Bacterial, but not baculoviral infections stimulate Hemolin expression in noctuid moths

CmHem, a hemolin-like gene identified from Cnaphalocrocis medinalis, involved in metamorphosis and baculovirus infection

Background

As a member of the immunoglobulin superfamily, hemolins play a vital role in insect development and defense against pathogens. However, the innate immune response of hemolin to baculovirus infection varies among different insects.

Methods and results

In this study, the hemolin-like gene from a Crambidae insect, Cnaphalocrocis medinalis, CmHem was cloned, and its role in insect development and baculovirus infection was analyzed. A 1,528 bp contig as potential hemolin-like gene of C. medinalis was reassembled from the transcriptome. Further, the complete hemolin sequence of C. medinalis (CmHem) was cloned and sequenced. The cDNA of CmHem was 1,515 bp in length and encoded 408 amino acids. The deduced amino acid of CmHem has relatively low identities (41.9-62.3%) to various insect hemolins. However, it contains four Ig domains similarity to other insect hemolins. The expression level of CmHem was the highest in eggs, followed by pupae and adults, and maintained a low expression level at larval stage. The synthesized siRNAs were injected into mature larvae, and the CmHem transcription decreased by 51.7%. Moreover, the abdominal somites of larvae became straightened, could not pupate normally, and then died. Infection with a baculovirus, C. medinalis granulovirus (CnmeGV), the expression levels of CmHem in the midgut and fat body of C. medinalis significantly increased at 12 and 24 h, respectively, and then soon returned to normal levels.

Conclusions

Our results suggested that hemolin may be related to the metamorphosis of C. medinalis. Exposure to baculovirus induced the phased expression of hemolin gene in the midgut and fat body of C. medinalis, indicated that hemolin involved in the immune recognition of Crambidae insects to baculovirus.

A Versatile Hemolin With Pattern Recognitional Contributions to the Humoral Immune Responses of the Chinese Oak Silkworm Antheraea pernyi

Hemolin is a distinctive immunoglobulin superfamily member involved in invertebrate immune events. Although it is believed that hemolin regulates hemocyte phagocytosis and microbial agglutination in insects, little is known about its contribution to the humoral immune system. In the present study, we focused on hemolin in Antheraea pernyi (Ap-hemolin) by studying its pattern recognition property and humoral immune functions. Tissue distribution analysis demonstrated the mRNA level of Ap-hemolin was extremely immune-inducible in different tissues. The results of western blotting and biolayer interferometry showed recombinant Ap-hemolin bound to various microbes and pathogen-associated molecular patterns. In further immune functional studies, it was detected that knockdown of hemolin regulated the expression level of antimicrobial peptide genes and decreased prophenoloxidase activation in the A. pernyi hemolymph stimulated by microbial invaders. Together, these data suggest that hemolin is a multifunctional pattern recognition receptor that plays critical roles in the humoral immune responses of A. pernyi.

Insect Defense Proteins and Peptides

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

Regulation by gut bacteria of immune response, Bacillus thuringiensis susceptibility and hemolin expression in Plodia interpunctella

Plodia interpunctella (Hübner) is an important stored grain insect pest worldwide, and the first lepidopteran with reported resistance to Bacillus thuringiensis (Bt) toxins. Since gut bacteria may affect Bt insecticidal activity, we evaluated whether P. interpunctella lacking gut bacteria had differences in immune responses and susceptibility to the Bt formulation, Bactospeine. In order to clear gut bacteria, third instar larvae were reared on artificial diet containing antibiotics, or were obtained from sterilized eggs and reared under sterile conditions, and larvae were fed diets with or without Bt. Mortality was significantly lower (p<0.05) in bacteria-free larvae treated with Bt, compared with Bt-treated larvae with unaffected gut bacteria. The number of hemocytes was lower in control and Bt-treated larvae, but was significantly higher (p<0.001) in larvae treated with antibiotics and Bt, and larvae from presterilized eggs and reared on sterile diet had the highest number of hemocytes. Phenoloxidase activity was significantly lower (p<0.05) in Bt-treated larvae from presterilized eggs reared on antibiotics for 24h or in larvae reared on antibiotic-treated diets prior to Bt introduction compared with those fed control diet. Hemolin gene expression was reduced in larvae fed Bt diets compared with control and was not detected in larvae treated with antibiotics. Larvae from sterilized eggs and fed sterile diet never reached the pupal stage. Therefore, the loss of gut bacteria in P. interpunctella larvae affected the host immune response and expression of the hemolin gene, and significantly reduced susceptibility to Bt.

Identification of a hemolin protein from Actias selene mediates immune response to pathogens

Hemolins play an important role in development and innate immunity in insects. In this study, a hemolin cDNA of 1412bp in Actias selene (As-HEM) was isolated and its open reading frames (ORFs) were 420 amino acid residues. Sequence analysis indicated As-HEM was homologous to those hemolins from other insects species. The recombinant protein of As-HEM was expressed in Escherichia coli, and anti-As-HEM antibodies were prepared. Real-time quantitative PCR (RT-qPCR) and western blot results revealed that mRNA and protein levels of As-HEM were mostly detected in hemocytes and hemolymph. Immune challenge assays showed that both the mRNA and protein levels of As-HEM could be induced significantly post Beauveria bassiana, E. coli, Micrococcus luteus and nuclear polyhedrosis virus challenges. Agglutination assays revealed that recombinant As-HEM could promote the agglutination of E. coli in the presence of calcium. Our results suggested that As-HEM was involved in the innate immunity of A. selene.

Gut immunity in Lepidopteran insects

Lepidopteran insects constitute one of the largest fractions of animals on earth, but are considered pests in their relationship with man. Key to the success of this order of insects is its ability to digest food and absorb nutrition, which takes place in the midgut. Because environmental microorganisms can easily enter Lepidopteran guts during feeding, the innate immune response guards against pathogenic bacteria, virus and microsporidia that can be devoured with food. Gut immune responses are complicated by both resident gut microbiota and the surrounding peritrophic membrane and are distinct from immune responses in the body cavity, which depend on the function of the fat body and hemocytes. Due to their relevance to agricultural production, studies of Lepidopteran insect midgut and immunity are receiving more attention, and here we summarize gut structures and functions, and discuss how these confer immunity against different microorganisms. It is expected that increased knowledge of Lepidopteran gut immunity may be utilized for pest biological control in the future.

Pollen feeding proteomics: Salivary proteins of the passion flower butterfly, Heliconius melpomene

While most adult Lepidoptera use flower nectar as their primary food source, butterflies in the genus Heliconius have evolved the novel ability to acquire amino acids from consuming pollen. Heliconius butterflies collect pollen on their proboscis, moisten the pollen with saliva, and use a combination of mechanical disruption and chemical degradation to release free amino acids that are subsequently re-ingested in the saliva. Little is known about the molecular mechanisms of this complex pollen feeding adaptation. Here we report an initial shotgun proteomic analysis of saliva from Heliconius melpomene. Results from liquid-chromatography tandem mass-spectrometry confidently identified 31 salivary proteins, most of which contained predicted signal peptides, consistent with extracellular secretion. Further bioinformatic annotation of these salivary proteins indicated the presence of four distinct functional classes: proteolysis (10 proteins), carbohydrate hydrolysis (5), immunity (6), and "housekeeping" (4). Additionally, six proteins could not be functionally annotated beyond containing a predicted signal sequence. The presence of several salivary proteases is consistent with previous demonstrations that Heliconius saliva has proteolytic capacity. It is likely that these proteins play a key role in generating free amino acids during pollen digestion. The identification of proteins functioning in carbohydrate hydrolysis is consistent with Heliconius butterflies consuming nectar, like other lepidopterans, as well as pollen. Immune-related proteins in saliva are also expected, given that ingestion of pathogens is a likely route to infection. The few "housekeeping" proteins are likely not true salivary proteins and reflect a modest level of contamination that occurred during saliva collection. Among the unannotated proteins were two sets of paralogs, each seemingly the result of a relatively recent tandem duplication. These results offer a first glimpse into the molecular foundation of Heliconius pollen feeding and provide a substantial advance towards comprehensively understanding this striking evolutionary novelty.

Identification and functional analysis of a Hemolin like protein from Litopenaeus vannamei

Hemolin is a specific immune protein belonging to immunoglobulin superfamily and firstly identified in insects. Growing evidences suggest that Hemolin can be activated by bacterial and viral infections and may play an important role in antimicrobial immunity. In this paper, we firstly identified a Hemolin-like protein from Litopenaeus vannamei (LvHemolin). Sequence analysis showed that LvHemolin shares high similarity with insect Hemolins and is mainly composed of seven immunoglobulin (Ig) domains which form a 'horseshoe' tertiary structure. Tissue distribution analysis demonstrated that LvHemolin mainly expressed in stomach, gill, epithelium and pyloric cecum of L. vannamei. After challenge with pathogens or stimulants, expression of LvHemolin was significantly up-regulated in both gill and stomach. Agglutination analysis demonstrated that recombinant LvHemolin protein purified from Escherichia coli could accelerate the agglutination of Vibrio parahaemolyticus, E. coli, Staphylococcus aureus, and Bacillus subtilis in the presence of Ca(2+). To verify the immune function of LvHemolin in vivo, shrimps were injected with gene-specific dsRNA, followed by challenge with white spot syndrome virus (WSSV) or V. parahaemolyticus. The results revealed that silence of LvHemolin could increase the cumulative mortalities of shrimps challenged by pathogens and increase the WSSV copies in shrimp tissues. These suggested that Hemolin could play an important role in shrimp innate immune defense against bacterial and viral infections.

Functional characterization of hesp018, a baculovirus-encoded serpin gene

The serpin family of serine proteinase inhibitors plays key roles in a variety of biochemical pathways. In insects, one of the important functions carried out by serpins is regulation of the phenoloxidase (PO) cascade - a pathway that produces melanin and other compounds that are important in insect humoral immunity. Recent sequencing of the baculovirus Hemileuca sp. nucleopolyhedrovirus (HespNPV) genome revealed the presence of a gene, hesp018, with homology to insect serpins. To our knowledge, hesp018 is the first viral serpin homologue to be characterized outside of the chordopoxviruses. The Hesp018 protein was found to be a functional serpin with inhibitory activity against a subset of serine proteinases. Hesp018 also inhibited PO activation when mixed with lepidopteran haemolymph. The Hesp018 protein was secreted when expressed in lepidopteran cells and a baculovirus expressing Hesp018 exhibited accelerated production of viral progeny during in vitro infection. Expression of Hesp018 also reduced caspase activity induced by baculovirus infection, but caused increased cathepsin activity. In infected insect larvae, expression of Hesp018 resulted in faster larval melanization, consistent with increased activity of viral cathepsin. Finally, expression of Hesp018 increased the virulence of a prototype baculovirus by fourfold in orally infected neonate Trichoplusia ni larvae. Based on our observations, we hypothesize that hesp018 may have been retained in HespNPV due to its ability to inhibit the activity of select host proteinases, possibly including proteinases involved in the PO response, during infection of host insects.

Functional Immunomics of the Squash Bug, Anasa tristis (De Geer) (Heteroptera: Coreidae)

The Squash bug, Anasa tristis (De Geer), is a major piercing/sucking pest of cucurbits, causing extensive damage to plants and fruits, and transmitting phytopathogens. No genomic resources to facilitate field and laboratory studies of this pest were available; therefore the first de novo exome for this destructive pest was assembled. RNA was extracted from insects challenged with bacterial and fungal immunoelicitors, insects fed on different cucurbit species, and insects from all life stages from egg to adult. All treatments and replicates were separately barcoded for subsequent analyses, then pooled for sequencing in a single lane using the Illumina HiSeq2000 platform. Over 211 million 100-base tags generated in this manner were trimmed, filtered, and cleaned, then assembled into a de novo reference transcriptome using the Broad Institute Trinity assembly algorithm. The assembly was annotated using NCBIx NR, BLAST2GO, KEGG and other databases. Of the >130,000 total assemblies 37,327 were annotated identifying the sequences of candidate gene silencing targets from immune, endocrine, reproductive, cuticle, and other physiological systems. Expression profiling of the adult immune response was accomplished by aligning the 100-base tags from each biological replicate from each treatment and controls to the annotated reference assembly of the A. tristis transcriptome.

Increase in gut microbiota after immune suppression in baculovirus-infected larvae

Spodoptera exigua microarray was used to determine genes differentially expressed in S. exigua cells challenged with the species-specific baculovirus SeMNPV as well as with a generalist baculovirus, AcMNPV. Microarray results revealed that, in contrast to the host transcriptional shut-off that is expected during baculovirus infection, S. exigua cells showed a balanced number of up- and down-regulated genes during the first 36 hours following the infection. Many immune-related genes, including pattern recognition proteins, genes involved in signalling and immune pathways as well as immune effectors and genes coding for proteins involved in the melanization cascade were found to be down-regulated after baculovirus infection. The down-regulation of immune-related genes was confirmed in the larval gut. The expression of immune-related genes in the gut is known to affect the status of gut microorganisms, many of which are responsible for growth and development functions. We therefore asked whether the down-regulation that occurs after baculovirus infection affects the amount of gut microbiota. An increase in the gut bacterial load was observed and we hypothesize this to be as a consequence of viral infection. Subsequent experiments on virus performance in the presence and absence of gut microbiota revealed that gut bacteria enhanced baculovirus virulence, pathogenicity and dispersion. We discuss the host immune response processes and pathways affected by baculoviruses, as well as the role of gut microbiota in viral infection.

Baculoviruses are large DNA viruses that infect invertebrates, mainly insects from the order Lepidoptera. They were first discovered to cause insects' epizootics and are now used worldwide as biocontrol agents. Extensive studies on baculovirus biology led to the discovery that they can serve as expression vectors in insect cells; recently they have also been considered as vectors for gene therapy. Baculovirus infection, like many other oral infections, starts with the invasion of the gut by viruses; the gut is a compartment colonized by a community of resident microbiota. In this study, we observed that baculovirus infection leads to the decreased expression of immune-related genes in a Spodoptera exigua cell culture as well as in the larval gut. Gut microbial loads were found to increase after baculovirus infection. A series of bioassays showed that the baculovirus performs better in the presence of microbiota in the gut. Our study shows that baculovirus infection leads to increase of microbiota loads in the gut and that the gut microbiota play a significant role in insect immunity and susceptibility to viral infections. These findings suggest that gut microbiota can be manipulated to improve biocontrol strategies that employ baculoviruses.

RNA-Seq Study of Microbially Induced Hemocyte Transcripts from Larval Heliothis virescens (Lepidoptera: Noctuidae)

Larvae of the tobacco budworm are major polyphagous pests throughout the Americas. Development of effective microbial biopesticides for this and related noctuid pests has been stymied by the natural resistance mediated innate immune response. Hemocytes play an early and central role in activating and coordinating immune responses to entomopathogens. To approach this problem we completed RNA-seq expression profiling of hemocytes collected from larvae following an in vivo challenge with bacterial and fungal cell wall components to elicit an immune response. A de novo exome assembly was constructed by combination of sequence tags from all treatments. Sequence tags from each treatment were aligned separately with the assembly to measure expression. The resulting table of differential expression had >22,000 assemblies each with a distinct combination of annotation and expression. Within these assemblies >1,400 were upregulated and >1,500 downregulated by immune activation with bacteria or fungi. Orthologs to innate immune components of other insects were identified including pattern recognition, signal transduction pathways, antimicrobial peptides and enzymes, melanization and coagulation. Additionally orthologs of components regulating hemocytic functions such as autophagy, apoptosis, phagocytosis and nodulation were identified. Associated cellular oxidative defenses and detoxification responses were identified providing a comprehensive snapshot of the early response to elicitation.

Baculovirus induced transcripts in hemocytes from the larvae of Heliothis virescens

Using RNA-seq digital difference expression profiling methods, we have assessed the gene expression profiles of hemocytes harvested from Heliothis virescens that were challenged with Helicoverpa zea single nucleopolyhedrovirus (HzSNPV). A reference transcriptome of hemocyte-expressed transcripts was assembled from 202 million 42-base tags by combining the sequence data of all samples, and the assembled sequences were then subject to BLASTx analysis to determine gene identities. We used the fully sequenced HzSNPV reference genome to align 477,264 Illumina sequence tags from infected hemocytes in order to document expression of HzSNPV genes at early points during infection. A comparison of expression profiles of control insects to those lethally infected with HzSNPV revealed differential expression of key cellular stress response genes and genes involved in lipid metabolism. Transcriptional regulation of specific insect hormones in baculovirus-infected insects was also altered. A number of transcripts bearing homology to retroviral elements that were detected add to a growing body of evidence for extensive invasion of errantiviruses into the insect genome. Using this method, we completed the first and most comprehensive gene expression survey of both baculoviral infection and host immune defense in lepidopteran larvae.

Immunity in lepidopteran insects

Lepidopteran insects provide important model systems for innate immunity of insects, particularly for cell biology of hemocytes and biochemical analyses of plasma proteins. Caterpillars are also among the most serious agricultural pests, and understanding of their immune systems has potential practical significance. An early response to infection in lepidopteran larvae is the activation of hemocyte adhesion, leading to phagocytosis, nodule formation, or encapsulation. Plasmatocytes and granular cells are the hemocyte types involved in these responses. Infectious microorganisms are recognized by binding of hemolymph plasma proteins to microbial surface components. This "pattern recognition" triggers phagocytosis and nodule formation, activation of prophenoloxidase and melanization and the synthesis of antimicrobial proteins that are secreted into the hemolymph. Many hemolymph proteins that function in such innate immune responses of insects were first discovered in lepidopterans. Microbial proteinases and nucleic acids released from lysed host cells may also activate lepidopteran immune responses. Hemolymph antimicrobial peptides and proteins can reach high concentrations and may have activity against a broad spectrum of microorganisms, contributing significantly to clearing of infections. Serine proteinase cascade pathways triggered by microbial components interacting with pattern recognition proteins stimulate activation of the cytokine Spätzle, which initiates the Toll pathway for expression of antimicrobial peptides. A proteinase cascade also results inproteolytic activation of phenoloxidase and production of melanin coatings that trap and kill parasites and pathogens. The proteinases in hemolymph are regulated by specific inhibitors, including members of the serpin superfamily. New developments in lepidopteran functional genomics should lead to much more complete understanding of the immune systems of this insect group.