Microbial activation of prophenoloxidase from immune insect larvae

Molecular mechanism and potential application of bacterial infection in the silkworm, Bombyx mori

As a representative species of Lepidoptera, Bombyx mori has been widely studied and applied. However, bacterial infection has always been an important pathogen threatening the growth of silkworms. Bombyx mori can resist various pathogenic bacteria through their own physical barrier and innate immune system. However, compared with other insects, such as Drosophila melanogaster, research on the antibacterial mechanism of silkworms is still in its infancy. This review systematically summarized the routes of bacterial infection in silkworms, the antibacterial mechanism of silkworms after ingestion or wounding infection, and the intestinal bacteria and infection of silkworms. Finally, we will discuss silkworms as a model animal for studying bacterial infectious diseases and screening antibacterial drugs.

Immune response and survival of Circulifer haematoceps to Spiroplasma citri infection requires expression of the gene hexamerin

Spiroplasma citri is a cell wall-less bacterium that infects plants. It is transmitted by the leafhopper Circulifer haematoceps, which hosts this bacterium in the haemocel and insect tissues. Bacterial factors involved in spiroplasma colonization of the insect host have been identified, but the immune response of the leafhopper to S. citri infection remains unknown. In this study, we showed that C. haematoceps activates both humoral and cellular immune responses when challenged with bacteria. When infected by S. citri, C. haematoceps displayed a specific immune response, evidenced by activation of phagocytosis and upregulation of a gene encoding the protein hexamerin. S. citri infection also resulted in decreased phenoloxidase-like activity. Inhibition of hexamerin by RNA interference resulted in a significant reduction in phenoloxidase-like activity and increased mortality of infected leafhoppers. Therefore, the gene hexamerin is involved in S. citri control by interfering with insect phenoloxidase activity.

Lysozyme and defense peptides as suppressors of phenoloxidase activity in Galleria mellonella

The prophenoloxidase (proPO) cascade supplies quinones and other reactive compounds for melanin formation, protein cross-linking, hemolymph coagulation, and killing of microbial invaders as well as parasites. The high cytotoxicity of the generated compounds requires a strict control of the activation of the proPO system and phenoloxidase (PO) activity to minimize damage to host tissues and cells. The PO activity in hemolymph of Escherichia coli challenged Galleria mellonella larvae increased, with a temporal drop 1 h after the challenge, reaching the highest level 24 h after the challenge. In the present study, a potential role of G. mellonella defense peptides and lysozyme in controlling the proPO system was investigated. The effects of purified defense peptides (anionic peptides 1 and 2, cecropin D-like peptide, Galleria defensin, proline-rich peptides 1 and 2) and lysozyme were analyzed. Four compounds, namely lysozyme, Galleria defensin, proline-rich peptide 1, and anionic peptide 2, decreased the hemolymph PO activity considerably, whereas the others did not affect the enzyme activity level. Our results indicate that these hemolymph factors could play multiple and distinct roles in the insect immune response.

Activation of autophagic programmed cell death and innate immune gene expression reveals immuno-competence of integumental epithelium in Bombyx mori infected by a dipteran parasitoid

In insects, the integument forms the primary barrier between the environment and internal milieu, but cellular and immune responses of the integumental epithelium to infection by micro- and macro-parasites are mostly unknown. We elucidated cellular and immune responses of the epithelium induced through infection by a dipteran endoparasitoid, Exorista bombycis in the economically important silkworm Bombyx mori. Degradative autophagic vacuoles, lamella-like bodies, a network of cytoplasmic channels with cellular cargo, and an RER network that opened to vacuoles were observed sequentially with increase in age after infection. This temporal sequence culminated in apoptosis, accompanied by the upregulation of the caspase gene and fragmentation of DNA. The infection significantly enhanced the tyrosine level and phenol oxidase activity in the integument. Proteomic analysis revealed enhanced expression of innate immunity components of toll and melanization pathways, cytokines, signaling molecules, chaperones, and proteolytic enzymes demonstrating diverse host responses. qPCR analysis revealed the upregulation of spatzle, BmToll, and NF kappa B transcription factors Dorsal and BmRel. NF kappa B inhibitor cactus showed diminished expression when Dorsal and BmRel were upregulated, revealing a negative correlation (R = (-)0.612). During melanization, prophenol oxidase 2 was expressed, a novel finding in integumental epithelium. The integument showed a low level of melanin metabolism and localized melanism in order to prevent the spreading of cytotoxic quinones. The gene-encoding proteolytic enzyme, beta-N-acetylglucosaminidase, was activated at 24 h post-infection, whereas chitinase, was activated at 96 h post-infection; however, most of the immune genes enhanced their expression in the early stages of infection. Thus the integument contributes to humoral immune responses that enhance resistance against macroparasite invasion.

The innate immune and systemic response in honey bees to a bacterial pathogen, Paenibacillus larvae

Background

There is a major paradox in our understanding of honey bee immunity: the high population density in a bee colony implies a high rate of disease transmission among individuals, yet bees are predicted to express only two-thirds as many immunity genes as solitary insects, e.g., mosquito or fruit fly. This suggests that the immune response in bees is subdued in favor of social immunity, yet some specific immune factors are up-regulated in response to infection. To explore the response to infection more broadly, we employ mass spectrometry-based proteomics in a quantitative analysis of honey bee larvae infected with the bacterium Paenibacillus larvae. Newly-eclosed bee larvae, in the second stage of their life cycle, are susceptible to this infection, but become progressively more resistant with age. We used this host-pathogen system to probe not only the role of the immune system in responding to a highly evolved infection, but also what other mechanisms might be employed in response to infection.

Results

Using quantitative proteomics, we compared the hemolymph (insect blood) of five-day old healthy and infected honey bee larvae and found a strong up-regulation of some metabolic enzymes and chaperones, while royal jelly (food) and energy storage proteins were down-regulated. We also observed increased levels of the immune factors prophenoloxidase (proPO), lysozyme and the antimicrobial peptide hymenoptaecin. Furthermore, mass spectrometry evidence suggests that healthy larvae have significant levels of catalytically inactive proPO in the hemolymph that is proteolytically activated upon infection. Phenoloxidase (PO) enzyme activity was undetectable in one or two-day-old larvae and increased dramatically thereafter, paralleling very closely the age-related ability of larvae to resist infection.

Conclusion

We propose a model for the host response to infection where energy stores and metabolic enzymes are regulated in concert with direct defensive measures, such as the massive enhancement of PO activity.

The ontogeny of immunity: development of innate immune strength in the honey bee (Apis mellifera)

Honey bees (Apis mellifera) are of vital economic and ecological importance. These eusocial animals display temporal polyethism, which is an age-driven division of labor. Younger adult bees remain in the hive and tend to developing brood, while older adult bees forage for pollen and nectar to feed the colony. As honey bees mature, the types of pathogens they experience also change. As such, pathogen pressure may affect bees differently throughout their lifespan. We provide the first direct tests of honey bee innate immune strength across developmental stages. We investigated immune strength across four developmental stages: larvae, pupae, nurses (1-day-old adults), and foragers (22-30 days old adults). The immune strength of honey bees was quantified using standard immunocompetence assays: total hemocyte count, encapsulation response, fat body quantification, and phenoloxidase activity. Larvae and pupae had the highest total hemocyte counts, while there was no difference in encapsulation response between developmental stages. Nurses had more fat body mass than foragers, while phenoloxidase activity increased directly with honey bee development. Immune strength was most vigorous in older, foraging bees and weakest in young bees. Importantly, we found that adult honey bees do not abandon cellular immunocompetence as has recently been proposed. Induced shifts in behavioral roles may increase a colony's susceptibility to disease if nurses begin foraging activity prematurely.

Effects of retinoids and juvenoids on moult and on phenoloxidase activity in the blood-sucking insect Rhodnius prolixus

Retinoic acid and insect juvenile hormone (JH) are structurally related terpenoids which are widespread in nature and are involved in many biological events such as morphogenesis, embryogenesis and cellular differentiation. Here, we investigated the effects of the retinoids 9-cis retinoic acid (9cisRA), all trans retinol (atROH), all trans retinoic acid (atRA) and the juvenoids methoprene (Met) and JH injection on moult and on phenoloxidase activity in the blood-sucking insect Rhodnius prolixus. Overall, we observed that injection of retinoids or juvenoids (120 pmols) in the hemocoel of 4th instar nymphs reduced the percentage of insects which appeared normal in morphology upon moult. Noteworthy, insects exposed to 9cisRA or JH underwent profound morphological changes upon moult, generating abnormal 5th instar nymphs and also markedly increased the death of insects during the moulting process. In addition, reduction in the percentage of insects that moult without any morphological alteration, induced by retinoids or juvenoids treatment, was negatively correlated with insects that both display abnormal moult and those that die during moult. Hemolymphatic phenoloxidase activity in adult male insects injected with 9cisRA, Met and JH were significantly reduced after a bacterial challenge. Together, these results indicate that not only juvenoids but also retinoids play an important role on morphogenesis and on immune response in R. prolixus, suggesting that the molecular mechanisms involved in these events recognize the terpenoid backbone as an important structural determinant in insects.

Two Proteases Defining a Melanization Cascade in the Immune System of Drosophila

The melanization reaction is used as an immune mechanism in arthropods to encapsulate and kill microbial pathogens. In Drosophila, the serpin Spn27A regulates melanization apparently by inhibiting the protease that activates phenoloxidase, the key enzyme in melanin synthesis. Here, we have described the genetic characterization of two immune inducible serine proteases, MP1 and MP2, which act in a melanization cascade regulated by Spn27A. MP1 is required to activate melanization in response to both bacterial and fungal infection, whereas MP2 is mainly involved during fungal infection. Pathogenic bacteria and fungi may therefore trigger two different melanization cascades that use MP1 as a common downstream protease to activate phenoloxidase. We have also shown that the melanization reaction activated by MP1 and MP2 plays an important role in augmenting the effectiveness of other immune reactions, thereby promoting resistance of Drosophila to microbial infection.

Damaged DNA binding protein 1 in Drosophila defense reactions

We have focused attention on functions of Drosophila damaged DNA binding protein 1 (D-DDB1) in Drosophila hematopoiesis and previously reported that its whole body dsRNA over-expression using a GAL4-UAS targeted expression system results in melanotic tumors and complete lethality. Since the lesions appear to arise as a normal and heritable response to abnormal development, forming groups of cells that are recognized by the immune system and encapsulated in melanized cuticle, D-DDB1 appears to be an essential development-associated factor in Drosophila. To probe the possibility that it contributes to hemocyte development, we used a collagen promoter-GAL4 strain to over-express dsRNA of D-DDB1 in Drosophila hemocytes. The D-DDB1 gene silencing caused melanotic tumors and mortality at the end of larval development. Similarly, it interfered with melanization and synthesis of antimicrobial peptides. Transgenic flies with D-DDB1 gene silencing were found to accumulate abnormal large blood cells, reminiscent of human leukemia, suggesting that D-DDB1 has functions in hemocyte development.

Carminic acid dye from the homopteran Dactylopius coccus hemolymph is consumed during treatment with different microbial elicitors

The activation of Dactylopius coccus (Costa) hemolymph with microbial polysaccharide molecules was studied. Hemolymph incubated in the presence of laminarin, zymosan, and N-acetyl glucosamine produced a dark fibrillar precipitated, and the red pigment (carminic acid) was consumed (measured spectrophotometrically at 495 nm). Lipopolysaccharide (LPS) did not induce any response. The reaction was inhibited with millimolar concentrations of serine and cysteine protease inhibitors, EGTA and phenyl thiourea. It was also diminished by prostaglandin synthesis inhibitors: dexamethasone, acetylsalicylic acid, and indomethacin. However, Mg2+ chelator EDTA did not inhibit hemolymph activation. Hemolymph proteins were depleted from soluble phase during treatment with laminarin, but a group of around 34 kDa remained unmodified. These results showed that D. coccus hemolymph is activated by microbial elicitors, its activation depends on eicosanoids, and suggest participation of a prophenoloxidase (PPO)-like activation system that could consume carminic acid. We are currently dissecting the molecular factors involved in D. coccus hemolymph activation to determine homologies and differences with other arthropods immune response pathways.

Suppression of the prophenoloxidase system in Rhodnius prolixus orally infected with Trypanosoma rangeli

Investigations were carried out to compare aspects of the prophenoloxidase (proPO)-activating pathway in Rhodnius prolixus hemolymph in response to oral infection and inoculation of the insects with two developmental forms of Trypanosoma rangeli epimastigotes strain H14. In vivo experiments demonstrated that in control insects fed on uninfected blood, inoculation challenge with short epimastigotes resulted in high phenoloxidase (PO) activity. In contrast, previous feeding on blood containing either short or long epimastigotes was able to suppress the proPO activation induced by thoracic inoculation of the short forms. In vitro assays in the presence of short epimastigotes demonstrated that control hemolymph or hemolymph provided by insects previously fed on blood containing epimastigotes incubated with fat body homogenates from control insects significantly increased the PO activity. However, fat body homogenates from insects previously fed on blood containing epimastigotes, incubated with hemolymph taken from insects fed on control blood or blood infected with epimastigotes, drastically reduced the proPO activation. The proteolytic activity in the fat body homogenates of control insects was significantly higher than in those obtained from fat body extracts of insects previously fed on blood containing epimastigotes. These findings indicate that the reduction of the proteolytic activities in the fat body from insects fed on infected blood no longer allows a significant response of the proPO system against parasite challenge. It also provides a better understanding of T. rangeli infection in the vector and offer novel insights into basic immune processes in their invertebrate hosts.

Virulence ofCandida albicansmutants toward larvalGalleria mellonella(Insecta, Lepidoptera, Galleridae)

Culture medium affected the virulence of a strain of Candida albicans toward Galleria mellonella larvae, but the yeast growth rates in yeast extract – peptone – dextrose broth and synthetic Galleria serum were not correlated with yeast virulence. Virulent C. albicans grew rapidly in larval serum, whereas, it limited nodulation and continued development in vivo, producing toxins that damaged the hemocytes and fat body. Nonpathogenic yeast-phase cells grew slowly in larval serum but induced extensively melanized nodules in vivo and developed no further. There was no discernible relationship in 14 exo-enzymes between the virulent and avirulent yeast strains and virulence. The avirulent myosin-I-defective yeast cells were rapidly removed from the hemolymph in vivo because of lysozyme-mediated yeast agglutination and the possible binding of the yeast cells by lysozyme and apolipophorin-III. Both lysozyme and apolipophorin-III are proteins that bind β-1,3-glucan. Finally, insects with nonpathogenic C. albicans exhibited induced immunity and were more resistant to candidiasis from the wild-type yeast cells than were noninduced insects.Key words: Candida, virulence, insect, nodule, melanization, apolipophorin-III.

The Drosophila immune system detects bacteria through specific peptidoglycan recognition

The Drosophila immune system discriminates between different classes of infectious microbes and responds with pathogen-specific defense reactions through selective activation of the Toll and the immune deficiency (Imd) signaling pathways. The Toll pathway mediates most defenses against Gram-positive bacteria and fungi, whereas the Imd pathway is required to resist infection by Gram-negative bacteria. The bacterial components recognized by these pathways remain to be defined. Here we report that Gram-negative diaminopimelic acid-type peptidoglycan is the most potent inducer of the Imd pathway and that the Toll pathway is predominantly activated by Gram-positive lysine-type peptidoglycan. Thus, the ability of Drosophila to discriminate between Gram-positive and Gram-negative bacteria relies on the recognition of specific forms of peptidoglycan.

A serpin mutant links Toll activation to melanization in the host defence of Drosophila

A prominent response during the Drosophila host defence is the induction of proteolytic cascades, some of which lead to localized melanization of pathogen surfaces, while others activate one of the major players in the systemic antimicrobial response, the Toll pathway. Despite the fact that gain-of-function mutations in the Toll receptor gene result in melanization, a clear link between Toll activation and the melanization reaction has not been firmly established. Here, we present evidence for the coordination of hemolymph-borne melanization with activation of the Toll pathway in the Drosophila host defence. The melanization reaction requires Toll pathway activation and depends on the removal of the Drosophila serine protease inhibitor Serpin27A. Flies deficient for this serpin exhibit spontaneous melanization in larvae and adults. Microbial challenge induces its removal from the hemolymph through Toll-dependent transcription of an acute phase immune reaction component.

An immune-responsive Serpin regulates the melanization cascade in Drosophila

In arthropods, the melanization reaction is associated with multiple host defense mechanisms leading to the sequestration and killing of invading microorganisms. Arthropod melanization is controlled by a cascade of serine proteases that ultimately activates the enzyme prophenoloxidase (PPO), which, in turn, catalyzes the synthesis of melanin. Here we report the biochemical and genetic characterization of a Drosophila serine protease inhibitor protein, Serpin-27A, which regulates the melanization cascade through the specific inhibition of the terminal protease prophenoloxidase-activating enzyme. Our data demonstrate that Serpin-27A is required to restrict the phenoloxidase activity to the site of injury or infection, preventing the insect from excessive melanization.

Immune suppression of Galleria mellonella (Insecta, Lepidoptera) humoral defenses induced by Steinernema feltiae (Nematoda, Rhabditida): involvement of the parasite cuticle

Immune depression of Galleria mellonella larvae was evaluated a short time after infection with the entomopathogenic nematode Steinernema feltiae. In the host the activity of the enzymatic cascade known as the proPO system was significantly reduced by the presence of either live or dead parasites. The presence of parasites decreased the LPS-elicited proPO system activity. In addition, this process seems to be related to a decrease in the activity of hemolymph proteases, more than to phenoloxidase damage. proPO inhibition was also achieved by injected isolated cuticle fragments, suggesting that the parasite body surface plays an important role in the early parasitation phase.

Oxyresveratrol and hydroxystilbene compounds. Inhibitory effect on tyrosinase and mechanism of action

Tyrosinase is responsible for the molting process in insects, undesirable browning of fruits and vegetables, and coloring of skin, hair, and eyes in animals. To clarify the mechanism of the depigmenting property of hydroxystilbene compounds, inhibitory actions of oxyresveratrol and its analogs on tyrosinases from mushroom and murine melanoma B-16 have been elucidated in this study. Oxyresveratrol showed potent inhibitory effect with an IC(50) value of 1.2 microm on mushroom tyrosinase activity, which was 32-fold stronger inhibition than kojic acid, a depigmenting agent used as the cosmetic material with skin-whitening effect and the medical agent for hyperpigmentation disorders. Hydroxystilbene compounds of resveratrol, 3,5-dihydroxy-4'-methoxystilbene, and rhapontigenin also showed more than 50% inhibition at 100 microm on mushroom tyrosinase activity, but other methylated or glycosylated hydroxystilbenes of 3,4'-dimethoxy-5-hydroxystilbene, trimethylresveratrol, piceid, and rhaponticin did not inhibit significantly. None of the hydroxystilbene compounds except oxyresveratrol exhibited more than 50% inhibition at 100 microm on l-tyrosine oxidation by murine tyrosinase activity; oxyresveratrol showed an IC(50) value of 52.7 microm on the enzyme activity. The kinetics and mechanism for inhibition of mushroom tyrosinase exhibited the reversibility of oxyresveratrol as a noncompetitive inhibitor with l-tyrosine as the substrate. The interaction between oxyresveratrol and tyrosinase exhibited a high affinity reflected in a K(i) value of 3.2-4.2 x 10(-7) m. Oxyresveratrol did not affect the promoter activity of the tyrosinase gene in murine melanoma B-16 at 10 and 100 microm. Therefore, the depigmenting effect of oxyresveratrol works through reversible inhibition of tyrosinase activity rather than suppression of the expression and synthesis of the enzyme. The number and position of hydroxy substituents seem to play an important role in the inhibitory effects of hydroxystilbene compounds on tyrosinase activity.

Effects of juvenile hormones and precocenes on the immune response of Spodoptera littoralis larvae to supernumerary larvae of the solitary parasitoid, Microplitis rufiventris Kok

The study represents one of the first direct tests of juvenile hormones (JHs) on the cellular immune response of a lepidopteran insect, Spodoptera littoralis (Boisd.), superparasitized by a solitary parasitoid, Microplitis rufiventris Kok. JHII-treated S. littoralis larvae exhibited a significant reduction in encapsulation response to the supernumerary M. rufiventris larvae at different developmental ages (0-4, 10-14 and 24-26 h) of supernumerary M. rufiventris embryos within 48 or 144 h after parasitization than either JHI-treated or control hosts. The reduction in encapsulation response was significantly greater in hosts which received higher JHII dose (5 &mgr;g) than those treated with lower one (1 &mgr;g).The progression of the encapsulation, culminating in melanization in the capsules occurred more frequently in precocene-treated hosts than either JH-treatment or control ones. Application of the anti-juvenile hormones (PI or PII) caused stronger encapsulation reaction to the injured supernumerary parasitoid larvae than JHI or JHII. In PI-treatments, the reaction was rapid and dramatic by lower (25 &mgr;g) dose treatments than higher ones (70 &mgr;g).The capsules formed around the supernumerary larvae of M. rufiventris showed signs of melanization within 48 h in PI-treated hosts. Melanization was more pronounced in either PI- or PII-treated hosts prior emergence of successful parasitoid larvae. Generally, the injured supernumerary larvae were seen within melanin-rich capsules in PI- and PII-treated hosts. Melanization was not observed in most capsules of JHI- and JHII-treated hosts. Additional physiological effects were observed: (1) in some cases, application of JHI to parasitized S. littoralis larvae inhibited subsequent emergence of its wasp, M. rufiventris, and (2) teratocytes (cells of wasp origin) from PI or PII-treated hosts were smaller in size than those observed in the JH-treated or control hosts.Finally, the dramatic increase in encapsulation responses of precocene-treated S. littoralis larvae of the supernumerary of M. rufiventris and definitely opposite reactions in typical hosts but JHII-treated hosts suggested that the cellular defense reaction may be under inhibitory hormonal control.The results of the study have implications for medically important insects such as mosquitoes and their interactions with parasites, as well as having significance for biocontrol and mass rearing programs using parasitoids.

Cuticular pro-phenoloxidase of the silkworm, Bombyx mori. Purification and demonstration of its transport from hemolymph

Pro-phenoloxidase (proPO) in insects is implicated in the defense against microbes and wounding. The presence of proPO in the cuticle was suggested more than 30 years ago, but it has not been purified. The extract of cuticles of the silkworm, Bombyx mori, was shown to contain two proPO isoforms (F-type and S-type proPOs, which have slightly different mobilities in polyacrylamide gel electrophoresis under nondenaturing conditions). The two isoforms were purified to homogeneity. From hemolymph of the same insect, two types of proPO with the same electrophoretic mobilities as those of cuticular isoforms were separated and were shown to be different at five amino acid residues in one of their subunits. The isoforms in the hemolymph and cuticle were activated by a specific activating enzyme. The resulting active phenoloxidases exhibited almost the same substrate specificities and specific activities toward o-diphenols. The substrate specificities and the susceptibilities to inhibitors, including carbon monoxide, indicated that the purified proPO isoforms were not zymogens of laccase-type phenoloxidase. The proPO in hemolymph was shown to be transported to the cuticle. This demonstration was corroborated by the failure to detect proPO transcripts by Northern analysis of total RNA from epidermal cells. In reversed-phase column chromatography, cuticular and hemolymph proPOs gave distinct elution profiles, indicating that some yet to be identified modification occurs in hemolymph proPO and results in the formation of cuticular proPO. There was little transportation of cuticular proPO to the cuticle when it was injected into the hemocoel. The nature of the modification is described in the accompanying paper (Asano, T., and Ashida, M. (2001) J. Biol. Chem. 276, 11113-11125).

A link between blood coagulation and prophenol oxidase activation in arthropod host defense

Phenol oxidase, a copper-containing enzyme, is widely distributed not only in animals but also in plants and fungi, which is responsible for initiating the biosynthesis of melanin. Activation of prophenol oxidase in arthropods is important in host defense. However, the prophenol oxidase-activating system remains poorly understood at the molecular level. Here we show that the coagulation cascade of the horseshoe crab Tachypleus tridentatus is linked to prophenol oxidase activation, with the oxygen carrier hemocyanin functioning as a substitute for prophenol oxidase. Tachypleus clotting enzyme functionally transforms hemocyanin to phenol oxidase, and the conversion reaches a plateau at 1:1 stoichiometry without proteolytic cleavage. The active site-masked clotting enzyme also has the same effect, suggesting that complex formation of the clotting enzyme with hemocyanin is critical for the conversion. The two systems of blood coagulation and prophenol oxidase activation may have evolved from a common ancestral protease cascade.

Rhodnius prolixus infected with Trypanosoma rangeli: In vivo and in vitro experiments

Studies were carried out on the activation of the prophenoloxidase (proPO) in adults of Rhodnius prolixus infected by short and long epimastigote forms of Trypanosoma rangeli. The in vitro activation of the proPO cascade using l-DOPA as substrate was very low in the absence of fat body extract, hemolymph, and parasites. On the other hand, a higher PO activity was observed when short, but not long, epimastigotes of T. rangeli were incubated with fresh hemolymph, fat body extract, and l-DOPA. Supernatant from lysed long epimastigotes increased the PO activity at levels identical to those observed with supernatants from lysed short epimastigotes. Similarly, the PO activity of hemolymph obtained from inoculated insects with long epimastigotes of T. rangeli showed a very low activity when incubated with l-DOPA compared to the PO activity of hemolymph taken from insects inoculated with short epimastigotes of T. rangeli. Control insects inoculated with sterile PBS showed no PO activity. These data indicate the presence of (a) factor(s) in the hemolymph as well as in the fat body extract that may be released (or induced) by the presence of short epimastigotes of T. rangeli and which results in the activation of the R. prolixus proPO system. The implications of these findings are discussed in relation to the development of T. rangeli and its ability to overcome the proPO system, survive, and successfully colonize the hemolymph of R. prolixus.

Hemocyte surface phenoloxidase (PO) and immune response to lipopolysaccharide (LPS) in Ceratitis capitata

Bacterial lipopolysaccharide (LPS) attachment at the hemocyte surface is based on the crosslinking of surface associated p47 to LPS, via the intermediacy of tyrosine derivatives generated by the action of phenoloxidase (PO). This attachment is an initial step for LPS internalization from hemocytes (Charalambidis et al., 1996). The results presented clearly show the critical role of hemocyte associated PO activity in the above processes. Biochemical and immunofluorescent analysis demonstrated unambiguously the presence of prophenoloxidase (proPO) on the hemocyte surface. The cell-surface expression of proPO appeared to be LPS-independent, whereas its activation was LPS-dependent. The activation of cell surface proPO involves a limited proteolysis, since upon activation with chymotrypsin proPO is converted to a set of smaller molecular weight proteins with PO activity. The activation appears to be due to enzyme activators, serine proteases, released upon LPS-stimulation. This hypothesis was supported from the activation of membrane proPO by the culture medium of hemocytes which have been triggered with LPS. In addition, proPO, activation was abolished by inhibitors of secretion and PMSF. The release of proPO activators upon LPS-stimulation is mediated via protein tyrosine phosphorylation, as genistein inhibited proPO activation, a situation similar to that reported by us for the release of the effector protein p47 (Charalambidis et al., 1995). The LPS-stimulated activation of cell-surface proPO is a prerequisite for LPS (either cell associated or cell free) internalization, as judged by the resistance of LPS binding to dissociation by proteinase K.

The prophenoloxidase from the wax moth Galleria mellonella: purification and characterization of the proenzyme

A prophenoloxidase (PPO) was purified from the hemolymph of the larvae of Galleria mellonella. A 135-fold purification of the proenzyme with 25% yield was achieved by a combination of different chromatographic methods. An alternative micropreparation of pure PPO by a novel method for native electrophoresis in polyacrylamide gel is also described. The molecular mass of the native PPO was estimated to be 300 kDa by the pore-limit gradient electrophoresis in polyacrylamide gel. In the presence of sodium dodecyl sulphate, two closely migrating subunits of 80 and 83 kDa were detected under non-reducing conditions. The PPO was shown to be a glycoprotein and its isoelectric point was 6.2. The amino-acid composition of the purified protein was similar to the PPO from Bombyx mori. The monospecific antibody raised against the purified PPO crossreacted with the (pro)phenoloxidase in hemolymph of Manduca sexta. The activation of the PPO with chymotrypsin was investigated and two proteins of 67 and 50 kDa were found to be products of the proteolytic cleavage. The N-terminus of the G. mellonella PPO was blocked, but eleven partial internal sequences were determined after fragmentation of the purified PPO with trypsin. Three of these peptides exhibited significant homology with highly conserved sequences found in arthopod hemocyanins and insect storage proteins, which indicates that the PPO belongs to this family.

A study of novel lectins and their involvement in the activation of the prophenoloxidase system in Blaberus discoidalis

Endogenous and exogenous lectins have been found to activate the prophenoloxidase (proPO) system of the cockroach, Blaberus discoidalis, to the same extent as laminarin, a previously known microbial activator of proPO. The lectins can also further enhance this laminarin activation of the proPO system. Non-lectin proteins did not display any activation properties. The time course of proPO activation was studied after reconstitution of the reaction system using purified lectins, a trypsin-like enzyme, a trypsin inhibitor and partially purified lectin-binding proteins from the cockroach haemolymph. Lectin activation of the proPO system is probably not mediated by the lectin sugar-binding sites, as specific inhibitory sugars failed to abrogate the enhanced effect. The results suggest that alternative binding site(s) on the lectins may be involved in the proPO activation process. Evidence also suggests that several different lectins are involved in the regulation of the proPO system through separate receptors or binding molecules on the haemocytes, and that they exert their effects early in the sequence of events leading to conversion of proPO into its active form, possibly via regulation of serine proteases and protease inhibitors.

Purification of the pro-phenol oxidase enzyme from haemocytes of the cockroach Blaberus discoidalis

Pro-phenol oxidase was purified from the haemocytes of the cockroach Blaberus discoidalis by Blue Sepharose chromatography, hydrophobic-interaction chromatography on a Phenyl-Superose column and, finally, gel filtration on a Superose 6 column. Results suggest that the molecule exists as a polymer of identical 76 kDa monomeric units. The enzyme is a glycoprotein with pI of 5.2 and can be converted by trypsin into phenol oxidase.

On the latency of blood phenoloxidase of painted locust Poeciloceros pictus

The blood phenoloxidase of Poeciloceros pictus exists as proenzyme. The amidase activity present in the haemocyte is responsible for the activation of haemocyte phenoloxidase. There is an amidase inhibitor in the plasma. Plasma phenoloxidase could be activated by various fatty acids, biological compounds like lipopolysaccharide zymosan and detergents. Electrophoretic studies suggest that activation of haemocyte phenoloxidase involves limited proteolysis. It is suggested that the natural activator of plasma phenoloxidase may be fatty acid and it does not undergo proteolysis during activation. The biological compounds like zymosan or LPS are easily recognised by the factor(s) from the membranes of haemocytes.

Reduced cellular immune competence of a temperature-sensitive dopa decarboxylase mutant strain of Drosophila melanogaster against the parasite Leptopilina boulardi

1. The melanotic encapsulation response made by larvae of a temperature-sensitive dopa decarboxylase (DDC) mutant strain of Drosophila against the parasitic wasp Leptopilina was severely compromised in hosts with reduced levels of DDC. 2. Dopa and 5,6-dihydroxyindole (DHI) were two hemolymph components identified in hosts exhibiting a melanotic encapsulation response. 3. This is the first study to implicate DDC in insect cellular immune responses, and to provide chemical evidence that the pigment formed during such responses is eumelanin derived from tyrosine.

Use of nitrocellulose membrane to activate and measure insect prophenol oxidase

A method for the activation and measurement of insect prophenol oxidase using nitrocellulose membrane is presented. Using this method we were able to conveniently activate both crude and purified prophenol oxidase from insects belonging to three different orders. This rapid method allows for prophenol oxidase activation, in the absence of a prophenol oxidase-activating system, and in the presence of high ionic strength, protease inhibitors, or chelator.

Mosquito host influences on development of filariae

A brief review is presented of the literature relating to factors which limit the capacity of filariae to develop in mosquitoes, with particular emphasis on immune mechanisms. Most insects respond to bacterial infection by the production of potent antibacterial proteins, but little is known of this aspect of the immune response in mosquitoes or of the possible influence of immune proteins on the fate of filarial infections in mosquitoes. A summary account is given of recent experiments with the mosquito Aedes aegypti which involve passive transfer of immune haemolymph together with its in vitro assay and SDS-PAGE examination for induced proteins. These experiments demonstrate the production, in response to inoculation with Brugia pahangi, Escherichia coli, and various components of microbial cell walls, of haemolymph factors which are protective against filarial infection. It remains to be seen whether mosquitoes can produce a specific protective response to infection with eukaryotic organisms such as filaria that is distinctive from that mobilized against bacteria.

Studies on prophenoloxidase activation in the mosquito Aedes aegypti L

This study, the first of its kind in a mosquito vector species, demonstrates the feasibility of studying prophenoloxidase activation in an insect containing not more than a few microliters of hemolymph. Mosquito phenoloxidase was found to be in an inactive proenzyme form, prophenoloxidase. Mosquito prophenoloxidase required bivalent cation for its activation; Ca2+ was found to be the most efficient for activation. Concomitant amidase activity was also observed prior to phenoloxidase activity. Through Western blotting, using a cross-reactive silkworm antiprophenoloxidase antibody, our results strongly suggest that mosquito prophenoloxidase activation resulted from limited proteolysis. Protease inhibitor studies reinforced this contention showing the involvement of (a) serine protease(s) with trypsin-like activity in the activation of mosquito prophenoloxidase.