Purification and molecular cloning of an inducible gram-negative bacteria-binding protein from the silkworm, Bombyx mori

Germline and somatic diversification of immune recognition elements in Metazoa

The histories of the immune systems of Metazoa during evolution are envisaged like as many adaptations to the continuous diversification of immune receptors and effectors genes under the pressure of changing environments. A basic diversity of potential immune receptor genes existed in primitive Metazoa. Their subsequent recruitment into immunity, their diversification revolving around the conservation of signaling cascades was paralleled by cell specialization and the introduction of regulatory networks. Polymorphism, duplication and somatic mechanisms of diversification affected independently and still affect different gene families in many phyla, creating a greater variety of immune system exhibiting sometimes little homology but much analogy to one another. Diversity and multiplicity of receptors was generated by duplication and creation of multigene families. Independently in several phyla further diversity is created somatically by alternate splicing, somatic mutation, gene conversion and gene rearrangement. In several instances combinatorial usage of polypeptide chains or genes segments increases the repertoire of the recognition structures. Metazoa had to adapt to the conditions generated by this diversity: the control of expression of multiple genes and the risk of autoimmunity.

Ookinete-induced midgut peroxidases detonate the time bomb in anopheline mosquitoes

Previous analysis of the temporal-spatial relationship between ookinete migration and the cellular localization of genes mediating midgut immune defense responses suggested that, in order to survive, parasites must complete invasion before toxic chemicals ("a bomb") are generated by the invaded cell. Recent studies indicate that ookinete invasion induces tyrosine nitration as a two-step reaction, in which NOS induction is followed by a localized increase in peroxidase activity. Peroxidases utilize nitrite and hydrogen peroxide as substrates, and detonate the time bomb by generating reactive nitrogen intermediates, such as nitrogen dioxide, which mediate nitration. There is evidence that peroxidases also mediate antimicrobial responses to bacteria, fungi and parasites in a broad range of biological systems including humans and plants. Defense reactions that generate toxic chemicals are also potentially harmful to the host mounting the response and often results in apoptosis. The two-step nitration pathway is probably an ancient response, as it has also been described in vertebrate leukocytes and probably evolved as a mechanism to circumscribe the toxic products generated during defense responses involving protein nitration.

Recent advances in the innate immunity of invertebrate animals

Invertebrate animals, which lack adaptive immune systems, have developed other systems of biological host defense, so called innate immunity, that respond to common antigens on the cell surfaces of potential pathogens. During the past two decades, the molecular structures and functions of various defense components that participated in innate immune systems have been established in Arthropoda, such as, insects, the horseshoe crab, freshwater crayfish, and the protochordata ascidian. These defense molecules include phenoloxidases, clotting factors, complement factors, lectins, protease inhibitors, antimicrobial peptides, Toll receptors, and other humoral factors found mainly in hemolymph plasma and hemocytes. These components, which together compose the innate immune system, defend invertebrate from invading bacterial, fungal, and viral pathogens. This review describes the present status of our knowledge concerning such defensive molecules in invertebrates.

Overview of innate immunity in Drosophila

Drosophila protects itself from infection by microbial organisms by means of its pivotal defense, the so-called innate immunity system. This is its sole defense as it lacks an adaptive immunity system such as is found in mammals. The strong conservation of innate immunity systems in organisms from Drosophila to mammals, and the ease with which Drosophila can be manipulated genetically, makes this fly a good model system for investigating the mechanisms of virulence of a number of medically important pathogens. Potentially damaging endogenous and/or exogenous challenges sensed by specific receptors initiate signals via the Toll and/or Imd signaling pathways. These in turn activate the transcription factors Dorsal, Dorsal-related immune factor (Dif) and Relish, culminating in transcription of genes involved in the production of antimicrobial peptides, melanization, phagocytosis, and the cytoskeletal rearrangement required for appropriate responses. Clarifying the regulatory interactions between the various pathways involved is very important for understanding the specificity and termination mechanism of the immune response.

Peptidoglycan molecular requirements allowing detection by the Drosophila immune deficiency pathway

Innate immune recognition of microbes is a complex process that can be influenced by both the host and the microbe. Drosophila uses two distinct immune signaling pathways, the Toll and immune deficiency (Imd) pathways, to respond to different classes of microbes. The Toll pathway is predominantly activated by Gram-positive bacteria and fungi, while the Imd pathway is primarily activated by Gram-negative bacteria. Recent work has suggested that this differential activation is achieved through peptidoglycan recognition protein (PGRP)-mediated recognition of specific forms of peptidoglycan (PG). In this study, we have further analyzed the specific PG molecular requirements for Imd activation through the pattern recognition receptor PGRP-LC in both cultured cell line and in flies. We found that two signatures of Gram-negative PG, the presence of diaminopimelic acid in the peptide bridge and a 1,6-anhydro form of N-acetylmuramic acid in the glycan chain, allow discrimination between Gram-negative and Gram-positive bacteria. Our results also point to a role for PG oligomerization in Imd activation, and we demonstrate that elements of both the sugar backbone and the peptide bridge of PG are required for optimum recognition. Altogether, these results indicate multiple requirements for efficient PG-mediated activation of the Imd pathway and demonstrate that PG is a complex immune elicitor.

In vivo and in vitro interactions of the Bombyx mori chymotrypsin inhibitor b1 with Escherichia coli

Various chymotrypsin inhibitors occur in the hemolymph of silkworm larvae. Interaction of chymotrypsin inhibitor b1 (CI-b1) with Escherichia coli was examined from the viewpoint of action against invading bacteria. Injection of dead E. coli cells into larva reduced the CI-b1 content of the hemolymph, suggesting in vivo binding of CI-b1 to the outer membrane of the cell. Results from incubation of E. coli in cell-free hemolymph in the presence or absence of lipopolysaccharide indicated that CI-b1 is the only CI bound to E. coli and that it interacts with lipopolysaccharide. CI-b1 formed a complex with lipopolysaccharide in vitro; the value of the dissociation constant was relatively large. Inhibitory activity of CI-b1 changed insignificantly in mixture with lipopolysaccharide. CI-b1 affected the growth of E. coli but never worked lethally. CI-b1 is speculated to be a mediator that scavenges intruding bacteria rather than a direct anti-bacterial factor. This is the first report confirming that CI-b1 is a lipopolysaccharide binding protein.

Molecular cloning and characterization of an endo-1,3-beta-D-glucanase from the mollusk Spisula sachalinensis

cDNA encoding the endo-1,3-beta-d-glucanase from Spisula sachalinensis (LIV) was amplified by PCR using oligonucleotides deduced from the N-terminal end peptide sequence. Predicted enzyme structure consists of 444 amino acids with a signal sequence. The mature enzyme has 316 amino acids and its deduced amino acid sequence coincides completely with the N-terminal end (38 amino acids) of the beta-1,3-glucanase (LIV) isolated from the mollusk. The enzyme sequence from Val 121 to Met 441 reveals closest homology with Pacifastacus leniusculus lipopolysaccharide- and beta-1,3-glucan-binding protein and with coelomic cytolytic factors from Lumbricus terrestris. The mollusk glucanase also shows 36% identity and 56% similarity with beta-1,3-glucanase of the sea urchin Strongylocentrotus purpuratus. It is generally considered that invertebrate glucanase-like proteins containing the bacterial glucanase motif have evolved from an ancient beta-1,3-glucanase gene, but most of them lost their glucanase activity in the course of evolution and retained only the glucan-binding activity. A more detailed evaluation of the protein folding elicited very interesting relationships between the active site of LIV and other enzymes, which hydrolyze native glucans.

The immune response of Drosophila melanogaster

The response of the fruit fly Drosophila melanogaster to various microorganism infections relies on a multilayered defense. The epithelia constitute a first and efficient barrier. Innate immunity is activated when microorganisms succeed in entering the body cavity of the fly. Invading microorganisms are killed by the combined action of cellular and humoral processes. They are phagocytosed by specialized blood cells, surrounded by toxic melanin, or lysed by antibacterial peptides secreted into the hemolymph by fat body cells. During the last few years, research has focused on the mechanisms of microbial recognition by various pattern recognition receptors and of the subsequent induction of antimicrobial peptide expression. The cellular arm of the Drosophila innate immune system, which was somehow neglected, now constitutes the new frontier.

Comparative and functional genomics of the innate immune system in the malaria vector Anopheles gambiae

In much of Africa, the mosquito Anopheles gambiae is the major vector of human malaria, a devastating infectious disease caused by Plasmodium parasites. Vector and parasite interact at multiple stages and locations, and the nature and effectiveness of this reciprocal interaction determines the success of transmission. Many of the interactions engage the mosquito's innate immunity, a primitive but very effective defense system. In some cases, the mosquito kills the parasite, thus blocking the transmission cycle. However, not all interactions are antagonistic; some represent immune evasion. The sequence of the A. gambiae genome revealed numerous potential components of the innate immune system, and it established that they evolve rapidly, as summarized in the present review. Their rapid evolution by gene family expansion diversification as well as the prevalence of haplotype alleles in the best-studied families may reflect selective adaptation of the immune system to the exigencies of multiple immune challenges in a variety of ecologic niches. As a follow-up to the comparative genomic analysis, the development of functional genomic methodologies has provided novel opportunities for understanding the immune system and the nature of its interactions with the parasite. In this context, identification of both Plasmodium antagonists and protectors in the mosquito represents a significant conceptual advance. In addition to providing fundamental understanding of primitive immune systems, studies of mosquito interactions with the parasite open unprecedented opportunities for novel interventions against malaria transmission. The generation of transgenic mosquitoes that resist malaria infection in the wild and the development of antimalarial 'smart sprays' capable of disrupting interactions that are protective of the parasite, or reinforcing others that are antagonistic, represent technical challenges but also immense opportunities for improvement of global health.

Sensing infection in Drosophila: Toll and beyond

Drosophila has evolved a potent immune system that is somewhat adapted to the nature of infections through the selective activation of either one of two NF-kappa B-like signalling pathways, the Toll and IMD (Immune deficiency) pathways. In contrast to the mammalian system, the Toll receptor does not act as a pattern recognition receptor (PRR) but as a cytokine receptor. The sensing of microbial infections is achieved by at least four PRRs that belong to two distinct families: the peptidoglycan recognition proteins (PGRPs) and the Gram-negative binding proteins (GNBPs)/beta-glucan recognition proteins (beta GRPs).

Innate immunity in the malaria vector Anopheles gambiae:comparative and functional genomics

The resurgence of malaria is at least partly attributed to the absence of an effective vaccine, parasite resistance to antimalarial drugs and resistance to insecticides of the anopheline mosquito vectors. Novel strategies are needed to combat the disease on three fronts: protection (vaccines),prophylaxis/treatment (antimalarial drugs) and transmission blocking. The latter entails either killing the mosquitoes (insecticides), preventing mosquito biting (bednets and repellents), blocking parasite development in the vector (transmission blocking vaccines), genetic manipulation or chemical incapacitation of the vector. During the past decade, mosquito research has been energized by several breakthroughs, including the successful transformation of anopheline vectors, analysis of gene function by RNAi,genome-wide expression profiling using DNA microarrays and, most importantly,sequencing of the Anopheles gambiae genome. These breakthroughs helped unravel some of the mechanisms underlying the dynamic interactions between the parasite and the vector and shed light on the mosquito innate immune system as a set of potential targets to block parasite development. In this context, putative pattern recognition receptors of the mosquito that act as positive and negative regulators of parasite development have been identified recently. Characterizing these molecules and others of similar function, and identifying their ligands on the parasite surface, will provide clues on the nature of the interactions that define an efficient parasite–vector system and open up unprecedented opportunities to control the vectorial capacity of anopheline mosquitoes.

Innate Immunity in a Pyralid Moth

Invertebrates, like vertebrates, utilize pattern recognition proteins for detection of microbes and subsequent activation of innate immune responses. We report structural and functional properties of two domains from a beta-1,3-glucan recognition protein present in the hemolymph of a pyralid moth, Plodia interpunctella. A recombinant protein corresponding to the first 181 amino-terminal residues bound to beta-1,3-glucan, lipopolysaccharide, and lipoteichoic acid, polysaccharides found on cell surfaces of microorganisms, and also activated the prophenoloxidase-activating system, an immune response pathway in insects. The amino-terminal domain consists primarily of an alpha-helical secondary structure with a minor beta-structure. This domain was thermally stable and resisted proteolytic degradation. The 290 residue carboxyl-terminal domain, which is similar in sequence to glucanases, had less affinity for the polysaccharides, did not activate the prophenoloxidase cascade, had a more complicated CD spectrum, and was heat-labile and susceptible to proteinase digestion. The carboxyl-terminal domain bound to laminarin, a beta-1,3-glucan with beta-1,6 branches, but not to curdlan, a beta-1,3-glucan that lacks branching. These results indicate that the two domains of Plodia beta-1,3-glucan recognition protein, separated by a putative linker region, bind microbial polysaccharides with differing specificities and that the amino-terminal domain, which is unique to this class of pattern recognition receptors from invertebrates, is responsible for stimulating prophenoloxidase activation.

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.

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.

Proteomic Analysis of the Systemic Immune Response of Drosophila

Improvements in two-dimensional gel electrophoresis, mass spectrometry, and bioinformatics provide new tools to characterize proteins involved in a physiological process, such as the immune response of the insect model Drosophila melanogaster. Profiling of the proteins present in the hemolymph (insect blood) of noninfected flies versus flies infected with bacteria or fungi was performed by two-dimensional gel electrophoresis, silver or Coomassie staining, and image analysis. Through this differential analysis, more than 70 out of 160 spots were up- or down-regulated by at least 5-fold after microbial infection. Coomassie staining, in-gel digestion, and database searches yielded the identity of a series of proteins that are directly involved in the Drosophila immune system. This included proteases, protease inhibitors, and recognition molecules such as prophenoloxydase-activating enzymes, serpins, and Gram-negative binding protein-like. Proteins with a potential function in the immune response were also identified, such as an odorant binding protein, peptidylglycine alpha-hydroxylating monooxygenase, and transferrin, affording new candidates for further investigation of innate immune mechanisms. Moreover, several molecules resulting from the cleavage of proteins were detected after the fungal infection. Altogether, this first differential proteomic analysis of the immune response of Drosophila paves the way for the study of proteins affected during innate immunity.

Beta-1,3-glucan recognition protein-2 (betaGRP-2)from Manduca sexta; an acute-phase protein that binds beta-1,3-glucan and lipoteichoic acid to aggregate fungi and bacteria and stimulate prophenoloxidase activation

We have isolated and characterized a new beta-1,3-glucan recognition protein that is present in Manduca sexta cuticle and hemolymph. This 52 kDa protein, designated betaGRP-2, is 57% identical in sequence to betaGRP-1 from larval hymolymph of the same insect. BetaGRP-2 differs from betaGRP-1 in its absence of the naive larvae before the wandering stage begins. Transcription of the betaGRP-2 was up-regulated in larvae challenged with yeast or bacteria. BetaGRP-2 contains a region with sequence similarity to several glucanases but lacks glucanase activity. It aggregates yeasts and bacteria to, perhaps, limit the spread of invading cells and ensure a localized defense reaction. BetaGRP-2 binds laminarin and lipoteichoic acid, but not lipopolysaccharide. Laminarin-triggered prophenoloxidase activation was greatly enhanced in the induced larval hemolymph supplemented with purified betaGRP-2. Complementing other studies on pattern recognition molecules in M. sexta, these results indicate that a complex system of protein sensors is an integral component of the insect immune system and that different recognition molecules have overlapping binding specificity and functions.

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.

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.

cDNA cloning and expression of bacteria-induced Hdd11 gene from eri-silkworm, Samia cynthia ricini

A cDNA clone encoding Hdd11 protein, a bacteria-induced protein of unknown function, was isolated from fat body of immunized Samia cynthia ricini larvae based on suppression subtractive hybridization. The cDNA encodes a 167 amino acid residue open reading frame with an 18 residue predicted signal peptide. The deduced amino acid sequence showed 54% and 55% identity with Hdd11 proteins from Hyphantria cunea and Manduca sexta, respectively. Expression of the gene was undetectable in naive larvae when measured by Northern blot hybridization, but strongly induced in fat body after injecting bacterial cells or peptidoglycan into the larvae. The mRNA expression in the fat body was detected as early as 3 h post injection, reached peak levels at 12 h and continued for further 60 h at significant levels. The transcript was detected at very low levels in midgut, hemocytes and malpighian tubules.

Characterization and properties of a 1,3-beta-D-glucan pattern recognition protein of Tenebrio molitor larvae that is specifically degraded by serine protease during prophenoloxidase activation

Although many different pattern recognition receptors recognizing peptidoglycan and 1,3-beta-D-glucan have been identified in vertebrates and insects, the molecular mechanism of these molecules in the pattern recognition and subsequent signaling is largely unknown. To gain insights into the action mechanism of 1,3-beta-D-glucan pattern recognition protein in the insect prophenoloxidase (proPO) activation system, we purified a 53-kDa 1,3-beta-D-glucan recognition protein (Tm-GRP) to homogeneity from the hemolymph of the mealworm, Tenebrio molitor, by using a 1,3-beta-d-glucan affinity column. The purified protein specifically bound to 1,3-beta-D-glucan but not to peptidoglycan. Subsequent molecular cloning revealed that Tm-GRP contains a region with close sequence similarity to bacterial glucanases. Strikingly, two catalytically important residues in glucanases are replaced with other nonhomologous amino acids in Tm-GRP. The finding suggests that Tm-GRP has evolved from an ancestral gene of glucanases but retained only the ability to recognize 1,3-beta-D-glucan. A Western blot analysis of the protein level of endogenous Tm-GRP showed that the protein was specifically degraded following the activation of proPO with 1,3-beta-D-glucan and calcium ion. The degradation was significantly retarded by the addition of serine protease inhibitors but not by cysteine or acidic protease inhibitor. These results suggest that 1,3-beta-D-glucan pattern recognition protein is specifically degraded by serine protease(s) during proPO activation, and we propose that this degradation is an important regulatory mechanism of the activation of the proPO system.

Evidence for a LPS-binding protein in medfly hemocyte surface: mediation in LPS internalization but not in LPS signaling

A doublet of medfly hemocyte proteins with a molecular mass of about 55 and 50 kDa were precipitated with LPS. Antibodies raised against human CD14 recognize the same doublet of proteins. These results support that mammalian CD14 and the doublet of protein bands in medfly hemocytes share common epitopes. This doublet of protein bands is released from hemocytes upon LPS triggering. A portion of the released protein is clustered on the surface of a distinct hemocyte type and the other remains soluble. The membrane-bound LPS-binding protein is involved in LPS internalization and Escherichia coli phagocytosis but not in LPS signaling.

cDNA cloning, purification, properties, and function of a beta-1,3-glucan recognition protein from a pyralid moth, Plodia interpunctella

Microorganisms possess distinctive biochemical or molecular patterns on their cell surfaces, such as those formed by the lipopolysaccharides, lipoteichoic acids, and/or peptidoglycans of bacteria and the beta-1,3-glucans of fungi. Pattern recognition proteins that bind to these surface moieties have been implicated in the activation of the innate immune response in insects and other invertebrates. We report the purification and cloning of a cDNA for a 53-kDa beta-1,3-glucan recognition protein (betaGRP) from the Indianmeal moth, Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae). BetaGRP cDNA contains an open reading frame that encodes 488 amino acids, of which the first 17 residues comprise the secretion signal peptide. The calculated molecular mass of the 471-residue mature protein is 53,311 Da. The protein consists of a carboxyl-terminal domain that is similar to other recognition proteins from invertebrates, beta-1,3-glucanases from bacteria, and a beta-1,3-glucanase from the sea urchin, Strongylocentrotus purpuratus. The amino-terminus of betaGRP shares sequence similarity with other invertebrate recognition molecules and the beta-1,3-glucanase from S. purpuratus. Affinity purification of a 53-kDa protein and subsequent sequencing of a peptide produced by tryptic cleavage confirmed the presence of the betaGRP in P. interpunctella larval hemolymph. RT-PCR analysis indicates that betaGRP is constitutively expressed in all life-stages, with no detectable induction following exposure of wandering larvae to microbial elicitors. Northern blot analysis indicates that the 1.8-kb betaGRP transcript is transcribed within the fat body. Recombinant betaGRP retains beta-1,3-glucan-binding activity, binds to lipopolysaccharide and lipoteichoic acid in vitro, causes aggregation of microorganisms, and activates the prophenoloxidase cascade in the presence of soluble beta-1,3-glucan. These data support the hypothesis that the 53-kDa betaGRP functions to recognize pathogen surface molecules as nonself and subsequently activates insect innate immune responses.

Insect immunity and its implication in mosquito-malaria interactions

Insects' resistance to infectious agents is essential for their own survival and also for the health of the plant, animal and human populations with which they closely interact. Several of the major human diseases are spread by insects and are rapidly expanding as a result of the development of insecticide resistance in vectors and drug resistance in parasites. A vector insects' permissiveness to a pathogen, and hence the spread of the disease, will largely depend on the compatibility of the molecular interactions between the two species and the capability of the insect immune system to recognize and kill the pathogen. The innate immune system comprises a variety of components and mechanisms that can discriminate between different microorganisms and mount specific responses to control pathogenic infections. An impressive body of knowledge on the insects' innate immunity has been generated from studies in the model organism Drosophila. These studies are now guiding the exploration of the immune system in the vector mosquito of human malaria, Anopheles, and its implication in the elimination of parasites. Anopheles immune responses have been linked to parasite losses and some refractory mosquitoes can kill all parasites through specific defence mechanisms. The recently sequenced Drosophila and Anopheles genomes provide a detailed and comparative view on their immune gene repertoires that in combination with post-genomic analyses is used to further dissect the complex mechanisms of Plasmodium killing in the mosquito.

Implications of Time Bomb model of ookinete invasion of midgut cells

In this review, we describe the experimental observations that led us to propose the Time Bomb model of ookinete midgut invasion and discuss potential implications of this model when considering malaria transmission-blocking strategies aimed at arresting parasite development within midgut cells. A detailed analysis of the molecular interactions between Anopheles stephensi midgut epithelial cells and Plasmodium berghei parasites, as they migrate through midgut cells, revealed that ookinetes induce nitric oxide synthase (NOS) expression, remodeling of the actin cytoskeleton and characteristic morphological changes in the invaded epithelial cells. Parasites inflict extensive damage that ultimately leads to genome fragmentation and cell death. During their migration through the cytoplasm, ookinetes release a subtilisin-like protease (PbSub2) and the surface protein (Pbs21). The model proposes that ookinetes must escape rapidly from the invaded cells, as the responses mediating cell death could be potentially lethal to the parasites. In other words, the physical and/or chemical damage triggered by the parasite can be thought of as a 'lethal bomb'. Once this cascade of events is initiated, the parasite must leave the cellular compartment within a limited time to escape unharmed from the 'bomb' it has activated. The midgut epithelium has the ability to heal rapidly by 'budding off' the damaged cells to the midgut lumen without losing its integrity.

Pattern recognition proteins in Manduca sexta plasma

Recognition of nonself is the first step in mounting immune responses. In the innate immune systems of both vertebrates and arthropods, such recognition, termed pattern recognition, is mediated by a group of proteins, known as pattern recognition proteins or receptors. Different pattern recognition proteins recognize and bind to molecules (molecular patterns) present on the surface of microorganisms but absent from animals. These molecular patterns include microbial cell wall components such as bacterial lipopolysaccharide, lipoteichoic acid and peptidoglycan, and fungal beta-1,3-glucans. Binding of pattern recognition proteins to these molecular patterns triggers responses such as phagocytosis, nodule formation, encapsulation, activation of proteinase cascades, and synthesis of antimicrobial peptides. In this article, we describe four classes of pattern recognition proteins, hemolin, peptidoglycan recognition protein, beta-1,3-glucan recognition proteins, and immulectins (C-type lectins) involved in immune responses of the tobacco hornworm, Manduca sexta.

A beta-1,3-glucan binding protein from the black tiger shrimp, Penaeus monodon

A beta-1,3-glucan binding protein (GBP) has been isolated from a shrimp hemocyte cDNA library. Its open reading frame consists of 1314 nucleotides with a polyadenylated sequence and a poly A tail. It encodes a polypeptide of 370 amino acids including a 17 amino acid-signal peptide. The mature protein has an estimated molecular mass of 39.5 kDa and a predicted pI of 5.5. Sequence comparison shows a high degree of similarity to invertebrate recognition proteins with glucanase-like domains for example, the lipopolysaccharide- and beta-1,3-glucan-binding protein (LGBP) from the freshwater crayfish, Pacifastacus leniusculus, coelomic cytolytic factor-1 from the earthworm, Eisenia foetida and the Gram negative bacteria binding protein from the mosquito, Anopheles gambiae as well as to sea urchin beta-1,3-glucanases and bacterial beta-1,3-glucanases or beta-1,3-, 1,4-glucanases. Northern blot analysis showed that the shrimp protein is constitutively expressed in hemocytes. Animals injected with curdlan or heat-killed bacterial cell of Vibrio harveyi, a shrimp pathogen, showed no significant change in the mRNA expression profile within 12h post-injection. After incubation of shrimp hemocyte lysate supernatant (HLS) with curdlan or zymosan, a protein with a molecular mass of 31 kDa was eluted from the incubated curdlan or zymosan, and, by immunoblotting, this 31-kDa band could be detected by an affinity-purified anti-crayfish LGBP antibody. In contrast, incubation of shrimp HLS with LPS showed no any reactive band detected on SDS-PAGE or by immunoblotting suggesting that the binding is specific for beta-1,3-glucan.

A genome-wide analysis of immune responses in Drosophila

Oligonucleotide DNA microarrays were used for a genome-wide analysis of immune-challenged Drosophila infected with Gram-positive or Gram-negative bacteria, or with fungi. Aside from the expression of an established set of immune defense genes, a significant number of previously unseen immune-induced genes were found. Genes of particular interest include corin- and Stubble-like genes, both of which have a type II transmembrane domain; easter- and snake-like genes, which may fulfil the roles of easter and snake in the Toll pathway; and a masquerade-like gene, potentially involved in enzyme regulation. The microarray data has also helped to greatly reduce the number of target genes in large gene groups, such as the proteases, helping to direct the choices for future mutant studies. Many of the up-regulated genes fit into the current conceptual framework of host defense, whereas others, including the substantial number of genes with unknown functions, offer new avenues for research.

Distinct carbohydrate recognition domains of an invertebrate defense molecule recognize Gram-negative and Gram-positive bacteria

Coelomic fluid of Eisenia foetida earthworms (Oligochaeta, Annelida) contains a 42-kDa defense molecule named CCF for coelomic cytolytic factor. By binding microbial antigens, namely the O-antigen of lipopolysaccharide (LPS), beta-1,3-glucans, or N,N'-diacetylchitobiose present, respectively, on Gram-negative bacteria or yeast cell walls, CCF triggers the prophenoloxidase activating pathway. We report that CCF recognizes lysozyme-predigested Gram-positive bacteria or the peptidoglycan constituent muramyl dipeptide as well as muramic acid. To identify the pattern recognition domains of CCF, deletion mutants were tested for their ability to reconstitute the prophenoloxidase cascade in E. foetida coelomic fluid depleted of endogenous CCF in the presence of LPS, beta-1,3-glucans, N,N'-diacetylchitobiose, and muramic acid. In addition, affinity chromatography of CCF peptides was performed on immobilized beta-1,3-glucans or N,N'-diacetylchitobiose. We found that the broad specificity of CCF for pathogen-associated molecular patterns results from the presence of two distinct pattern recognition domains. One domain, which shows homology with the polysaccharide and glucanase motifs of beta-1,3-glucanases and invertebrate defense molecules located in the central part of the CCF polypeptide chain, interacts with LPS and beta-1,3-glucans. The C-terminal tryptophan-rich domain mediates interactions of CCF with N,N'-diacetylchitobiose and muramic acid. These data provide evidence for the presence of spatially distinct carbohydrate recognition domains within this invertebrate defense molecule.

Drosophila scavenger receptor CI is a pattern recognition receptor for bacteria

One hallmark of innate immunity apparently conserved from primitive life forms through to humans is the ability of the host to recognize pathogen-associated molecular patterns (PAMPs). Since macrophage pattern recognition receptors are not well defined in Drosophila, we set out to identify such receptors. Our findings reveal that Drosophila macrophages express multiple pattern recognition receptors and that the Drosophila scavenger receptor, dSR-CI, is one such receptor capable of recognizing both gram-negative and gram-positive bacteria, but not yeast. Our data indicate that scavenger receptor bacterial recognition is conserved from insects to humans and may represent one of the most primitive forms of microbial recognition.

Genome-wide analysis of the Drosophila immune response by using oligonucleotide microarrays

To identify new Drosophila genes involved in the immune response, we monitored the gene expression profile of adult flies in response to microbial infection by using high-density oligonucleotide microarrays encompassing nearly the full Drosophila genome. Of 13,197 genes tested, we have characterized 230 induced and 170 repressed by microbial infection, most of which had not previously been associated with the immune response. Many of these genes can be assigned to specific aspects of the immune response, including recognition, phagocytosis, coagulation, melanization, activation of NF-kappaB transcription factors, synthesis of antimicrobial peptides, production of reactive oxygen species, and regulation of iron metabolism. Additionally, we found a large number of genes with unknown function that may be involved in control and execution of the immune response. Determining the function of these genes represents an important challenge for improving our knowledge of innate immunity. Complete results may be found at http://www.fruitfly.org/expression/immunity/.

The evolution and genetics of innate immunity

The immune system provides protection from a wide range of pathogens. One component of immunity, the phylogenetically ancient innate immune response, fights infections from the moment of first contact and is the fundamental defensive weapon of multicellular organisms. The Toll family of receptors has a crucial role in immune defence. Studies in fruitflies and in mammals reveal that the defensive strategies of invertebrates and vertebrates are highly conserved at the molecular level, which raises the exciting prospects of an increased understanding of innate immunity.

Innate immune defense against malaria infection in the mosquito

Anopheles gambiae, the most important vector of malaria, employs its innate immune system in the fight against Plasmodium. This can affect the propagative capacity of Plasmodium in the vector and, in some cases, leads to total refractoriness to the parasite. The components operating in the mosquito's innate immune system and their potential relevance to antimalarial responses are being systematically dissected.

A family of peptidoglycan recognition proteins in the fruit fly Drosophila melanogaster

Peptidoglycans from bacterial cell walls trigger immune responses in insects and mammals. A peptidoglycan recognition protein, PGRP, has been cloned from moths as well as vertebrates and has been shown to participate in peptidoglycan-mediated activation of prophenoloxidase in the silk moth. Here we report that Drosophila expresses 12 PGRP genes, distributed in 8 chromosomal loci on the 3 major chromosomes. By analyzing cDNA clones and genomic databases, we grouped them into two classes: PGRP-SA, SB1, SB2, SC1A, SC1B, SC2, and SD, with short transcripts and short 5'-untranslated regions; and PGRP-LA, LB, LC, LD, and LE, with long transcripts and long 5'-untranslated regions. The predicted structures indicate that the first group encodes extracellular proteins and the second group, intracellular and membrane-spanning proteins. Most PGRP genes are expressed in all postembryonic stages. Peptidoglycan injections strongly induce five of the genes. Transcripts from the different PGRP genes were found in immune competent organs such as fat body, gut, and hemocytes. We demonstrate that at least PGRP-SA and SC1B can bind peptidoglycan, and a function in immunity is likely for this family.

Gram-negative bacteria-binding protein, a pattern recognition receptor for lipopolysaccharide and beta-1,3-glucan that mediates the signaling for the induction of innate immune genes in Drosophila melanogaster cells

Pattern recognition receptors, non-clonal immune proteins recognizing common microbial components, are critical for non-self recognition and the subsequent induction of Rel/NF-kappaB-controlled innate immune genes. However, the molecular identities of such receptors are still obscure. Here, we present data showing that Drosophila possesses at least three cDNAs encoding members of the Gram-negative bacteria-binding protein (DGNBP) family, one of which, DGNBP-1, has been characterized. Western blot, flow cytometric, and confocal laser microscopic analyses demonstrate that DGNBP-1 exists in both a soluble and a glycosylphosphatidylinositol-anchored membrane form in culture medium supernatant and on Drosophila immunocompetent cells, respectively. DGNBP-1 has a high affinity to microbial immune elicitors such as lipopolysaccharide (LPS) and beta-1,3-glucan whereas no binding affinity is detected with peptidoglycan, beta-1,4-glucan, or chitin. Importantly, the overexpression of DGNBP-1 in Drosophila immunocompetent cells enhances LPS- and beta-1,3-glucan-induced innate immune gene (NF-kappaB-dependent antimicrobial peptide gene) expression, which can be specifically blocked by pretreatment with anti-DGNBP-1 antibody. These results suggest that DGNBP-1 functions as a pattern recognition receptor for LPS from Gram-negative bacteria and beta-1, 3-glucan from fungi and plays an important role in non-self recognition and the subsequent immune signal transmission for the induction of antimicrobial peptide genes in the Drosophila innate immune system.

Vibrio fischeri lipopolysaccharide induces developmental apoptosis, but not complete morphogenesis, of the Euprymna scolopes symbiotic light organ

During initiation of the association between the squid host Euprymna scolopes and its bacterial partner Vibrio fischeri, the bacteria induce dramatic morphogenesis of the host symbiotic organ, a portion of which involves the signaling of widespread apoptosis of the cells in a superficial ciliated epithelium on the colonized organ. In this study, we investigated the role in this process of lipopolysaccharide (LPS), a bacterial cell-surface molecule implicated in the induction of animal cell apoptosis in other systems. Purified V. fischeri LPS, as well as the LPS of V. cholerae, Haemophilus influenzae, Escherichia coli, and Shigella flexneri, added in the concentration range of pg/ml to ng/ml, induced apoptosis in epithelial cells 10- to 100-fold above background levels. The absence of species specificity suggested that the conserved lipid A portion of the LPS was the responsible component of the LPS molecule. Lipid A from V. fischeri, E. coli, or S. flexneri induced apoptosis. In addition, strains of H. influenzae carrying a mutation in the htrB gene, which is involved in the synthesis of virulent lipid A, showed a diminished ability to induce apoptosis of host cells. Confocal microscopy using fluorescently labeled LPS indicated that the LPS behaves similar to intact bacterial symbionts, interacting with host cells in the internal crypt spaces and not directly with the superficial epithelium. Although LPS was able to induce apoptosis, it did not induce the full morphogenesis of the ciliated surface, suggesting that multiple signals are necessary to mediate the development of this animal-bacterial mutualism.

Genes that fight infection: what the Drosophila genome says about animal immunity

From deciphering the principles of heredity to identifying the genes that control development, the fruit fly Drosophila melanogaster is being used to deconstruct an increasing number of biological processes. Genetic studies of Drosophila responses to microbial infection have identified regulators of innate immunity that are functionally conserved in mammals. These recent findings highlight the ancient origins of animal immune responses and demonstrate the potential of Drosophila for dissecting host-pathogen interactions. The sequencing of the Drosophila genome both enhances genetic approaches and provides new clues for the identification of key components of innate immunity. This article summarizes how information gained from genomic analysis contributes to our understanding of how animals cope with infectious disease.

Mosquito immune responses and malaria transmission: lessons from insect model systems and implications for vertebrate innate immunity and vaccine development

The introduction of novel biochemical, genetic, molecular and cell biology tools to the study of insect immunity has generated an information explosion in recent years. Due to the biodiversity of insects, complementary model systems have been developed. The conceptual framework built based on these systems is used to discuss our current understanding of mosquito immune responses and their implications for malaria transmission. The areas of insect and vertebrate innate immunity are merging as new information confirms the remarkable extent of the evolutionary conservation, at a molecular level, in the signaling pathways mediating these responses in such distant species. Our current understanding of the molecular language that allows the vertebrate innate immune system to identify parasites, such as malaria, and direct the acquired immune system to mount a protective immune response is very limited. Insect vectors of parasitic diseases, such as mosquitoes, could represent excellent models to understand the molecular responses of epithelial cells to parasite invasion. This information could broaden our understanding of vertebrate responses to parasitic infection and could have extensive implications for anti-malarial vaccine development.

Comparative genomics of the eukaryotes

A comparative analysis of the genomes of Drosophila melanogaster, Caenorhabditis elegans, and Saccharomyces cerevisiae-and the proteins they are predicted to encode-was undertaken in the context of cellular, developmental, and evolutionary processes. The nonredundant protein sets of flies and worms are similar in size and are only twice that of yeast, but different gene families are expanded in each genome, and the multidomain proteins and signaling pathways of the fly and worm are far more complex than those of yeast. The fly has orthologs to 177 of the 289 human disease genes examined and provides the foundation for rapid analysis of some of the basic processes involved in human disease.

A beta1,3-glucan recognition protein from an insect, Manduca sexta, agglutinates microorganisms and activates the phenoloxidase cascade

Pattern recognition proteins function in innate immune responses by binding to molecules on the surface of invading pathogens and initiating host defense reactions. We report the purification and molecular cloning of a cDNA for a 53-kDa beta1,3-glucan-recognition protein from the tobacco hornworm, Manduca sexta. This protein is constitutively expressed in fat body and secreted into hemolymph. The protein contains a region with sequence similarity to several glucanases, but it lacks glucanase activity. It binds to the surface of and agglutinates yeast, as well as gram-negative and gram-positive bacteria. Beta1,3-glucan-recognition protein in the presence of laminarin, a soluble glucan, stimulated activation of prophenoloxidase in plasma, whereas laminarin alone did not. These results suggest that beta1,3-glucan-recognition protein serves as a pattern recognition molecule for beta1,3-glucan on the surface of fungal cell walls. After binding to beta1,3-glucan, the protein may interact with a serine protease, leading to the activation of the prophenoloxidase cascade, a pathway in insects for defense against microbial infection.

A pattern-recognition protein for beta-1,3-glucan. The binding domain and the cDNA cloning of beta-1,3-glucan recognition protein from the silkworm, Bombyx mori

The beta-1,3-glucan recognition protein (betaGRP) has strong specific affinity for beta-1,3-glucan, a component of the fungal cell wall. Its interaction with beta-1,3-glucan initiates the activation of the prophenoloxidase cascade, which is an important defense system in invertebrates of many species. We cloned the cDNA of the betaGRP of the silkworm Bombyx mori. The betaGRP mRNA transcript was constitutively expressed in the hemocytes, fat body, and epithelial cells of the naive silkworm. At the same time, a bacterial or yeast challenge was indicated to intensify the transcription. Comparison of the deduced amino acid sequence with known sequences revealed that the betaGRP contained a region (Thr(264) to Pro(386)) displaying significant similarity to the catalytic regions of bacterial beta-1,3-glucanases and much higher similarity to the glucanase-like regions of Gram-negative bacteria-binding proteins found in the silkworm B. mori and the mosquito Anopheles gambiae. The region (Thr(264) to Pro(386)) of the betaGRP, however, was demonstrated not to have appreciable affinity for beta-1,3-glucan. A recombinant peptide corresponding to an N-terminal region (Tyr(1) to Ala(102)) of the betaGRP bound strongly to beta-1,3-glucan. These results indicate that the binding domain of the betaGRP for beta-1,3-glucan is located in the N-terminal region. Glucanases and the current pattern-recognition proteins that contain a glucanase-like region seem to have a common origin in their molecular evolution.

A lipopolysaccharide- and beta-1,3-glucan-binding protein from hemocytes of the freshwater crayfish Pacifastacus leniusculus. Purification, characterization, and cDNA cloning

A lipopolysaccharide- and beta-1,3-glucan-binding protein (LGBP) was isolated and characterized from blood cells (hemocytes) of the freshwater crayfish Pacifastacus leniusculus. The LGBP was purified by chromatography on Blue-Sepharose and phenyl-Sepharose, followed by Sephacryl S-200. The LGBP has a molecular mass of 36 kDa and 40 kDa on 10% SDS-polyacrylamide gel electrophoresis under reducing and nonreducing conditions, respectively. The calculated mass of LGBP is 39,492 Da, which corresponds to the native size of LGBP; the estimated pI of the mature LGBP is 5.80. LGBP has binding activity to lipopolysaccharides as well as to beta-1,3-glucans such as laminarin and curdlan, but peptidoglycan could not bind to LGBP. Cloning and sequencing of LGBP showed significant homology with several putative Gram-negative bacteria-binding proteins and beta-1, 3-glucanases. Interestingly, LGBP also has a structure and functions similar to those of the coelomic cytolytic factor-1, a lipopolysaccharide- and glucan-binding protein from the earthworm Eisenia foetida. To evaluate the involvement of LGBP in the prophenoloxidase (proPO) activating system, a polyclonal antibody against LGBP was made and used for the inhibition of phenoloxidase (PO) activity triggered by the beta-1,3-glucan laminarin in the hemocyte lysate of crayfish. The PO activity was blocked completely by the anti-LGBP antibody. Moreover, the PO activity could be recovered by the addition of purified LGBP. These results suggest that the 36-kDa LGBP plays a role in the activation of the proPO activating system in crayfish and thus seems to play an important role in the innate immune system of crayfish.

Antimicrobial defense of the earthworm

Discrimination of self and nonself is one of the features of all animal species but the ways of elimination of nonself are different. Defense strategies of invertebrates, which lack antibodies and lymphocytes, are based on innate defense mechanisms. The study of such, undoubtedly less complex, defense mechanisms in invertebrates may shed a new light on the more sophisticated immunity of vertebrates. The main aim of this review is to show on one experimental model--an oligochaete annelid--cellular and humoral defense pathways protecting against microbial infection.

Purification and characterization of chitin-binding proteins from the hemolymph of sweet potato hornworm, Agrius convolvuli

Three chitin-binding proteins (CBPs: CBP9, CBP15, CBP66) were identified from the larval hemolymph of sweet potato hornworm, Agrius convolvuli. Two (CBP9 and CBP15) of them have been isolated and purified by gel filtration (Superdex HR 75), cation-exchange chromatography (Mono S), and reverse-phase chromatography (muRPC PC 2.1/3). In experiments to detect CBPs in hemolymph, we examined whether ionic strength and existence of bovine serum albumin in the incubation solution influenced binding affinity of CBPs to chitin. The N-terminal sequences of three CBPs were determined by the automated Edman degradation and showed the sequence homology in basic local alignment search tool search CBP15 and CBP66 were quite similar to lysozymes and bovine serum albumins, respectively. In contrast, CBP9 is not similar to any other known protein, as judged from databank comparisons. Therefore, we concluded that CBP9 is a novel protein with binding capacity to chitin that is a component of the fungal cell wall. CBP9 has no antibacterial activity against Escherichia coli and Micrococcus luteus, and also showed negative response in hemagglutination assay. CBP9 is confirmed as a monomer with a molecular mass of 9.14 kDa by electron spray ionization and matrix-assisted laser desorption ionization mass spectrometry.

Lipopolysaccharide-binding protein of Bombyx mori participates in a hemocyte-mediated defense reaction against gram-negative bacteria

BmLBP is a lipopolysaccharide-binding protein in B. mori and participates in bacterial clearance in vivo. Here, we investigated the function of BmLBP more specifically. More than 90% of injected gram-negative rough strains to which BmLBP binds were removed from the plasma within 30 min post-injection, whereas it required 8h for the clearance of smooth strains to which BmLBP does not bind. Observation of the hemocoel after the injection of Escherichia coli rough strain showed that melanized nodules were formed at 30 min post-injection when the clearance of injected E. coli cells had occurred. Fluorescence microscope observation revealed that E. coli cells were actually trapped in the nodules formed in vivo. Furthermore, plasma pre-treated E. coli rough cells (BmLBP bound) added to hemocytes isolated in vitro caused vigorous hemocyte aggregations with the bacteria, while plasma pre-treated smooth cells did not. The formation of aggregates was inhibited by anti-BmLBP serum pre-treatment, suggesting that BmLBP causes the clearance of bacteria by promoting hemocyte nodule formation.

Induction and regulation of antimicrobial peptides in Drosophila

Activation of the innate immune response involves recognition of the infectious agent and the subsequent activation of cellular and humoral reactions. In insects, a number of immunity genes are activated at the level of transcription leading to the synthesis of antimicrobial peptides. Genetic analyses in Drosophila have identified several signal transduction pathways that promote activation of these immunity genes. Recent data suggest that the insect immune system is able to discriminate between a bacterial and a fungal infection, and responds by higher levels of activation of the appropriate peptides to repel the infection. These and other recent data on transcription factors and regulation of antimicrobial genes are integrated into a model to suggest how differential activation of antifungal and antibacterial peptides can occur in response to fungal and bacterial infection.