Hemolymph protease-5 links the melanization and Toll immune pathways in the tobacco hornworm, Manduca sexta

Lack of signal peptide in insect prophenoloxidase to avoid glycosylation to damage the zymogen activity

Insect prophenoloxidases (PPOs) are important immunity proteins for defending against the invading pathogens and parasites. As a Type-Ⅲ copper-containing proteins, unlike Homo sapiens tyrosinases, the insect PPOs and most bacterial tyrosinases contain no signal peptides for unknown reason, however they can still be released. To this end, we fused different signal peptides to Drosophila melanogaster PPOs for in vitro and in vivo expression, respectively. We demonstrate that an artificial signal peptide can help PPO secretion in vitro. The secreted PPO appeared larger than wild-type PPO on molecular weight sizes due to glycosylation when expressed in S2 cells. Two asparagine residues for potential glycosylation in PPO1 were identified when a signal peptide was fused. After purification, the glycosylated PPO1 lost zymogen activity. When PPO1 containing a signal peptide was over-expressed in Drosophila larvae, the glycosylation and secretion of PPO1 was detected in vivo. Unlike insect PPO, human tyrosinase needs a signal peptide for protein expression and maintaining enzyme activity. An artificial signal peptide fused to bacterial tyrosinase had no influence on the protein expression and enzyme activity. These Type-Ⅲ copper-containing proteins from different organisms may evolve to perform their specific functions. Intriguingly, our study revealed that the addition of calcium inhibits PPO secretion from the transiently cultured larval hindguts in vitro, indicating that the calcium concentration may regulate PPO secretion. Taken together, insect PPOs can maintain enzyme activities without any signal peptide.

Identification of immunity-related genes distinctly regulated by Manduca sexta Spӓtzle-1/2 and Escherichia coli peptidoglycan

The immune system of Manduca sexta has been well studied to understand molecular mechanisms of insect antimicrobial responses. While evidence supports the existence of major immune signaling pathways in this species, it is unclear how induced production of defense proteins is specifically regulated by the Toll and Imd pathways. Our previous studies suggested that diaminopimelic acid-type peptidoglycans (DAP-PG) from Gram-negative and some Gram-positive bacteria, more than Lys-type peptidoglycans (Lys-PG) from other Gram-positive bacteria, triggers both pathways through membrane-bound receptors orthologous to Drosophila Toll and PGRP-LC. In this study, we produced M. sexta proSpätzle-1 and proSpätzle-2 in Sf9 cells, identified their processing enzymes, and used prophenoloxidase activating protease-3 to activate the cytokine precursors. After Spätzle-1 and -2 were isolated from the reaction mixtures, we separately injected the purified cytokines into larval hemocoel to induce gene transcription in fat body through the Toll pathway solely. On the other hand, we treated a M. sexta cell line with E. coli DAP-PG to only induce the Imd pathway and target gene expression. RNA-Seq analysis of the fat body and cultured cells collected at 0, 6, and 24 h after treatment indicated that expression of diapausin-4, -10, -12, -13, cecropin-2, -4, -5, attacin-5, -11, and lebocin D is up-regulated predominantly via Toll signaling, whereas transcription of cecropin-6, gloverin, lysozyme-1, and gallerimycin-2 is mostly induced by DAP-PG via Imd signaling. Other antimicrobial peptides are expressed in response to both pathways. Transcripts of most Toll-specific genes (e.g., lebocin D) peaked at 6 h, contrasting the gradual increase and plateauing of drosomycin mRNA level at 24-48 h in Drosophila. We also used T (oll)-I (md) ratios to estimate relative contributions of the two pathways to transcriptional regulation of other components of the immune system. The differences in pathway specificity and time course of transcriptional regulation call for further investigations in M. sexta and other insects.

The immune response mechanism of Nilaparvata lugens against a combined infection of rice ragged stunt virus and Metarhizium anisopliae

BACKGROUND

Previous studies of brown planthopper (BPH), Nilaparvata lugens, showed that carrying the plant pathogenic virus, rice ragged stunt virus (RRSV), enhanced the lethality of the entomopathogenic fungus, Metarhizium anisopliae (YTTR). The underlying mechanism for this was not established but a serine protease cascade was hypothesized to be involved.

RESULTS

Two immune response genes, NlKPI and NlVenomase, were identified and shown to be involved. The synthesized double-strand RNA (dsRNA) techniques used in this study to explore gene function revealed that treatment with dsRNA to silence either gene led to a higher BPH mortality from M. anisopliae infection than the dsRNA control treatment. NlKPI and NlVenomase play vital roles in BPH immunity to defend against alien pathogens. Both genes participate in the immune response process of BPH against co-infection with RRSV and M. anisopliae YTTR by regulating the expression of antimicrobial peptides and phenoloxidase activity.

CONCLUSION

Our study provided new targets for BPH biocontrol and laid a solid foundation for further research on the interaction of virus-insect-EPF (entomopathogenic fungus). © 2023 Society of Chemical Industry.

Suppression of Drosophila antifungal immunity by a parasite effector via blocking GNBP3 and GNBP-like 3, the dual receptors for β-glucans

The tactics used by animal pathogens to combat host immunity are largely unclear. Here, we report the depiction of the virulence-required effector Tge1 deployed by the entomopathogen Metarhizium robertsii to suppress Drosophila antifungal immunity. Tge1 can target both GNBP3 and GNBP-like 3 (GL3), and the latter can bind to β-glucans like GNBP3, whereas the glucan binding by both receptors can be attenuated by Tge1. As opposed to the surveillance GNBP3, GL3 is inducible in Drosophila depending on the Toll pathway via a positive feedback loop mechanism. Losses of GNBP3 and GL3 genes result in the deregulations of protease cascade, Spätzle maturation, and antimicrobial gene expressions in Drosophila upon fungal challenges. Fly survival assays confirm that GL3 plays a more essential role than GNBP3 in combating fungal infections. In addition to evidencing the gene-for-gene interactions between fungi and insects, our data advance insights into Drosophila antifungal immunity.

The accumulation of modular serine protease mediated by a novel circRNA sponging miRNA increases Aedes aegypti immunity to fungus

BACKGROUND

Mosquitoes transmit many infectious diseases that affect human health. The fungus Beauveria bassiana is a biological pesticide that is pathogenic to mosquitoes but harmless to the environment.

RESULTS

We found a microRNA (miRNA) that can modulate the antifungal immunity of Aedes aegypti by inhibiting its cognate serine protease. Fungal infection can induce the expression of modular serine protease (ModSP), and ModSP knockdown mosquitoes were more sensitive to B. bassiana infection. The novel miRNA-novel-53 is linked to antifungal immune response and was greatly diminished in infected mosquitoes. The miRNA-novel-53 could bind to the coding sequences of ModSP and impede its expression. Double fluorescence in situ hybridization (FISH) showed that this inhibition occurred in the cytoplasm. The amount of miRNA-novel-53 increased after miRNA agomir injection. This resulted in a significant decrease in ModSP transcript and a significant increase in mortality after fungal infection. An opposite effect was produced after antagomir injection. The miRNA-novel-53 was also knocked out using CRISPR-Cas9, which increased mosquito resistance to the fungus B. bassiana. Moreover, mosquito novel-circ-930 can affect ModSP mRNA by interacting with miRNA-novel-53 during transfection with siRNA or overexpression plasmid.

CONCLUSIONS

Novel-circ-930 affects the expression level of ModSP by a novel-circ-930/miRNA-novel-53/ModSP mechanism to modulate antifungal immunity, revealing new information on innate immunity in insects.

Insight into the structural hierarchy of the protease cascade that regulates the mosquito melanization response

Serine protease cascades regulate important insect immune responses, including melanization and Toll pathway activation. In the context of melanization, central components of these cascades are clip domain serine proteases (CLIPs) including the catalytic, clip domain serine proteases (cSPs) and their non-catalytic homologs (cSPHs). Here, we define partially the structural hierarchy of An. gambiae cSPs of the CLIPB family, central players in melanization, and characterize their relative contributions to bacterial melanization and to mosquito susceptibility to bacterial infections. Using in vivo genetic analysis we show that the protease cascade branches downstream of the cSPs CLIPB4 and CLIPB17 into two branches one converging on CLIPB10 and the second on CLIPB8. We also show that the contribution of key cSPHs to melanization in vivo in response to diverse microbial challenges is more significant than any of the individual cSPs, possibly due to partial functional redundancy among the latter. Interestingly, we show that the key cSPH CLIPA8 which is essential for the efficient activation cleavage of CLIPBs in vivo is efficiently cleaved itself by several CLIPBs in vitro, suggesting that cSPs and cSPHs regulate signal amplification and propagation in melanization cascades by providing positive reinforcement upstream and downstream of each other.

Serpin-4 Negatively Regulates Prophenoloxidase Activation and Antimicrobial Peptide Synthesis in the Silkworm, Bombyx mori

The prophenoloxidase (PPO) activation and Toll antimicrobial peptide synthesis pathways are two critical immune responses in the insect immune system. The activation of these pathways is mediated by the cascade of serine proteases, which is negatively regulated by serpins. In this study, we identified a typical serpin, BmSerpin-4, in silkworms, whose expression was dramatically up-regulated in the fat body and hemocytes after bacterial infections. The pre-injection of recombinant BmSerpin-4 remarkably decreased the antibacterial activity of the hemolymph and the expression of the antimicrobial peptides (AMPs) gloverin-3, cecropin-D, cecropin-E, and moricin in the fat body under Micrococcus luteus and Yersinia pseudotuberculosis serotype O: 3 (YP III) infection. Meanwhile, the inhibition of systemic melanization, PO activity, and PPO activation by BmSerpin-4 was also observed. Hemolymph proteinase 1 (HP1), serine protease 2 (SP2), HP6, and SP21 were predicted as the candidate target serine proteases for BmSerpin-4 through the analysis of residues adjacent to the scissile bond and comparisons of orthologous genes in Manduca sexta. This suggests that HP1, SP2, HP6, and SP21 might be essential in the activation of the serine protease cascade in both the Toll and PPO pathways in silkworms. Our study provided a comprehensive characterization of BmSerpin-4 and clues for the further dissection of silkworm PPO and Toll activation signaling.

An evolutionarily conserved serine protease network mediates melanization and Toll activation in Drosophila

Melanization and Toll pathway activation are essential innate immune mechanisms in insects, which result in the generation of reactive compounds and antimicrobial peptides, respectively, to kill pathogens. These two processes are mediated by phenoloxidase (PO) and Spätzle (Spz) through an extracellular network of serine proteases. While some proteases have been identified in Drosophila melanogaster in genetic studies, the exact order of proteolytic activation events remains controversial. Here, we reconstituted the serine protease framework in Drosophila by biochemical methods. This system comprises 10 proteases, i.e., ModSP, cSP48, Grass, Psh, Hayan-PA, Hayan-PB, Sp7, MP1, SPE and Ser7, which form cascade pathways that recognize microbial molecular patterns and virulence factors, and generate PO1, PO2, and Spz from their precursors. Furthermore, the serpin Necrotic negatively regulates the immune response progression by inhibiting ModSP and Grass. The biochemical approach, when combined with genetic analysis, is crucial for addressing problems that long stand in this important research field.

Parasitoid Serpins Evolve Novel Functions to Manipulate Host Homeostasis

Parasitoids introduce various virulence factors when parasitism occurs, and some taxa generate teratocytes to manipulate the host immune system and metabolic homeostasis for the survival and development of their progeny. Host-parasitoid interactions are extremely diverse and complex, yet the evolutionary dynamics are still poorly understood. A category of serpin genes, named CvT-serpins, was discovered to be specifically expressed and secreted by the teratocytes of Cotesia vestalis, an endoparasitoid of the diamondback moth Plutella xylostella. Genomic and phylogenetic analysis indicated that the C. vestalis serpin genes are duplicated and most of them are clustered into 1 monophyletic clade. Intense positive selection was detected at the residues around the P1-P1' cleavage sites of the Cv-serpin reactive center loop domain. Functional analyses revealed that, in addition to the conserved function of melanization inhibition (CvT-serpins 1, 16, 18, and 21), CvT-serpins exhibited novel functions, i.e. bacteriostasis (CvT-serpins 3 and 5) and nutrient metabolism regulation (CvT-serpins 8 and 10). When the host-parasitoid system is challenged with foreign bacteria, CvT-serpins act as an immune regulator to reprogram the host immune system through sustained inhibition of host melanization while simultaneously functioning as immune effectors to compensate for this suppression. In addition, we provided evidence that CvT-serpin8 and 10 participate in the regulation of host trehalose and lipid levels by affecting genes involved in these metabolic pathways. These findings illustrate an exquisite tactic by which parasitoids win out in the parasite-host evolutionary arms race by manipulating host immune and nutrition homeostasis via adaptive gene evolution and neofunctionalization.

Insight into the structural hierarchy of the protease cascade that regulates the mosquito melanization response

Serine protease cascades regulate important insect immune responses, including melanization and Toll pathway activation. In the context of melanization, central components of these cascades are clip domain serine proteases (CLIPs) including the catalytic, clip domain serine proteases (cSPs) and their non-catalytic homologs (cSPHs). Here, we define partially the structural hierarchy of An. gambiae cSPs of the CLIPB family, central players in melanization, and characterize their relative contributions to bacterial melanization and to mosquito susceptibility to bacterial infections. Using in vivo genetic analysis we show that the protease cascade branches downstream of the cSPs CLIPB4 and CLIPB17 into two branches one converging on CLIPB10 and the second on CLIPB8. We also show that the contribution of key cSPHs to melanization in vivo in response to diverse microbial challenges is more significant than any of the individual cSPs, possibly due to partial functional redundancy among the latter. Interestingly, we show that the key cSPH CLIPA8 which is essential for the efficient activation cleavage of CLIPBs in vivo is efficiently cleaved itself by several CLIPBs in vitro, suggesting that cSPs and cSPHs regulate signal amplification and propagation in melanization cascades by providing positive reinforcement upstream and downstream of each other.

Immune responses of the Asian onion moth, Acrolepiopsis sapporensis, and their genetic factors from RNA-Seq analysis

A nonmodel insect, Acrolepiopsis sapporensis, has been analyzed in immune responses. The total hemocytes in the fifth instar larvae were 2.33 × 106  cells/mL. These hemocytes comprised at least five different types and different relative ratios: 47% granulocytes, 26% plasmatocytes, 11% oenocytoid, 8% prohemocytes, and 5% spherulocytes. Upon bacterial challenge, some of the hemocytes exhibited typical hemocyte-spreading behaviors, such as focal adhesion, and filopodial and lamellipodial cytoplasmic extensions. The hemocyte behaviors induced cellular immune responses demonstrated by nodule formation. In addition, the plasma collected from the immune-challenged larvae exhibited humoral immune responses by bacterial growth inhibition along with enhanced phenoloxidase enzyme activity. These cellular and humoral immune responses were further analyzed by determining the immune-associated genes from a transcriptome generated by RNA-Seq. A total of about 12 Gb sequences led to about 218,116 contigs, which were predicted to encode about 46,808 genes. Comparative expression analysis showed 8392 uniquely expressed genes in the immune-challenged larvae. Differentially expressed gene (DEG) analysis among the commonly expressed genes indicated that 782 genes were upregulated and 548 genes were downregulated in the expressions after bacterial challenge. These immune-associated genes included pattern recognition receptors, immune mediation/signaling genes, and various immune effectors. Specifically, the genetic components of the Toll, IMD, and JAK/STAT immune signaling pathways were included in the DEG database. These results demonstrate the immune responses of A. sapporensis larvae and suggest the genes associated with the immune responses in this nonmodel insect.

Serpin-1a and serpin-6 regulate the Toll pathway immune homeostasis by synergistically inhibiting the Spätzle-processing enzyme CLIP2 in silkworm, Bombyx mori

The Toll receptor signaling pathway is an important innate immune response of insects to pathogen infection; its extracellular signal transduction involves serine protease cascade activation. However, excessive or constitutive activation of the Toll pathway can be detrimental. Hence, the balance between activation and inhibition of the extracellular protease cascade must be tightly regulated to achieve favorable outcomes. Previous studies have shown that serpins-serine protease inhibitors-negatively regulate insect innate immunity by inhibiting extracellular protease cascade signaling. Although the roles of serpins in insect innate immunity are well described, the physiological mechanisms underlying their synergistic effects remain poorly understand. Here, we characterize the molecular mechanism by which serpin-1a and serpin-6 synergistically maintain immune homeostasis of the silkworm Toll pathway under physiological and pathological conditions. Through in vitro biochemical assays and in vivo bioassays, we demonstrate that clip-domain serine protease 2 (CLIP2), as the Toll cascade-activating terminal protease, is responsible for processing proSpätzle1 to induce the expression of antimicrobial peptides. Further biochemical and genetic analyses indicate that constitutively expressed serpin-1a and inducible serpin-6 synergistically target CLIP2 to maintain homeostasis of the silkworm Toll pathway under physiological and pathological conditions. Taken together, this study provides new insights into the precise regulation of Toll cascade activation signals in insect innate immune responses and highlights the importance and complexity of insect immune homeostasis regulation.

CLIPB4 Is a Central Node in the Protease Network that Regulates Humoral Immunity in Anopheles gambiae Mosquitoes

Insect humoral immune responses are regulated in part by protease cascades, whose components circulate as zymogens in the hemolymph. In mosquitoes, these cascades consist of clip-domain serine proteases (cSPs) and/or their non-catalytic homologs, which form a complex network, whose molecular make-up is not fully understood. Using a systems biology approach, based on a co-expression network of gene family members that function in melanization and co-immunoprecipitation using the serine protease inhibitor (SRPN)2, a key negative regulator of the melanization response in mosquitoes, we identify the cSP CLIPB4 from the African malaria mosquito Anopheles gambiae as a central node in this protease network. CLIPB4 is tightly co-expressed with SRPN2 and forms protein complexes with SRPN2 in the hemolymph of immune-challenged female mosquitoes. Genetic and biochemical approaches validate our network analysis and show that CLIPB4 is required for melanization and antibacterial immunity, acting as a prophenoloxidase (proPO)-activating protease, which is inhibited by SRPN2. In addition, we provide novel insight into the structural organization of the cSP network in An. gambiae, by demonstrating that CLIPB4 is able to activate proCLIPB8, a cSP upstream of the proPO-activating protease CLIPB9. These data provide the first evidence that, in mosquitoes, cSPs provide branching points in immune protease networks and deliver positive reinforcement in proPO activation cascades.

CLIPB4 is a central node in the protease network that regulates humoral immunity in Anopheles gambiae mosquitoes

Insect humoral immune responses are regulated in part by protease cascades, whose components circulate as zymogens in the hemolymph. In mosquitoes, these cascades consist of clip domain serine proteases (cSPs) and/or their non-catalytic homologs (cSPHs), which form a complex network, whose molecular make-up is not fully understood. Using a systems biology approach, based on a co-expression network of gene family members that function in melanization and co-immunoprecipitation using the serine protease inhibitor (SRPN)2, a key negative regulator of the melanization response in mosquitoes, we identify the cSP CLIPB4 from the African malaria mosquito Anopheles gambiae as a central node in this protease network. CLIPB4 is tightly co-expressed with SRPN2 and forms protein complexes with SRPN2 in the hemolymph of immune-challenged female mosquitoes. Genetic and biochemical approaches validate our network analysis and show that CLIPB4 is required for melanization and antibacterial immunity, acting as a prophenoloxidase (proPO)-activating protease, which is inhibited by SRPN2. In addition, we provide novel insight into the structural organization of the cSP network in An. gambiae, by demonstrating that CLIPB4 is able to activate proCLIPB8, a cSP upstream of the proPO-activating protease CLIPB9. These data provide the first evidence that, in mosquitoes, cSPs provide branching points in immune protease networks and deliver positive reinforcement in proPO activation cascades.

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.

Serine Protease Networks Mediate Immune Responses in Extra-Embryonic Tissues of Eggs in the Tobacco Hornworm, Manduca sexta

The melanization and Toll pathways, regulated by a network of serine proteases and noncatalytic serine protease homologs (SPHs), have been investigated mostly in adult and larval insects. However, how these innate immune reactions are regulated in insect eggs remains unclear. Here we present evidence from transcriptome and proteome analyses that extra-embryonic tissues (yolk and serosa) of early-stage Manduca sexta eggs are immune competent, with expression of immune effector genes including prophenoloxidase and antimicrobial peptides. We identified gene products of the melanization and Toll pathways in M. sexta eggs. Through in vitro reconstitution experiments, we demonstrated that constitutive and infection-induced serine protease cascade modules that stimulate immune responses exist in the extra-embryonic tissues of M. sexta eggs. The constitutive module (HP14b-SP144-GP6) may promote rapid early immune signaling by forming a cascade activating the cytokine Spätzle and regulating melanization by activating prophenoloxidase (proPO). The inducible module (HP14a-HP21-HP5) may trigger enhanced activation of Spätzle and proPO at a later phase of infection. Crosstalk between the two modules may occur in transition from the constitutive to the induced response in eggs inoculated with bacteria. Examination of data from two other well-studied insect species, Tribolium castaneum and Drosophila melanogaster, supports a role for a serosa-dependent constitutive protease cascade in protecting early embryos against invading pathogens.

Characterization and functional analysis of a myeloid differentiation factor 88 in Ostrinia furnacalis Guenée larvae infected by Bacillus thuringiensis

Myeloid differentiation factor 88 (MyD88) is a pivotal adapter protein involved in activating nuclear factor NF-κB of the Toll pathway in insect innate immunity. MyD88 has been extensively studied in vertebrates and Drosophila. However, the information ascribed to MyD88 in Lepidoptera is scarce. In the present study, an Ostrinia furnacalis MyD88 (OfMyD88) cDNA was cloned and functionally characterized (GenBank accession no. MN906311). The complete cDNA sequence of OfMyD88 is 804 bp, and contains a 630 bp open reading frame encoding 209 amino acid residues. OfMyD88 has the death domain (DD), an intermediate domain, and the Toll/interleukin 1 receptor (TIR) domain. OfMyD88 was widely expressed in immune-related tissues such as hemocytes, fat body, midgut, and integument, with the highest expression level in hemocytes, and the lowest expression level in integument. To clarify the immune function of MyD88, O. furnacalis larvae were challenged with Bacillus thuringiensis (Bt) through feeding. Bt oral infection had significantly up-regulated the expression of OfMyD88 and immune genes, including PPO2 (prophenoloxidase 2), Attacin, Gloverin, Cecropin, Moricin, GRP3 (β-1, 3-Glucan recognition protein 3), and Lysozyme, and increased the activities of PO and lysozyme in hemolymph of O. furnacalis larvae. Knockdown of OfMyD88 by RNA interference suppressed the expression levels of immune related genes, but not PPO2 in the larvae orally infected with Bt, suggesting that OfMyD88 is involved in defending against Bt invasion through the Toll signaling pathway, but does not affect the PPO expression in O. furnacalis larvae.

Proteomic analysis of pharate pupal molting fluid from the tobacco hornworm, Manduca sexta

The insect cuticle is a key component of their success, being important for protection, communication, locomotion, and support. Conversely, as an exoskeleton, it also limits the size of the insect and must be periodically molted and a new one synthesized, to permit growth. To achieve this, the insect secretes a solution of chitinases, proteases and other proteins, known collectively as molting fluid, during each molting process to break down and recycle components of the old cuticle. Previous research has focused on the degradative enzymes in molting fluid and offered some characterization of their biochemical properties. However, identification of the specific proteins involved remained to be determined. We have used 2D SDS-PAGE and LC/MS-based proteomic analysis to identify proteins in the molting fluid of the tobacco hornworm, Manduca sexta, undergoing the larval to pupal molt. We categorized these proteins based on their proposed functions including chitin metabolism, proteases, peptidases, and immunity. This analysis complements previous reported work on M. sexta molting fluid and identifies candidate genes for enzymes involved in cuticle remodeling. Proteins classified as having an immune function highlight potential for molting fluid to act as an immune barrier to prevent infections during the cuticle degradation and ecdysis processes. Several proteins known to function in melanin synthesis as an immune response in hemolymph were present in molting fluid. We demonstrated that the bacterium Micrococcus luteus and the entomopathogenic fungus Beauveria bassiana can stimulate activation of phenoloxidase in molting fluid, indicating that the recognition proteins, protease cascade, and prophenoloxidase needed for melanin synthesis are present as a defense against infection during cuticle degradation. This analysis offers insights for proteins that may be important not only for molting in M. sexta but for insects in general.

Comparative analysis of the immune system and expression profiling of Lymantria dispar infected by Beauveria bassiana

Lymantria dispar is one of the most devastating forest pests worldwide. Fungal biopesticides have great potential as alternatives owing to their high lethality to pests and eco-friendly feature, which is, however, often severely compromised by the pests' innate immunity. A better understanding of the antifungal immune system in L. dispar would significantly facilitate the development of the biopesticide. Here, we investigated phylogenetic characteristics of immunity-related genes as well as the tissue expression patterns in L. dispar after the infection of an entomopathogen Beauveria bassiana using RNA-sequencing data. Results showed most immune genes remain at a low level of response after 24 h post-infection (HPI). Almost all genes in the Toll pathway were significantly up-regulated at 48 HPI, and SPH1, SPN6, Toll6, Toll12, Myd88, pelle, and Drosal were significantly down-regulated at 72 HPI. Immunoblotting analysis revealed that the protein levels of βGRP3 and PPO1 were significantly upregulated at 24 and 48 HPI, while Myd88 was downregulated at 24 HPI, which was further confirmed by Quantitative real-time PCR experiments. Moreover, the relative content of H₂O₂, a potent reactive oxygen species (ROS), was significantly increased with the decrease of the total antioxidant capacity, indicating that oxidative stress system positively participates in the clearance of the pathogenic fungus. Together, our study provides detailed genetic characteristics of antifungal immunity as well as profiling of the host defense against entomopathogenic infection, and comprehensive insight into molecular interaction between L. dispar and the entomopathogen.

A mechanistic analysis of bacterial recognition and serine protease cascade initiation in larval hemolymph of Manduca sexta

Serine protease cascades have evolved in vertebrates and invertebrates to mediate rapid defense responses. Previous biochemical studies showed that in hemolymph of a caterpillar, Manduca sexta, recognition of fungi by β-1,3-glucan recognition proteins (βGRP1 and βGRP2) or recognition of bacteria by peptidoglycan recognition protein-1 (PGRP1) and microbe binding protein (MBP) results in autoactivation of hemolymph protease-14 precursor (proHP14). HP14 then activates downstream members of a protease cascade leading to the melanization immune response. ProHP14 has a complex domain architecture, with five low-density lipoprotein receptor class A repeats at its amino terminus, followed by a Sushi domain, a Sushi domain variant called Wonton, and a carboxyl-terminal serine protease catalytic domain. Its zymogen form is activated by specific proteolytic cleavage at the amino-terminal end of the protease domain. While a molecular mechanism for recognition and triggering the response to β-1,3-glucan has been delineated, it is unclear how bacterial recognition stimulates proHP14 activation. To fill this knowledge gap, we expressed the two domains of M. sexta MBP and found that the amino-terminal domain binds to diaminopimelic acid-peptidoglycan (DAP-PG). ProHP14 bound to both the carboxyl-terminal domain (MBP-C) and amino-terminal domain (MBP-N) of MBP. In the mixture of DAP-PG, MBP, and larval plasma, inclusion of an HP14 fragment composed of LDLa repeats 2-5 (LDLa2-5) or MBP-C significantly reduced prophenoloxidase activation, likely by competing with the interactions of the full-length proteins, and suggesting that molecular interactions involving these regions of proHP14 and MBP take part in proHP14 activation in response to peptidoglycan. Using a series of N-terminally truncated versions of proHP14, we found that autoactivation required LDLa2-5. The optimal ratio of PGRP1, MBP, and proHP14 is close to 3:2:1. In summary, proHP14 autoactivation by DAP-type peptidoglycan requires binding of DAP-PG by PGRP1 and the MBP N-terminal domain and association of the LDLa2-5 region of proHP14 with the MBP C-terminal domain. These interactions may concentrate the proHP14 zymogen at the bacterial cell wall surface and promote autoactivation.

Preferential binding of DAP-PGs by major peptidoglycan recognition proteins found in cell-free hemolymph of Manduca sexta

Peptidoglycan recognition proteins (PGRPs) detect invading bacteria to trigger or modulate immune responses in insects. While these roles are established in Drosophila, functional studies are not yet achieved at the PGRP family level in other insects. To attain this goal, we selected Manduca sexta PGRP12 and five of the nine secreted PGRPs for recombinant expression and biochemical characterization. We cloned PGRP2-5, 12 and 13 cDNAs, produced the proteins in full (PGRP2-5, 13) or in part (PGRP3s, 12e, 13N, 13C) in Sf9 cells, and tested their bindings of two muramyl pentapeptides by surface plasmon resonance, two soluble peptidoglycans by competitive ELISA, and four insoluble peptidoglycans and eight whole bacteria by a pull-down assay. Preferential binding of meso-diaminopimelic acid-peptidoglycans (DAP-PGs) was observed in all the proteins containing a peptidoglycan binding domain and, since PGRP6, 7 and 9 proteins were hardly detected in cell-free hemolymph, the reportoire of PGRPs (including PGRP1 published previously) in M. sexta hemolymph is likely adapted to mainly detect Gram-negative bacteria and certain Gram-positive bacteria with DAP-PGs located on their surface. After incubation with plasma from naïve larvae, PGRP2, 3f, 4, 5, 13f and 13N considerably stimulated prophenoloxidase activation in the absence of a bacterial elicitor. PGRP3s and 12e had much smaller effects. Inclusion of the full-length PGRPs and their regions in the plasma also led to proHP8 activation, supporting their connections to the Toll pathway, since HP8 is a Spӓtzle-1 processing enzyme in M. sexta. Together, these findings raised concerns on the common belief that the Toll-pathway is specific for Gram-positive bacteria in insects.

The Micrococcus luteus infection activates a novel melanization pathway of cSP10, cSP4, and cSP8 in Helicoverpa armigera

Melanization is a key immune response mediated by serine protease (SP) cascade in insects. Multiple SP pathways exist in different species and it is unclear how conserved these cascades are. The cotton bollworm Helicoverpa armigera is a major worldwide agricultural pest. We reported a conserved melanization pathway in this species, which consists of SP41, cSP1, and cSP6. In this study, we attempted to identify an insect pathogen that elicits the cascade and test whether or not there are other SP cascades in H. armigera. After Micrococcus luteus, Enterobacter cloacae, Beauveria bassiana, or Helicoverpa armigera nucleopolyhedrovirus were injected into larvae, pathogen-induced hemolymph samples were collected for in vitro biochemical assays, which failed to detect proSP41 or procSP1 activation. In contrast, we found that procSP4, a protein proposed to participate in H. armigera melanization, was activated in M. luteus infected hemolymph. We further revealed that cSP8 was a prophenoloxidase (PPO) activating protease downstream of cSP4, and cSP4 was activated by cSP10. The pathway of cSP10-cSP4-cSP8 activated PPO in vitro. Efficiently cleaved procSPH11 and procSPH50 by cSP8 substantially enhanced phenoloxidase activity, suggesting they work together as a cofactor for cSP8 mediated PPO activation. Hemolymph from larvae challenged with M. luteus or its peptidoglycan effectively activated procSP10. Collectively, these results revealed a new PPO activation cascade specifically triggered by the bacterium. In addition, we found that the PPO activation cascades in H. armigera and Manduca sexta are conserved.

Antimicrobial peptides: Defending the mucosal epithelial barrier

The recent epidemic caused by aerosolized SARS-CoV-2 virus illustrates the importance and vulnerability of the mucosal epithelial barrier against infection. Antimicrobial proteins and peptides (AMPs) are key to the epithelial barrier, providing immunity against microbes. In primitive life forms, AMPs protect the integument and the gut against pathogenic microbes. AMPs have also evolved in humans and other mammals to enhance newer, complex innate and adaptive immunity to favor the persistence of commensals over pathogenic microbes. The canonical AMPs are helictical peptides that form lethal pores in microbial membranes. In higher life forms, this type of AMP is exemplified by the defensin family of AMPs. In epithelial tissues, defensins, and calprotectin (complex of S100A8 and S100A9) have evolved to work cooperatively. The mechanisms of action differ. Unlike defensins, calprotectin sequesters essential trace metals from microbes, which inhibits growth. This review focuses on defensins and calprotectin as AMPs that appear to work cooperatively to fortify the epithelial barrier against infection. The antimicrobial spectrum is broad with overlap between the two AMPs. In mice, experimental models highlight the contribution of both AMPs to candidiasis as a fungal infection and periodontitis resulting from bacterial dysbiosis. These AMPs appear to contribute to innate immunity in humans, protecting the commensal microflora and restricting the emergence of pathobionts and pathogens. A striking example in human innate immunity is that elevated serum calprotectin protects against neonatal sepsis. Calprotectin is also remarkable because of functional differences when localized in epithelial and neutrophil cytoplasm or released into the extracellular environment. In the cytoplasm, calprotectin appears to protect against invasive pathogens. Extracellularly, calprotectin can engage pathogen-recognition receptors to activate innate immune and proinflammatory mechanisms. In inflamed epithelial and other tissue spaces, calprotectin, DNA, and histones are released from degranulated neutrophils to form insoluble antimicrobial barriers termed neutrophil extracellular traps. Hence, calprotectin and other AMPs use several strategies to provide microbial control and stimulate innate immunity.

Aedes aegypti CLIPB9 activates prophenoloxidase-3 in the presence of CLIPA14 after fungal infection

Melanization is an integral part of the insect defense system and is often induced by pathogen invasion. Phenoloxidases (POs) are critical enzymes that catalyze melanin formation. PO3 is associated with the antifungal response of the mosquito, Aedes aegypti, but the molecular mechanism of the prophenoloxidase-3 (PPO3) activation is unclear. Here we report that PPO3 cleavage activation is mediated by a clip-domain serine protease, CLIPB9. We purified recombinant CLIPB9 and found that it cleaved PPO3 and increased PO activity in the hemolymph. We then identified CLIPA14 (a serine protease homolog) by co-immunoprecipitation using anti-CLIPB9 antibody. After being cleaved by CLIPB9, Ae. aegypti CLIPA14 acted as a cofactor for PPO3 activation. In addition, dsRNA co-silencing of CLIPB9 and CLIPA14 genes reduced melanization after infection with the entomopathogen, Beauveria bassiana, making the adult mosquitoes more sensitive to fungal infection. These results illustrate the roles of CLIPB9 and CLIPA14 in the PPO activation pathway and revealed the complexity of the upstream serine protease network controlling melanization.

Serpin-4 Facilitates Baculovirus Infection by Inhibiting Melanization in Asian Corn Borer, Ostrinia furnacalis (Guenée)

Phenoloxidase (PO)-catalyzed melanization is a vital immune response in insects for defense against pathogen infection. This process is mediated by clip domain serine proteases and regulated by members of the serpin superfamily. We here revealed that the infection of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) significantly inhibited the PO activity in Ostrinia furnacalis hemolymph and induced the expression of O. furnacalis serpin-4. Addition of recombinant serpin-4 protein to O. furnacalis hemolymph resulted in a great increase of AcMNPV copies. Serpin-4 significantly suppressed the PO activity and the amidase activity in cleaving colorimetric substrate IEARpNA (IEARase activity) of hemolymph. Further experiments indicated it formed covalent complexes with three serine proteases (SP1, SP13 and SP105) and prevented them from cleaving their cognate downstream proteases in vitro. Altogether, O. furnacalis melanization restricted AcMNPV replication and serpin-4 facilitated AcMNPV infection by inhibiting serine proteases, SP1, SP13, and SP105 which were all involved in the melanization response.

KPI5 Is Involved in the Regulation of the Expression of Antibacterial Peptide Genes and Hemolymph Melanization in the Silkworm, Bombyx mori

Kunitz-type protease inhibitors (KPIs) are ubiquitously found in many organisms, and participate in various physiological processes. However, their function in insects remains to be elucidated. In the present study, we characterized and functionally analyzed silkworm KPI5. Sequence analysis showed that KPI5 contains 85 amino acids with six conserved cysteine residues, and the P1 site is a phenylalanine residue. Inhibitory activity and stability analyses indicated that recombinant KPI5 protein significantly inhibited the activity of chymotrypsin and was highly tolerant to temperature and pH. The spatio-temporal expression profile analysis showed that KPI5 was synthesized in the fat body and secreted into the hemolymph. In vivo induction analysis showed that the expression of KPI5 in the fat body was significantly upregulated by pathogen-associated molecular patterns (PAMPs). Binding assays suggested that KPI5 can bind to pathogens and PAMPs. In vitro pathogen growth inhibition assay and encapsulation analysis indicated that KPI5 can neither kill pathogenic bacteria directly nor promote the encapsulation of agarose beads by silkworm hemocytes. Recombinant protein injection test and CRISPR/Cas9-mediated knockdown showed that KPI5 promotes the expression of antimicrobial peptides (AMPs) in the fat body. Moreover, the survival rate of individuals in the KPI5 knockdown group was significantly lower than that of the control group after pathogen infection. Phenoloxidase (PO) activity assays showed that KPI5 significantly inhibited the hemolymph PO activity and melanization induced by PAMPs. These findings suggested that KPI5 plays a dual regulatory role in innate immunity by promoting the expression of antimicrobial peptides in the fat body and inhibiting hemolymph melanization. Our study furthers the understanding of the function of insect KPIs and provides new insights into the regulatory mechanism of insect immune homeostasis.

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

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

Serine protease SP7 cleaves prophenoloxidase and is regulated by two serpins in Ostrinia furnacalis melanization

Melanization is an innate immune response in insects to defend against the invading pathogens and parasites. During melanization, prophenoloxidase (PPO) requires proteolytic activation by its upstream prophenoloxidase-activating protease (PAP). We here cloned a full-length cDNA for a serine protease, named as SP7, from Ostrinia furnacalis. The open reading frame of SP7 encodes 421-amino acid residue protein with a 19-residue signal peptide. qRT-PCR analysis showed that SP7 mRNA levels were significantly upregulated upon exposure to microbial infection. Recombinant SP7 zymogen was activated by serine protease SP2. The active SP7 could cleave O. furnacalis PPOs including PPO2, PPO1b and PPO3. Additionally, active SP7 could form covalent complexes with serine protease inhibitor serpin-3 and serpin-4. The activity of SP7 in cleaving a colorimetric substrate IEARpNA or O. furnacalis PPOs was efficiently blocked by either serpin-3 or serpin-4. Our work thus revealed that SP7 and SP2 partially constituted a PPO activation cascade in which SP7 was activated by SP2 and then likely worked as a PAP. SP7 was effectively regulated by serpin-3 and serpin-4. The results would allow further advances in the understanding of melanization mechanisms in O. furnacalis.

Characterization and functional analysis of a clip domain serine protease (MncSP) and its alternative transcript (MncSP-isoform) from Macrobrachium nipponense

Clip domain serine protease (cSPs) play an important role in the innate immune defense of crustaceans. In this study, a clip domain serine protease (MncSP) and its alternative transcript (MncSP-isoform) were identified from Macrobrachium nipponense. The full-length cDNA sequences of MncSP and MncSP-isoform were 2447 and 2351 bp with open reading frames comprising 1497 and 1401 bp nucleotides and encoding 498 and 466 amino acids, respectively. The genome of MncSP had 10 exons and 9 introns. MncSP contained all 10 exons, whereas MncSP-isoform lacked the second exon. MncSP and MncSP-isoform contained a signal peptide, a clip domain, and a Tryp_SPc domain. Phylogenetic tree analysis showed that MncSP and MncSP-isoform clustered with cSPs from Palaemonidae. MncSP and MncSP-isoform were widely distributed in hemocytes, heart, hepatopancreas, gills, stomach, and intestine. The expression profiles of MncSP and MncSP-isoform in the hemocytes of M. nipponense changed after simulation by Vibrio parahaemolyticus or Staphylococcus aureus. The RNAi of MncSP could inhibit the expression of antimicrobial peptides (AMPs), including crustins and anti-lipopolysaccharide factors. Phenoloxidase activity was also down-regulated in MncSP-silenced prawns. This study indicated that MncSP participated in the synthesis of AMPs and the activation of prophenoloxidase.

Toll signal pathway activating eicosanoid biosynthesis shares its conserved upstream recognition components in a lepidopteran Spodoptera exigua upon infection by Metarhizium rileyi, an entomopathogenic fungus

Eicosanoids play crucial roles in mediating immune responses in insects. Upon a fungal infection, Toll signal pathway can mediate immune responses of Spodoptera exigua, a lepidopteran insect, by activating eicosanoid biosynthesis. However, upstream signal components of the Toll signal pathway activating eicosanoid biosynthesis remain unclear. This study predicted pattern recognition receptors (PRRs) and serine proteases (SPs) as upstream components of the Toll pathway with reference to known signal components of Manduca sexta, another lepidopteran insect. S. exigua infected with Metarhizium rileyi, an entomopathogenic fungus, activated phospholipase A₂ (PLA₂) and phenoloxidase (PO) enzymes along with marked increases of expression levels of genes encoding three specific antimicrobial peptides, cecropin, gallerimycin, and hemolin. Among ten Toll receptors encoded in the genome of S. exigua, seven Toll genes were associated with immune responses against fungal infection by M. rileyi through individual RNA interference (RNAi) screening. In addition, two Spätzles (ligands of Toll receptor) were required for Toll signaling against the fungal infection. All predicted upstream components of the Toll pathway were inducible by the fungal infection. Individual RNAi screening showed that three PRRs (βGRP-1, βGRP-2, and GNBP3) and five SPs (ModSP, HP21, HP5, HP6, and HP8) were required for immune responses of S. exigua mediated by Toll signal pathway against the fungal infection. However, two PO-activating proteases (PAP1 and PAP3) were not required for PLA₂ activation, although they were required for PO activation. These results suggest that PRRs and SPs conserved as upstream components in Toll signal pathway play crucial roles in triggering eicosanoid biosynthesis of S. exigua to mediate various immune responses against fungal infection.

Immune mediation of HMG-like DSP1 via Toll-Spätzle pathway and its specific inhibition by salicylic acid analogs

Xenorhabdus hominickii, an entomopathogenic bacterium, inhibits eicosanoid biosynthesis of target insects to suppress their immune responses by inhibiting phospholipase A₂ (PLA₂) through binding to a damage-associated molecular pattern (DAMP) molecule called dorsal switch protein 1 (DSP1) from Spodoptera exigua, a lepidopteran insect. However, the signalling pathway between DSP1 and PLA₂ remains unknown. The objective of this study was to determine whether DSP1 could activate Toll immune signalling pathway to activate PLA₂ activation and whether X. hominickii metabolites could inhibit DSP1 to shutdown eicosanoid biosynthesis. Toll-Spätzle (Spz) signalling pathway includes two Spz (SeSpz1 and SeSpz2) and 10 Toll receptors (SeToll1-10) in S. exigua. Loss-of-function approach using RNA interference showed that SeSpz1 and SeToll9 played crucial roles in connecting DSP1 mediation to activate PLA₂. Furthermore, a deletion mutant against SeToll9 using CRISPR/Cas9 abolished DSP1 mediation and induced significant immunosuppression. Organic extracts of X. hominickii culture broth could bind to DSP1 at a low micromolar range. Subsequent sequential fractionations along with binding assays led to the identification of seven potent compounds including 3-ethoxy-4-methoxyphenol (EMP). EMP could bind to DSP1 and prevent its translocation to plasma in response to bacterial challenge and suppress the up-regulation of PLA₂ activity. These results suggest that X. hominickii inhibits DSP1 and prevents its DAMP role in activating Toll immune signalling pathway including PLA₂ activation, leading to significant immunosuppression of target insects.

Immune responses of insects are highly effective in defending various entomopathogens. Xenorhabdus hominickii is an entomopathogenic bacterium that uses a pathogenic strategy of suppressing host insect immunity by inhibiting phospholipase A₂ (PLA₂) which catalyzes the committed step for eicosanoid biosynthesis. Eicosanoids mediate both cellular and humoral immune responses in insects. This study discovers an upstream signalling pathway to activate PLA₂ in response to bacterial challenge. Se-DSP1 is an insect homolog of vertebrate HMGB1 that acts as a damage-associated molecular pattern. Upon bacterial infection, Se-DSP1 is released to the circulatory system to activate Spätzle, an insect cytokine that can bind to Toll receptor. Toll immune signalling pathway can activate antimicrobial peptide gene expression and PLA₂. A deletion mutant against a Toll gene abolished immune responses mediated by Se-DSP1. Indeed, X. hominickii can produce and secrete secondary metabolites including salicylic acid analogs that can strongly bind to Se-DSP1. These bacterial metabolites prevented the release of Se-DSP1, which impaired the activation of PLA₂ and resulted in a significant immunosuppression of target insects against bacterial infection.

Eicosanoid Signaling in Insect Immunology: New Genes and Unresolved Issues

This paper is focused on eicosanoid signaling in insect immunology. We begin with eicosanoid biosynthesis through the actions of phospholipase A₂, responsible for hydrolyzing the C18 polyunsaturated fatty acid, linoleic acid (18:2n-6), from cellular phospholipids, which is subsequently converted into arachidonic acid (AA; 20:4n-6) via elongases and desaturases. The synthesized AA is then oxygenated into one of three groups of eicosanoids, prostaglandins (PGs), epoxyeicosatrienoic acids (EETs) and lipoxygenase products. We mark the distinction between mammalian cyclooxygenases and insect peroxynectins, both of which convert AA into PGs. One PG, PGI₂ (also called prostacyclin), is newly discovered in insects, as a negative regulator of immune reactions and a positive signal in juvenile development. Two new elements of insect PG biology are a PG dehydrogenase and a PG reductase, both of which enact necessary PG catabolism. EETs, which are produced from AA via cytochrome P450s, also act in immune signaling, acting as pro-inflammatory signals. Eicosanoids signal a wide range of cellular immune reactions to infections, invasions and wounding, including nodulation, cell spreading, hemocyte migration and releasing prophenoloxidase from oenocytoids, a class of lepidopteran hemocytes. We briefly review the relatively scant knowledge on insect PG receptors and note PGs also act in gut immunity and in humoral immunity. Detailed new information on PG actions in mosquito immunity against the malarial agent, Plasmodium berghei, has recently emerged and we treat this exciting new work. The new findings on eicosanoid actions in insect immunity have emerged from a very broad range of research at the genetic, cellular and organismal levels, all taking place at the international level.

Rhodnius prolixus uses the peptidoglycan recognition receptor rpPGRP-LC/LA to detect Gram-negative bacteria and activate the IMD pathway

Insects rely on an innate immune system to recognize and eliminate pathogens. Key components of this system are highly conserved across all invertebrates. To detect pathogens, insects use Pattern recognition receptors (PRRs) that bind to signature motifs on the surface of pathogens called Pathogen Associated Molecular Patterns (PAMPs). In general, insects use peptidoglycan recognition proteins (PGRPs) in the Immune Deficiency (IMD) pathway to detect Gram-negative bacteria, and other PGRPs and Gram-negative binding proteins (GNBPs) in the Toll pathway to detect Gram-positive bacteria and fungi, although there is crosstalk and cooperation between these and other pathways. Once pathogens are recognized, these pathways activate the production of potent antimicrobial peptides (AMPs). Most PRRs in insects have been reported from genome sequencing initiatives but few have been characterized functionally. The initial studies on insect PRRs were done using established dipteran model organisms such as Drosophila melanogaster, but there are differences in the numbers and functional role of PRRs in different insects. Here we describe the genomic repertoire of PGRPs in Rhodnius prolixus, a hemimetabolous hemipteran vector of the parasite Trypanosoma cruzi that causes Chagas disease in humans. Using a de novo transcriptome from the fat body of immune activated insects, we found 5 genes encoding PGRPs. Phylogenetic analysis groups R. prolixus PGRPs with D. melanogaster PGRP-LA, which is involved in the IMD pathway in the respiratory tract. A single R. prolixus PGRP gene encodes isoforms that contain an intracellular region or motif (cryptic RIP Homotypic Interaction Motif-cRHIM) that is involved in the IMD signaling pathway in D. melanogaster. We characterized and silenced this gene using RNAi and show that the PGRPs that contain cRHIMs are involved in the recognition of Gram-negative bacteria, and activation of the IMD pathway in the fat body of R. prolixus, similar to the PGRP-LC of D. melanogaster. This is the first functional characterization of a PGRP containing a cRHIM motif that serves to activate the IMD pathway in a hemimetabolous insect.

Changes in composition and levels of hemolymph proteins during metamorphosis of Manduca sexta

The tobacco hornworm, Manduca sexta, is a lepidopteran model species widely used to study insect biochemical processes. Some of its larval hemolymph proteins are well studied, and a detailed proteomic analysis of larval plasma proteins became available in 2016, revealing features such as correlation with transcriptome data, formation of immune complexes, and constitution of an immune signaling system in hemolymph. It is unclear how the composition of these proteins may change in other developmental stages. In this paper, we report the proteomes of cell-free hemolymph from prepupae, pupae on day 4 and day 13, and young adults. Of the 1824 proteins identified, 907 have a signal peptide and 410 are related to immunity. Drastic changes in abundance of the storage proteins, lipophorins and vitellogenin, for instance, reflect physiological differences among prepupae, pupae, and adults. Considerably more proteins lacking signal peptide are present in the late pupae, suggesting that plasma contains relatively low concentrations of intracellular components released from remodeling tissues during metamorphosis. The defense proteins detected include 43 serine proteases and 11 serine protease homologs. Some of these proteins are members of the extracellular immune signaling network found in feeding larvae, and others may play additional roles and hence confer new features in the later life stages. In summary, the proteins and their levels revealed in this study, together with their transcriptome data, are expected to stimulate focused explorations of humoral immunity and other physiological systems in wandering larvae, pupae, and adults of M. sexta and shed light upon functional and comparative genomic research in other holometabolous insects.