Lack of an antibacterial response defect in Drosophila Toll-9 mutant

Immunological function of Bombyx Toll9-2 in the silkworm (Bombyx mori) larval midgut: Activation by Escherichia coli/lipopolysaccharide and regulation of growth

Toll receptors are important regulators of insects' innate immune system which, upon binding of pathogen molecules, activate a conserved signal transduction cascade known as the Toll pathway. RNA interference (RNAi) is a powerful tool to study the function of genes via reverse genetics. However, due to the reported refractory of RNAi efficiency in lepidopteran insects, successful reports of silencing of Toll receptors in the silkworm Bombyx mori have not been reported yet. In this study, a Toll receptor of the silkworm Bombyx Toll9-2 (BmToll9-2) was cloned and its expression and function were analyzed. The results showed that BmToll9-2 contains an ectodomain (ECD) with a signal peptide and nine leucine-rich repeats, a transmembrane helix, and a cytoplasmic region with a Toll/interleukin-1 domain. Phylogenetic analysis indicates that BmToll9-2 clusters with other insect Toll9 receptors and mammalian Toll-like receptor 4. Oral infection of exogenous pathogens showed that the Gram-negative bacterium Escherichia coli and its main cell wall component lipopolysaccharide (LPS), as well as the Gram-positive bacterium Staphylococcus aureus and its main cell wall component peptidoglycan, significantly induce BmToll9-2 expression in vivo. LPS also induced the expression of BmToll9-2 in BmN4 cells in vitro. These observations indicate its role as a sensor in the innate immunity to exogenous pathogens and as a pathogen-associated receptor that is responsive to LPS. RNAi of BmToll9-2 was effective in the midgut and epidermis. RNAi-mediated knock-down of BmToll9-2 reduced the weight and growth of the silkworm. Bacterial challenge following RNAi upregulated the expression of BmToll9-2 and rescued the weight differences of the silkworm, which may be related to its participation in the immune response and the regulation of the microbiota in the midgut lumen of the silkworm larvae.

Drosophila melanogaster Toll-9 elicits antiviral immunity against Drosophila C virus

The Toll pathway plays a pivotal role in innate immune responses against pathogens. The evolutionary conserved pathogen recognition receptors (PRRs), including Toll like receptors (TLRs), play a crucial role in recognition of pathogen associated molecular patterns (PAMPs). The Drosophila genome encodes nine Toll receptors that are orthologous to mammalian TLRs. While mammalian TLRs directly recognize PAMPs, most Drosophila Tolls recognize the proteolytically cleaved ligand Spätzle to activate downstream signaling cascades. In this study, we demonstrated that Toll-9 is crucial for antiviral immunity against Drosophila C virus (DCV), a natural pathogen of Drosophila . A transposable element insertion in the Toll-9 gene renders the flies more susceptible to DCV. The stable expression of Toll-9 in S2 cells confers resistance against DCV infection by upregulation of the RNAi pathway. Toll-9 promotes the dephosphorylation of AKT, resulting in the induction of antiviral RNAi genes to inhibit DCV replication. Toll-9 localizes to the endosome where it binds dsRNA, suggesting its role to detect viral dsRNA. Toll-9 also induces apoptosis during DCV infection, contributing to its antiviral role. Together, this work identifies the role of Toll-9 in antiviral immunity against DCV infection through its ability to bind dsRNA and induce AKT-mediated RNAi antiviral immunity.

IMPORTANCE

Insects rely on innate immunity and RNA interference (RNAi) to combat viral infections. Our study underscores the pivotal role of Drosophila Toll-9 in antiviral immunity, aligning with findings in Bombyx mori , where Toll-9 activation upregulates the RNAi component Dicer2 . We demonstrate that Drosophila Toll-9 functions as a pattern recognition receptor (PRR) for double-stranded RNA (dsRNA) during Drosophila C virus (DCV) infection, akin to mammalian TLRs. Toll-9 activation leads to the upregulation of key RNAi components, Dicer2 and Argonaute2 , and dephosphorylation of AKT triggers apoptosis via induction of proapoptotic genes Hid and Reaper . This study also reveals that Toll-9 localizes in endosomal compartments where it interacts with dsRNA. These insights enhance our understanding of Drosophila innate immune mechanisms, reflecting the evolutionary conservation of immune responses across diverse species and providing impetus for further research into the conserved roles of TLRs across the animal kingdom.

Innate Immune Response of TmToll-3 Following Systemic Microbial Infection in Tenebrio molitor

Although Toll-like receptors have been widely identified and functionally characterized in mammalian models and Drosophila, the immunological function of these receptors in other insects remains unclear. Here, we explored the relevant innate immune response of Tenebrio molitor (T. molitor) Toll-3 against Gram-negative bacteria, Gram-positive bacteria, and fungal infections. Our findings indicated that TmToll-3 expression was mainly induced by Candida albicans infections in the fat bodies, gut, Malpighian tubules, and hemolymph of young T. molitor larvae. Surprisingly, Escherichia coli systemic infection caused mortality after TmToll-3 knockdown via RNA interference (RNAi) injection, which was not observed in the control group. Further analyses indicated that in the absence of TmToll-3, the final effector of the Toll signaling pathway, antimicrobial peptide (AMP) genes and relevant transcription factors were significantly downregulated after E. coli challenge. Our results indicated that the expression of almost all AMP genes was suppressed in silenced individuals, whereas the expression of relevant genes was positively regulated after fungal injection. Therefore, this study revealed the immunological involvement of TmToll-3 in T. molitor in response to systematic infections.

Drosophila Toll-9 is induced by aging and neurodegeneration to modulate stress signaling and its deficiency exacerbates tau-mediated neurodegeneration

Drosophila Toll-9 is most closely related to mammalian Toll-like receptors; however, physiological functions of Toll-9 remain elusive. We examined the roles of Toll-9 in fly brains in aging and neurodegeneration. Toll-9 mRNA levels were increased in aged fly heads accompanied by activation of nuclear factor-kappa B (NF-kB) and stress-activated protein kinase (SAPK) signaling, and many of these changes were modulated by Toll-9 in glial cells. The loss of Toll-9 did not affect lifespan or brain integrity, whereas it exacerbated hydrogen peroxide-induced lethality. Toll-9 expression was also induced by nerve injury but did not affect acute stress response or glial engulfment activity, suggesting Toll-9 may modulate subsequent neurodegeneration. In a fly tauopathy model, Toll-9 deficiency enhanced neurodegeneration and disease-related tau phosphorylation with reduced SAPK activity, and blocking SAPK enhanced tau phosphorylation and neurodegeneration. In sum, Toll-9 is induced upon aging and nerve injury and affects neurodegeneration by modulating stress kinase signaling.

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.

Toll-9 interacts with Toll-1 to mediate a feedback loop during apoptosis-induced proliferation in Drosophila

Drosophila Toll-1 and all mammalian Toll-like receptors regulate innate immunity. However, the functions of the remaining eight Toll-related proteins in Drosophila are not fully understood. Here, we show that Drosophila Toll-9 is necessary and sufficient for a special form of compensatory proliferation after apoptotic cell loss (undead apoptosis-induced proliferation [AiP]). Mechanistically, for AiP, Toll-9 interacts with Toll-1 to activate the intracellular Toll-1 pathway for nuclear translocation of the NF-κB-like transcription factor Dorsal, which induces expression of the pro-apoptotic genes reaper and hid. This activity contributes to the feedback amplification loop that operates in undead cells. Given that Toll-9 also functions in loser cells during cell competition, we define a general role of Toll-9 in cellular stress situations leading to the expression of pro-apoptotic genes that trigger apoptosis and apoptosis-induced processes such as AiP. This work identifies conceptual similarities between cell competition and AiP.

Evolution of Cellular Immunity Effector Cells; Perspective on Cytotoxic and Phagocytic Cellular Lineages

The immune system has evolved to protect organisms from infections caused by bacteria, viruses, and parasitic pathogens. In addition, it provides regenerative capacities, tissue maintenance, and self/non-self recognition of foreign tissues. Phagocytosis and cytotoxicity are two prominent cellular immune activities positioned at the base of immune effector function in mammals. Although these immune mechanisms have diversified into a wide heterogeneous repertoire of effector cells, it appears that they share some common cellular and molecular features in all animals, but also some interesting convergent mechanisms. In this review, we will explore the current knowledge about the evolution of phagocytic and cytotoxic immune lineages against pathogens, in the clearance of damaged cells, for regeneration, for histocompatibility recognition, and in killing virally infected cells. To this end, we give different immune examples of multicellular organism models, ranging from the roots of bilateral organisms to chordate invertebrates, comparing to vertebrates' lineages. In this review, we compare cellular lineage homologies at the cellular and molecular levels. We aim to highlight and discuss the diverse function plasticity within the evolved immune effector cells, and even suggest the costs and benefits that it may imply for organisms with the meaning of greater defense against pathogens but less ability to regenerate damaged tissues and organs.

Toll family members bind multiple Spätzle proteins and activate antimicrobial peptide gene expression in Drosophila

The Toll signaling pathway in Drosophila melanogaster regulates several immune-related functions, including the expression of antimicrobial peptide (AMP) genes. The canonical Toll receptor (Toll-1) is activated by the cytokine Spätzle (Spz-1), but Drosophila encodes eight other Toll genes and five other Spz genes whose interactions with one another and associated functions are less well-understood. Here, we conducted in vitro assays in the Drosophila S2 cell line with the Toll/interleukin-1 receptor (TIR) homology domains of each Toll family member to determine whether they can activate a known target of Toll-1, the promoter of the antifungal peptide gene drosomycin. All TIR family members activated the drosomycin promoter, with Toll-1 and Toll-7 TIRs producing the highest activation. We found that the Toll-1 and Toll-7 ectodomains bind Spz-1, -2, and -5, and also vesicular stomatitis virus (VSV) virions, and that Spz-1, -2, -5, and VSV all activated the promoters of drosomycin and several other AMP genes in S2 cells expressing full-length Toll-1 or Toll-7. In vivo experiments indicated that Toll-1 and Toll-7 mutants could be systemically infected with two bacterial species (Enterococcus faecalis and Pseudomonas aeruginosa), the opportunistic fungal pathogen Candida albicans, and VSV with different survival times in adult females and males compared with WT fly survival. Our results suggest that all Toll family members can activate several AMP genes. Our results further indicate that Toll-1 and Toll-7 bind multiple Spz proteins and also VSV, but they differentially affect adult survival after systemic infection, potentially because of sex-specific differences in Toll-1 and Toll-7 expression.

TmToll-7 Plays a Crucial Role in Innate Immune Responses Against Gram-Negative Bacteria by Regulating 5 AMP Genes in Tenebrio molitor

Although it is known that the Drosophila Toll-7 receptor plays a critical role in antiviral autophagy, its function in other insects has not yet been reported. Here, we have identified a Toll-like receptor 7 gene, TmToll-7, in the coleopteran insect T. molitor and examined its potential role in antibacterial and antifungal immunity. We showed that TmToll-7 expression was significantly induced in larvae 6 h after infection with Escherichia coli and Staphylococcus aureus and 9 h after infection with Candida albicans. However, even though TmToll-7 was induced by all three pathogens, we found that TmToll-7 knockdown significantly reduced larval survival to E. coli, but not to S. aureus, and C. albicans infections. To understand the reasons for this difference, we examined the effects of TmToll-7 knockdown on antimicrobial peptide (AMP) gene expression and found a significant reduction of E. coli-induced expression of AMP genes such as TmTenecin-1, TmDefensin-1, TmDefensin-2, TmColeoptericin-1, and TmAttacin-2. Furthermore, TmToll-7 knockdown larvae infected with E. coli showed significantly higher bacterial growth in the hemolymph compared to control larvae treated with Vermilion dsRNA. Taken together, our results suggest that TmToll-7 plays an important role in regulating the immune response of T. molitor to E. coli.

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 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.

Rapidly Evolving Toll-3/4 Genes Encode Male-Specific Toll-Like Receptors in Drosophila

Animal Toll-like receptors (TLRs) have evolved through a pattern of duplication and divergence. Whereas mammalian TLRs directly recognize microbial ligands, Drosophila Tolls bind endogenous ligands downstream of both developmental and immune signaling cascades. Here, we find that most Toll genes in Drosophila evolve slowly with little gene turnover (gains/losses), consistent with their important roles in development and indirect roles in microbial recognition. In contrast, we find that the Toll-3/4 genes have experienced an unusually rapid rate of gene gains and losses, resulting in lineage-specific Toll-3/4s and vastly different gene repertoires among Drosophila species, from zero copies (e.g., D. mojavensis) to nineteen copies (e.g., D. willistoni). In D. willistoni, we find strong evidence for positive selection in Toll-3/4 genes, localized specifically to an extracellular region predicted to overlap with the binding site of Spätzle, the only known ligand of insect Tolls. However, because Spätzle genes are not experiencing similar selective pressures, we hypothesize that Toll-3/4s may be rapidly evolving because they bind to a different ligand, akin to TLRs outside of insects. We further find that most Drosophila Toll-3/4 genes are either weakly expressed or expressed exclusively in males, specifically in the germline. Unlike other Toll genes in D. melanogaster, Toll-3, and Toll-4 have apparently escaped from essential developmental roles, as knockdowns have no substantial effects on viability or male fertility. Based on these findings, we propose that the Toll-3/4 genes represent an exceptionally rapidly evolving lineage of Drosophila Toll genes, which play an unusual, as-yet-undiscovered role in the male germline.

Toll-receptor 9 gene in the black tiger shrimp (Penaeus monodon) induced the activation of the TLR-NF-κB signaling pathway

Toll receptors are important pathogen recognition receptors (PRRs) in shrimps, which play a vital role in defending against virus and bacterial challenge. In this paper, the characterization and functional analysis of a Toll9 receptor gene from Penaeus monodon was performed in HEK293T cells. Data showed that PmToll9 can activate the NF-κB promoter activities of TLR pathway, while ISRE and IFN-β promoter cannot be activated obviously in HEK293T cells using dual-luciferase reporter system. The downstream immune factors of IL-8, IκB-α, and TRAF6 were activated by PmToll9 and IL-8 showed the most significant up-regulation in expression levels, indicating the activities of NF-κB can be mediated by PmToll9. Six LRRs-deletion mutants were constructed and results showed these mutants had obvious declines in luciferase activities, among which the mutant pCMV-DeLRR4 showed the most significant decline. qPCR data indicated LRRs-deletion mutants efficiently impaired the activities of the downstream immune factors IL-8, IκB-α, and TRAF6. It demonstrates that LRRs-deletion mutants could result in the weaken abilities of PmToll9 in signaling transduction. Overexpression of PmToll9-GFP fusion protein in Hela cells revealed the primary cellular localization of PmToll9 is in the cytoplasm.

Molecular analysis of Culex quinquefasciatus larvae responses to Lysinibacillus sphaericus Bin toxin

Lysinibacillus sphaericus produces the mosquito larvicidal binary toxin consisting of BinA and BinB, which are both required for toxicity against Culex and Anopheles larvae. The molecular mechanisms behind Bin toxin-induced damage remain unexplored. We used whole-genome microarray-based transcriptome analysis to better understand how Culex larvae respond to Bin toxin treatment at the molecular level. Our analyses of Culex quinquefasciatus larvae transcriptome changes at 6, 12, and 18 h after Bin toxin treatment revealed a wide range of transcript signatures, including genes linked to the cytoskeleton, metabolism, immunity, and cellular stress, with a greater number of down-regulated genes than up-regulated genes. Bin toxin appears to mainly repress the expression of genes involved in metabolism, the mitochondrial electron transport chain, and the protein transporter of the outer/inner mitochondrial membrane. The induced genes encode proteins linked to mitochondrial-mediated apoptosis and cellular detoxification including autophagic processes and lysosomal compartments. This study is, to our knowledge, the first microarray analysis of Bin toxin-induced transcriptional responses in Culex larvae, providing a basis for an in-depth understanding of the molecular nature of Bin toxin-induced damage.

Analysis of the Contribution of Hemocytes and Autophagy to Drosophila Antiviral Immunity

Antiviral immunity in the model organism Drosophila melanogaster involves the broadly active intrinsic mechanism of RNA interference (RNAi) and virus-specific inducible responses. Here, using a panel of six viruses, we investigated the role of hemocytes and autophagy in the control of viral infections. Injection of latex beads to saturate phagocytosis, or genetic depletion of hemocytes, resulted in decreased survival and increased viral titers following infection with Cricket paralysis virus (CrPV), Flock House virus (FHV), and vesicular stomatitis virus (VSV) but had no impact on Drosophila C virus (DCV), Sindbis virus (SINV), and Invertebrate iridescent virus 6 (IIV6) infection. In the cases of CrPV and FHV, apoptosis was induced in infected cells, which were phagocytosed by hemocytes. In contrast, VSV did not trigger any significant apoptosis but we confirmed that the autophagy gene Atg7 was required for full virus resistance, suggesting that hemocytes use autophagy to recognize the virus. However, this recognition does not depend on the Toll-7 receptor. Autophagy had no impact on DCV, CrPV, SINV, or IIV6 infection and was required for replication of the sixth virus, FHV. Even in the case of VSV, the increases in titers were modest in Atg7 mutant flies, suggesting that autophagy does not play a major role in antiviral immunity in Drosophila Altogether, our results indicate that, while autophagy plays a minor role, phagocytosis contributes to virus-specific immune responses in insects.

IMPORTANCE

Phagocytosis and autophagy are two cellular processes that involve lysosomal degradation and participate in Drosophila immunity. Using a panel of RNA and DNA viruses, we have addressed the contribution of phagocytosis and autophagy in the control of viral infections in this model organism. We show that, while autophagy plays a minor role, phagocytosis contributes to virus-specific immune responses in Drosophila This work brings to the front a novel facet of antiviral host defense in insects, which may have relevance in the control of virus transmission by vector insects or in the resistance of beneficial insects to viral pathogens.

Drosophila Rabex-5 restricts Notch activity in hematopoietic cells and maintains hematopoietic homeostasis

Hematopoietic homeostasis requires the maintenance of a reservoir of undifferentiated blood cell progenitors and the ability to replace or expand differentiated blood cell lineages when necessary. Multiple signaling pathways function in these processes, but how their spatiotemporal control is established and their activity is coordinated in the context of the entire hematopoietic network are still poorly understood. We report here that loss of the gene Rabex-5 in Drosophila causes several hematopoietic abnormalities, including blood cell (hemocyte) overproliferation, increased size of the hematopoietic organ (the lymph gland), lamellocyte differentiation and melanotic mass formation. Hemocyte-specific Rabex-5 knockdown was sufficient to increase hemocyte populations, increase lymph gland size and induce melanotic masses. Rabex-5 negatively regulates Ras, and we show that Ras activity is responsible for specific Rabex-5 hematopoietic phenotypes. Surprisingly, Ras-independent Notch protein accumulation and transcriptional activity in the lymph gland underlie multiple distinct hematopoietic phenotypes of Rabex-5 loss. Thus, Rabex-5 plays an important role in Drosophila hematopoiesis and might serve as an axis coordinating Ras and Notch signaling in the lymph gland.

Characterization of the effect of Cr(VI) on humoral innate immunity using Drosophila melanogaster

With the advancement of human race, different anthropogenic activities have heaped the environment with chemicals that can cause alteration in the immune system of exposed organism. As a first line of barrier, the evolutionary conserved innate immunity is crucial for the health of an organism. However, there is paucity of information regarding in vivo assessment of the effect of environmental chemicals on innate immunity. Therefore, we examined the effect of a widely used environmental chemical, Cr(VI), on humoral innate immune response using Drosophila melanogaster. The adverse effect of Cr(VI) on host humoral response was characterized by decreased gene expression of antimicrobial peptides (AMPs) in the exposed organism. Concurrently, a significantly decreased transcription of humoral pathway receptors (Toll and PGRP) and triglyceride level along with inhibition of antioxidant enzyme activities were observed in exposed organism. This in turn weakened the immune response of exposed organism that was manifested by their reduced resistance against bacterial infection. In addition, overexpression of the components of humoral immunity particularly Diptericin benefits Drosophila from Cr(VI)-induced humoral immune-suppressive effect. To our knowledge, this is the first report regarding negative impact of an environmental chemical on humoral innate immune response of Drosophila along with subsequent protection by AMPs, which may provide novel insight into host-chemical interactions. Also, our data validate the utility and sensitivity of Drosophila as a model that could be used for screening the possible risk of environmental chemicals on innate immunity with minimum ethical concern that can be further extrapolated to higher organisms.

Comparative Genomics Reveals the Origins and Diversity of Arthropod Immune Systems

Insects are an important model for the study of innate immune systems, but remarkably little is known about the immune system of other arthropod groups despite their importance as disease vectors, pests, and components of biological diversity. Using comparative genomics, we have characterized the immune system of all the major groups of arthropods beyond insects for the first time--studying five chelicerates, a myriapod, and a crustacean. We found clear traces of an ancient origin of innate immunity, with some arthropods having Toll-like receptors and C3-complement factors that are more closely related in sequence or structure to vertebrates than other arthropods. Across the arthropods some components of the immune system, such as the Toll signaling pathway, are highly conserved. However, there is also remarkable diversity. The chelicerates apparently lack the Imd signaling pathway and beta-1,3 glucan binding proteins--a key class of pathogen recognition receptors. Many genes have large copy number variation across species, and this may sometimes be accompanied by changes in function. For example, we find that peptidoglycan recognition proteins have frequently lost their catalytic activity and switch between secreted and intracellular forms. We also find that there has been widespread and extensive duplication of the cellular immune receptor Dscam (Down syndrome cell adhesion molecule), which may be an alternative way to generate the high diversity produced by alternative splicing in insects. In the antiviral short interfering RNAi pathway Argonaute 2 evolves rapidly and is frequently duplicated, with a highly variable copy number. Our results provide a detailed analysis of the immune systems of several important groups of animals for the first time and lay the foundations for functional work on these groups.

A novel vertebrates Toll-like receptor counterpart regulating the anti-microbial peptides expression in the freshwater crayfish, Procambarus clarkii

Toll-like receptors (TLRs) play an important role in regulation of anti-microbial peptides (AMPs) expression. A novel vertebrates TLR counterpart named PcToll, was firstly identified from the freshwater crayfish, Procambarus clarkii. Phylogenetic analysis showed that PcToll together with Drosophila melanogaster and Anopheles gambiae Toll9 were clustered with human Tolls. PcToll was mainly expressed in hepatopancreas and gills and it also could be detected in hemocytes, heart, stomach and intestine. PcToll was upregulated in hemocytes and gills post 24 h Vibrio anguillarum challenge. In hepatopancreas and intestine, the highest expression level of PcToll could be observed at 12 h V. anguillarum challenge. In hemocytes, PcToll went up post 24 h Staphylococcus aureus challenge and in gills, the expression level of PcToll showed no obvious change from 2 to 24 h S. aureus challenge. In hepatopancreas post 12 h S. aureus challenge, PcToll was upregulated and it showed obvious upregulation post 12 h S. aureus challenge in intestine. RNAi results showed that PcToll was involved in regulation of crustins (Cru1, Cru2), anti-lipopolysaccharide factor 2 (ALF2) and lysozyme 1 (Lys1) expression. Overexpression of PcToll in Drosophila S2 cells could induce Drosophila Attacin (Atta), Metchnikowin (Mtk), Drosomycin (Drs) and shrimp Penaeidin (PEN4) expression. From the results, it could be speculated that PcToll might play important roles in crayfish innate immune defense.

Diversity of animal immune receptors and the origins of recognition complexity in the deuterostomes

Invertebrate animals are characterized by extraordinary diversity in terms of body plan, life history and life span. The past impression that invertebrate immune responses are controlled by relatively simple innate systems is increasingly contradicted by genomic analyses that reveal significant evolutionary novelty and complexity. One accessible measure of this complexity is the multiplicity of genes encoding homologs of pattern recognition receptors. These multigene families vary significantly in size, and their sequence character suggests that they vary in function. At the same time, certain aspects of downstream signaling appear to be conserved. Here, we analyze five major classes of immune recognition receptors from newly available animal genome sequences. These include the Toll-like receptors (TLR), Nod-like receptors (NLR), SRCR domain scavenger receptors, peptidoglycan recognition proteins (PGRP), and Gram negative binding proteins (GNBP). We discuss innate immune complexity in the invertebrate deuterostomes, which was first recognized in sea urchins, within the wider context of emerging genomic information across animal phyla.

Evolution of Immunity and Pathogens

Host and pathogen engage in a constant evolutionary struggle known as a "Red Queen Paradigm". In this struggle, natural selection favours the pathogen which evolves effective virulence mechanisms and the host which is able to field adequate resistance strategies. A number of factors limit what each side can do. These include the fact that the elaboration of virulence or resistance mechanisms results in costs in genetic fitness and requires the use of ever more of the limited number of genes available in the genome. In addition, since the pathogen usually has a very much shorter generation time than the host, it can fix new virulence mutations much more quickly than the host can evolve matching resistance mechanisms. Finally, the host must ensure that its defence system does not result in unacceptable levels of collateral damage to its own tissues. This chapter briefly outlines how these considerations shape host-pathogen interactions.

Modulation of the transcriptional response of innate immune and RNAi genes upon exposure to dsRNA and LPS in silkmoth-derived Bm5 cells overexpressing BmToll9-1 receptor

Injection or feeding of dsRNA is commonly used to induce specific gene silencing by RNAi in insects but very little research has been carried out to investigate non-specific effects on gene expression of dsRNA as pathogen-associated molecular pattern (PAMP). This study focuses on the potential role of the BmToll9-1 receptor to modulate the transcriptional response of innate immune and RNAi genes to dsRNA and lipopolysaccharide (LPS), which was used for comparison. To study this role, we took advantage of the silkmoth-derived Bm5 cell line, which does not express BmToll9-1 endogenously, and engineered a transformed cell line that permanently expresses BmToll9-1. Quantitative mRNA expression studies showed that BmToll9-1 can significantly alter the transcriptional response to dsRNA and LPS: (1) BmToll9-1 promotes the transcriptional response of Dicer2, encoding a key component of the RNAi machinery, and, to a lesser extent, that of transcription factors in the Jak-STAT and Toll pathways; and (2) BmToll9-1 represses the transcriptional induction of the IMD and Jak-STAT pathway genes, as well as the antimicrobial peptide (AMP) effector genes, by LPS. Thus, BmToll9-1 was identified as a modulator of innate immune and RNAi machinery gene expression that could be related to its preferential expression in the larval gut, the major barrier of pathogen entry. While BmToll9-1 was found to modulate RNAi-related gene expression, a reporter-based RNAi assay established no evidence for a direct interaction of BmToll9-1 with the intracellular RNAi machinery.

Conventional and non-conventional Drosophila Toll signaling

The discovery of Toll in Drosophila and of the remarkable conservation in pathway composition and organization catalyzed a transformation in our understanding of innate immune recognition and response. At the center of that picture is a cascade of interactions in which specific microbial cues activate Toll receptors, which then transmit signals driving transcription factor nuclear localization and activity. Experiments gave substance to the vision of pattern recognition receptors, linked phenomena in development, gene regulation, and immunity into a coherent whole, and revealed a rich set of variations for identifying non-self and responding effectively. More recently, research in Drosophila has illuminated the positive and negative regulation of Toll activation, the organization of signaling events at and beneath membranes, the sorting of information flow, and the existence of non-conventional signaling via Toll-related receptors. Here, we provide an overview of the Toll pathway of flies and highlight these ongoing realms of research.

Antimicrobial autophagy: a conserved innate immune response in Drosophila

Autophagy is a highly conserved degradative pathway that has rapidly emerged as a critical component of immunity and host defense. Studies have implicated autophagy genes in restricting the replication of a diverse array of pathogens, including bacteria, viruses and protozoans. However, in most cases, the in vivo role of antimicrobial autophagy against pathogens has been undefined. Drosophila provides a genetically tractable model system that can be easily adapted to study autophagy in innate immunity, and recent studies in flies have demonstrated that autophagy is an essential antimicrobial response against bacteria and viruses in vivo. These findings reveal striking conservation of antimicrobial autophagy between flies and mammals, and in particular, the role of pathogen-associated pattern recognition in triggering this response. This review discusses our current understanding of antimicrobial autophagy in Drosophila and its potential relevance to human immunity.

Signaling pathways regulating innate immune responses in shrimp

The first line of defense against microbial infections in animals is innate immune response which triggers diverse humoral and cellular activities via signal transduction pathways. Toll, IMD and JAK/STAT pathways are regarded as the main pathways regulating the immune response of invertebrates. This paper reviews the main progress of the investigation on the immune response to pathogen's infection in shrimp and supposes that these three signal pathways exist in shrimp. Most of the components (proteins or genes) involved in Toll pathway of Drosophila have been cloned also in shrimp which suggested the existence of Toll pathway in shrimp. The data update shows that the Toll pathway of shrimp is responsive not only to Gram-positive bacteria, Gram-negative bacteria, but also to WSSV. Challenge of WSSV can lead to the variation of transcription level of all identified components in shrimp Toll pathway, which supported that Toll pathway in shrimp played important roles during WSSV infection. Two major homologs to the components of IMD pathway of Drosophila, IMD and Relish, have been identified in shrimp, which indicated that IMD pathway should be existed in shrimp and might play important roles in regulating the immune response of shrimp to bacteria and virus infection. Relish in IMD pathway and dorsal in Toll pathway of shrimp were both involved in the immune response of shrimp to bacteria and virus infection, which implied that these two pathways are not completely separated during the immune response of shrimp. The transcription of STAT in shrimp was modulated after WSSV infection, which suggested that a putative JAK/STAT pathway might exist in shrimp and be very important to virus infection. Study on the signaling pathway regulating the immune response in shrimp could help us to understand the innate immune system, and would provide instructions to shrimp disease control. Obviously, to get more clear ideas about the innate immunological pathways in shrimp, more solid functional studies should be done in the future.

Dynamic evolution of toll-like receptor multigene families in echinoderms

The genome sequence of the purple sea urchin, Strongylocentrotus purpuratus, a large and long-lived invertebrate, provides a new perspective on animal immunity. Analysis of this genome uncovered a highly complex immune system in which the gene families that encode homologs of the pattern recognition receptors that form the core of vertebrate innate immunity are encoded in large multigene families. The sea urchin genome contains 253 Toll-like receptor (TLR) sequences, more than 200 Nod-like receptors and 1095 scavenger receptor cysteine-rich domains, a 10-fold expansion relative to vertebrates. Given their stereotypic protein structure and simple intron-exon architecture, the TLRs are the most tractable of these families for more detailed analysis. A role for these receptors in immune defense is suggested by their similarity to TLRs in other organisms, sequence diversity, and expression in immunologically active tissues, including phagocytes. The complexity of the sea urchin TLR multigene families is largely derived from expansions independent of those in vertebrates and protostomes, although a small family of TLRs with structure similar to that of Drosophila Toll can be traced to an ancient eumetazoan ancestor. Several other echinoderm sequences are now available, including Lytechinus variegatus, as well as partial sequences from two other sea urchin species. Here, we present an analysis of the invertebrate deuterostome TLRs with emphasis on the echinoderms. Representatives of most of the S. purpuratus TLR subfamilies and homologs of the mccTLR sequences are found in L. variegatus, although the L. variegatus TLR gene family is notably smaller (68 TLR sequences). The phylogeny of these genes within sea urchins highlights lineage-specific expansions at higher resolution than is evident at the phylum level. These analyses identify quickly evolving TLR subfamilies that are likely to have novel immune recognition functions and other, more stable, subfamilies that may function more similarly to those of vertebrates.

Virus recognition by Toll-7 activates antiviral autophagy in Drosophila

Innate immunity is highly conserved and relies on pattern recognition receptors (PRRs) such as Toll-like receptors (identified through their homology to Drosophila Toll) for pathogen recognition. Although Drosophila Toll is vital for immune recognition and defense, roles for the other eight Drosophila Tolls in immunity have remained elusive. Here we have shown that Toll-7 is a PRR both in vitro and in adult flies; loss of Toll-7 led to increased vesicular stomatitis virus (VSV) replication and mortality. Toll-7, along with additional uncharacterized Drosophila Tolls, was transcriptionally induced by VSV infection. Furthermore, Toll-7 interacted with VSV at the plasma membrane and induced antiviral autophagy independently of the canonical Toll signaling pathway. These data uncover an evolutionarily conserved role for a second Drosophila Toll receptor that links viral recognition to autophagy and defense and suggest that other Drosophila Tolls may restrict specific as yet untested pathogens, perhaps via noncanonical signaling pathways.

Molecular cloning, characterization and expression analysis of two novel Tolls (LvToll2 and LvToll3) and three putative Spätzle-like Toll ligands (LvSpz1-3) from Litopenaeus vannamei

Toll-like receptor-mediated NF-κB pathways are essential for inducing immune related-gene expression in the defense against bacterial, fungal and viral infections in insects and mammals. Although a Toll receptor (LvToll1) was cloned in Litopenaeus vannamei, relatively little is known about other types of Toll-like receptors and their endogenous cytokine-like ligand, Spätzle. Here, we report two novel Toll-like receptors (LvToll2 and LvToll3) and three Spätzle-like proteins (LvSpz1-3) from L. vannamei. LvToll2 has 1009 residues with an extracellular domain containing 18 leucine-rich repeats (LRRs) and a cytoplasmic Toll/interleukin-1 receptor (TIR) domain of 139 residues. LvToll3 is 1244 residues long with an extracellular domain containing 23 LRRs and a cytoplasmic TIR domain of 138 residues. The Spätzle-like proteins LvSpz1, LvSpz2 and LvSpz3 are 237, 245 and 275 residues in length, respectively, and all of them have a putative C-terminal cystine-knot domain. In Drosophila Schneider 2 (S2) cells, LvToll1 and LvToll3 were localized to the membrane and cytoplasm, and LvToll2 was confined to the cytoplasm. In Drosophila S2 cells, LvToll2 could significantly activate the promoters of NF-κB-pathway-controlled antimicrobial peptide genes, whereas LvToll1 and LvToll3 had no effect on them. LvSpz1 exerted some degree of inhibition on the promoter activities of Drosophila Attacin A and L. vannamei Penaeidin4. LvSpz3 also inhibited the Drosophila Attacin A promoter, but LvSpz2 could only slightly activate it. LvToll1, LvToll2 and LvToll3 were constitutive expressed in various tissues, while LvSpz1, LvSpz2 and LvSpz3 exhibited tissue-specific expression in the epithelium, eyestalk, intestine, gill and muscle. In the gill, after Vibrio alginolyticus challenge, LvToll1 was upregulated, but LvToll2 and LvToll3 showed no obvious changes. LvSpz1 and LvSpz3 were also strongly induced by V. alginolyticus challenge, but LvSpz2 only showed a slight downregulation. In the gill, after white spot syndrome virus (WSSV) challenge, LvToll1, LvToll2, LvToll3, LvSpz1 and LvSpz3 were upregulated, but LvSpz2 showed no obvious change, except for a slight downregulation at 12h post-injection of WSSV. These findings might be valuable in understanding the innate immune signal pathways of shrimp and enabling future studies on the host-pathogen interactions in V. alginolyticus and WSSV infections.

Toll-8/Tollo negatively regulates antimicrobial response in the Drosophila respiratory epithelium

Barrier epithelia that are persistently exposed to microbes have evolved potent immune tools to eliminate such pathogens. If mechanisms that control Drosophila systemic responses are well-characterized, the epithelial immune responses remain poorly understood. Here, we performed a genetic dissection of the cascades activated during the immune response of the Drosophila airway epithelium i.e. trachea. We present evidence that bacteria induced-antimicrobial peptide (AMP) production in the trachea is controlled by two signalling cascades. AMP gene transcription is activated by the inducible IMD pathway that acts non-cell autonomously in trachea. This IMD-dependent AMP activation is antagonized by a constitutively active signalling module involving the receptor Toll-8/Tollo, the ligand Spätzle2/DNT1 and Ect-4, the Drosophila ortholog of the human Sterile alpha and HEAT/ARMadillo motif (SARM). Our data show that, in addition to Toll-1 whose function is essential during the systemic immune response, Drosophila relies on another Toll family member to control the immune response in the respiratory epithelium.

Invertebrates solely rely on innate immune responses for defense against microbial infections. Taking advantage of its powerful genetics, the fly Drosophila melanogaster has been extensively used as a model system to dissect the molecular mechanisms that control innate immunity. This work led to the discovery of the essential role of the Toll-1 receptor in triggering the systemic immune response in flies, and paved the way for the discovery of the function of members of the Toll-like receptor (TLR) family in mammalian immunity. Whereas all TLRs are implicated in the mammalian immune response, Toll-1 was, so far, the only Drosophila Toll family member to be involved in the regulation of the immune response. In the present study, we show that another Toll family member, Toll-8 (Tollo), plays an important role in controlling the respiratory epithelium immune response. Our data indicate that, by antagonizing the IMD pathway, Tollo is preventing over-activation of the antibacterial response in the airway epithelium.