Pathogen recognition and signalling in the Drosophila innate immune response

Grass carp (Ctenopharyngodon idella) NLK2 inhibits IFN I response through blocking MAVS-IRF3 axis

In mammals, nemo-like kinase 2 (NLK2) is a conservative protein kinase involved in Wnt/β-catenin signaling pathway and immune response. However, the role of NLK2 in immune response in teleost remain unclear. In this study, we identified an ortholog of mammalian NLK from grass carp (Ctenopharyngodon idellus) named CiNLK2. CiNLK2 shares a high level of homology with the counterparts, especially with that of Cyprinus carpio. CiNLK2 was ubiquitously expressed in all tested tissues (liver, brain, spleen, gill, kidney and eye) and its expression was up-regulated under the treatment with poly I:C or GCRV. Overexpression of CiNLK2 suppressed the production of IFN I in CIK cells whether or not treated with poly I:C. However, knockdown of CiNLK2 increased the expression level of IFN I. The analysis of subcellular localization showed that CiNLK2 protein was scattered throughout the cytoplasm and nucleus. In terms of mechanism, CiNLK2 can directly interact with MAVS and inhibit MAVS-induced IFN I response. Moreover, CiNLK2 increased the phosphorylation level of MAVS, which led to the degradation of MAVS protein. On the other hand, CiNLK2 suppressed the phosphorylation and nuclear translocation of IRF3. In general, CiNLK2 served as an inhibitor for IFN I response by targeting MAVS-IRF3 signal axis.

Downregulation of SOCS1 increases interferon-induced ISGylation during differentiation of induced-pluripotent stem cells to hepatocytes

Background & Aims

Increased expression of IFN-stimulated gene 15 (ISG15) and subsequently increased ISGylation are key factors in the host response to viral infection. In this study, we sought to characterize the expression of ISG15, ISGylation, and associated enzymes at each stage of differentiation from induced pluripotent stem cells (iPSCs) to hepatocytes.

Methods

To study the regulation of ISGylation, we utilized patient samples and in vitro cell culture models including iPSCs, hepatocytes-like cells, immortalized cell lines, and primary human hepatocytes. Protein/mRNA expression were measured following treatment with poly(I:C), IFNα and HCV infection.

Results

When compared to HLCs, we observed several novel aspects of the ISGylation pathway in iPSCs. These include a lower baseline expression of the ISGylation-activating enzyme, UBE1L, a lack of IFN-induced expression of the ISGylation-conjugation enzyme UBE2L6, an attenuated activation of the transcription factor STAT1 and constitutive expression of SOCS1. ISGylation was observed in iPSCs following downregulation of SOCS1, which facilitated STAT1 activation and subsequently increased expression of UBE2L6. Intriguingly, HCV permissive transformed hepatoma cell lines demonstrated higher intrinsic expression of SOCS1 and weaker ISGylation following IFN treatment. SOCS1 downregulation in HCV-infected Huh 7.5.1 cells led to increased ISGylation.

Conclusions

Herein, we show that high basal levels of SOCS1 inhibit STAT1 activation and subsequently IFN-induced UBE2L6 and ISGylation in iPSCs. Furthermore, as iPSCs differentiate into hepatocytes, epigenetic mechanisms regulate ISGylation by modifying UBE1L and SOCS1 expression levels. Overall, this study demonstrates that the development of cell-intrinsic innate immunity during the differentiation of iPSCs to hepatocytes provides insight into cell type-specific regulation of host defense responses and related oncogenic processes.

Impact and implications

To elucidate the mechanism underlying regulation of ISGylation, a key process in the innate immune response, we studied changes in ISGylation-associated genes at the different stages of differentiation from iPSCs to hepatocytes. We found that high basal levels of SOCS1 inhibit STAT1 activation and subsequently IFN-induced UBE2L6 and ISGylation in iPSCs. Importantly, epigenetic regulation of SOCS1 and subsequently ISGylation may be important factors in the development of cell type-specific host defense responses in hepatocytes that should be considered when studying chronic infections and oncogenic processes in the liver.

Infection increases activity via Toll dependent and independent mechanisms in Drosophila melanogaster

Host behavioural changes are among the most apparent effects of infection. ‘Sickness behaviour’ can involve a variety of symptoms, including anorexia, depression, and changed activity levels. Here, using a real-time tracking and behavioural profiling platform, we show that in Drosophila melanogaster, several systemic bacterial infections cause significant increases in physical activity, and that the extent of this activity increase is a predictor of survival time in some lethal infections. Using multiple bacteria and D. melanogaster immune and activity mutants, we show that increased activity is driven by at least two different mechanisms. Increased activity after infection with Micrococcus luteus, a Gram-positive bacterium rapidly cleared by the immune response, strictly requires the Toll ligand spätzle. In contrast, increased activity after infection with Francisella novicida, a Gram-negative bacterium that cannot be cleared by the immune response, is entirely independent of both Toll and the parallel IMD pathway. The existence of multiple signalling mechanisms by which bacterial infections drive increases in physical activity implies that this effect may be an important aspect of the host response.

Sickness behaviours are often observed during infection. Animals have been shown to change their feeding, mating, social and resting (sleeping) behaviours in response to infection. We show here that fruit-flies infected with bacteria respond by increasing their physical activity and decreasing the amount of time spent sleeping. This increase in activity is seen in some, but not all, bacterial infections, and appears to be driven by at least two different mechanisms: with some bacteria, activating the immune response is the only requirement to induce increased activity, while other bacteria induce increased activity independently of known immune detection pathways. The biological role of increased activity is unclear; flies in the wild may be driven to flee sites where infection risk or pathogen burden is high. Alternatively, increased activity could serve a less direct anti-microbial function. For example, active animals may be more likely to encounter potential mates or food resource.

The Silkworm as a Source of Natural Antimicrobial Preparations: Efficacy on Various Bacterial Strains

The global spread of multi-resistant pathogens responsible for infections, which cannot be treated with existing drugs such as antibiotics, is of particular concern. Antibiotics are becoming increasingly ineffective and drug resistance is leading to more difficult-to-treat infections; therefore, new bioactive compounds with antimicrobial activity are needed and new alternative sources should be found. Antimicrobial peptides (AMPs) are synthesized by processes typical of the innate immune system and are present in almost all organisms. Insects are extremely resistant to bacterial infections as they can produce a wide range of AMPs, providing an effective first line of defense. The AMPs produced by insects therefore represent a possible source of natural antimicrobial molecules. In this paper, the possibility of using plasma preparations from silkworm (Bombyx mori) larvae as a source of antimicrobials was evaluated. After simple purification steps, insect plasma was analyzed and tested on different Gram-positive and Gram-negative bacterial strains. The results obtained are encouraging as the assays on Escherichia coli and Enterobacter cloacae showed significant decrease in the growth of these Gram-negative bacteria. Similar results were obtained on Gram-positive bacteria, such as Micrococcus luteus and Bacillus subtilis, which showed strong susceptibility to the silkworm AMPs pool. In contrast, Staphylococcus aureus displayed high resistance to Bombyx mori plasma. Finally, the tested plasma formulations were assessed for possible storage not only at 4 °C, but also above room temperature. In conclusion, partially purified plasma from silkworm could be a promising source of AMPs which could be used in formulations for topical applications, without additional and expensive purification steps.

Disparate regulation of IMD signaling drives sex differences in infection pathology in Drosophila melanogaster

Sex differences in infection outcome are a widely observed phenomenon. While it is known that biological sex can influence an animal’s response to infection, the mechanisms through which these differences emerge are less clear. Here, we describe a mechanism through which heightened regulation of the IMD signaling pathway by female—but not male—Drosophila melanogaster reduces the cost of immune activity at the expense of resistance to bacterial infection. Through the masculinization of the main organ responsible for antimicrobial peptide activity in the fly (fat body), this work demonstrates that this heightened immune regulation is mediated by sex-determining pathways.

Male and female animals exhibit differences in infection outcomes. One possible source of sexually dimorphic immunity is the sex-specific costs of immune activity or pathology, but little is known about the independent effects of immune- versus microbe-induced pathology and whether these may differ for the sexes. Here, by measuring metabolic and physiological outputs in Drosophila melanogaster with wild-type and mutant immune responses, we test whether the sexes are differentially impacted by these various sources of pathology and identify a critical regulator of this difference. We find that the sexes exhibit differential immune activity but similar bacteria-derived metabolic pathology. We show that female-specific immune-inducible expression of PGRP-LB, a negative regulator of the immune deficiency (IMD) pathway, enables females to reduce immune activity in response to reductions in bacterial numbers. In the absence of PGRP-LB, females are more resistant to infection, confirming the functional importance of this regulation and suggesting that female-biased immune restriction comes at a cost.

Phosphorylation of MAVS/VISA by Nemo-like kinase (NLK) for degradation regulates the antiviral innate immune response

MAVS is essential for antiviral immunity, but the molecular mechanisms responsible for its tight regulation remain poorly understood. Here, we show that NLK inhibits the antiviral immune response during viral infection by targeting MAVS for degradation. NLK depletion promotes virus-induced antiviral cytokine production and decreases viral replication, which is potently rescued by the reintroduction of NLK. Moreover, the depletion of NLK promotes antiviral effects and increases the survival times of mice after infection with VSV. NLK interacts with and phosphorylates MAVS at multiple sites on mitochondria or peroxisomes, thereby inducing the degradation of MAVS and subsequent inactivation of IRF3. Most importantly, a peptide derived from MAVS promotes viral-induced IFN-β production and antagonizes viral replication in vitro and in vivo. These findings provide direct insights into the molecular mechanisms by which phosphorylation of MAVS regulates its degradation and influences its activation and identify an important peptide target for propagating antiviral responses.

Sexual dimorphism in Drosophila melanogaster survival of Beauveria bassiana infection depends on core immune signaling

In many animal species, females and males differ in physiology, lifespan, and immune function. The magnitude and direction of the sexual dimorphism in immune function varies greatly and the genetic and mechanistic bases for this dimorphism are often unknown. Here we show that Drosophila melanogaster females are more likely than males to die from infection with several strains of the fungal entomopathogen Beauveria bassiana. The sexual dimorphism is not exclusively due to barrier defenses and persists when flies are inoculated by injection as well as by surface exposure. Loss of function mutations of Toll pathway genes remove the dimorphism in survivorship. Surprisingly, loss of function mutation of relish, a gene in the Imd pathway, also removes the dimorphism, but the dimorphism persists in flies carrying other Imd pathway mutations. The robust sexual dimorphism in D. melanogaster survival to B. bassiana presents opportunities to further dissect its mechanistic details, with applications for biological control of insect vectors of human disease and insect crop pests.

Differential modulation of the cellular and humoral immune responses in Drosophila is mediated by the endosomal ARF1-Asrij axis

How multicellular organisms maintain immune homeostasis across various organs and cell types is an outstanding question in immune biology and cell signaling. In Drosophila, blood cells (hemocytes) respond to local and systemic cues to mount an immune response. While endosomal regulation of Drosophila hematopoiesis is reported, the role of endosomal proteins in cellular and humoral immunity is not well-studied. Here we demonstrate a functional role for endosomal proteins in immune homeostasis. We show that the ubiquitous trafficking protein ADP Ribosylation Factor 1 (ARF1) and the hemocyte-specific endosomal regulator Asrij differentially regulate humoral immunity. Asrij and ARF1 play an important role in regulating the cellular immune response by controlling the crystal cell melanization and phenoloxidase activity. ARF1 and Asrij mutants show reduced survival and lifespan upon infection, indicating perturbed immune homeostasis. The ARF1-Asrij axis suppresses the Toll pathway anti-microbial peptides (AMPs) by regulating ubiquitination of the inhibitor Cactus. The Imd pathway is inversely regulated- while ARF1 suppresses AMPs, Asrij is essential for AMP production. Several immune mutants have reduced Asrij expression, suggesting that Asrij co-ordinates with these pathways to regulate the immune response. Our study highlights the role of endosomal proteins in modulating the immune response by maintaining the balance of AMP production. Similar mechanisms can now be tested in mammalian hematopoiesis and immunity.

The Toll Signaling Pathway in the Chinese Oak Silkworm, Antheraea pernyi: Innate Immune Responses to Different Microorganisms

The Toll pathway is one of the most important signaling pathways regulating insect innate immunity. Spatzle is a key protein that functions as a Toll receptor ligand to trigger Toll-dependent expression of immunity-related genes. In this study, a novel spatzle gene (ApSPZ) from the Chinese oak silkworm Antheraea pernyi was identified. The ApSPZ cDNA is 1065 nucleotides with an open reading frame (ORF) of 777 bp encoding a protein of 258 amino acids. The protein has an estimated molecular weight of 29.71 kDa and an isoelectric point (PI) of 8.53. ApSPZ is a nuclear and secretory protein with no conserved domains or membrane helices and shares 40% amino acid identity with SPZ from Manduca sexta. Phylogenetic analysis indicated that ApSPZ might be a new member of the Spatzle type 1 family, which belongs to the Spatzle superfamily. The expression patterns of several genes involved in the Toll pathway were examined at different developmental stages and various tissues in 5th instar larvae. The examined targets included A. pernyi spatzle, GNBP, MyD88, Tolloid, cactus and dorsalA. The RT-PCR results showed that these genes were predominantly expressed in immune-responsive fat body tissue, indicating that the genes play a crucial role in A. pernyi innate immunity. Moreover, A. pernyi infection with the fungus Nosema pernyi and the gram-positive bacterium Enterococcus pernyi, but not the gram-negative bacterium Escherichia coli, activated the Toll signaling pathway. These results represent the first study of the Toll pathway in A. pernyi, which provides insight into the A. pernyi innate immune system.

Identification of a Novel Nematotoxic Protein by Challenging the Model Mushroom Coprinopsis cinerea with a Fungivorous Nematode

The dung of herbivores, the natural habitat of the model mushroom Coprinopsis cinerea, is a nutrient-rich but also very competitive environment for a saprophytic fungus. We showed previously that C. cinerea expresses constitutive, tissue-specific armories against antagonists such as animal predators and bacterial competitors. In order to dissect the inducible armories against such antagonists, we sequenced the poly(A)-positive transcriptome of C. cinerea vegetative mycelium upon challenge with fungivorous and bacterivorous nematodes, Gram-negative and Gram-positive bacteria and mechanical damage. As a response to the fungivorous nematode Aphelenchus avenae, C. cinerea was found to specifically induce the transcription of several genes encoding previously characterized nematotoxic lectins. In addition, a previously not characterized gene encoding a cytoplasmic protein with several predicted Ricin B-fold domains, was found to be strongly upregulated under this condition. Functional analysis of the recombinant protein revealed a high toxicity toward the bacterivorous nematode Caenorhabditis elegans. Challenge of the mycelium with A. avenae also lead to the induction of several genes encoding putative antibacterial proteins. Some of these genes were also induced upon challenge of the mycelium with the bacteria Escherichia coli and Bacillus subtilis. These results suggest that fungi have the ability to induce specific innate defense responses similar to plants and animals.

The role of fibrinogen-related proteins in the gastropod immune response

Fibrinogen-related proteins or FREPs constitute a large family of molecules, defined by the presence of a fibrinogen-related domain (FReD). These molecules are found in all animals and are diverse in both form and function. Here, we review the current understanding of gastropod FREPs, which are characterized by the presence of a fibrinogen domain connected to one or two immunoglobulin superfamily domains by way of a short interceding region. We present a historical perspective on the discovery of FREPs in gastropods followed by a summary of advances made in the nearly two decades of research focused on the characterization of FREPs in Biomphalaria glabrata (BgFREPs). Topics covered include BgFREP genomic architecture, predicted structure and known functions, structural comparisons between BgFREPs, and evidence of somatic diversification. Also examined are the expression patterns of BgFREPs during snail development and immunological challenges. Recent functional characterization of the role BgFREPs play in the defence response against digenean trematodes is also presented, as well as new data investigating the nucleotide-level genomic conservation of FREPs among Pulmonate gastropods. Finally, we identify areas in need of further research. These include confirming and identifying the specific binding targets of BgFREPs and elucidating how they later engage snail haemocytes to elicit an immunological response, precise mechanisms and importance of BgFREP diversification, characterizing the tissue expression patterns of BgFREPs, as well as addressing whether gastropod FREPs retain immunological importance in alternative snail-trematode associations or more broadly in snail-pathogen interactions.

A Fas associated factor negatively regulates anti-bacterial immunity by promoting Relish degradation in Bombyx mori

Negative regulation is required to keep NF-κB-dependent immune response under tight control. In previous study, we have identified a Fas associated factor (FAF) family member in Bombyx mori, BmFAF, and proposed it may act as a negative regulator in immune response. In this study, we found knock-down of BmFAF by RNAi led to a remarkable increase in transcriptional level of several antimicrobial peptide genes, including BmCecropinA1 and BmMoricin, and higher survival rate to Gram-negative bacterial infection. We also confirmed the regulatory role of BmFAF in suppressing NF-κB-dependent transcription by employing an inducible promoter in BmE cells. Consistent with these physiological phenotypes, BmFAF suppressed the activity of the essential transcription factor, Relish, in IMD signaling pathway by promoting its proteasomal degradation through direct interaction. In addition, by constructing various truncation mutants, we further demonstrated that UBA domain in BmFAF is required for the inhibitory role, and potential ubiquitination also occurs in this domain. Taken together, our results suggest that BmFAF is a negative regulator of IMD pathway by mediating degradation of Relish.

Tweedle cuticular protein BmCPT1 is involved in innate immunity by participating in recognition of Escherichia coli

Bombyx mori, a lepidopteran insect, is one of the earliest models for pattern recognition of Gram-negative bacteria, which may induce the IMD pathway for production of antibacterial peptides. So far, several recognition proteins have been reported in B. mori. However, the connection between pattern recognition of Gram negative bacteria and activation of BmRelish1, a transcription factor controlled by the IMD pathway remains largely unknown. In the present study, we identify BmCPT1, a cuticle protein bearing a Tweedle domain. Its gene expression is co-regulated by NF-kappaB and juvenile hormone signals. BmCPT1 is induced by Escherichia coli in fat bodies and hemocytes, but is constitutively expressed in the epidermis. In vitro binding assays indicate that BmCPT1 protein recognizes and binds to E. coli peptidoglycan. Post-transcriptionally modified BmCPT1 in the hemolymph binds to E. coli cells through interactions with peptidoglycan recognition protein-5 (BmPGRP5) and lipopolysaccharide binding protein (BmLBP). Transgenic overexpression of BmCPT1 causes the upregulated expression of BmRelish1 and clear induction of two gloverin genes. Therefore, BmCPT1 may work along with BmPGRP-S5 and BmLBP to recognize E. coli in the hemolymph and indirectly activate BmRelish1 to induce antimicrobial peptide synthesis.

The insect microbiome modulates vector competence for arboviruses

Diseases caused by arthropod-borne viruses (arboviruses), such as Dengue, West Nile, and Chikungunya, constitute a major global health burden and are increasing in incidence and geographic range. The natural microbiota of insect vectors influences various aspects of host biology, such as nutrition, reproduction, metabolism, and immunity, and recent studies have highlighted the ability of insect-associated bacteria to reduce vector competence for arboviruses and other pathogens. This reduction can occur through mechanisms, such as immune response activation, resource competition, or the production of anti-viral molecules. Studying the interactions between insect vectors and their microbiota is an important step toward developing alternative strategies for arbovirus transmission control.

The mosquito microbiota influences vector competence for human pathogens

The midgut of insect vectors of human disease contains not only pathogens harmful to human health, but also a diverse microbiota. This microbiota can influence insects' susceptibility to human pathogens, and the capacity to transmit them, through different mechanisms. Understanding the interaction between the vector, its microbiota and transmitted pathogens will provide novel opportunities to limit disease transmission.

Multiple targets of the microRNA miR-8 contribute to immune homeostasis in Drosophila

MicroRNAs (miRNAs) are noncoding, around 22-nucleotide-long RNAs that exert significant modulatory roles in gene expression throughout the genome. As such, miRNAs take part in and modulate almost all biological processes like cell growth, development, and immunity. We previously showed that miR-8 miRNA plays a role in maintaining immune homeostasis in Drosophila. Here, we further discovered that targeting of multiple coding genes by miR-8 contributes to the maintenance of immune homeostasis. Toll and Dorsal, respectively the receptor and transcription factor in the Toll immune pathway, were found to be miR-8 targets, as shown by reporter assays and miR-8 null flies. Moreover, U-shaped (Ush), a previously verified miR-8 target, was seen to mediate miR-8 regulation of immune homeostasis. Consistently, overexpression of either Dorsal or Ush in the fat body led to increased Drosomycin expression, mimicking that induced by deletion of miR-8. Furthermore, mutation in Toll immune pathway or Ush rescues the abnormal expression of Drosomycin and lethality in miR-8 mutant. Thus, miR-8 regulates Drosophila immune homeostasis by targeting multiple immune genes, thereby contributing to survival.

Identification of gamma-interferon-inducible lysosomal thiol reductase (GILT) homologues in the fruit fly Drosophila melanogaster

Gamma-interferon-inducible lysosomal thiol reductase (GILT) has been demonstrated to be involved in the immune response to bacterial challenge in various organisms. However, little is known about GILT function in innate immunity. Drosophila has been commonly used as a model for the study of the innate immune response of invertebrates. Here, we identify the CG9796, CG10157, and CG13822 genes of fruit fly Drosophila melanogaster as GILT homologues. All deduced Drosophila GILT coding sequences contained the major characteristic features of the GILT protein family: the GILT signature CQHGX2ECX2NX4C sequence and the active site CXXC or CXXS motif. The mRNA transcript levels of the Drosophila GILT genes were up-regulated after Gram-negative bacteria Escherichia coli DH5α infection. Moreover, a bacterial load assay showed that over-expression of Drosophila GILT in fat body or hemocytes led to a low bacterial colony number whereas knock-down of Drosophila GILT in fat body or hemocytes led to a high bacterial colony number when compared to a wild-type control. These results indicate that the Drosophila GILTs are very likely to play a role in the innate immune response upon bacterial challenge of Drosophila host defense. This study may provide the basis for further study on GILT function in innate immunity.

Identification of immunity-related genes in Ostrinia furnacalis against entomopathogenic fungi by RNA-seq analysis

BACKGROUND

The Asian corn borer (Ostrinia furnacalis (Guenée)) is one of the most serious corn pests in Asia. Control of this pest with entomopathogenic fungus Beauveria bassiana has been proposed. However, the molecular mechanisms involved in the interactions between O. furnacalis and B. bassiana are unclear, especially under the conditions that the genomic information of O. furnacalis is currently unavailable. So we sequenced and characterized the transcriptome of O. furnacalis larvae infected by B. bassiana with special emphasis on immunity-related genes.

METHODOLOGY/PRINCIPAL FINDINGS

Illumina Hiseq2000 was used to sequence 4.64 and 4.72 Gb of the transcriptome from water-injected and B. bassiana-injected O. furnacalis larvae, respectively. De novo assembly generated 62,382 unigenes with mean length of 729 nt. All unigenes were searched against Nt, Nr, Swiss-Prot, COG, and KEGG databases for annotations using BLASTN or BLASTX algorithm with an E-value cut-off of 10(-5). A total of 35,700 (57.2%) unigenes were annotated to at least one database. Pairwise comparisons resulted in 13,890 differentially expressed genes, with 5,843 up-regulated and 8,047 down-regulated. Based on sequence similarity to homologs known to participate in immune responses, we totally identified 190 potential immunity-related unigenes. They encode 45 pattern recognition proteins, 33 modulation proteins involved in the prophenoloxidase activation cascade, 46 signal transduction molecules, and 66 immune responsive effectors, respectively. The obtained transcriptome contains putative orthologs for nearly all components of the Toll, Imd, and JAK/STAT pathways. We randomly selected 24 immunity-related unigenes and investigated their expression profiles using quantitative RT-PCR assay. The results revealed variant expression patterns in response to the infection of B. bassiana.

CONCLUSIONS/SIGNIFICANCE

This study provides the comprehensive sequence resource and expression profiles of the immunity-related genes of O. furnacalis. The obtained data gives an insight into better understanding the molecular mechanisms of innate immune processes in O. furnacalis larvae against B. bassiana.

Evolutionary rate patterns of genes involved in the Drosophila Toll and Imd signaling pathway

BACKGROUND

To survive in a hostile environment, insects have evolved an innate immune system to defend against infection. Studies have shown that natural selection may drive the evolution of immune system-related proteins. Yet, how network architecture influences protein sequence evolution remains unclear. Here, we analyzed the molecular evolutionary patterns of genes in the Toll and Imd innate immune signaling pathways across six Drosophila genomes within the context of a functional network.

RESULTS

Based on published literature, we identified 50 genes that are directly involved in the Drosophila Toll and Imd signaling pathways. Of those genes, only two (Sphinx1 and Dnr1) exhibited signals of positive selection. There existed a negative correlation between the strength of purifying selection and gene position within the pathway; the downstream genes were more conserved, indicating that they were subjected to stronger evolutionary constraints. Interestingly, there was also a significantly negative correlation between the rate of protein evolution and the number of regulatory microRNAs, implying that genes regulated by more miRNAs experience stronger functional constraints and therefore evolve more slowly.

CONCLUSION

Taken together, our results suggested that both network architecture and miRNA regulation affect protein sequence evolution. These findings improve our understanding of the evolutionary patterns of genes involved in Drosophila innate immune pathways.

Translationally controlled tumor protein, a dual functional protein involved in the immune response of the silkworm, Bombyx mori

Insect gut immunity is the first line of defense against oral infection. Although a few immune-related molecules in insect intestine has been identified by genomics or proteomics approach with comparison to well-studied tissues, such as hemolymph or fat body, our knowledge about the molecular mechanism underlying the gut immunity which would involve a variety of unidentified molecules is still limited. To uncover additional molecules that might take part in pathogen recognition, signal transduction or immune regulation in insect intestine, a T7 phage display cDNA library of the silkworm midgut is constructed. By use of different ligands for biopanning, Translationally Controlled Tumor Protein (TCTP) has been selected. BmTCTP is produced in intestinal epithelial cells and released into the gut lumen. The protein level of BmTCTP increases at the early time points during oral microbial infection and declines afterwards. In vitro binding assay confirms its activity as a multi-ligand binding molecule and it can further function as an opsonin that promotes the phagocytosis of microorganisms. Moreover, it can induce the production of anti-microbial peptide via a signaling pathway in which ERK is required and a dynamic tyrosine phosphorylation of certain cytoplasmic membrane protein. Taken together, our results characterize BmTCTP as a dual-functional protein involved in both the cellular and the humoral immune response of the silkworm, Bombyx mori.

Reproductive status alters transcriptomic response to infection in female Drosophila melanogaster

Mating and consequent reproduction significantly reduce the ability of female Drosophila melanogaster to defend against systemic bacterial infection. The goal of the present study was to identify genes likely to inform the mechanism of this post-mating immunosuppression. We used microarrays to contrast genome-wide transcript levels in virgin vs. mated females before and after infection. Because the immunosuppressive effect of mating is contingent on the presence of a germline in females, we repeated the entire experiment by using female mutants that do not form a germline. We found that multiple genes involved in egg production show reduced expression in response to infection, and that this reduction is stronger in virgins than it is in mated females. In germline-less females, expression of egg-production genes was predictably low and not differentially affected by infection. We also identified several immune responsive genes that are differentially induced after infection in virgins vs. mated females. Immune genes affected by mating status and egg production genes altered by infection are candidates to inform the mechanism of the trade-off between mating and immune defense.

Insect immune system maintains long-term resident bacteria through a local response

Long-term associations between bacteria and animals are widely represented in nature and play an important role in animal adaptation and evolution. In insects thriving on nutritionally unbalanced diets, intracellular symbiotic bacteria (endosymbionts) complement the host nutrients with amino acids and vitamins and interfere with host physiology and reproduction. Endosymbionts permanently infect host cells, called bacteriocytes, which express an adapted local immune response that permits symbiont maintenance and control. Among the immune players in bacteriocytes, the coleoptericin A (ColA) antimicrobial peptide of the cereal weevil, Sitophilus zeamais, was recently found to specifically trigger endosymbionts and to inhibit their cytokinesis, thereby limiting bacterial cell division and dispersion throughout the insect tissues. This review focuses on the biological and evolutionary features of Sitophilus symbiosis, and discusses the possible interactions of ColA with weevil endosymbiont proteins and pathways.

Phylogeny of Toll-like receptor signaling: adapting the innate response

The Toll-like receptors represent a largely evolutionarily conserved pathogen recognition machinery responsible for recognition of bacterial, fungal, protozoan, and viral pathogen associated microbial patterns and initiation of inflammatory response. Structurally the Toll-like receptors are comprised of an extracellular leucine rich repeat domain and a cytoplasmic Toll/Interleukin 1 receptor domain. Recognition takes place in the extracellular domain where as the cytoplasmic domain triggers a complex signal network required to sustain appropriate immune response. Signal transduction is regulated by the recruitment of different intracellular adaptors. The Toll-like receptors can be grouped depending on the usage of the adaptor, MyD88, into MyD88-dependent and MyD88 independent subsets. Herein, we present a unique phylogenetic analysis of domain regions of these receptors and their cognate signaling adaptor molecules. Although previously unclear from the phylogeny of full length receptors, these analyses indicate a separate evolutionary origin for the MyD88-dependent and MyD88-independent signaling pathway and provide evidence of a common ancestor for the vertebrate and invertebrate orthologs of the adaptor molecule MyD88. Together these observations suggest a very ancient origin of the MyD88-dependent pathway Additionally we show that early duplications gave rise to several adaptor molecule families. In some cases there is also strong pattern of parallel duplication between adaptor molecules and their corresponding TLR. Our results further support the hypothesis that phylogeny of specific domains involved in signaling pathway can shed light on key processes that link innate to adaptive immune response.

Lack of phenotypic and evolutionary cross-resistance against parasitoids and pathogens in Drosophila melanogaster

Background

When organisms are attacked by multiple natural enemies, the evolution of a resistance mechanism to one natural enemy will be influenced by the degree of cross-resistance to another natural enemy. Cross-resistance can be positive, when a resistance mechanism against one natural enemy also offers resistance to another; or negative, in the form of a trade-off, when an increase in resistance against one natural enemy results in a decrease in resistance against another. Using Drosophila melanogaster, an important model system for the evolution of invertebrate immunity, we test for the existence of cross-resistance against parasites and pathogens, at both a phenotypic and evolutionary level.

Methods

We used a field strain of D. melanogaster to test whether surviving parasitism by the parasitoid Asobara tabida has an effect on the resistance against Beauveria bassiana, an entomopathogenic fungus; and whether infection with the microsporidian Tubulinosema kingi has an effect on the resistance against A. tabida. We used lines selected for increased resistance to A. tabida to test whether increased parasitoid resistance has an effect on resistance against B. bassiana and T. kingi. We used lines selected for increased tolerance against B. bassiana to test whether increased fungal resistance has an effect on resistance against A. tabida.

Results/Conclusions

We found no positive cross-resistance or trade-offs in the resistance to parasites and pathogens. This is an important finding, given the use of D. melanogaster as a model system for the evolution of invertebrate immunity. The lack of any cross-resistance to parasites and pathogens, at both the phenotypic and the evolutionary level, suggests that evolution of resistance against one class of natural enemies is largely independent of evolution of resistance against the other.

Female Drosophila melanogaster suffer reduced defense against infection due to seminal fluid components

Reduced defense against infection is commonly observed as a consequence of reproductive activity, but little is known about how post-mating immunosuppression occurs. In this work, we use Drosophila melanogaster as a model to test the role of seminal fluid components and egg production in suppressing post-mating immune defense. We also evaluate whether systemic immune system activity is altered during infection in mated females. We find that post-mating reduction in female defense depends critically on male transfer of sperm and seminal fluid proteins, including the accessory gland protein known as "sex peptide." However, the effect of these male factors is dependent on the presence of the female germline. We find that mated females have lower antimicrobial peptide gene expression than virgin females in response to systemic infection, and that this lower expression correlates with higher systemic bacterial loads. We conclude that, upon receipt of sperm and seminal fluid proteins, females experience a germline-dependent physiological shift that directly or indirectly reduces their overall ability to defend against infection, at least in part through alteration of humoral immune system activity.

Conserved microRNA miR-8 in fat body regulates innate immune homeostasis in Drosophila

Antimicrobial peptides (AMPs) constitute a major arm of the innate immune system across diverse organisms. In Drosophila, septic injury by microbial pathogens rapidly induces the production of the AMPs in fat body via well elucidated pathways such as Toll and IMD. However, several epithelial tissues were reported to locally express AMPs without septic injury via poorly characterized ways. Here, we report that microRNA miR-8 regulates the levels of AMPs basally expressed in Drosophila. The levels of AMPs such as Drosomycin and Diptericin are significantly increased in miR-8 null animals in non-pathogen stimulated conditions. Analysis of various larval tissues revealed that the increase of Drosomycin is fat body specific. Supporting this observation, re-introduction of miR-8 only in the fat body restored the altered AMP expression in miR-8 null flies. Although loss of miR-8 impedes PI3K in the fat body, inhibition of PI3K does not phenocopy the AMP expression of miR-8 null flies, indicating that miR-8 regulates AMP independently of PI3K. Together, our findings suggest a role of miR-8 in systemic immune homeostasis in generally non-pathogenic conditions in flies.

Toll-like receptors on the fork roads between innate and adaptive immunity

There is a permanent interaction amid the innate and adaptive immune systems that leads to a defensive immune response against pathogens and contributes substantially to self-nonself discrimination. Toll-like receptors (TLRs) are essential molecules of the innate immune system that stimulate numerous inflammatory pathways and harmonize systemic defense against a wide array of pathogens. In addition to identifying unique molecular patterns associated with various sections of pathogens, TLRs may also recognize a number of self proteins and endogenous nucleic acids. Several reports have indicated that inappropriate stimulation of the TLR pathway via endogenous or exogenous ligands in animal models or humans may lead to the induction and/or prolongation of autoimmune response and tissue injury.

L-arginine enhances immunity to parasitoids in Drosophila melanogaster and increases NO production in lamellocytes

Drosophila melanogaster was used as a model system to explore the link between nutrition and immunity, and to investigate the role of nitric oxide (NO) in enhancing immunity following dietary enhancement with L-arginine. First, we show that adding L-arginine to the food medium increases the ability of D. melanogaster larvae to encapsulate the eggs of the parasitoid Asobara tabida. Secondly, we show that the increase in immunity is specific to L-arginine, and not to an enhanced calorific content, and that immunity decreases when larvae are fed food with added L-NAME, an inhibitor of nitric oxide synthase. Finally, we show that parasitised larvae fed L-arginine have increased haemocyte numbers, and that the lamellocytes (haemocytes which play a key role in encapsulation) show evidence of an increased production of NO. These results suggest that NO plays a key role in immunity and that the effect of NO is mostly targeted via the lamellocytes.

Pathogen and host factors are needed to provoke a systemic host response to gastrointestinal infection of Drosophila larvae by Candida albicans

Candida albicans systemic dissemination in immunocompromised patients is thought to develop from initial gastrointestinal (GI) colonisation. It is unclear what components of the innate immune system are necessary for preventing C. albicans dissemination from the GI tract, but studies in mice have indicated that both neutropenia and GI mucosal damage are crucial for allowing widespread invasive C. albicans disease. Mouse models, however, provide limited applicability to genome-wide screens for pathogen or host factors - factors that might influence systemic dissemination following GI colonisation. For this reason we developed a Drosophila model to study intestinal infection by Candida. We found that commensal flora aided host survival following GI infection. Candida provoked extensive JNK-mediated death of gut cells and induced antimicrobial peptide expression in the fat body. From the side of the host, nitric oxide and blood cells influenced systemic antimicrobial responses. The secretion of SAP4 and SAP6 (secreted aspartyl proteases) from Candida was also essential for activating systemic Toll-dependent immunity.

Differentially expressed genes in silkworm cell cultures in response to infection by Wolbachia and Cardinium endosymbionts

Wolbachia and Cardinium are bacterial endosymbionts that are widely distributed amongst arthropods. Both cause reproductive alterations, such as cytoplasmic incompatibility, parthenogenesis and feminization. Here we studied differentially expressed genes in Wolbachia- and Cardinium-infected Bm-aff3 silkworm cells using a silkworm microarray. Wolbachia infection did not alter gene expression or induce or suppress immune responses. In contrast, Cardinium infection induced many immune-related genes, including antimicrobial peptides, pattern recognition receptors and a serine protease. Host immune responses differed, possibly because of the different cell wall structures of Wolbachia and Cardinium because the former lacks genes encoding lipopolysaccharide components and two racemases for peptidoglycan formation. A few possibly non-immune-related genes were differentially expressed, but their involvement in host reproductive alteration was unclear.

AlgW regulates multiple Pseudomonas syringae virulence strategies

Gram-negative bacterial pathogens have evolved a number of virulence-promoting strategies including the production of extracellular polysaccharides such as alginate and the injection of effector proteins into host cells. The induction of these virulence mechanisms can be associated with concomitant downregulation of the abundance of proteins that trigger the host immune system, such as bacterial flagellin. In Pseudomonas syringae, we observed that bacterial motility and the abundance of flagellin were significantly reduced under conditions that induce the type III secretion system. To identify genes involved in this negative regulation, we conducted a forward genetic screen with P. syringae pv. maculicola ES4326 using motility as a screening phenotype. We identified the periplasmic protease AlgW as a key negative regulator of flagellin abundance that also positively regulates alginate biosynthesis and the type III secretion system. We also demonstrate that AlgW constitutes a major virulence determinant of P. syringae required to dampen plant immune responses. Our findings support the conclusion that P. syringae co-ordinately regulates virulence strategies through AlgW in order to effectively suppress host immunity.

Regulation of hemocytes in Drosophila requires dappled cytochrome b5

A major category of mutant hematopoietic phenotypes in Drosophila is melanotic tumors or nodules, which consist of abnormal and overproliferated blood cells, similar to granulomas. Our analyses of the melanotic mutant dappled have revealed a novel type of gene involved in blood cell regulation. The dappled gene is an essential gene that encodes cytochrome b5, a conserved hemoprotein that participates in electron transfer in multiple biochemical reactions and pathways. Viable mutations of dappled cause melanotic nodules and hemocyte misregulation during both hematopoietic waves of development. The sexes are similarly affected, but hemocyte number is different in females and males of both mutants and wild type. Additionally, initial tests show that curcumin enhances the dappled melanotic phenotype and establish screening of endogenous and xenobiotic compounds as a route for analysis of cytochrome b5 function. Overall, dappled provides a tractable genetic model for cytochrome b5, which has been difficult to study in higher organisms.

Structural and functional characterization of a highly specific serpin in the insect innate immunity

The Toll signaling pathway, an essential innate immune response in invertebrates, is mediated via the serine protease cascade. Once activated, the serine proteases are irreversibly inactivated by serine protease inhibitors (serpins). Recently, we identified three serpin-serine protease pairs that are directly involved in the regulation of Toll signaling cascade in a large beetle, Tenebrio molitor. Of these, the serpin SPN48 was cleaved by its target serine protease, Spätzle-processing enzyme, at a noncanonical P1 residue of the serpin's reactive center loop. To address this unique cleavage, we report the crystal structure of SPN48, revealing that SPN48 exhibits a native conformation of human antithrombin, where the reactive center loop is partially inserted into the center of the largest β-sheet of SPN48. The crystal structure also shows that SPN48 has a putative heparin-binding site that is distinct from those of the mammalian serpins. Ensuing biochemical studies demonstrate that heparin accelerates the inhibition of Spätzle-processing enzyme by a proximity effect in targeting the SPN48. Our finding provides the molecular mechanism of how serpins tightly regulate innate immune responses in invertebrates.

A different repertoire of Galleria mellonella antimicrobial peptides in larvae challenged with bacteria and fungi

To date, functioning of insect humoral immune response is especially well described in Diptera. The mechanisms of pathogen recognition, activation of signaling pathways and regulation of antimicrobial defense peptide expression are relatively well known. The present paper demonstrates evidence that the immune system of the Lepidoptera moth, Galleria mellonella, is also able to distinguish between different classes of microorganisms and responds to the invading pathogen accordingly. G. mellonella larvae were challenged with Gram-negative and Gram-positive bacteria as well as with yeast and filamentous fungus cells. Subsequently, 24, 48 and 72 h after immunization, the concentrations of lysozyme and six defense peptides were determined in the hemolymph by the HPLC technique. The compounds studied demonstrated variability both in the kinetics of the increase as well as in the concentrations reached. The Gram-negative bacterium and filamentous fungus were particularly effective immunogens, especially affecting the levels of lysozyme, Galleria defensin, proline-rich peptide 2 and cecropin D-like peptide.

Molecular cloning and characterization of a short peptidoglycan recognition protein (PGRP-S) with antibacterial activity from the bumblebee Bombus ignitus

Peptidoglycan recognition proteins (PGRPs) are pattern recognition molecules of the innate immune system that recognize peptidoglycan, a unique bacterial cell wall component. Here we cloned and characterized PGRP-S from the bumblebee Bombus ignitus (BiPGRP-S). The BiPGRP-S gene consists of four exons that encode 194 amino acid residues. Comparative analysis indicates that the predicted amino acid sequence of BiPGRP-S shares a high identity with enzymatically active PGRP-S proteins and contains the amino acids required for amidase activity. BiPGRP-S in B. ignitus worker bees is constitutively expressed in both the fat body and epidermis, and it is secreted into the hemolymph. Quantitative real-time PCR assays revealed that the BiPGRP-S gene is highly induced in both the fat body and the epidermis after an injection of Bacillus thuringiensis. In addition, recombinant BiPGRP-S expressed as a 19-kDa protein in baculovirus-infected insect cells can bind to Bacillus megaterium and B. thuringiensis but not to Staphylococcus aureus, Escherichia coli or Beauveria bassiana. Consistent with these data, BiPGRP-S shows antibacterial activity against B. megaterium and B. thuringiensis. After B. thuringiensis injection, the expression profiles of four antibacterial peptide genes in the fat body of RNA interference (RNAi)-mediated BiPGRP-S-knock-down B. ignitus worker bees was similar to that of control worker bees, indicating that BiPGRP-S does not affect the activation of antibacterial peptide gene expression. These results indicate that BiPGRP-S is an inducible protein that may function as an amidase-type PGRP-S during the immune response against Bacillus bacteria.

Drosophila as a model for antiviral immunity

The fruit fly Drosophila melanogaster has been successfully used to study numerous biological processes including immune response. Flies are naturally infected with more than twenty RNA viruses making it a valid model organism to study host-pathogen interactions during viral infections. The Drosophila antiviral immunity includes RNA interference, activation of the JAK/STAT and other signaling cascades and other mechanisms such as autophagy and interactions with other microorganisms. Here we review Drosophila as an immunological research model as well as recent advances in the field of Drosophila antiviral immunity.

HP1 recruitment in the absence of argonaute proteins in Drosophila

Highly repetitive and transposable element rich regions of the genome must be stabilized by the presence of heterochromatin. A direct role for RNA interference in the establishment of heterochromatin has been demonstrated in fission yeast. In metazoans, which possess multiple RNA-silencing pathways that are both functionally distinct and spatially restricted, whether RNA silencing contributes directly to heterochromatin formation is not clear. Previous studies in Drosophila melanogaster have suggested the involvement of both the AGO2-dependent endogenous small interfering RNA (endo-siRNA) as well as Piwi-interacting RNA (piRNA) silencing pathways. In order to determine if these Argonaute genes are required for heterochromatin formation, we utilized transcriptional reporters and chromatin immunoprecipitation of the critical factor Heterochromatin Protein 1 (HP1) to monitor the heterochromatic state of piRNA clusters, which generate both endo-siRNAs and the bulk of piRNAs. Surprisingly, we find that mutation of AGO2 or piwi increases silencing at piRNA clusters corresponding to an increase of HP1 association. Furthermore, loss of piRNA production from a single piRNA cluster results in genome-wide redistribution of HP1 and reduction of silencing at a distant heterochromatic site, suggesting indirect effects on HP1 recruitment. Taken together, these results indicate that heterochromatin forms independently of endo-siRNA and piRNA pathways.

Toll-dependent antimicrobial responses in Drosophila larval fat body require Spätzle secreted by haemocytes

In Drosophila, the humoral response characterised by the synthesis of antimicrobial peptides (AMPs) in the fat body (the equivalent of the mammalian liver) and the cellular response mediated by haemocytes (blood cells) engaged in phagocytosis represent two major reactions that counter pathogens. Although considerable analysis has permitted the elucidation of mechanisms pertaining to the two responses individually, the mechanism of their coordination has been unclear. To characterise the signals with which infection might be communicated between blood cells and fat body, we ablated circulating haemocytes and defined the parameters of AMP gene activation in larvae. We found that targeted ablation of blood cells influenced the levels of AMP gene expression in the fat body following both septic injury and oral infection. Expression of the AMP gene drosomycin (a Toll target) was blocked when expression of the Toll ligand Spätzle was knocked down in haemocytes. These results show that in larvae, integration of the two responses in a systemic reaction depend on the production of a cytokine (spz), a process that strongly parallels the mammalian immune response.

Evolution of host resistance and parasitoid counter-resistance

By their nature, parasitoids will exert a selection pressure on their hosts to evolve a mechanism through which to resist parasitoid attack. In turn, such a resistance mechanism will lead to parasitoids evolving counter-resistance. In this chapter, we present an overview of the research on the (co)evolutionary interaction between Drosophila and their parasitoids, with the main focus on the cellular immune response of D. melanogaster, and the counter-resistance mechanism of one of its main parasitoids, Asobara tabida. A key aspect of this interaction is the existence of genetic variation: in the field, host resistance and parasitoid counter-resistance vary, both between and within populations. Host resistance and parasitoid counter-resistance are costly, and both these costs turn out to be density dependent. These tradeoffs can explain the existence of genetic variation. We briefly touch upon behavioral aspects of the interaction and the parasites and pathogens that the parasitoids themselves suffer from. We end this chapter by considering the data coming from gene chip experiments: early indications suggest that the genes involved in the actual immune response against parasitoids are mostly different from the genes involved in the evolution of resistance.

Humoral immune response of Galleria mellonella larvae after infection by Beauveria bassiana under optimal and heat-shock conditions

Natural infection of Galleria mellonella larvae with the entomopathogenic fungus Beauveria bassiana led to antifungal, but not antibacterial host response. This was manifested by induction of gallerimycin and galiomicin gene expression and, consequently, the appearance of antifungal activity in the hemolymph of the infected larvae. The activity of lysozyme increased at the beginning of infection and dropped while infection progressed. Exposure of the naturally infected animals to 43 degrees C for 15 min extended their life time. Galleria mellonella larvae were injected with 10(4), 10(5) and 10(6) fungal blastospores, resulting in the appearance of strong antifungal activity and a significant increase in lysozyme activity in larval hemolymph after 24h. Antibacterial activity was detectable only when 10(5) and increased when 10(6) blastospores were injected. The number of the injected B. bassiana blastospores also determined the survival rate of animals. We found that exposure of the larvae to 38 degrees C for 30 min before infection extended their life time when 10(3) and 10(4) spores were injected. The increase in the survival rate of the pre-heat-shocked animals may be explained by higher expression of antimicrobial peptides and higher antifungal and lysozyme activities in their hemolymph in comparison to non-heat-shocked animals.

Short-term starvation of immune deficient Drosophila improves survival to gram-negative bacterial infections

Background

Primary immunodeficiencies are inborn errors of immunity that lead to life threatening conditions. These predispositions describe human immunity in natura and highlight the important function of components of the Toll-IL-1- receptor-nuclear factor kappa B (TIR-NF-κB) pathway. Since the TIR-NF-κB circuit is a conserved component of the host defence in higher animals, genetically tractable models may contribute ideas for clinical interventions.

Methodology/Principal Findings

We used immunodeficient fruit flies (Drosophila melanogaster) to address questions pertaining to survival following bacterial infection. We describe here that flies lacking the NF-κB protein Relish, indispensable for countering Gram-negative bacteria, had a greatly improved survival to such infections when subject to dietary short-term starvation (STS) prior to immune challenge. STS induced the release of Nitric Oxide (NO), a potent molecule against pathogens in flies, mice and humans. Administering the NO Synthase-inhibitory arginine analog N-Nitro-L-Arginine-Methyl-Ester (L-NAME) but not its inactive enantiomer D-NAME increased once again sensitivity to infection to levels expected for relish mutants. Surprisingly, NO signalling required the NF-κB protein Dif, usually needed for responses against Gram-positive bacteria.

Conclusions/Significance

Our results show that NO release through STS may reflect an evolutionary conserved process. Moreover, STS could be explored to address immune phenotypes related to infection and may offer ways to boost natural immunity.