Insect immunity

Broad activity against porcine bacterial pathogens displayed by two insect antimicrobial peptides moricin and cecropin B

In response to infection, insects produce a variety of antimicrobial peptides (AMPs) to kill the invading pathogens. To study their physicochemical properties and bioactivities for clinical and commercial use in the porcine industry, we chemically synthesized the mature peptides Bombyx mori moricin and Hyalophora cecropia cecropin B. In this paper, we described the antimicrobial activity of the two AMPs. Moricin exhibited antimicrobial activity on eight strains tested with minimal inhibitory concentration values (MICs) ranging between 8 and 128 μg/ml, while cecropin B mainly showed antimicrobial activity against the Gramnegative strains with MICs ranging from 0.5 to 16 μg/ml. Compared to the potent antimicrobial activity these two AMPs displayed against most of the bacterial pathogens tested, they exhibited limited hemolytic activity against porcine red blood cells. The activities of moricin and cecropin B against Haemophilus parasuis SH 0165 were studied in further detail. Transmission electron microscopy (TEM) of moricin and cecropin B treated H. parasuis SH 0165 indicated extensive damage to the membranes of the bacteria. Insights into the probable mechanism utilized by moricin and cecropin B to eliminate pathogens are also presented. The observations from this study are important for the future application of AMPs in the porcine industry.

Drosophilagenescutandminiatureare associated with the susceptibility to infection bySerratia marcescens

A mutant strain ofDrosophila melanogasterwith five markers on theX-chromosome was found to be more sensitive than the wild type when infected with an insect-pathogenic strain ofSerratia marcescens. Two of the five mutations in this fly strain,cutandminiature, were found to be responsible for this sensitivity. A double-mutant, with bothcutandminiature, was as sensitive toSerratiainfection as was the original sensitiveDrosophilastrain with all five mutations. Recombinant flies with other alleles ofcutandminiaturewere also sensitive. A revertant ofcutwas found to be less sensitive than the parental flies. Our insect pathogenic strain ofSerratiaproduces several proteases and a chitinase. A bacterial mutant, lacking proteases and chitinase, was found to be less virulent than wild-type bacteria. When pupal shells from resistant andcut-miniatureflies were incubated with a mixture of protease and chitinase there was a release ofN-acetyl glucosamine, and 50% more material was liberated from pupal shells of sensitive flies. Sensitive flies reared on sucrose infected withSerratiashowed bacteria in their hemolymph earlier than wild-type flies. We conclude thatDrosophilagenes forcutandminiatureare associated with the sensitivity toSerratiainfection, presumably because the gut peritrophic membrane is more susceptible to bacterial proteases and chitinase.

Purification and characterization of an extracellular protease from Xenorhabdus nematophila involved in insect immunosuppression

Xenorhabdus nematophila, a bacterium pathogenic for insects associated with the nematode Steinernema carpocapsae, releases high quantities of proteases, which may participate in the virulence against insects. Zymogram assays and cross-reactions of antibodies suggested that two distinct proteases were present. The major one, protease II, was purified and shown to have a molecular mass of 60 kDa and an estimated isoelectric point of 8.5. Protease II digested the chromogenic substrate N-tosyl-Gly-Pro-Arg-paranitroanilide (pNA) with V(max) and K(m) values of 0.0551 microM/min and 234 microM, respectively, and the substrate DL-Val-Leu-Arg-pNA with V(max) and K(m) values of 0.3830 microM/min and 429 microM, respectively. Protease II activity was inhibited 93% by Pefabloc SC and 45% by chymostatin. The optimum pH for protease II was 7, and the optimum temperature was 23C. Proteolytic activity was reduced by 90% at 60 degrees C for 10 min. Sequence analysis was performed on four internal peptides that resulted from the digestion of protease II. Fragments 29 and 45 are 75 and 68% identical to alkaline metalloproteinase produced by Pseudomonas aeruginosa. Fragment 29 is 79% identical to a metalloprotease of Erwinia amylovora and 75% identical to the protease C precursor of Erwinia chrysanthemi. Protease II showed no toxicity to hemocytes but destroyed antibacterial activity on the hemolymph of inoculated insects' larvae and reduced 97% of the cecropin A bacteriolytic activity.

Viresin. A novel antibacterial protein from immune hemolymph of Heliothis virescens pupae

Immune hemolymph was collected from fifth instar larvae and 1-day-old pupae of Heliothis virescens after injection of prepupae with live Enterobacter cloacae. Induction of antibacterial activity against Escherichia coli K12 D31 was 7.5 times greater in pupal than in larval immune hemolymph. Lysozyme activity of immune pupal hemolymph against Micrococcus lysodeikticus was 11 times greater when compared with lysozyme activity of immune larval hemolymph. Early pupal immune response with regard to antibacterial activity was much greater than larval immune response in H. virescens. Normal pupal hemolymph showed an increase in antibacterial activity and lysozyme that was induced during metamorphosis. Antibacterial protein was isolated together with lysozyme by gel filtration chromatography and then separated from lysozyme by sequential electrophoresis with a native acid gel and SDS gel. Molecular mass of antibacterial protein was estimated to be 12 kDa. The N-terminal amino acid sequence of 12-kDa protein was different from those of antibacterial molecules found in other insects and has not been identified before. A sample containing 12-kDa protein was negative for immunoblotting with anti-synthetic cecropin B antibody. We have named the novel 12-kDa antibacterial protein viresin. Viresin showed antibacterial activity against several Gram-negative bacteria including E. cloacae but not against Gram-positive bacteria.

Attacin--an insect immune protein--binds LPS and triggers the specific inhibition of bacterial outer-membrane protein synthesis

Attacin is a 20 kDa antibacterial protein, originally isolated from the immune haemolymph of Hyalophora cecropia. It has been demonstrated previously that attacin causes increased permeability of the outer membrane of Escherichia coli and inhibition of outer-membrane protein synthesis at the transcriptional level. This is accompanied by inhibition of growth. Here, LPS is shown to serve as the receptor for attacin and evidence is presented that attacin does not need to enter the cell to exert its activity. The increase in outer-membrane permeability precedes any increase in inner-membrane permeability by at least one generation time (approximately 45 min), and the inhibiting effect of attacin on synthesis of outer-membrane proteins is detectable after only 10 min. It is also shown that attacin causes induction of several stress proteins and increased synthesis of LPS within, respectively, 25 and 60 min of treatment. Based on the results presented, it is proposed that attacin has the unique ability to specifically interfere with synthesis of outer-membrane proteins without entering the inner membrane or cytoplasm.

A novel protease in the pupal yellow body of Sarcophaga peregrina (flesh fly). Its purification and cDNA cloning

We purified a novel serine protease with a molecular mass of 26 kDa from Sarcophaga pupae. This protease appeared almost exclusively in the yellow body, an organ that develops temporarily in the pupae of dipteran insects and expands to form the adult midgut by engulfing the larval midgut. cDNA analysis revealed that this protease consists of 239 amino acid residues and has significant structural similarity with bovine trypsin (about 40% sequence identity). The 26-kDa protease gene was transiently activated in 1-day-old pupae. The protease was found to cross-react immunologically with antibody against sarcotoxin IA, an antibacterial protein produced by this insect. It is suggested that this protease participates in the decomposition of the larval midgut in the yellow body during metamorphosis.

Formation of pores in Escherichia coli cell membranes by a cecropin isolated from hemolymph of Heliothis virescens larvae

The insect humoral defense system produces antibacterial peptides called cecropins. Cecropins were initially isolated from Hyalophora cecropia pupae and have since been isolated and identified in various insects. In this study, we have isolated and identified a cecropin from Heliothis virescens larvae. Rabbit IgG were raised against synthetic cecropin B. Affinity chromatography with the rabbit anti-(cecropin B) IgG was used to isolate a cecropin from hemolymph of H. virescens larvae. Acid gel electrophoresis followed by a bacterial-overlay analysis showed that Heliothis cecropin is a basic peptide of low molecular mass with bactericidal activity against Escherichia coli K12 D31. Heliothis cecropin is therefore analogous to synthetic cecropin B. One unresolved issue concerning cecropins and other antibiotic peptides is the mode of action by which they kill bacteria. By means of electron microscopy and immunocytochemistry with gold-labeled rabbit anti-cecropin IgG, binding of purified and synthetic cecropin to the cell membranes of E. coli K12 D31 cells was observed. Small lesions in the cell membrane were seen that had a diameter of 9.6 nm and internal pore of 4.2 nm. The Heliothis cecropin was found to be a pore-forming molecule that causes lesions in the cell membrane of E. coli K12 D31. The lesions lead to leakage of cytoplasmic contents and death of bacteria.

The lysozyme locus in Drosophila melanogaster: an expanded gene family adapted for expression in the digestive tract

Lysozyme has been studied in insects as part of the system of inducible antibacterial defence in the haemolymph. We recently found two Drosophila lysozyme genes that are constitutively expressed in the digestive tract, and are probably involved in the digestion of bacteria in the food. To obtain an overview of the lysozyme genes in this species and their possible roles in immunity and digestion, we have now characterized all six lysozyme genes in the cloned part of the lysozyme locus at 61F, and a seventh gene that maps to the same chromosomal location. The expression of the genes follows four different patterns: firstly, four closely related genes, LysB, C, D and E, are all strongly expressed in the midgut of larvae and adults; secondly, LysP is expressed in the adult salivary gland; thirdly, LysS is expressed mainly in the gastric caecae of larvae; and finally, LysX is primarily expressed in the metamorphosing midgut of late larvae and early pupae. The LysD-like genes and LysS are strongly repressed in artificially infected animals, possibly reflecting a malaise reaction in the digestive tract. None of the genes is expressed in the fat body or haemocytes. Thus rather than being a component of the haemolymph, the Drosophila lysozymes are found mainly in the digestive tract where they are expressed at a high level. Furthermore all genes, except LysP, encode acidic proteins, in contrast to the strongly basic "typical" lysozymes. This is highly reminiscent of the situation in ruminants, where the lysozymes have been recruited for the digestion of symbiotic bacteria in the stomach.

Purification and primary structure of ceratotoxin A and B, two antibacterial peptides from the female reproductive accessory glands of the medfly Ceratitis capitata (Insecta:Diptera)

In the present article we report the purification and the amino acid sequence of two antibacterial peptides present in the secretion of the female reproductive accessory glands of the dipteran insect Ceratitis capitata. Both peptides consist of 29 amino acid residues, are heat stable, strongly basic and differ from each other for the substitution of two amino acids. Their primary sequence and predicted secondary structure are related to other families of peptides known to have lytic and/or antibacterial activity. We propose the name ceratotoxins (from Ceratitis) for these antibacterial peptides.

The humoral antibacterial response of Drosophila

Drosophila, like other insects, responds to the injection of bacteria by the rapid and transient synthesis of a battery of potent antibacterial peptides. Only a few of these peptides have been fully characterized to date. We review our recent data on the control of the expression of a gene encoding one of the induced peptides, i.e. diptericin. Our data highlight the role of proximal cis-regulatory motifs similar to regulatory elements binding NF-kappa B and NF-IL6 in promoters of some immune genes of mammals. We argue that the Drosophila host defense is homologous to the mammalian acute phase response.

Expression and characterization of cDNAs for cecropin B, an antibacterial protein of the silkworm, Bombyx mori

To analyze the induction mechanism of antibacterial protein gene expression, cDNAs coding for cecropin B have been cloned from the B. mori fat body cDNA library. Nucleotide sequences of two positive clones were determined and their amino acid sequences deduced. They revealed that these clones coded for the same cecropin, which is identical to purified cecropin B. However, the cDNAs contained different nucleotides at the third codon position and 5' or 3' non-coding regions. Results obtained by Northern blot analysis showed that the gene expression of B. mori cecropin B was rapidly induced by Escherichia coli and reached maximum levels 8 h after immunization. The expression of cecropin B gene occurred specifically in tissues, mainly in the fat body and hemocytes.

The lysozyme locus in Drosophila melanogaster: different genes are expressed in midgut and salivary glands

As part of a study of the genes involved in antibacterial defense in Drosophila melanogaster, we have isolated genomic clones harboring a family of chicken-type lysozyme genes, using a lepidopteran lysozyme cDNA as probe. The locus was mapped to the cytological location 61F1-4 on the third chromosome and two of the genes at this locus, LysD and LysP, were analyzed in detail. In contrast to the bacteria-induced lysozymes in the hemolymph of many insects, the transcription levels of both Drosophila genes decrease after bacterial injections into the hemocoel. Apparently, these gene products, like the specifically adapted lysozymes in mammalian foregut fermenters, have been recruited for the digestion of bacteria present in fermenting food. The LysD gene is expressed in an anterior section of the midgut during all feeding stages of development in both larvae and adults. The LysP gene is only active in the adult where it is expressed in the salivary glands. The transcription units for both genes are very compact and they lack introns. Lysozyme D is unusual in that it is predicted to have an acidic isoelectric point whereas lysozyme P appears to be a typical basic lysozyme.

CecC, a cecropin gene expressed during metamorphosis in Drosophila pupae

Cecropins are antibacterial peptides, induced in insects in response to bacterial infections. In Drosophila, three cecropin genes have previously been characterized, CecA1, CecA2, and CecB, in a dense cluster at 99E on the third chromosome. From the same locus, we now describe a fourth member of the cecropin gene family, CecC, which is mainly expressed at the early pupal stage. In situ hybridization to immunized pupae show that CecC is induced in the anterior end of the larval hindgut and in other larval tissues that are undergoing histolysis. Within these other tissues it is often expressed in distinct foci that may correspond to hemocytes. A similar pattern of expression in the metamorphosing pupa is also observed for the CecA and CecB genes. Comparing the DNA sequences of the cecropin genes, a conserved region is observed about 30 bp upstream of the TATA box. It consists of three shorter motifs, two of which are reminiscent of a putative promoter element in immune protein genes from the cecropia moth.

Involvement of sapecin in embryonic cell proliferation of Sarcophaga peregrina (flesh fly)

Addition of antibodies against sapecin to the culture medium of NIH-Sape-4 cells derived from a Sarcophaga embryo greatly inhibited cell proliferation, whereas addition of sapecin stimulated cell proliferation. These results suggest that sapecin is involved in the proliferation of embryonic cells of Sarcophaga. Sapecin is known to have potent antibacterial activity, so it seems to have two different biological functions: i.e. protection against bacterial infection and stimulation of embryonic cell proliferation.

Structure and expression of the attacin genes in Hyalophora cecropia

To study the regulation of the immune genes in insects, we have cloned and sequenced the attacin gene locus of the giant silk moth Hyalophora cecropia. The locus contains one acidic and one basic attacin gene as well as two pseudogenes, which are remnants of basic attacin genes. A small insertion element was found within the locus. The two functional attacin genes are transcribed in opposite directions and have two introns inserted at homologous positions. A common sequence, GGGGATTCCT, is found at nucleotide position -48 in the acidic gene and at nucleotide position -58 in the basic gene. Interestingly, this decanucleotide is similar to the consensus of the NF-k B-binding site. Expression studies revealed that both attacins are strongly induced by phorbol 12-myristate 13-acetate, lipopolysaccharide and bacteria. However, only the acidic attacin gene showed a clear response to injury.