Parasite-induced changes in nitric oxide levels in Drosophila paramelanica.

In larvae of Drosophila paramelanica, eggs and larvae of the endoparasitic wasp Leptopilina heterotoma succumb to an effective host reaction that does not involve blood cell-mediated melanotic encapsulation, a response that characterizes cellular immunity in various species of Drosophila and in many insects and other arthropods. A significant increase occurs, however, in the number of lamellocytes, a type of blood cell that functions in encapsulation reactions. The appearance of activated lamellocytes in D. paramelanica is viewed as an early response to infection, one most likely initiated by non-self-recognition processes that similarly function in other wasp-infected Drosophila. However, ensuing cytotoxic responses, about which little is presently known, are not accompanied by melanotic encapsulation in D. paramelanica. Concurrent analyses of the cell-signaling molecule nitric oxide (*NO) revealed significant alterations in the levels of this free radical during the early stages of infection, most notably a dramatic increase immediately upon infection, and precipitous decreases occurring at times when parasites were killed. Injections of a specific inhibitor of nitric oxide synthase (NOS) into the host's body cavity prior to infection significantly increased parasite survival. These observations suggest some involvement of *NO in the host immune response, either in recruiting hemocytes to sites of infection or as a component of the insect's cytotoxic arsenal, given the capacity of the radical to generate toxic molecules through interactions with various intermediates of oxygen and nitrogen.

Melanogenesis and associated cytotoxic reactions: applications to insect innate immunity.

Insects transmit the causative agents for such debilitating diseases as malaria, lymphatic filariases, sleeping sickness, Chagas' disease, leishmaniasis, river blindness, Dengue, and yellow fever. The persistence of these diseases provides testimony to the genetic capacity of parasites to evolve strategies that ensure their successful development in two genetically diverse host species: insects and mammals. Current efforts to address the problems posed by insect-borne diseases benefit from a growing understanding of insect and mammalian immunity. Of considerable interest are recent genomic investigations that show several similarities in the innate immune effector responses and associated regulatory mechanisms manifested by insects and mammals. One notable exception, however, is the nearly universal presence of a brown-black pigment accompanying cellular innate immunity in insects. This response, which is unique to arthropods and certain other invertebrates, has focused attention on the elements involved in pigment synthesis as causing or contributing to the death of the parasite, and has even prompted speculation that the enzyme cascade mediating melanogenesis constitutes an ill-defined recognition mechanism. Experimental evidence defining the role of melanin and its precursors in insect innate immunity is severely lacking. A great deal of what is known about melanogenesis comes from studies of the process occurring in mammalian systems, where the pigment is synthesized by such diverse cells as those comprising portions of the skin, hair, inner ear, brain, and retinal epithelium. Fortunately, many of the components in the metabolic pathways leading to the formation of melanin have been found to be common to both insects and mammals. This review examines some of the factors that influence enzyme-mediated melanogenic responses, and how these responses likely contribute to blood cell-mediated, target-specific cytotoxicity in immune challenged insects.

Drosophila serpin 27A is a likely target for immune suppression of the blood cell-mediated melanotic encapsulation response.

Avirulent strains of the endoparasitoid Leptopilina boulardi succumb to a blood cell-mediated melanotic encapsulation response in host larvae of Drosophila melanogaster. Virulent wasp strains effectively abrogate the cellular response with substances introduced into the host that specifically target and effectively suppress one or more immune signaling pathways, including elements that control phenoloxidase-mediated melanotic encapsulation. The present study implicates involvement of the Drosophila Toll pathway in cellular innate immunity by regulating the serine protease inhibitor Serpin 27A (Spn27A), which normally functions as a negative regulator of phenoloxidase. The introduction of Spn27A into normally highly immune competent D. melanogaster larvae significantly reduced their ability to form melanotic capsules around eggs of L. boulardi. This study confirms the role of Spn27A in the melanization cascade and establishes that this pathway and associated blood cell responses can be activated by parasitization. The activation of phenoloxidase and the site-specific localization of the ensuing melanotic response are such critical components of the blood cell response that Spn27A and the signaling elements mediating its activity are likely to represent prime targets for immune suppression by L. boulardi.

Genetics of anti-parasite resistance in invertebrates.

This review summarizes and compares available data on genetic and molecular aspects of resistance in four well-described invertebrate host-parasite systems: snail-schistosome, mosquito-malaria, mosquito-filarial worm, and Drosophila-wasp associations. It underlies that the major components of the immune reaction, such as hemocyte proliferation and/or activation, and production of cytotoxic radicals are common to invertebrate hosts. Identifying genes responsible for naturally occurring resistance will then be helpful to understand the mechanisms of invertebrate immune defenses and to determine how virulence factors are used by parasites to overcome host resistance. Based on these four well-studied models, invertebrate resistance appears as generally determined by one major locus or a few loci, displaying at least partial dominance. Interestingly, specificity of resistance is highly variable and would involve processes other than simple recognition mechanisms. Finally, resistance was shown to be generally costly but is nevertheless observed at high frequencies in many natural populations, suggesting a high potential for host parasite coevolution.

The effects of parasite-derived immune-suppressive factors on the cellular innate immune and autoimmune responses of Drosophila melanogaster.

Immune-suppressive factors (ISFs) introduced into larvae of Drosophila melanogaster during infection by virulent endoparasitic wasps effectively block the innate immune response mediated by blood cells (hemocytes) but have little influence on the autoimmune response made by a tumor strain in which the blood cells manifest a similar response but instead target and destroy endogenous tissues. Quantitative hemocyte analyses indicate that ISFs interfere with the immune effector responses downstream of nonself recognition, hemocyte activation and differentiation, because these responses were manifested by tumor hosts, in which the parasitoids developed. The data suggest that once activated to encapsulate aberrant tissues, the target specificity of the autoimmune-activated hemocytes, and the genetic program underlying tumor formation, cannot be blocked by parasitoid-derived ISFs, which effectively inhibit identical hemocyte-mediated responses during parasitization.

Immunogenetic aspects of the cellular immune response of Drosophilia against parasitoids.

Host-parasite relationships represent integrating adaptations of considerable complexity involving the host's immune capacity to both recognize and destroy the parasite, and the latter's ability to successfully invade the host and to circumvent its immune response. Compatibility in Drosophila-parasitic wasp (parasitoid) associations has been shown to have a genetic basis, and to be both species and strain specific. Studies using resistant and susceptible strains of Drosophila melanogaster infected with virulent and avirulent strains of the wasp Leptopilina boulardi demonstrate that the success of the host cellular immune response depends on the genetic status of both host and parasitoid. Immunological, physiological, biochemical, and genetic data form the bases of a two-component model proposed here to account for the observed specificity and complexity of two coevolved adaptations, host nonself recognition and parasitoid virulence.

The effects of nitric oxide on the oxidations of l-dopa and dopamine mediated by tyrosinase and peroxidase.

The effects of nitric oxide (NO) on both tyrosinase/O(2)- and horseradish peroxidase/H(2)O(2)-mediated oxidations of dopamine and its o-dihydric phenol precursor l-dopa were compared with autoxidative processes and quantitatively assessed by oxidative and reductive electrochemical detection systems. In peroxidase/H(2)O(2)/NO-catalyzed reactions, significantly more substrate was oxidized than in the corresponding control incubations lacking NO. In tyrosinase/O(2)/NO-promoted reactions the total amounts of l-dopa and dopamine oxidized were significantly less than the amounts of the substrates oxidized by enzyme alone. These data indicate that the activity of the heme protein peroxidase was enhanced by NO, whereas tyrosinase, a copper-containing monoxygenase, was inhibited. The NO-mediated reduction of tyrosinase/O(2) activity may be attributed to the formation of an inhibitory copper.nitrosyl complex. An oxidized nitrodopamine derivative, considered to be either the quinone or semiquinone of 6-nitrosodopamine, was generated in peroxidase/H(2)O(2)/NO-mediated reactions with dopamine along with two oxidized melanin precursors, dopamine quinone and dopaminechrome. No corresponding nitroso compound was formed in reactions involving l-dopa or in any of the tyrosinase-mediated reactions. The formation of such a noncyclized nitrosodopamine represents an important alternative pathway in catecholamine metabolism, one that by-passes the formation of cytoprotective indole precursors of melanin. The results of this investigation suggest that cellular integrity and function can be adversely affected by NO-promoted oxidations of dopamine and other catechols, reactions that not only accelerate their conversion to reactive quinones but also form potentially cytotoxic noncyclized nitroso derivatives. Reduced levels of dopamine in the brain through NO-enhanced oxidation of the catecholamine will almost certainly be manifested by diminished levels of the dopamine-derived brain pigment neuromelanin.

Nitric oxide involvement in Drosophila immunity.

The augmented production of nitric oxide (NO) was observed during the hemocyte-mediated melanotic encapsulation responses of Drosophila melanogaster and D. teissieri. When introduced into the hemocoel of D. melanogaster larvae, NO activated the gene encoding the antimicrobial peptide Diptericin. These observations, together with previous studies documenting the production of superoxide anion (O(*-)(2)) and H(2)O(2) in immune-challenged Drosophila, provide evidence that reactive intermediates of both oxygen (ROI) and nitrogen (RNI) constitute a part of the cytotoxic arsenal employed by Drosophila in defense against both microbial pathogens and eukaryotic parasites. These ROI and RNI appear to represent an evolutionarily conserved innate immune response that is mediated by regulatory proteins that are homologous to those of mammalian species.

Iron, metalloenzymes and cytotoxic reactions.

There is considerable evidence implicating iron and other redox-active transition metals as progenitors of reactive intermediates of oxygen (ROI), molecules which lead to oxidative stress and contribute to various neurodegenerative processes. An important aspect of such metal-mediated damage to biomolecules is the site-specific nature of such pathological activity. Iron sequestering molecules, such as ferritin, transferrin, lactotransferrin, melanotransferrin, hemosiderin and heme can serve as cytoprotectants against metal-mediated oxidant damage. Metalloenzymes also constitute an important group of iron sequestering molecules. Metalloenzyme-catalyzed reactions in which metal ions at the enzyme active site undergo redox-cycling in association with O2 are site-specific in nature, and may represent a potential source of ROI-mediated damage to biomolecules. Dysregulation of brain iron and alterations in the levels of metalloenzymes involved in reactions with O2 derived molecules can contribute to neuronal damage. Iron may increase the cytotoxicity of neuronal dopamine by increasing its rate of oxidation to quinones and semiquinones, thereby reducing the level of this neurotransmitter. Interestingly, dopamine also may play an important role in the maintenance of transition-metal homeostasis as an iron chelator, since it can form both catecholate and hydroxamate groups, molecules employed by many microorganisms to sequester iron.

Cytotoxicity and cytotoxic molecules in invertebrates.

Although lacking the components that characterize the acquired immunity systems of vertebrates, invertebrates nevertheless possess effective general innate immune mechanisms which exhibit striking parallels with those of vertebrates. These innate immune systems include both cellular and humoral elements. Invertebrate phagocytes synthesize both oxygen-dependent and oxygen-independent molecules to combat infectious agents. Cytotoxic substances employed by invertebrates include reactive intermediates of oxygen and nitrogen, antimicrobial peptides, lectins, cytokine- and complement-like molecules, and quinoid intermediates of melanin. The signal transduction pathways that are involved in mediating the production of these substances appear to be very similar among animal species, suggesting a common ancestral origin for the innate immune systems.

Developmental and immunological aspects of Drosophila-parasitoid relationships.

The Drosophila-parasitic wasp (parasitoid) associations involve integrating adaptations of considerable complexity. This review focuses on some of the factors that influence these interactions including host immunity, nutrition and hormonal changes, and parasitoid virulence and mechanisms of immune suppression.

Contrasting effects of catecholic and O-methylated tetrahydroisoquinolines on hydroxyl radical production.

Tetrahydroisoquinolines (TIQs) are intraneuronal, catecholamine-derived alkaloids that have been implicated in the etiology of Parkinson's disease and in alcohol related disorders. The in vitro production of the cytotoxic hydroxyl radical (*OH) was recorded during the autoxidation of salsolinol (SAL) and salsolinol-1-carboxylic acid (SAL-1C), but not when these two catecholic TIQs were oxidized by tyrosinase. Significantly higher levels of the radical were produced when these catecholic TIQs were incubated with *OH generating complexes, or with chelated iron. In contrast, mono-O-methylated TIQs such as salsoline (SLN) and salsoline-1-carboxylic acid (SLN-1C) did not generate *OH during autoxidation or when incubated with chelated iron or tyrosinase. Radical production by *OH-generating complexes was reduced in the presence of O-methylated TIQs. The neurotoxicity of TIQs may result from their propensity to autoxidize and generate reactive quinoids and ensuing oxygen radicals. The functional significance of the replacement of a hydroxyl group attached to C-7 of SAL or SAL-1C with a methoxyl group remains to be determined. This single structural modification may prevent mono-O-methylated TIQs from participating in catalytic redox cycling reactions that would otherwise augment *OH production. If true, then O-methylation and other cellular mechanisms that circumvent the autoxidation of catecholamine-derived TIQs may reduce the likelihood of these substances forming cytotoxic quinoids and influencing endogenous *OH-generating reactions.

Hydroxyl radical formation via iron-mediated Fenton chemistry is inhibited by methylated catechols.

The differing effects of O-methylated catecholamines and their dihydroxyphenyl precursors on the production of *OH were quantified using a previously established specific salicylate hydroxylation assay in conjunction with a sensitive electrochemical detection system. The production of *OH by the Fenton reaction was diminished significantly by O-methylated catecholamines (O-methyldopa, O-methyldopamine, O-methyltyrosine, and N-acetyl-O-methyldopamine), whereas radical production was augmented by dihydroxyphenyls (DOPA, dopamine, and N-acetyldopamine), including those with methylated side chains (N-methyldopamine and alpha-methyldopa). Monohydroxyphenyls such as octopamine, tyramine, tyrosine, and alpha-methyltyrosine had little or no effect on radical production. These data show that a methyl group positioned on the side chain of a catecholamine does not alter its pro-oxidant behavior, while a methyl group positioned on the aromatic ring renders the catecholamine sterically or kinetically unfavorable for coordination with transition metals, thus preventing the promotion of Fenton chemistry. These results highlight the importance of O-methylation in forming catechols that are less reactive than their dihydroxyphenyl precursors. Thus, factors regulating the methylation of brain catecholamines may play a crucial role in mediating neuronal integrity during aging and in the pathogenesis of certain neurodegenerative disorders. Competitive side-chain methylation reactions may sustain or perpetuate some dihydroxyphenyls, creating an oxidatively less favorable environment for cells than would result from compounds formed by O-methylation.

The effects of dietary yeast on the cellular immune response of Drosophila melanogaster against the larval parasitoid, Leptopilina boulardi.

The role of dietary yeast in Drosophila melanogaster cellular immunity was investigated. Host larvae deprived of yeast immediately after parasitization by the cynipid wasp Leptopilina boulardi encapsulated a significantly lower percentage of the parasitoid's eggs than hosts transferred to a medium with yeast. When the transfers of hosts were made 24 hr after exposure to the parasite, diet had no effect on the immune response that had commenced prior to the transfers. This study demonstrates for the first time the effect of a specific dietary component on the immune responsiveness of Drosophila against a larval parasitoid.

Prolonged oviposition decreases the ability of the parasitoid Leptopilina boulardi to suppress the cellular immune response of its host Drosophila melanogaster.

The cellular immune response of Drosophila against metazoan parasites is characterized by the production of melanotic capsules comprised mostly of host blood cells (hemocytes). During the latter part of the ovipositional period of the cynipid wasp parasitoid Leptopilina boulardi, eggs are deposited into host larvae of Drosophila melanogaster that are more susceptible to destruction by melanotic encapsulation than are eggs laid earlier. The increase in parasitoid mortality is attributed to a decline in the wasp's ability to suppress the host immune response. The decrease in active immune suppression is dependent on the reproductive physiology of the wasp, and this correlates with the extent of her prior ovipositional experience and not on her chronological age nor on the number of eggs remaining in the ovarioles. Such females with prior ovipositional experience which lack the ability to immune suppress infect far fewer hosts than females with no prior ovipositional experience. The reluctance of experienced wasps to infect hosts is not due to egg depletion, but instead is attributed to a depletion in immune suppressive substances. Perhaps by ovipositional restraints, retaining eggs that would otherwise become encapsulated reduces selection pressure in host populations for specific immune reactivity.

Hydroxyl radical formation resulting from the interaction of nitric oxide and hydrogen peroxide.

The highly reactive and cytotoxic hydroxyl radical (OH) was found by electrochemical detection to be produced in reactions involving hydrogen peroxide (H2O2) and the nitric oxide (NO) donor diethylamine- NO complex. Using aromatic hydroxylation of salicylate as a specific indicator of OH, three salicylate hydroxylation products were identified; catechol, 2,3- and 2,5-dihydroxybenzoic acid. Four additional compounds were detected but not identified. The interactions of H2O2 and NO represent a biologically feasible reaction mechanism that can account for OH-induced damage in cellular environments where transition metal ions are unavailable for participation in the superoxide-mediated Fenton reaction. The ability of the NO/H2O2 complex to generate OH independently of iron or other transition metals provides a new focus for studies concerned with the origin of tissue-specific damage caused by oxygen-derived species.

Hydrogen peroxide production in immune-reactive Drosophila melanogaster.

Upon infection with the wasp parasitoid Leptopilina boulardi, the blood cells or hemocytes of Drosophila melanogaster larvae become activated and manifest a type of communal phagocytosis wherein eggs of the parasitoid are enveloped by multicellular, melanotic capsules. Hemocytes engaged in this collaborative response generate reactive oxygen intermediates (ROI). These molecules, together with melanogenic intermediates, are believed to destroy intrahemocoelic parasites. Cellular uptake of 2',7'-dichlorofluorescin diacetate (DCF-DA) and the oxidation of its deacetylated form (DCF) to yield the fluorescent product dichlorofluorescein (DC) was used as an intracellular probe for oxidant generation. The selective uptake of the fluorescent probe only by activated plasmatocytes from immune-reactive larvae identified these hemocytes as the source of ROI. Inhibition of DCF oxidation by catalase established hydrogen peroxide (H2O2) as 1 of the principal oxidants generated during melanotic encapsulation. A sensitive spectrometric assay for assessing iron oxidation and complex formation with xylenol orange (FOX assay) also was used to document in vitro-enhanced H2O2-mediated oxidations by hemolymph from immune-competent larvae. Cumulative evidence now establishes both superoxide anion (O2-*) and its dismutation product H2O2 in the cellular encapsulation response of D. melanogaster.

Comparative studies of enhanced iron-mediated production of hydroxyl radical by glutathione, cysteine, ascorbic acid, and selected catechols.

A sensitive electrochemical detection system was employed together with a specific salicylate hydroxylation assay to comparatively assess the effects of various substances on the iron-mediated generation of the hydroxyl radical (.OH). Hydroxyl radical production was found to be enhanced significantly by reduced glutathione, cysteine, ascorbic acid, and selected catechols, but not by mannitol, melatonin or tyramine. The data showed that over the range of concentrations examined, the augmented effects were linearly proportional to the amount of added reductant for a given amount of iron in the system. The pro-oxidant activity of thiols and ascorbate reduced and recycled iron providing both hydrogen peroxide (H2O2) and catalytic ferrous ions for augmented .OH production by the Fenton reaction. The enhanced production of .OH by catechols resulted from their oxidation either by molecular oxygen or ferric ions, with the accompanying formation of semiquinones, superoxide anion and H2O2. These data caution against therapeutic applications of thiols and ascorbate for ameliorating oxy-radical-induced tissue damage in environments where free redox-active metal ions may be present to function both as foci for site-specific peroxidative activity, and as catalysts to promote the pro-oxidant properties of certain endogenous reductants, thereby elevating rather than diminishing .OH levels.

Drosophila cellular immunity against parasitoids.

Insect host-parasite interrelations involve co-adaptations of considerable complexity. Against endoparasites, immune competent insect hosts initiate a hemocyte-mediated response that quickly destroys the intruders and envelops them in multilayered, melanotic capsules. In this review, Yves Carton and Anthony Nappi focus on recent studies of the cytological, biochemical and genetic mechanisms involved in the cellular immune response of Drosophila against wasp parasitoids.