Two Nimrod receptors, NimC1 and Eater, synergistically contribute to bacterial phagocytosis in Drosophila melanogaster

Single von Willebrand factor C-domain protein-2 confers immune defense against bacterial infections in the silkworm, Bombyx mori

Single-domain von Willebrand factor type C proteins (SVWCs), primarily found in arthropods, responds to infections caused by various pathogens. Three SVWCs have been identified in the silkworm and BmSVWC2 might play a crucial role in the immune system. However, the regulatory mechanism of BmSVWC2 remains largely unknown. This study aimed to investigate the biochemical functions of BmSVWC2 in the immune system of B. mori comprehensively. Phylogenetic analysis revealed that BmSVWC1, BmSVWC3, and BmSVWC2 were distributed in diverse groups, suggesting distinct biochemical functions. The mRNA and protein levels of BmSVWC2 increased significantly in response to bacterial infection. BmSVWC2 exhibited clear binding activity to the polysaccharide pathogen-associated molecular patterns of bacteria and fungi, enhancing bacterial clearance in vivo but not in vitro. RNA-sequencing assays of the fat body and hemocytes showed that numerous immune genes were markedly up-regulated with higher level of BmSVWC2, primarily affecting recognition, signaling, and response production of the Toll and immune deficiency (IMD) signaling pathways. This led to the production of various antimicrobial peptides and significant antibacterial activities in the hemolymph. BmSVWC2 up-regulated phagocytosis-related genes in the fat body and hemocytes, and phagocytosis assays confirmed that BmSVWC2 improved the phagocytic ability of hemocytes against bacteria. Additionally, BmSVWC2 induced the expression of nitric oxide synthetase (NOS) in the fat body, and bioassays confirmed that BmSVWC2 increased NOS activity in the fat body and hemolymph, resulting in nitric oxide accumulation. However, BmSVWC2 did not affect phenoloxidase activity, despite it caused differential expression of a few serine proteases and serine protease inhibitors. Co-immunoprecipitation and mass spectrometry assays showed that BmSVWC2 interacted with 30 K proteins, such as 30 K protein 2, 30 K pBmHPC-19, 30 K 19G1-like, 30 K protein 8, 30 K protein 7, 30 K pBmHPC-23, and low molecular mass lipoprotein 4-like. Our study provides a comprehensive characterization of BmSVWC2 and elucidates the mechanism underlying its regulation of immune responses activation.

Professional phagocytes are recruited for the clearance of obsolete nonprofessional phagocytes in the Drosophila ovary

Cell death is an important process in the body, as it occurs throughout every tissue during development, disease, and tissue regeneration. Phagocytes are responsible for clearing away dying cells and are typically characterized as either professional or nonprofessional phagocytes. Professional phagocytes, such as macrophages, are found in nearly every part of the body while nonprofessional phagocytes, such as epithelial cells, are found in every tissue type. However, there are organs that are considered "immune-privileged" as they have little to no immune surveillance and rely on nonprofessional phagocytes to engulf dying cells. These organs are surrounded by barriers to protect the tissue from viruses, bacteria, and perhaps even immune cells. The Drosophila ovary is considered immune-privileged, however the presence of hemocytes, the macrophages of Drosophila, around the ovary suggests they may have a potential function. Here we analyze hemocyte localization and potential functions in response to starvation-induced cell death in the ovary. Hemocytes were found to accumulate in the oviduct in the vicinity of mature eggs and follicle cell debris. Genetic ablation of hemocytes revealed that the presence of hemocytes affects oogenesis and that they phagocytose ovarian cell debris and in their absence fecundity decreases. Unpaired3, an IL-6 like cytokine, was found to be required for the recruitment of hemocytes to the oviduct to clear away obsolete follicle cells. These findings demonstrate a role for hemocytes in the ovary, providing a more thorough understanding of phagocyte communication and cell clearance in a previously thought immune-privileged organ.

Transient caspase-mediated activation of caspase-activated DNase causes DNA damage required for phagocytic macrophage differentiation

Phagocytic macrophages are crucial for innate immunity and tissue homeostasis. Most tissue-resident macrophages develop from embryonic precursors that populate every organ before birth to lifelong self-renew. However, the mechanisms for versatile macrophage differentiation remain unknown. Here, we use in vivo genetic and cell biological analysis of the Drosophila larval hematopoietic organ, the lymph gland that produces macrophages. We show that the developmentally regulated transient activation of caspase-activated DNase (CAD)-mediated DNA strand breaks in intermediate progenitors is essential for macrophage differentiation. Insulin receptor-mediated PI3K/Akt signaling regulates the apoptosis signal-regulating kinase 1 (Ask1)/c-Jun kinase (JNK) axis to control sublethal levels of caspase activation, causing DNA strand breaks during macrophage development. Furthermore, caspase activity is also required for embryonic-origin macrophage development and efficient phagocytosis. Our study provides insights into developmental signaling and CAD-mediated DNA strand breaks associated with multifunctional and heterogeneous macrophage differentiation.

Immune Gene Repertoire of Soft Scale Insects (Hemiptera: Coccidae)

Insects possess an effective immune system, which has been extensively characterized in several model species, revealing a plethora of conserved genes involved in recognition, signaling, and responses to pathogens and parasites. However, some taxonomic groups, characterized by peculiar trophic niches, such as plant-sap feeders, which are often important pests of crops and forestry ecosystems, have been largely overlooked regarding their immune gene repertoire. Here we annotated the immune genes of soft scale insects (Hemiptera: Coccidae) for which omics data are publicly available. By using immune genes of aphids and Drosophila to query the genome of Ericerus pela, as well as the transcriptomes of Ceroplastes cirripediformis and Coccus sp., we highlight the lack of peptidoglycan recognition proteins, galectins, thaumatins, and antimicrobial peptides in Coccidae. This work contributes to expanding our knowledge about the evolutionary trajectories of immune genes and offers a list of promising candidates for developing new control strategies based on the suppression of pests' immunity through RNAi technologies.

Senescent cells and macrophages cooperate through a multi-kinase signaling network to promote intestinal transformation in Drosophila

Cellular senescence is a conserved biological process that plays a crucial and context-dependent role in cancer. The highly heterogeneous and dynamic nature of senescent cells and their small numbers in tissues make in vivo mechanistic studies of senescence challenging. As a result, how multiple senescence-inducing signals are integrated in vivo to drive senescence in only a small number of cells is unclear. Here, we identify cells that exhibit multiple features of senescence in a Drosophila model of intestinal transformation, which emerge in response to concurrent activation of AKT, JNK, and DNA damage signaling within transformed tissue. Eliminating senescent cells, genetically or by treatment with senolytic compounds, reduces overgrowth and improves survival. We find that senescent cells promote tumorigenesis by recruiting Drosophila macrophages to the transformed tissue, which results in non-autonomous activation of JNK signaling. These findings identify senescent cell-macrophage interactions as an important driver of epithelial transformation.

Research progress of aphid immunity system: Potential effective target for green pest management

Due to the absence of acquired immunity, insects primarily rely on their innate immune system to resist pathogenic microorganisms and parasitoids in natural habitats. This innate immune system can be classified into cellular immunity and humoral immunity. Cellular immunity is mediated by hemocytes, which perform phagocytosis, aggregation, and encapsulation to fight against invaders, whereas the humoral immunity primarily activates the immune signaling pathways and induces the generation of immune effectors. Existing studies have revealed that the hemipteran aphids lack some crucial immune genes compared to other insect species, indicating the different immune mechanisms in aphids. The current review summarizes the adverse impacts of pathogenic microorganisms and parasitoids on aphids, introduces the cellular and humoral immune systems in insects, and analyzes the differences between aphids and other insect species. Furthermore, our review also discussed the existing prospects and challenges in aphid immunity research, and proposed the potential application of immune genes in green pest management.

Molecular traces of Drosophila hemocytes reveal transcriptomic conservation with vertebrate myeloid cells

Drosophila hemocytes serve as the primary defense system against harmful threats, allowing the animals to thrive. Hemocytes are often compared to vertebrate innate immune system cells due to the observed functional similarities between the two. However, the similarities have primarily been established based on a limited number of genes and their functional homologies. Thus, a systematic analysis using transcriptomic data could offer novel insights into Drosophila hemocyte function and provide new perspectives on the evolution of the immune system. Here, we performed cross-species comparative analyses using single-cell RNA sequencing data from Drosophila and vertebrate immune cells. We found several conserved markers for the cluster of differentiation (CD) genes in Drosophila hemocytes and validated the role of CG8501 (CD59) in phagocytosis by plasmatocytes, which function much like macrophages in vertebrates. By comparing whole transcriptome profiles in both supervised and unsupervised analyses, we showed that Drosophila hemocytes are largely homologous to vertebrate myeloid cells, especially plasmatocytes to monocytes/macrophages and prohemocyte 1 (PH1) to hematopoietic stem cells. Furthermore, a small subset of prohemocytes with hematopoietic potential displayed homology with hematopoietic progenitor populations in vertebrates. Overall, our results provide a deeper understanding of molecular conservation in the Drosophila immune system.

Identifying new players in structural synaptic plasticity through dArc1 interrogation

The formation, expansion, and pruning of synapses, known as structural synaptic plasticity, is needed for learning and memory, and perturbation of plasticity is associated with many neurological disorders and diseases. Previously, we observed that the Drosophila homolog of Activity-regulated cytoskeleton-associated protein (dArc1), forms a capsid-like structure, associates with its own mRNA, and is transported across synapses. We demonstrated that this transfer is needed for structural synaptic plasticity. To identify mRNAs that are modified by dArc1 in presynaptic neuron and postsynaptic muscle, we disrupted the expression of dArc1 and performed genomic analysis with deep sequencing. We found that dArc1 affects the expression of genes involved in metabolism, phagocytosis, and RNA-splicing. Through immunoprecipitation we also identified potential mRNA cargos of dArc1 capsids. This study suggests that dArc1 acts as a master regulator of plasticity by affecting several distinct and highly conserved cellular processes.

Understanding the diversity and dynamics of in vivo efferocytosis: Insights from the fly embryo

The clearance of dead and dying cells, termed efferocytosis, is a rapid and efficient process and one that is critical for organismal health. The extraordinary speed and efficiency with which dead cells are detected and engulfed by immune cells within tissues presents a challenge to researchers who wish to unravel this fascinating process, since these fleeting moments of uptake are almost impossible to catch in vivo. In recent years, the fruit fly (Drosophila melanogaster) embryo has emerged as a powerful model to circumvent this problem. With its abundance of dying cells, specialist phagocytes and relative ease of live imaging, the humble fly embryo provides a unique opportunity to catch and study the moment of cell engulfment in real-time within a living animal. In this review, we explore the recent advances that have come from studies in the fly, and how live imaging and genetics have revealed a previously unappreciated level of diversity in the efferocytic program. A variety of efferocytic strategies across the phagocytic cell population ensure efficient and rapid clearance of corpses wherever death is encountered within the varied and complex setting of a multicellular living organism.

The Effects of Diet on the Immune Responses of the Oriental Armyworm Mythimna separata

Nutrients can greatly affect host immune defenses against infection. Possessing a simple immune system, insects have been widely used as models to address the relationships between nutrition and immunity. The effects of high versus low protein-to-carbohydrate ratio (P:C) diets on insect immune responses vary in different studies. To reveal the dietary manipulation of immune responses in the polyphagous agricultural pest oriental armyworm, we examined immune gene expression, phenoloxidase (PO) activity, and phagocytosis to investigate the immune traits of bacteria-challenged oriental armyworms, which were fed different P:C ratio diets. We found the oriental armyworms that were fed a 35:7 (P:C) diet showed higher phenoloxidase (PO) activity and stronger melanization, and those reared on a 28:14 (P:C) diet showed higher antimicrobial activity. However, different P:C diets had no apparent effect on the hemocyte number and phagocytosis. These results overall indicate that high P:C diets differently optimize humoral immune defense responses in oriental armyworms, i.e., PO-mediated melanization and antimicrobial peptide synthesis in response to bacteria challenge.

Senescent cells and macrophages cooperate through a multi-kinase signaling network to promote intestinal transformation in Drosophila

Cellular senescence is a conserved biological process essential for embryonic development, tissue remodeling, repair, and a key regulator of aging. Senescence also plays a crucial role in cancer, though this role can be tumor-suppressive or tumor-promoting, depending on the genetic context and the microenvironment. The highly heterogeneous, dynamic, and context-dependent nature of senescence-associated features and the relatively small numbers of senescent cells in tissues makes in vivo mechanistic studies of senescence challenging. As a result, which senescence-associated features are observed in which disease contexts and how they contribute to disease phenotypes remain largely unknown. Similarly, the specific mechanisms by which various senescence-inducing signals are integrated in vivo to induce senescence and why some cells become senescent while their immediate neighbors do not are unclear. Here, we identify a small number of cells that exhibit multiple features of senescence in a genetically complex model of intestinal transformation we recently established in the developing Drosophila larval hindgut epithelium. We demonstrate that these cells emerge in response to concurrent activation of AKT, JNK, and DNA damage response pathways within transformed tissue. Eliminating senescent cells, genetically or by treatment with senolytic compounds, reduces overgrowth and improves survival. We find that this tumor-promoting role is mediated by Drosophila macrophages recruited to the transformed tissue by senescent cells, which results in non-autonomous activation of JNK signaling within the transformed epithelium. These findings emphasize complex cell-cell interactions underlying epithelial transformation and identify senescent cell-macrophage interactions as a potential druggable node in cancer.

Rickettsia parkeri hijacks tick hemocytes to manipulate cellular and humoral transcriptional responses

Introduction

Blood-feeding arthropods rely on robust cellular and humoral immunity to control pathogen invasion and replication. Tick hemocytes produce factors that can facilitate or suppress microbial infection and pathogenesis. Despite the importance of hemocytes in regulating microbial infection, understanding of their basic biology and molecular mechanisms remains limited.

Methods

Here we combined histomorphology and functional analysis to identify five distinct phagocytic and non-phagocytic hemocyte populations circulating within the Gulf Coast tick Amblyomma maculatum.

Results and discussion

Depletion of phagocytic hemocytes using clodronate liposomes revealed their function in eliminating bacterial infection. We provide the first direct evidence that an intracellular tick-borne pathogen, Rickettsia parkeri, infects phagocytic hemocytes in Am. maculatum to modify tick cellular immune responses. A hemocyte-specific RNA-seq dataset generated from hemocytes isolated from uninfected and R. parkeri-infected partially blood-fed ticks generated ~40,000 differentially regulated transcripts, >11,000 of which were immune genes. Silencing two differentially regulated phagocytic immune marker genes (nimrod B2 and eater-two Drosophila homologs), significantly reduced hemocyte phagocytosis.

Conclusion

Together, these findings represent a significant step forward in understanding how hemocytes regulate microbial homeostasis and vector competence.

Heightened immune surveillance in Drosophila melanogaster populations selected for faster development and extended longevity

Maximization of life-history traits is under constraints due to both, limitations of resource acquisition and the restricted pathways of resource allocation. Drosophila melanogaster has served as an excellent model organism to not only unravel various trade-offs among life history traits but also numerous aspects of host immune response. Drosophila larvae are semi-aquatic that live, feed and excrete inside the food source-often over-ripe fruits and vegetables that are rich in both commensal and pathogenic microbiota that can impact the larval survival. In this study, we have used six populations of D. melanogaster, three of which are selected for faster pre-adult development and extended adult longevity, and their three ancestral controls, to explore the impact of selection on the basal immune activity in the larval stage. The larvae from selected populations had nearly significantly upregulated plasmatocyte density, significantly higher percent phagocytosis, phagocytic index and higher transcript levels of Tep3, eater and NimC1. Selected populations also had significantly upregulated crystal cell number along with higher transcript of PPO2. Out of seven tested AMPs level, Drosomycin was significantly upregulated in selected populations while Drosocin was significantly higher in control populations. ROS levels were comparable in the selected and control populations. Our results strongly suggest that enhanced basal immune activity during larval stage manages the faster development and could be responsible for comparable larval survival of selected and control populations.

Drosophila Innate Immunity Involves Multiple Signaling Pathways and Coordinated Communication Between Different Tissues

The innate immune response provides the first line of defense against invading pathogens, and immune disorders cause a variety of diseases. The fruit fly Drosophila melanogaster employs multiple innate immune reactions to resist infection. First, epithelial tissues function as physical barriers to prevent pathogen invasion. In addition, macrophage-like plasmatocytes eliminate intruders through phagocytosis, and lamellocytes encapsulate large particles, such as wasp eggs, that cannot be phagocytosed. Regarding humoral immune responses, the fat body, equivalent to the mammalian liver, secretes antimicrobial peptides into hemolymph, killing bacteria and fungi. Drosophila has been shown to be a powerful in vivo model for studying the mechanism of innate immunity and host-pathogen interactions because Drosophila and higher organisms share conserved signaling pathways and factors. Moreover, the ease with which Drosophila genetic and physiological characteristics can be manipulated prevents interference by adaptive immunity. In this review, we discuss the signaling pathways activated in Drosophila innate immunity, namely, the Toll, Imd, JNK, JAK/STAT pathways, and other factors, as well as relevant regulatory networks. We also review the mechanisms by which different tissues, including hemocytes, the fat body, the lymph gland, muscles, the gut and the brain coordinate innate immune responses. Furthermore, the latest studies in this field are outlined in this review. In summary, understanding the mechanism underlying innate immunity orchestration in Drosophila will help us better study human innate immunity-related diseases.

Phagocytosis Is the Sole Arm of Drosophila melanogaster Known Host Defenses That Provides Some Protection Against Microsporidia Infection

Microsporidia are obligate intracellular parasites able to infest specifically a large range of species, including insects. The knowledge about the biology of microsporidial infections remains confined to mostly descriptive studies, including molecular approaches such as transcriptomics or proteomics. Thus, functional data to understand insect host defenses are currently lacking. Here, we have undertaken a genetic analysis of known host defenses of the Drosophila melanogaster using an infection model whereby Tubulinosema ratisbonensis spores are directly injected in this insect. We find that phagocytosis does confer some protection in this infection model. In contrast, the systemic immune response, extracellular reactive oxygen species, thioester proteins, xenophagy, and intracellular antiviral response pathways do not appear to be involved in the resistance against this parasite. Unexpectedly, several genes such as PGRP-LE seem to promote this infection. The prophenol oxidases that mediate melanization have different functions; PPO1 presents a phenotype similar to that of PGRP-LE whereas that of PPO2 suggests a function in the resilience to infection. Similarly, eiger and Unpaired3, which encode two cytokines secreted by hemocytes display a resilience phenotype with a strong susceptibility to T. ratisbonensis.

Haemocyte-mediated immunity in insects: Cells, processes and associated components in the fight against pathogens and parasites

The host defence of insects includes a combination of cellular and humoral responses. The cellular arm of the insect innate immune system includes mechanisms that are directly mediated by haemocytes (e.g., phagocytosis, nodulation and encapsulation). In addition, melanization accompanying coagulation, clot formation and wound healing, nodulation and encapsulation processes leads to the formation of cytotoxic redox-cycling melanin precursors and reactive oxygen and nitrogen species. However, demarcation between cellular and humoral immune reactions as two distinct categories is not straightforward. This is because many humoral factors affect haemocyte functions and haemocytes themselves are an important source of many humoral molecules. There is also a considerable overlap between cellular and humoral immune functions that span from recognition of foreign intruders to clot formation. Here, we review these immune reactions starting with the cellular mechanisms that limit haemolymph loss and participate in wound healing and clot formation and advancing to cellular functions that are critical in restricting pathogen movement and replication. This information is important because it highlights that insect cellular immunity is controlled by a multilayered system, different components of which are activated by different pathogens or during the different stages of the infection.

A secreted factor NimrodB4 promotes the elimination of apoptotic corpses by phagocytes in Drosophila

Programmed cell death plays a fundamental role in development and tissue homeostasis. Professional and non‐professional phagocytes achieve the proper recognition, uptake, and degradation of apoptotic cells, a process called efferocytosis. Failure in efferocytosis leads to autoimmune and neurodegenerative diseases. In Drosophila, two transmembrane proteins of the Nimrod family, Draper and SIMU, mediate the recognition and internalization of apoptotic corpses. Beyond this early step, little is known about how apoptotic cell degradation is regulated. Here, we study the function of a secreted member of the Nimrod family, NimB4, and reveal its crucial role in the clearance of apoptotic cells. We show that NimB4 is expressed by macrophages and glial cells, the two main types of phagocytes in Drosophila. Similar to draper mutants, NimB4 mutants accumulate apoptotic corpses during embryogenesis and in the larval brain. Our study points to the role of NimB4 in phagosome maturation, more specifically in the fusion between the phagosome and lysosomes. We propose that similar to bridging molecules, NimB4 binds to apoptotic corpses to engage a phagosome maturation program dedicated to efferocytosis.

Scavenger receptor B1 mediates phagocytosis and the antimicrobial peptide pathway in the endoparasitic wasp Micropilits mediator

Scavenger receptors (SRs) are a family of pattern recognition receptors (PRRs) in the immune system. They are required for phagocytosis and act as co-receptors of Toll-like receptors to regulate immune signaling pathways in the fight against pathogens. Little is known about the function of SRs in insects. Here, we reported on a member of the SR family from the parasitic wasp Micropilits mediator (designated MmSR-B1) that is responsive to bacterial infection. The recombinant extracellular CD36 domain of MmSR-B1 produced in Escherichia coli cells is capable of binding to peptidoglycans and bacterial cells, causing agglutination of bacteria. Furthermore, we demonstrated that double-stranded RNA-mediated knockdown of MmSR-B1 impedes hemocyte phagocytosis and downregulates the expression of antimicrobial peptide (AMP) genes defensins and hymenoptaecins. Knockdown of MmSR-B1 led to increased death of the wasps when challenged by bacteria. Our study suggests that MmSR-B1 mediates phagocytosis and the production of AMPs in M. mediator wasps.

Drosophila melanogaster as a Model System to Assess the Effect of Epstein-Barr Virus DNA on Inflammatory Gut Diseases

The Epstein-Barr virus (EBV) commonly infects humans and is highly associated with different types of cancers and autoimmune diseases. EBV has also been detected in inflamed gastrointestinal mucosa of patients suffering from prolonged inflammation of the digestive tract such as inflammatory bowel disease (IBD) with no clear role identified yet for EBV in the pathology of such diseases. Since we have previously reported immune-stimulating capabilities of EBV DNA in various models, in this study we investigated whether EBV DNA may play a role in exacerbating intestinal inflammation through innate immune and regeneration responses using the Drosophila melanogaster model. We have generated inflamed gastrointestinal tracts in adult fruit flies through the administration of dextran sodium sulfate (DSS), a sulfated polysaccharide that causes human ulcerative colitis- like pathologies due to its toxicity to intestinal cells. Intestinal damage induced by inflammation recruited plasmatocytes to the ileum in fly hindguts. EBV DNA aggravated inflammation by enhancing the immune deficiency (IMD) pathway as well as further increasing the cellular inflammatory responses manifested upon the administration of DSS. The study at hand proposes a possible immunostimulatory role of the viral DNA exerted specifically in the fly hindgut hence further developing our understanding of immune responses mounted against EBV DNA in the latter intestinal segment of the D. melanogaster gut. These findings suggest that EBV DNA may perpetuate proinflammatory processes initiated in an inflamed digestive system. Our findings indicate that D. melanogaster can serve as a model to further understand EBV-associated gastroinflammatory pathologies. Further studies employing mammalian models may validate the immunogenicity of EBV DNA in an IBD context and its role in exacerbating the disease through inflammatory mediators.

Macrophages and Their Organ Locations Shape Each Other in Development and Homeostasis - A Drosophila Perspective

Across the animal kingdom, macrophages are known for their functions in innate immunity, but they also play key roles in development and homeostasis. Recent insights from single cell profiling and other approaches in the invertebrate model organism Drosophila melanogaster reveal substantial diversity among Drosophila macrophages (plasmatocytes). Together with vertebrate studies that show genuine expression signatures of macrophages based on their organ microenvironments, it is expected that Drosophila macrophage functional diversity is shaped by their anatomical locations and systemic conditions. In vivo evidence for diverse macrophage functions has already been well established by Drosophila genetics: Drosophila macrophages play key roles in various aspects of development and organogenesis, including embryogenesis and development of the nervous, digestive, and reproductive systems. Macrophages further maintain homeostasis in various organ systems and promote regeneration following organ damage and injury. The interdependence and interplay of tissues and their local macrophage populations in Drosophila have implications for understanding principles of organ development and homeostasis in a wide range of species.

Immunoprofiling of Drosophila Hemocytes by Single-cell Mass Cytometry

Single-cell mass cytometry (SCMC) combines features of traditional flow cytometry (i.e., fluorescence-activated cell sorting) with mass spectrometry, making it possible to measure several parameters at the single-cell level for a complex analysis of biological regulatory mechanisms. In this study, weoptimizedSCMC to analyze hemocytes of the Drosophila innate immune system. We used metal-conjugated antibodies (against cell surface antigens H2, H3, H18, L1, L4, and P1, and intracellular antigens 3A5 and L2) and anti-IgM (against cell surface antigen L6) to detect the levels of antigens, while anti-GFP was used to detect crystal cells in the immune-induced samples. We investigated the antigen expression profile of single cells and hemocyte populations in naive states, in immune-induced states, in tumorous mutants bearing a driver mutation in the Drosophila homologue of Janus kinase (hop^Tum) and carrying a deficiency of the tumor suppressor gene lethal(3)malignant blood neoplasm-1  [l(3)mbn¹], as well as in stem cell maintenance-defective hdc^Δ84 mutant larvae. Multidimensional analysis enabled the discrimination of the functionally different major hemocyte subsets for lamellocytes, plasmatocytes, and crystal cells, anddelineated the unique immunophenotype of Drosophila mutants. We have identified subpopulations of L2⁺/P1⁺ and L2⁺/L4⁺/P1⁺ transitional phenotype cells in the tumorous strains l(3)mbn¹ and hop^Tum, respectively, and a subpopulation of L4⁺/P1⁺ cells upon immune induction. Our results demonstrated for the first time that SCMC, combined with multidimensional bioinformatic analysis, represents a versatile and powerful tool to deeply analyze the regulation of cell-mediated immunity of Drosophila.

Pea Aphid Rearing, Bacterial Infection and Hemocyte Phagocytosis Assay

Insects rely on the simple but effective innate immune system to combat infection. Cellular and humoral responses are interconnected and synergistic in insects' innate immune system. Phagocytosis is one major cellular response. It is difficult to collect clean hemolymph from the small insect like pea aphid. Here, we provide a practicable method for small insects hemocyte phagocytosis assay by taking pea aphid as an example. Furthermore, we provide the protocols for pea aphid rearing and bacterial infection, which offer referential method for related research.

Widely Used Mutants of eiger, Encoding the Drosophila Tumor Necrosis Factor, Carry Additional Mutations in the NimrodC1 Phagocytosis Receptor

The process of apoptosis in epithelia involves activation of caspases, delamination of cells, and degradation of cellular components. Corpses and cellular debris are then rapidly cleared from the tissue by phagocytic blood cells. In studies of the Drosophila TNF, Eiger (Egr) and cell death in wing imaginal discs, the epithelial primordia of fly wings, we noticed that dying cells appeared to transiently accumulate in egr3 mutant wing discs, raising the possibility that their phagocytic engulfment by hemocytes was impaired. Further investigation revealed that lymph glands and circulating hemocytes from egr3 mutant larvae were completely devoid of NimC1 staining, a marker of phagocytic hemocytes. Genome sequencing uncovered mutations in the NimC1 coding region that are predicted to truncate the NimC1 protein before its transmembrane domain, and provide an explanation for the lack of NimC staining. The work that we report here demonstrates the presence of these NimC1 mutations in the widely used egr3 mutant, its sister allele, egr1 , and its parental strain, Regg1GS9830 As the egr3 and egr1 alleles have been used in numerous studies of immunity and cell death, it may be advisable to re-evaluate their associated phenotypes.

The Route of Infection Influences the Contribution of Key Immunity Genes to Antibacterial Defense in Anopheles gambiae

Insect systemic immune responses to bacterial infections have been mainly studied using microinjections, whereby the microbe is directly injected into the hemocoel. While this methodology has been instrumental in defining immune signaling pathways and enzymatic cascades in the hemolymph, it remains unclear whether and to what extent the contribution of systemic immune defenses to host microbial resistance varies if bacteria invade the hemolymph after crossing the midgut epithelium subsequent to an oral infection. Here, we address this question using the pathogenic Serratia marcescens (Sm) DB11 strain to establish systemic infections of the malaria vector Anopheles gambiae, either by septic Sm injections or by midgut crossing after feeding on Sm. Using functional genetic studies by RNAi, we report that the two humoral immune factors, thioester-containing protein 1 and C-type lectin 4, which play key roles in defense against Gram-negative bacterial infections, are essential for defense against systemic Sm infections established through injection, but they become dispensable when Sm infects the hemolymph following oral infection. Similar results were observed for the mosquito Rel2 pathway. Surprisingly, blocking phagocytosis by cytochalasin D treatment did not affect mosquito susceptibility to Sm infections established through either route. Transcriptomic analysis of mosquito midguts and abdomens by RNA-seq revealed that the transcriptional response in these tissues is more pronounced in response to feeding on Sm. Functional classification of differentially expressed transcripts identified metabolic genes as the most represented class in response to both routes of infection, while immune genes were poorly regulated in both routes. We also report that Sm oral infections are associated with significant downregulation of several immune genes belonging to different families, specifically the clip-domain serine protease family. In sum, our findings reveal that the route of infection not only alters the contribution of key immunity genes to host antimicrobial defense but is also associated with different transcriptional responses in midguts and abdomens, possibly reflecting different adaptive strategies of the host.

There and back again: The mechanisms of differentiation and transdifferentiation in Drosophila blood cells

Transdifferentiation is a conversion of an already differentiated cell type into another cell type without the involvement of stem cells. This transition is well described in the case of vertebrate immune cells, as well as in Drosophila melanogaster, which therefore serves as a suitable model to study the process in detail. In the Drosophila larva, the latest single-cell sequencing methods enabled the clusterization of the phagocytic blood cells, the plasmatocytes, which are capable of transdifferentiation into encapsulating cells, the lamellocytes. Here we summarize the available data of the past years on the plasmatocyte-lamellocyte transition, and make an attempt to harmonize them with transcriptome-based blood cell clustering to better understand the underlying mechanisms of transdifferentiation in Drosophila, and in general.

Eater cooperates with Multiplexin to drive the formation of hematopoietic compartments

Blood development in multicellular organisms relies on specific tissue microenvironments that nurture hematopoietic precursors and promote their self-renewal, proliferation, and differentiation. The mechanisms driving blood cell homing and their interactions with hematopoietic microenvironments remain poorly understood. Here, we use the Drosophila melanogaster model to reveal a pivotal role for basement membrane composition in the formation of hematopoietic compartments. We demonstrate that by modulating extracellular matrix components, the fly blood cells known as hemocytes can be relocated to tissue surfaces where they function similarly to their natural hematopoietic environment. We establish that the Collagen XV/XVIII ortholog Multiplexin in the tissue-basement membranes and the phagocytosis receptor Eater on the hemocytes physically interact and are necessary and sufficient to induce immune cell-tissue association. These results highlight the cooperation of Multiplexin and Eater as an integral part of a homing mechanism that specifies and maintains hematopoietic sites in Drosophila.

Comparative RNA-Seq analyses of Drosophila plasmatocytes reveal gene specific signatures in response to clean injury and septic injury

Drosophila melanogaster's blood cells (hemocytes) play essential roles in wound healing and are involved in clearing microbial infections. Here, we report the transcriptional changes of larval plasmatocytes after clean injury or infection with the Gram-negative bacterium Escherichia coli or the Gram-positive bacterium Staphylococcus aureus compared to hemocytes recovered from unchallenged larvae via RNA-Sequencing. This study reveals 676 differentially expressed genes (DEGs) in hemocytes from clean injury samples compared to unchallenged samples, and 235 and 184 DEGs in E. coli and S. aureus samples respectively compared to clean injury samples. The clean injury samples showed enriched DEGs for immunity, clotting, cytoskeleton, cell migration, hemocyte differentiation, and indicated a metabolic reprogramming to aerobic glycolysis, a well-defined metabolic adaptation observed in mammalian macrophages. Microbial infections trigger significant transcription of immune genes, with significant differences between the E. coli and S. aureus samples suggesting that hemocytes have the ability to engage various programs upon infection. Collectively, our data bring new insights on Drosophila hemocyte function and open the route to post-genomic functional analysis of the cellular immune response.

JNK pathway plays a key role in the immune system of the pea aphid and is regulated by microRNA-184

Different from holometabolous insects, the hemipteran species such as pea aphid Acyrthosiphon pisum exhibit reduced immune responses with the absence of the genes coding for antimicrobial peptide (AMP), immune deficiency (IMD), peptidoglycan recognition proteins (PGRPs), and other immune-related molecules. Prior studies have proved that phenoloxidase (PO)-mediated melanization, hemocyte-mediated phagocytosis, and reactive oxygen species (ROS) participate in pea aphid defense against bacterial infection. Also, the conserved signaling, Jun N-terminal kinase (JNK) pathway, has been suggested to be involved in pea aphid immune defense. However, the precise role of the JNK signaling, its interplay with other immune responses and its regulation in pea aphid are largely unknown. In this study, using in vitro biochemical assays and in vivo bioassays, we demonstrated that the JNK pathway regulated hemolymph PO activity, hydrogen peroxide concentration and hemocyte phagocytosis in bacteria infected pea aphids, suggesting that the JNK pathway plays a central role in regulating immune responses in pea aphid. We further revealed the JNK pathway is regulated by microRNA-184 in response to bacterial infection. It is possible that in common the JNK pathway plays a key role in immune system of hemipteran insects and microRNA-184 regulates the JNK pathway in animals.

Synthesis and Antibacterial Evaluation of Novel 1,3,4-Oxadiazole Derivatives Containing Sulfonate/Carboxylate Moiety

Abstract: In order to discover new lead compounds with high antibacterial activity, a series of new derivatives were designed and synthesized by introducing a sulfonate or carboxylate moiety into the 1,3,4-oxadiazole structure. Antibacterial activity against two phytopathogens, Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas axonopodis pv. citri (Xac), was assayed in vitro. The preliminary results indicated that ten compounds including 4a-1-4a-4 and 4a-11-4a-16 had good antibacterial activity against Xoo, with EC50 values ranging from 50.1-112.5 µM, which was better than those of Bismerthiazol (253.5 µM) and Thiodiazole copper (467.4 µM). Meanwhile, 4a-1, 4a-2, 4a-3 and 4a-4 demonstrated good inhibitory effect against Xanthomonas axonopodis pv. citri with EC50 values around 95.8-155.2 µM which were better than those of bismerthiazol (274.3 µM) and thiodiazole copper (406.3 µM). In addition, in vivo protection activity of compound 4a-2 and 4a-3 against rice bacterial leaf blight was 68.6% and 62.3%, respectively, which were better than bismerthiazol (49.6%) and thiodiazole copper (42.2%). Curative activity of compound 4a-2 and 4a-3 against rice bacterial leaf blight was 62.3% and 56.0%, which were better than bismerthiazol (42.9%) and thiodiazole copper (36.1%). Through scanning electron microscopy (SEM) analysis, it was observed that compound 4a-2 caused the cell membrane of Xanthomonas oryzae pv. oryzae ruptured or deformed. The present results indicated novel derivatives of 5-phenyl sulfonate methyl 1,3,4-oxadiazole might be potential antibacterial agents.