Antimicrobial autophagy: a conserved innate immune response in Drosophila

Evolution of innate immunity: lessons from mammalian models shaping our current view of insect immunity

The innate immune system, a cornerstone for organismal resilience against environmental and microbial insults, is highly conserved across the evolutionary spectrum, underpinning its pivotal role in maintaining homeostasis and ensuring survival. This review explores the evolutionary parallels between mammalian and insect innate immune systems, illuminating how investigations into these disparate immune landscapes have been reciprocally enlightening. We further delve into how advancements in mammalian immunology have enriched our understanding of insect immune responses, highlighting the intertwined evolutionary narratives and the shared molecular lexicon of immunity across these organisms. Therefore, this review posits a holistic understanding of innate immune mechanisms, including immunometabolism, autophagy and cell death. The examination of how emerging insights into mammalian and vertebrate immunity inform our understanding of insect immune responses and their implications for vector-borne disease transmission showcases the imperative for a nuanced comprehension of innate immunity's evolutionary tale. This understanding is quintessential for harnessing innate immune mechanisms' potential in devising innovative disease mitigation strategies and promoting organismal health across the animal kingdom.

Cross-validation of chemical and genetic disruption approaches to inform host cellular effects on Wolbachia abundance in Drosophila

Introduction

Endosymbiotic Wolbachia bacteria are widespread in nature, present in half of all insect species. The success of Wolbachia is supported by a commensal lifestyle. Unlike bacterial pathogens that overreplicate and harm host cells, Wolbachia infections have a relatively innocuous intracellular lifestyle. This raises important questions about how Wolbachia infection is regulated. Little is known about how Wolbachia abundance is controlled at an organismal scale.

Methods

This study demonstrates methodology for rigorous identification of cellular processes that affect whole-body Wolbachia abundance, as indicated by absolute counts of the Wolbachia surface protein (wsp) gene.

Results

Candidate pathways, associated with well-described infection scenarios, were identified. Wolbachia-infected fruit flies were exposed to small molecule inhibitors known for targeting those same pathways. Sequential tests in D. melanogaster and D. simulans yielded a subset of chemical inhibitors that significantly affected whole-body Wolbachia abundance, including the Wnt pathway disruptor, IWR-1 and the mTOR pathway inhibitor, Rapamycin. The implicated pathways were genetically retested for effects in D. melanogaster, using inducible RNAi expression driven by constitutive as well as chemically-induced somatic GAL4 expression. Genetic disruptions of armadillo, tor, and ATG6 significantly affected whole-body Wolbachia abundance.

Discussion

As such, the data corroborate reagent targeting and pathway relevance to whole-body Wolbachia infection. The results also implicate Wnt and mTOR regulation of autophagy as important for regulation of Wolbachia titer.

Transcutaneous electrical acupoint stimulation attenuated neuroinflammation and oxidative stress by activating SIRT1-induced signaling pathway in MCAO/R rat models

BACKGROUND

Silent information regulator 1 (SIRT1) plays a beneficial role in cerebral ischemic injury. Previous reports have demonstrated that transcutaneous electrical acupoint stimulation (TEAS) exerts a beneficial effect on ischemic stroke; however, whether SIRT1 participates in the underlying mechanism for the neuroprotective effects of TEAS against ischemic brain damage has not been confirmed.

METHODS

The rat models of middle cerebral artery occlusion/reperfusion (MCAO/R) were utilized in the current experiment. After MCAO/R surgery, rats in TEAS, EC and EX group received TEAS intervention with or without the injection of EX527, the SIRT1 inhibitor. Neurological deficit scores, infarct volume, hematoxylin eosin (HE) staining and apoptotic cell number were measured. The results of RNA sequencing were analyzed to determine the differential expression changes of genes among sham, MCAO and TEAS groups, in order to investigate the possible pathological processes involved in cerebral ischemia and explore the protective mechanisms of TEAS. Moreover, oxidative stress markers including MDA, SOD, GSH and GSH-Px were measured with assay kits. The levels of the proinflammatory cytokines, such as IL-6, IL-1β and TNF-α, were detected by ELISA assay, and Iba-1 (the microglia marker protein) positive cells was measured by immunofluorescence (IF). Western blot and IF were utilized to examine the levels of key molecules in SIRT1/FOXO3a and SIRT1/BRCC3/NLRP3 signaling pathways.

RESULTS

TEAS significantly decreased brain infarcted size and apoptotic neuronal number, and alleviated neurological deficit scores and morphological injury by activating SIRT1. The results of RNA-seq and bioinformatic analysis revealed that oxidative stress and inflammation were the key pathological mechanisms, and TEAS alleviated oxidative injury and inflammatory reactions following ischemic stroke. Then, further investigation indicated that TEAS notably attenuated neuronal apoptosis, neuroinflammation and oxidative stress damage in the hippocampus of rats with MCAO/R surgery. Moreover, TEAS intervention in the MCAO/R model significantly elevated the expressions of SIRT1, FOXO3a, CAT, BRCC3, NLRP3 in the hippocampus. Furthermore, EX527, as the inhibitor of SIRT1, obviously abolished the anti-oxidative stress and anti-neuroinflammatory roles of TEAS, as well as reversed the TEAS-mediated elevation of SIRT1, FOXO3a, CAT and reduction of BRCC3 and NLRP3 mediated by following MCAO/R surgery.

CONCLUSIONS

In summary, these findings clearly suggested that TEAS attenuated brain damage by suppressing apoptosis, oxidative stress and neuroinflammation through modulating SIRT1/FOXO3a and SIRT1/BRCC3/NLRP3 signaling pathways following ischemic stroke, which can be a promising treatment for stroke patients.

Toll and IMD Immune Pathways Are Important Antifungal Defense Components in a Pupal Parasitoid, Pteromalus puparum

Insects employ multifaceted strategies to combat invading fungi, with immunity being a promising mechanism. Immune pathways function in signal transduction and amplification, ultimately leading to the activation of antimicrobial peptides (AMPs). Although several studies have shown that immune pathways are responsible for defending against fungi, the roles of parasitoid immune pathways involved in antifungal responses remain unknown. In this study, we evaluated the roles of the Toll and IMD pathways of a pupal parasitoid, Pteromalus puparum (Hymenoptera: Pteromalidae), in fighting against Beauveria bassiana (Hypocreales: Cordycipitaceae). Successful colonization of B. bassiana on P. puparum adults was confirmed by scanning electron microscopy (SEM). AMPs were induced upon B. bassiana infection. The knockdown of key genes, PpTollA and PpIMD, in Toll and IMD signaling pathways, respectively, significantly compromised insect defense against fungal infection. The knockdown of either PpTollA or PpIMD in P. puparum dramatically promoted the proliferation of B. bassiana, resulting in a decreased survival rate and downregulated expression levels of AMPs against B. bassiana compared to controls. These data indicated that PpTollA and PpIMD participate in Toll and IMD-mediated activation of antifungal responses, respectively. In summary, this study has greatly broadened our knowledge of the parasitoid antifungal immunity against fungi.

Atg2 Regulates Cellular and Humoral Immunity in Drosophila

Autophagy is a process that promotes the lysosomal degradation of cytoplasmic proteins and is highly conserved in eukaryotic organisms. Autophagy maintains homeostasis in organisms and regulates multiple developmental processes, and autophagy disruption is related to human diseases. However, the functional roles of autophagy in mediating innate immune responses are largely unknown. In this study, we sought to understand how Atg2, an autophagy-related gene, functions in the innate immunity of Drosophila melanogaster. The results showed that a large number of melanotic nodules were produced upon inhibition of Atg2. In addition, inhibiting Atg2 suppressed the phagocytosis of latex beads, Staphylococcus aureus and Escherichia coli; the proportion of Nimrod C1 (one of the phagocytosis receptors)-positive hemocytes also decreased. Moreover, inhibiting Atg2 altered actin cytoskeleton patterns, showing longer filopodia but with decreased numbers of filopodia. The expression of AMP-encoding genes was altered by inhibiting Atg2. Drosomycin was upregulated, and the transcript levels of Attacin-A, Diptericin and Metchnikowin were decreased. Finally, the above alterations caused by the inhibition of Atg2 prevented flies from resisting invading pathogens, showing that flies with low expression of Atg2 were highly susceptible to Staphylococcus aureus and Erwinia carotovora carotovora 15 infections. In conclusion, Atg2 regulated both cellular and humoral innate immunity in Drosophila. We have identified Atg2 as a crucial regulator in mediating the homeostasis of immunity, which further established the interactions between autophagy and innate immunity.

Host-pathogen interaction in arthropod vectors: Lessons from viral infections

Haematophagous arthropods can harbor various pathogens including viruses, bacteria, protozoa, and nematodes. Insects possess an innate immune system comprising of both cellular and humoral components to fight against various infections. Haemocytes, the cellular components of haemolymph, are central to the insect immune system as their primary functions include phagocytosis, encapsulation, coagulation, detoxification, and storage and distribution of nutritive materials. Plasmatocytes and granulocytes are also involved in cellular defense responses. Blood-feeding arthropods, such as mosquitoes and ticks, can harbour a variety of viral pathogens that can cause infectious diseases in both human and animal hosts. Therefore, it is imperative to study the virus-vector-host relationships since arthropod vectors are important constituents of the ecosystem. Regardless of the complex immune response of these arthropod vectors, the viruses usually manage to survive and are transmitted to the eventual host. A multidisciplinary approach utilizing novel and strategic interventions is required to control ectoparasite infestations and block vector-borne transmission of viral pathogens to humans and animals. In this review, we discuss the arthropod immune response to viral infections with a primary focus on the innate immune responses of ticks and mosquitoes. We aim to summarize critically the vector immune system and their infection transmission strategies to mammalian hosts to foster debate that could help in developing new therapeutic strategies to protect human and animal hosts against arthropod-borne viral infections.

Meta-Analysis of the Public RNA-Seq Data of the Western Honeybee Apis mellifera to Construct Reference Transcriptome Data

The Western honeybee (Apis mellifera) is valuable in biological research and agriculture. Its genome sequence was published before those for other insect species. RNA-Seq data for A. mellifera have been applied in several recently published studies. Nevertheless, these data have not been prepared for use in subsequent meta-analyses. To promote A. mellifera transcriptome analysis, we constructed reference transcriptome data using the reference genome sequence and RNA-Seq data curated from about 1,000 runs of public databases. The new reference transcriptome data construct comprised 149,685 transcripts, and 194,174 protein sequences were predicted. Approximately 50-60% of the predicted protein sequences were functionally annotated using the protein sequence data for several model and insect species. Novel candidate immune-related transcripts were searched by meta-analysis using immune-response-related RNA-Seq and reference transcriptome data. Three to twenty candidate transcripts including autophagy-related protein 3 were upregulated or downregulated in response to both viral and bacterial infections. The constructed reference transcriptome data may facilitate future transcriptome analyses of A. mellifera.

A fat body transcriptome analysis of the immune responses of Rhodnius prolixus to artificial infections with bacteria

Background

Rhodnius prolixus is an important vector of Trypanosoma cruzi, the causal agent of Chagas disease in humans. Despite the medical importance of this and other triatomine vectors, the study of their immune responses has been limited to a few molecular pathways and processes. Insect immunity studies were first described for holometabolous insects such as Drosophila melanogaster, and it was assumed that their immune responses were conserved in all insects. However, study of the immune responses of triatomines and other hemimetabolous insects has revealed discrepancies between these and the Drosophila model.

Methods

To expand our understanding of innate immune responses of triatomines to pathogens, we injected fifth instar nymphs of R. prolixus with the Gram-negative (Gr−) bacterium Enterobacter cloacae, the Gram-positive (Gr+) bacterium Staphylococcus aureus, or phosphate-buffered saline (PBS), and evaluated transcript expression in the fat body 8 and 24 h post-injection (hpi). We analyzed the differential expression of transcripts at each time point, and across time, for each treatment.

Results

At 8 hpi, the Gr− bacteria-injected group had a large number of differentially expressed (DE) transcripts, and most of the changes in transcript expression were maintained at 24 hpi. In the Gr+ bacteria treatment, few DE transcripts were detected at 8 hpi, but a large number of transcripts were DE at 24 hpi. Unexpectedly, the PBS control also had a large number of DE transcripts at 24 hpi. Very few DE transcripts were common to the different treatments and time points, indicating a high specificity of the immune responses of R. prolixus to different pathogens. Antimicrobial peptides known to be induced by the immune deficiency pathway were induced upon Gr− bacterial infection. Many transcripts of genes from the Toll pathway that are thought to participate in responses to Gr+ bacteria and fungi were induced by both bacteria and PBS treatment. Pathogen recognition receptors and serine protease cascade transcripts were also overexpressed after Gr− bacteria and PBS injections. Gr- injection also upregulated transcripts involved in the metabolism of tyrosine, a major substrate involved in the melanotic encapsulation response to pathogens.

Conclusions

These results reveal time-dependent pathogen-specific regulation of immune responses in triatomines, and hint at strong interactions between the immune deficiency and Toll pathways.

Graphical abstract

Supplementary Information

The online version contains supplementary material, which is available at 10.1186/s13071-022-05358-9.

Activation of the Host Immune Response in Hyphantria cunea (Drury) (Lepidoptera: Noctuidae) Induced by Serratia marcescens Bizio

Simple Summary

Hyphantria cunea (Drury) is a quarantine pest, due to its extensive host, leading to serious economic losses in the agricultural and forestry industries. To control this pest, it is increasingly important to use microbial pesticides because they are biologically active and ecologically safe. Serratia marcescens Bizio (SM1) is a potential biocontrol bacterium. Although SM1 has a pathogenic role in H. cunea, H. cunea self-defense reduces the pathogenic effect of SM1. In this study, immune-related differentially expressed genes (DEGs) in H. cunea were first identified after SM1 infection, and the immune regulation mode of H. cunea in response to SM1, including antimicrobial peptide synthesis pathways, melanization and cellular immunity, was revealed. According to the analysis, the immune system of H. cunea was induced by SM1. In summary, our study demonstrates how the immune systems of the H. cunea work to resist the infection of SM1, which provides the theoretical basis for researching more efficient microbial pesticides for H. cunea.

Abstract

Host–pathogen interactions are essential to our understanding of biological pesticides. Hyphantria cunea (Drury) is an important forest pest worldwide. The immune mechanism of the interaction between H. cunea and Serratia marcescens Bizio (SM1) is unclear. First, transcriptome sequencing and quantitative real-time PCR (qRT-PCR) analysis described the H. cunea immune response to SM1. A total of 234 immune-related differentially expressed genes (DEGs) were found. Many immune regulatory genes in three classical pathways were found. Antimicrobial peptides, including attacin B, cecropin A, gloverin, lebocin and diapausin, are involved in defending against SM1 challenge, and are mainly produced by Toll and immune deficiency (IMD) pathways. Some melanization genes were changed in H. cunea, which suggested that H. cunea melanization was activated by SM1. Furthermore, phagocytosis, autophagolysosome and apoptosis pathways in cellular immunity were activated in H. cunea against SM1. Finally, the expression patterns of 10 immune genes were analyzed systematically by qRT-PCR, and most of the genes were upregulated compared to the control. Our studies provide useful information about the immune response of H. cunea under the stress of SM1, which is important to understand how SM1 affects the immune system of H. cunea and provides new ideas to control H. cunea by using SM1.

Activation of the autophagy pathway decreases dengue virus infection in Aedes aegypti cells

Background

Mosquito-borne dengue virus (DENV) causes major disease worldwide, impacting 50–100 million people every year, and is spread by the major mosquito vector Aedes aegypti. Understanding mosquito physiology, including antiviral mechanisms, and developing new control strategies have become an important step towards the elimination of DENV disease. In the study reported here, we focused on autophagy, a pathway suggested as having a positive influence on virus replication in humans, as a potential antiviral target in the mosquito.

Methods

To understand the role played by autophagy in Ae. aegypti, we examined the activation of this pathway in Aag-2 cells, an Ae. aegypti-derived cell line, infected with DENV. Rapamycin and 3-methyladenine, two small molecules that have been shown to affect the function of the autophagy pathway, were used to activate or suppress, respectively, the autophagy pathway.

Results

At 1-day post-DENV infection in Aag-2 cells, transcript levels of both the microtubule-associated protein light chain 3-phosphatidylethanolamine conjugate (LC3-II) and autophagy-related protein 1 (ATG1) increased. Rapamycin treatment activated the autophagy pathway as early as 1-h post-treatment, and the virus titer had decreased in the Aag-2 cells at 2 days post-infection; in contrast, the 3-methyladenine treatment did not significantly affect the DENV titer. Treatment with these small molecules also impacted the ATG12 transcript levels in DENV-infected cells.

Conclusions

Our studies revealed that activation of the autophagy pathway through rapamycin treatment altered DENV infection in the mosquito cells, suggesting that this pathway could be a possible antiviral mechanism in the mosquito system. Here we provide fundamental information needed to proceed with future experiments and to improve our understanding of the mosquito’s immune response against DENV.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-05066-w.

RNAi-mediated silencing of the autophagy-related gene NlATG3 inhibits survival and fecundity of the brown planthopper, Nilaparvata lugens

BACKGROUND

The brown planthopper (BPH), Nilaparvata lugens, is a serious insect pest of rice. Autophagy and its related gene ATG3 play multiple roles in insects. However, information about the functions of ATG3 in BPH (NlATG3) is unavailable, and its potential as a target for pest control remains unclear.

RESULTS

RT-qPCR results showed a relatively low expression of NlATG3 in 1st-4th-instar nymphs, which increased through 9-day-old adults. The expression of NlATG3 increased continuously in 1-day-old through 5-day-old eggs, whereas it decreased thereafter. The mRNA level of NlATG3 was markedly higher in the ovary (1.16) and head (1.00) compared to the rest body parts of BPH adults. Injecting nymphs with dsNlATG3 at doses from 62.5 to 250 ng per insect had strong lethal effect upon them. For the 5th-instar nymphs, all individuals died within 5 days after receiving the dsNlATG3, and importantly, no individual successfully molted. Transmission electron microscopy revealed the new cuticle of nymphs injected with dsNlATG3 became loose and curved, which is clearly different from that of the control. Correspondingly, the obvious vesicles in epidermal cells disappeared after dsNlATG3-treatment. RNAi of NlATG3 significantly reduced the total number of eggs laid per female as well as the eggs' hatchability, especially in the dsNlATG3♀ × dsGFP♂ group, whose total number of eggs laid per female largely decreased by 80.4%, and whose eggs' hatchability was significantly reduced from 95.7% to zero, when compared with the control (dsGFP♀ × dsGFP♂).

CONCLUSION

NlATG3 is a promising target for developing RNAi-based insect management strategies. © 2021 Society of Chemical Industry.

The molecular interplay of the establishment of an infection - gene expression of Diaphorina citri gut and Candidatus Liberibacter asiaticus

BACKGROUND

Candidatus Liberibacter asiaticus (CLas) is one the causative agents of greening disease in citrus, an unccurable, devastating disease of citrus worldwide. CLas is vectored by Diaphorina citri, and the understanding of the molecular interplay between vector and pathogen will provide additional basis for the development and implementation of successful management strategies. We focused in the molecular interplay occurring in the gut of the vector, a major barrier for CLas invasion and colonization.

RESULTS

We investigated the differential expression of vector and CLas genes by analyzing a de novo reference metatranscriptome of the gut of adult psyllids fed of CLas-infected and healthy citrus plants for 1-2, 3-4 and 5-6 days. CLas regulates the immune response of the vector affecting the production of reactive species of oxygen and nitrogen, and the production of antimicrobial peptides. Moreover, CLas overexpressed peroxiredoxin, probably in a protective manner. The major transcript involved in immune expression was related to melanization, a CLIP-domain serine protease we believe participates in the wounding of epithelial cells damaged during infection, which is supported by the down-regulation of pangolin. We also detected that CLas modulates the gut peristalsis of psyllids through the down-regulation of titin, reducing the elimination of CLas with faeces. The up-regulation of the neuromodulator arylalkylamine N-acetyltransferase implies CLas also interferes with the double brain-gut communication circuitry of the vector. CLas colonizes the gut by expressing two Type IVb pilin flp genes and several chaperones that can also function as adhesins. We hypothesized biofilm formation occurs by the expression of the cold shock protein of CLas.

CONCLUSIONS

The thorough detailed analysis of the transcritome of Ca. L. asiaticus and of D. citri at different time points of their interaction in the gut tissues of the host led to the identification of several host genes targeted for regulation by L. asiaticus, but also bacterial genes coding for potential effector proteins. The identified targets and effector proteins are potential targets for the development of new management strategies directed to interfere with the successful utilization of the psyllid vector by this pathogen.

Multiplexed functional metagenomic analysis of the infant microbiome identifies effectors of NF-κB, autophagy, and cellular redox state

The human microbiota plays a critical role in host health. Proper development of the infant microbiome is particularly important. Its dysbiosis leads to both short-term health issues and long-term disorders lasting into adulthood. A central way in which the microbiome interacts with the host is through the production of effector molecules, such as proteins and small molecules. Here, a metagenomic library constructed from 14 infant stool microbiomes is analyzed for the production of effectors that modulate three distinct host pathways: immune response (nuclear factor κB [NF-κB] activation), autophagy (LC3-B puncta formation), and redox potential (NADH:NAD ratio). We identify microbiome-encoded bioactive metabolites, including commendamide and hydrogen sulfide and their associated biosynthetic genes, as well as a previously uncharacterized autophagy-inducing operon from Klebsiella spp. This work extends our understanding of microbial effector molecules that are known to influence host pathways. Parallel functional screening of metagenomic libraries can be easily expanded to investigate additional host processes.

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Construction of a metagenomic library from stool of infants

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A multiplexed screen for bacterial effectors of host cellular processes

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Identification of microbiome-encoded effectors hydrogen sulfide and commendamide

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The products of a Klebsiella pneumoniae operon induce autophagy

Probiotic based-diet effect on the immune response and induced stress in irradiated mass reared Ceratitis capitata males (Diptera: Tephritidae) destined for the release in the sterile insect technique programs

Ceratitis capitata (medfly) is one of the most devastating crop pests worldwide. The Sterile Insect Technique (SIT) is a control method that is based on the mass rearing of males, their sterilization, and release in the field. However, the effectiveness of the technique depends on the quality of the released males and their fitness. We previously isolated and selected a probiotic bacteria (Enterobacter sp.), from wild-caught medflies, according to criteria that improved biological quality traits of reared medfly males.We firstly evaluated the impact of the irradiation on the expression of different immune and stress genes in the medfly sterile males. Expression was measured at differents time points ranging from 0 to 168 h after irradiation to capture the response of genes with distinct temporal expression patterns. Then, we supplemented the larval diet with previously isolated Enterobacter sp.strain, live and autoclaved at various concentrations to see whether the probiotic treatments affect, through their protective role, the gene expression level, and quality traits. The irradiation had significant effect on the genes attacin, cecropin, PGPR-LC, hsp23, and hsp70 level expression. The expression of attacin and PGPR-LC was up-regulated while that of cecropin was down-regulated. Hsp genes showed decreased levels between 0 and 18 h to peak at 72 h. However, the supplementation of the probiotic strain, either live or autoclaved, was statistically significant only for attacingene. However, significant interaction time x probiotic was noticed for attacin, cecropin, hsp23 and hsp70. The probiotic treatments also improved the quality control parameters like pupal weight. From this work we can conclude that a consortium of parabiotics (autoclaved probiotics) treatment will be recommended in insectaries considering both the beneficial effects on mass reared insects and its general safety for insectary workers and for environment.

Immune Response of Eastern Honeybee Worker to Nosema ceranae Infection Revealed by Transcriptomic Investigation

Simple Summary

Currently, knowledge regarding Apis cerana–Nosema ceranae interaction is very limited, though A. cerana is the original host of N. ceranae. Apis cerana cerana is a subspecies of A. cerana and a major bee species used in the beekeeping industry in China and other countries. Here, the effective infection of A. c. cerana workers by N. ceranae was verified, followed by transcriptomic investigation of host responses. Furthermore, immune responses between A. c. cerana and Apis mellifera ligustica were deeply compared and discussed. In total, 1127 and 957 N. ceranae-responsive genes were identified in the infected midguts at 7 d post-inoculation (dpi) and 10 dpi, respectively. Additionally, DEGs in workers’ midguts at both 7 dpi and 10 dpi were associated with six cellular immune pathways and three humoral immune pathways. Noticeably, one up-regulated gene was enriched in the NF-κB signaling pathway in the midgut at 10 dpi. Further analysis indicated that different cellular and humoral immune responses were employed by A. c. cerana and A. m. ligustica workers to combat N. ceranae. Our findings provide a foundation for clarifying the mechanisms regulating the immune response of A. c. cerana workers to N. ceranae invasion and developing new approaches to control bee microsporidiosis.

Abstract

Here, a comparative transcriptome investigation was conducted based on high-quality deep sequencing data from the midguts of Apis cerana cerana workers at 7 d post-inoculation (dpi) and 10 dpi with Nosema ceranae and corresponding un-inoculated midguts. PCR identification and microscopic observation of paraffin sections confirmed the effective infection of A. c. cerana worker by N. ceranae. In total, 1127 and 957 N. ceranae-responsive genes were identified in the infected midguts at 7 dpi and 10 dpi, respectively. RT-qPCR results validated the reliability of our transcriptome data. GO categorization indicated the differentially expressed genes (DEGs) were respectively engaged in 34 and 33 functional terms associated with biological processes, cellular components, and molecular functions. Additionally, KEGG pathway enrichment analysis showed that DEGs at 7 dpi and 10 dpi could be enriched in 231 and 226 pathways, respectively. Moreover, DEGs in workers’ midguts at both 7 dpi and 10 dpi were involved in six cellular immune pathways such as autophagy and phagosome and three humoral immune pathways such as the Toll/Imd signaling pathway and Jak-STAT signaling pathway. In addition, one up-regulated gene (XM_017055397.1) was enriched in the NF-κB signaling pathway in the workers’ midgut at 10 dpi. Further investigation suggested the majority of these DEGs were engaged in only one immune pathway, while a small number of DEGs were simultaneously involved in two immune pathways. These results together demonstrated that the overall gene expression profile in host midgut was altered by N. ceranae infection and some of the host immune pathways were induced to activation during fungal infection, whereas some others were suppressed via host–pathogen interaction. Our findings offer a basis for clarification of the mechanism underlying the immune response of A. c. cerana workers to N. ceranae infection, but also provide novel insights into eastern honeybee-microsporodian interaction.

Tick Immune System: What Is Known, the Interconnections, the Gaps, and the Challenges

Ticks are ectoparasitic arthropods that necessarily feed on the blood of their vertebrate hosts. The success of blood acquisition depends on the pharmacological properties of tick saliva, which is injected into the host during tick feeding. Saliva is also used as a vehicle by several types of pathogens to be transmitted to the host, making ticks versatile vectors of several diseases for humans and other animals. When a tick feeds on an infected host, the pathogen reaches the gut of the tick and must migrate to its salivary glands via hemolymph to be successfully transmitted to a subsequent host during the next stage of feeding. In addition, some pathogens can colonize the ovaries of the tick and be transovarially transmitted to progeny. The tick immune system, as well as the immune system of other invertebrates, is more rudimentary than the immune system of vertebrates, presenting only innate immune responses. Although simpler, the large number of tick species evidences the efficiency of their immune system. The factors of their immune system act in each tick organ that interacts with pathogens; therefore, these factors are potential targets for the development of new strategies for the control of ticks and tick-borne diseases. The objective of this review is to present the prevailing knowledge on the tick immune system and to discuss the challenges of studying tick immunity, especially regarding the gaps and interconnections. To this end, we use a comparative approach of the tick immune system with the immune system of other invertebrates, focusing on various components of humoral and cellular immunity, such as signaling pathways, antimicrobial peptides, redox metabolism, complement-like molecules and regulated cell death. In addition, the role of tick microbiota in vector competence is also discussed.

Rhodnius prolixus uses the peptidoglycan recognition receptor rpPGRP-LC/LA to detect Gram-negative bacteria and activate the IMD pathway

Insects rely on an innate immune system to recognize and eliminate pathogens. Key components of this system are highly conserved across all invertebrates. To detect pathogens, insects use Pattern recognition receptors (PRRs) that bind to signature motifs on the surface of pathogens called Pathogen Associated Molecular Patterns (PAMPs). In general, insects use peptidoglycan recognition proteins (PGRPs) in the Immune Deficiency (IMD) pathway to detect Gram-negative bacteria, and other PGRPs and Gram-negative binding proteins (GNBPs) in the Toll pathway to detect Gram-positive bacteria and fungi, although there is crosstalk and cooperation between these and other pathways. Once pathogens are recognized, these pathways activate the production of potent antimicrobial peptides (AMPs). Most PRRs in insects have been reported from genome sequencing initiatives but few have been characterized functionally. The initial studies on insect PRRs were done using established dipteran model organisms such as Drosophila melanogaster, but there are differences in the numbers and functional role of PRRs in different insects. Here we describe the genomic repertoire of PGRPs in Rhodnius prolixus, a hemimetabolous hemipteran vector of the parasite Trypanosoma cruzi that causes Chagas disease in humans. Using a de novo transcriptome from the fat body of immune activated insects, we found 5 genes encoding PGRPs. Phylogenetic analysis groups R. prolixus PGRPs with D. melanogaster PGRP-LA, which is involved in the IMD pathway in the respiratory tract. A single R. prolixus PGRP gene encodes isoforms that contain an intracellular region or motif (cryptic RIP Homotypic Interaction Motif-cRHIM) that is involved in the IMD signaling pathway in D. melanogaster. We characterized and silenced this gene using RNAi and show that the PGRPs that contain cRHIMs are involved in the recognition of Gram-negative bacteria, and activation of the IMD pathway in the fat body of R. prolixus, similar to the PGRP-LC of D. melanogaster. This is the first functional characterization of a PGRP containing a cRHIM motif that serves to activate the IMD pathway in a hemimetabolous insect.

Immunological Function of the Antibacterial Peptide Attacin-Like in the Chinese Oak Silkworm, Antheraea pernyi

BACKGROUND

Antibacterial peptides play important roles in the innate immune system of insects and are divided into four categories according to their structures. Although many antibacterial peptides have been reported in lepidopteran insects, the roles of an attacin-like gene in immune response of Antheraea pernyi remain unclear.

OBJECTIVE

In this study, the cloning and immunological functions of an attacin-like gene from Antheraea pernyi were investigated.

METHODS

The open reading frame of Ap-attacin-like gene was cloned by PCR using the specific primers and then was ligated to the pET-32a vector to construct the recombinant plasmids Ap-attacin- like-pET-32a. The recombinant Ap-attacin-like protein was expressed in E. coli (BL21 DE3) cells and purified by Ni-NTA affinity chromatography. The expression patterns of Ap-attacin-like in different tissues or under microorganism challenges were investigated by real-time PCR and western blotting. Finally, agar well diffusion assay was performed to determine the antimicrobial activity of the recombinant Ap-attacin-like proteins based on the inhibition rate.

RESULTS

The expression level of Ap-attacin-like was highest in the fat body compared with the other examined tissues. The expression of Ap-attacin-like in the fat body was significantly elevated after E. coli, Beauveria bassiana, Micrococcus luteus or Nuclear Polyhedrosis Virus challenges. In addition, the recombinant Ap-attacin-like proteins had obvious antibacterial activity against E. coli.

CONCLUSION

Ap-attacin-like was highly expressed in immune-related tissues and its expression level was significantly induced by different microorganism challenges, suggesting that Ap-attacin-like participated in the innate immunity of A. pernyi.

Anopheles aquasalis transcriptome reveals autophagic responses to Plasmodium vivax midgut invasion

Background

Elimination of malaria depends on mastering transmission and understanding the biological basis of Plasmodium infection in the vector. The first mosquito organ to interact with the parasite is the midgut and its transcriptomic characterization during infection can reveal effective antiplasmodial responses able to limit the survival of the parasite. The vector response to Plasmodium vivax is not fully characterized, and its specificities when compared with other malaria parasites can be of fundamental interest for specific control measures.

Methods

Experimental infections were performed using a membrane-feeding device. Three groups were used: P. vivax-blood-fed, blood-fed on inactivated gametocytes, and unfed mosquitoes. Twenty-four hours after feeding, the mosquitoes were dissected and the midgut collected for transcriptomic analysis using RNAseq. Nine cDNA libraries were generated and sequenced on an Illumina HiSeq2500. Readings were checked for quality control and analysed using the Trinity platform for de novo transcriptome assembly. Transcript quantification was performed and the transcriptome was functionally annotated. Differential expression gene analysis was carried out. The role of the identified mechanisms was further explored using functional approaches.

Results

Forty-nine genes were identified as being differentially expressed with P. vivax infection: 34 were upregulated and 15 were downregulated. Half of the P. vivax-related differentially expressed genes could be related to autophagy; therefore, the effect of the known inhibitor (wortmannin) and activator (spermidine) was tested on the infection outcome. Autophagic activation significantly reduced the intensity and prevalence of infection. This was associated with transcription alterations of the autophagy regulating genes Beclin, DRAM and Apg8.

Conclusions

Our data indicate that P. vivax invasion of An. aquasalis midgut epithelium triggers an autophagic response and its activation reduces infection. This suggests a novel mechanism that mosquitoes can use to fight Plasmodium infection.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3506-8) contains supplementary material, which is available to authorized users.

The Oxysterol 7-Ketocholesterol Reduces Zika Virus Titers in Vero Cells and Human Neurons

Zika virus (ZIKV) is an emerging flavivirus responsible for a major epidemic in the Americas beginning in 2015. ZIKV associated with maternal infection can lead to neurological disorders in newborns, including microcephaly. Although there is an abundance of research examining the neurotropism of ZIKV, we still do not completely understand the mechanism by which ZIKV targets neural cells or how to limit neural cell infection. Recent research suggests that flaviviruses, including ZIKV, may hijack the cellular autophagy pathway to benefit their replication. Therefore, we hypothesized that ZIKV replication would be impacted when infected cells were treated with compounds that target the autophagy pathway. We screened a library of 94 compounds known to affect autophagy in both mammalian and insect cell lines. A subset of compounds that inhibited ZIKV replication without affecting cellular viability were tested for their ability to limit ZIKV replication in human neurons. From this second screen, we identified one compound, 7-ketocholesterol (7-KC), which inhibited ZIKV replication in neurons without significantly affecting neuron viability. Interestingly, 7-KC induces autophagy, which would be hypothesized to increase ZIKV replication, yet it decreased virus production. Time-of-addition experiments suggest 7-KC inhibits ZIKV replication late in the replication cycle. While 7-KC did not inhibit RNA replication, it decreased the number of particles in the supernatant and the relative infectivity of the released particles, suggesting it interferes with particle budding, release from the host cell, and particle integrity.

Tenebrio molitor Gram-negative-binding protein 3 (TmGNBP3) is essential for inducing downstream antifungal Tenecin 1 gene expression against infection with Beauveria bassiana JEF-007

The Toll signaling pathway is responsible for defense against both Gram-positive bacteria and fungi. Gram-negative binding protein 3 (GNBP3) has a strong affinity for the fungal cell wall component, β-1,3-glucan, which can activate the prophenoloxidase (proPO) cascade and induce the Toll signaling pathway. Myeloid differentiation factor 88 (MyD88) is an intracellular adaptor protein involved in the Toll signaling pathway. In this study, we monitored the response of 5 key genes (TmGNBP3, TmMyD88, and Tenecin 1, 2, and 3) in the Toll pathway of the mealworm Tenebrio molitor immune system against the fungus Beauveria bassiana JEF-007 using RT-PCR. TmGNBP3, Tenecin 1, and Tenecin 2 were significantly upregulated after fungal infection. To better understand the roles of the Toll signaling pathway in the mealworm immune system, TmGNBP3 and TmMyD88 were knocked down by RNAi silencing. Target gene expression levels decreased at 2 d postknockdown and were dramatically reduced at 6 d post-dsRNA injection. Therefore, mealworms were compromised by B. bassiana JEF-007 at 6 d post-dsRNA injection. Silencing of TmMyD88 and TmGNBP3 resulted in reduced resistance of the host to fungal infection. Particularly, reducing TmGNBP3 levels obviously downregulated Tenecin 1 and Tenecin 2 expression levels, whereas silencing TmMyD88 expression resulted in decreased Tenecin 2 expression. These results indicate that TmGNBP3 is essential to induce downstream antifungal peptide Tenecin 1 expression against B. bassiana JEF-007.

Population genetic analysis of autophagy and phagocytosis genes in Drosophila melanogaster and D. simulans

Autophagy and phagocytosis are cellular immune mechanisms for internalization and elimination of intracellular and extracellular pathogens. Some pathogens have evolved the ability to inhibit or manipulate these processes, raising the prospect of adaptive reciprocal co-evolution by the host. We performed population genetic analyses on phagocytosis and autophagy genes in Drosophila melanogaster and D. simulans to test for molecular evolutionary signatures of immune adaptation. We found that phagocytosis and autophagy genes as a whole exhibited an elevated level of haplotype homozygosity in both species. In addition, we detected signatures of recent selection, notably in the Atg14 and Ykt6 genes in D. melanogaster and a pattern of elevated sequence divergence in the genderblind (gb) gene on the D. simulans lineage. These results suggest that the evolution of the host cellular immune system as a whole may be shaped by a dynamic conflict between Drosophila and its pathogens even without pervasive evidence of strong adaptive evolution at the individual gene level.

Natural Variation in Resistance to Virus Infection in Dipteran Insects

The power and ease of Drosophila genetics and the medical relevance of mosquito-transmitted viruses have made dipterans important model organisms in antiviral immunology. Studies of virus-host interactions at the molecular and population levels have illuminated determinants of resistance to virus infection. Here, we review the sources and nature of variation in antiviral immunity and virus susceptibility in model dipteran insects, specifically the fruit fly Drosophila melanogaster and vector mosquitoes of the genera Aedes and Culex. We first discuss antiviral immune mechanisms and describe the virus-specificity of these responses. In the following sections, we review genetic and microbiota-dependent variation in antiviral immunity. In the final sections, we explore less well-studied sources of variation, including abiotic factors, sexual dimorphism, infection history, and endogenous viral elements. We borrow from work on other pathogen types and non-dipteran species when it parallels or complements studies in dipterans. Understanding natural variation in virus-host interactions may lead to the identification of novel restriction factors and immune mechanisms and shed light on the molecular determinants of vector competence.

Autophagy and innate immunity: Insights from invertebrate model organisms

Macroautophagy/autophagy is a fundamental intracellular degradation process with multiple roles in immunity, including direct elimination of intracellular microorganisms via 'xenophagy.' In this review, we summarize studies from the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans that highlight the roles of autophagy in innate immune responses to viral, bacterial, and fungal pathogens. Research from these genetically tractable invertebrates has uncovered several conserved immunological paradigms, such as direct targeting of intracellular pathogens by xenophagy and regulation of autophagy by pattern recognition receptors in D. melanogaster. Although C. elegans has no known pattern recognition receptors, this organism has been particularly useful in understanding many aspects of innate immunity. Indeed, work in C. elegans was the first to show xenophagic targeting of microsporidia, a fungal pathogen that infects all animals, and to identify TFEB/HLH-30, a helix-loop-helix transcription factor, as an evolutionarily conserved regulator of autophagy gene expression and host tolerance. Studies in C. elegans have also highlighted the more recently appreciated relationship between autophagy and tolerance to extracellular pathogens. Studies of simple, short-lived invertebrates such as flies and worms will continue to provide valuable insights into the molecular mechanisms by which autophagy and immunity pathways intersect and their contribution to organismal survival. Abbreviations Atg autophagy related BECN1 Beclin 1 CALCOCO2 calcium binding and coiled-coil domain 2 Cry5B crystal toxin 5B Daf abnormal dauer formation DKF-1 D kinase family-1 EPG-7 Ectopic P Granules-7 FuDR fluorodeoxyuridine GFP green fluorescent protein HLH-30 Helix Loop Helix-30 Imd immune deficiency ins-18 INSulin related-18; LET-363, LEThal-363 lgg-1 LC3, GABARAP and GATE-16 family-1 MAPK mitogen-activated protein kinase MATH the meprin and TRAF homology MTOR mechanistic target of rapamycin NBR1 neighbor of BRCA1 gene 1 NFKB nuclear factor of kappa light polypeptide gene enhancer in B cells NOD nucleotide-binding oligomerization domain containing OPTN optineurin PAMPs pathogen-associated molecular patterns Park2 Parkinson disease (autosomal recessive, juvenile) 2, parkin pdr-1 Parkinson disease related PFTs pore-forming toxins PGRP peptidoglycan-recognition proteins PIK3C3 phosphatidylinositol 3- kinase catalytic subunit type 3 pink-1 PINK (PTEN-I induced kinase) homolog PRKD protein kinase D; PLC, phospholipase C PRKN parkin RBR E3 ubiquitin protein ligase PRRs pattern-recognition receptors PtdIns3P phosphatidylinositol-3-phosphate rab-5 RAB family-5 RB1CC1 RB1-inducible coiled-coil 1 RNAi RNA interference sqst SeQueSTosome related SQSTM1 sequestosome 1 TBK1 TANK-binding kinase 1 TFEB transcription factor EB TGFB/TGF-β transforming growth factor beta TLRs toll-like receptors unc-51 UNCoordinated-51 VPS vacuolar protein sorting; VSV, vesicular stomatitis virus VSV-G VSV surface glycoprotein G Wipi2 WD repeat domain, phosphoinositide interacting 2.

Life or death: disease-tolerant coral species activate autophagy following immune challenge

Global climate change has increased the number and severity of stressors affecting species, yet not all species respond equally to these stressors. Organisms may employ cellular mechanisms such as apoptosis and autophagy in responding to stressful events. These two pathways are often mutually exclusive, dictating whether a cell adapts or dies. In order to examine differences in cellular response to stress, we compared the immune response of four coral species with a range of disease susceptibility. Using RNA-seq and novel pathway analysis, we were able to identify differences in response to immune stimulation between these species. Disease-susceptible species Orbicella faveolata activated pathways associated with apoptosis. By contrast, disease-tolerant species Porites porites and Porites astreoides activated autophagic pathways. Moderately susceptible species Pseudodiploria strigosa activated a mixture of these pathways. These findings were corroborated by apoptotic caspase protein assays, which indicated increased caspase activity following immune stimulation in susceptible species. Our results indicate that in response to immune stress, disease-tolerant species activate cellular adaptive mechanisms such as autophagy, while susceptible species turn on cell death pathways. Differences in these cellular maintenance pathways may therefore influence the organismal stress response. Further study of these pathways will increase understanding of differential stress response and species survival in the face of changing environments.

Listeriolysin O Regulates the Expression of Optineurin, an Autophagy Adaptor That Inhibits the Growth of Listeria monocytogenes

Autophagy, a well-established defense mechanism, enables the elimination of intracellular pathogens including Listeria monocytogenes. Host cell recognition results in ubiquitination of L. monocytogenes and interaction with autophagy adaptors p62/SQSTM1 and NDP52, which target bacteria to autophagosomes by binding to microtubule-associated protein 1 light chain 3 (LC3). Although studies have indicated that L. monocytogenes induces autophagy, the significance of this process in the infectious cycle and the mechanisms involved remain poorly understood. Here, we examined the role of the autophagy adaptor optineurin (OPTN), the phosphorylation of which by the TANK binding kinase 1 (TBK1) enhances its affinity for LC3 and promotes autophagosomal degradation, during L. monocytogenes infection. In LC3- and OPTN-depleted host cells, intracellular replicating L. monocytogenes increased, an effect not seen with a mutant lacking the pore-forming toxin listeriolysin O (LLO). LLO induced the production of OPTN. In host cells expressing an inactive TBK1, bacterial replication was also inhibited. Our studies have uncovered an OPTN-dependent pathway in which L. monocytogenes uses LLO to restrict bacterial growth. Hence, manipulation of autophagy by L. monocytogenes, either through induction or evasion, represents a key event in its intracellular life style and could lead to either cytosolic growth or persistence in intracellular vacuolar structures.

The genetic architecture of defence as resistance to and tolerance of bacterial infection in Drosophila melanogaster

Defence against pathogenic infection can take two forms: resistance and tolerance. Resistance is the ability of the host to limit a pathogen burden, whereas tolerance is the ability to limit the negative consequences of infection at a given level of infection intensity. Evolutionarily, a tolerance strategy that is independent of resistance could allow the host to avoid mounting a costly immune response and, theoretically, to avoid a co-evolutionary arms race between pathogen virulence and host resistance. Biomedically, understanding the mechanisms of tolerance and how they relate to resistance could potentially yield treatment strategies that focus on health improvement instead of pathogen elimination. To understand the impact of tolerance on host defence and identify genetic variants that determine host tolerance, we defined genetic variation in tolerance as the residual deviation from a binomial regression of fitness under infection against infection intensity. We then performed a genomewide association study to map the genetic basis of variation in resistance to and tolerance of infection by the bacterium Providencia rettgeri. We found a positive genetic correlation between resistance and tolerance, and we demonstrated that the level of resistance is highly predictive of tolerance. We identified 30 loci that predict tolerance, many of which are in genes involved in the regulation of immunity and metabolism. We used RNAi to confirm that a subset of mapped genes have a role in defence, including putative wound repair genes grainy head and debris buster. Our results indicate that tolerance is not an independent strategy from resistance, but that defence arises from a collection of physiological processes intertwined with canonical immunity and resistance.

De Novo Transcriptome Analysis and Detection of Antimicrobial Peptides of the American Cockroach Periplaneta americana (Linnaeus)

Cockroaches are surrogate hosts for microbes that cause many human diseases. In spite of their generally destructive nature, cockroaches have recently been found to harbor potentially beneficial and medically useful substances such as drugs and allergens. However, genomic information for the American cockroach (Periplaneta americana) is currently unavailable; therefore, transcriptome and gene expression profiling is needed as an important resource to better understand the fundamental biological mechanisms of this species, which would be particularly useful for the selection of novel antimicrobial peptides. Thus, we performed de novo transcriptome analysis of P. americana that were or were not immunized with Escherichia coli. Using an Illumina HiSeq sequencer, we generated a total of 9.5 Gb of sequences, which were assembled into 85,984 contigs and functionally annotated using Basic Local Alignment Search Tool (BLAST), Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) database terms. Finally, using an in silico antimicrobial peptide prediction method, 86 antimicrobial peptide candidates were predicted from the transcriptome, and 21 of these peptides were experimentally validated for their antimicrobial activity against yeast and gram positive and -negative bacteria by a radial diffusion assay. Notably, 11 peptides showed strong antimicrobial activities against these organisms and displayed little or no cytotoxic effects in the hemolysis and cell viability assay. This work provides prerequisite baseline data for the identification and development of novel antimicrobial peptides, which is expected to provide a better understanding of the phenomenon of innate immunity in similar species.

Insect immunology and hematopoiesis

Insects combat infection by mounting powerful immune responses that are mediated by hemocytes, the fat body, the midgut, the salivary glands and other tissues. Foreign organisms that have entered the body of an insect are recognized by the immune system when pathogen-associated molecular patterns bind host-derived pattern recognition receptors. This, in turn, activates immune signaling pathways that amplify the immune response, induce the production of factors with antimicrobial activity, and activate effector pathways. Among the immune signaling pathways are the Toll, Imd, Jak/Stat, JNK, and insulin pathways. Activation of these and other pathways leads to pathogen killing via phagocytosis, melanization, cellular encapsulation, nodulation, lysis, RNAi-mediated virus destruction, autophagy and apoptosis. This review details these and other aspects of immunity in insects, and discusses how the immune and circulatory systems have co-adapted to combat infection, how hemocyte replication and differentiation takes place (hematopoiesis), how an infection prepares an insect for a subsequent infection (immune priming), how environmental factors such as temperature and the age of the insect impact the immune response, and how social immunity protects entire groups. Finally, this review highlights some underexplored areas in the field of insect immunobiology.

Clinical application: Restoration of immune homeostasis by autophagy as a potential therapeutic target in sepsis

Sepsis-induced lymphocyte and dendritic cell apoptosis contributes to immunosuppression, resulting in an inability to eradicate the primary infection and a propensity to acquire secondary infections. However, the inhibition of apoptosis may produce unexpected and undesirable consequences. Another cellular process, autophagy, is also activated in immune cells. There is increasing evidence to suggest that autophagy confers a protective effect in sepsis. The protective mechanisms underlying this effect include limiting apoptotic cell death and maintaining cellular homeostasis. Therefore, understanding the regulation of immune cell autophagy and apoptosis may provide insight into novel therapeutic strategies. The present review examined potential novel therapeutic strategies aimed at restoring immune homeostasis by inducing autophagy. The restoration of balance between apoptosis and autophagy may be a novel approach for improving sepsis-induced immunosuppression and decreasing susceptibility to sepsis.

Sensing of Mycobacterium tuberculosis and consequences to both host and bacillus

Mycobacterium tuberculosis (Mtb), the primary causative agent of human tuberculosis, has killed more people than any other bacterial pathogen in human history and remains one of the most important transmissible diseases worldwide. Because of the long-standing interaction of Mtb with humans, it is no surprise that human mucosal and innate immune cells have evolved multiple mechanisms to detect Mtb during initial contact. To that end, the cell surface of human cells is decorated with numerous pattern recognition receptors for a variety of mycobacterial ligands. Furthermore, once Mtb is ingested into professional phagocytes, other host molecules are engaged to report on the presence of an intracellular pathogen. In this review, we discuss the role of specific mycobacterial products in modulating the host's ability to detect Mtb. In addition, we describe the specific host receptors that mediate the detection of mycobacterial infection and the role of individual receptors in mycobacterial pathogenesis in humans and model organisms.

Medium from γ-irradiated Escherichia coli bacteria stimulates a unique immune response in Drosophila cells

It is well known that γ-irradiated, non-dividing bacteria can elicit potent immune responses in mammals. Compared to traditional heat or chemical inactivation of microbes, γ-irradiation likely preserves metabolic activity and antigenic features to a larger extent. We have previously shown that antimicrobial peptides are induced in Drosophila by peptidoglycan fragments secreted into the medium of exponentially growing bacterial cultures. In this study, we γ-irradiated Escherichiacoli cells at a dose that halted cell division. The temporal synthesis and release of peptidoglycan fragments were followed as well as the potential of bacterial supernatants to induce immune responses in Drosophila S2 cells. We demonstrate that peptidoglycan synthesis continues for several days post irradiation and that monomeric peptidoglycan is shed into the medium. Whole transcriptome analysis revealed a strong immune response against the bacterial medium. The response to medium taken directly post irradiation shows a large overlap to that of peptidoglycan. Medium from prolonged bacterial incubation does, however, stimulate a selective set of immune genes. A shift towards a stress response was instead observed with a striking induction of several heat shock proteins. Our findings suggest that γ-irradiated bacteria release elicitors that stimulate a novel response in Drosophila.

Autophagy in Drosophila: from historical studies to current knowledge

The discovery of evolutionarily conserved Atg genes required for autophagy in yeast truly revolutionized this research field and made it possible to carry out functional studies on model organisms. Insects including Drosophila are classical and still popular models to study autophagy, starting from the 1960s. This review aims to summarize past achievements and our current knowledge about the role and regulation of autophagy in Drosophila, with an outlook to yeast and mammals. The basic mechanisms of autophagy in fruit fly cells appear to be quite similar to other eukaryotes, and the role that this lysosomal self-degradation process plays in Drosophila models of various diseases already made it possible to recognize certain aspects of human pathologies. Future studies in this complete animal hold great promise for the better understanding of such processes and may also help finding new research avenues for the treatment of disorders with misregulated autophagy.

Insect antimicrobial peptides and their applications

Insects are one of the major sources of antimicrobial peptides/proteins (AMPs). Since observation of antimicrobial activity in the hemolymph of pupae from the giant silk moths Samia Cynthia and Hyalophora cecropia in 1974 and purification of first insect AMP (cecropin) from H. cecropia pupae in 1980, over 150 insect AMPs have been purified or identified. Most insect AMPs are small and cationic, and they show activities against bacteria and/or fungi, as well as some parasites and viruses. Insect AMPs can be classified into four families based on their structures or unique sequences: the α-helical peptides (cecropin and moricin), cysteine-rich peptides (insect defensin and drosomycin), proline-rich peptides (apidaecin, drosocin, and lebocin), and glycine-rich peptides/proteins (attacin and gloverin). Among insect AMPs, defensins, cecropins, proline-rich peptides, and attacins are common, while gloverins and moricins have been identified only in Lepidoptera. Most active AMPs are small peptides of 20-50 residues, which are generated from larger inactive precursor proteins or pro-proteins, but gloverins (~14 kDa) and attacins (~20 kDa) are large antimicrobial proteins. In this mini-review, we will discuss current knowledge and recent progress in several classes of insect AMPs, including insect defensins, cecropins, attacins, lebocins and other proline-rich peptides, gloverins, and moricins, with a focus on structural-functional relationships and their potential applications.

Role for σ38 in prolonged survival of Escherichia coli in Drosophila melanogaster

Bacteria adapt themselves to host environments by altering the pattern of gene expression. The promoter-recognizing subunit σ of bacterial RNA polymerase plays a major role in the selection of genes to be transcribed. Among seven σ factors of Escherichia coli, σ(38) is responsible for the transcription of genes in the stationary phase and under stressful conditions. We found a transient increase of σ(38) when E. coli was injected into the hemocoel of Drosophila melanogaster. The loss of σ(38) made E. coli rapidly eliminated in flies, and flies infected with σ(38)-lacking E. coli stayed alive longer than those infected with the parental strain. This was also observed in fly lines defective in humoral immune responses, but not in flies in which phagocytosis was impaired. The lack of σ(38) did not influence the susceptibility of E. coli to phagocytosis, but made them vulnerable to killing after engulfment. The changes caused by the loss of σ(38) were recovered by the forced expression of σ(38)-encoding rpoS as well as σ(38)-regulated katE and katG coding for enzymes that detoxify reactive oxygen species. These results collectively suggested that σ(38) contributes to the prolonged survival of E. coli in Drosophila by inducing the production of enzymes that protect bacteria from killing in phagocytes. Considering the similarity in the mechanism of innate immunity against invading bacteria between fruit flies and humans, the products of these genes could be new targets for the development of more effective antibacterial remedies.

The ubiquitin ligase parkin mediates resistance to intracellular pathogens

Ubiquitin-mediated targeting of intracellular bacteria to the autophagy pathway is a key innate defense mechanism against invading microbes, including the important human pathogen Mycobacterium tuberculosis. However, the ubiquitin ligases responsible for catalyzing ubiquitin chains that surround intracellular bacteria are poorly understood. PARKIN is a ubiquitin ligase with a well-established role in mitophagy, and mutations in the PARKIN gene (Park2) lead to increased susceptibility to Parkinson’s disease. Surprisingly, genetic polymorphisms in the Park2 regulatory region are also associated with increased susceptibility to intracellular bacterial pathogens in humans, including Mycobacterium leprae and Salmonella typhi, but the function of PARKIN in immunity remains unexplored. Here we show that PARKIN plays a role in ubiquitin-mediated autophagy of M. tuberculosis. Both PARKIN-deficient mice and flies are sensitive to various intracellular bacterial infections, suggesting PARKIN plays a conserved role in metazoan innate defense. Moreover, our work reveals an unexpected functional link between mitophagy and infectious disease.