A heterotrimeric death domain complex in Toll signaling

Functions of Drosophila Toll/NF-κB signaling in imaginal tissue homeostasis and cancer

The Toll/NF-κB pathway plays a central role in patterning the Drosophila embryo and in orchestrating the innate immune response against microbial infections. Both discoveries were associated with a Nobel Prize award and led to the recognition of the Toll-like receptor pathway in mammals, which has significant implications for diseases. Recent discoveries have revealed that the Toll/NF-κB pathway also maintains epithelial homeostasis of imaginal tissues during development: local Toll/NF-κB signaling activity monitors internal cellular fitness, and precancerous mutant cells can trigger systemic Toll/NF-κB pathway activation. However, this signaling can be exploited in diseases like cancer, in which Toll/NF-κB signaling is often co-opted or subverted. Various models have been proposed to explain how Toll/NF-κB signaling contributes to different types of cancer. Here we provide an overview of the functions of Toll/NF-κB signaling in imaginal tissue homeostasis with a focus on their misuse in pathological contexts, particularly their significance for tumor formation.

Anti-Tumor Effects of Cecropin A and Drosocin Incorporated into Macrophage-like Cells Against Hematopoietic Tumors in Drosophila mxc Mutants

Five major antimicrobial peptides (AMPs) in Drosophila are induced in multiple sex combs (mxc) mutant larvae harboring lymph gland (LG) tumors, and they exhibit anti-tumor effects. The effects of other well-known AMPs, Cecropin A and Drosocin, remain unexplored. We investigated the tumor-elimination mechanism of these AMPs. A half-dose reduction in either the Toll or Imd gene reduced the induction of these AMPs and enhanced tumor growth in mxcmbn1 mutant larvae, indicating that their anti-tumor effects depend on the innate immune pathway. Overexpression of these AMPs in the fat body suppressed tumor growth without affecting cell proliferation. Apoptosis was promoted in the mutant but not in normal LGs. Conversely, knockdown of them inhibited apoptosis and enhanced tumor growth; therefore, they inhibit LG tumor growth by inducing apoptosis. The AMPs from the fat body were incorporated into the hemocytes of mutant but not normal larvae. Another AMP, Drosomycin, was taken up via phagocytosis factors. Enhanced phosphatidylserine signals were observed on the tumor surface. Inhibition of the signals exposed on the cell surface enhanced tumor growth. AMPs may target phosphatidylserine in tumors to induce apoptosis and execute their tumor-specific effects. AMPs could be beneficial anti-cancer drugs with minimal side effects for clinical development.

Maintenance of persistent transmission of a plant arbovirus in its insect vector mediated by the Toll-Dorsal immune pathway

Throughout evolution, arboviruses have developed various strategies to counteract the host's innate immune defenses to maintain persistent transmission. Recent studies have shown that, in addition to bacteria and fungi, the innate Toll-Dorsal immune system also plays an essential role in preventing viral infections in invertebrates. However, whether the classical Toll immune pathway is involved in maintaining the homeostatic process to ensure the persistent and propagative transmission of arboviruses in insect vectors remain unclear. In this study, we revealed that the transcription factor Dorsal is actively involved in the antiviral defense of an insect vector (Laodelphax striatellus) by regulating the target gene, zinc finger protein 708 (LsZN708), which mediates downstream immune-related effectors against infection with the plant virus (Rice stripe virus, RSV). In contrast, an antidefense strategy involving the use of the nonstructural-protein (NS4) to antagonize host antiviral defense through competitive binding to Dorsal from the MSK2 kinase was employed by RSV; this competitive binding inhibited Dorsal phosphorylation and reduced the antiviral response of the host insect. Our study revealed the molecular mechanism through which Toll-Dorsal-ZN708 mediates the maintenance of an arbovirus homeostasis in insect vectors. Specifically, ZN708 is a newly documented zinc finger protein targeted by Dorsal that mediates the downstream antiviral response. This study will contribute to our understanding of the successful transmission and spread of arboviruses in plant or invertebrate hosts.

Immune Reactions of Vector Insects to Parasites and Pathogens

This overview initially describes insect immune reactions and then brings together present knowledge of the interactions of vector insects with their invading parasites and pathogens. It is a way of introducing this Special Issue with subsequent papers presenting the latest details of these interactions in each particular group of vectors. Hopefully, this paper will fill a void in the literature since brief descriptions of vector immunity have now been brought together in one publication and could form a starting point for those interested and new to this important area. Descriptions are given on the immune reactions of mosquitoes, blackflies, sandflies, tsetse flies, lice, fleas and triatomine bugs. Cellular and humoral defences are described separately but emphasis is made on the co-operation of these processes in the completed immune response. The paper also emphasises the need for great care in extracting haemocytes for subsequent study as appreciation of their fragile nature is often overlooked with the non-sterile media, smearing techniques and excessive centrifugation sometimes used. The potential vital role of eicosanoids in the instigation of many of the immune reactions described is also discussed. Finally, the priming of the immune system, mainly in mosquitoes, is considered and one possible mechanism is presented.

Regulating metabolism to shape immune function: Lessons from Drosophila

Infection with pathogenic microbes is a severe threat that hosts manage by activating the innate immune response. In Drosophila melanogaster, the Toll and Imd signaling pathways are activated by pathogen-associated molecular patterns to initiate cellular and humoral immune processes that neutralize and kill invaders. The Toll and Imd signaling pathways operate in organs such as fat body and gut that control host nutrient metabolism, and infections or genetic activation of Toll and Imd signaling also induce wide-ranging changes in host lipid, carbohydrate and protein metabolism. Metabolic regulation by immune signaling can confer resistance to or tolerance of infection, but it can also lead to pathology and susceptibility to infection. These immunometabolic phenotypes are described in this review, as are changes in endocrine signaling and gene regulation that mediate survival during infection. Future work in the field is anticipated to determine key variables such as sex, dietary nutrients, life stage, and pathogen characteristics that modify immunometabolic phenotypes and, importantly, to uncover the mechanisms used by the immune system to regulate metabolism.

Drosophila caspases as guardians of host-microbe interactions

An intact cell death machinery is not only crucial for successful embryonic development and tissue homeostasis, but participates also in the defence against pathogens and contributes to a balanced immune response. Centrally involved in the regulation of both cell death and inflammatory immune responses is the evolutionarily conserved family of cysteine proteases named caspases. The Drosophila melanogaster genome encodes for seven caspases, several of which display dual functions, participating in apoptotic signalling and beyond. Among the Drosophila caspases, the caspase-8 homologue Dredd has a well-characterised role in inflammatory signalling activated by bacterial infections, and functions as a driver of NF-κB-mediated immune responses. Regarding the other Drosophila caspases, studies focusing on tissue-specific immune signalling and host-microbe interactions have recently revealed immunoregulatory functions of the initiator caspase Dronc and the effector caspase Drice. The aim of this review is to give an overview of the signalling cascades involved in the Drosophila humoral innate immune response against pathogens and of their caspase-mediated regulation. Furthermore, the apoptotic role of caspases during antibacterial and antiviral immune activation will be discussed.

The role of micro RNAs (miRNAs) in the regulation of Drosophila melanogaster's innate immunity

MicroRNAs (miRNAs) are a class of small non-coding RNAs ~19-22 nt long which post-transcriptionally regulate gene expression. Their ability to exhibit dynamic expression patterns coupled with their wide variety of targets allows miRNAs to regulate many processes, including the innate immune response of Drosophila melanogaster. Recent studies have identified miRNAs in Drosophila which are differentially expressed during infection with different pathogens as well as miRNAs that may affect immune signalling when differentially expressed. This review provides an overview of miRNAswhich have been identified to play a role in the immune response of Drosophila through targeting of the Toll and IMD signalling pathways and other immune processes. It will also explore the role of miRNAs in fine-tuning the immune response in Drosophila and highlight current gaps in knowledge regarding the role of miRNAs in immunity and areas for further research.

Drosophila hemocytes recognize lymph gland tumors of mxc mutants and activate the innate immune pathway in a reactive oxygen species-dependent manner

Mechanisms of cancer cell recognition and elimination by the innate immune system remains unclear. The immune signaling pathways are activated in the fat body to suppress the tumor growth in mxcmbn1 hematopoietic tumor mutants in Drosophila by inducing antimicrobial peptides (AMP). Here, we investigated the regulatory mechanism underlying the activation in the mutant. Firstly, we found that reactive oxygen species (ROS) accumulated in the hemocytes due to induction of dual oxidase and one of its activators. This was required for the AMP induction and the tumor growth suppression. Next, more hemocytes transplanted from normal larvae were associated with the mutant tumor than normal lymph glands (LGs). Matrix metalloproteinase 1 and 2 (MMP2) were highly expressed in the tumors. The basement membrane components in the tumors were reduced and ultimately lost inside. Depletion of the MMP2 rather than MMP1 resulted in a significantly reduced AMP expression in the mutant larvae. The hemocytes may recognize the disassembly of basement membrane in the tumors and activate the ROS production. Our findings highlight the mechanism via which macrophage-like hemocytes recognize tumor cells and subsequently convey the information to induce AMPs in the fat body. They contribute to uncover the role of innate immune system against cancer.

Drosophila as a Model Organism to Study Basic Mechanisms of Longevity

The spatio-temporal regulation of gene expression determines the fate and function of various cells and tissues and, as a consequence, the correct development and functioning of complex organisms. Certain mechanisms of gene activity regulation provide adequate cell responses to changes in environmental factors. Aside from gene expression disorders that lead to various pathologies, alterations of expression of particular genes were shown to significantly decrease or increase the lifespan in a wide range of organisms from yeast to human. Drosophila fruit fly is an ideal model system to explore mechanisms of longevity and aging due to low cost, easy handling and maintenance, large number of progeny per adult, short life cycle and lifespan, relatively low number of paralogous genes, high evolutionary conservation of epigenetic mechanisms and signalling pathways, and availability of a wide range of tools to modulate gene expression in vivo. Here, we focus on the organization of the evolutionarily conserved signaling pathways whose components significantly influence the aging process and on the interconnections of these pathways with gene expression regulation.

Dynamic Regulation of NF-κB Response in Innate Immunity: The Case of the IMD Pathway in Drosophila

Metazoans have developed strategies to protect themselves from pathogenic attack. These preserved mechanisms constitute the immune system, composed of innate and adaptive responses. Among the two kinds, the innate immune system involves the activation of a fast response. NF-κB signaling pathways are activated during infections and lead to the expression of timely-controlled immune response genes. However, activation of NF-κB pathways can be deleterious when uncontrolled. Their regulation is necessary to prevent the development of inflammatory diseases or cancers. The similarity of the NF-κB pathways mediating immune mechanisms in insects and mammals makes Drosophila melanogaster a suitable model for studying the innate immune response and learning general mechanisms that are also relevant for humans. In this review, we summarize what is known about the dynamic regulation of the central NF-κB-pathways and go into detail on the molecular level of the IMD pathway. We report on the role of the nuclear protein Akirin in the regulation of the NF-κB Relish immune response. The use of the Drosophila model allows the understanding of the fine-tuned regulation of this central NF-κB pathway.

Drosophila Innate Immunity Involves Multiple Signaling Pathways and Coordinated Communication Between Different Tissues

The innate immune response provides the first line of defense against invading pathogens, and immune disorders cause a variety of diseases. The fruit fly Drosophila melanogaster employs multiple innate immune reactions to resist infection. First, epithelial tissues function as physical barriers to prevent pathogen invasion. In addition, macrophage-like plasmatocytes eliminate intruders through phagocytosis, and lamellocytes encapsulate large particles, such as wasp eggs, that cannot be phagocytosed. Regarding humoral immune responses, the fat body, equivalent to the mammalian liver, secretes antimicrobial peptides into hemolymph, killing bacteria and fungi. Drosophila has been shown to be a powerful in vivo model for studying the mechanism of innate immunity and host-pathogen interactions because Drosophila and higher organisms share conserved signaling pathways and factors. Moreover, the ease with which Drosophila genetic and physiological characteristics can be manipulated prevents interference by adaptive immunity. In this review, we discuss the signaling pathways activated in Drosophila innate immunity, namely, the Toll, Imd, JNK, JAK/STAT pathways, and other factors, as well as relevant regulatory networks. We also review the mechanisms by which different tissues, including hemocytes, the fat body, the lymph gland, muscles, the gut and the brain coordinate innate immune responses. Furthermore, the latest studies in this field are outlined in this review. In summary, understanding the mechanism underlying innate immunity orchestration in Drosophila will help us better study human innate immunity-related diseases.

Cell mechanics and cell-cell recognition controls by Toll-like receptors in tissue morphogenesis and homeostasis

Signal transduction by the Toll-like receptors (TLRs) is conserved and essential for innate immunity in metazoans. The founding member of the TLR family, Drosophila Toll-1, was initially identified for its role in dorsoventral axis formation in early embryogenesis. The Drosophila genome encodes nine TLRs that display dynamic expression patterns during development, suggesting their involvement in tissue morphogenesis and homeostasis. Recent progress on the developmental functions of TLRs beyond dorsoventral patterning has revealed not only their diverse functions in various biological processes, but also unprecedented molecular mechanisms in directly regulating cell mechanics and cell-cell recognition independent of the canonical signal transduction pathway involving transcriptional regulation of target genes. In this review, I feature and discuss the non-immune functions of TLRs in the control of epithelial tissue homeostasis, tissue morphogenesis, and cell-cell recognition between cell populations with different cell identities.

The antiviral role of NF-κB-mediated immune responses and their antagonism by viruses in insects

The antiviral role of innate immune responses mediated by the NF-κB family of transcription factors is well established in vertebrates but was for a long time less clear in insects. Insects encode two canonical NF-κB pathways, the Toll and Imd ('immunodeficiency') pathways, which are best characterised for their role in antibacterial and antifungal defence. An increasing body of evidence has also implicated NF-κB-mediated innate immunity in antiviral responses against some, but not all, viruses. Specific pattern recognition receptors (PRRs) and molecular events leading to NF-κB activation by viral pathogen-associated molecular patterns (PAMPs) have been elucidated for a number of viruses and insect species. Particularly interesting are recent findings indicating that the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway detects viral RNA to activate NF-κB-regulated gene expression. We summarise the literature on virus-NF-κB pathway interactions across the class Insecta, with a focus on the dipterans Drosophila melanogaster and Aedes aegypti. We discuss potential reasons for differences observed between different virus-host combinations, and highlight similarities and differences between cGAS-STING signalling in insects versus vertebrates. Finally, we summarise the increasing number of known molecular mechanisms by which viruses antagonise NF-κB responses, which suggest that NF-κB-mediated immunity exerts strong evolutionary pressures on viruses. These developments in our understanding of insect antiviral immunity have relevance to the large number of insect species that impact on humans through their transmission of human, livestock and plant diseases, exploitation as biotechnology platforms, and role as parasites, pollinators, livestock and pests.

cGMP signaling pathway that modulates NF-κB activation in innate immune responses

The nuclear factor-kappa B (NF-κB) pathway is an evolutionarily conserved signaling pathway that plays a central role in immune responses and inflammation. Here, we show that Drosophila NF-κB signaling is activated via a pathway in parallel with the Toll receptor by receptor-type guanylate cyclase, Gyc76C. Gyc76C produces cyclic guanosine monophosphate (cGMP) and modulates NF-κB signaling through the downstream Tollreceptor components dMyd88, Pelle, Tube, and Dif/Dorsal (NF-κB). The cGMP signaling pathway comprises a membrane-localized cGMP-dependent protein kinase (cGK) called DG2 and protein phosphatase 2A (PP2A) and is crucial for host survival against Gram-positive bacterial infections in Drosophila. A membrane-bound cGK, PRKG2, also modulates NF-κB activation via PP2A in human cells, indicating that modulation of NF-κB activation in innate immunity by the cGMP signaling pathway is evolutionarily conserved.

Activation of the Host Immune Response in Hyphantria cunea (Drury) (Lepidoptera: Noctuidae) Induced by Serratia marcescens Bizio

Simple Summary

Hyphantria cunea (Drury) is a quarantine pest, due to its extensive host, leading to serious economic losses in the agricultural and forestry industries. To control this pest, it is increasingly important to use microbial pesticides because they are biologically active and ecologically safe. Serratia marcescens Bizio (SM1) is a potential biocontrol bacterium. Although SM1 has a pathogenic role in H. cunea, H. cunea self-defense reduces the pathogenic effect of SM1. In this study, immune-related differentially expressed genes (DEGs) in H. cunea were first identified after SM1 infection, and the immune regulation mode of H. cunea in response to SM1, including antimicrobial peptide synthesis pathways, melanization and cellular immunity, was revealed. According to the analysis, the immune system of H. cunea was induced by SM1. In summary, our study demonstrates how the immune systems of the H. cunea work to resist the infection of SM1, which provides the theoretical basis for researching more efficient microbial pesticides for H. cunea.

Abstract

Host–pathogen interactions are essential to our understanding of biological pesticides. Hyphantria cunea (Drury) is an important forest pest worldwide. The immune mechanism of the interaction between H. cunea and Serratia marcescens Bizio (SM1) is unclear. First, transcriptome sequencing and quantitative real-time PCR (qRT-PCR) analysis described the H. cunea immune response to SM1. A total of 234 immune-related differentially expressed genes (DEGs) were found. Many immune regulatory genes in three classical pathways were found. Antimicrobial peptides, including attacin B, cecropin A, gloverin, lebocin and diapausin, are involved in defending against SM1 challenge, and are mainly produced by Toll and immune deficiency (IMD) pathways. Some melanization genes were changed in H. cunea, which suggested that H. cunea melanization was activated by SM1. Furthermore, phagocytosis, autophagolysosome and apoptosis pathways in cellular immunity were activated in H. cunea against SM1. Finally, the expression patterns of 10 immune genes were analyzed systematically by qRT-PCR, and most of the genes were upregulated compared to the control. Our studies provide useful information about the immune response of H. cunea under the stress of SM1, which is important to understand how SM1 affects the immune system of H. cunea and provides new ideas to control H. cunea by using SM1.

Verloren negatively regulates the expression of IMD pathway dependent antimicrobial peptides in Drosophila

Drosophila immune deficiency (IMD) pathway is similar to the human tumor necrosis factor receptor (TNFR) signaling pathway and is preferentially activated by Gram-negative bacterial infection. Recent studies highlighted the importance of IMD pathway regulation as it is tightly controlled by numbers of negative regulators at multiple levels. Here, we report a new negative regulator of the IMD pathway, Verloren (Velo). Silencing of Velo led to constitutive expression of the IMD pathway dependent antimicrobial peptides (AMPs), and Escherichia coli stimulation further enhanced the AMP expression. Epistatic analysis indicated that Velo knock-down mediated AMP upregulation is dependent on the canonical members of the IMD pathway. The immune fluorescent study using overexpression constructs revealed that Velo resides both in the nucleus and cytoplasm, but the majority (~ 75%) is localized in the nucleus. We also observed from in vivo analysis that Velo knock-down flies exhibit significant upregulation of the AMP expression and reduced bacterial load. Survival experiments showed that Velo knock-down flies have a short lifespan and are susceptible to the infection of pathogenic Gram-negative bacteria, P. aeruginosa. Taken together, these data suggest that Velo is an additional new negative regulator of the IMD pathway, possibly acting in both the nucleus and cytoplasm.

Evolution of Toll, Spatzle and MyD88 in insects: the problem of the Diptera bias

BACKGROUND

Arthropoda, the most numerous and diverse metazoan phylum, has species in many habitats where they encounter various microorganisms and, as a result, mechanisms for pathogen recognition and elimination have evolved. The Toll pathway, involved in the innate immune system, was first described as part of the developmental pathway for dorsal-ventral differentiation in Drosophila. Its later discovery in vertebrates suggested that this system was extremely conserved. However, there is variation in presence/absence, copy number and sequence divergence in various genes along the pathway. As most studies have only focused on Diptera, for a comprehensive and accurate homology-based approach it is important to understand gene function in a number of different species and, in a group as diverse as insects, the use of species belonging to different taxonomic groups is essential.

RESULTS

We evaluated the diversity of Toll pathway gene families in 39 Arthropod genomes, encompassing 13 different Insect Orders. Through computational methods, we shed some light into the evolution and functional annotation of protein families involved in the Toll pathway innate immune response. Our data indicates that: 1) intracellular proteins of the Toll pathway show mostly species-specific expansions; 2) the different Toll subfamilies seem to have distinct evolutionary backgrounds; 3) patterns of gene expansion observed in the Toll phylogenetic tree indicate that homology based methods of functional inference might not be accurate for some subfamilies; 4) Spatzle subfamilies are highly divergent and also pose a problem for homology based inference; 5) Spatzle subfamilies should not be analyzed together in the same phylogenetic framework; 6) network analyses seem to be a good first step in inferring functional groups in these cases. We specifically show that understanding Drosophila's Toll functions might not indicate the same function in other species.

CONCLUSIONS

Our results show the importance of using species representing the different orders to better understand insect gene content, origin and evolution. More specifically, in intracellular Toll pathway gene families the presence of orthologues has important implications for homology based functional inference. Also, the different evolutionary backgrounds of Toll gene subfamilies should be taken into consideration when functional studies are performed, especially for TOLL9, TOLL, TOLL2_7, and the new TOLL10 clade. The presence of Diptera specific clades or the ones lacking Diptera species show the importance of overcoming the Diptera bias when performing functional characterization of Toll pathways.

Antimicrobial Peptides from Black Soldier Fly (Hermetia illucens) as Potential Antimicrobial Factors Representing an Alternative to Antibiotics in Livestock Farming

Functional antimicrobial peptides (AMPs) are an important class of effector molecules of innate host immune defense against pathogen invasion. Inability of microorganisms to develop resistance against the majority of AMPs has made them alternatives to antibiotics, contributing to the development of a new generation of antimicrobials. Due to extensive biodiversity, insects are one of the most abundant sources of novel AMPs. Notably, black soldier fly insect (BSF; Hermetia illucens (Diptera: Stratiomyidae)) feeds on decaying substrates and displays a supernormal capacity to survive under adverse conditions in the presence of abundant microorganisms, therefore, BSF is one of the most promising sources for identification of AMPs. However, discovery, functional investigation, and drug development to replace antibiotics with AMPs from Hermetia illucens remain in a preliminary stage. In this review, we provide general information on currently verified AMPs of Hermetia illucens, describe their potential medical value, discuss the mechanism of their synthesis and interactions, and consider the development of bacterial resistance to AMPs in comparison with antibiotics, aiming to provide a candidate for substitution of antibiotics in livestock farming or, to some extent, for blocking the horizontal transfer of resistance genes in the environment, which is beneficial to human and animal welfare.

Genetic determinants of antiviral immunity in dipteran insects - Compiling the experimental evidence

The genetic basis of antiviral immunity in dipteran insects is extensively studied in Drosophila melanogaster and advanced technologies for genetic manipulation allow a better characterization of immune responses also in non-model insect species. Especially, immunity in vector mosquitoes is recently in the spotlight, due to the medical impact that these insects have by transmitting viruses and other pathogens. Here, we review the current state of experimental evidence that supports antiviral functions for immune genes acting in different cellular pathways. We discuss the well-characterized RNA interference mechanism along with the less well-defined JAK-STAT, Toll, and IMD signaling pathways. Furthermore, we highlight the initial evidence for antiviral activity observed for the autophagy pathway, transcriptional pausing, as well as piRNA production from endogenous viral elements. We focus our review on studies from Drosophila and mosquito species from the lineages Aedes, Culex, and Anopheles, which contain major vector species responsible for virus transmission.

Host Factors That Control Mosquito-Borne Viral Infections in Humans and Their Vector

Mosquito-borne viral infections are responsible for a significant degree of morbidity and mortality across the globe due to the severe diseases these infections cause, and they continue to increase each year. These viruses are dependent on the mosquito vector as the primary means of transmission to new vertebrate hosts including avian, livestock, and human populations. Due to the dynamic host environments that mosquito-borne viruses pass through as they are transmitted between vector and vertebrate hosts, there are various host factors that control the response to infection over the course of the pathogen's life cycle. In this review, we discuss these host factors that are present in either vector or vertebrate models during infection, how they vary or are conserved between hosts, and their implications in future research pertaining to disease prevention and treatment.

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.

Deltex positively regulates Toll signaling in a JNK independent manner in Drosophila

Toll pathway is the center for the function of immune system in both Drosophila and mammals. Toll pathway in Drosophila gets activated upon binding of the ligand Spätzle to the receptor, Toll, triggering a series of proteolytic cascade culminating into the activation of the NF-κB factors Dorsal and/or Dif (Dorsal-related immunity factor). Inappropriate activation of the Toll pathway is often associated with systemic inflammation phenotype in the absence of infection, and thus, it is important to understand the regulation of Toll signaling. Deltex (Dx) is a context-dependent regulator of Notch signaling and has been linked with cell-mediated immunity in the mammalian system lately. However, the unambiguous role of Dx in humoral and cell-mediated immunity is yet to be explored. Our study unravels the novel role of Dx in Toll pathway activation. Gain of function of dx in Drosophila larvae results in increased melanotic mass formation and increased lamellocyte production. Our results also reveal the nuclear accumulation of transcription factors Dorsal and Dif and expression of Toll-associated antimicrobial peptides (AMP) in Dx over-expression background. Further, we also tried to elucidate the role of Dx in JNK-independent Toll activation. Here we present Dx as a novel candidate in the regulation of Toll pathway.

Activation of Toll Immune Pathway in an Insect Vector Induced by a Plant Virus

The Toll pathway plays an important role in defense against infection of various pathogenic microorganisms, including viruses. However, current understanding of Toll pathway was mainly restricted in mammal and some model insects such as Drosophila and mosquitoes. Whether plant viruses can also activate the Toll signaling pathway in vector insects is still unknown. In this study, using rice stripe virus (RSV) and its insect vector (small brown planthopper, Laodelphax striatellus) as a model, we found that the Toll pathway was activated upon RSV infection. In comparison of viruliferous and non-viruliferous planthoppers, we found that four Toll pathway core genes (Toll, Tube, MyD88, and Dorsal) were upregulated in viruliferous planthoppers. When the planthoppers infected with RSV, the expressions of Toll and MyD88 were rapidly upregulated at the early stage (1 and 3 days post-infection), whereas Dorsal was upregulated at the late stage (9 days post-infection). Furthermore, induction of Toll pathway was initiated by interaction between a Toll receptor and RSV nucleocapsid protein (NP). Knockdown of Toll increased the proliferation of RSV in vector insect, and the dsToll-treated insects exhibited higher mortality than that of dsGFP-treated ones. Our results provide the first evidence that the Toll signaling pathway of an insect vector is potentially activated through the direct interaction between Toll receptor and a protein encoded by a plant virus, indicating that Toll immune pathway is an important strategy against plant virus infection in an insect vector.

Abnormal brain structure and behavior in MyD88-deficient mice

While the original protein Toll in Drosophila melanogaster regulates both host defense and morphogenesis, the role of its ortholog Toll-like receptors (TLRs), the interleukin 1 receptor (IL-1R) family, and the associated signaling pathways in mammalian brain development and structure is poorly understood. Because the adaptor protein myeloid differentiation primary response protein 88 (MyD88) is essential for downstream signaling of most TLRs and IL-1R, we systematically investigated the effect of MyD88 deficiency on murine brain structure during development and on behavior. In neonatal Myd88^-/- mice, neocortical thickness was reduced, while density of cortical neurons was increased. In contrast, microglia, astrocyte, oligodendrocyte, and proliferating cell numbers were unchanged in these mice compared to wild-type mice. In adult Myd88^-/- mice, neocortical thickness was unaltered, but neuronal density in neocortex and hippocampus was increased. Neuron arborization was less pronounced in adult Myd88^-/- mice compared to wild-type animals. In addition, numbers of microglia and proliferating cells were increased in the neocortex and subventricular zone, respectively, with unaltered astrocyte and oligodendrocyte numbers, and myelinization was enhanced in the adult Myd88^-/- neocortex. These morphologic changes in the brain of adult Myd88^-/- mice were accompanied by specific behavioral traits, such as decreased locomotor activity, increased anxiety-like behavior, but normal day/light activity, satisfactory learning, short- and long-term spatial memory, potential cognitive inflexibility, and increased hanging and locomotor behavior within their home cage. Taken together, MyD88 deficiency results in morphologic and cellular changes in the mouse brain, as well as in altered natural and specific behaviors. Our data indicate a pathophysiological significance of MyD88 for mammalian CNS development, structure, and function.

A Toll-Spätzle Pathway in the Immune Response of Bombyx mori

The Toll-Spätzle pathway is a crucial defense mechanism in insect innate immunity, it plays an important role in fighting against pathogens through the regulation of antimicrobial peptide gene expression. Although Toll and Spätzle (Spz) genes have been identified in Bombyx mori, little is known regarding the specific Spz and Toll genes members involved in innate immunity. There is also limited direct evidence of the interaction between Spz and Toll. In this study, the dual-luciferase reporter assay results showed that BmToll11 and BmToll9-1 could activate both drosomycin and diptericin promoters in S2 cells. Furthermore, BmToll11, BmToll9-1, and five BmSpzs genes were found to be significantly upregulated in B. mori infected by Escherichia coli and Staphylococcus aureus. Additionally, the yeast two-hybrid assay results confirmed that BmSpz2, but not other BmSpzs, could interact with both BmToll11 and BmToll9-1. These findings suggest that the activated BmSpz2 can bind with BmToll11 and BmToll9-1 to induce the expression of AMPs after the silkworm is infected by pathogens.

Molecular and Functional Analysis of Pore-Forming Toxin Monalysin From Entomopathogenic Bacterium Pseudomonas entomophila

Pseudomonas entomophila is a highly pathogenic bacterium that infects insects. It is also used as a suitable model pathogen to analyze Drosophila's innate immunity. P. entomophila's virulence is largely derived from Monalysin, a β-barrel pore-forming toxin that damages Drosophila tissues, inducing necrotic cell death. Here we report the first and efficient purification of endogenous Monalysin and its characterization. Monalysin is successfully purified as a pro-form, and trypsin treatment results in a cleaved mature form of purified Monalysin which kills Drosophila cell lines and adult flies. Electrophysiological measurement of Monalysin in a lipid membrane with an on-chip device confirms that Monalysin forms a pore, in a cleavage-dependent manner. This analysis also provides a pore-size estimate of Monalysin using current amplitude for a single pore and suggests lipid preferences for the insertion. Atomic Force Microscope (AFM) analysis displays its structure in a solution and shows that active-Monalysin is stable and composed of an 8-mer complex; this observation is consistent with mass spectrometry data. AFM analysis also shows the 8-mer structure of active-Monalysin in a lipid bilayer, and real-time imaging demonstrates the moment at which Monalysin is inserted into the lipid membrane. These results collectively suggest that endogenous Monalysin is indeed a pore-forming toxin composed of a rigid structure before pore formation in the lipid membrane. The endogenous Monalysin characterized in this study could be a desirable tool for analyzing host defense mechanisms against entomopathogenic bacteria producing damage-inducing toxins.

Functional Variation of IL-1R-Associated Kinases in the Conserved MyD88-TRAF6 Pathway during Evolution

IL-1R-associated kinases (IRAK) are important regulators in the TLR/IL-1R pathways, but their function appears inconsistent between Drosophila, bony fishes, and vertebrates. This causes a difficulty to understand the IRAK functions. As a step to reveal the evolution of IRAKs, in this study, we performed comparative and functional analysis of IRAKs by exploiting the amphioxus, a pivotal taxon connecting invertebrates and vertebrates. Sequence and phylogenetic analysis indicated three major IRAK lineages: IRAK1/2/3 is a vertebrate-specific lineage, IRAK4 is an ancient lineage conserved between invertebrate and vertebrates, and Pelle is another ancient lineage that is preserved in protostomes and invertebrate deuterostomes but lost in vertebrate deuterostomes. Pelle is closer neither to IRAK4 nor to IRAK1/2/3, hence suggesting no clear functional analogs to IRAK1/2/3 in nonvertebrates. Functional analysis showed that both amphioxus IRAK4 and Pelle could suppress NF-κB activation induced by MyD88 and TRAF6, which are unlike mammalian and Drosophila IRAKs, but, surprisingly, similar to bony fish IRAK4. Also unlike Drosophila IRAKs, no interaction was detected between amphioxus IRAK4 and Pelle, although both of them were shown capable of binding MyD88. These findings, together with previous reports, show that unlike other signal transducers in the TLR/IL-1R pathways, such as MyD88 and TRAF6, the functions of IRAKs are highly variable during evolution and very specialized in different major animal taxa. Indeed, we suggest that the functional variability of IRAKs might confer plasticity to the signal transduction of the TLR/IL-1R pathways, which in return helps the species to evolve against the pathogens.

Antimicrobial peptides fromBombyx mori: a splendid immune defense response in silkworms

Bombyx mori L., a primary producer of silk, is the main tool in the sericulture industry and provides the means of livelihood to a large number of people. Silk cocoon crop losses due to bacterial infection pose a major threat to the sericulture industry. Bombyx mori L., a silkworm of the mulberry type, has a sophisticated inherent innate immune mechanism to combat such invasive pathogens. Among all the components in this defense system, antimicrobial peptides (AMPs) are notable due to their specificity towards the invading pathogens without harming the normal host cells. Bombyx mori L. so far has had AMPs identified that belong to six different families, namely cecropin, defensin, moricin, gloverin, attacin and lebocin, which are produced by the Toll and immune deficiency (IMD) pathways. Their diverse modes of action depend on microbial pathogens and are still under investigation. This review examines the recent progress in understanding the immune defense mechanism of Bombyx mori based on AMPs.

AMPs produced by B. mori induced by microbial challenge in the fat body.

Innate immunity in the simplest animals - placozoans

BACKGROUND

Innate immunity provides the core recognition system in animals for preventing infection, but also plays an important role in managing the relationship between an animal host and its symbiont. Most of our knowledge about innate immunity stems from a few animal model systems, but substantial variation between metazoan phyla has been revealed by comparative genomic studies. The exploration of more taxa is still needed to better understand the evolution of immunity related mechanisms. Placozoans are morphologically the simplest organized metazoans and the association between these enigmatic animals and their rickettsial endosymbionts has recently been elucidated. Our analyses of the novel placozoan nuclear genome of Trichoplax sp. H2 and its associated rickettsial endosymbiont genome clearly pointed to a mutualistic and co-evolutionary relationship. This discovery raises the question of how the placozoan holobiont manages symbiosis and, conversely, how it defends against harmful microorganisms. In this study, we examined the annotated genome of Trichoplax sp. H2 for the presence of genes involved in innate immune recognition and downstream signaling.

RESULTS

A rich repertoire of genes belonging to the Toll-like and NOD-like receptor pathways, to scavenger receptors and to secreted fibrinogen-related domain genes was identified in the genome of Trichoplax sp. H2. Nevertheless, the innate immunity related pathways in placozoans deviate in several instances from well investigated vertebrates and invertebrates. While true Toll- and NOD-like receptors are absent, the presence of many genes of the downstream signaling cascade suggests at least primordial Toll-like receptor signaling in Placozoa. An abundance of scavenger receptors, fibrinogen-related domain genes and Apaf-1 genes clearly constitutes an expansion of the immunity related gene repertoire specific to Placozoa.

CONCLUSIONS

The found wealth of immunity related genes present in Placozoa is surprising and quite striking in light of the extremely simple placozoan body plan and their sparse cell type makeup. Research is warranted to reveal how Placozoa utilize this immune repertoire to manage and maintain their associated microbiota as well as to fend-off pathogens.

The Toll pathway inhibits tissue growth and regulates cell fitness in an infection-dependent manner

The Toll pathway regulates the cellular response to infection via the transcriptional upregulation of antimicrobial peptides. In Drosophila, apart from its role in innate immunity, this pathway has also been reported to be important for the elimination of loser cells in a process referred to as cell competition, which can be locally triggered by secreted factors released from winner cells. In this work, we provide evidence that the inhibition of Toll signaling not only increases the fitness of loser cells, but also bestows a clonal growth advantage on wild-type cells. We further demonstrate that this growth advantage depends on basal infection levels since it is no longer present under axenic conditions but exacerbated upon intense pathogen exposure. Thus, the Toll pathway functions as a fine-tuned pro-apoptotic and anti-proliferative regulator, underlining the existence of a trade-off between innate immunity and growth during development.

Back to homeostasis: Negative regulation of NF-κB immune signaling in insects

Maintenance of homeostasis requires prompt activation and down-regulation of immune signaling pathways. This review attempts to summarize our current knowledge regarding the negative regulation of two NF-κB signaling pathways in insects, Toll and IMD pathway, which are mostly essential for host defense against bacteria and fungus. Various types of negative regulators and their mechanisms are discussed here with the emphasis on the prominent roles of ubiquitination. The counterbalance between these two pathways, the crosstalk with other physiological pathways, and the difference in their repertoires of negative regulators are also discussed.

The Fly Way of Antiviral Resistance and Disease Tolerance

Like humans, insects face the threat of viral infection. Despite having repercussions on human health and disease, knowledge gaps exist for how insects cope with viral pathogens. Drosophila melanogaster serves as an ideal insect model due to its genetic tractability. When encountering a pathogen, two major approaches to fight disease are resistance strategies and tolerance strategies. Disease resistance strategies promote the health of the infected host by reducing pathogen load. Multiple disease resistance mechanisms have been identified in Drosophila: RNA interference, Jak/STAT signaling, Toll signaling, IMD signaling, and autophagy. Disease tolerance mechanisms, in contrast, do not reduce pathogen load directly, but rather mitigate the stress and damage incurred by infection. The main benefit of tolerance mechanisms may therefore be to provide the host with time to engage antiviral resistance mechanisms that eliminate the threat. In this review, antiviral resistance mechanisms used by Drosophila will be described and compared to mammalian antiviral mechanisms. Disease tolerance will then be explained in a broader context as this is a burgeoning field of study.

Toll immune signal activates cellular immune response via eicosanoids

Upon immune challenge, insects recognize nonself. The recognition signal will propagate to nearby immune effectors. It is well-known that Toll signal pathway induces antimicrobial peptide (AMP) gene expression. Eicosanoids play crucial roles in mediating the recognition signal to immune effectors by enhancing humoral immune response through activation of AMP synthesis as well as cellular immune responses, suggesting a functional cross-talk between Toll and eicosanoid signals. This study tested a cross-talk between these two signals. Two signal transducing factors (MyD88 and Pelle) of Toll immune pathway were identified in Spodoptera exigua. RNA interference (RNAi) of either SeMyD88 or SePelle expression interfered with the expression of AMP genes under Toll signal pathway. Bacterial challenge induced PLA₂ enzyme activity. However, RNAi of these two immune factors significantly suppressed the induction of PLA₂ enzyme activity. Furthermore, RNAi treatment prevented gene expression of cellular PLA₂. Inhibition of PLA₂ activity reduced phenoloxidase activity and subsequent suppression in cellular immune response measured by hemocyte nodule formation. However, immunosuppression induced by RNAi of Toll signal molecules was significantly reversed by addition of arachidonic acid (AA), a catalytic product of PLA₂. The addition also significantly reduced the enhanced fungal susceptibility of S. exigua treated by RNAi against two Toll signal molecules. These results indicate that there is a cross-talk between Toll and eicosanoid signals in insect immunity.

Toll and Toll-like receptor signalling in development

The membrane receptor Toll and the related Toll-like receptors (TLRs) are best known for their universal function in innate immunity. However, Toll/TLRs were initially discovered in a developmental context, and recent studies have revealed that Toll/TLRs carry out previously unanticipated functions in development, regulating cell fate, cell number, neural circuit connectivity and synaptogenesis. Furthermore, knowledge of their molecular mechanisms of action is expanding and has highlighted that Toll/TLRs function beyond the canonical NF-κB pathway to regulate cell-to-cell communication and signalling at the synapse. Here, we provide an overview of Toll/TLR signalling and discuss how this signalling pathway regulates various aspects of development across species.

Comparative genomic analysis of innate immunity reveals novel and conserved components in crustacean food crop species

BACKGROUND

Growing global demands for crustacean food crop species have driven large investments in aquaculture research worldwide. However, large-scale production is susceptible to pathogen-mediated destruction particularly in developing economies. Thus, a thorough understanding of the immune system components of food crop species is imperative for research to combat pathogens.

RESULTS

Through a comparative genomics approach utilising extant data from 55 species, we describe the innate immune system of the class Malacostraca, which includes all food crop species. We identify 7407 malacostracan genes from 39 gene families implicated in different aspects of host defence and demonstrate dynamic evolution of innate immunity components within this group. Malacostracans have achieved flexibility in recognising infectious agents through divergent evolution and expansion of pathogen recognition receptors genes. Antiviral RNAi, Toll and JAK-STAT signal transduction pathways have remained conserved within Malacostraca, although the Imd pathway appears to lack several key components. Immune effectors such as the antimicrobial peptides (AMPs) have unique evolutionary profiles, with many malacostracan AMPs not found in other arthropods. Lastly, we describe four putative novel immune gene families, potentially representing important evolutionary novelties of the malacostracan immune system.

CONCLUSION

Our analyses across the broader Malacostraca have allowed us to not only draw analogies with other arthropods but also to identify evolutionary novelties in immune modulation components and form strong hypotheses as to when key pathways have evolved or diverged. This will serve as a key resource for future immunology research in crustacean food crops.

Annotation of the Asian Citrus Psyllid Genome Reveals a Reduced Innate Immune System

Citrus production worldwide is currently facing significant losses due to citrus greening disease, also known as Huanglongbing. The citrus greening bacteria, Candidatus Liberibacter asiaticus (CLas), is a persistent propagative pathogen transmitted by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae). Hemipterans characterized to date lack a number of insect immune genes, including those associated with the Imd pathway targeting Gram-negative bacteria. The D. citri draft genome was used to characterize the immune defense genes present in D. citri. Predicted mRNAs identified by screening the published D. citri annotated draft genome were manually searched using a custom database of immune genes from previously annotated insect genomes. Toll and JAK/STAT pathways, general defense genes Dual oxidase, Nitric oxide synthase, prophenoloxidase, and cellular immune defense genes were present in D. citri. In contrast, D. citri lacked genes for the Imd pathway, most antimicrobial peptides, 1,3-β-glucan recognition proteins (GNBPs), and complete peptidoglycan recognition proteins. These data suggest that D. citri has a reduced immune capability similar to that observed in A. pisum, P. humanus, and R. prolixus. The absence of immune system genes from the D. citri genome may facilitate CLas infections, and is possibly compensated for by their relationship with their microbial endosymbionts.

Evolution and integration of innate immune systems from fruit flies to man: lessons and questions

Despite broad differences in morphology, ecology and behavior, the fruit fly Drosophila melanogaster and humans show a remarkably high degree of conservation for many molecular, cellular, and developmental aspects of their biology. During the last decade, similarities have also been discovered in some of the mechanisms regulating their innate immune system. These parallels regard mainly the Toll-like receptor family and the intracellular signaling pathways involved in the control of the immune response. However, if the overall similarities are important, the detailed pathogen recognition mechanisms differ significantly between fly and humans, highlighting a complicated evolutionary history of the metazoan innate defenses. In this review, we will discuss the main similarities and differences between the two types of organisms. We hope that this current knowledge will be used as a starting point for a more comprehensive view of innate immunity within the broad variety of metazoan phyla.

The Friend of GATA Transcriptional Co-Regulator, U-Shaped, Is a Downstream Antagonist of Dorsal-Driven Prohemocyte Differentiation in Drosophila

Recent studies suggest that mammalian hematopoietic stem and progenitor cells (HSPCs) respond directly to infection and inflammatory signaling. These signaling pathways also regulate HSPCs during steady-state conditions (absence of infection), and dysregulation may lead to cancer or age-related loss of progenitor repopulation capacity. Toll-like receptors (TLRs) are a major class of pathogen recognition receptors, and are expressed on the surface of immune effector cells and HSPCs. TLR/NF-κB activation promotes HSPCs differentiation; however, the mechanisms by which this signaling pathway alters the intrinsic transcriptional landscape are not well understood. Although Drosophila prohemocytes are the functional equivalent of mammalian HSPCs, a prohemocyte-specific function for Toll signaling has not been reported. Using Drosophila transgenics, we identified prohemocyte-specific roles for Toll pathway members, Dorsal and Cactus. We showed that Dorsal is required to limit the size of the progenitor pool. Additionally, we showed that activation of Toll signaling in prohemocytes drives differentiation in a manner that is analogous to TLR/NF-κB-driven HSPC differentiation. This was accomplished by showing that over-expression of Dorsal, or knockdown of Cactus, promotes differentiation. We also investigated whether Dorsal and Cactus control prohemocyte differentiation by regulating a key intrinsic prohemocyte factor, U-shaped (Ush), which is known to promote multipotency and block differentiation. We showed that Dorsal repressed Ush expression levels to promote differentiation, whereas Cactus maintained Ush levels to block differentiation. Additionally, we showed that another Toll antagonist, Lesswright, also maintained the level of Ush to block differentiation and promote proliferative quiescence. Collectively, these results identify a novel role for Ush as a downstream target of Toll signaling.

Toll pathway modulates TNF-induced JNK-dependent cell death in Drosophila

Signalling networks that control the life or death of a cell are of central interest in modern biology. While the defined roles of the c-Jun N-terminal kinase (JNK) pathway in regulating cell death have been well-established, additional factors that modulate JNK-mediated cell death have yet to be fully elucidated. To identify novel regulators of JNK-dependent cell death, we performed a dominant-modifier screen in Drosophila and found that the Toll pathway participates in JNK-mediated cell death. Loss of Toll signalling suppresses ectopically and physiologically activated JNK signalling-induced cell death. Our epistasis analysis suggests that the Toll pathway acts as a downstream modulator for JNK-dependent cell death. In addition, gain of JNK signalling results in Toll pathway activation, revealed by stimulated transcription of Drosomycin (Drs) and increased cytoplasm-to-nucleus translocation of Dorsal. Furthermore, the Spätzle (Spz) family ligands for the Toll receptor are transcriptionally upregulated by activated JNK signalling in a non-cell-autonomous manner, providing a molecular mechanism for JNK-induced Toll pathway activation. Finally, gain of Toll signalling exacerbates JNK-mediated cell death and promotes cell death independent of caspases. Thus, we have identified another important function for the evolutionarily conserved Toll pathway, in addition to its well-studied roles in embryonic dorso-ventral patterning and innate immunity.

Cytokines in Drosophila immunity

Cytokines are a large and diverse group of small proteins that can affect many biological processes, but most commonly cytokines are known as mediators of the immune response. In the event of an infection, cytokines are produced in response to an immune stimulus, and they function as key regulators of the immune response. Cytokines come in many shapes and sizes, and although they vary greatly in structure, their functions have been well conserved in evolution. The immune signaling pathways that respond to cytokines are remarkably conserved from fly to man. Therefore, Drosophila melanogaster, provides an excellent platform for studying the biology and function of cytokines. In this review, we will describe the cytokines and cytokine-like molecules found in the fly and discuss their roles in host immunity.

Ex vivo genome-wide RNAi screening of the Drosophila Toll signaling pathway elicited by a larva-derived tissue extract

Damage-associated molecular patterns (DAMPs), so-called "danger signals," play important roles in host defense and pathophysiology in mammals and insects. In Drosophila, the Toll pathway confers damage responses during bacterial infection and improper cell-fate control. However, the intrinsic ligands and signaling mechanisms that potentiate innate immune responses remain unknown. Here, we demonstrate that a Drosophila larva-derived tissue extract strongly elicits Toll pathway activation via the Toll receptor. Using this extract, we performed ex vivo genome-wide RNAi screening in Drosophila cultured cells, and identified several signaling factors that are required for host defense and antimicrobial-peptide expression in Drosophila adults. These results suggest that our larva-derived tissue extract contains active ingredients that mediate Toll pathway activation, and the screening data will shed light on the mechanisms of damage-related Toll pathway signaling in Drosophila.

Transcriptomic insight into the immune defenses in the ghost moth, Hepialus xiaojinensis, during an Ophiocordyceps sinensis fungal infection

Hepialus xiaojinensis is an economically important species of Lepidopteran insect. The fungus Ophiocordyceps sinensis can infect its larvae, which leads to mummification after 5-12 months, providing a valuable system with which to study interactions between the insect hosts and pathogenic fungi. However, little sequence information is available for this insect. A time-course analysis of the fat body transcriptome was performed to explore the host immune response to O. sinensis infection. In total, 50,164 unigenes were obtained by assembling the reads from two high-throughput approaches: 454 pyrosequencing and Illumina Hiseq2000. Hierarchical clustering and functional examination revealed four major gene clusters. Clusters 1-3 included transcripts markedly induced by the fungal infection within 72 h. Cluster 4, with a lower number of transcripts, was suppressed during the early phase of infection but returned to normal expression levels sometime before 1 year. Based on sequence similarity to orthologs known to participate in immune defenses, 258 candidate immunity-related transcripts were identified, and their functions were hypothesized. The genes were more primitive than those in other Lepidopteran insects. In addition, lineage-specific family expansion of the clip-domain serine proteases and C-type lectins were apparent and likely caused by selection pressures. Global expression profiles of immunity-related genes indicated that H. xiaojinensis was capable of a rapid response to an O. sinensis challenge; however, the larvae developed tolerance to the fungus after prolonged infection, probably due to immune suppression. Specifically, antimicrobial peptide mRNAs could not be detected after chronic infection, because key components of the Toll pathway (MyD88, Pelle and Cactus) were downregulated. Taken together, this study provides insights into the defense system of H. xiaojinensis, and a basis for understanding the molecular aspects of the interaction between the host and the entomopathogen.

Identification, characterization, and functional studies of a Pelle gene in the Chinese mitten crab, Eriocheir sinensis

The toll-like receptor/NF-κB signaling pathways play an important role in the innate immune system. In the present study, one Pelle gene (named EsPelle) was identified for the first time from the Chinese mitten crab Eriocheir sinensis. The full-length cDNA of EsPelle is 3797 bp with a 3156 bp-long open reading frame that encodes a 1051 amino acid protein. EsPelle protein contains a death domain at the N-terminal and a serine/threonine kinase domain at the C-terminal. A neighbor joining phylogenetic tree showed that the EsPelle protein, which is closest to those of Scylla paramamosain Pelle and Litopenaeus vannamei Pelle, was clustered to a group of crustacean Pelle proteins. EsPelle was expressed in all tested tissues of normal crabs, and its expression was regulated in hemocytes and hepatopancreas of crabs challenged with lipopolysaccharide, peptidoglycan, Staphyloccocus aureus, Vibrio parahaemolyticus, and Aeromonas hydrophila. Overexpression of EsPelle in Drosophila Schneider 2 cells could upregulate the expression of Drosophila antimicrobial peptides, namely, metchnikowin (Mtk), attacinA (Atta), drosomycin (Drs), and cecropinA (CecA). Moreover, EsPelle silencing by siRNA reduced the transcription of anti-lipopolysaccharide factor 1 and 2, crustin 2, and lysozyme in crabs challenged with V. parahaemolyticus. From the results, we speculated that EsPelle was involved in innate immune defense against V. parahaemolyticus in E. sinensis.

Hm-MyD88 and Hm-SARM: two key regulators of the neuroimmune system and neural repair in the medicinal leech

Unlike mammals, the CNS of the medicinal leech can regenerate damaged neurites, thus restoring neural functions after lesion. We previously demonstrated that the injured leech nerve cord is able to mount an immune response promoting the regenerative processes. Indeed neurons and microglia express sensing receptors like Hm-TLR1, a leech TLR ortholog, associated with chemokine release in response to a septic challenge or lesion. To gain insights into the TLR signaling pathways involved during these neuroimmune responses, members of the MyD88 family were investigated. In the present study, we report the characterization of Hm-MyD88 and Hm-SARM. The expression of their encoding gene was strongly regulated in leech CNS not only upon immune challenge but also during CNS repair, suggesting their involvement in both processes. This work also showed for the first time that differentiated neurons of the CNS could respond to LPS through a MyD88-dependent signalling pathway, while in mammals, studies describing the direct effect of LPS on neurons and the outcomes of such treatment are scarce and controversial. In the present study, we established that this PAMP induced the relocalization of Hm-MyD88 in isolated neurons.

Cytoplasmic actin is an extracellular insect immune factor which is secreted upon immune challenge and mediates phagocytosis and direct killing of bacteria, and is a Plasmodium Antagonist

Actin is a highly versatile, abundant, and conserved protein, with functions in a variety of intracellular processes. Here, we describe a novel role for insect cytoplasmic actin as an extracellular pathogen recognition factor that mediates antibacterial defense. Insect actins are secreted from cells upon immune challenge through an exosome-independent pathway. Anopheles gambiae actin interacts with the extracellular MD2-like immune factor AgMDL1, and binds to the surfaces of bacteria, mediating their phagocytosis and direct killing. Globular and filamentous actins display distinct functions as extracellular immune factors, and mosquito actin is a Plasmodium infection antagonist.

Actin is one of the best studied, evolutionary conserved and most abundant intracellular proteins. Actin can exists in globular and filamentous functionally distinct forms, and is involved in a variety of biological processes, such as muscle contraction, cell motility, cell division, vesicle and organelle movement, endocytosis, and cell signaling. Here we show a novel function of insect cytoplasmic actin, as an extracellular immune factor. Actin is externalized by insect immune competent cells upon immune challenge with bacteria or bacterial surface components, and once externalized, actin binds with high affinity to the surface of bacteria. A functional role of actin’s interaction with bacteria is to mediate their killing through either phagocytosis or direct antibacterial action. The globular and filamentous forms of actins appear to play distinct functions as extracellular immune factors. Actin also plays a role as a Plasmodium antagonist as it limits parasite infection of the mosquito gut tissue.

Novel myeloid differentiation factor 88, EsMyD88, exhibits EsTube-binding activity in Chinese mitten crab Eriocheir sinensis

Myeloid differentiation factor 88 (MyD88) is a universal and essential adapter protein that participates in the activation of the Toll-like receptor/interleukin-1 receptor-mediated signaling pathway. In the present study, a new MyD88 gene (named EsMyD88) was identified in the Chinese mitten crab Eriocheir sinensis. The cDNA of EsMyD88 was 2210 bp long with a 1416 bp open reading frame that encoded a protein with 472 amino acids. Predicted EsMyD88 protein had a death domain at the N-terminal and a TIR domain at the C-terminal. BLASTP and phylogenetic analysis results showed that EsMyD88 was clustered in one group together with other crustaceans MyD88 (SpMyD88, FcMyD88, LvMyD88, and LvMyD88-1). EsMyD88 was detected in all the examined tissues of healthy crabs, and was mainly expressed in the hemocytes and nerves. When normal crabs were challenged with lipopolysaccharide, peptidoglycan, Staphylococcus aureus, Vibrio parahaemolyticus, or Aeromonas hydrophila, the expression levels of EsMyD88 significantly increased either in the hepatopancreas or hemocytes. Results of the pull-down assay showed that EsMyD88 could bind to downstream cytosolic adaptor EsTube. Overexpression of EsMyD88 protein in Drosophila Schneider 2 cells led to the activation of antimicrobial peptide genes. RNA interference assay showed that EsMyD88 is involved in regulating the transcription of ALF1 and ALF2, Cru1 and Cru2, and Lys in crab challenged with V. parahaemolyticus. All the results mentioned earlier indicated that EsMyD88 gene has a key function in antibacterial innate immune defense.

Maternal control of the Drosophila dorsal-ventral body axis

The pathway that generates the dorsal-ventral (DV) axis of the Drosophila embryo has been the subject of intense investigation over the previous three decades. The initial asymmetric signal originates during oogenesis by the movement of the oocyte nucleus to an anterior corner of the oocyte, which establishes DV polarity within the follicle through signaling between Gurken, the Drosophila Transforming Growth Factor (TGF)-α homologue secreted from the oocyte, and the Drosophila Epidermal Growth Factor Receptor (EGFR) that is expressed by the follicular epithelium cells that envelop the oocyte. Follicle cells that are not exposed to Gurken follow a ventral fate and express Pipe, a sulfotransferase that enzymatically modifies components of the inner vitelline membrane layer of the eggshell, thereby transferring DV spatial information from the follicle to the egg. These ventrally sulfated eggshell proteins comprise a localized cue that directs the ventrally restricted formation of the active Spätzle ligand within the perivitelline space between the eggshell and the embryonic membrane. Spätzle activates Toll, a transmembrane receptor in the embryonic membrane. Transmission of the Toll signal into the embryo leads to the formation of a ventral-to-dorsal gradient of the transcription factor Dorsal within the nuclei of the syncytial blastoderm stage embryo. Dorsal controls the spatially specific expression of a large constellation of zygotic target genes, the Dorsal gene regulatory network, along the embryonic DV circumference. This article reviews classic studies and integrates them with the details of more recent work that has advanced our understanding of the complex pathway that establishes Drosophila embryo DV polarity. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.