Potent virucidal activity in larval Heliothis virescens plasma against Helicoverpa zea single capsid nucleopolyhedrovirus

Increased plasma selenium levels correlate with elevated resistance of Heliothis virescens larvae against baculovirus infection

We reported that dietary selenium (Se) impacted the growth and development of Trichoplusia ni reared for many generations on diet containing extremely low levels of Se. Larvae had an elevated resistance to per os infection with a baculovirus. In this study, we examine how dietary Se (in the form of selenite) affects the growth, development, and Se content of Heliothis virescens that have been laboratory reared for less than two years. Larvae fed a commercial tobacco budworm diet supplemented with greater than 20 ppm Se grew at a slower rate than insects fed lower levels of Se and had an increase in the amount of Se sequestered in pupae. Larvae fed diets containing from 10-60 ppm Se exhibited elevated plasma concentrations of the micronutrient and increased plasma virucidal activity against Helicoverpa zea single nucleopolyhedrovirus (HzSNPV). Larvae reared on diet supplemented with 10 or 60 ppm Se until the onset of the penultimate instar were then infected per os or by injection with increasing concentrations of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). Larvae fed dietary Se and infected with occluded virus per os displayed a significantly lower mortality compared with infected larvae not fed Se. Our results suggest that dietary Se levels are directly correlated with plasma Se levels, and that plasma Se levels are in turn correlated with baculovirus resistance.

Eicosanoid biosynthesis inhibitors increase the susceptibility of Lymantria dispar to nucleopolyhedrovirus LdMNPV

Eighteen pharmaceutical inhibitors of eicosanoid biosynthesis were tested for their effects on gypsy moth, Lymantria dispar and its susceptibility to the nucleopoly-hedrovirus LdMNPV. None of the inhibitors tested had any detrimental effects upon larval growth and development. Treatment with nine inhibitor/NPV combinations (e.g., bromophenacylbromide, clotrimazole, dexamethasone, esculetin, flufenamic acid, indomethacin, nimesulide, sulindac, tolfenamic acid) resulted in 3.5- to 6.6-fold reductions in LC(50)s. Larvae treated with several other COX inhibitors did not yield significant LC(50) reductions. We infer that eicosanoids act in insect defense responses to viral infection. Eicosanoids may act at three levels of insect immune reactions to viral infection, organismal (febrile response), cellular (hemocytic microaggregation, nodulation and plasmatocytes spreading reactions) and intracellular level (mechanisms responsible for insect permissiveness to viral replication).

Eicosanoids mediate Galleria mellonella cellular immune response to viral infection

Nodulation is the predominant insect cellular immune response to bacterial and fungal infections and it can also be induced by some viral infections. Treating seventh instar larvae of greater wax moth Galleria mellonella with Bovine herpes simplex virus-1 (BHSV-1) induced nodulation reactions in a dose-dependent manner. Because eicosanoids mediate nodulation reactions to bacterial and fungal infection, we hypothesized that eicosanoids also mediate nodulation reactions to viral challenge. To test this idea, we injected G. mellonella larvae with indomethacin, a nonsteroidal anti-inflammatory drug immediately prior to intrahemocoelic injection of BHSV-1. Relative to vehicle-treated controls, indomethacin-treated larvae produced significantly reduced numbers of nodules following viral infection (down from approximately 190 nodules/larva to <50 nodules/larva). In addition to injection treatments, increasing dietary indomethacin dosages (from 0.01% to 1%) were associated with decreasing nodulation (by 10-fold) and phenoloxidase activity (by 3-fold) reactions to BHSV-1 injection. We infer from these findings that cyclooxygenase products, prostaglandins, mediate nodulation response to viral infection in G. mellonella.

Uptake of dietary micronutrients from artificial diets by larval Heliothis virescens

Micronutrient assimilation from artificial diet by larvae of Heliothis virescens during selenium (Se) supplementation was studied. The metal content of pupae and plugs of the artificial diet on which they had developed from hatching was analyzed by inductively coupled plasma-mass spectrometry. Levels of the metals Cr, Co, Fe, Mg, Mn, Ni, Se, Na, and Zn were not bioaccumulated from the diet regardless of the amount of Se added to the diet. Only pupal Cu and Mo bioaccumulation were found to be altered significantly by dietary Se supplementation. Larvae fed Zn, which was found in higher levels in pupae than diet, had a deleterious response to increasing levels of dietary Zn. Larvae fed Cr, found in higher levels in diet than in pupae, were not adversely affected when increasing levels of Cr were added to the diet. Based on this analysis, metals were identified that might well impact the fitness of a given colony of insects in relation to their diet.

Gene expression profiling of Spodoptera frugiperda hemocytes and fat body using cDNA microarray reveals polydnavirus-associated variations in lepidopteran host genes transcript levels

Background

Genomic approaches provide unique opportunities to study interactions of insects with their pathogens. We developed a cDNA microarray to analyze the gene transcription profile of the lepidopteran pest Spodoptera frugiperda in response to injection of the polydnavirus HdIV associated with the ichneumonid wasp Hyposoter didymator. Polydnaviruses are associated with parasitic ichneumonoid wasps and are required for their development within the lepidopteran host, in which they act as potent immunosuppressive pathogens. In this study, we analyzed transcriptional variations in the two main effectors of the insect immune response, the hemocytes and the fat body, after injection of filter-purified HdIV.

Results

Results show that 24 hours post-injection, about 4% of the 1750 arrayed host genes display changes in their transcript levels with a large proportion (76%) showing a decrease. As a comparison, in S. frugiperda fat body, after injection of the pathogenic JcDNV densovirus, 8 genes display significant changes in their transcript level. They differ from the 7 affected by HdIV and, as opposed to HdIV injection, are all up-regulated. Interestingly, several of the genes that are modulated by HdIV injection have been shown to be involved in lepidopteran innate immunity. Levels of transcripts related to calreticulin, prophenoloxidase-activating enzyme, immulectin-2 and a novel lepidopteran scavenger receptor are decreased in hemocytes of HdIV-injected caterpillars. This was confirmed by quantitative RT-PCR analysis but not observed after injection of heat-inactivated HdIV. Conversely, an increased level of transcripts was found for a galactose-binding lectin and, surprisingly, for the prophenoloxidase subunits. The results obtained suggest that HdIV injection affects transcript levels of genes encoding different components of the host immune response (non-self recognition, humoral and cellular responses).

Conclusion

This analysis of the host-polydnavirus interactions by a microarray approach indicates that the presence of HdIV induces, directly or indirectly, variations in transcript levels of specific host genes, changes that could be responsible in part for the alterations observed in the parasitized host physiology. Development of such global approaches will allow a better understanding of the strategies employed by parasites to manipulate their host physiology, and will permit the identification of potential targets of the immunosuppressive polydnaviruses.

Prostaglandins and other eicosanoids in insects: biological significance

Prostaglandins and other eicosanoids are oxygenated metabolites of certain polyunsaturated fatty acids. These compounds are well known for their important actions in mammalian physiology and disease. Recent work has revealed the presence and biological actions of eicosanoids in insects and many other invertebrate animals. In insects, eicosanoids mediate cellular immunity to microbial and metazoan challenge. Notably, some infectious organisms secrete factors responsible for impairing host insect immune reactions by inhibiting biosynthesis of eicosanoids. Eicosanoids also act in insect reproductive biology, in ion transport physiology, and in fever response to infection as well as in protein exocytosis in tick salivary glands. Aside from ongoing actions in homeostasis, certain eicosanoid actions occur at crucial points in insect life histories, such as during infectious challenge and important events in reproduction.

Plasma phenoloxidase of the larval tobacco budworm, Heliothis virescens, is virucidal

Heliothis virescens larval plasma contains high levels of an antiviral activity against the budded form of the Helicoverpa zea single nucleopolyhedrovirus (HzSNPV) in vitro. Preliminary results indicated that phenoloxidase is primarily responsible for this virucidal effect. However it is known that other enzymes that generate antimicrobial reactive oxygen intermediates and reactive nitrogen intermediates are present in hemolymph that could contribute to the observed virucidal activity. To elucidate the contributions of phenoloxidase and other candidate activities to plasma innate immune response against baculovirus infection specific metabolic inhibitors were used. In vitro the general inhibitors of melanization (N-acetyl cysteine, ascorbate and glutathione), and specific inhibitors of phenoloxidase (phenylthiourea and Kojic acid), completely blocked virucidal activity up to the level seen in controls. Addition of the enzyme superoxide dismutase to plasma did not affect virucidal activity; however addition of catalase had an inhibitory effect. Inhibitors of nitric oxide synthase activity did not affect virucidal activity. Our results confirm that phenoloxidase is the predominate activity in larval plasma accounting for inactivation of HzSNPV in vitro, and that phenoloxidase-dependent H(2)O(2) production may contribute to this virucidal activity.