Insect immunity

Antiparasitic peptides

: The most important parasitic diseases, malaria, leishmaniasis, trypanosomiasis, and schistosomiasis, are a great burden to mankind, threatening the life of millions of people worldwide and mostly affecting the poorest. Because drug resistance is increasing and vaccines are rarely available, novel chemotherapeutic compounds are necessary in order to treat these devastating diseases. Insects serve as vectors of many human parasitic diseases and have been shown to express a huge variety of antimicrobial peptides (AMPs). Therefore, research activity on insect-derived AMPs has been increasing in the last 40 years. This chapter summarizes the current state of research on the possible role of AMPs as potential chemotherapeutic compounds against human parasitic diseases. As a representative antimicrobial peptide with antiparasitic activity, the structure of insect defensin A is shown [PDB accession code: 1ICA]. The molecule is surrounded by schematic representations of the human pathogenic parasites Plasmodium, Leishmania and Trypanosoma.

Immunosuppressive effect of cyclosporin A on insect humoral immune response

Cyclosporin A suppressed humoral immune response of Galleria mellonella larvae. Insects were immunized with LPS Pseudomonas aeruginosa and then injected with cyclosporin A. Immunosuppressive effects were expressed both, in larvae treated with cyclosporin A at the initial phase of immune response and at the effector phase of antibacterial immunity. Cyclosporin A moderately decreased lysozyme activity and significantly decreased antibacterial activity peptides against Escherichia coli. Immunosuppressive effects of cyclosporin A were observed after immunoblotting with antibodies anti-G. mellonella lysozyme. Tricine SDS/PAGE shown that synthesis of antibacterial peptides of larvae treated with cyclosporin A was considerably inhibited. Insects of impaired immune response by cyclosporin A action lost protective immunity to insect bacterial pathogen P. aeruginosa.

Antibacterial peptides in insect vectors of tropical parasitic disease

The induction and characterization of immune peptides in two groups of medically important insects, the mosquitoes and blackflies, is currently an important research area. Mosquitoes transmit a variety of viral and parasitic diseases including yellow fever, dengue, malaria and lymphatic filariasis. Simuliid black flies are vectors of river blindness. The diseases are together responsible for death and morbidity in millions of people each year. The relationship between inducible peptides and bacterial and parasitic infections in these insects is proving to be a complex one. The identification of an insect defensin (4 kDa) in Aedes aegypti, the yellow fever mosquito, has proved to be the first peptide characterized in a vector of human disease. This inducible molecule appears in the haemolymph in response to bacterial and to a lesser extent filarial infection. The characterization of inducible blackfly peptides has revealed potent inducible anti-Gram-positive as well as anti-Gram-negative activity. In addition, non-self recognition molecules such as phenoloxidase may play a part in differentiating one species of eukaryotic pathogen from another of the same genus. The interactions between the peptides and these other proteins are likely to be important in the establishment of a successful immune response against a parasitic pathogen, particularly as we now know these peptides to have anti-eukaryotic activity (against a range of parasite species). As well as being of fundamental interest in our understanding of host-parasite relationships, the indication that antibacterial peptides are toxic to parasitic organisms has implications for their possible use in the disease vector control strategies of the future. It may also mean that a revision in our understanding of their mode of action, loose as it is, has to take place.

Analysis of fat body transcriptome from the adult tsetse fly, Glossina morsitans morsitans

Tsetse flies (Diptera: Glossinidia) are vectors of pathogenic African trypanosomes. To develop a foundation for tsetse physiology, a normalized expressed sequence tag (EST) library was constructed from fat body tissue of immune-stimulated Glossina morsitans morsitans. Analysis of 20,257 high-quality ESTs yielded 6372 unique genes comprised of 3059 tentative consensus (TC) sequences and 3313 singletons (available at http://aksoylab.yale.edu). We analysed the putative fat body transcriptome based on homology to other gene products with known functions available in the public domain. In particular, we describe the immune-related products, reproductive function related yolk proteins and milk-gland protein, iron metabolism regulating ferritins and transferrin, and tsetse's major energy source proline biosynthesis. Expression analysis of the three yolk proteins indicates that all are detected in females, while only the yolk protein with similarity to lipases, is expressed in males. Milk gland protein, apparently important for larval nutrition, however, is primarily synthesized by accessory milk gland tissue.

Immune responses and parasite transmission in blood-feeding insects

The detailed model of insect immunity being built for Drosophila, allied to mass sequencing programs for blood-feeding insects, has led to advances in our understanding of the interaction between pathogens and insect vectors. An outline of insect immunity is given here based on the Drosophila studies, which is used as a framework to discuss recent work on Plasmodium-mosquito and Trypanosoma-tsetse interactions.

Interactions between tsetse and trypanosomes with implications for the control of trypanosomiasis

Tsetse flies (Diptera: Glossinidae) are vectors of several species of pathogenic trypanosomes in tropical Africa. Human African trypanosomiasis (HAT) is a zoonosis caused by Trypanosoma brucei rhodesiense in East Africa and T. b. gambiense in West and Central Africa. About 100000 new cases are reported per year, with many more probably remaining undetected. Sixty million people living in 36 countries are at risk of infection. Recently, T. b. gambiense trypanosomiasis has emerged as a major public health problem in Central Africa, especially in the Democratic Republic of Congo, Angola and southern Sudan where civil war has hampered control efforts. African trypanosomes also cause nagana in livestock. T. vivax and T. congolense are major pathogens of cattle and other ruminants, while T. simiae causes high mortality in domestic pigs; T. brucei affects all livestock, with particularly severe effects in equines and dogs. Central to the control of these diseases is control of the tsetse vector, which should be very effective since trypanosomes rely on this single insect for transmission. However, the area infested by tsetse has increased in the past century. Recent advances in molecular technologies and their application to insects have revolutionized the field of vector biology, and there is hope that such new approaches may form the basis for future tsetse control strategies. This article reviews the known biology of trypanosome development in the fly in the context of the physiology of the digestive system and interactions of the immune defences and symbiotic flora.

Control of tsetse flies and trypanosomes using molecular genetics

TSETSE FLIES (DIPTERA: Glossinidae) are important agricultural and medical vectors transmitting the African trypanosomes, the agents of sleeping sickness disease in humans and various diseases in animals (nagana). While the prevalence of disease has increased to epidemic proportions, lack of a mammalian vaccine and affordable and effective drugs have hindered disease control. Trypanosomiasis management relies heavily on the control of its single insect vector, the tsetse fly. Despite the effectiveness of some of these tools, their impact on disease control has not been sustainable due to their local nature and extensive dependence on community participation. Recent advances in molecular technologies and their application to insects have revolutionized the field of vector biology, and there is hope that such new approaches may form the basis for future tsetse interventions. The success of the genetic approaches aiming to disrupt the transmission cycle of the parasite in their invertebrate host depends on full understanding of the interaction between tsetse and trypanosomes. This article reviews the biology of trypanosome development in the fly and the multiple bacterial symbionts that inhabit the same gut environment. The availability of a genetic transformation system for the midgut symbiont allows for gene products to be expressed in vivo in the tsetse gut where they can produce a hostile environment for pathogen transmission. The characterization of gene product(s) with anti-pathogenic properties and their expression in vivo is discussed. A strategy is outlined where the replacement of susceptible insect phenotypes with their engineered refractory counterparts can result in decreased disease transmission.

Tsetse immune responses and trypanosome transmission: implications for the development of tsetse-based strategies to reduce trypanosomiasis

Tsetse flies are the medically and agriculturally important vectors of African trypanosomes. Information on the molecular and biochemical nature of the tsetse/trypanosome interaction is lacking. Here we describe three antimicrobial peptide genes, attacin, defensin, and diptericin, from tsetse fat body tissue obtained by subtractive cloning after immune stimulation with Escherichia coli and trypanosomes. Differential regulation of these genes shows the tsetse immune system can discriminate not only between molecular signals specific for bacteria and trypanosome infections but also between different life stages of trypanosomes. The presence of trypanosomes either in the hemolymph or in the gut early in the infection process does not induce transcription of attacin and defensin significantly. After parasite establishment in the gut, however, both antimicrobial genes are expressed at high levels in the fat body, apparently not affecting the viability of parasites in the midgut. Unlike other insect immune systems, the antimicrobial peptide gene diptericin is constitutively expressed in both fat body and gut tissue of normal and immune stimulated flies, possibly reflecting tsetse immune responses to the multiple Gram-negative symbionts it naturally harbors. When flies were immune stimulated with bacteria before receiving a trypanosome containing bloodmeal, their ability to establish infections was severely blocked, indicating that up-regulation of some immune responsive genes early in infection can act to block parasite transmission. The results are discussed in relation to transgenic approaches proposed for modulating vector competence in tsetse.

Purification of an insect defensin from the mosquito, Aedes aegypti

Using a new, sensitive assay of bacterial growth inhibition, inducible antibacterial activity has been identified in the haemolymph of the mosquito, Aedes aegypti following inoculation with bacteria or with microfilariae of the filarial nematode Brugia pahangi, but not after inoculation with sterile culture medium. A lower level of antibacterial activity has also been observed in untreated individual mosquitoes. Following bacterial inoculation, a basic, inducible antibacterial peptide has been detected using native PAGE at pH 4, which corresponds with a 4.5 kDa peptide detected by tricine SDS-PAGE followed by silver staining. A peptide has been purified from immune haemolymph by ultrafiltration, followed by reversed-phase HPLC, yielding a single major peak with antibacterial activity. Partial amino acid sequence analysis of this fraction has revealed substantial homology with insect defensins. The data are consistent with the peptide being another member of this family, and we propose the name Aedes aegypti defensin.

Approaches to vector control: new and trusted. 1. Humoral immune responses in blackfly and mosquito vectors of filariae

The vectors of filariasis, mosquitoes and blackflies, are capable of mounting a defence response to the infection. This selective review describes the molecules that are involved in these immune systems. Several antibacterial peptides are known to be induced and secreted into the haemolymph by the fat body and the circulating haemocytes. In addition, haemagglutinating lectins with carbohydrate specificities to the surface of the developing filarial larvae appear. Activation of a range of proteases occurs rapidly as does activation of the prophenoloxidase pathway. The possible roles of these and other molecules is discussed, together with mention of a working hypothesis as to how these molecules may be regulated.

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.

Antibacterial activity inducible in the haemolymph of the silkworm, Bombyx mori, by injection of formalin-treated Escherichia coli K-12 during the fifth larval instar and pharate adult development

1. Antibacterial activity inducible in the haemolymph of the silkworm, Bombyx mori, by immunization, i.e. by injection of formalin-treated Escherichia coli (E. coli) K-12 during the fifth larval instar and pharate adult development that was reared aseptically on an artificial diet was determined by inhibition zone assay using the same bacterium as a test organism. 2. A peak of antibacterial activity was observed in each development stage; approximately 8 mm in diameter of a clear zone at days 3 or 4 in the fifth larval instar and approximately 5 mm at day 1 in the pharate adults. 3. Acid polyacrylamide gel electrophoresis of immunized haemolymph followed by overlay assay showed that an activity band was associated with two peptide bands that were similar to the cecropin-like peptides A and B that were reported in the silkworm (Morishima et al., 1988, Agri. Biol. Chem. 52, 929-934). Any other activity bands were not observed. No activity band was detectable from the haemolymph of non-immunized insects. 4. Fractionation of antibacterial peptides in immunized haemolymph on a CM-cellulose column resulted in separation of two groups of activity, both in the fifth instar larvae and in the pharate adults with a slight difference in elution conditions. 5. Duration of high antibacterial activity induced by a single immunization was approximately 12 hr in the fifth instar day 3 larvae and 48 hr in the day 2 pharate adults.

Insect antibacterial proteins: not just for insects and against bacteria

In response to a bacterial infection, insects launch an array of countermeasures. Among these are the antibacterial proteins, which effectively lyse bacteria or are bacteriostatic. These proteins were generally assumed to be restricted to insects, yet recent information has shown some homologous counterparts in vertebrates, including humans. Recent data have revealed that at least some of these proteins can also act against eukaryotic cells, including human infectious parasites. The latter activities have opened up new possibilities for disease control.

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.