Structural changes of the Cry1Ac oligomeric pre-pore from bacillus thuringiensis induced by N-acetylgalactosamine facilitates toxin membrane insertion

The primary action of Cry toxins produced by Bacillus thuringiensis is to lyse midgut epithelial cells in their target insect by forming lytic pores. The toxin-receptor interaction is a complex process, involving multiple interactions with different receptor and carbohydrate molecules. It has been proposed that Cry1A toxins sequentially interact with a cadherin receptor, leading to the formation of a pre-pore oligomer structure, and that the oligomeric structure binds to glycosylphosphatidyl-inositol-anchored aminopeptidase-N (APN) receptor. The Cry1Ac toxin specifically recognizes the N-acetylgalactosamine (GalNAc) carbohydrate present in the APN receptor from Manduca sexta larvae. In this work, we show that the Cry1Ac pre-pore oligomer has a higher binding affinity with APN than the monomeric toxin. The effects of GalNAc binding on the toxin structure were studied in the monomeric Cry1Ac, in the soluble pre-pore oligomeric structure, and in its membrane inserted state by recording the fluorescence status of the tryptophan (W) residues. Our results indicate that the W residues of Cry1Ac have a different exposure to the solvent when compared with that of the closely related Cry1Ab toxin. GalNAc binding specifically affects the exposure of W545 in the pre-pore oligomer in contrast to the monomer where GalNAc binding did not affect the fluorescence of the toxin. These results indicate a subtle conformational change in the GalNAc binding pocket in the pre-pore oligomer that could explain the increased binding affinity of the Cry1Ac pre-pore to APN. Although our analysis did not reveal major structural changes in the pore-forming domain I upon GalNAc binding, it showed that sugar interaction enhanced membrane insertion of soluble pre-pore oligomeric structure. Therefore, the data presented here permits to propose a model in which the interaction of Cry1Ac pre-pore oligomer with APN receptor facilitates membrane insertion and pore formation.

Discovery of Bacillus thuringiensis virulence genes using signature-tagged mutagenesis in an insect model of septicaemia

Transposon Tn917 was used to identify Bacillus thuringiensis genes required for virulence and survival in a Manduca sexta (tobacco hornworm) septicaemia model. Uniquely tagged transposons, n = 72, were constructed and used to generate 1152 insertion mutants. Sixteen pools of 72 mutants were screened in the infection model, and 12 virulence-attenuated mutants were unable to survive the infection. Analysis of the mutated DNA sequences implicated an arsR family transcriptional regulator, a histone-like DNA-binding protein, a transposon, and several sequences of unknown function in B. thuringiensis pathogenesis.

Agr interference between clinical Staphylococcus aureus strains in an insect model of virulence

Repression of virulence by Staphylococcus aureus strains from different Agr groups has been demonstrated in vitro and is proposed as a means of competitive interference. Here, using the insect Manduca sexta, we show for the first time that this interference also occurs in vivo within a mixed population.

Expression of Cry1Ac cadherin receptors in insect midgut and cell lines

Cadherin-like proteins have been identified as putative receptors for the Bacillus thuringiensis Cry1A proteins in Heliothis virescens and Manduca sexta. Immunohistochemistry showed the cadherin-like proteins are present in the insect midgut apical membrane, which is the target site of Cry toxins. This subcellular localization is distinct from that of classical cadherins, which are usually present in cell-cell junctions. Immunoreactivity of the cadherin-like protein in the insect midgut was enhanced by Cry1Ac ingestion. We also generated a stable cell line Flp-InT-REX-293/Full-CAD (CAD/293) that expressed the H. virescens cadherin. As expected, the cadherin-like protein was mainly localized in the cell membrane. Interestingly, toxin treatment of CAD/293 cells caused this protein to relocalize to cell membrane subdomains. In addition, expression of H. virescens cadherin-like protein affects cell-cell contact and cell membrane integrity when the cells are exposed to activated Cry1Ab/Cry1Ac.

Influence of the biophysical and biochemical environment on the kinetics of pore formation by Cry toxins

The effect of Bacillus thuringiensis toxins on the permeability of the luminal membrane of Manduca sexta midgut columnar epithelial cells is strongly influenced by several biophysical and biochemical factors, including pH, ionic strength, and divalent cations, suggesting an important role for electrostatic interactions. The influence of these factors can differ greatly, however, depending on the toxin being studied, even for closely related toxins such as Cry1Ac and Cry1Ca. In the present study, the possibility of using temperature changes as a tool for controlling the rate and extent of pore formation in midgut brush border membrane vesicles was evaluated. Lowering temperature gradually decreased the rate of pore formation, but had little effect on the permeability of vesicles previously incubated with toxin at room temperature. The formation of new pores, following incubation of the vesicles with toxin, could thus be almost abolished by rapidly cooling the vesicles to 2 degrees C. Using this approach, changes in the rate of pore formation could be more easily distinguished from alterations in the properties of the pores formed, thus allowing a more detailed analysis of the kinetics and mechanism of pore formation.

Prior infection of Manduca sexta with non-pathogenic Escherichia coli elicits immunity to pathogenic Photorhabdus luminescens: roles of immune-related proteins shown by RNA interference

Prior infection of Manduca sexta caterpillars with the non-pathogenic bacterium Escherichia coli elicits effective immunity against subsequent infection by the usually lethal and highly virulent insect pathogen Photorhabdus luminescens TT01. Induction of this protective effect is associated with up-regulation of both microbial pattern recognition protein genes (hemolin, immulectin-2 and peptidoglycan recognition protein) and anti-bacterial effector genes (attacin, cecropin, lebocin, lysozyme and moricin). We used RNA interference to knock down over-transcription of members of both these sets of genes one at a time. Interfering with expression of individual recognition proteins had a drastic adverse effect on the E. coli elicited immunity. RNAi knock-down of immulectin-2 caused the greatest reduction in immunity, followed by hemolin and peptidoglycan recognition protein (PGRP) in that order, to the extent that knock-down of any one of these three proteins left the insects more susceptible to P. luminescens infection than insects that had not experienced prior infection with E. coli. Interfering with the expression of individual antibacterial effector proteins and peptides had a much less marked effect on immunity. Knock-down of attacin, cecropin or moricin caused treated insects to be more susceptible to P. luminescens infection than controls that had been pre-infected with E. coli but which had not received the specific RNAi reagents, but they were still less susceptible than insects that had not been pre-infected with E. coli. RNAi knock-down with expression of lebocin or lysozyme had no effect on E. coli-induced immunity to P. luminescens, indicating that these effectors are not involved in the response. By bleeding pre-infected caterpillars and growing the pathogen directly within cell-free insect haemolymph, we showed that at least part of the protection elicited by previous exposure to E. coli is due to the presence of factors within the blood plasma that inhibit the growth of P. luminescens. The production of these factors is inhibited by RNAi treatment with ds-RNA reagents that knock down hemolin, immulectin-2, and PGRP. These results demonstrate that the insect immune system can be effectively primed by prior infection with non-pathogenic bacteria against subsequent infection by a highly virulent pathogen. Given the continuous normal exposure of insects to environmental and symbiotic bacteria, we suggest that prior infection is likely to play a significant and underestimated role in determining the level of insect immunity found in nature.

Immulectin-4 from the tobacco hornworm Manduca sexta binds to lipopolysaccharide and lipoteichoic acid

Insect C-type lectins function as pattern recognition receptors in innate immunity. In the tobacco hornworm Manduca sexta, we have previously isolated three C-type lectins named immulectins, which are involved in innate immune responses. Here, we report a new member of the immulectin family, immulectin-4 (IML-4). IML-4 mRNA was detected in the fat body of control larvae and was induced in the fat body when larvae were injected with bacteria. Recombinant IML-4 bound to bacterial lipopolysaccharide (LPS) and lipoteichoic acid (LTA), and the binding activity was not affected by addition of calcium or EGTA. IML-4 agglutinated bacteria and yeast, and agglutination of Escherichia coli by IML-4 was concentration- and calcium-dependent. IML-4 also enhanced haemocyte encapsulation and melanization.

RNAi suppression of recognition protein mediated immune responses in the tobacco hornworm Manduca sexta causes increased susceptibility to the insect pathogen Photorhabdus

Bacterial pathogens either hide from or overcome the immune response of their hosts. Here we show that two different species of insect pathogenic bacteria, Photorhabdus luminescens TT01 and Photorhabdus asymbiotica ATCC43949, were both recognized by the immune system of their host Manduca sexta, as indicated by a rapid increase in the levels of mRNAs encoding three different inducible microbial recognition proteins, Hemolin, Immulectin-2 and peptidoglycan recognition protein. RNA interference (RNAi)-mediated inhibition of expression ("knock-down") of each of these genes at the level of both mRNA and protein was achieved through injection of double-stranded RNA (dsRNA). Knock-down of any one of these genes markedly decreased the ability of the insects to withstand infection when exposed to either species of Photorhabdus, as measured by the rate at which infected insects died. RNAi against Immulectin-2 caused the greatest reduction in host resistance to infection. The decreased resistance to infection was associated with reduced hemolymph phenoloxidase activity. These results show not only that Photorhabdus is recognized by the Manduca sexta immune system but also that the insect's immune system plays an active, but ultimately ineffective, role in countering infection.

Pyrimidine nucleoside salvage confers an advantage to Xenorhabdus nematophila in its host interactions

Xenorhabdus nematophila is a mutualist of entomopathogenic nematodes and a pathogen of insects. To begin to examine the role of pyrimidine salvage in nutrient exchange between X. nematophila and its hosts, we identified and mutated an X. nematophila tdk homologue. X. nematophila tdk mutant strains had reduced virulence toward Manduca sexta insects and a competitive defect for nematode colonization in plate-based assays. Provision of a wild-type tdk allele in trans corrected the defects of the mutant strain. As in Escherichia coli, X. nematophila tdk encodes a deoxythymidine kinase, which converts salvaged deoxythymidine and deoxyuridine nucleosides to their respective nucleotide forms. Thus, nucleoside salvage may confer a competitive advantage to X. nematophila in the nematode intestine and be important for normal entomopathogenicity.

Parasitic suppression of feeding in the tobacco hornworm, Manduca sexta: parallels with feeding depression after an immune challenge

The parasitic wasp, Cotesia congregata, suppresses feeding in its host Manduca sexta. Feeding suppression in the host coincides with the emergence of the wasps through the host's cuticle. During wasp emergence, host hemocyte number declined, suggesting that the host mounts a wound/immune response against the exiting parasitoids and/or resulting tissue damage. Eliciting a different type of immune response by injecting heat-killed Serratia marcescens also resulted in a decline in feeding and a reduction in hemocyte number. Both the emerging wasps and the bacteria induced an increase in hemolymph octopamine concentration and a decrease in foregut peristalsis in M. sexta. The emerging parasitoids produced the largest changes. The source of the additional octopamine appeared to be the host in both cases. S. marcescens was found to contain no detectable amounts of octopamine. The parasitoids had insufficient octopamine to account for the amount found in host hemolymph and they did not secrete octopamine in vitro. One cause for the high concentration of octopamine in parasitized M. sexta was that octopamine was removed from the hemolymph approximately 23 times more slowly after the wasps emerged than prior to wasp emergence. The striking similarity between the effects of parasitoids and bacteria on M. sexta feeding, hemocyte number, hemolymph octopamine concentration, and foregut peristalsis supports the possibility that the immune/wound reaction induced by the emerging wasps could play a role in the suppression of host feeding. These results also support the hypothesis that M. sexta exhibit an immune-activated anorexia.

Differential Effects of Ionic Strength, Divalent Cations and pH on the Pore-forming Activity of Bacillus thuringiensis Insecticidal Toxins

The combined effects of ionic strength, divalent cations, pH and toxin concentration on the pore-forming activity of Cry1Ac and Cry1Ca were studied using membrane potential measurements in isolated midguts of Manduca sexta and a brush border membrane vesicle osmotic swelling assay. The effects of ionic strength and divalent cations were more pronounced at pH 10.5 than at pH 7.5. At the higher pH, lowering ionic strength in isolated midguts enhanced Cry1Ac activity but decreased considerably that of Cry1Ca. In vesicles, Cry1Ac had a stronger pore-forming ability than Cry1Ca at a relatively low ionic strength. Increasing ionic strength, however, decreased the rate of pore formation of Cry1Ac relative to that of Cry1Ca. The activity of Cry1Ca, which was small at the higher pH, was greatly increased by adding calcium or by increasing ionic strength. EDTA inhibited Cry1Ac activity at pH 10.5, but not at pH 7.5, indicating that trace amounts of divalent cations are necessary for Cry1Ac activity at the higher pH. These results, which clearly demonstrate a strong effect of ionic strength, divalent cations and pH on the pore-forming activity of Cry1Ac and Cry1Ca, stress the importance of electrostatic interactions in the mechanism of pore formation by B. thuringiensis toxins.

Differential gene expression by Metarhizium anisopliae growing in root exudate and host (Manduca sexta) cuticle or hemolymph reveals mechanisms of physiological adaptation

Like many other fungal pathogens Metarhizium anisopliae is a facultative saprophyte with both soil-dwelling and insect pathogenic life-stages. In addition, as M. anisopliae traverses the cuticle and enters the hemolymph it must adapt to several different host environments. In this study, we used expressed sequence tags and cDNA microarray analyses to demonstrate that physiological adaptation by M. anisopliae to insect cuticle, insect hemolymph, bean root exudate (a model for life in the soil), and nutrient rich Sabouraud dextrose broth (SDB) involves different subsets of genes. Overall, expression patterns in cuticle and hemolymph clustered separately from expression patterns in root exudates and SDB, indicative of critical differences in transcriptional control during pathogenic and saprophytic growth. However, there were differences in gene expression between hemolymph and cuticle and these mostly involved perception mechanisms, carbon metabolism, proteolysis, cell surface properties, and synthesis of toxic metabolites. These differences suggest previously unsuspected stratagems of fungal pathogenicity that can be tested experimentally. Examples include the switch-off of cuticle-degrading proteases and a dramatic cell wall reorganization during growth in hemolymph.

Bacillus cereus Fur regulates iron metabolism and is required for full virulence

A homologue of the Bacillus subtilis fur gene was identified in Bacillus cereus and characterized. The predicted amino acid sequence of the cloned gene was found to be highly similar to other members of the Fur family of transcriptional regulators. The B. cereus fur gene was shown to partially complement an Escherichia coli fur mutant. Purified B. cereus Fur bound specifically to a 19 bp DNA sequence homologous to the B. subtilis Fur box in a metal-dependent manner. Analysis of the available B. cereus genome data identified a number of genes which contain predicted Fur box sequences in the promoter region. Many of these genes are predicted to play a role in bacterial iron uptake and metabolism, but several have also been implicated as having a role in virulence. Fur and iron regulation of a siderophore biosynthesis operon was confirmed in a beta-galactosidase assay. A B. cereus fur null strain was constructed by allelic replacement of the chromosomal gene with a copy disrupted with a kanamycin resistance cassette. The Deltafur mutant was found to constitutively express siderophores, to accumulate iron intracellularly to a level approximately threefold greater than the wild-type, and to be hypersensitive to hydrogen peroxide. In an insect infection model, the virulence of the fur null strain was found to be significantly attenuated, highlighting the essential role played by Fur in the virulence of this pathogen.

Variation in gene expression patterns as the insect pathogen Metarhizium anisopliae adapts to different host cuticles or nutrient deprivation in vitro

Metarhizium anisopliaeinfects a broad range of insects by direct penetration of the host cuticle. To explore the molecular basis of this process, its gene expression responses to diverse insect cuticles were surveyed, using cDNA microarrays constructed from an expressed sequence tag (EST) clone collection of 837 genes. During growth in culture containing caterpillar cuticle (Manduca sexta),M. anisopliaeupregulated 273 genes, representing a broad spectrum of biological functions, including cuticle-degradation (e.g. proteases), amino acid/peptide transport and transcription regulation. There were also many genes of unknown function. The 287 down-regulated genes were also distinctive, and included a large set of ribosomal protein genes. The response to nutrient deprivation partially overlapped with the response toMan. sextacuticle, but unique expression patterns in response to cuticles from another caterpillar (Lymantria dispar), a cockroach (Blaberus giganteus) and a beetle (Popilla japonica) indicate that the pathogen can respond in a precise and specialized way to specific conditions. The subtilisins provided an example of a large gene family in which differences in regulation could potentially allow virulence determinants to target different hosts and stages of infection. Comparisons betweenM. anisopliaeand published data onTrichoderma reeseiandSaccharomyces cerevisiaeidentified differences in the regulation of glycolysis-related genes and citric acid cycle/oxidative phosphorylation functions. In particular,M. anisopliaehas multiple forms of several catabolic enzymes that are differentially regulated in response to sugar levels. These may increase the flexibility ofM. anisopliaeas it responds to nutritional changes in its environment.

An entomopathogenic bacterium, Xenorhabdus nematophila, inhibits hemocytic phospholipase A2 (PLA2) in tobacco hornworms Manduca sexta

The entomopathogenic bacterium, Xenorhabdus nematophila, induces immunodepression in target insects and finally leads to lethal septicemia of the infected hosts. A hypothesis has been raised that the bacteria inhibit eicosanoid-biosynthesis pathway to interrupt immune signaling of the infected hosts. Here, we show direct evidence that X. nematophila inhibits the activity of phospholipase A2 (PLA2), the initial step in the eicosanoid-biosynthesis pathway. Inhibition of PLA2 was dependent on both incubation time with X. nematophila and the bacterial concentration in in vitro PLA2 preparations of Manduca sexta hemocytes. While living bacteria inhibited PLA2 activity, heat-killed X. nematophila rather increased PLA2 activity. X. nematophila secreted PLA2 inhibitor(s) which were detected in the organic, but not aqueous, extract of the bacterial culture medium. The PLA2 inhibitory activity of the organic extract was lost after heat treatment. These results clearly indicate that X. nematophila inhibits PLA2 activity, and thereby inhibits eicosanoid biosynthesis which leads to immunodepression of the infected hosts.

Expression and activity of a Xenorhabdus nematophila haemolysin required for full virulence towards Manduca sexta insects

As an insect pathogen, the gamma-proteobacterium Xenorhabdus nematophila likely possesses an arsenal of virulence factors, one of which is described in this work. We present evidence that the X . nematophilahaemolysin XhlA is required for full virulence towards Manduca sexta larvae. Lrp (leucine-responsive regulatory protein), FlhDC (regulator of flagella synthesis), and iron (II) limitation positively influenced xhlA transcript levels, suggesting XhlA expression is linked with nutrient acquisition and motility regulons. To help understand the role of XhlA in virulence, we examined its cellular targets and found that XhlA was a cell-surface associated haemolysin that lysed the two most prevalent types of insect immune cells (granulocytes and plasmatocytes) as well as rabbit and horse erythrocytes. Taken together, the need for xhlA for full virulence and XhlA activity towards insect immune cells suggest this haemolysin functions in X. nematophila immune evasion during infection. Analysis of a gene located immediately upstream of the xhlA locus, hcp (haemolysin co-regulated protein) revealed that its transcript levels were elevated during iron (III) limitation and its expression was Lrp-dependent. Further characterization of xhlA, hcp, and lrp will clarify their regulatory and functional relationships and their individual roles during the infectious process.

A Pilot-Scale Expressed Sequence Tag Analysis of Beauveria Bassiana Gene Expression Reveals a Tripeptidyl Peptidase that is Differentially Expressed in vivo

The entomopathogen Beauveria bassiana is a dimorphic fungus that displays an in vivo-specific, yeast-like parasitic phase. In order to study the transcriptome of B. bassiana during this unique developmental phase, we developed a method to harvest in vivo B. bassiana cells from infected Manduca sexta larvae. The infected hemolymph was collected just prior to insect death and subjected to gradient centrifugation, which allowed for separation of the B. bassiana in vivo-produced cells from remaining insect hemocytes. Total RNA was extracted from the harvested fungal cells and used to construct a cDNA library that is representative of B. bassiana gene expression in vivo. Expressed Sequence Tags (ESTs) were generated and led to the cloning of two protease genes. One of these proteases was identified as a tripeptidyl peptidase (Bb TPP). The Bb TPP protease was shown to be up-regulated during infection, and identification of a signal peptide suggested that the enzyme is secreted in the host hemolymph. Although its activity and role have yet to be characterized, the Bb TPP protease appears as a likely candidate for being involved in B. bassiana pathogenesis. The identification of this novel, up-regulated protease also suggests that random sequencing from our in vivo cDNA library may be a valuable step towards identifying biologically active metabolites produced in vivo by B. bassiana.

Microbial infection causes the appearance of hemocytes with extreme spreading ability in monolayers of the tobacco hornworm Manduca sexta

The ability to adhere to and spread on a surface is a common property of insect blood cells. Spreading on a glass surface by insect hemocytes is often used as a measure of immune fitness that can be inhibited by some insect pathogens and parasites. Here, we report that upon infection of the tobacco hornworm Manduca sexta with either a fungus (Beauveria bassiana) or a bacterium (Photorhabdus luminescens), a new type of hemocyte, not previously observed in healthy insects, was found in hemocyte monolayers. These cells have a distinctive morphology, characterised by extreme spreading ability. They achieve a diameter of up to 120 microm after 1 h on glass coverslips and are therefore extremely thin. These hyper-spreading cells first appear in fungal-infected insects prior to hyphal growth. Their numbers later fall to zero as the pathogen begins to proliferate. The same hyper-spreading cells are induced after a 24 h delay following an injection of laminarin, a source of the fungal cell wall polymer beta-1,3-glucans. Wounding, on the other hand, did not cause the appearance of hyper-spreading cells. Evidence is presented here that is consistent with these spreading cells having a role in the cellular immune response of nodule formation.

Effect of the insect pathogenic bacterium Photorhabdus on insect phagocytes

Photorhabdus are insect pathogenic bacteria that replicate within the insect haemocoel following release from their entomopathogenic nematode symbionts. To investigate how they escape the cellular immune response we examined the effects of two strains of Photorhabdus, W14 and K122, on Manduca sexta phagocytes (haemocytes), in vitro and in vivo. Following injection of Esherichia coli into Manduca larvae, these non-pathogenic bacteria are rapidly cleared from the haemolymph and the number of free haemocytes transiently increases. In contrast, following injection of either strain of pathogenic Photorhabdus, the bacteria grow rapidly while the number of haemocytes decreases dramatically. In vitro incubation of haemocytes with either Photorhabdus supernatant reduced haemocyte viability, and the W14 supernatant caused distinct changes in the actin cytoskeleton morphology of different haemocyte cell types. In phagocytosis assays both Photorhabdus strains can inhibit their own phagocytosis whether the bacterial cells are alive or dead. Further, the supernatant of W14 also contains a factor capable of inhibiting the phagocytosis of labelled E. coli. Together these results suggest that Photorhabdus evades the cellular immune response by killing haemocytes and suppressing phagocytosis by mechanisms that differ between strains.

Endospore Degradation in an Oligosporogenic, Crystalliferous Mutant of Bacillus thuringiensis

We isolated a new oligosporogenic mutant from Bacillus thuringiensis var. kurstaki HD73 that retains the ability to produce insecticidal crystal inclusions. Sporulation in this mutant initiates in a manner similar to the wild-type strain, and under the electron microscope endospores are seen, but these do not reach maturity (except for 0.2% of them). At a late stage, the coat surrounding the forespore seems to lack shape and to be empty. Most mutant cells exhibit a well-formed bipyramidal crystal but are completely devoid of the forespore. The mutant has a functional SigK holoenzyme, which is required for the expression of genes involved in the formation of spore coat and cortex and for cry1A transcription from the BtII promoter. Defective maturation of spores could be due to an inadequate forespore coat or cortex structure resulting in the arrest of sporulation at late stage III or early stage IV.

Prophenoloxidase-activating proteinase-3 (PAP-3) from Manduca sexta hemolymph: a clip-domain serine proteinase regulated by serpin-1J and serine proteinase homologs

Phenoloxidase (PO) is a key enzyme implicated in several defense mechanisms in insects and crustaceans. It is converted from prophenoloxidase (proPO) through limited proteolysis by prophenoloxidase-activating proteinase (PAP). We previously isolated PAP-1 from integument and PAP-2 from hemolymph of the tobacco hornworm, Manduca sexta. Here, we report the purification, characterization, and regulation of PAP-3 from the hemolymph. Similar to M. sexta PAP-2, PAP-3 consists of two amino-terminal clip domains followed by a carboxyl-terminal catalytic domain, whereas PAP-1 contains only one clip domain at its amino-terminus. Purified PAP-3 cleaved proPO at Arg51 and generated a low level of PO activity. However, the enzyme efficiently activated proPO when M. sexta serine proteinase homolog-1 and -2 were present. These proteinase-like proteins associate with immulectin-2, a pattern-recognition receptor for lipopolysaccharide. M. sexta PAP-3 was inhibited by recombinant serpin-1J, which formed an SDS-stable complex with the enzyme. PAP-3 mRNA was detected at a low level in the fat body or hemocytes of naive larvae, but was elevated in insects that had been challenged with bacteria. These data, along with our previous results on PAP-1 and PAP-2, indicate that proPO activation by PAPs is a tightly regulated process. Individual PAPs could play different roles during immune responses and developmental processes.

Prostaglandins, not lipoxygenase products, mediate insect microaggregation reactions to bacterial challenge in isolated hemocyte preparations

Nodulation is the predominant cellular defense reaction to bacterial challenge in insects. Eicosanoids mediate several steps in the nodulation process, including formation of hemocyte microaggregations. Isolated hemocyte preparations synthesize and secrete eicosanoids, which mediate hemocytic immune reactions. Two major groups of eicosanoids are prostaglandins (products of cyclooxygenase pathways) and various products of lipoxygenase pathways. In this study, we test the hypothesis that prostaglandins, but not lipoxygenase products, mediate hemocyte microaggregation reactions in response to bacterial challenge. Our results indicate that isolated hemocyte preparations pretreated with the cyclooxygenase inhibitors indomethacin and naproxen yielded fewer microaggregates than untreated control groups (3.7 x 10(5) microaggregates/ml hemolymph vs. 11.0 x 10(5) microaggregates/ml hemolymph). These inhibitors influence hemocyte microaggregate formation in a dose-dependent manner in treatments ranging from 0 to 200 microM. The lipoxygenase inhibitors esculetin and caffeic acid did not impact the formation of microaggregates in this system. The influence of the phospholipase A(2) inhibitor dexamethasone was reversed by amending experimental (dexamethasone-treated) preparations with prostaglandin H(2), but not prostaglandin D(2), prostaglandin E(2), nor 5(S)-hydroperoxy-6E,8Z,11Z,14Z-eicosatetraenoic acid, a product of the lipoxygenase pathway. We infer that prostaglandins are the primary mediators of microaggregation reactions to bacterial challenge in insect hemocyte preparations.

Manduca sexta lipopolysaccharide-specific immulectin-2 protects larvae from bacterial infection

We previously reported the isolation of a lipopolysaccharide (LPS)-specific immulectin-2 from the tobacco hornworm, Manduca sexta [J. Biol. Chem. 275 (2000) 37373]. Immulectin-2 is a C-type lectin that is present at a constitutively low level in hemolymph of naive larvae, and its synthesis is induced after injection of Gram-negative bacteria or LPS. Immulectin-2 contains two carbohydrate recognition domains. It binds to LPS and stimulates prophenoloxidase activation in plasma. In this paper, we focus on properties of carbohydrate recognition domain-2 of immulectin-2 and the biological functions of immulectin-2 in immune responses. The carboxyl-terminal carbohydrate recognition domain (CRD2) of immulectin-2 was able to bind bacterial LPS. Binding of recombinant CRD2 to LPS stimulated activation of prophenoloxidase in plasma. Injection of antiserum against immulectin-2 into M. sexta larvae inhibited clearance of a Gram-negative bacterial pathogen, Serratia marcescens, and decreased survival of infection. These results suggest that immulectin-2 plays an important role in the immune system of M. sexta, and helps to protect the animal from Gram-negative bacterial infections.

The bacterium Xenorhabdus nematophilus depresses nodulation reactions to infection by inhibiting eicosanoid biosynthesis in tobacco hornworms, Manduca sexta

The bacterium, Xenorhabdus nematophilus, is a virulent insect pathogen. We tested the hypothesis that this bacterium impairs insect cellular immune defense reactions by inhibiting biosynthesis of eicosanoids involved in mediating cellular defense reactions. Fifth instar tobacco hornworms, Manduca sexta, produced melanized nodules in reaction to challenge with living and heat-killed X. nematophilus. However, the nodulation reactions were much attenuated in insects challenged with living bacteria (approximately 20 nodules/larva for living bacteria vs. approximately 80 nodules/larva in insects challenged with heat-killed bacteria). The nodule-inhibiting action of living X. nematophilus was due to a factor that was present in the organic, but not aqueous, fraction of the bacterial cultural medium. The nodule-inhibiting factor in the organic fraction was labile to heat treatments. The immunodepressive influence of the factor in the organic fraction was reversed by treating challenged hornworms with arachidonic acid. The factor also depressed nodulation reactions to challenge with the plant pathogenic bacteria, Pseudomonas putida and Ralstonia solanacearum. These findings indicate that one or more factors from X. nematophilus depress nodulation reactions in tobacco hornworms by inhibiting eicosanoid biosynthesis.

Eicosanoids in insect immunity: bacterial infection stimulates hemocytic phospholipase A2 activity in tobacco hornworms

Intracellular phospholipase A(2) (PLA(2)) is responsible for releasing arachidonic acid from cellular phospholipids, and is thought to be the first step in eicosanoid biosynthesis. Intracellular PLA(2)s have been characterized in fat body and hemocytes from tobacco hornworms, Manduca sexta. Here we show that bacterial challenge stimulated increased PLA(2) activity in isolated hemocyte preparations, relative to control hemocyte preparations that were challenged with water. The increased activity was detected as early as 15 s post-challenge and lasted for at least 1 h. The increased activity depended on a minimum bacterial challenge dose, and was inhibited in reactions conducted in the presence of oleyoxyethylphosphorylcholine, a site-specific PLA(2) inhibitor. In independent experiments with serum prepared from whole hemolymph, we found no PLA(2) activity was secreted into serum during the first 24 h following bacterial infection. We infer that a hemocytic intracellular PLA(2) activity is increased immediately an infection is detected. The significance of this enzyme lies in its role in launching the biosynthesis of eicosanoids, which mediate cellular immune reactions to bacterial infection.