Innate immunity of insects

Innate recognition systems in insect immunity and development: new approaches in Drosophila

Phagocytosis is important in immune defense and development in vertebrates and invertebrates. It leads to swift ingestion of microorganisms and dying cells by phagocytes. How particles are recognized is not well understood. Studies in insects, and Drosophila in particular, argue that these are powerful model systems to dissect this process.

Insect immunity: a genetic factor (hrtp) is essential for antibacterial peptide expression in Drosophila after infection by parasitoid wasps

We have used a parasitoid wasp Drosophila melanogaster system to investigate the relationship between the humoral and cellular immune responses in insects. Expression of the gene encoding diptericin, an antibacterial peptide in various D. melanogaster strains parasitized by several species of parasitoid wasps, was studied by Northern blot. These strains have the capacity to encapsulate parasitoid eggs. Two strains appeared to produce diptericin mRNA after parasitoid challenge, regardless of their cellular immune reaction to the wasp species. This suggests that a specific genetic factor, or factors, here designated humoral response to parasitoid (hrtp), is present in these two strains of D. melanogaster and is implicated in the expression of the antibacterial gene after parasite infection. This hrtp genetic factor is recessively expressed and located on the second chromosome, suggesting that it is monofactorial. The transgenic strain Dipt.2.2-lacZ:1, in which the transgene is present on the first chromosome, is normally susceptible to the parasitoid wasp. The chromosome bearing the hrtp factor was transferred to this transgenic strain, which then became reactive when triggered by wasp infection. The hrtp factor appears necessary for the activation of diptericin by the parasitoid wasp. No correlation between the cellular immune capacity and the humoral response was observed, suggesting that the two components of insect immunity are regulated independently. Arch.

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.

Cationic peptide antimicrobials induce selective transcription of micF and osmY in Escherichia coli

Cationic antimicrobial peptides, such as polymyxin and cecropin, activated transcription of osmY and micF in growing Escherichia coli independently of each other. The micF response required the presence of a functional rob gene. It is intriguing that in this and other assays an identical response profile was also seen with hyperosmotic salt or sucrose gradient, two of the most commonly used traditional food preservatives. The osmY and micF transcription was not induced by hypoosmotic gradient, ionophoric peptides, uncouplers, or with other classes of membrane perturbing agents. The antibacterial peptides did not promote transcription of genes that respond to macromolecular or oxidative damage, fatty acid biosynthesis, heat shock, or depletion of proton or ion gradients. These and other results show that the antibacterial cationic peptides induce stasis in the early growth phase, and the transcriptional efficacy of antibacterial peptides correlates with their minimum inhibitory concentration, and also with their ability to mediate direct exchange of phospholipids between vesicles. The significance of these results is developed as the hypothesis that the cationic peptide antimicrobials stress growth of Gram-negative organisms by making contacts between the two phospholipid interfaces in the periplasmic space and prevent the hyperosmotic wrinkling of the cytoplasmic membrane. Broader significance of these results, and of the hypothesis that the peptide mediated contacts between the periplasmic phospholipid interfaces are the primary triggers, is discussed in relation to antibacterial resistance.

Transcriptional regulation of beta-defensin gene expression in tracheal epithelial cells

Innate immunity provides an ever-present or rapidly inducible initial defense against microbial infection. Among the effector molecules of this defense in many species are broad-spectrum antimicrobial peptides. Tracheal antimicrobial peptide (TAP) was the first discovered member of the beta-defensin family of mammalian antimicrobial peptides. TAP is expressed in the ciliated epithelium of the bovine trachea, and its mRNA levels are dramatically increased upon stimulation with bacteria or bacterial lipopolysaccharide (LPS). We report here that this induction by LPS is regulated at the level of transcription. Furthermore, the transfection of reporter gene constructs into tracheal epithelial cells indicates that DNA sequences in the 5' flanking region of the TAP gene, within 324 nucleotides of the transcription start site, are responsible in part for mediating gene induction. This region includes consensus binding sites for NF-kappaB and nuclear factor interleukin-6 (NF IL-6) transcription factors. Gel mobility shift assays indicate that LPS induces NF-kappaB binding activity in the nuclei of these cells, while NF IL-6 binding activity is constitutively present. The gene encoding human beta-defensin 2, a human homologue of TAP with similar inducible expression patterns in the airway, was cloned and found to have conserved NF-kappaB and NF IL-6 consensus binding sites in its 5' flanking region. Previous studies of antimicrobial peptides from insects indicated that their induction by infectious microbes and microbial products also occurs via activation of NF-kappaB-like and NF IL-6-like transcription factors. Together, these observations indicate that a strategy for the induction of peptide-based antimicrobial innate immunity is conserved among evolutionarily diverse organisms.

Structure and organization of hemolytic and nonhemolytic diastereomers of antimicrobial peptides in membranes

Recently, we reported on a new group of diastereomers of short-model peptides (12 amino acids long) composed of leucine and lysine with varying ratios, possessing several properties that make them potentially better than native or de novo-designed all L-amino acid antimicrobial peptides. Preliminary studies have revealed that modulating the hydrophobicity and positive charges of these diastereomers is sufficient to confer antibacterial activity and cell selectivity. However, the relationship between their biological function, structure, and mode of action was not investigated. Here we synthesized and investigated three types of linear model diastereomers (12 amino acids long) with varying lysine:leucine (or tryptophan) ratios (i.e., K(3)L(8)W, K(5)L(6)W, and K(7)L(4)W), which confer different levels of lytic activities. For each K:L ratio, tryptophan was introduced in the middle or the N- or C-terminus of the peptides, as an intrinsic fluorescent probe. Only the hemolytic peptide K(3)L(8)W binds to both negatively charged and zwitterionic phospholipid membranes. K(5)L(6)W and K(7)L(4)W bind similarly, but only to negatively charged membranes, despite the fact that K(5)L(6)W is substantially more lytic to bacteria than K(7)L(4)W. Interestingly, although K(3)L(8)W contains 33% D-amino acids, ATR-FTIR spectroscopy revealed a structure of approximately 90% alpha-helix in both types of membranes. In addition, K(5)L(6)W contains approximately 40% 3(10)-helix and K(7)L(4)W is predominantly a random coil in membranes. Polarized ATR-FTIR and tryptophan-quenching experiments, using brominated phospholipids, revealed a similar depth of penetration and an orientation that was parallel to the membrane surface for all the peptides, but with K(3)L(8)W affecting the lipid order more than the others. The results provide insight into the mode of action of this group of diastereomeric peptides, and the effect of hydrophobicity and positive charges on their membrane structure, function, and cell selectivity. Moreover, this research should assist in the development of suitable diastereomeric peptide antibiotics for therapeutic use that would overcome the problem the increasing resistance of bacteria to conventional antibiotics.

Origin of the interferon-inducible (2'-5')oligoadenylate synthetases: cloning of the (2'-5')oligoadenylate synthetase from the marine sponge Geodia cydonium

In vertebrates cytokines mediate innate (natural) immunity and protect them against viral infections. The cytokine interferon causes the induction of the (2'-5')oligoadenylate synthetase [(2-5)A synthetase], whose product, (2'-5')oligoadenylate, activates the endoribonuclease L which in turn degrades (viral) RNA. Three isoforms of (2-5)A synthetases exist, form I (40-46 kDa), form II (69 kDa), and form III (100 kDa). Until now (2-5)A synthetases have only been cloned from birds and mammals. Here we describe the cloning of the first putative invertebrate (2-5)A synthetase from the marine sponge Geodia cydonium. The deduced amino acid sequence shows signatures characteristic for (2-5)A synthetases of form I. Phylogenetic analysis of the putative sponge (2-5)A synthetase indicates that it diverged first from a common ancestor of the hitherto known members of (vertebrate) (2-5)A synthetases I, (2-5)A synthetases II and III. Moreover, it is suggested that the (2-5)A synthetases II and III evolved from this common ancestor (very likely) by gene duplication. Together with earlier results on the existence of the (2'-5')oligoadenylates in G. cydonium, the data presented here demonstrate that also invertebrates, here sponges, are provided with the (2-5)A system. At present, it is assumed that this system might be involved in growth control, including control of apoptosis, and acquired its additional function in innate immune response in evolutionarily younger animals, in vertebrates.

Lethal effects of apidaecin on Escherichia coli involve sequential molecular interactions with diverse targets

Apidaecins, short proline-arginine-rich peptides from insects, are highly bactericidal through a mechanism that includes stereoselective elements but is completely devoid of any pore-forming activity. The spectrum of antibacterial activity, always limited to Gram-negatives, is further dependent on a small number of variable residues and can be manipulated. We show here that mutations in the evolutionary conserved regions result in a more general loss of function, and we have used such analogs to probe molecular interactions in Escherichia coli. First, an assay was developed to measure selectively chiral association with cellular targets. By using this method, we find that apidaecin uptake is energy-driven and irreversible and yet can be partially competed by proline in a stereospecific fashion, results upholding a model of a permease/transporter-mediated mechanism. This putative transporter is not the end point of apidaecin action, for failure of certain peptide analogs to kill cells after entering indicates the existence of another downstream target. Tetracycline-induced loss of bactericidal activity and dose-dependent in vivo inhibition of translation by apidaecin point at components of the protein synthesis machinery as likely candidates. These findings provide new insights into the antibacterial mechanism of a unique group of peptides and perhaps, by extension, for distant mammalian relatives such as PR-39.

Maternal transmission of immunity to white spot syndrome associated virus (WSSV) in shrimp (Penaeus monodon)

Beta-1,3-1,6-glucan, derived from bakers' yeast Saccharomyces cerevisiae, was used in the present study to investigate the extent to which glucan is able to protect spawners from white spot syndrome associated virus (WSSV), and whether this protection (if any) can be passed on to hatchlings via maternal transmission of immunity. Results showed that fewer spawners in the glucan-injected groups showed the clinical symptoms of red body coloration and white spots on the shell during the 15 days between eyestalk ablation and the end of repeated spawning. This suggests that the application of glucan might lead to a slight enhancement of disease resistance in spawners, although the differences were not statistically significant within the confidence limit chosen. Challenge results showed a significant increase in relative percent survival for larvae derived from groups of glucan-injected spawners compared to those derived from groups of untreated spawners. It therefore seems that a maternally transmitted disease resistance induced by glucan, protected the larvae against a WSSV infection. Glucan immersion was not only shown to be effective for nauplii derived from spawners that were not injected with glucan, it also provided additional, cumulative protection for nauplii which already had a maternally transmitted resistance to WSSV. This is the first documented demonstration of a maternal transmission of immunity in invertebrates.

The anti-microbial activity of maggot secretions: results of a preliminary study

The ability of larval secretions to kill or prevent the growth of a range of potentially pathogenic bacteria was investigated in a preliminary laboratory study. Marked anti-microbial activity was detected against Streptococcus A and B and Staphylococcus aureus. Some activity was also detected against Pseudomonas sp. and a clinical isolate of a resistant strain of S. aureus (MRSA). No evidence of inhibition was recorded against Enterococcus or the Gram-negative bacteria Escherichia coli and Proteus. These findings are consistent with clinical observations made by us and others that larvae of the common greenbottle are able to combat clinical infections in a variety of wound types including those caused by antibiotic-resistant strains of bacteria.

Molecular cloning of an apoptosis-inducing protein, pierisin, from cabbage butterfly: possible involvement of ADP-ribosylation in its activity

We have previously reported that the cabbage butterfly, Pieris rapae, contains a 98-kDa protein, named pierisin, that induces apoptosis in a variety of human cancer cell lines. In the present study, sequencing and cloning of a cDNA encoding pierisin was accomplished. PCR-direct sequencing showed that the gene encodes an 850-amino acid protein with a calculated molecular weight of 98,081. An intact clone at the amino acid level encompassing the entire coding region was obtained by recombination of two independent clones, and the molecular mass of its in vitro expressed protein was about 100 kDa on SDS/PAGE, the same as that of purified native pierisin. The expressed protein induced apoptosis in human gastric carcinoma TMK-1 and cervical carcinoma HeLa cells, like the native protein, indicating functional activity. The deduced amino acid sequence of pierisin showed 32% homology with a 100-kDa mosquitocidal toxin from Bacillus sphaericus SSII-1. In addition, pierisin showed regional sequence similarities with ADP-ribosylating toxins, such as the A subunit of cholera toxin. A glutamic acid residue at the putative NAD-binding site, conserved in all ADP-ribosylating toxins, was also found in pierisin. Substitution of another amino acid for glutamic acid 165 resulted in a great decrease in cytotoxicity and induction of apoptosis. Moreover, inhibitors of ADP-ribosylating enzymes reduced pierisin-induced apoptosis. These results suggest that the apoptosis-inducing protein pierisin might possess ADP-ribosylation activity that leads to apoptosis of the cells.

Induction of apoptosis of monocyte-macrophage lineage cells by 5-S-GAD

We found that 5-S-GAD, an insect-derived antibacterial peptide, inhibited murine osteoclast formation in vitro. We examined the specific time point of the inhibitory action of 5-S-GAD on osteoclast formation and found that it mainly suppressed differentiation of osteoclasts in the middle of the culture period. Using HL60 cells that are able to differentiate into multinucleated macrophage-like cells, we found that 5-S-GAD induced apoptosis of HL60 cells by producing H(2)O(2). Thus, the inhibition of osteoclast formation by 5-S-GAD could be, in part, due to apoptosis of the cells of an osteoclast lineage.

Structure of a paralytic peptide from an insect, Manduca sexta

Paralytic peptide 1 (PP1) from a moth, Manduca sexta, is a 23-residue peptide (Glu-Asn-Phe-Ala-Gly-Gly-Cys-Ala-Thr-Gly-Tyr-Leu-Arg-Thr-Ala-Asp-Gly-Arg -Cys-Lys-Pro-Thr-Phe) that was first found to have paralytic activity when injected into M. sexta larvae. Recent studies demonstrated that PP1 also stimulated the spreading and aggregation of a blood cell type called plasmatocytes and inhibited bleeding from wounds. We determined the solution structure of PP1 by two-dimensional 1H NMR spectroscopy to begin to understand structural-functional relationships of this peptide. PP1 has an ordered structure, which is composed of a short antiparallel beta-sheet at residues Tyr11-Thr14 and Arg18-Pro21, three beta turns at residues Phe3-Gly6, Ala8-Tyr11 and Thr14-Gly17, and a half turn at the carboxyl-terminus (residues Lys20-Phe23). The well-defined secondary and tertiary structure was stabilized by hydrogen bonding and side-chain hydrophobic interactions. In comparison with two related insect peptides, whose structures have been solved recently, the amino-terminal region of PP1 is substantially more ordered. The short antiparallel beta-sheet of PP1 has a folding pattern similar to the carboxyl-terminal subdomain of epidermal growth factor (EGF). Therefore, PP1 may interact with EGF receptor-like molecules to trigger its different biological activities.

Immulectin, an inducible C-type lectin from an insect, Manduca sexta, stimulates activation of plasma prophenol oxidase

Immulectin, a C-type lectin from the tobacco hornworm, Manduca sexta, was cloned from a larval fat body cDNA library. The immulectin cDNA encodes a 309 residue polypeptide. Immulectin synthesis was induced by injection of killed gram-positive or gram-negative bacteria or yeast. After injection of bacteria, immulectin mRNA appeared in fat body and immulectin protein was detected in hemolymph. Immulectin contains two carbohydrate recognition domains. The carboxyl-terminal carbohydrate recognition domain is most similar (36% identity) to a lipopolysaccharide-binding protein from the American cockroach, Periplaneta americana. It also shares 26-35% identity to carbohydrate recognition domains of various mammalian C-type lectins. Two immulectin isoforms were identified in the hemolymph of bacteria-injected larvae. Recombinant immulectin agglutinated gram-positive and gram-negative bacteria and yeast. Addition of recombinant immulectin to M. sexta plasma stimulated activation of phenol oxidase. A combination of immulectin with lipopolysaccharide from E. coli activated phenol oxidase more rapidly and to a higher level than immulectin alone, whereas lipopolysaccharide by itself had little effect on phenol oxidase activation. Immulectin synthesized in response to bacterial or fungal infection may help to trigger protective responses in M. sexta in a manner similar to mannose-binding protein, a C-type lectin that functions in the mammalian innate immune system.

Antibiotic peptides from higher eukaryotes: biology and applications

Gene-encoded antibiotic peptides are increasingly being recognized as effector molecules of host defense in plants and animals. Studies of antimicrobial peptides are providing new insights into the dynamic interactions between microbes and their hosts, and are generating new paradigms for the pathogenesis and treatment of diseases. Because antimicrobial peptides of higher eukaryotes differ structurally from conventional antibiotics produced by bacteria and fungi, they offer novel templates for pharmaceutical compounds that could be effective against increasingly resistant microbes.

Cytotoxic activity of pierisin, from the cabbage butterfly, Pieris rapae, in various human cancer cell lines

Pierisin, a protein purified from pupae of the cabbage butterfly, Pieris rapae, exhibits cytotoxic effects against the human gastric cancer TMK-1 cell line, inducing apoptosis. The present study was performed to determine whether pierisin might exert a similar influence on nine other human cancer cell lines and human umbilical vein endothelial cells (HUVECs). Pierisin showed cytotoxic effects in all the human cells tested, with IC50 values ranging from 0.043 ng/ml to 150 ng/ml. Among the target cells, the cervical carcinoma cell line, HeLa, was the most sensitive to pierisin, showing a 1000-fold less IC50 value than that of HUVECs. While pierisin clearly induced apoptotic cell death in most cancer cell lines and HUVECs, the pathway appeared to be probably different from that involving anti-Fas, TNF-alpha and p53. Pierisin may thus be a promising new candidate for cancer therapy.

Killing of Caenorhabditis elegans by Pseudomonas aeruginosa used to model mammalian bacterial pathogenesis

We show that a single clinical isolate of the human opportunistic pathogen Pseudomonas aeruginosa (strain PA14), which previously was shown to be pathogenic in mice and plants, also kills Caenorhabditis elegans. The rate of PA14-mediated killing of C. elegans depends on the composition of the agar medium on which PA14 is grown. When PA14 is grown on minimal medium, killing occurs over the course of several days and is referred to as "slow" killing. When PA14 is grown on high-osmolarity medium, killing occurs over the course of several hours and is referred to as "fast" killing. Several lines of evidence, including the fact that heat-killed bacteria are still capable of fast but not slow killing of C. elegans, indicate that fast and slow killing occur by distinct mechanisms. Slow killing involves an infection-like process and correlates with the accumulation of PA14 within worm intestines. Among 10 PA14 virulence-related mutants that had been shown previously to affect pathogenicity in plants and mice, 6 were less effective in killing C. elegans under both fast- and slow-killing conditions, indicating a high degree of commonalty among the P. aeruginosa factors required for pathogenicity in disparate eukaryotic hosts. Thus, we show that a C. elegans pathogenicity model that is genetically tractable from the perspectives of both host and pathogen can be used to model mammalian bacterial pathogenesis.

Acaloleptins A: inducible antibacterial peptides from larvae of the beetle, Acalolepta luxuriosa

We purified and characterized three structurally related antibacterial peptides with a molecular mass of 8 kDa (acaloleptins A1, A2, and A3) from the hemolymph of immunized larvae of the Udo longicorn beetle, Acalolepta luxuriosa. These peptides have the same 6 N-terminal amino acid residues and show potent antibacterial activity against some Gram-negative bacteria. The three peptides are thought to be isoforms. Reverse phase HPLC analysis of the hemolymph of immunized and naive larvae showed that acaloleptins A1, A2, and A3 were inducible and suggested that all three peptides were produced in a single insect. We determined the complete amino acid sequence of acaloleptin A1: Acaloleptin A1 consists of 71 amino acid residues and shares significant sequence similarity with coleoptericin and holotricin 2, which were isolated from other coleopteran insects. Furthermore, the 29 C-terminal residues of acaloleptin A1 had 40% identity with the 30 C-terminal residues of hymenoptaecin found in honeybees. Arch. Insect Biochem.

Snakin-1, a peptide from potato that is active against plant pathogens

A new type of antimicrobial peptide, snakin-1 (SN1), has been isolated from potato tubers and found to be active, at concentrations < 10 microM, against bacterial and fungal pathogens from potato and other plant species. The action of SN1 and potato defensin PTH1 was synergistic against the bacterium Clavibacter michiganensis subsp. sepedonicus and additive against the fungus Botrytis cinerea. Snakin-1 causes aggregation of both gram-positive and gram-negative bacteria. The peptide has 63 amino acid residues (M(r) 6,922), 12 of which are cysteines, and is unrelated to any previously isolated protein, although it is homologous to amino acid sequences deduced from cloned cDNAs that encode gibberellin-inducible mRNAs and has some sequence motifs in common with kistrin and other hemotoxic snake venoms. A degenerate oligonucleotide probe based on the internal sequence CCEECKC has been used to clone an SN1 cDNA. With the cDNA used as probe, one copy of the StSN1 gene per haploid genome has been estimated and expression of the gene has been detected in tubers, stems, axillary buds, and young floral buds. Expression levels in petals and carpels from fully developed flowers were much higher than in sepals and stamens. The expression pattern of gene StSN1 suggests that protein SN1 may be a component of constitutive defense barriers, especially those of storage and reproductive plant organs.

Cathelicidin gene expression in porcine tissues: roles in ontogeny and tissue specificity

Cathelicidins constitute a family of mammalian antimicrobial peptides that are synthesized in the bone marrow as prepropeptides, stored in neutrophil granules as propeptides, and released as active, mature peptides upon neutrophil degranulation. We investigated the developmental expression of two porcine cathelicidins, PR-39 and protegrin. Both cathelicidins were expressed constitutively in the bone marrow of all pigs at all of the ages tested. Peripheral blood neutrophils from young pigs expressed PR-39 and protegrin mRNA, which were not detectable at 42 days of age. At earlier ages, expression of PR-39 mRNA was detected in the kidney and liver and several lymphoid organs, including the thymus, spleen, and mesenteric lymph nodes, but disappeared at 4 weeks of age. These data provide the first evidence of cathelicidin gene expression in peripheral leukocytes and may indicate a role for these antimicrobial peptides in the development of host defense mechanisms.

Cecropins induce the hyperosmotic stress response in Escherichia coli

Cecropin A and B, below or near their minimum inhibitory concentrations in viable Escherichia coli, interfered with the rapid NaCl-induced hyperosmotic shrinkage of the cytoplasmic volume (plasmolysis), and also activated the promoter of the hyperosmotic stress gene osmY. The same promoter was also expressed by hyperosmolar NaCl or sucrose, two of the most commonly used antimicrobial food preservatives. Stress responses were monitored during the logarithmic growth phase of E. coli strains that contain specific promoters fused to a luxCDABE operon on a plasmid. The luminescence assay, developed to monitor the transcriptional response to stresses, is based on the premise that organisms often respond and adapt to sublethal environmental adversities by increased expression of stress proteins to restore homeostasis. The luminescence response from these fusion strains to a specific stress occurs as the transcription at the promoter site is activated. Cecropins induced luminescence response only from the osmY-luxCDABE fusion, but not the corresponding stress promoter activation associated with macromolecular or oxidative damage, or leakage of the cytoplasmic content including the proton gradient. The inhibitory effect of cecropins on plasmolysis is interpreted to suggest that the primary locus of action of these antimicrobial peptides in the periplasmic space is on the coupling between the inner and outer membrane.

Isolation, characterization and chemical synthesis of a new insect defensin from Chironomus plumosus (Diptera)

Injection of low doses of bacteria into the aquatic larvae of the dipteran insect Chironomus plumosus induces the appearance in their hemolymph of a potent antibacterial activity. We have isolated two 36-residue peptides from this hemolymph which are active against Gram-positive bacteria. The peptides are novel members of the insect defensin family and their sequences present marked differences with those of insect defensins isolated from other dipteran species. We have developed a method for efficient renaturation of this cysteine-rich molecule and obtained a highly pure synthetic Chironomus defensin.

Haemolymph and fat body metallo-protease associated with Enterobacter cloacae infection in the bloodsucking insect, Rhodnius prolixus

Analysis of zymograms with SDS-polyacrilamide gel electrophoresis containing gelatin as substrate, and performed on samples of haemolymph or fat body taken from Rhodnius prolixus inoculated or not with Enterobacter cloacae, demonstrated distinct patterns of protease activities: (i) in the haemolymph two proteases were induced in insects inoculated with bacteria; (ii) two proteases were detected in the fat bodies derived from non-inoculated controls or insect inoculated with sterile culture medium; (iii) haemolymph and fat body had both the same apparent molecular weights proteases (46 and 56 kDa); and (iv) these enzymes were characterized as metallo-proteases. The association of these enzymes in Rhodnius infected with bacteria was discussed.

From noxiustoxin to Shiva-3, a peptide toxic to the sporogonic development of Plasmodium berghei

This communication reviews shortly the main structural and functional characteristics of Noxiustoxin, a 39 amino acid residue peptide, maintained closely packed by three-disulfide bridges and its effects on excitable membranes. Shiva-3, a cecropin like-peptide composed of 38 amino acid residues is also briefly reviewed. Its design and synthesis was made possible by the expertise gained through the work previously performed with Noxiustoxin. One of the most prominent functional characteristics of Shiva-3 is the toxic effect upon the sporogonic development of Plasmodium berghei (responsible for a murine version of malaria). A synthetic Shiva-3 gene was constructed by recursive polymerase-chain reaction (PCR) methodology and expressed using the vector pGEX2T as a hybrid protein between the glutathione-S-transferase at the N-terminal and Shiva-3 in the C-terminal part of the hybrid. The recombinant protein kills bacteria and Plasmodium berghei. The future aim of this work is to produce a transgenic mosquito that carries the message for synthesis and excretion of Shiva-3 and similar peptides, in the midgut of mosquitoes, in an attempt to control the spreading of human malaria.

The role of surface characteristics in eliciting humoral encapsulation of foreign bodies in Plasmodium-refractory and -susceptible strains of Anopheles gambiae

A refractory strain of the mosquito, Anopheles gambiae, melanotically encapsulates and kills many species of malaria parasites, whereas susceptible strains allow the parasites to develop normally. To study the role of surface characteristics in eliciting this immune response, 27 types of chromatography beads that differed in matrix type, charge, functional group, and functional group density were assayed for degree of melanotic encapsulation in refractory and susceptible mosquitoes. Overall, two glucan-based matrices, Sephadex (dextran) and cellulose, stimulated the strongest responses, regardless of functional group. Substituting matrix hydroxyl groups with functional groups on Sephadex and cellulose beads decreased the level of encapsulation. These results demonstrate that glucans induce melanotic encapsulation in An. gambiae. Beads with agarose, polystyrene, and acrylic matrices, and most methacrylate-based beads elicited little or no melanization; however, epoxide-methacrylate beads were encapsulated, demonstrating that glucans are not essential for eliciting a response. Comparisons between the two strains demonstrated that refractory mosquitoes melanized many bead types to a greater degree than did susceptible mosquitoes. On this basis, we propose that an important difference between the two strains is that one of the enzymes involved in the melanization pathway functions at a higher level in the refractory strain. Finally, of all beads tested, only 85% substituted CM-Sephadex beads were virtually unmelanized in susceptible mosquitoes but highly melanized in the refractory strain; thus, a specific surface microenvironment is necessary to demonstrate this effect.

A conserved p38 mitogen-activated protein kinase pathway regulates Drosophila immunity gene expression

Accumulating evidence suggests that the insect and mammalian innate immune response is mediated by homologous regulatory components. Proinflammatory cytokines and bacterial lipopolysaccharide stimulate mammalian immunity by activating transcription factors such as NF-kappaB and AP-1. One of the responses evoked by these stimuli is the initiation of a kinase cascade that leads to the phosphorylation of p38 mitogen-activated protein (MAP) kinase on Thr and Tyr within the motif Thr-Gly-Tyr, which is located within subdomain VIII. We have investigated the possible involvement of the p38 MAP kinase pathway in the Drosophila immune response. Two genes that are highly homologous to the mammalian p38 MAP kinase were molecularly cloned and characterized. Furthermore, genes that encode two novel Drosophila MAP kinase kinases, D-MKK3 and D-MKK4, were identified. D-MKK3 is an efficient activator of both Drosophila p38 MAP kinases, while D-MKK4 is an activator of D-JNK but not D-p38. These data establish that Drosophila indeed possesses a conserved p38 MAP kinase signaling pathway. We have examined the role of the D-p38 MAP kinases in the regulation of insect immunity. The results revealed that one of the functions of D-p38 is to attenuate antimicrobial peptide gene expression following exposure to lipopolysaccharide.

Differential induction of the Arabidopsis thaliana Thi2.1 gene by Fusarium oxysporum f. sp. matthiolae

The Arabidopsis thaliana Thi2.1 gene is inducible by necrotrophic fungi through a signal transduction pathway different from that for pathogenesis-related (PR) proteins. We have identified three ecotypes that are susceptible (Col-2, Ler, and Ws) and two ecotypes that are resistant (Mt-0 and Uk-4) to spray inoculation with Fusarium oxysporum f. sp. matthiolae. The Thi2.1 transcript level after infection correlates with resistance, being 5 to 10 times higher in the resistant than in the susceptible ecotypes. The beta-glucuronidase (GUS) expression of a Thi2.1-promoter-uidA fusion (with a promoter derived from Col-2) is on the average almost 10 times higher in the Uk-4 background than in the Col-2 background. This confirms the results obtained by Northern (RNA) blots and indicates that Uk-4, and probably other resistant ecotypes too, might have a more sensitive recognition system for F. oxysporum f. sp. matthiolae or might have a signal transduction system that gives a higher amplification of the original recognition signal. Our results suggest a role of the Thi2.1 gene in resistance against F. oxysporum f. sp. matthiolae and perhaps other necrotrophic fungi.

Osmotic stress in viable Escherichia coli as the basis for the antibiotic response by polymyxin B

Cationic antimicrobial peptides, such as polymyxin B (PxB), below growth inhibitory concentration induce expression of osmY gene in viable E. coli without leakage of solutes and protons. osmY expression is also a locus of hyperosmotic stress response induced by common food preservatives, such as hypertonic NaCl or sucrose. High selectivity of PxB against Gram-negative organisms and the basis for the hyperosmotic stress response at sublethal PxB concentrations is attributed to PxB-induced mixing of anionic phospholipid between the outer layer of the cytoplasmic membrane with phospholipids in the inner layer of the outer membrane. This explanation is supported by PxB-mediated rapid and direct exchange of anionic phospholipid between vesicles. This mechanism is consistent with the observation that genetically stable resistance against PxB could not be induced by mutagenesis.

Purification of pierisin, an inducer of apoptosis in human gastric carcinoma cells, from cabbage butterfly, Pieris rapae

A substance strongly cytotoxic to human carcinoma cell line TMK-1 has been found in pupae, larvae and adults of the cabbage butterfly, Pieris rapae, and named pierisin. Pierisin was purified from the pupae of P. rapae by ammonium sulfate precipitation followed by DEAE-cellulose, Phenyl-Sepharose and hydroxyapatite column chromatographies. The molecular weight of the purified pierisin, which was homogenous on SDS-polyacrylamide gel, was analyzed by mass spectrometry and found to be 98 kDa. Pierisin showed a strong cytotoxic effect, with and IC50 of 0.75 ng/ml for human gastric carcinoma TMK-1 cells. The dying cells exhibited characteristic morphological features of apoptosis, such as cell shrinkage, chromatin condensation and nuclear fragmentation. Oligonucleosomal DNA fragmentation was also observed in DNA isolated from pierisin-treated cells. Moreover, similar characteristic changes showing apoptotic cell death were observed in TMK-1 cells treated with a crude extract of pupae of P. rapae. These results indicate that pierisin from the pupae of P. rapae induces apoptosis in human carcinoma cells.

Acidic peptide-mediated expression of the antimicrobial peptide buforin II as tandem repeats in Escherichia coli

Antimicrobial peptides have received increasing attention as a new pharmaceutical substance, because of their broad spectrum of antimicrobial activities and the rapid development of multidrug-resistant pathogenic microorganisms. The main obstacle to the wide application of antimicrobial peptides has been the lack of a cost-effective, mass-production method. A novel mass-production method for an antimicrobial peptide of 21 amino acids, buforin II, which was isolated from the stomach of the amphibian Bufo bufo gargarizans, has been developed. This method is based on the neutralization of the positive charges of buforin II by fusing to an acidic peptide to avoid the lethal effect of the expressed antimicrobial peptide on the host cells. The fusion peptide was expressed in Escherichia coli as tandem repeats to increase the product yield. Multimers of the acidic peptide-buforin II fusion peptide were expressed at high levels without causing damage to the cells. The presence of cysteine residues in the acidic peptide was critical for the high level expression of the fusion peptide multimers. Multimers of this fusion peptide were expressed as inclusion bodies, and about 107 mg of pure buforin II was obtained from 1 L of E. coli culture by cleaving the multimers with CNBr. Recombinant buforin II had an antimicrobial activity identical to that of natural buforin II. These results may lead to a general, cost-effective solution to the mass production of antimicrobial peptides and other basic peptides which are lethal to the host strain.

An ancient system of host defense

Research over the past few years has begun to provide significant advances in our understanding of the interplay between the innate and adaptive immune systems. New findings in several model systems reveal remarkable parallels and conservation of ancient host defense pathways in organisms separated by over a billion years of evolution.

Drosophila host defense: differential induction of antimicrobial peptide genes after infection by various classes of microorganisms

Insects respond to microbial infection by the rapid and transient expression of several genes encoding potent antimicrobial peptides. Herein we demonstrate that this antimicrobial response of Drosophila is not aspecific but can discriminate between various classes of microorganisms. We first observe that the genes encoding antibacterial and antifungal peptides are differentially expressed after injection of distinct microorganisms. More strikingly, Drosophila that are naturally infected by entomopathogenic fungi exhibit an adapted response by producing only peptides with antifungal activities. This response is mediated through the selective activation of the Toll pathway.

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.

The macrophage scavenger receptor type A is expressed by activated macrophages and protects the host against lethal endotoxic shock

During gram-negative bacterial infections, lipopolysaccharide (LPS) stimulates primed macrophages (Mphi) to release inflammatory mediators such as tumor necrosis factor (TNF)-alpha, which can cause hypotension, organ failure, and often death. Several different receptors on Mphi have been shown to bind LPS, including the type A scavenger receptor (SR-A). This receptor is able to bind a broad range of polyanionic ligands such as modified lipoproteins and lipoteichoic acid of gram-positive bacteria, which suggests that SR-A plays a role in host defense. In this study, we used mice lacking the SR-A (SRKO) to investigate the role of SR-A in acquired immunity using a viable bacillus Calmette Guérin (BCG) infection model. We show that activated Mphi express SR-A and that this molecule is functional in assays of adhesion and endocytic uptake. After BCG infection, SRKO mice are able to recruit Mphi to sites of granuloma formation where they become activated and restrict BCG replication. However, infected mice lacking the SR-A are more susceptible to endotoxic shock and produce more TNF-alpha and interleukin-6 in response to LPS. In addition, we show that an antibody which blocks TNF-alpha activity reduces LPS-induced mortality in these mice. Thus SR-A, expressed by activated Mphi, plays a protective role in host defense by scavenging LPS as well as by reducing the release by activated Mphi of proinflammatory cytokines. Modulation of SR-A may provide a novel therapeutic approach to control endotoxic shock.

Antimicrobial peptide defense in Drosophila

Drosophila responds to a septic injury by the rapid synthesis of antimicrobial peptides. These molecules are predominantly produced by the fat body, a functional equivalent of mammalian liver, and are secreted into the hemolymph where their concentrations can reach up to 100 microM. Six distinct antibacterial peptides (plus isoforms) and one antifungal peptide have been characterized in Drosophila and their genes cloned. The induction of the gene encoding the antifungal peptide relies on the spätzle/Toll/cactus gene cassette, which is involved in the control of dorsoventral patterning in the embryo, and shows interesting structural and functional similarities with cytokine-induced activation of NF-kappa B in mammalian cells. An additional pathway, dependent on the as yet unidentified imd (for immune-deficiency) gene, is required for the full induction of the antibacterial peptide genes. Mutants deficient for the Toll and imd pathways exhibit a severely reduced survival to fungal and bacterial infections, respectively. Recent data on the molecular mechanisms underlying recognition of non-self are also discussed in this review.

The 18-wheeler mutation reveals complex antibacterial gene regulation in Drosophila host defense

Mammals and insects employ similar Rel/NF-kappaB signaling cascades in their humoral immune responses. The mammalian interleukin-1 type I receptor (IL-1R) is one way of activating this cascade. The Drosophila Toll protein, whose cytoplasmic domain shows striking similarity to that of the IL-1R, acts in the humoral antimicrobial response. Here we demonstrate that a second IL-1R-related Drosophila protein, 18-Wheeler (18W), is a critical component of the humoral immune response. 18-wheeler is expressed in the larval fat body, the primary organ of antimicrobial peptide synthesis. In the absence of the 18W receptor, larvae are more susceptible to bacterial infection. Nuclear translocation of the Rel protein Dorsal-like immunity factor (Dif) is inhibited, though nuclear translocation of another Rel protein, Dorsal, is unaffected. Induction of several antibacterial genes is reduced following infection, relative to wild-type: attacin is reduced by 95%, cecropin by 65% and diptericin by 12%. Finally, 18-wheeler (18w) expression is induced in response to infection and, in addition to the receptor form, four immune-specific transcripts and proteins are produced.

The concentration-dependent membrane activity of cecropin A

Cecropin A is a naturally occurring, linear, cationic, 37-residue antimicrobial peptide. The precise mechanism by which it kills bacteria is not known, but its site of action is believed to be the cell membrane. To investigate the nature of its membrane activity, we examined the ability of cecropin A to alter membrane permeability in synthetic lipid vesicles and in Gram-negative bacteria. Cecropin A exerted distinctly different types of membrane activity depending on its concentration. In synthetic lipid vesicles, cecropin A dissipated transmembrane electrochemical ion gradients at relatively low concentrations, but much higher concentrations were required to release an encapsulated fluorescent probe. Cecropin A dissipated ion gradients whether or not the vesicle membranes contained anionic lipid, although the presence of anionic lipid dramatically increased peptide binding, and modestly increased the release of an encapsulated probe. Cholesterol did not prevent the dissipation of ion gradients by low concentrations of peptide, but it did inhibit release of the encapsulated probe by high concentrations of peptide. At the highest concentrations examined, cecropin A remained monomeric in solution, and did not aggregate, lyse, or otherwise alter vesicle size. In Gram-negative bacteria, cecropin A was potently bactericidal at concentrations which dissipated ion gradients in lipid vesicles, but much higher concentrations were required to cause the release of cytoplasmic contents. These findings point to the conclusion that cecropin A kills bacteria by dissipating transmembrane electrochemical ion gradients. They weigh against theories comparing the antimicrobial activity of cecropin A to the release of encapsulated probes from lipid vesicles, and against roles for cholesterol or anionic lipid headgroups in the selectivity of peptide action against bacteria.

Lipopolysaccharide-binding proteins and their involvement in the bacterial clearance from the hemolymph of the silkworm Bombyx mori

Proteins having the ability to bind to Escherichia coli K12W3110 (rough (R) mutant) were isolated and purified by affinity precipitation from the larval hemolymph of the silkworm Bombyx mori. These proteins were found to consist of two components with molecular masses of 43 kDa and 40 kDa by SDS/PAGE. They bound to all E. coli R mutants (Ra, Rb1, Rc, Rd1 and Re) and Salmonella minnesota R mutants. However, they did not bind to smooth types of the above bacteria. They bound to both lipopolysaccharide(LPS)-coated and lipid-A-coated microtiter plates and have similar dissociation constants for LPS and lipid A. This indicates that the binding proteins recognize the lipid A portion of LPS and thus, we have named these proteins BmLBP (B. mori LPS-binding proteins). We also found that BmLBP participated in the clearance of E. coli cells injected into the body cavity of the silkworm.

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.

Genetic engineering for fungal and bacterial diseases

Significant new advances at the molecular level in the field of plant-pathogen interactions form the basis for novel transgenic approaches to crop protection. The cloning of disease resistance genes and the dissection of the signal transduction components of the hypersensitive response and systemic acquired resistance pathways have greatly increased the diversity of options available for transgenic disease resistance. These new approaches will supplement our rapidly increasing repertoire of antimicrobial peptides, defense-related proteins and antimicrobial compounds. The combinatorial deployment of these strategies will be exploited for engineering effective and durable resistance to pathogens in the field. The integration of transgenic approaches with classical resistance breeding offers a potentially chemical-free and environmentally friendly solution for controlling pathogens.

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 Activity of the Limulus Antimicrobial Peptide Tachyplesin I on Marine Bivalve Pathogens

Tachyplesin 1 is an antimicrobial peptide extracted from hemocytes of the Japanese horseshoe crab Tachypleus tridentatus. We studied the in vitro activity of tachyplesin I against bivalve pathogens: the oyster parasites Bonamia ostreae, the intrahemocytic parasite of the flat oyster Ostrea edulis and Perkinsus marinus, the histozoic parasite of the Eastern oyster Crassostrea virginica, and the bacterium Vibrio P1, pathogenic for the clam Tapes philippinarum. Viability of the protozoans was assessed microscopically by the uptake of the vital dyes acridine orange and ethidium bromide. Following exposure to tachyplesin I, B. ostreae and P. marinus viabilities were reduced in a dose-dependent manner, up to, respectively, 94 and 62% within a 500 μg/ml peptide concentration. The fine structure of P. marinus was highly altered by the peptide. Tachyplesin I also displayed a potent activity against marine vibrios, with a MIC of 0.4-0.8 μg/ml against Vibrio P1. We examined the morphology of oyster hemocytes treated by tachyplesin I, together with the cell functional capabilities to produce chemiluminescence. No effect of the peptide was found on bivalve host cells. As transgenic technology is currently being applied to marine invertebrates, these results indicate that tachyplesin I may provide effective gene sequences to be manipulated in order to produce disease-resistant bivalves.

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.

Characterization of novel cysteine-rich antimicrobial peptides from scorpion blood

We have isolated, from the hemolymph of unchallenged scorpions of the species Androctonus australis, three distinct antimicrobial peptides, which we have fully characterized by Edman degradation, electrospray ionization mass spectrometry, and matrix-assisted laser desorption/ionization mass spectrometry. Two are novel molecules: (i) androctonin, a 25-residue peptide with two disulfide bridges, active against both bacteria (Gram-positive and Gram-negative) and fungi and showing marked sequence homology to tachyplesins and polyphemusins from horseshoe crabs; and (ii) buthinin, a 34-residue antibacterial (Gram-positive and Gram-negative) peptide with three disulfide bridges. The third peptide contains 37 residues and three disulfide bridges and clearly belongs to the family of anti-Gram-positive insect defensins. We have synthesized androctonin and explored its activity spectrum and mode of action.

Identification and characterization of differentially expressed cDNAs of the vector mosquito, Anopheles gambiae

The isolation and study of Anopheles gambiae genes that are differentially expressed in development, notably in tissues associated with the maturation and transmission of the malaria parasite, is important for the elucidation of basic molecular mechanisms underlying vector-parasite interactions. We have used the differential display technique to screen for mRNAs specifically expressed in adult males, females, and midgut tissues of blood-fed and unfed females. We also screened for mRNAs specifically induced upon bacterial infection of larval stage mosquitoes. We have characterized 19 distinct cDNAs, most of which show developmentally regulated expression specificity during the mosquito life cycle. The most interesting are six new sequences that are midgut-specific in the adult, three of which are also modulated by blood-feeding. The gut-specific sequences encode a maltase, a V-ATPase subunit, a GTP binding protein, two different lectins, and a nontrypsin serine protease. The latter sequence is also induced in larvae subjected to bacterial challenge. With the exception of a mitochondrial DNA fragment, the other 18 sequences constitute expressed genomic sequence tags, 4 of which have been mapped cytogenetically.