus luteus, Escherichia coli, and Saccharomyces cerevisiae. Antisera raised against C-type lectins, BmLBP, and BmMBP also inhibited nodule-like aggregate formation, while those against β-glucan recognition proteins and peptidoglycan recognition protein-S1 did not. Microorganisms pretreated with hemolymph, which contains HP8 and C-type lectins, also induced nodule-like aggregate formation, indicating that nodulation factors are present on microbial cells. Furthermore, antisera raised against HP8, BmLBP, and BmMBP showed inhibitory activities in the in vivo nodule formation system using Bombyx mori larvae. Thus, two humoral factors in the hemolymph of B. mori larvae, BmHP8 and C-type lectins, were found to play significant roles in eliciting the cellular defense response of nodule formation.

Identification of 35 C-Type Lectins in the Oriental Armyworm, Mythimna separata (Walker)

Simple Summary

The oriental armyworm Mythimna separata is a lepidopteral agricultural pest that causes serious damage to many crops, such as maize, wheat, and sorghum. To control this pest, it is advisable to take comprehensive measures, including the use of chemical pesticides, microbial pesticides, and cultural practices. However, microbial pesticides (entomopathogens) can be eliminated by the insect immune system. C-type lectins (CTLs) are a family of pattern-recognition receptors that recognize carbohydrates and mediate immune responses. C-type lectins in the oriental armyworm have not yet been identified and characterized. In this study, a transcriptome of M. separata larvae was constructed and a total of 35 CTLs containing single or dual carbohydrate-recognition domains (CRDs) were identified from unigenes. Phylogenetic analyses, sequence alignments and structural predictions were performed. Gene expression profiles in different developmental stages, naïve larval tissues, and bacteria/fungi-challenged larvae were analyzed. Overall, our findings indicate that most dual-CRD CTLs are expressed in mid-late-stage larvae, pupae, and adults. Bacterial and fungal challenges can stimulate the expression of many CTLs in larval hemocytes, fat body, and midgut. Our data suggest the importance of CTLs in immune responses of M. separata.

Abstract

Insect C-type lectins (CTLs) play vital roles in modulating humoral and cellular immune responses. The oriental armyworm, Mythimna separata (Walker) (Lepidoptera: Noctuidae) is a migratory pest that causes significant economic loss in agriculture. CTLs have not yet been systematically identified in M. separata. In this study, we first constructed a transcriptome of M. separata larvae, generating a total of 45,888 unigenes with an average length of 910 bp. Unigenes were functionally annotated in six databases: NR, GO, KEGG, Pfam, eggNOG, and Swiss-Prot. Unigenes were enriched in functional pathways, such as those of signal transduction, endocrine system, cellular community, and immune system. Thirty-five unigenes encoding C-type lectins were identified, including CTL-S1~CTL-S6 (single CRD) and IML-1~IML-29 (dual CRD). Phylogenetic analyses showed dramatic lineage-specific expansions of IMLs. Sequence alignment and structural modeling identified potential ligand-interacting residues. Real-time qPCR revealed that CTL-Ss mainly express in eggs and early stage larvae, while IMLs mainly express in mid-late-stage larvae, pupae, and adults. In naïve larvae, hemocytes, fat body, and epidermis are the major tissues that express CTLs. In larvae challenged by Escherichia coli, Staphylococcus aureus, or Beauveria bassiana, the expression of different CTLs was stimulated in hemocytes, fat body and midgut. The present study will help further explore functions of M. separata CTLs.

Hepatopancreas-Specific Lectin Participates in the Antibacterial Immune Response by Regulating the Expression of Antibacterial Proteins

The hepatopancreas is an important digestive and immune organ in crustacean. There were low but stable numbers of microbes living in the hemolymph of crustacean, whereas the organs (including hepatopancreas) of crustacean were immersed in the hemolymph. It is very important to study the immune mechanism of the hepatopancreas against bacteria. In this study, a novel CTL (HepCL) with two CRDs, which was mainly expressed in the hepatopancreas, was identified in red swamp crayfish (Procambarus clarkii). HepCL binds to bacteria in vitro and could enhance bacterial clearance in vivo. Compared with the C-terminal CRD of HepCL (HepCL-C), the N-terminal CRD (HepCL-N) showed weaker bacterial binding ability in vitro and stronger bacterial clearance activity in vivo. The expression of some antimicrobial proteins, such as FLP, ALF1 and ALF5, was downregulated under knockdown of HepCL or blocked with Anti-HepCL after challenge with Vibrio in crayfish. These results demonstrated that HepCL might be involved in the antibacterial immune response by regulating the expression of antimicrobial proteins.

Identification and Characterization of Genes for the Allo A Lectins in Japanese Rhinoceros Beetle (Trypoxylus dichotomus [Allomyrina dichotoma])

The Japanese rhinoceros beetle (Trypoxylus dichotomus [Allomyrina dichotoma]) produces the lectins allo A-I and allo A-II, which have strong N-acetyllactosamine (Galβ1-4GlcNAc)-binding activity. It has been suggested that the two lectins are formed from three subunits (α, β, and γ), with allo A-I comprising α and γ subunits and allo A-II comprising β and γ subunits. Here, we determined the cDNA sequences of these subunits using both conventional polymerase chain reaction (PCR)-cloning-sequencing and transcriptome-sequencing analyses. For the α and β subunits, one gene (locus) for each was predicted, whereas for the γ subunit, two types of cDNA sequences were obtained, which we named γ1 and γ2. These two types probably have distinct loci. Average nucleotide sequence identities among the subunits ranged from 87.6% (between α and γ1) to 92.6% (between γ1 and γ2), suggesting that they form a gene family. Although no homology was found between the sequences of allo A and other known lectin proteins in a protein database search, some unknown proteins containing the DUF3421 domain were identified. Those DUF3421 domain-encoding proteins are upregulated in the insect larval midgut. Thus, we infer that allo A genes also play an important role in larvae and that their lectin activity may have been obtained collaterally.

Identification of the Ricin-B-Lectin LdRBLk in the Colorado Potato Beetle and an Analysis of Its Expression in Response to Fungal Infections

Ricin-B-lectins (RBLs) have been identified in many groups of organisms, including coleopterans insects, particularly the Colorado potato beetle Leptinotarsa decemlineata (LdRBLs). We hypothesized that one of these LdRBLs (LdRBLk) may be involved in the immune response to fungal infections. We performed a theoretical analysis of the structure of this protein. Additionally, the expression levels of the LdRBlk gene were measured in L. decemlineata in response to infections with the fungi Metarhizium robertsii and Beauveria bassiana. The expression levels of LdRBlk in the L. decemlineata cuticle and fat body were increased in response to both infections. The induction of LdRBlk expression was dependent on the susceptibility of larvae to the fungi. Upregulation of the LdRBlk gene was also observed in response to other stresses, particularly thermal burns. Elevation of LdRBlk expression was frequently observed to be correlated with the expression of the antimicrobial peptide attacin but was not correlated with hsp90 regulation. Commercially available β-lectin of ricin from Ricinuscommunis was observed to inhibit the germination of conidia of the fungi. We suggest that LdRBLk is involved in antifungal immune responses in the Colorado potato beetle, either exerting fungicidal properties directly or acting as a modulator of the immune response.

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.

Fungal immunity and pathogenesis in mammals versus the invertebrate model organism Galleria mellonella

In recent decades, Galleria mellonella (Lepidoptera: Pyralidae) have emerged as a model system to explore experimental aspects of fungal pathogenesis. The benefits of the G. mellonella model include being faster, cheaper, higher throughput and easier compared with vertebrate models. Additionally, as invertebrates, their use is subject to fewer ethical and regulatory issues. However, for G. mellonella models to provide meaningful insight into fungal pathogenesis, the G. mellonella-fungal interactions must be comparable to mammalian-fungal interactions. Indeed, as discussed in the review, studies suggest that G. mellonella and mammalian immune systems share many similarities, and fungal virulence factors show conserved functions in both hosts. While the moth model has opened novel research areas, many comparisons are superficial and leave large gaps of knowledge that need to be addressed concerning specific mechanisms underlying G. mellonella-fungal interactions. Closing these gaps in understanding will strengthen G. mellonella as a model for fungal virulence in the upcoming years. In this review, we provide comprehensive comparisons between fungal pathogenesis in mammals and G. mellonella from immunological and virulence perspectives. When information on an antifungal immune component is unknown in G. mellonella, we include findings from other well-studied Lepidoptera. We hope that by outlining this information available in related species, we highlight areas of needed research and provide a framework for understanding G. mellonella immunity and fungal interactions.

A C-type lectin with dual-CRD from Tribolium castaneum is induced in response to bacterial challenge

BACKGROUND

C-type lectins (CTLs), a group of pattern recognition receptors, are involved in regulating the immune response of insects and could be used as potential targets for pest control. However, information about roles of CTLs in the innate immunity of Tribolium castaneum, a serious, worldwide pest that damages stored grain products, is relatively scarce.

RESULTS

Here, a CTL with dual carbohydrate recognition domains (CRDs) containing a highly conserved WHD (Trp⁵³ -His⁵⁴ -Asp⁵⁵ ) motif was identified in T. castaneum and named as TcCTL3. Spatiotemporal analysis showed that TcCTL3 was highly expressed in all developmental stages except early eggs, and mainly distributed in central nervous system and hemolymph. The transcript levels of TcCTL3 were significantly increased after lipopolysaccharide (LPS) and peptidoglycan (PGN) stimulation. Recombinant TcCTL3 was able to bind directly to LPS, PGN and all tested bacteria and induce a broad spectrum of microbial agglutination in the presence of Ca²⁺ . The binding was shown mainly through CRD1 domain of TcCTL3. When TcCTL3 was knocked down by RNA interference, expression of nine antimicrobial peptides (AMPs) (attacin1, attacin2, attacin3, defensins1, defensins2, coleoptericin1, coleoptericin2, cecropins2 and cecropins3) and four transcription factors (TFs) (dif1, dif2, relish and jnk) were significantly decreased under LPS and PGN stimulation, leading to increased mortality of T. castaneum when infected with Gram-positive Staphylococcus aureus or Gram-negative Escherichia coli infection.

CONCLUSION

TcCTL3 could mediate the immune response in T. castaneum via the pattern recognition, agglutination and AMP expression. These findings indicate a potential mechanism of TcCTL3 in resisting bacteria and provide an alternative molecular target for pest control. © 2020 Society of Chemical Industry.

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 Lectins Link Immunological and Reproductive Processes in Aedes aegypti

Physiological trade-offs between mosquito immune response and reproductive capability can arise due to insufficient resource availability. C-type lectin family members may be involved in these processes. We established a GCTL-3-/- mutant Aedes aegypti using CRISPR/Cas9 to investigate the role of GCTL-3 in balancing the costs associated with immune responses to arboviral infection and reproduction. GCTL-3-/- mutants showed significantly reduced DENV-2 infection rate and gut commensal microbiota populations, as well as upregulated JAK/STAT, IMD, Toll, and AMPs immunological pathways. Mutants also had significantly shorter lifespans than controls and laid fewer eggs due to defective germ line development. dsRNA knock-down of Attacin and Gambicin, two targets of the AMPs pathway, partially rescued this reduction in reproductive capabilities. Upregulation of immune response following GCTL-3 knock-out therefore comes at a cost to reproductive fitness. Knock-out of other lectins may further improve our knowledge of the molecular and genetic mechanisms underlying reproduction-immunity trade-offs in mosquitoes.

Physiological evidence of integrin-antibody reactive proteins influencing the innate cellular immune responses of larval Galleria mellonella hemocytes

Larval Galleria mellonella (L.) hemocytes form microaggregates in response to stimulation by Gram-positive bacteria. Hemocyte adhesion to foreign materials is mediated by the cAMP/ protein kinase A pathway and the β-subunit of cholera toxin using a cAMP-independent mechanism. Cholera toxin-induced microaggregation was inhibited by the integrin inhibitory RGDS peptide, implying integrins may be part of the mechanism. Based on the types of mammalian integrin-antibody reactive proteins affecting hemocyte adhesion and bacterial-induced responses α₅ , α_v , β₁ , and β₃ subunits occurred on both granular cell and plasmatocyte hemocyte subtypes. A fluorescent band representing the binding of rabbit α₅ -integrin subunit antibodies occurred between adhering heterotypic hemocytes. The frequency of the bands was increased by cholera toxin. The α₅ and β₁ rabbit integrin subunit antibodies inhibited removal of Bacillus subtilis (Cohn) from the hemolymph in vivo. A α₅ β₁ -specific synthetic peptide blocker similarly diminished hemocyte function whereas the α_v β₃ -specific inhibitory peptide and the corresponding integrin subunit antibodies did not influence nonself hemocyte activities. Western blots revealed several proteins reacting with a given integrin-antibody subtype. Thus integrin-antibody reactive proteins (which may include integrins) with possible α₅ and β₁ epitopes modulate immediate hemocyte function. Confocal microscopy established plasmatocyte adhesion to and rosetting over substrata followed by granular cell microaggregate adhesion to plasmatocytes during early stage nodulation.

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.

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.

Comparative Proteomic Analysis Reveals Immune Competence in Hemolymph of Bombyx mori Pupa Parasitized by Silkworm Maggot Exorista sorbillans

The silkworm maggot, Exoristasorbillans, is a well-known larval endoparasitoid of the silkworm Bombyxmori that causes considerable damage to the silkworm cocoon crop. To gain insights into the response mechanism of the silkworm at the protein level, we applied a comparative proteomic approach to investigate proteomic differences in the hemolymph of the female silkworm pupae parasitized by E. sorbillans. In total, 50 differentially expressed proteins (DEPs) were successfully identified, of which 36 proteins were upregulated and 14 proteins were downregulated in response to parasitoid infection. These proteins are mainly involved in disease, energy metabolism, signaling pathways, and amino acid metabolism. Eight innate immune proteins were distinctly upregulated to resist maggot parasitism. Apoptosis-related proteins of cathepsin B and 14-3-3 zeta were significantly downregulated in E. sorbillans-parasitized silkworm pupae; their downregulation induces apoptosis. Quantitative PCR was used to further verify gene transcription of five DEPs, and the results are consistent at the transcriptional and proteomic levels. This was the first report on identification of possible proteins from the E. bombycis-parasitized silkworms at the late stage of parasitism, which contributes to furthering our understanding of the response mechanism of silkworms to parasitism and dipteran parasitoid biology.

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.

A C-type lectin with a single carbohydrate-recognition domain involved in the innate immune response of Tribolium castaneum

C-type lectins are one of the pattern-recognition proteins involved in innate immunity in invertebrates. Although there are 16 C-type lectin genes that have been identified in the genome of Tribolium castaneum, their functions and mechanisms in innate immunity remain unknown. Here, we identified one C-type lectin orthologue, TcCTL6 (TC003708), by sequencing random clones from the cDNA library of the coleopteran beetle, T. castaneum. TcCTL6 contains a 654 bp open reading frame encoding a protein of 217 amino acids that includes a single carbohydrate-recognition domain. The expression of TcCTL6 was significantly induced by Escherichia coli, Staphylococcus aureus and stimulation with carbohydrates, including lipopolysaccharide and peptidoglycan. A binding assay suggested that the recombinant TcCTL6 not only bound to lipopolysaccharide and peptidoglycan but also bound to Gram-positive (S. aureus, Bacillus subtilis and Bacillus thuringiensis) and Gram-negative bacteria (E. coli and Pseudomonas aeruginosa) in the presence of calcium ions. Furthermore, when TcCTL6 was knocked down by RNA interference, four antimicrobial peptides (attacin1, attacin2, coleoptericin1 and coleoptericin2) were significantly decreased. These results demonstrate that TcCTL6 plays a vital role in the immune response towards pathogen infection by influencing the expression of antimicrobial peptides and the agglutination of bacteria in the presence of calcium ions in T. castaneum.

Comparative analysis of C-type lectin domain proteins in the ghost moth, Thitarodes xiaojinensis (Lepidoptera: Hepialidae)

Insects have a large family of C-type lectins involved in cell adhesion, pathogen recognition and activation of immune responses. In this study, 32 transcripts encoding C-type lectin domain proteins (CTLDPs) were identified from the Thitarodes xiaojinensis transcriptome. According to their domain structures, six CTLDPs with one carbohydrate-recognition domain (CRD) were classified into the CTL-S subfamily. The other 23 CTLDPs with two CRDs were grouped into the immulectin (IML) subfamily. The remaining three with extra regulatory domains were sorted into the CTL-X subfamily. Phylogenetic analysis showed that CTL-S and CTL-X members from different insects could form orthologous groups. In contrast, no T. xiaojinensis IML orthologues were found in other insects. Remarkable lineage-specific expansion in this subfamily was observed reflecting that these CTLDPs, as important receptors, have evolved diversified members in response to a variety of microbes. Prediction of binding ligands revealed that T. xiaojinensis, a cold-adapted species, conserved the ability of CRDs to combine with Ca²⁺ to keep its receptors from freezing. Comparative analysis of induction of CTLDP genes after different immune challenges indicated that IMLs might play critical roles in immune defenses. This study examined T. xiaojinensis CTLDPs and provides a basis for further studies of their characteristics.

Staphylococcus aureus aggregation in the plasma fraction of silkworm hemolymph

Staphylococcus aureus formed bacterial aggregates in the plasma fraction of the hemolymph of silkworm, the larva of Bombyx mori, in a growth-dependent manner. The addition of arabinose or galactose inhibited the formation of S. aureus aggregates in the silkworm plasma. Formation of the bacterial aggregates depended on S. aureus genes required for the synthesis of bacterial surface polysaccharides–ypfP and ltaA, which are involved in lipoteichoic acid synthesis, and the tagO gene, which is involved in wall teichoic acid synthesis. These findings suggest that S. aureus forms bacterial aggregates in the silkworm plasma via bacterial surface teichoic acids.

A multi-CRD C-type lectin gene Cnlec-1 enhance the immunity response in noble scallop Chlamys nobilis with higher carotenoids contents through up-regulating under different immunostimulants

C-type lectins have a variety of immunological functions in invertebrates. In order to investigate whether C-type lectin gene and carotenoids do have immune influences on noble scallop Chlamys nobilis under pathogen stress, acute challenges lasting 48 h to Vibrio parahaemolyticus, lipopolysaccharide (LPS), polyinosinic polycytidylic acid (Poly I: C), and PBS were conducted in noble scallop with different carotenoids content. A multi-CRD C-type lectin gene called Cnlec-1 was cloned and its transcripts under different challenges were determined. Full length cDNA of Cnlec-1 is 2267 bp with an open reading frame (ORF) of 1845 bp encoding 614 deduced amino acids, containing four carbohydrate recognition domains (CRD1, CRD2, CRD3 and CRD4). Phylogenetic tree analysis showed that CRDs of Cnlec-1 were clustered with CRDs of shellfish C-type lectins, especially closely related to Chlamys farreri and Argopecten irradians CRDs. Cnlec-1 transcripts were detected in hemocytes, mantle, gonad, kidney, intestines, gill and adductor. Compared with PBS control group, Cnlec-1 transcripts were up-regulated in V. parahaemolyticus, LPS and Poly I: C groups. Furthermore, Cnlec-1 transcript levels of Golden scallops were significantly higher than that of Brown ones at 3-48 h (P < 0.05) in V. parahemolyticus groups, at 24 h in LPS groups and at 12-24 h in Poly I: C groups. These results suggesting that Cnlec-1 is an important immune factor involved in the defense against pathogens in the noble scallop, and carotenoids can enhance the immunity of noble scallop through up-regulating Cnlec-1 to different immunostimulants.

Innate immune responses in the Chinese oak silkworm, Antheraea pernyi

Innate immunity, the evolutionarily conserved defense system, has been extensively analyzed in insect models over recent decades. The significant progress in this area has formed our dominant conceptual framework of the innate immune system, but critical advances in other insects have had a profound impact on our insights into the mystery of innate immunity. In recent years, we focused on the immune responses in Antheraea pernyi, an important commercial silkworm species reared in China. Here, we review the immune responses of A. pernyi based on immune-related gene-encoded proteins that are divided into five categories, namely pattern recognition receptors, hemolymph proteinases and their inhibitors, prophenoloxidase, Toll pathway factors and antimicrobial peptides, and others. Although the summarized information is limited since the research on A. pernyi immunity is in its infancy, we hope to provide evidence for further exploration of innate immune mechanisms.

Insect C-type lectins in innate immunity

C-type lectins (CTLs) are a family of proteins that contain characteristic modules of carbohydrate-recognition domains (CRDs) and they possess the binding activity to ligands in a calcium-dependent manner. CTLs play important roles in animal immune responses, and in insects, they are involved in opsonization, nodule formation, agglutination, encapsulation, melanization, and prophenoloxidase activation, as well as in maintaining gut microbiome homeostasis. In this review, we will summarize insect CTLs, compare the properties of insect CTLs with vertebrate CTLs, and focus mainly on the domain organization and functions of insect CTLs in innate immunity.

Characterizing the Mechanisms of Nonopsonic Uptake of Cryptococci by Macrophages

The pathogenic fungus Cryptococcus enters the human host via inhalation into the lung and is able to reside in a niche environment that is serum- (opsonin) limiting. Little is known about the mechanism by which nonopsonic phagocytosis occurs via phagocytes in such situations. Using a combination of soluble inhibitors of phagocytic receptors and macrophages derived from knockout mice and human volunteers, we show that uptake of nonopsonized Cryptococcus neoformans and C. gattii via the mannose receptor is dependent on macrophage activation by cytokines. However, although uptake of C. neoformans is via both dectin-1 and dectin-2, C. gattii uptake occurs largely via dectin-1. Interestingly, dectin inhibitors also blocked phagocytosis of unopsonized Cryptococci in wax moth (Galleria mellonella) larvae and partially protected the larvae from infection by both fungi, supporting a key role for host phagocytes in augmenting early disease establishment. Finally, we demonstrated that internalization of nonopsonized Cryptococci is not accompanied by the nuclear translocation of NF-κB or its concomitant production of proinflammatory cytokines such as TNF-α. Thus, nonopsonized Cryptococci are recognized by mammalian phagocytes in a manner that minimizes proinflammatory cytokine production and potentially facilitates fungal pathogenesis.

Characterization of two groups of Spodoptera exigua Hübner (Lepidoptera: Noctuidae) C-type lectins and insights into their role in defense against the densovirus JcDV

Insect innate immunity relies on numerous soluble and membrane-bound receptors, named pattern recognition proteins (PRPs), which enable the insect to recognize pathogen-associated molecular patterns. C-type lectins are among the best-studied PRPs and constitute the most diverse family of animal lectins. Here we have characterized two groups of Spodoptera exigua C-type lectins that differ in their phylogeny, domain architecture, and expression pattern. One group includes C-type lectins with similar characteristics to other lepidopteran lectins, and a second group includes bracoviral-related lectins (bracovirus-like lectins, Se-BLLs) recently acquired by horizontal gene transfer. Subsequently, we have investigated the potential role of some selected lectins in the susceptibility to Junonia coenia densovirus (JcDV). For this purpose, three of the bracoviral-related lectins were expressed, purified, and their effect on the densovirus infection to two different Spodoptera species was assessed. The results showed that Se-BLL3 specifically reduce the mortality of Spodoptera frugiperda larvae caused by JcDV. In contrast, no such effect was observed with S. exigua larvae. In a previous work, we have also shown that Se-BLL2 increased the tolerance of S. exigua larvae to baculovirus infection. Taken together, these results confirm the implication of two different C-type lectins in antiviral response and reflect the biological relevance of the acquisition of bracoviral genes in Spodoptera spp.

Molecular cloning and analysis of a C-type lectin from silkworm Bombyx mori

C-type lectins (CTLs) play a variety of roles in plants and animals. They are involved in animal development, pathogen recognition, and the activation of immune responses. CTLs carry one or more non-catalytic carbohydrate-recognition domains (CRDs) to bind specific carbohydrates reversibly. Here, we report the molecular cloning and functional analysis of a single-CRD CTL, named C-type lectin-S2 (BmCTL-S2) from the domesticated silkmoth Bombyx mori (Lepidoptera: Bombycidae). The ORF of CTL-S2 is 666 bp, which encodes a putative protein of 221 amino acids. BmCTL-S2 is expressed in a variety of immune-related tissues, including hemocytes and fat body among others. BmCTL-S2 mRNA level in the midgut and the fat body was significantly increased by bacterial challenges. The recombinant protein (rBmCTL-S2) bound different bacterial cell wall components and bacterial cells. rBmCTL-S2 also inhibited the growth of Bacillus subtilis and Staphylococcus aureus. Taken together, we infer that BmCTL-S2 is a pattern-recognition receptor with antibacterial activities.

Pathogen recognition of a novel C-type lectin from Marsupenaeus japonicus reveals the divergent sugar-binding specificity of QAP motif

C-type lectins (CTLs) are calcium-dependent carbohydrate-binding proteins known to assist the innate immune system as pattern recognition receptors (PRRs). The binding specificity of CTLs lies in the motif of their carbohydrate recognition domain (CRD), the tripeptide motifs EPN and QPD bind to mannose and galactose, respectively. However, variants of these motifs were discovered including a QAP sequence reported in shrimp believed to have the same carbohydrate specificity as QPD. Here, we characterized a novel C-type lectin (MjGCTL) possessing a CRD with a QAP motif. The recombinant MjGCTL has a calcium-dependent agglutinating capability against both Gram-negative and Gram-positive bacteria, and its sugar specificity did not involve either mannose or galactose. In an encapsulation assay, agarose beads coated with rMjGCTL were immediately encapsulated from 0 h followed by melanization at 4 h post-incubation with hemocytes. These results confirm that MjGCTL functions as a classical CTL. The structure of QAP motif and carbohydrate-specificity of rMjGCTL was found to be different to both EPN and QPD, suggesting that QAP is a new motif. Furthermore, MjGCTL acts as a PRR binding to hemocytes to activate their adherent state and initiate encapsulation.

Novel Pattern Recognition Receptor Protects Shrimp by Preventing Bacterial Colonization and Promoting Phagocytosis

The recognition of pathogen-associated molecular patterns is accomplished by the recognition modules of pattern recognition receptors (PRRs). Leucine-rich repeats (LRRs) and C-type lectin-like domain (CTLD) represent the two most universal categories of recognition modules. In the current study, we identified a novel soluble and bacteria-inducible PRR comprising LRRs and a CTLD from the hepatopancreas of kuruma shrimp Marsupenaeus japonicus and named it Leulectin. The module arrangement of Leulectin is unique among all organisms. Both modules, together with the whole molecule, protected shrimp against Vibrio infection. By screening the pathogen-associated molecular patterns that shrimp might encounter, Leulectin was found to sense Vibrio flagellin through the LRRs and to recognize LPS through CTLD. The LRR-flagellin interaction was confirmed by pull-down and far-Western assays and was found to rely on the fourth LRR of Leulectin and the N terminus of flagellin. The recognition of LPS was determined by the long loop region of CTLD in a calcium-independent manner. By sensing the flagellin, LRRs could prevent its attachment to shrimp cells, thereby inhibiting Vibrio colonization. With the ability to recognize LPS, CTLD could agglutinate the bacteria and promote hemocytic phagocytosis. Our study clearly showed the division of labor and the synergy between different recognition modules and provided new insights into the concept of pattern recognition and the function of soluble PRRs in the antibacterial response.

High Bacterial Agglutination Activity in a Single-CRD C-Type Lectin from Spodoptera exigua (Lepidoptera: Noctuidae)

Lectins are carbohydrate-interacting proteins that play a pivotal role in multiple physiological and developmental aspects of all organisms. They can specifically interact with different bacterial and viral pathogens through carbohydrate-recognition domains (CRD). In addition, lectins are also of biotechnological interest because of their potential use as biosensors for capturing and identifying bacterial species. In this work, three C-type lectins from the Lepidoptera Spodoptera exigua were produced as recombinant proteins and their bacterial agglutination properties were characterized. The lowest protein concentration producing bacterial agglutination against a panel of different Gram+ and Gram− as well as their carbohydrate binding specificities was determined for the three lectins. One of these lectins, BLL2, was able to agglutinate cells from a broad range of bacterial species at an extremely low concentration, becoming a very interesting protein to be used as a biosensor or for other biotechnological applications involving bacterial capture.

The sequence variation and functional differentiation of CRDs in a scallop multiple CRDs containing lectin

A C-type lectin of multiple CRDs (CfLec-4) from Chlamys farreri was selected to investigate the sequence variation and functional differentiation of its CRDs. Its four CRDs with EPD/LSD, EPN/FAD, EPN/LND and EPN/YND key motifs were recombined separately. The recombinant proteins of CRD1 and CRD2 (designated as rCRD1 and rCRD2) could bind LPS and mannan, while the recombinant proteins of CRD3 and CRD4 (designated as rCRD3 and rCRD4) could bind LPS, PGN, mannan and glucan. Moreover, rCRD3 displayed broad microbe binding spectrum towards Gram-positive bacteria Staphylococcus aureus and Micrococcus luteus, Gram-negative bacteria Escherichia coli and Vibrio anguillarum, as well as fungi Pichia pastoris and Yarrowia lipolytica. These results indicated CRD3 contributed more to CfLec-4's nonself-recognition ability. Furthermore, CRD1, CRD3 and CRD4 functioned as opsonin participating in the clearance against invaders in scallops. The sequence variation in Ca²⁺ binding site 2 among CRDs was suspected to be associated with such functional differentiation.

Comparative study of two single CRD C-type lectins, CgCLec-4 and CgCLec-5, from pacific oyster Crassostrea gigas

C-type lectins (CTLs), a superfamily of Ca²⁺-dependent carbohydrate-recognition proteins, are involved in nonself-recognition and pathogen elimination, and play crucial roles in the innate immunity. In the present study, two single CRD C-type lectins, CgCLec-4 and CgCLec-5, were identified from oyster Crassostrea gigas. The open reading frame (ORF) of CgCLec-4 and CgCLec-5 encoded polypeptides of 152 and 150 amino acids, respectively. Both CgCLec-4 and CgCLec-5 contained one CRD with six conserved cysteines to form three disulfide bridges. The motif in Ca²⁺-binding site 2 of CgCLec-4 was QPE, while it was QYE, a non-a typical motif in CgCLec-5. CgCLec-4 was a secreted lectin with a signal peptide which was highly expressed in hepatopancreas, mantle and hemocytes. CgCLec-5 was an intracellular lectin which was mostly expressed in hemocytes. The lipopolysaccharide stimulation could induce the expressions of CgCLec-4 and CgCLec-5. The recombinant proteins of CgCLec-4 and CgCLec-5 (rCgCLec-4 and rCgCLec-5) could bind to various PAMPs including LPS, PGN, GLU and mannan, while the binding affinity of rCgCLec-5 was stronger than that of rCgCLec-4. Meanwhile, rCgCLec-4 and rCgCLec-5 could bind to different kinds of microorganisms, including Staphylococcus aureus, Escherichia coli and Vibro anguillarum and Yarrowia lipolytica, and the microbial agglutinating ability of rCgCLec-4 was stronger than that of CgCLec-5. Moreover, rCgCLec-4 exhibited anti-microbial activity against bacteria and fungi, but anti-microbial activity of CgCLec-5 was not obvious. All these results suggested that CgCLec-4 and CgCLec-5 could function as an important PRR involved in immune defense against invading pathogen in oyster, and the diversity and complexity of motifs in Ca²⁺ binding site 2 in CRDs determined their comprehensive recognition spectrum and multiple immune functions.

Mechanisms of nodule-specific melanization in the hemocoel of the silkworm, Bombyx mori

In the insect immune system, nodules are known to be a product of the cellular response against microorganisms and may be a preferential target for melanization. However, the mechanism of nodule-preferential melanization remains to be explored. In this study, we identified several mechanisms of nodule-preferential melanization by analyzing congregation and the activation of several factors involved in the prophenoloxidase (proPO)-activating system in the silkworm, Bombyx mori. Microorganism-binding assays revealed that B. mori larval plasma have an effective invading microorganism-surveillance network consisting of at least six pattern-recognition receptors (PRRs). We also found that a hemolymph serine proteinase, BmHP14, can bind to Saccharomyces cerevisiae. Pull-down assays showed that PRR C-type lectins form protein complexes with serine proteinase homologs, BmSPH1 and BmSPH2, which leads to the activated forms of BmSPH1 and BmSPH2 being gathered on microorganisms and trapped in nodules. Immunostaining analysis revealed that most factors in the proPO-activating system and some factors in the triggering system for antimicrobial peptide production exist in the granules of hemocytes which can gather in nodules. Western blot analysis showed that factors in the proPO-activating system are congregated in formed nodules by their concentration in plasma and aggregating hemocytes.

Identification of C-type lectin-domain proteins (CTLDPs) in silkworm Bombyx mori

C-type lectins (CTLs) represent a large family of proteins that can bind carbohydrate moieties normally in a calcium-dependent manner. CTLs play important roles in mediating cell adhesion and the recognition of pathogens in the immune system. In the present study, we have identified 23 CTL genes in domestic silkworm Bombyx mori. CTL-domain proteins (CTLDPs) are classified into three groups based on the number of carbohydrate-recognition domains (CRDs) and the domain architectures. These include twelve CTL-S (Single-CRD), six immulectins (Dual-CRD) and five CTL-X (CRD with other domains). We studied their phylogenetic features, analyzed the conserved residues, predicted tertiary structures, and examined the tissue expression profile and immune inducibility. Through bioinformatics analysis, we have putatively identified ten secretory and two cytoplasmic CTL-S; four secretory and two cytoplasmic immulectins; one secretory, one cytoplasmic and three transmembrane forms of CTL-X. Most B. mori CTLDPs form monophyletic groups with orthologs from Lepidoptera, Diptera, Coleoptera and Hymenoptera species. Immulectins of B. mori and Manduca sexta evolved from common ancestor genes perhaps due to gene duplication events of CTL-S ancestor genes. Homology modeling revealed that the overall structures of B. mori CTL domains are analogous to those of humans with a variable loop region. We examined the expression profile of CTLDP genes in naïve and immune-stimulated tissues. The expression and induction of CTLDP genes were related to the tissues and microorganisms. Together, our gene identification, sequence comparison, phylogenetic analysis, homology modeling and expression analysis laid a good foundation for the further studies of B. mori CTLDPs and comparative genomics.