Cecropia immunoresponsive factor, an insect immunoresponsive factor with DNA-binding properties similar to nuclear-factor kappa B

Secreted C-type lectin regulation of neuromuscular junction synaptic vesicle dynamics modulates coordinated movement

The synaptic cleft manifests enriched glycosylation, with structured glycans coordinating signaling between presynaptic and postsynaptic cells. Glycosylated signaling ligands orchestrating communication are tightly regulated by secreted glycan-binding lectins. Using the Drosophila neuromuscular junction (NMJ) as a model glutamatergic synapse, we identify a new Ca2+-binding (C-type) lectin, Lectin-galC1 (LGC1), which modulates presynaptic function and neurotransmission strength. We find that LGC1 is enriched in motoneuron presynaptic boutons and secreted into the NMJ extracellular synaptomatrix. We show that LGC1 limits locomotor peristalsis and coordinated movement speed, with a specific requirement for synaptic function, but not NMJ architecture. LGC1 controls neurotransmission strength by limiting presynaptic active zone (AZ) and postsynaptic glutamate receptor (GluR) aligned synapse number, reducing both spontaneous and stimulation-evoked synaptic vesicle (SV) release, and capping SV cycling rate. During high-frequency stimulation (HFS), mutants have faster synaptic depression and impaired recovery while replenishing depleted SV pools. Although LGC1 removal increases the number of glutamatergic synapses, we find that LGC1-null mutants exhibit decreased SV density within presynaptic boutons, particularly SV pools at presynaptic active zones. Thus, LGC1 regulates NMJ neurotransmission to modulate coordinated movement.

Fungal immunity and pathogenesis in mammals versus the invertebrate model organism Galleria mellonella

In recent decades, Galleria mellonella (Lepidoptera: Pyralidae) have emerged as a model system to explore experimental aspects of fungal pathogenesis. The benefits of the G. mellonella model include being faster, cheaper, higher throughput and easier compared with vertebrate models. Additionally, as invertebrates, their use is subject to fewer ethical and regulatory issues. However, for G. mellonella models to provide meaningful insight into fungal pathogenesis, the G. mellonella-fungal interactions must be comparable to mammalian-fungal interactions. Indeed, as discussed in the review, studies suggest that G. mellonella and mammalian immune systems share many similarities, and fungal virulence factors show conserved functions in both hosts. While the moth model has opened novel research areas, many comparisons are superficial and leave large gaps of knowledge that need to be addressed concerning specific mechanisms underlying G. mellonella-fungal interactions. Closing these gaps in understanding will strengthen G. mellonella as a model for fungal virulence in the upcoming years. In this review, we provide comprehensive comparisons between fungal pathogenesis in mammals and G. mellonella from immunological and virulence perspectives. When information on an antifungal immune component is unknown in G. mellonella, we include findings from other well-studied Lepidoptera. We hope that by outlining this information available in related species, we highlight areas of needed research and provide a framework for understanding G. mellonella immunity and fungal interactions.

Towards a paradigm shift in innate immunity-seminal work by Hans G. Boman and co-workers

Four decades ago, immunological research was dominated by the field of lymphoid biology. It was commonly accepted that multicellular eukaryotes defend themselves through phagocytosis. The lack of lymphoid cells in insects and other simpler animals, however, led to the common notion that they might simply lack the capacity defend themselves with humoral factors. This view was challenged by microbiologist Hans G. Boman and co-workers in a series of publications that led to the advent of antimicrobial peptides as a universal arm of the immune system. Besides ingenious research, Boman ignited his work by posing the right questions. He started off by asking himself a simple question: 'Antibodies take weeks to produce while many microbes divide hourly; so how come we stay healthy?'. This led to two key findings in the field: the discovery of an inducible and highly potent antimicrobial immune response in Drosophila in 1972, followed by the characterization of cecropin in 1981. Despite broadly being considered an insect-specific response at first, the work of Boman and co-workers eventually created a bandwagon effect that unravelled various aspects of innate immunity.This article is part of the themed issue 'Evolutionary ecology of arthropod antimicrobial peptides'.

Overview of Drosophila immunity: a historical perspective

The functional analysis of genes from the model organism Drosophila melanogaster has provided invaluable information for many cellular and developmental or physiological processes, including immunity. The best-understood aspect of Drosophila immunity is the inducible humoral response, first recognized in 1972. This pioneering work led to a remarkable series of findings over the next 30 years, ranging from the identification and characterization of the antimicrobial peptides produced, to the deciphering of the signalling pathways activating the genes that encode them and, ultimately, to the discovery of the receptors sensing infection. These studies on an insect model coincided with a revival of the field of innate immunity, and had an unanticipated impact on the biomedical field.

WITHDRAWN: Overview of Drosophila immunity: A historical perspective

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The fruit fly Drosophila melanogaster unfolds the secrets of innate immunity

In 2011, the Nobel Prize in Physiology and Medicine was rewarded, in part, for research on the Drosophila immune response. The research described the role of the Drosophila Toll receptor in antifungal resistance, and the subsequent characterization of Toll-like receptors in mammals reshaped our understanding of the immune system. This review summarizes the potential of the Drosophila model and describes the path that has lead Drosophila to become an important model to study immunity.

CONCLUSION

Drosophila melanogaster has been one of the most fruitful models to study innate immunity.

NF-κB: where did it come from and why?

The vast majority of research on nuclear factor κB (NF-κB) signaling in the past 25 years has focused on its roles in normal and disease-related processes in vertebrates, especially mice and humans. Recent genome and transcriptome sequencing efforts have shown that homologs of NF-κB transcription factors, inhibitor of NF-κB (IκB) proteins, and IκB kinases are present in a variety of invertebrates, including several in phyla simpler than Arthropoda, the phylum containing insects such Drosophila. Moreover, many invertebrates also contain genes encoding homologs of upstream signaling proteins in the Toll-like receptor signaling pathway, which is well-known for its downstream activation of NF-κB for innate immunity. This review describes what we now know or can infer and speculate about the evolution of the core elements of NF-κB signaling as well as the biological processes controlled by NF-κB in invertebrates. Further research on NF-κB in invertebrates is likely to uncover information about the evolutionary origins of this key human signaling pathway and may have relevance to our management of the responses of ecologically and economically important organisms to environmental and adaptive pressures.

Peli: a family of signal-responsive E3 ubiquitin ligases mediating TLR signaling and T-cell tolerance

E3 ubiquitin ligases play a crucial role in regulating immune receptor signaling and modulating immune homeostasis and activation. One emerging family of such E3s is the Pelle-interacting (Peli) proteins, characterized by the presence of a cryptic forkhead-associated domain involved in substrate binding and an atypical RING domain mediating formation of both lysine (K) 63- and K48-linked polyubiquitin chains. A well-recognized function of Peli family members is participation in the signal transduction mediated by Toll-like receptors (TLRs) and IL-1 receptor. Recent gene targeting studies have provided important insights into the in vivo functions of Peli1 in the regulation of TLR signaling and inflammation. These studies have also extended the biological functions of Peli1 to the regulation of T-cell tolerance. Consistent with its immunoregulatory functions, Peli1 responds to different immune stimuli for its gene expression and catalytic activation. In this review, we discuss the recent progress, as well as the historical perspectives in the regulation and biological functions of Peli.

The noncanonical NF-κB pathway

The noncanonical nuclear factor-κB (NF-κB) signaling pathway mediates activation of the p52/RelB NF-κB complex and, thereby, regulates specific immunological processes. This NF-κB pathway relies on the inducible processing of NF-κB2 precursor protein, p100, as opposed to the degradation of IκBα in the canonical NF-κB pathway. A central signaling component of the noncanonical NF-κB pathway is NF-κB-inducing kinase (NIK), which functions together with a downstream kinase, IKKα (inhibitor of NF-κB kinase α), to induce phosphorylation-dependent ubiquitination and processing of p100. Under normal conditions, NIK is targeted for continuous degradation by a tumor necrosis factor (TNF) receptor-associated factor-3 (TRAF3)-dependent E3 ubiquitin ligase. In response to signals mediated by a subset of TNF receptor superfamily members, NIK becomes stabilized as a result of TRAF3 degradation, leading to the activation of noncanonical NF-κB. This review discusses both the historical perspectives and the recent progress in the regulation and biological function of the noncanonical NF-κB pathway.

The Drosophila Toll signaling pathway

The identification of the Drosophila melanogaster Toll pathway cascade and the subsequent characterization of TLRs have reshaped our understanding of the immune system. Ever since, Drosophila NF-κB signaling has been actively studied. In flies, the Toll receptors are essential for embryonic development and immunity. In total, nine Toll receptors are encoded in the Drosophila genome, including the Toll pathway receptor Toll. The induction of the Toll pathway by gram-positive bacteria or fungi leads to the activation of cellular immunity as well as the systemic production of certain antimicrobial peptides. The Toll receptor is activated when the proteolytically cleaved ligand Spatzle binds to the receptor, eventually leading to the activation of the NF-κB factors Dorsal-related immunity factor or Dorsal. In this study, we review the current literature on the Toll pathway and compare the Drosophila and mammalian NF-κB pathways.

Hemolin-A lepidopteran anti-viral defense factor?

Immunity in insects has largely focused on responses towards bacteria and fungi, but recently the study of immune responses against viral infections has also received attention. In Lepidoptera, phagocytosis and encapsulation mediated by hemocytes, and apoptosis are part of the response against virus infection; however, many studies also suggest the presence of unknown factors involved in the anti-viral defense. An up-regulation of the lepidopteran-specific pattern recognition protein Hemolin after baculovirus infection in the Chinese oak silkmoth and discovery of putative virus responsive elements in the up-stream regions of Hemolin in the Cecropia moth and the Tobacco horn worm could suggest that Hemolin is involved in virus defense. In this paper, a number of studies investigating baculovirus pathogenesis, and others analyzing Hemolin expression have been revisited leading to the speculation that Hemolin could be engaged in several anti-viral processes.

Drosophila as a model system for antibacterial peptides

As a defence against bacteria, infected insects synthesize cecropins and a large number of other bactericidal proteins and peptides. To understand this response and its possible relationship with similar systems in mammals, we need to characterize the induced components and how they act, as well as how this antibacterial response is initiated. To study the molecular basis for this response we cloned the genes for cecropins and other bactericidal peptides from Drosophila, 14 genes in total. The cecropin genes were selected as convenient markers for the immune response because they are strongly induced by different microbial substances. In contrast the lysozyme gene family is constitutively expressed in the digestive tract. We have developed an inducible blood cell line from Drosophila for studying the immune response in vitro. Using this system we are now investigating the function of membrane proteins and signal pathways in the transcriptional activation of immune genes in Drosophila.

Participation of a galactose-specific C-type lectin in Drosophila immunity

A galactose-specific C-type lectin has been purified from a pupal extract of Drosophila melanogaster. This lectin gene, named DL1 (Drosophila lectin 1), is part of a gene cluster with the other two galactose-specific C-type lectin genes, named DL2 (Drosophila lectin 2) and DL3 (Drosophila lectin 3). These three genes are expressed differentially in fruit fly, but show similar haemagglutinating activities. The present study characterized the biochemical and biological properties of the DL1 protein. The recombinant DL1 protein bound to Escherichia coli and Erwinia chrysanthemi, but not to other Gram-negative or any other kinds of microbial strains that have been investigated. In addition, DL1 agglutinated E. coli and markedly intensified the association of a Drosophila haemocytes-derived cell line with E. coli. For in vivo genetic analysis of the lectin genes, we also established a null-mutant Drosophila. The induction of inducible antibacterial peptide genes was not impaired in the DL1 mutant, suggesting that the galactose-specific C-type lectin does not participate in the induction of antibacterial peptides, but possibly participates in the immune response via the haemocyte-mediated mechanism.

20-Hydroxyecdysone indirectly regulates Hemolin gene expression in Hyalophora cecropia

Development and innate immune defence are two vital processes that have been demonstrated to use the same or similar molecules and signalling pathways in insects. Hemolin is a moth haemolymph protein belonging to the immunoglobulin superfamily. It is strongly induced upon bacterial infection. However, recent studies indicate a developmental regulation of hemolin. We show that the steroid hormone 20-hydroxyecdysone (20E) can activate the expression of Hyalophora cecropia Hemolin (HcHemolin) in the fat body of diapausing pupae. Using the protein synthesis inhibitor cycloheximide we demonstrate that Hemolin up-regulation by 20E requires ongoing protein synthesis. Moreover, 20E enhances transcription of the Hemolin gene in response to bacteria. Comparing the upstream regions of Manduca sexta Hemolin (MsHemolin) and HcHemolin, we identified four putative regulatory sites. Two are putative hormone response elements (HREs), one with an imperfect inverted repeat (HRE-IR) and one with a monomeric site (HRE-M). An additional monomeric hormone receptor site (MRE) is present only in HcHemolin. The third conserved motif is similar to the interferon (IFN) regulatory factor binding element (IRF-E) and IFN-stimulated response element (ISRE). The fourth conserved element is a kappaB motif situated between the Cap-site and the TATA-box. Finally, by electrophoresis mobility shift assay we demonstrate that the HRE-IR forms specific complexes with nuclear extract proteins of normal pupae that increase after 20E stimulation.

Gene structure and expression profile of Manduca sexta prophenoloxidase-activating proteinase-3 (PAP-3), an immune protein containing two clip domains

Prophenoloxidase-activating proteinase-3 (PAP-3) is a component of the defence system in Manduca sexta. We have isolated genomic clones and elucidated the organization of this gene. The 3' end of exon 2, the entire exon 3 and the 5' end of exon 4 encode the two amino-terminal clip domains. Southern blot analysis suggested a single copy of the PAP-3 gene in the genome. We identified several putative immune-responsive elements in the upstream region. The PAP-3 gene is not highly expressed in the fat body during larval development until the wandering stage begins. The mRNA level is high in the epithelium, fat body and haemocytes. Tissue-specific alternative splicing occurs in the fat body and trachea. A bacterial injection markedly induced the gene expression in the fat body and haemocytes.

Ikappabeta-related vankyrin genes in the Campoletis sonorensis ichnovirus: temporal and tissue-specific patterns of expression in parasitized Heliothis virescens lepidopteran hosts

Polydnaviruses (PDVs) are unusual insect viruses that occur in obligate symbiotic associations with parasitic ichneumonid (ichnoviruses, or IVs) and braconid (bracoviruses, or BVs) wasps. PDVs are injected with eggs, ovarian proteins, and venom during parasitization. Following infection of cells in host tissues, viral genes are expressed and their products function to alter lepidopteran host physiology, enabling endoparasitoid development. Here we describe the Campoletis sonorensis IV viral ankyrin (vankyrin) gene family and its transcription. The seven members of this gene family possess ankyrin repeat domains that resemble the inhibitory domains of the Drosophila melanogaster NF-kappabeta transcription factor inhibitor (Ikappabeta) cactus. vankyrin gene expression is detected within 2 to 4 h postparasitization (p.p.) in Heliothis virescens hosts and reaches peak levels by 3 days p.p. Our data indicate that vankyrin genes from the C. sonorensis IV genome are differentially expressed in the tissues of parasitized hosts and can be divided into two subclasses: those that target the host fat body and those that target host hemocytes. Polyclonal antibodies raised against a fat-body targeting vankyrin detected a 19-kDa protein in crude extracts prepared from the 3 days p.p. fat body. Vankyrin-specific Abs localized to 3-day p.p. fat-body and hemocyte nuclei, suggesting a role for vankyrin proteins in the nuclei of C. sonorensis IV-infected cells. These data are evidence for divergent tissue specificities and targeting of multigene families in IVs. We hypothesize that PDV vankyrin genes may suppress NF-kappabeta activity during immune responses and developmental cascades in parasitized lepidopteran hosts of C. sonorensis.

Manduca sexta prophenoloxidase activating proteinase-1 (PAP-1) gene: organization, expression, and regulation by immune and hormonal signals

Insect phenoloxidase (PO) participates in melanotic encapsulation, wound healing, and cuticle sclerotization. It is converted from prophenoloxidase (proPO) by a proPO-activating proteinase (PAP). Manduca sexta PAP-1, the final component of a serine proteinase cascade, cleaves proPO to generate active PO. In an effort to understand the transcriptional regulation, we isolated a genomic clone of the PAP-1 gene, determined its nucleotide sequence, and elucidated its exon-intron organization. Computer analysis revealed several immune and hormone responsive elements in the upstream region. Southern blot analysis suggested that the M. sexta genome contains a single copy of PAP-1 gene. Reverse transcription-polymerase chain reaction showed that PAP-1 was constitutively expressed in fat body, trachea, and nerve tissue of the fifth instar larvae. The mRNA levels in hemocytes and fat body markedly increased in response to a bacterial challenge. We also observed tissue-specific and developmental regulation of the gene's transcription. Treating M. sexta fat body culture with 20-hydroxyecdysone reduced the PAP-1 mRNA level. These data indicated that the expression of PAP-1 gene is under the dual control of immune and hormonal signals.

Peptidoglycan recognition proteins: on and off switches for innate immunity

Insects rely on innate immune mechanisms to defend themselves against microbes. The inducible anti-microbial peptides constitute an important arm of this defense. In Drosophila, the Toll and the Imd pathways are the major routes to induce the peptides, and it has become clear that to a certain extent, these pathways can discriminate between different microbes and mount an appropriate response to eliminate the intruder. This review discusses the proteins responsible for this discriminatory recognition, the peptidoglycan recognition proteins (PGRPs). The serum protein PGRP-SA triggers a humoral cascade of proteases upon infection by certain gram-positive bacteria to activate the Toll pathway. The membrane-bound receptor PGRP-LC activates the Imd pathway in response to certain gram-negative bacteria or their peptidoglycans. Other PGRPs have enzymatic activity, cleaving lactylamide bonds in peptidoglycan to eliminate its immunogenicity, thus turning off the immune response. The PGRP family is conserved from insects to man. Short mammalian PGRP variants are synthesized in neutrophils and stored in granules. These PGRPs seem to influence the survival of phagocytosed non-pathogenic bacteria. Long PGRP variants are expressed in the liver and secreted into the bloodstream where their peptidoglycan-degrading activity might serve scavenger functions.

Baculovirus and dsRNA induce Hemolin, but no antibacterial activity, in Antheraea pernyi

Hemolin is one of the haemolymph proteins most strongly induced upon bacterial infection in Lepidoptera. When we applied RNA interference (RNAi) to suppress Hemolin expression in the Chinese oak silk moth Antheraea pernyi, we discovered that Hemolin is induced by double-stranded RNA (dsRNA) per se. As dsRNA is recognized as a virus pattern molecule, we then investigated the effect of a baculovirus (ApNPV) infection. We found that Hemolin is induced and expressed with similar kinetics as upon dsRNA injection. Notably, no Attacin gene expression or antibacterial activity was recorded. When baculovirus and high amounts of dsRNA were coinjected, the viral symptoms appeared earlier with Hemolin dsRNA than with GFP dsRNA. This indicates that silencing of hemolin affected the progress of the viral infection.

Characterization of a homologue of the Rel/NF-kappaB transcription factor from a beetle, Allomyrina dichotoma

A cDNA encoding a Rel/NF-kappaB homologue was cloned from a beetle, Allomyrina dichotoma, by reverse transcriptase-polymerase chain reactions (RT-PCR) taking advantage of the conserved Rel homology domain (RHD) to synthesize primers. The Rel/NF-kappaB homologue was designated A. dichotoma (A.d.) Rel A. The amino acid sequence of the A.d. Rel A RHD was compared with those of insect RHDs. The result showed that it has 70% identity with Tribolium castaneum Dorsal, 66% with Drosophila melanogaster Dorsal, 61% with Anopheles gambiae Gambif1, and 55% with D. melanogaster Dif. A putative phosphorylation site in the RHD, RRPS, and two putative nuclear localization signals were conserved in A.d. Rel A. A recombinant fusion protein containing the A.d. Rel A RHD was confirmed to bind specifically to the NF-kappaB site of a gene encoding A.d. coleoptericin A, an antibacterial peptide from A. dichotoma. The activity of A.d. Rel A in modulating a gene construct of the A.d. coleoptericin A promoter-luciferase reporter by expressing the A.d. coleoptericin A cDNA in a Bombyx mori cell line was analyzed. The result showed that A.d. Rel A strongly activates the A.d. coleoptericin A gene construct, whereas A.d. Rel A failed to activate the gene construct containing the mutated NF-kappaB site, suggesting the importance of the interaction between the NF-kappaB site and A.d. Rel A in the signal transduction for gene expression of antibacterial peptides in A. dichotoma.

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.

An intron enhancer activates the immunoglobulin-related Hemolin gene in Hyalophora cecropia

Hemolin is the only insect member of the immunoglobulin (Ig) superfamily reported to be up-regulated during an immune response. In diapausing pupae of Hyalophora cecropia the gene is expressed in fat body cells and in haemocytes. Like the mammalian Ig kappa light chain gene, the Hemolin gene harbours an enhancer including a kappaB motif in one of its introns. This motif binds the H. cecropia Rel factor Cif (Cecropia immunoresponsive factor). The Hemolin third intron also mediates transient reporter gene expression in immunoresponsive Drosophila mbn-2 cells. Co-transfections of Drosophila SL2 cells showed that the Drosophila Rel factor Dif (Dorsal-related immunity factor), transactivates reporter gene constructs through the intron. Moreover, a 4.8-fold synergistic activation was obtained when Dif is combined with the rat C/EBP (CCAAT/enhancer element-binding protein) and human HMGI (high mobility group protein I). This is the first report of an insect immune-related gene that is up-regulated by an enhancer activity conferred through an intron.

Regulation of antibacterial and antifungal innate immunity in fruitflies and humans

Insects have been very successful in adapting to their environment, and the ability of the insect immune system to detect and elicit the appropriate response against various invading pathogens has helped in this success. Unlike the vertebrate immune system, which consists of both innate and adaptive components, insect immunity probably consists entirely of an innate immune response, as no evidence of an adaptive response has been found. The innate immune response is described as either a reaction against "lack of self," or the interaction between host germline-encoded receptors and molecules unique to a particular class of invading organisms. Once the invading organism is recognized, the host immune response can be activated via signaling pathways that lead to the appropriate reaction. This review endeavors to put forth how through genetic, molecular, and biochemical studies of the fruit fly Drosophila melanogaster, as well as other insects, it is now understood that aspects of the insect and vertebrate innate immune system are very similar.

Regulation of midgut defensin production in the blood-sucking insect Stomoxys calcitrans

The Stomoxys midgut defensin (Smd) family of genes are exclusively expressed in the anterior midgut of adult flies. Their putative function is protection of the stored bloodmeal from microbial attack. Smd genes are constitutively expressed, up-regulated in response to a bloodmeal and further up-regulated by immune stimulation per os but only in the presence of a bloodmeal not a sugar meal. Smd genes are down-regulated in response to a systemic immune challenge. Smd gene constructs transfected into l(2)mbn cells undertake constitutive expression but are not up-regulated by immune challenge. Electrophoretic mobility shift assays (EMSA) suggest the rel-like sites in the proximal promoter region of Smd genes do not bind midgut factors and so are non-functional.

Exploration of mosquito immunity using cells in culture

The propagation of immune-responsive cells in vitro has provided the basis for substantial contributions to our understanding of many aspects of the mammalian immune response. In contrast, the potential for exploring the innate immune response of insects using cultured cells is only beginning to be developed, particularly with various mosquito cell lines from the genera Aedes and Anopheles. Immune-reactive mosquito cell lines express various defensive factors, including transferrin, lysozyme, cecropin, defensin, and prophenoloxidase activities. In this review, we discuss insect immunity in the context of key concepts that have emerged in the study of the mammalian immune system, with emphasis on the properties of the cells that participate in the immune response. The nature of established cell lines and their contributions to our understanding of immune functions in humans and insects is described, with emphasis on our own work with the C7-10 and Aag-2 mosquito cell lines from Aedes albopictus and Aedes aegypti, respectively. Finally, we offer some speculation on further advances in insect immunology that may be facilitated by work with cells in culture.

Lipopolysaccharide-activated kinase, an essential component for the induction of the antimicrobial peptide genes in Drosophila melanogaster cells

Eukaryotic organisms use a similar Rel/NF-kappaB signaling cascade for the induction of innate immune genes. In Drosophila, lipopolysaccharide (LPS) signal-induced activation of the Rel/NF-kappaB family transcription factors is an essential step in the transcriptional activation of inducible antimicrobial peptide genes. However, the mechanism by which the LPS-induced signaling pathway proceeds remains largely unknown. Here we have cloned a novel Drosophila LPS-activated kinase (DLAK) that is structurally related to mammalian IkappaB kinases. DLAK is expressed and transiently activated in LPS-responsive Drosophila cells following LPS stimulation. Furthermore, DLAK can interact with Cactus, a Drosophila IkappaB and phosphorylate recombinant Cactus, in vitro. Overexpression of dominant-negative mutant DLAK (DLAK(K50A)) blocks LPS-induced Cactus degradation. DLAK-bound Cactus can be degraded in a LPS signal-dependent fashion, whereas the DLAK(K50A) mutant-bound Cactus is completely resistant to degradation in the presence of LPS. The DLAK(K50A) mutant also inhibits nuclear kappaB binding activity and kappaB-dependent diptericin reporter gene activity in a dose-dependent manner, but the kappaB-dependent diptericin reporter gene activity can be rescued by overexpression of wild type DLAK. Moreover, mRNA analysis of various kappaB-dependent antimicrobial peptide genes shows that LPS inducibility of these genes is greatly impaired in cells overexpressing DLAK(K50A). These results establish that DLAK is a novel LPS-activated kinase, which is an essential signaling component for the induction of antimicrobial peptide genes following LPS treatment in Drosophila cells.

Interaction and specificity of Rel-related proteins in regulating Drosophila immunity gene expression

NF-kappaB/Rel family proteins regulate genes that are critical for many cellular processes including apoptosis, inflammation, immune response, and development. NF-kappaB/Rel proteins function as homodimers or heterodimers, which recognize specific DNA sequences within target promoters. We examined the activity of different Drosophila Rel-related proteins in modulating Drosophila immunity genes by expressing the Rel proteins in stably transfected cell lines. We also compared how different combinations of these transcriptional regulators control the activity of various immunity genes. The results show that Rel proteins are directly involved in regulating the Drosophila antimicrobial response. Furthermore, the drosomycin and defensin expression is best induced by the Relish/Dif and the Relish/Dorsal heterodimers, respectively, whereas the attacin activity can be efficiently up-regulated by the Relish homodimer and heterodimers. These results illustrate how the formation of Rel protein dimers differentially regulate target gene expression.

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.

Immune proteins and their gene expression in the silkworm, Bombyx mori

Several self-defense proteins have been isolated from the silkworm, Bombyx mori and their amino acid sequences determined. These proteins include novel antibacterial proteins designated lebocin and moricin, and a novel lectin designated hemocytin, an insect homologue of mammalian von Willebrand factor. Antibacterial mechanisms of lebocin and moricin have been analyzed and their ability to form ion channels in bacterial membranes play an important role in defense against bacterial infection. cDNAs and genes encoding these proteins have been cloned to examine their induction mechanisms upon bacterial infection. Regulatory motifs such as the kappaB-like and GATA sequence have been identified in the B. mori antibacterial proteins. On the other hand, hemocytin gene expression was confirmed to occur upon bacterial infection and before pupation under naive conditions, suggesting that hemocytin plays an important role in both immunity and metamorphosis. Moreover, this review also describes the releasing mechanisms of a bacterial cell wall component, lipopolysaccharide (LPS), from intact bacteria, clearance of LPS from B. mori hemolymph and a possible signal transduction pathway for antibacterial protein gene expression.

Toll receptor-mediated Drosophila immune response requires Dif, an NF-kappaB factor

The induction of immunity genes in Drosophila has been proposed to be dependent on Dorsal, Dif, and Relish, the NF-kappaB-related factors. Here we provide genetic evidence that Dif is required for the induction of only a subset of antimicrobial peptide genes. The results show that the presence of Dif without Dorsal is sufficient to mediate the induction of drosomycin and defensin. We also demonstrate that Dif is a downstream component of the Toll signaling pathway in activating the drosomycin expression. These results reveal that individual members of the NF-kappaB family in Drosophila have distinct roles in immunity and development.

Isolation of p-hydroxycinnamaldehyde as an antibacterial substance from the saw fly, Acantholyda parki S

We purified an antibacterial substance from larvae of the saw fly, Acantholyda parki S., and identified its molecular structure as p-hydroxycinnamaldehyde. We then synthesized it by reduction of p-hydroxycinnamic acid. The antibacterial activity of the synthetic p-hydroxycinnamaldehyde was equal to that of the authentic substance. This molecule was found to have a broad antibacterial spectrum against not only Gram-negative, but also Gram-positive bacteria. Furthermore, it showed antifungal activity against Candida albicans. We suggest that this substance may play a role in the defense system of this insect. This is the first report of p-hydroxycinnamaldehyde of animal origin.

Cecropin D-like antibacterial peptides from the sphingid moth, Agrius convolvuli

Two major antibacterial peptides were isolated and purified from immunized larval hemolymph of Agrius convolvuli. Acid extraction, gel filtration, ultrafiltration, and reversed-phase FPLC were used for purification of peptides. These peptides had similar molecular mass and amino acid composition. Moreover, 21 of the first 23 N terminal residues were identical. The peptides were highly homologous with cecropin D in size and primary sequence, and named Agrius cecropin D1 and D2. The molecular masses of Agrius cecropin D1 and D2 were 3,879.39 and 3,839.27, respectively. In antibacterial and hemolytic assays, Agrius cecropin D showed potent antibacterial activities against a panel of Gram positive and negative bacteria without hemolytic activity against human red blood cells. Notably, our antibacterial assay revealed Agrius cecropin D possessed stronger or at least equivalent activities against B. megaterium than cecropin A. It suggests that Agrius cecropin D, which has an alternative structure from cecropin D, could be the model for the development of peptide antibiotics. Arch.

Gene-encoded peptide antibiotics and the concept of innate immunity: an update review

Antibacterial peptides were first considered rather species-specific. However, the perspective began to change in 1987-89. Five years later there were two symposium volumes and several reviews on gene-encoded peptide antibiotics which covered the known peptides irrespective of origin. The field is rapidly growing and a first update was published in this Journal in 1996. At that time a database was made with about 100 entries; now it has over 400, with some redundancy. Recently a methodological handbook was published and there are many specialized reviews covering only defensins or insect immunity. In the last 2 years, the larger perspective of innate immunity and the role of gene-mediated peptide antibiotics have evolved in ways which justify a new update. Today insects and plants are known to have similar overall design of their defensins while insects and mammals have very similar control mechanisms. The signal pathways are beginning to appear and the future perspective may involve additional changes.

Differential gene expression in insects: transcriptional control

Studies on transcriptional control of gene expression play a pivotal role in many areas of biology. In non-Drosophilid insects, the cuticle, chorion, immune response, silk gland, storage proteins, and vitellogenin are foci for advances in basic research on promoter elements and transcription factors. Insects offer other advantages for gene regulation studies, including the availability of applied problems. In non-Drosophilid insects, the most serious problem for transcriptional control studies is the lack of homologous in vivo expression systems. Once this deficiency is addressed, the full impact of research on transcription control will be realized throughout the field of entomology.

The genes encoding the antibacterial sex-specific peptides ceratotoxins are clustered in the genome of the medfly Ceratitis capitata

Ceratotoxins are antibacterial peptides produced in the female reproductive accessory glands of the medfly Ceratitis capitata. Their expression is not affected by bacterial infection, but is enhanced after mating and is modulated by juvenile hormone. Three different peptides, named ceratotoxins A, B and C, have been previously purified from the female accessory gland secretion and their amino acid and cDNA sequences have been determined. We report here the complete nucleotide sequences of four genes encoding closely related ceratotoxin peptides. One of them encodes a novel peptide, which we named ceratotoxin D. Restriction and nucleotide sequence analysis indicate that these ceratotoxin genes are organized in a large cluster spanning more than 26 kilobases of DNA. All ceratotoxin genes are coordinately expressed. Ceratotoxin transcripts appear in 2-3 day old adult females, and they reach a maximum in 6-7 day old females. The presence of highly conserved motifs in the upstream regions of all the sequenced ceratotoxin genes suggests the presence of common regulatory elements for all ceratotoxins.

Drosophila immunity

Septic injury induces in Drosophila the rapid and transient transcription of several genes encoding potent antimicrobial peptides. Significant structural and functional similarities exist between the injury-induced signalling cascades leading to antimicrobial peptide gene expression in Drosophila and cytokine-induced expression of mammalian acute-phase proteins. Here, the authors discuss their understanding of these pathways and their relationships to those found in mammalian cells. They also analyse non-self recognition and the role of blood cells in Drosophila host defence.

Adjacent GATA and kappa B-like motifs regulate the expression of a Drosophila immune gene

The GATA motif is a well known positive cis -regulatory element in vertebrates. In this work we report experimental evidence for the direct participation of a GATA motif in the expression of the Drosophila antibacterial peptide gene Cecropin A1 . Previously we have shown that a kappaB-like site is necessary for Cecropin A1 gene expression. Here we present evidence that the Drosophila Rel protein which binds to the kappaB-like site requires an intact GATA site for maximal Dif-mediated transactivation of the Cecropin A1 gene. We show that a Drosophila blood cell line contains factors binding specifically to the GATA motif of the Cecropin A1 gene. The GATA binding activity is likely to include member(s) of the GATA family of transcriptional regulators. We show that the promoters of several inducible insect immune genes possess GATA sites 0-12 base pairs away from kappaB-like sites in functionally important promoter regions. Clusters of GATA and kappaB sites are also observed in the promoters of two important mammalian immune genes, namely IL6 and IL3. The consistent proximity of GATA and kappaB sites appears to be a common theme in the immune gene expression of insects and mammals.

In vitro phagocytosis of Escherichia coli and release of lipopolysaccharide by adhering hemocytes of the silkworm, Bombyx mori

A primary culture containing adhering hemocytes mainly granular cells from the silkworm, Bombyx mori, was used to investigate in vitro phagocytosis of Escherichia coli. Phagocytosis was confirmed to occur in this system by microscopic observation. Lipopolysaccharide (LPS) concentration in the culture medium was measured by a Limulus test and a higher LPS concentration was detected in phagocytosis-occurred samples than in control samples, which omitted either E. coli cells or adhering hemocytes. Moreover, it was found that LPS containing sample but not control samples strongly induces gene expression of cecropin B, an antibacterial protein. These results suggest that bacterial cell wall components like LPS released by phagocytosis play an important role in the induction of insect antibacterial proteins.

Biological mediators of insect immunity

Infection in insects stimulates a complex defensive response. Recognition of pathogens may be accomplished by plasma or hemocyte b1p4eins that bind specifically to bacterial or fungal polysaccharides. Several morphologically distinct hemocyte cell types cooperate in the immune response. Hemocytes attach to invading organisms and then isolate them by phagocytosis, by trapping them in hemocyte aggregates called nodules, or by forming an organized multicellular capsule around large parasites. These responses are often accompanied by proteolytic activation of the phenoloxidase zymogen that is present in the hemolymph. A component of insect immune responses to bacteria is the synthesis by fat body and hemocytes of a variety of antibacterial proteins and peptides, which are secreted into the hemolymph. These molecules attack bacteria by several mechanisms. Inducible antifungal proteins have also been recently discovered in insect hemolymph. The promoters for several antibacterial protein genes in insects are regulated by transcription factors similar to those involved in mammalian acute phase responses.

Differential induction of antibacterial transcripts in Drosophila susceptible and resistant to parasitism by Leptopilina boulardi

Two well-described elements of the immune response of insects include encapsulation of metazoan parasites (blood-cell-mediated) and the production of antibacterial peptides (humoral and/or cellular). However, the possible functional interrelationship between cellular encapsulation and antibacterial responses, and the extent to which the two components may be co-regulated, are poorly understood. We used a novel approach involving strains of Drosophila resistant (R) or susceptible (S) to the wasp parasitoid Leptopilina boulardi to study the expression of three genes involved in the antibacterial response: Dorsal-related immunity factor (Dif), Cecropin (CecA1) and Diptericin (Dip). Both S and R strains produced high levels of all antibacterial transcripts upon bacterial injection. However, when parasitized the R strain showed no induction whilst the S strain did. This lack of antibacterial transcript induction in the parasitized R strain not only clarifies the separation of these two types of immune response but also raises the fascinating possibility of a link in their genetic regulation.

Purification and characterization of N-beta-alanyl-5-S-glutathionyl-3,4-dihydroxyphenylalanine, a novel antibacterial substance of Sarcophaga peregrina (flesh fly)

We purified a novel antibacterial substance from immunized adult Sarcophaga and determined its molecular structure to be N-beta-alanyl-5-S-glutathionyl-3,4-dihydroxyphenylalanine (5-S-GAD). We synthesized 5-S-GAD enzymatically from N-beta-alanyl-3, 4-dihydroxyphenylalanine (beta-Ala-Dopa) and reduced glutathione (GSH). The antibacterial activity of 5-S-GAD was found to be due to its production of H2O2. This is a novel antibacterial mechanism as it differs from the mechanisms of known antibacterial peptides. Two possible roles of 5-S-GAD in insect immunity, suppression of bacterial growth and activation of a Rel family transcription factor, are proposed.

Drosophila immunity: a comparative analysis of the Rel proteins dorsal and Dif in the induction of the genes encoding diptericin and cecropin

In Drosophila, bacterial challenge induces the rapid transcription of several genes encoding potent antibacterial peptides. The upstream sequences of the diptericin and cecropin Al genes, which have been investigated in detail, contain two, respectively one sequence element homologous to the binding site of the mammalian nuclear factor kappaB. These elements have been shown to be mandatory for immune-induced transcription of both genes. Functional studies have shown that these kappaB-related elements can be the target for the Drosophila Rel proteins dorsal and Dif. Here we present a comparative analysis of the transactivating capacities of these proteins on reporter genes fused to either the diptericin or the cecropin kappaB-related motifs. We conclude from our results: (i) the kappaB motifs of the diptericin and cecropin genes are not functionally equivalent; (ii) the dorsal and Dif proteins have distinct DNA-binding characteristics; (iii) dorsal and Dif can heterodimerize in vitro; (iv) mutants containing no copies of dorsal and a single copy of Dif retain their full capacity to express the diptericin and cecropin genes in response to challenge.

Insect lysozymes

Lysozymes, related to the chicken-type lysozymes in vertebrates, are ubiquitous components in the bacteriolytic armamentarium of insects. The enzyme is normally present in the blood, and together with other bactericidal factors lysozyme is often strongly induced when the insect is infected. This response is regulated by mechanisms that are related to those that activate inflammatory, acute-phase and immune responses in mammals, and the induction of lysozyme and other factors is now being investigated as a model for innate immune reactions in general. A special adaptation is seen in flies like Musca and Drosophila. These animals live on the microorganisms in decompositing matter, and they have developed a specialized set of lysozymes that are expressed in the alimentary tract. In Drosophila, at least seven different lysozyme genes are clustered in a small region on the third chromosome. The different genes are expressed in different parts of the digestive tract, and at different time points during development, and they are highly divergent in sequence. The major lysozymes in the fly gut have acidic isoelectric points and/or pH optima, and their evolution provides an interesting parallel to the ruminants.

Immune response in insects: the role of phenoloxidase in defense reactions in relation to melanization and sclerotization

It is well known that activated prophenoloxidase (proPO) plays an important role in cuticular melanization and sclerotization. In addition, studies dealing with immune response of insects suggest that phenoloxidase (PO) is also critical in the defense reactions of insects against invaders. proPO is activated by elicitors derived from microbial cell wall components such as peptidoglycan, beta-1,3-glucan, and lipopolysaccharide (LPS). According to our recent studies we proposed a model clarifying the role of PO in both cellular and humoral immune responses. LPS triggers Ceratitis capitata hemocytes via induced protein tyrosine phosphorylation to release biologically active molecules, including p47 and proPO-activators. Furthermore, hemocytes in response to LPS facilitate clearance of LPS from the hemocoel of medfly. The effector molecules involved in the LPS clearance are hemocyte surface-associated p47 (mp47), soluble p47 (sp47), activated proPO, and tyrosine. A similar LPS clearance system in the integument of medfly in vitro was also demonstrated. According to our data, the proposed mechanism for LPS clearance from hemocoel and from integument is the crosslinking of LPS to p47 or certain integumental proteins via the intermediacy of reactive tyrosine derivatives generated by PO activity, as is the case for cuticular protein-chitin crosslinks during sclerotization. We also demonstrated that metabolites of the eumelanin biosynthesis and not melanin itself or N-acetyldopamine (NADA), the key precursor of sclerotizing agent, were necessary for the immune responses by hemocytes and integument.

Structures of genes for two cathelin-associated antimicrobial peptides: prophenin-2 and PR-39

We characterized genes for prophenin (PF)-2 and PR-39, two cathelin-associated antimicrobial peptides found in porcine leukocytes. Both contained 4 exons and 3 introns and were compact, contiguous and highly homologous. Exons I-III encoded most of their cathelin domains. Exon IV specified the final few cathelin residues, including its conserved C-terminal valine, followed by the mature PR-39 peptide or a PF-2 precursor. The highly conserved 5' flanking sequences of this gene family contained NF-kappa B, IL-6, GM-CSF and NF-1 binding motifs and the introns were unusually conserved. These data suggest that the panoply of porcine cathelin-associated antimicrobial peptides arose relatively recently via gene reduplications and exon shuffling, and that in vivo expression of cathelin-associated antimicrobial peptides may respond to mediators generated early during infection.

Parallel induction of cecropin and lysozyme in larvae of the silkworm, Bombyx mori

Lysozyme activity in the hemolymph of Bombyx mori increased in parallel with cecropin activity after injection of the larvae with soluble peptidoglycan or UV-killed bacteria. The lysozyme and cecropin A genes were expressed in parallel in the fat body after injection of peptidoglycan as detected by northern blot hybridization. The elicitor specificity for lysozyme induction was identical to that for cecropin, suggesting a common mechanism for recognition of bacteria and following signal transduction introducing to the simultaneous synthesis of cecropin and lysozyme. Bacterial cells killed by UV-irradiation were also effective as elicitor when added to the fat body culture, suggesting that phagocytosis of bacteria by hemocytes may not be an essential process for the induction of antibacterial protein synthesis in the silkworm.

Structure and expression of Hemolin, an insect member of the immunoglobulin gene superfamily

Hemolin is an insect protein which belongs to the immunoglobulin superfamily and is strongly induced upon bacterial infection. It has been isolated from two moths, Hyalophora cecropia and Manduca sexta. We have isolated and sequenced a genomic clone for hemolin in H. cecropia, in order to resolve its organization and as a basis for investigating hemolin gene regulation. According to Southern-blot analysis, hemolin is encoded by a single gene, Hemolin. It contains six exons ranging over 32-603 bp. The introns are positioned both within and between the immunoglobulin-like domains, a feature typical for cell-adhesion molecules belonging to the immunoglobulin superfamily. By an RNase protection assay, we show that the Hemolin transcript is strongly induced not only by bacteria, but also by lipopolysaccharide and phorbol 12-myristate 13-acetate. Analysis of the upstream region and introns revealed potential binding sites for the Cecropia immunoresponsive factor (CIF), which recognizes the kappa B-like consensus GGGRA YYYYY.