The cell-mediated immunity of Drosophila melanogaster: hemocyte lineages, immune compartments, microanatomy and regulation

Experimental horizontal transfer of phage-derived genes to Drosophila confers innate immunity to parasitoids

Metazoan parasites have played a major role in shaping innate immunity in animals. Insect hosts and parasitoid wasps are excellent models for illuminating how animal innate immune systems have evolved to neutralize these enemies. One such strategy relies on symbioses between insects and intracellular bacteria that express phage-encoded toxins. In some cases, the genes that encode these toxins have been horizontally transferred to the genomes of the insects. Here, we used genome editing in Drosophila melanogaster to recapitulate the evolution of two toxin genes-cytolethal distending toxin B (cdtB) and apoptosis inducing protein of 56kDa (aip56)-that were horizontally transferred likely from phages of endosymbiotic bacteria to insects millions of years ago. We found that a cdtB::aip56 fusion gene (fusionB), which is conserved in D. ananassae subgroup species, dramatically promoted fly survival and suppressed parasitoid wasp development when heterologously expressed in D. melanogaster immune tissues. We found that FusionB was a functional nuclease and was secreted into the host hemolymph where it targeted the parasitoid embryo's serosal tissue. Although the mechanism of toxicity remains unknown, when expressed ubiquitously, fusionB resulted in delayed development of late-stage fly larvae and eventually killed pupating flies. These results point to the salience of regulatory constraint in mitigating autoimmunity during the domestication process following horizontal transfer. Our findings demonstrate how horizontal gene transfer can instantly provide new, potent innate immune modules in animals.

Dual role for Headcase in hemocyte progenitor fate determination in Drosophila melanogaster

The hematopoietic organ of the Drosophila larva, the lymph gland, is a simplified representation of mammalian hematopoietic compartments, with the presence of hemocyte progenitors in the medullary zone (MZ), differentiated hemocytes in the cortical zone (CZ), and a hematopoietic niche called the posterior signaling centre (PSC) that orchestrates progenitor differentiation. Our previous work has demonstrated that the imaginal cell factor Headcase (Hdc, Heca) is required in the hematopoietic niche to control the differentiation of hemocyte progenitors. However, the downstream mechanisms of Hdc-mediated hematopoietic control remained unknown. Here we show that Hdc exerts this function by negatively regulating the insulin/mTOR signaling in the niche. When Hdc is depleted in the PSC, the overactivation of this pathway triggers reactive oxygen species (ROS) accumulation and, in turn, the differentiation of effector lamellocytes non-cell-autonomously. Although overactivation of insulin/mTOR signaling normally leads to an increase in the size of the hematopoietic niche, this effect is concealed by cell death caused by hdc loss-of-function. Moreover, we describe here that hdc silencing in progenitors causes cell-autonomous ROS elevation and JNK pathway activation, resulting in decreased MZ size and differentiation of lamellocytes. Similarly to the PSC niche, knocking down hdc in the MZ also leads to caspase activation. Notably, depleting Hdc in the progenitors triggers proliferation, an opposing effect to what is observed in the niche. These findings further our understanding of how progenitor maintenance in the larval lymph gland is controlled autonomously and non-cell-autonomously, and point towards new mechanisms potentially regulating HSC maintenance across vertebrates.

Notch signaling promotes differentiation, cell death and autophagy in Drosophila hematopoietic system

Notch signaling is a highly conserved pathway between mammals and Drosophila and plays a key role in various biological processes. Drosophila has emerged as a powerful model for studying hematopoiesis and leukemia. In exception to crystal cells, the strength of Notch signaling in Drosophila lymph gland cortical zone (CZ)/intermediate zone (IZ) cells is weak. However, the influence of Notch activation in the lymph gland CZ/IZ cells and circulating hemocytes on hematopoietic homeostasis maintenance is unclear. Here, we showed that Notch activation in lymph gland CZ/IZ cells induced overdifferentiation of progenitors. Moreover, Notch activation promoted lamellocyte generation via NFκB/Toll signaling activation and increased reactive oxygen species (ROS). In addition, we found that Notch activation in lymph gland CZ/IZ cells and circulating hemocytes caused caspase-independent and nonautophagic cell death. However, crystal cell autophagy was activated by upregulation of the expression of the target gene of the Hippo/Yki pathway Diap1. Moreover, we showed that Notch activation could alleviate cytokine storms and improve the survival of Rasv12 leukemia model flies. Our study revealed the various mechanisms of hematopoietic dysregulation induced by Notch activation in healthy flies and the therapeutic effect of Notch activation on leukemia model flies.

Polyunsaturated fatty acids stimulate immunity and eicosanoid production in Drosophila melanogaster

Eicosanoids are a class of molecules derived from C20 polyunsaturated fatty acids (PUFAs) that play a vital role in mammalian and insect biological systems, including development, reproduction, and immunity. Recent research has shown that insects have significant but lower levels of C20 PUFAs in circulation in comparison to C18 PUFAs. It has been previously hypothesized in insects that eicosanoids are synthesized from C18 precursors, such as linoleic acid (LA), to produce downstream eicosanoids. In this study, we show that introduction of arachidonic acid (AA) stimulates production of cyclooxygenase, lipoxygenase, and cytochrome P450-derived eicosanoids. Downstream immune readouts showed that LA stimulates phagocytosis by hemocytes, while both LA and AA stimulate increased antimicrobial peptide production when D. melanogaster is exposed to a heat-killed bacterial pathogen. In totality, this work identifies PUFAs that are involved in insect immunity and adds evidence to the notion that Drosophila utilizes immunostimulatory lipid signaling to mitigate bacterial infections. Our understanding of immune signaling in the fly and its analogies to mammalian systems will increase the power and value of Drosophila as a model organism in immune studies.

Longitudinal tracking of hemocyte populations in vivo indicates lineage relationships and supports neural progenitor identity in adult neurogenesis

Adult neurogenesis, which takes place in both vertebrate and invertebrate species, is the process by which new neurons are born and integrated into existing functional neural circuits, long after embryonic development. Most studies in mammals suggest that self-renewing stem cells are the source of the new neurons, although the extent of self-renewal is a matter of debate. In contrast, research in the crayfish Procambarus clarkii has demonstrated that the neural progenitors producing adult-born neurons are capable of both self-renewing and consuming (non-self-renewing) divisions. However, self-renewing divisions are relatively rare, and therefore the production of adult-born neurons depends heavily on progenitors that are not replenishing themselves. Because the small pool of neural progenitors in the neurogenic niche is never exhausted throughout the long lives of these animals, we hypothesized that there must also be an extrinsic source of these cells. It was subsequently demonstrated that the neural progenitors originate in hemocytes (blood cells) produced by the immune system that travel in the circulation before ultimately integrating into niches where the neural lineage begins. The current study examines the developmental lineage of the three hemocyte types - hyaline (HC), semigranular (SGC) and granular (GC) cells - with the goal of understanding the origins of the progenitor cells that produce adult-born neurons. Longstanding qualitative metrics for hemocyte classification were validated quantitatively. Then, in a longitudinal study, proliferation markers were used to label the hemocytes in vivo, followed by sampling the circulating hemocyte population over the course of two months. Hemolymph samples were taken at intervals to track the frequencies of the different hemocyte types. These data reveal sequential peaks in the relative frequencies of HCs, SGCs and GCs, which were identified using qualitative and quantitative measures. These findings suggest that the three hemocyte types comprise a single cellular lineage that occurs in the circulation, with each type as a sequential progressive stage in hemocyte maturation beginning with HCs and ending with GCs. When combined with previously published data, this timeline provides additional evidence that HCs serve as the primary neural progenitor during adult neurogenesis in P. clarkii.

Analysis of X-ray irradiation effects on the mortality values and hemolymph immune cell composition of Apis mellifera and its parasite, Varroa destructor

Varroa destructor is one of the most destructive enemies of the honey bee, Apis mellifera all around the world. Several control methods are known to control V. destructor, but the efficacy of several alternative control methods remains unexplored. Irradiation can be one of these unknown solutions but before practical application, the effectiveness, and the physiological effects of ionizing radiation on the host and the parasite are waiting to be tested. Therefore, the objective of our study was to investigate the effects of different doses (15, 50, 100, and 150 Gy) of high-energy X-ray irradiation through mortality rates and hemocyte composition changes in A. mellifera workers and record the mortality rates of the parasite. The mortality rate was recorded during short-term (12, 24, and 48 h) and long-term periods (3, 6, 12, 18, and 24d). The sensitivity of the host and the parasite in case of the higher doses of radiation tested (50, 100, and 150 Gy) been demonstrated by total mortality of the host and 90 % of its parasite has been observed on the 18th day after the irradiation. V. destructor showed higher sensitivity (1.52-times higher than the adult honey bee workers) at the lowest dose (15 Gy). A. mellifera hemocytes were influenced significantly by radiation dosage and the elapsed time after treatment. The higher radiation doses increased plasmatocyte numbers in parallel with the decrease in prohemocyte numbers. On the contrary, the numbers of granulocytes and oencoytes increased in the treated samples, but the putative effects of the different dosages on the recorded number of these hemocyte types could not be statistically proven. In summary, based on the outcome of our study X-ray irradiation can be deemed an effective tool for controlling phoretic V. destructor. However, further research is needed to understand the physiological response of the affected organisms.

Recognition of nonself is necessary to activate Drosophila's immune response against an insect parasite

BACKGROUND

Innate immune responses can be activated by pathogen-associated molecular patterns (PAMPs), danger signals released by damaged tissues, or the absence of self-molecules that inhibit immunity. As PAMPs are typically conserved across broad groups of pathogens but absent from the host, it is unclear whether they allow hosts to recognize parasites that are phylogenetically similar to themselves, such as parasitoid wasps infecting insects.

RESULTS

Parasitoids must penetrate the cuticle of Drosophila larvae to inject their eggs. In line with previous results, we found that the danger signal of wounding triggers the differentiation of specialized immune cells called lamellocytes. However, using oil droplets to mimic infection by a parasitoid wasp egg, we found that this does not activate the melanization response. This aspect of the immune response also requires exposure to parasite molecules. The unidentified factor enhances the transcriptional response in hemocytes and induces a specific response in the fat body.

CONCLUSIONS

We conclude that a combination of danger signals and the recognition of nonself molecules is required to activate Drosophila's immune response against parasitic insects.

Molecular Mechanisms of Drosophila Hematopoiesis

As a model organism, the fruit fly (Drosophila melanogaster) has assumed a leading position in modern biological research. The Drosophila genetic system has a number of advantages making it a key model in investigating the molecular mechanisms of metazoan developmental processes. Over the past two decades, significant progress has been made in understanding the molecular mechanisms regulating Drosophila hematopoiesis. This review discusses the major advances in investigating the molecular mechanisms involved in maintaining the population of multipotent progenitor cells and their differentiation into mature hemocytes in the hematopoietic organ of the Drosophila larva. The use of the Drosophila hematopoietic organ as a model system for hematopoiesis has allowed to characterize the complex interactions between signaling pathways and transcription factors in regulating the maintenance and differentiation of progenitor cells through the signals from the hematopoietic niche, autocrine and paracrine signals, and the signals emanated by differentiated cells.

Antagonistic effect of the beneficial bacterium Enterobacter hormaechei against the heavy metal Cu2+ in housefly larvae

Vermicomposting via housefly larvae can be used to efficiently treat manure and regenerate biofertilizer; however, the uptake of heavy metals could negatively influence the growth and development of larvae. Intestinal bacteria play an important role in the development of houseflies, but their effects on resistance to heavy metal damage in houseflies are still poorly understood. In this study, the life history traits and gut microbiota of housefly larvae were evaluated after exposure to an environment with Cu2+ -Enterobacter hormaechei. The data showed that exposure to 300 μg/mL Cu2+ significantly inhibited larval development and locomotor activity and reduced immune capacity. However, dietary supplementation with a Cu2+ -Enterobacter hormaechei mixture resulted in increased body weight and length, and the immune capacity of the larvae returned to normal levels. The abundances of Providencia and Klebsiella increased when larvae were fed Cu2+ -contaminated diets, while the abundances of Enterobacter and Bacillus increased when larvae were exposed to a Cu2+ -Enterobacter hormaechei mixture-contaminated environment. In vitro scanning electron microscopy analysis revealed that Enterobacter hormaechei exhibited obvious adsorption of Cu2+ when cultured in the presence of Cu2+, which reduced the damage caused by Cu2+ to other bacteria in the intestine and protected the larvae from Cu2+ injury. Overall, our results showed that Enterobacter hormaechei can absorb Cu2+ and increase the abundance of beneficial bacteria, thus protecting housefly larvae from damage caused by Cu2+. These results may fill the gaps in our understanding of the interactions between heavy metals and beneficial intestinal bacteria, offering valuable insights into the interplay between housefly larvae and metal contaminants in the environment. This approach could enhance the efficiency of converting manure contaminated with heavy metals to resources using houseflies.

Distinctive features of Zaprionus indianus hemocyte differentiation and function revealed by transcriptomic analysis

Background

Insects have specialized cell types that participate in the elimination of parasites, for instance, the lamellocytes of the broadly studied species Drosophila melanogaster. Other drosophilids, such as Drosophila ananassae and the invasive Zaprionus indianus, have multinucleated giant hemocytes, a syncytium of blood cells that participate in the encapsulation of the eggs or larvae of parasitoid wasps. These cells can be formed by the fusion of hemocytes in circulation or originate from the lymph gland. Their ultrastructure highly resembles that of the mammalian megakaryocytes.

Methods

Morphological, protein expressional, and functional features of blood cells were revealed using epifluorescence and confocal microscopy. The respective hemocyte subpopulations were identified using monoclonal antibodies in indirect immunofluorescence assays. Fluorescein isothiocyanate (FITC)-labeled Escherichia coli bacteria were used in phagocytosis tests. Gene expression analysis was performed following mRNA sequencing of blood cells.

Results

D. ananassae and Z. indianus encapsulate foreign particles with the involvement of multinucleated giant hemocytes and mount a highly efficient immune response against parasitoid wasps. Morphological, protein expressional, and functional assays of Z. indianus blood cells suggested that these cells could be derived from large plasmatocytes, a unique cell type developing specifically after parasitoid wasp infection. Transcriptomic analysis of blood cells, isolated from naïve and wasp-infected Z. indianus larvae, revealed several differentially expressed genes involved in signal transduction, cell movements, encapsulation of foreign targets, energy production, and melanization, suggesting their role in the anti-parasitoid response. A large number of genes that encode proteins associated with coagulation and wound healing, such as phenoloxidase activity factor-like proteins, fibrinogen-related proteins, lectins, and proteins involved in the differentiation and function of platelets, were constitutively expressed. The remarkable ultrastructural similarities between giant hemocytes and mammalian megakaryocytes, and presence of platelets, and giant cell-derived anucleated fragments at wound sites hint at the involvement of this cell subpopulation in wound healing processes, in addition to participation in the encapsulation reaction.

Conclusion

Our observations provide insights into the broad repertoire of blood cell functions required for efficient defense reactions to maintain the homeostasis of the organism. The analysis of the differentiation and function of multinucleated giant hemocytes gives an insight into the diversification of the immune mechanisms.

The Drosophila melanogaster prophenoloxidase system participates in immunity against Zika virus infection

Drosophila melanogaster relies on an evolutionarily conserved innate immune system to protect itself from a wide range of pathogens, making it a convenient genetic model to study various human pathogenic viruses and host antiviral immunity. Here we explore for the first time the contribution of the Drosophila phenoloxidase (PO) system to host survival and defenses against Zika virus (ZIKV) infection by analyzing the role of mutations in the three prophenoloxidase (PPO) genes in female and male flies. We show that only PPO1 and PPO2 genes contribute to host survival and appear to be upregulated following ZIKV infection in Drosophila. Also, we present data suggesting that a complex regulatory system exists between Drosophila PPOs, potentially allowing for a sex-dependent compensation of PPOs by one another or other immune responses such as the Toll, Imd, and JAK/STAT pathways. Furthermore, we show that PPO1 and PPO2 are essential for melanization in the hemolymph and the wound site in flies upon ZIKV infection. Our results reveal an important role played by the melanization pathway in response to ZIKV infection, hence highlighting the importance of this pathway in insect host defense against viral pathogens and potential vector control strategies to alleviate ZIKV outbreaks.

The human leukemic oncogene MLL-AF4 promotes hyperplastic growth of hematopoietic tissues in Drosophila larvae

MLL-rearranged (MLL-r) leukemias are among the leukemic subtypes with poorest survival, and treatment options have barely improved over the last decades. Despite increasing molecular understanding of the mechanisms behind these hematopoietic malignancies, this knowledge has had poor translation into the clinic. Here, we report a Drosophila melanogaster model system to explore the pathways affected in MLL-r leukemia. We show that expression of the human leukemic oncogene MLL-AF4 in the Drosophila hematopoietic system resulted in increased levels of circulating hemocytes and an enlargement of the larval hematopoietic organ, the lymph gland. Strikingly, depletion of Drosophila orthologs of known interactors of MLL-AF4, such as DOT1L, rescued the leukemic phenotype. In agreement, treatment with small-molecule inhibitors of DOT1L also prevented the MLL-AF4-induced leukemia-like phenotype. Taken together, this model provides an in vivo system to unravel the genetic interactors involved in leukemogenesis and offers a system for improved biological understanding of MLL-r leukemia.

Insect-pathogen crosstalk and the cellular-molecular mechanisms of insect immunity: uncovering the underlying signaling pathways and immune regulatory function of non-coding RNAs

Multicellular organisms are constantly subjected to pathogens that might be harmful. Although insects lack an adaptive immune system, they possess highly effective anti-infective mechanisms. Bacterial phagocytosis and parasite encapsulation are some forms of cellular responses. Insects often defend themselves against infections through a humoral response. This phenomenon includes the secretion of antimicrobial peptides into the hemolymph. Specific receptors for detecting infection are required for the recognition of foreign pathogens such as the proteins that recognize glucans and peptidoglycans, together referred to as PGRPs and βGRPs. Activation of these receptors leads to the stimulation of signaling pathways which further activates the genes encoding for antimicrobial peptides. Some instances of such pathways are the JAK-STAT, Imd, and Toll. The host immune response that frequently accompanies infections has, however, been circumvented by diseases, which may have assisted insects evolve their own complicated immune systems. The role of ncRNAs in insect immunology has been discussed in several notable studies and reviews. This paper examines the most recent research on the immune regulatory function of ncRNAs during insect-pathogen crosstalk, including insect- and pathogen-encoded miRNAs and lncRNAs, and provides an overview of the important insect signaling pathways and effector mechanisms activated by diverse pathogen invaders.

Parasitic nematode secreted phospholipase A2 suppresses cellular and humoral immunity by targeting hemocytes in Drosophila melanogaster

A key aspect of parasitic nematode infection is the nematodes' ability to evade and/or suppress host immunity. This immunomodulatory ability is likely driven by the release of hundreds of excretory/secretory proteins (ESPs) during infection. While ESPs have been shown to display immunosuppressive effects on various hosts, our understanding of the molecular interactions between individual proteins released and host immunity requires further study. We have recently identified a secreted phospholipase A2 (sPLA2) released from the entomopathogenic nematode (EPN) Steinernema carpocapsae we have named Sc-sPLA2. We report that Sc-sPLA2 increased mortality of Drosophila melanogaster infected with Streptococcus pneumoniae and promoted increased bacterial growth. Furthermore, our data showed that Sc-sPLA2 was able to downregulate both Toll and Imd pathway-associated antimicrobial peptides (AMPs) including drosomycin and defensin, in addition to suppressing phagocytosis in the hemolymph. Sc-sPLA2 was also found to be toxic to D. melanogaster with the severity being both dose- and time-dependent. Collectively, our data highlighted that Sc-sPLA2 possessed both toxic and immunosuppressive capabilities.

Gradual specialization of phagocytic ameboid cells may have impaired regenerative capacities in metazoan lineages

Animal regeneration is a fascinating field of research that has captured the attention of many generations of scientists. Among the cellular mechanisms underlying tissue and organ regeneration, we highlight the role of phagocytic ameboid cells (PACs). Beyond their ability to engulf nutritional particles, microbes, and apoptotic cells, their involvement in regeneration has been widely documented. It has been extensively described that, at least in part, animal regenerative mechanisms rely on PACs that serve as a hub for a range of critical physiological functions, both in health and disease. Considering the phylogenetics of PAC evolution, and the loss and gain of nutritional, immunological, and regenerative potential across Metazoa, we aim to discuss when and how phagocytic activity was first co-opted to regenerative tissue repair. We propose that the gradual specialization of PACs during metazoan derivation may have contributed to the loss of regenerative potential in animals, with critical impacts on potential translational strategies for regenerative medicine.

A Novel Method for Primary Blood Cell Culturing and Selection in Drosophila melanogaster

The blood cells of the fruit fly Drosophila melanogaster show many similarities to their vertebrate counterparts, both in their functions and their differentiation. In the past decades, a wide palette of immunological and transgenic tools and methods have been developed to study hematopoiesis in the Drosophila larva. However, the in vivo observation of blood cells is technically restricted by the limited transparency of the body and the difficulty in keeping the organism alive during imaging. Here we describe an improved ex vivo culturing method that allows effective visualization and selection of live blood cells in primary cultures derived from Drosophila larvae. Our results show that cultured hemocytes accurately represent morphological and functional changes following immune challenges and in case of genetic alterations. Since cell culturing has hugely contributed to the understanding of the physiological properties of vertebrate blood cells, this method provides a versatile tool for studying Drosophila hemocyte differentiation and functions ex vivo.

Irradiation and parasitism affect the ability of larval hemocytes of Anastrepha obliqua for phagocytosis and the production of reactive oxygen species

The development of the parasitoid Doryctobracon crawfordi (Viereck) (Hymenoptera: Braconidae) in Anastrepha obliqua (McQuart) (Diptera: Tephritidae) larvae is unviable in nature; however, if the host larva is irradiated at 160 Gy, the parasitoid develops and emerges successfully. This suggests that radiation affects the immune responses of A. obliqua larvae, while the underlying mechanisms remain to be revealed. Using optical and electronic microscopies we determined the number and type of hemocyte populations found inside the A. obliqua larvae, either nonirradiated, irradiated at 160 Gy, parasitized by D. crawfordi, or irradiated and parasitized. Based on flow cytometry, the capacity to produce reactive oxygen species (ROS) was determined by the 123-dihydrorhodamine method in those hemocyte cells. Five cell populations were found in the hemolymph of A. obliqua larvae, two of which (granulocytes and plasmatocytes) can phagocytize and produce ROS. A reduction in the number of cells, mainly of the phagocytic type, was observed, as well as the capacity of these cells to produce ROS, when A. obliqua larvae were irradiated. Both radiation and parasitization decreased the ROS production, and when A. obliqua larvae were irradiated followed by parasitization by D. crawfordi, the reduction of the ROS level was even greater. In contrast, a slight increase in the size of these cells was observed in the hemolymph of the parasitized larvae compared to those in nonparasitized larvae. These results suggest that radiation significantly affects the phagocytic cells of A. obliqua and thus permits the development of the parasitoid D. crawfordi.

Hemocytes and fat body cells, the only professional immune cell types in Drosophila, show strikingly different responses to systemic infections

The fruit fly Drosophila is an excellent model to study the response of different immunocompetent organs during systemic infection. In the present study, we intended to test the hypothesis that the only professional immune organs of the fly, the fat body and hemocytes, show substantial similarities in their responses to systemic infection. However, comprehensive transcriptome analysis of isolated organs revealed highly divergent transcript signatures, with the few commonly regulated genes encoding mainly classical immune effectors from the antimicrobial peptide family. The fat body and the hemocytes each have specific reactions that are not present in the other organ. Fat body-specific responses comprised those enabling an improved peptide synthesis and export. This reaction is accompanied by transcriptomic shifts enabling the use of the energy resources of the fat body more efficiently. Hemocytes, on the other hand, showed enhanced signatures related to phagocytosis. Comparing immune-induced signatures of both cell types with those of whole-body responses showed only a minimal correspondence, mostly restricted again to antimicrobial peptide genes. In summary, the two major immunocompetent cell types of Drosophila show highly specific responses to infection, which are closely linked to the primary function of the respective organ in the landscape of the systemic immune response.

Drosophila melanogaster as a model to study innate immune memory

Over the last decades, research regarding innate immune responses has gained increasing importance. A growing body of evidence supports the notion that the innate arm of the immune system could show memory traits. Such traits are thought to be conserved throughout evolution and provide a survival advantage. Several models are available to study these mechanisms. Among them, we find the fruit fly, Drosophila melanogaster. This non-mammalian model has been widely used for innate immune research since it naturally lacks an adaptive response. Here, we aim to review the latest advances in the study of the memory mechanisms of the innate immune response using this animal model.

Hematopoietic plasticity mapped in Drosophila and other insects

Hemocytes, similar to vertebrate blood cells, play important roles in insect development and immunity, but it is not well understood how they perform their tasks. New technology, in particular single-cell transcriptomic analysis in combination with Drosophila genetics, may now change this picture. This review aims to make sense of recently published data, focusing on Drosophila melanogaster and comparing to data from other drosophilids, the malaria mosquito, Anopheles gambiae, and the silkworm, Bombyx mori. Basically, the new data support the presence of a few major classes of hemocytes: (1) a highly heterogenous and plastic class of professional phagocytes with many functions, called plasmatocytes in Drosophila and granular cells in other insects. (2) A conserved class of cells that control melanin deposition around parasites and wounds, called crystal cells in D. melanogaster, and oenocytoids in other insects. (3) A new class of cells, the primocytes, so far only identified in D. melanogaster. They are related to cells of the so-called posterior signaling center of the larval hematopoietic organ, which controls the hematopoiesis of other hemocytes. (4) Different kinds of specialized cells, like the lamellocytes in D. melanogaster, for the encapsulation of parasites. These cells undergo rapid evolution, and the homology relationships between such cells in different insects are uncertain. Lists of genes expressed in the different hemocyte classes now provide a solid ground for further investigation of function.

Peeling Back the Layers of Lymph Gland Structure and Regulation

During the past 60 years, the fruit fly, Drosophila melanogaster, has proven to be an excellent model to study the regulation of hematopoiesis. This is not only due to the evolutionarily conserved signalling pathways and transcription factors contributing to blood cell fate, but also to convergent evolution that led to functional similarities in distinct species. An example of convergence is the compartmentalization of blood cells, which ensures the quiescence of hematopoietic stem cells and allows for the rapid reaction of the immune system upon challenges. The lymph gland, a widely studied hematopoietic organ of the Drosophila larva, represents a microenvironment with similar features and functions to classical hematopoietic stem cell niches of vertebrates. Lymph gland studies were effectively supported by the unparalleled toolkit developed in Drosophila, which enabled the high-resolution investigation of the cellular composition and regulatory interaction networks of the lymph gland. In this review, we summarize how our understanding of lymph gland structure and hematopoietic cell-to-cell communication evolved during the past decades and compare their analogous features to those of the vertebrate hematopoietic stem cell niche.

Advances in the Immune Regulatory Role of Non-Coding RNAs (miRNAs and lncRNAs) in Insect-Pathogen Interactions

Insects are by far the most abundant and diverse living organisms on earth and are frequently prone to microbial attacks. In other to counteract and overcome microbial invasions, insects have in an evolutionary way conserved and developed immune defense mechanisms such as Toll, immune deficiency (Imd), and JAK/STAT signaling pathways leading to the expression of antimicrobial peptides. These pathways have accessory immune effector mechanisms, such as phagocytosis, encapsulation, melanization, nodulation, RNA interference (RNAi), lysis, autophagy, and apoptosis. However, pathogens evolved strategies that circumvent host immune response following infections, which may have helped insects further sophisticate their immune response mechanisms. The involvement of ncRNAs in insect immunity is undeniable, and several excellent studies or reviews have investigated and described their roles in various insects. However, the functional analyses of ncRNAs in insects upon pathogen attacks are not exhaustive as novel ncRNAs are being increasingly discovered in those organisms. This article gives an overview of the main insect signaling pathways and effector mechanisms activated by pathogen invaders and summarizes the latest findings of the immune modulation role of both insect- and pathogen-encoded ncRNAs, especially miRNAs and lncRNAs during insect–pathogen crosstalk.

Intrinsic and Extrinsic Regulation of Hematopoiesis in Drosophila

Drosophila melanogaster lymph gland, the primary site of hematopoiesis, contains myeloid-like progenitor cells that differentiate into functional hemocytes in the circulation of pupae and adults. Fly hemocytes are dynamic and plastic, and they play diverse roles in the innate immune response and wound healing. Various hematopoietic regulators in the lymph gland ensure the developmental and functional balance between progenitors and mature blood cells. In addition, systemic factors, such as nutrient availability and sensory inputs, integrate environmental variabilities to synchronize the blood development in the lymph gland with larval growth, physiology, and immunity. This review examines the intrinsic and extrinsic factors determining the progenitor states during hemocyte development in the lymph gland and provides new insights for further studies that may extend the frontier of our collective knowledge on hematopoiesis and innate immunity.

Activin and BMP Signaling Activity Affects Different Aspects of Host Anti-Nematode Immunity in Drosophila melanogaster

The multifaceted functions ranging from cellular and developmental mechanisms to inflammation and immunity have rendered TGF-ß signaling pathways as critical regulators of conserved biological processes. Recent studies have indicated that this evolutionary conserved signaling pathway among metazoans contributes to the Drosophila melanogaster anti-nematode immune response. However, functional characterization of the interaction between TGF-ß signaling activity and the mechanisms activated by the D. melanogaster immune response against parasitic nematode infection remains unexplored. Also, it is essential to evaluate the precise effect of entomopathogenic nematode parasites on the host immune system by separating them from their mutualistic bacteria. Here, we investigated the participation of the TGF-ß signaling branches, activin and bone morphogenetic protein (BMP), to host immune function against axenic or symbiotic Heterorhabditis bacteriophora nematodes (parasites lacking or containing their mutualistic bacteria, respectively). Using D. melanogaster larvae carrying mutations in the genes coding for the TGF-ß extracellular ligands Daw and Dpp, we analyzed the changes in survival ability, cellular immune response, and phenoloxidase (PO) activity during nematode infection. We show that infection with axenic H. bacteriophora decreases the mortality rate of dpp mutants, but not daw mutants. Following axenic or symbiotic H. bacteriophora infection, both daw and dpp mutants contain only plasmatocytes. We further detect higher levels of Dual oxidase gene expression in dpp mutants upon infection with axenic nematodes and Diptericin and Cecropin gene expression in daw mutants upon infection with symbiotic nematodes compared to controls. Finally, following symbiotic H. bacteriophora infection, daw mutants have higher PO activity relative to controls. Together, our findings reveal that while D. melanogaster Dpp/BMP signaling activity modulates the DUOX/ROS response to axenic H. bacteriophora infection, Daw/activin signaling activity modulates the antimicrobial peptide and melanization responses to axenic H. bacteriophora infection. Results from this study expand our current understanding of the molecular and mechanistic interplay between nematode parasites and the host immune system, and the involvement of TGF-ß signaling branches in this process. Such findings will provide valuable insight on the evolution of the immune role of TGF-ß signaling, which could lead to the development of novel strategies for the effective management of human parasitic nematodes.

Broad Ultrastructural and Transcriptomic Changes Underlie the Multinucleated Giant Hemocyte Mediated Innate Immune Response against Parasitoids

Multinucleated giant hemocytes (MGHs) represent a novel type of blood cell in insects that participate in a highly efficient immune response against parasitoid wasps involving isolation and killing of the parasite. Previously, we showed that circulating MGHs have high motility and the interaction with the parasitoid rapidly triggers encapsulation. However, structural and molecular mechanisms behind these processes remained elusive. Here, we used detailed ultrastructural analysis and live cell imaging of MGHs to study encapsulation in Drosophila ananassae after parasitoid wasp infection. We found dynamic structural changes, mainly driven by the formation of diverse vesicular systems and newly developed complex intracytoplasmic membrane structures, and abundant generation of giant cell exosomes in MGHs. In addition, we used RNA sequencing to study the transcriptomic profile of MGHs and activated plasmatocytes 72 h after infection, as well as the uninduced blood cells. This revealed that differentiation of MGHs was accompanied by broad changes in gene expression. Consistent with the observed structural changes, transcripts related to vesicular function, cytoskeletal organization, and adhesion were enriched in MGHs. In addition, several orphan genes encoding for hemolysin-like proteins, pore-forming toxins of prokaryotic origin, were expressed at high level, which may be important for parasitoid elimination. Our results reveal coordinated molecular and structural changes in the course of MGH differentiation and parasitoid encapsulation, providing a mechanistic model for a powerful innate immune response.

Methotrexate negatively acts on inflammatory responses triggered in Drosophila larva with hyperactive JAK/STAT pathway

Drosophila is a valuable paradigm for studying tumorigenesis and cancer. Mutations causing hematopoietic aberrations and melanotic-blood-tumors found in Drosophila mutants are vastly studied. Clear understanding about the blood cells, signaling pathways and the tissues affected during hematopoietic tumor formation provide an opportunity to delineate the effects of cancer therapeutics. Using this simple hematopoietic archetype, we elucidated the effects of the anti-cancer drug, Methotrexate (MTX) on immune responses in two scenarios i.e. against wasp infection and in hematopoietic mutant, hop^Tum-l. Through this in vivo study we show that MTX impedes the immune responses against wasp infection including the encapsulation response. We further observed that MTX reduces the tumor penetrance in gain-of-function mutants of JAK/STAT pathway, hop^Tum-l. MTX is anti-inflammatory as it hinders not only the immune responses of acute inflammation as observed after wasp infestation, but also chronic inflammatory responses associated with constitutively activated JAK/STAT pathway mutant (hop^Tum-l) carrying blood tumors.

Extracellular matrix protein N-glycosylation mediates immune self-tolerance in Drosophila melanogaster

In order to respond to infection, hosts must distinguish pathogens from their own tissues. This allows for the precise targeting of immune responses against pathogens and also ensures self-tolerance, the ability of the host to protect self tissues from immune damage. One way to maintain self-tolerance is to evolve a self signal and suppress any immune response directed at tissues that carry this signal. Here, we characterize the Drosophila tuSz1 mutant strain, which mounts an aberrant immune response against its own fat body. We demonstrate that this autoimmunity is the result of two mutations: 1) a mutation in the GCS1 gene that disrupts N-glycosylation of extracellular matrix proteins covering the fat body, and 2) a mutation in the Drosophila Janus Kinase ortholog that causes precocious activation of hemocytes. Our data indicate that N-glycans attached to extracellular matrix proteins serve as a self signal and that activated hemocytes attack tissues lacking this signal. The simplicity of this invertebrate self-recognition system and the ubiquity of its constituent parts suggests it may have functional homologs across animals.

Deciphering of Candida parapsilosis induced immune response in Drosophila melanogaster

Candida infections are the most prevalent cause of serious human mycoses and are the third most common pathogens isolated from bloodstream infections in hospitalized patients. C. parapsilosis is a member of the non-albicans spp., which have a predilection for causing life-threatening disease in neonates and hospitalized pediatric patients. In this study, we utilized a Drosophila melanogaster infection model to analyze the immunological responses to C. parapsilosis. Our results demonstrate that the Toll pathway in Drosophila controls C. parapsilosis proliferation as the Toll signaling mutant MyD88^-/- flies are highly susceptible to C. parapsilosis. We also confirmed that the MyD88^-/- fly is a convenient invertebrate animal model to analyze virulence properties of different species and strains from the C. parapsilosis sensu lato complex as C. orthopsilosis, C. metapsilosis proved to be less virulent than C. parapsilosis sensu stricto and the N-mannan deficient C. parapsilosis och1Δ/Δ strain showed attenuated pathogenicity in this immunodeficient Drosophila background. We also found that Persephone protease is not required for detection and activation of Toll pathway during C. parapsilosis infection. Furthermore, we observed that Drosophila β-glucan receptor deficient flies where more sensitive to C. parapsilosis compared to wild-type flies; however, we could not find a clear dependence on the recognition of this receptor and the cell wall β-glucan exposure-induced host response. These studies establish this D. melanogaster infection model as an efficient tool in deciphering immune responses to C. parapsilosis as well as for assessing virulence factors produced by this emerging fungal predator.

Hemocyte RNA-Seq analysis of Indian malarial vectors Anopheles stephensi and Anopheles culicifacies: From similarities to differences

Anopheles stephensi and Anopheles culicifacies are dominant malarial vectors in urban and rural India, respectively. Both species carry significant biological differences in their behavioral adaptation and immunity, but the genetic basis of these variations are still poorly understood. Here, we uncovered the genetic differences of immune blood cells, that influence several immune-physiological responses. We generated, analyzed and compared the hemocyte RNA-Seq database of both mosquitoes. A total of 5,837,223,769 assembled bases collapsed into 7,595 and 3,791 transcripts, originating from hemocytes of laboratory-reared 3-4 days old naïve (sugar-fed) mosquitoes, Anopheles stephensi and Anopheles culicifacies respectively. Comparative GO annotation analysis revealed that both mosquito hemocytes encode similar proteins. Furthermore, while An. stephensi hemocytes showed a higher percentage of immune transcripts encoding APHAG (Autophagy), IMD (Immune deficiency pathway), PRDX (Peroxiredoxin), SCR (Scavenger receptor), IAP (Inhibitor of apoptosis), GALE (galactoside binding lectins), BGBPs (1,3 beta D glucan binding proteins), CASPs (caspases) and SRRP (Small RNA regulatory pathway), An. culicifacies hemocytes yielded a relatively higher percentage of transcripts encoding CLIP (Clip domain serine protease), FREP (Fibrinogen related proteins), PPO (Prophenol oxidase), SRPN (Serpines), ML (Myeloid differentiation 2-related lipid recognition protein), Toll path and TEP (Thioester protein), family proteins. However, a detailed comparative Interproscan analysis showed An. stephensi mosquito hemocytes encode proteins with increased repeat numbers as compared to An. culicifacies. Notably, we observed an abundance of transcripts showing significant variability of encoded proteins with repeats such as LRR (Leucine rich repeat), WD40 (W-D dipeptide), Ankyrin, Annexin, Tetratricopeptide and Mitochondrial substrate carrier repeat-containing family proteins, which may have a direct influence on species-specific immune-physiological responses. Summarily, our deep sequencing analysis unraveled that An. stephensi evolved with an expansion of repeat sequences in hemocyte proteins as compared to An. culicifacies, possibly providing an advantage for better adaptation to diverse environments.

Mechanisms and biological impacts of graphene and multi-walled carbon nanotubes on Drosophila melanogaster: Oxidative stress, genotoxic damage, phenotypic variations, locomotor behavior, parasitoid resistance, and cellular immune response

The use of graphene and multi-walled carbon nanotubes (MWCNTs) has now become rather common in medical applications as well as several other areas thanks to their useful physicochemical properties. While in vitro testing offers some potential, in vivo research into toxic effects of graphene and MWCNTs could yield much more reliable data. Drosophila melanogaster has recently gained significant popularity as a dynamic eukaryotic model in examining toxicity, genotoxicity, and biological effects of exposure to nanomaterials, including oxidative stress, cellular immune response against two strains (NSRef and G486) of parasitoid wasp (Leptopilina boulardi), phenotypic variations, and locomotor behavior risks. D. melanogaster was used as a model organism in our study to identify the potential risks of exposure to graphene (thickness: 2-18 nm) and MWCNTs in different properties (as pure [OD: 10-20 nm short], modified by amide [NH₂ ] [OD: 7-13 nm length: 55 μm], and modified by carboxyl [COOH] [OD: 30-50 nm and length: 0.5-2 μm]) at concentrations ranging from 0.1 to 250 μg/ml. Significant effects were observed at two high doses (100 and 250 μg/ml) of graphene or MWCNTs. This is the first study to report findings of cellular immune response against hematopoiesis and parasitoids, nanogenotoxicity, phenotypic variations, and locomotor behavior in D. melanogaster.

Morphological Characterisation of Haemocytes in the Mealworm Beetle Tenebrio molitor (Coleoptera, Tenebrionidae)

The immunocompetence of the mealworm beetle Tenebrio molitor has been well investigated at molecular and physiological levels, but information on morphological and functional characteristics of its immune cells (haemocytes) is still scarce and fragmentary. This study provides an updated overview of the morphology of circulating immune cells from mealworm beetle adults, using light and transmission electron microscopy. Based on their affinities for May-Grünwald Giemsa stain, haemocytes were defined as either eosinophilic, basophilic or neutral. Ultrastructural descriptions allowed to detect four main cell types in the haemolymph: prohaemocytes, plasmatocytes, granular cells and oenocytoids. The morphological plasticity of haemocytes and the evidence of mitotic circulating cells, intermediate cell stages, as well as autophagic activities suggest haemocyte proliferation, turnover and transdifferentiation as constantly active processes in the haemolymph. Cytochemical tests revealed differences in the distribution of carbohydrates among cell types underling the great plasticity of the immune response and the direct involvement of circulating immune cells in the resource allocation. In addition, our results provide a detailed morphological description of vesicle trafficking, macro- and microautophagy, apoptotic and necrotic processes, confirming the suitability of T. molitor haemocytes as a model for studying evolutionarily conserved cellular mechanisms.

Regression plane concept for analysing continuous cellular processes with machine learning

Biological processes are inherently continuous, and the chance of phenotypic discovery is significantly restricted by discretising them. Using multi-parametric active regression we introduce the Regression Plane (RP), a user-friendly discovery tool enabling class-free phenotypic supervised machine learning, to describe and explore biological data in a continuous manner. First, we compare traditional classification with regression in a simulated experimental setup. Second, we use our framework to identify genes involved in regulating triglyceride levels in human cells. Subsequently, we analyse a time-lapse dataset on mitosis to demonstrate that the proposed methodology is capable of modelling complex processes at infinite resolution. Finally, we show that hemocyte differentiation in Drosophila melanogaster has continuous characteristics.

Identification of Silkworm Hemocyte Subsets and Analysis of Their Response to Baculovirus Infection Based on Single-Cell RNA Sequencing

A wide range of hemocyte types exist in insects but a full definition of the different subclasses is not yet established. The current knowledge of the classification of silkworm hemocytes mainly comes from morphology rather than specific markers, so our understanding of the detailed classification, hemocyte lineage and functions of silkworm hemocytes is very incomplete. Bombyx mori nucleopolyhedrovirus (BmNPV) is a representative member of the baculoviruses and a major pathogen that specifically infects silkworms (Bombyx mori) and causes serious losses in sericulture industry. Here, we performed single-cell RNA sequencing (scRNA-seq) of hemocytes in BmNPV and mock-infected larvae to comprehensively identify silkworm hemocyte subsets and determined specific molecular and cellular characteristics in each hemocyte subset before and after viral infectmadion. A total of 20 cell clusters and their potential marker genes were identified in silkworm hemocytes. All of the hemocyte clusters were infected by BmNPV at 3 days after inoculation. Interestingly, BmNPV infection can cause great changes in the distribution of hemocyte types. The cells appearing in the infection group mainly belong to prohemocytes (PR), while plasmatocytes (PL) and granulocytes (GR) are very much reduced. Furthermore, we found that BmNPV infection suppresses the RNA interference (RNAi) and immune response in the major hemocyte types. In summary, our results revealed the diversity of silkworm hemocytes and provided a rich resource of gene expression profiles for a systems-level understanding of their functions in the uninfected condition and as a response to BmNPV.

Chitin binding protein from the kuruma shrimp Marsupenaeus japonicus facilitates the clearance of Vibrio anguillarum

Peritrophic membrane (PM) refers to a vital physical barrier enabling shrimp to resist pathogen invasion. It primarily consists of chitin and proteins, mostly chitin-binding protein (CBP). CBPs have been identified from microorganisms to higher organisms. In the present study, a CBP, designated MjCBP, was reported from Marsupenaeus japonicus. The open reading frame of MjCBP was 1854 bp, encoding a protein with 618 amino acids (MH544098). To be specific, the theoretical pI and molecular mass of mature MjCBP reached 5.43 and 66064.00 Da, respectively. MjCBP consisted of seven type Ⅱ chitin-binding domains (ChtB D2), which was up-regulated after being challenged with Vibrio anguillarum and then agglutinating several bacteria. In addition, MjCBP and the first chitin-binding domain (CBD1) could bind to several Gram-positive and Gram-negative bacteria via the binding process to lipopolysaccharides and peptidoglycans, whereas CBD1 was not capable of agglutinating bacteria. Moreover, the anterior and posterior segments of CBD1 were synthesized in vitro, and the posterior segment could bind to lipopolysaccharides. However, both segments fail to agglutinate bacteria. Furthermore, MjCBP and CBD1 facilitated the clearance of V. anguillarum in vivo, and the silencing of MjCBP via RNA interference reduced the ability of bacterial clearance. As revealed from the mentioned results, MjCBP acts as an opsonin or pattern recognition receptor to achieve antibacterial immune response in shrimp.

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.

The S1A protease family members CG10764 and CG4793 regulate cellular immunity in Drosophila

In nature, Drosophila melanogaster larvae are infected by parasitoid wasps and mount a cellular immune response to this infection. Several conserved signaling pathways have been implicated in coordinating this response, however our understanding of the integration and regulation of these pathways is incomplete. Members of the S1A serine protease family have been previously linked to immune functions, and our findings suggest roles for two S1A family members, CG10764 and CG4793 in the cellular immune response to parasitoid infection.

Signaling cross-talk during development: Context-specific networking of Notch, NF-κB and JNK signaling pathways in Drosophila

Multicellular organisms depend on a handful of core signaling pathways that regulate a variety of cell fate choices. Often these relatively simple signals integrate to form a large and complex signaling network to achieve a distinct developmental fate in a context-specific manner. Various pathway-dependent and independent events control the assembly of signaling complexes. Notch pathway is one such conserved signaling mechanism that integrates with other signaling pathways to exhibit a context-dependent pleiotropic output. To understand how Notch signaling provides a spectrum of distinct outputs, it is important to understand various regulatory switches involved in mediating signaling cross-talk of Notch with other pathways. Here, we review our current understanding as to how Notch signal integrates with JNK and NF-κB signaling pathways in Drosophila to regulate various developmental events such as sensory organ precursor formation, innate immunity, dorsal closure, establishment of planar cell polarity as well as during proliferation and tumor progression. We highlight the importance of conserved signaling molecules during these cross-talks and debate further possibilities of novel switches that may be involved in mediating these cross-talk events.

Comparative hematopoiesis and signal transduction in model organisms

Hematopoiesis is a continuous phenomenon involving the formation of hematopoietic stem cells (HSCs) giving rise to diverse functional blood cells. This developmental process of hematopoiesis is evolutionarily conserved, yet comparably different in various model organisms. Vertebrate HSCs give rise to all types of mature cells of both the myeloid and the lymphoid lineages sequentially colonizing in different anatomical tissues. Signal transduction in HSCs facilitates their potency and specifies branching of lineages. Understanding the hematopoietic signaling pathways is crucial to gain insights into their deregulation in several blood-related disorders. The focus of the review is on hematopoiesis corresponding to different model organisms and pivotal role of indispensable hematopoietic pathways. We summarize and discuss the fundamentals of blood formation in both invertebrate and vertebrates, examining the requirement of key signaling nexus in hematopoiesis. Knowledge obtained from such comparative studies associated with developmental dynamics of hematopoiesis is beneficial to explore the therapeutic options for hematopoietic diseases.

The preference of Trichopria drosophilae for pupae of Drosophila suzukii is independent of host size

Controlling the cosmopolitan pest Drosophila suzukii (spotted wing drosophila) is a challenge for fruit growers. A promising agent for biological control of that pest are parasitoid wasps. Especially the widespread pupal parasitoid Trichopria drosophilae had shown the ability to parasitise the pest fly. However, as a biocontrol agent, parasitoids can only be effective when they prefer the pest to other insects. Until now studies have been inconsistent concerning the preference of T. drosophilae for D. suzukii and whether the preference depends on pupal volume. To clarify this inconsistency, we used video recordings of parasitisation experiments with a set up to observe the direct host preference of the parasitoid. Additionally, the volume of each host pupa was measured. We found significant preference of T. drosophilae for D. suzukii pupae independent of the pupal size and of the host species the wasps were reared on. The article also discusses the sex ratio and the success of the parasitoid in the different pupae characteristics.

Immune Cell Production Is Targeted by Parasitoid Wasp Virulence in a Drosophila-Parasitoid Wasp Interaction

The interactions between Drosophila melanogaster and the parasitoid wasps that infect Drosophila species provide an important model for understanding host-parasite relationships. Following parasitoid infection, D. melanogaster larvae mount a response in which immune cells (hemocytes) form a capsule around the wasp egg, which then melanizes, leading to death of the parasitoid. Previous studies have found that host hemocyte load; the number of hemocytes available for the encapsulation response; and the production of lamellocytes, an infection induced hemocyte type, are major determinants of host resistance. Parasitoids have evolved various virulence mechanisms to overcome the immune response of the D. melanogaster host, including both active immune suppression by venom proteins and passive immune evasive mechanisms. We identified a previously undescribed parasitoid species, Asobara sp. AsDen, which utilizes an active virulence mechanism to infect D. melanogaster hosts. Asobara sp. AsDen infection inhibits host hemocyte expression of msn, a member of the JNK signaling pathway, which plays a role in lamellocyte production. Asobara sp. AsDen infection restricts the production of lamellocytes as assayed by hemocyte cell morphology and altered msn expression. Our findings suggest that Asobara sp. AsDen infection alters host signaling to suppress immunity.

Constitutive activation of cellular immunity underlies the evolution of resistance to infection in Drosophila

Organisms rely on inducible and constitutive immune defences to combat infection. Constitutive immunity enables a rapid response to infection but may carry a cost for uninfected individuals, leading to the prediction that it will be favoured when infection rates are high. When we exposed populations of Drosophila melanogaster to intense parasitism by the parasitoid wasp Leptopilina boulardi, they evolved resistance by developing a more reactive cellular immune response. Using single-cell RNA sequencing, we found that immune-inducible genes had become constitutively upregulated. This was the result of resistant larvae differentiating precursors of specialized immune cells called lamellocytes that were previously only produced after infection. Therefore, populations evolved resistance by genetically hard-wiring the first steps of an induced immune response to become constitutive.

Parasitoid wasp venom vesicles (venosomes) enter Drosophila melanogaster lamellocytes through a flotillin/lipid raft-dependent endocytic pathway

Venosomes are extracellular vesicles found in the venom of Leptopilina endoparasitoids wasps, which transport and target virulence factors to impair the parasitoid egg encapsulation by the lamellocytes of their Drosophila melanogaster host larva. Using the co-immunolocalization of fluorescent L. boulardi venosomes and one of the putative-transported virulence factors, LbGAP, with known markers of cellular endocytosis, we show that venosomes endocytosis by lamellocytes is not a process dependent on clathrin or macropinocytosis and internalization seems to bypass the early endosomal compartment Rab5. After internalization, LbGAP colocalizes strongly with flotillin-1 and the GPI-anchored protein Atilla/L1 (a lamellocyte surface marker) suggesting that entry occurs via a flotillin/lipid raft-dependent pathway. Once internalized, venosomes reach all intracellular compartments, including late and recycling endosomes, lysosomes, and the endoplasmic reticulum network. Venosomes therefore enter their target cells by a specific mechanism and the virulence factors are widely distributed in the lamellocytes' compartments to impair their functions.

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.

C-type lectin-mediated microbial homeostasis is critical for Helicoverpa armigera larval growth and development

The immune system of a host functions critically in shaping the composition of the microbiota, and some microbes are involved in regulating host endocrine system and development. However, whether the immune system acts on endocrine and development by shaping the composition of the microbiota remains unclear, and few molecular players or microbes involved in this process have been identified. In the current study, we found that RNA interference of a C-type lectin (HaCTL3) in the cotton bollworm Helicoverpa armigera suppresses ecdysone and juvenile hormone signaling, thus reducing larval body size and delaying pupation. Depletion of HaCTL3 also results in an increased abundance of Enterocuccus mundtii in the hemolymph, which may escape from the gut. Furthermore, HaCTL3 and its controlled antimicrobial peptides (attacin, lebocin, and gloverin) are involved in the clearance of E. mundtii from the hemolymph via phagocytosis or direct bactericidal activity. Injection of E. mundtii into larval hemocoel mimics HaCTL3-depleted phenotypes and suppresses ecdysone and juvenile hormone signaling. Taken together, we conclude that HaCTL3 maintains normal larval growth and development of H. armigera via suppressing the abundance of E. mundtii in the hemolymph. Our results provide the first evidence of an immune system acting on an endocrine system to modulate development via shaping the composition of microbiota in insect hemolymph. Thus, this study will deepen our understanding of the interaction between immunity and development.

Considering that a large number of hemocytes and multiple soluble effectors are present in insect hemolymph, it is conventionally believed that healthy insect hemolymph is a hostile environment for bacteria and is, therefore, sterile. However, increasing evidences disprove this opinion, although the interactive mechanism between hemolymph microbiota and insect host, as well as the function of hemolymph microbiota, remain unclear. C-type lectin (CTL), as pattern recognition receptor (PRR), plays important roles in defending against various pathogens. Here we found that various bacteria colonized the hemolymph of the cotton bollworm Helicoverpa armigera. We first reported that an H. armigera CTL (HaCTL3) was involved in modulating larval growth and development. Further study indicated that HaCTL3-mediated homeostasis of Enterocuccus mundtii in the hemolymph is critical for normal larval growth and development. Our study demonstrated that this PRR modulated insect development through shaping hemolymph microbiota, which may represent a novel mechanism of immune system regulation during insect development.

Single-cell transcriptome maps of myeloid blood cell lineages in Drosophila

The Drosophila lymph gland, the larval hematopoietic organ comprised of prohemocytes and mature hemocytes, has been a valuable model for understanding mechanisms underlying hematopoiesis and immunity. Three types of mature hemocytes have been characterized in the lymph gland: plasmatocytes, lamellocytes, and crystal cells, which are analogous to vertebrate myeloid cells, yet molecular underpinnings of the lymph gland hemocytes have been less investigated. Here, we use single-cell RNA sequencing to comprehensively analyze heterogeneity of developing hemocytes in the lymph gland, and discover previously undescribed hemocyte types including adipohemocytes, stem-like prohemocytes, and intermediate prohemocytes. Additionally, we identify the developmental trajectory of hemocytes during normal development as well as the emergence of the lamellocyte lineage following active cellular immunity caused by wasp infestation. Finally, we establish similarities and differences between embryonically derived- and larval lymph gland hemocytes. Altogether, our study provides detailed insights into the hemocyte development and cellular immune responses at single-cell resolution.

How the Drosophila lymph gland hemocytes develop and are regulated at a single-cell level is unclear. Here, the authors use single-cell RNA sequencing to show heterogeneity of developing hemocytes in the lymph gland and how they react to wasp infestation, and compare hemocytes from two independent origins.

Identification of reference markers for characterizing honey bee (Apis mellifera) hemocyte classes

Cell mediated immunity of the honey bee (Apis mellifera) involves the activity of several hemocyte populations, currently defined by morphological features and lectin binding characteristics. The objective of the present study was to identify molecular markers capable of characterizing subsets of honey bee hemocytes. We developed and employed monoclonal antibodies with restricted reactions to functionally distinct hemocyte subpopulations. Melanizing cells, known as oenocytoids, were defined by an antibody to prophenoloxidase, aggregating cells were identified by the expression of Hemolectin, and phagocytic cells were identified by a marker expressed on granulocytes. We anticipate that this combination of antibodies not only allows for the detection of functionally distinct hemocyte subtypes, but will help to further the exploration of hematopoietic compartments, as well as reveal details of the honey bee cellular immune defense against parasites and microbes.

Temporal specificity and heterogeneity of Drosophila immune cells

Immune cells provide defense against non-self and have recently been shown to also play key roles in diverse processes such as development, metabolism, and tumor progression. The heterogeneity of Drosophila immune cells (hemocytes) remains an open question. Using bulk RNA sequencing, we find that the hemocytes display distinct features in the embryo, a closed and rapidly developing system, compared to the larva, which is exposed to environmental and metabolic challenges. Through single-cell RNA sequencing, we identify fourteen hemocyte clusters present in unchallenged larvae and associated with distinct processes, e.g., proliferation, phagocytosis, metabolic homeostasis, and humoral response. Finally, we characterize the changes occurring in the hemocyte clusters upon wasp infestation, which triggers the differentiation of a novel hemocyte type, the lamellocyte. This first molecular atlas of hemocytes provides insights and paves the way to study the biology of the Drosophila immune cells in physiological and pathological conditions.