The Toll and Imd pathways are the major regulators of the immune response in Drosophila

Drosophila: The Genetics of Innate Immune Recognition and Response

Because of the evolutionary conservation of innate mechanisms of host defense, Drosophila has emerged as an ideal animal in which to study the genetic control of immune recognition and responses. The discovery that the Toll pathway is required for defense against fungal infection in Drosophila was pivotal in studies of both mammalian and Drosophila immunity. Subsequent genetic screens in Drosophila to isolate additional mutants unable to induce humoral responses to infection have identified and ordered the function of components of two signaling cascades, the Toll and Imd pathways, that activate responses to infection. Drosophila blood cells also contribute to host defense through phagocytosis and signaling, and may carry out a form of self-nonself recognition that is independent of microbial pattern recognition. Recent work suggests that Drosophila will be a useful model for dissecting virulence mechanisms of several medically important pathogens.

TLR-independent control of innate immunity in Caenorhabditis elegans by the TIR domain adaptor protein TIR-1, an ortholog of human SARM

Both plants and animals respond to infection by synthesizing compounds that directly inhibit or kill invading pathogens. We report here the identification of infection-inducible antimicrobial peptides in Caenorhabditis elegans. Expression of two of these peptides, NLP-29 and NLP-31, was differentially regulated by fungal and bacterial infection and was controlled in part by tir-1, which encodes an ortholog of SARM, a Toll-interleukin 1 receptor (TIR) domain protein. Inactivation of tir-1 by RNA interference caused increased susceptibility to infection. We identify protein partners for TIR-1 and show that the small GTPase Rab1 and the f subunit of ATP synthase participate specifically in the control of antimicrobial peptide gene expression. As the activity of tir-1 was independent of the single nematode Toll-like receptor, TIR-1 may represent a component of a previously uncharacterized, but conserved, innate immune signaling pathway.

Genetic Basis of Natural Variation in D. melanogaster Antibacterial Immunity

Many genes involved in Drosophila melanogaster innate immune processes have been identified, but whether naturally occurring polymorphism in these genes leads to variation in immune competence among wild flies has not been tested. We report here substantial variability among wild-derived D. melanogaster in the ability to suppress infection by a Gram-negative entomopathogen, Serratia marcescens. Variability in immune competence was significantly associated with nucleotide polymorphism in 16 innate immunity genes, corresponding primarily to pathogen recognition and intracellular signaling loci, and substantial epistasis was detected between intracellular signaling and antimicrobial peptide genes. Variation in these genes, therefore, seems to drive variability in immunocompetence among wild Drosophila.

Exploration of host-pathogen interactions using Listeria monocytogenes and Drosophila melanogaster

The facultative intracellular bacterial pathogen Listeria monocytogenes is capable of replicating within a broad range of host cell types and host species. We report here the establishment of the fruit fly Drosophila melanogaster as a new model host for the exploration of L. monocytogenes pathogenesis and host response to infection. Listeria monocytogenes was capable of establishing lethal infections in adult fruit flies and larvae with extensive bacterial replication occurring before host death. Bacteria were found in the cytosol of insect phagocytic cells, and were capable of directing host cell actin polymerization. Bacterial gene products necessary for intracellular replication and cell-to-cell spread within mammalian cells were similarly found to be required within insect cells, and although previous work has suggested that L. monocytogenes virulence gene expression requires temperatures above 30 degrees C, bacteria within insect cells were found to express virulence determinants at 25 degrees C. Mutant strains of Drosophila that were compromised for innate immune responses demonstrated increased susceptibility to L. monocytogenes infection. These data indicate L. monocytogenes infection of fruit flies shares numerous features of mammalian infection, and thus that Drosophila has the potential to serve as a genetically tractable host system that will facilitate the analysis of host cellular responses to L. monocytogenes infection.

Immunity regulatory DNAs share common organizational features in Drosophila

Infection results in the rapid activation of immunity genes in the Drosophila fat body. Two classes of transcription factors have been implicated in this process: the REL-containing proteins, Dorsal, Dif, and Relish, and the GATA factor Serpent. Here we present evidence that REL-GATA synergy plays a pervasive role in the immune response. SELEX assays identified consensus binding sites that permitted the characterization of several immunity regulatory DNAs. The distribution of REL and GATA sites within these DNAs suggests that most or all fat-specific immunity genes contain a common organization of regulatory elements: closely linked REL and GATA binding sites positioned in the same orientation and located near the transcription start site. Aspects of this "regulatory code" are essential for the immune response. These results suggest that immunity regulatory DNAs contain constrained organizational features, which may be a general property of eukaryotic enhancers.

In Vivo RNA Interference Analysis Reveals an Unexpected Role for GNBP1 in the Defense against Gram-positive Bacterial Infection in Drosophila Adults

The Drosophila immune system discriminates between different classes of infectious microbes and responds with pathogen-specific defense reactions via the selective activation of the Toll and the immune deficiency (Imd) signaling pathways. The Toll pathway mediates most defenses against Gram-positive bacteria and fungi, whereas the Imd pathway is required to resist Gram-negative bacterial infection. Microbial recognition is achieved through peptidoglycan recognition proteins (PGRPs); Gram-positive bacteria activate the Toll pathway through a circulating PGRP (PGRP-SA), and Gram-negative bacteria activate the Imd pathway via PGRP-LC, a putative transmembrane receptor, and PGRP-LE. Gram-negative binding proteins (GNBPs) were originally identified in Bombyx mori for their capacity to bind various microbial compounds. Three GNBPs and two related proteins are encoded in the Drosophila genome, but their function is not known. Using inducible expression of GNBP1 double-stranded RNA, we now demonstrate that GNBP1 is required for Toll activation in response to Gram-positive bacterial infection; GNBP1 double-stranded RNA expression renders flies susceptible to Gram-positive bacterial infection and reduces the induction of the antifungal peptide encoding gene Drosomycin after infection by Gram-positive bacteria but not after fungal infection. This phenotype induced by GNBP1 inactivation is identical to a loss-of-function mutation in PGRP-SA, and our genetic studies suggest that GNBP1 acts upstream of the Toll ligand Spätzle. Altogether, our results demonstrate that the detection of Gram-positive bacteria in Drosophila requires two putative pattern recognition receptors, PGRP-SA and GNBP1.

Studies of the role of the Drosophila scs and scs' insulators in defining boundaries of a chromosome puff

Insulators are DNA elements that establish independent transcriptional domains within eukaryotic genomes. The Drosophila scs and scs' insulators localize near the borders of a structural domain in the polytene chromosomes, known as a puff, produced by transcription of the 87A heat shock protein (hsp) genes. It has been suggested that scs and scs' are boundary elements that delimit this decondensed chromatin domain, reflecting the mechanism by which these sequences act to constrain regulatory interactions. This model was tested using transposons that carried a yellow gene to assess enhancer blocking and an hsp70-lacZ gene to examine the structure of a heat shock puff in the presence and absence of insulators. We found that although scs and scs' blocked enhancer function, these sequences did not prevent the spread of decondensation resulting from hsp70-lacZ transcription. Further analysis of the endogenous 87A locus demonstrated that scs and scs' reside within, not at, the borders of the puff. Taken together, our studies suggest that scs and scs' are not boundary elements that block the propagation of an altered chromatin state associated with puff formation. We propose that these insulators may have a direct role in limiting regulatory interactions in the gene-dense 87A region.

Parasite-specific immune response in adult Drosophila melanogaster: a genomic study

Insects of the order Diptera are vectors for parasitic diseases such as malaria, sleeping sickness and leishmania. In the search for genes encoding proteins involved in the antiparasitic response, we have used the protozoan parasite Octosporea muscaedomesticae for oral infections of adult Drosophila melanogaster. To identify parasite-specific response molecules, other flies were exposed to virus, bacteria or fungi in parallel. Analysis of gene expression patterns after 24 h of microbial challenge, using Affymetrix oligonucleotide microarrays, revealed a high degree of microbe specificity. Many serine proteases, key intermediates in the induction of insect immune responses, were uniquely expressed following infection of the different organisms. Several lysozyme genes were induced in response to Octosporea infection, while in other treatments they were not induced or downregulated. This suggests that lysozymes are important in antiparasitic defence.

A proteomic approach for the analysis of instantly released wound and immune proteins in Drosophila melanogaster hemolymph

Insects respond to microbial infection by the rapid and transient expression of several genes encoding antibacterial peptides. In this paper we describe a powerful technique, two-dimensional difference gel electrophoresis, that, when combined with mass spectrometry, can be used to study the immune response of Drosophila melanogaster at the protein level. By comparatively analyzing the hemolymph proteome of 2,000 third-instar Drosophila larvae, we identified 10 differential proteins that appear in the fruit fly hemolymph very early after an immune-challenge with lipopolysaccharides. These proteins can be assigned to the immune response, because they are not induced after sterile injury. Reduction of integral variability or quantification problems related to conventional two-dimensional electrophoresis and improvement of image analysis were achieved by the use of two fluorescent dyes to label the two different protein samples. Some of the immune-induced proteins, such as thioester-containing protein 2, can be assigned to specific aspects of the immune response; others were already reported as being involved in stress response. An immune-induced protein (CG18594) is homologous to a mammalian serine protease inhibitor that mediates the mitogen-activated protein kinase and the NF-kappa B signaling pathways. In addition, a number of proteins that had not been associated with the immune response before were isolated and identified, and some of these were still present in the hemolymph 4 h after injury. Determining the function of all of these immune-induced proteins represents an exciting challenge for increasing our knowledge of insect immunity.

Dorsoventral Patterning: A Serpin Pinned down at Last

Dorsoventral patterning in Drosophila has long been known to involve a cascade of proteases, held in the inactive zymogen state prior to signaling. At long last, the prediction that a protease inhibitor is involved in this pathway has been shown to be true, with the identification of a serpin that plays a key part in Drosophila embryonic patterning.

Dual Activation of the Drosophila Toll Pathway by Two Pattern Recognition Receptors

The Toll-dependent defense against Gram-positive bacterial infections in Drosophila is mediated through the peptidoglycan recognition protein SA (PGRP-SA). A mutation termed osiris disrupts the Gram-negative binding protein 1 (GNBP1) gene and leads to compromised survival of mutant flies after Gram-positive infections, but not after fungal or Gram-negative bacterial challenge. Our results demonstrate that GNBP1 and PGRP-SA can jointly activate the Toll pathway. The potential for a combination of distinct proteins to mediate detection of infectious nonself in the fly will refine the concept of pattern recognition in insects.

Regulated assembly of the Toll signaling complex drives Drosophila dorsoventral patterning

In Drosophila, the Toll pathway establishes the embryonic dorsoventral axis and triggers innate immune responses to infection. The transmembrane receptor Toll acts through three death domain-containing proteins, the kinase Pelle and the adapters Tube and MyD88, in signaling to downstream NF-kappaB-like transcription factors. Here, we delineate the critical events in the earliest stages of Toll signaling. Mutational studies based on structural modeling reveal that the direct interaction of the bivalent Tube death domain with MyD88 is critical for signaling in vivo. The complex of MyD88 and Tube forms prior to signaling and is localized to the embryonic plasma membrane by MyD88. Upon Toll homodimerization, this complex is rapidly recruited to Toll. Binding of Pelle to the MyD88-Tube complex promotes Pelle activation, leading to degradation of the IkappaB-like inhibitor, Cactus. Together, these experiments convert a linear picture of gene function into a dynamic mechanistic and structural understanding of signaling complex assembly and function.

The immune response of Drosophila

Drosophila mounts a potent host defence when challenged by various microorganisms. Analysis of this defence by molecular genetics has now provided a global picture of the mechanisms by which this insect senses infection, discriminates between various classes of microorganisms and induces the production of effector molecules, among which antimicrobial peptides are prominent. An unexpected result of these studies was the discovery that most of the genes involved in the Drosophila host defence are homologous or very similar to genes implicated in mammalian innate immune defences. Recent progress in research on Drosophila immune defence provides evidence for similarities and differences between Drosophila immune responses and mammalian innate immunity.

Functional characterization of a novel promoter element required for an innate immune response in Drosophila

Innate immune reactions are crucial processes of metazoans to protect the organism against overgrowth of faster replicating microorganisms. Drosophila melanogaster is a precious model for genetic and molecular studies of the innate immune system. In response to infection, the concerted action of a battery of antimicrobial peptides ensures efficient killing of the microbes. The induced gene expression relies on translocation of the Drosophila Rel transcription factors Relish, Dif, and Dorsal to the nucleus where they bind to kappaB-like motifs in the promoters of the inducible genes. We have identified another putative promoter element, called region 1 (R1), in a number of antimicrobial peptide genes. Site-directed mutagenesis of the R1 site diminished Cecropin A1 (CecA1) expression in transgenic Drosophila larvae and flies. Infection of flies induced a nuclear R1-binding activity that was unrelated to the kappaB-binding activity in the same extracts. Although the R1 motif was required for Rel protein-mediated CecA1 expression in cotransfection experiments, our data argue against it being a direct target for the Drosophila Rel proteins. We propose that the R1 and kappaB motifs are targets for distinct regulatory complexes that act in concert to promote high levels of antimicrobial peptide gene expression in response to infection.

The Drosophila melanogaster genome

Drosophila's importance as a model organism made it an obvious choice to be among the first genomes sequenced, and the Release 1 sequence of the euchromatic portion of the genome was published in March 2000. This accomplishment demonstrated that a whole genome shotgun (WGS) strategy could produce a reliable metazoan genome sequence. Despite the attention to sequencing methods, the nucleotide sequence is just the starting point for genome-wide analyses; at a minimum, the genome sequence must be interpreted using expressed sequence tag (EST) and complementary DNA (cDNA) evidence and computational tools to identify genes and predict the structures of their RNA and protein products. The functions of these products and the manner in which their expression and activities are controlled must then be assessed-a much more challenging task with no clear endpoint that requires a wide variety of experimental and computational methods. We first review the current state of the Drosophila melanogaster genome sequence and its structural annotation and then briefly summarize some promising approaches that are being taken to achieve an initial functional annotation.

An endogenous suppressor of hairy-wing insulator separates regulatory domains in Drosophila

Insulators define independent domains of gene function throughout the genome. The Drosophila gypsy insulator was isolated from the gypsy retrotransposon as a region that contains a cluster of binding sites for the Suppressor of Hairy-wing [Su(Hw)] protein. To study the effects of the gypsy insulator on gene expression within a single genomic domain, targeted gene replacement was used to exchange the endogenous yellow gene, located at cytological location 1A, with a set of gypsy-modified yellow genes. Replaced yellow genes carried a gypsy insulator positioned between the yellow promoter and either the upstream or the downstream tissue-specific enhancers. Whereas the gypsy insulator blocked the function of the upstream enhancers at the endogenous location, the downstream enhancers were not blocked. Investigation of the 1A region revealed two clustered Su(Hw)-binding sites downstream of the yellow gene, named 1A-2, that bind Su(Hw) in vivo and possess enhancer blocking function. We propose that interaction between 1A-2 and the gypsy insulator permits activation of yellow expression by enhancers in the neighboring achaete-scute complex, causing an apparent absence of the block of the downstream yellow enhancers. Based on these data, we suggest that 1A-2 is an endogenous Su(Hw) insulator that separates regulatory domains within the Drosophila genome.

Construction of a cDNA-based microarray forDrosophila melanogaster: a comparison of gene transcription profiles from SL2 and Kc167 cells

We have constructed a DNA microarray that represents approximately 6900 of the estimated 13 598 genes in the Drosophila melanogaster genome. The microarray contains 5756 target cDNAs from the Berkeley Drosophila Genome Project, 1078 cDNAs from the National Institutes of Health Drosophila testis cDNA library, and 546 gene fragments that were amplified from genomic DNA. The methods for DNA amplification and microarray manufacture are presented. Academic researchers can obtain the microarray from the Canadian Drosophila Microarray Centre. To evaluate the utility of these arrays, we compared the gene transcription profiles of two commonly used Drosophila cell lines. Analysis revealed that 5412 spot pairs gave signals consistently above the average background in Kc167 cells, whereas 5636 spot pairs met this criterion in SL2 cells. When the expression profiles of the cell lines were compared, 1437 genes displayed at least a 1.5-fold difference, and 170 genes had a threefold or greater difference between the two cell lines. In each case, with respect to Kc167 when compared with SL2 cells, the number of genes that were upregulated was nearly equal to the number of downregulated genes. This result demonstrates that despite the similar embryonic derivation of both cell lines, their transcriptional profiles are very different.Key words: DNA microarray, Drosophila, transcriptional regulation, SL2, Kc167.

Binding of the Drosophila cytokine Spätzle to Toll is direct and establishes signaling

The extracellular protein Spätzle is required for activation of the Toll signaling pathway in the embryonic development and innate immune defense of Drosophila. Spätzle is synthesized as a pro-protein and is processed to a functional form by a serine protease. We show here that the mature form of Spätzle triggers a Toll-dependent immune response after injection into the hemolymph of flies. Spätzle specifically bound to Drosophila cells and to Cos-7 cells expressing Toll. Furthermore, in vitro experiments showed that the mature form of Spätzle bound to the Toll ectodomain with high affinity and with a stoichiometry of one Spätzle dimer to two receptors. The Spätzle pro-protein was inactive in all these assays, indicating that the pro-domain sequence, which is natively unstructured, acts to prevent interaction of the cytokine and its receptor Toll. These results show that, in contrast to the human Toll-like receptors, Drosophila Toll requires only an endogenous protein ligand for activation and signaling.

Model systems in drug discovery: chemical genetics meets genomics

Animal model systems are an intricate part of the discovery and development of new medicines. The sequencing of not only the human genome but also those of the various pathogenic bacteria, the nematode Caenorhabditis elegans, the fruitfly Drosophila, and the mouse has enabled the discovery of new drug targets to push forward at an unprecedented pace. The knowledge and tools in these "model" systems are allowing researchers to carry out experiments more efficiently and are uncovering previously hidden biological connections. While the history of bacteria, yeast, and mice in drug discovery are long, their roles are ever evolving. In contrast, the history of Drosophila and C. elegans at pharmaceutical companies is short. We will briefly review the historic role of each model organism in drug discovery and then update the readers as to the abilities and liabilities of each model within the context of drug development.

The Drosophila melanogaster toll pathway participates in resistance to infection by the gram-negative human pathogen Pseudomonas aeruginosa

Pseudomonas aeruginosa is a gram-negative pathogen that infects immunocompromised and cystic fibrosis patients. The molecular basis of the host-P. aeruginosa interaction and the effect of specific P. aeruginosa virulence factors on various components of the innate immunity pathways are largely unknown. We examine interactions between P. aeruginosa virulence factors and components of innate immunity response in the Drosophila melanogaster model system to reveal the importance of the Toll signaling pathway in resistance to infection by the P. aeruginosa human isolate PA14. Using the two PA14-isogenic mutants plcS and dsbA, we show that Drosophila loss-of-function mutants of Spatzle, the extracellular ligand of Toll, and Dorsal and Dif, two NF-kappa B-like transcription factors, allow increased P. aeruginosa infectivity within fly tissues. In contrast, a constitutively active Toll mutant and a loss-of-function mutant of Cactus, an I kappa B-like factor that inhibits the Toll signaling, reduce infectivity. Our finding that Dorsal activity is required to restrict P. aeruginosa infectivity in Drosophila provides direct in vivo evidence for Dorsal function in adult fly immunity. Additionally, our results provide the basis for future studies into interactions between P. aeruginosa virulence factors and components of the Toll signaling pathway, which is functionally conserved between flies and humans.

Drosophila melanogaster antimicrobial defense

The Drosophila melanogaster host defense is complex but remarkably efficient. It is a multifaceted response to a variety of fungal, bacterial, and parasitic invaders. Current knowledge is discussed on recognition of infectious microorganisms and on the activation of intracellular signaling cascades that concur with the expression of numerous immune-responsive genes, among which, to date, the most prominent appear to encode potent antimicrobial peptides.

Drosophila melanogaster is a genetically tractable model host for Mycobacterium marinum

Mycobacterium marinum is a pathogenic mycobacterial species that is closely related to Mycobacterium tuberculosis and causes tuberculosis-like disease in fish and frogs. We infected the fruit fly Drosophila melanogaster with M. marinum. This bacterium caused a lethal infection in the fly, with a 50% lethal dose (LD(50)) of 5 CFU. Death was accompanied by widespread tissue damage. M. marinum initially proliferated inside the phagocytes of the fly; later in infection, bacteria were found both inside and outside host cells. Intracellular M. marinum blocked vacuolar acidification and failed to colocalize with dead Escherichia coli, similar to infections of mouse macrophages. M. marinum lacking the mag24 gene were less virulent, as determined both by LD(50) and by death kinetics. Finally, in contrast to all other bacteria examined, mycobacteria failed to elicit the production of antimicrobial peptides in Drosophila.We believe that this system should be a useful genetically tractable model for mycobacterial infection.

The Drosophila immune system detects bacteria through specific peptidoglycan recognition

The Drosophila immune system discriminates between different classes of infectious microbes and responds with pathogen-specific defense reactions through selective activation of the Toll and the immune deficiency (Imd) signaling pathways. The Toll pathway mediates most defenses against Gram-positive bacteria and fungi, whereas the Imd pathway is required to resist infection by Gram-negative bacteria. The bacterial components recognized by these pathways remain to be defined. Here we report that Gram-negative diaminopimelic acid-type peptidoglycan is the most potent inducer of the Imd pathway and that the Toll pathway is predominantly activated by Gram-positive lysine-type peptidoglycan. Thus, the ability of Drosophila to discriminate between Gram-positive and Gram-negative bacteria relies on the recognition of specific forms of peptidoglycan.

Caenorhabditis elegans: an emerging genetic model for the study of innate immunity

Invaluable insights into how animals, humans included, defend themselves against infection have been provided by more than a decade of genetic studies that have used fruitflies. In the past few years, attention has also turned to another simple animal model, the nematode worm Caenorhabditis elegans. What exactly have we learned from the work in Drosophila? And will research with C. elegans teach us anything new about our response to pathogen attack?

Tamo selectively modulates nuclear import in Drosophila

BACKGROUND

The NF-kappaB/Rel pathway functions in the establishment of dorsal-ventral polarity and in the innate humoral and cellular immune response in Drosophila. An important aspect of all NF-kappaB/Rel pathways is the translocation of the Rel proteins from the cytoplasm to the nucleus, where they function as transcription factors.

RESULTS

We have identified a new protein, Tamo, which binds to Drosophila Rel protein Dorsal, but not to Dorsal lacking the nuclear localization sequence. Tamo does not bind to the other Drosophila Rel proteins, Dif and Relish. The Tamo-Dorsal complex forms in the cytoplasm and Tamo also interacts with a cytoplasmically orientated nucleoporin. In addition Tamo binds the Ras family small GTPase, Ran. Tamo functions during oogenesis and, based on phenotypic analysis, controls the levels of nuclear Dorsal in early embryos. It further regulates the accumulation of Dorsal in the nucleus after immune challenge.

CONCLUSIONS

Tamo has an essential function during oogenesis. Tamo interacts with Dorsal and proteins that are part of the nuclear import machinery. We propose that tamo modulates the levels of import of Dorsal and other proteins.

Relish-mediated immune deficiency in the transgenic mosquito Aedes aegypti

The lack of genetic means has been a serious limitation in studying mosquito immunity. We generated Relish-mediated immune deficiency (RMID) by transforming Aedes aegypti with the Delta Rel transgene driven by the vitellogenin (Vg) promoter using the pBac[3xP3-EGFP, afm] vector. A stable transformed line had a single copy of the Vg-Delta Rel transgene. The Vg-Delta Rel transgene expression was highly activated by blood feeding, and transgenic mosquitoes were extremely susceptible to the infection by Gram-negative bacteria. This RMID phenotype was characterized by severely reduced postinfection levels of antimicrobial peptides genes, defensin and cecropin. Crossing the RMID line with the wild-type strain produced the same RMID phenotype, indicating its dominant nature, whereas crossing with the Vg-def transgenic line, in which Defensin A was activated by blood feeding, restored the immunity to Enterobacter cloacae.

Characterization of three Toll-like genes from mosquito Aedes aegypti

Three Toll-related genes (AeToll1A, AeToll1B and AeToll5) were cloned and characterized from the yellow fever vector mosquito, Aedes aegypti. All three genes exhibited high levels of amino acid sequence similarity with Drosophila melanogaster (Dm)Toll1 and DmTehao (Toll5). AeToll1A and AeToll1B are 1124 and 1076 amino acid residues long, respectively. Both contain a carboxyl extension downstream of the Toll/interleukin-1 receptor (TIR) domain. AeToll5 is 1007 residues long and, like DmTehao, lacks the carboxyl terminal extension. Expression of these three genes was examined throughout development and after immune challenge. Both AeToll1A and AeToll5, like their Drosophila counterparts, activate transcription of drosomycin promoter in both Aedes and Drosophila cell lines. Deletion of the carboxyl extension of AeToll1A did not result in a further elevated level of the antifungal response. The intracellular signalling process appears to be species specific based on two observations. (1) DmToll is completely inactive in an Aedes cell line, suggesting a higher specificity requirement for DmToll in the intracellular signalling process. (2) Only one of three amino acid residues essential for DmToll function is required for AeToll1A function.

A serpin mutant links Toll activation to melanization in the host defence of Drosophila

A prominent response during the Drosophila host defence is the induction of proteolytic cascades, some of which lead to localized melanization of pathogen surfaces, while others activate one of the major players in the systemic antimicrobial response, the Toll pathway. Despite the fact that gain-of-function mutations in the Toll receptor gene result in melanization, a clear link between Toll activation and the melanization reaction has not been firmly established. Here, we present evidence for the coordination of hemolymph-borne melanization with activation of the Toll pathway in the Drosophila host defence. The melanization reaction requires Toll pathway activation and depends on the removal of the Drosophila serine protease inhibitor Serpin27A. Flies deficient for this serpin exhibit spontaneous melanization in larvae and adults. Microbial challenge induces its removal from the hemolymph through Toll-dependent transcription of an acute phase immune reaction component.

Evolution of signal transduction in intracellular symbiosis

Plant roots form intracellular symbioses with fungi and bacteria resulting in arbuscular mycorrhiza and nitrogen-fixing root nodules, respectively. A novel receptor like-kinase has been discovered that is required for the transduction of both bacterial and fungal symbiotic signals. This kinase defines an ancient signalling pathway that probably evolved in the context of arbuscular mycorrhiza and has been recruited subsequently for endosymbiosis with bacteria. An ancestral symbiotic interaction of roots with intracellular bacteria might have emerged from such a recruitment, in the progenitor of the nodulating clade of plants. Analysis of symbiotic mutants of host plants and bacterial microsymbionts has revealed that present-day endosymbioses require the coordinated induction of more than one signalling pathway for development.

Immunity-related genes and gene families in Anopheles gambiae

We have identified 242 Anopheles gambiae genes from 18 gene families implicated in innate immunity and have detected marked diversification relative to Drosophila melanogaster. Immune-related gene families involved in recognition, signal modulation, and effector systems show a marked deficit of orthologs and excessive gene expansions, possibly reflecting selection pressures from different pathogens encountered in these insects' very different life-styles. In contrast, the multifunctional Toll signal transduction pathway is substantially conserved, presumably because of counterselection for developmental stability. Representative expression profiles confirm that sequence diversification is accompanied by specific responses to different immune challenges. Alternative RNA splicing may also contribute to expansion of the immune repertoire.

An immune-responsive Serpin regulates the melanization cascade in Drosophila

In arthropods, the melanization reaction is associated with multiple host defense mechanisms leading to the sequestration and killing of invading microorganisms. Arthropod melanization is controlled by a cascade of serine proteases that ultimately activates the enzyme prophenoloxidase (PPO), which, in turn, catalyzes the synthesis of melanin. Here we report the biochemical and genetic characterization of a Drosophila serine protease inhibitor protein, Serpin-27A, which regulates the melanization cascade through the specific inhibition of the terminal protease prophenoloxidase-activating enzyme. Our data demonstrate that Serpin-27A is required to restrict the phenoloxidase activity to the site of injury or infection, preventing the insect from excessive melanization.