The Nimrod transmembrane receptor Eater is required for hemocyte attachment to the sessile compartment in Drosophila melanogaster

Transdifferentiation of plasmatocytes to crystal cells in the lymph gland of Drosophila melanogaster

Under homeostatic conditions, haematopoiesis in Drosophila larvae occurs in the lymph gland and sessile haemocyte clusters to produce two functionally and morphologically different cells: plasmatocytes and crystal cells. It is well-established that in the lymph gland both cell types stem from a binary decision of the medullary prohaemocyte precursors. However, in sessile clusters and dorsal vessel, crystal cells have been shown to originate from the transdifferentiation of plasmatocytes in a Notch/Serrate-dependent manner. We show that transdifferentiation occurs also in the lymph gland. In vivo phagocytosis assays confirm that cortical plasmatocytes are functionally differentiated phagocytic cells. We uncover a double-positive population in the cortical zone that lineage-tracing and long-term live imaging experiments show will differentiate into crystal cells. The reduction of Notch levels within the lymph gland plasmatocyte population reduces crystal cell number. This extension of a transdifferentiation mechanism reinforces the growing role of haematopoietic plasticity in maintaining homeostasis in Drosophila and vertebrate systems. Future work should test the regulation and relative contribution of these two processes under different immunological and/or metabolic conditions.

Drosophila immune cells transport oxygen through PPO2 protein phase transition

Insect respiration has long been thought to be solely dependent on an elaborate tracheal system without assistance from the circulatory system or immune cells1,2. Here we describe that Drosophila crystal cells-myeloid-like immune cells called haemocytes-control respiration by oxygenating Prophenoloxidase 2 (PPO2) proteins. Crystal cells direct the movement of haemocytes between the trachea of the larval body wall and the circulation to collect oxygen. Aided by copper and a neutral pH, oxygen is trapped in the crystalline structures of PPO2 in crystal cells. Conversely, PPO2 crystals can be dissolved when carbonic anhydrase lowers the intracellular pH and then reassembled into crystals in cellulo by adhering to the trachea. Physiologically, larvae lacking crystal cells or PPO2, or those expressing a copper-binding mutant of PPO2, display hypoxic responses under normoxic conditions and are susceptible to hypoxia. These hypoxic phenotypes can be rescued by hyperoxia, expression of arthropod haemocyanin or prevention of larval burrowing activity to expose their respiratory organs. Thus, we propose that insect immune cells collaborate with the tracheal system to reserve and transport oxygen through the phase transition of PPO2 crystals, facilitating internal oxygen homeostasis in a process that is comparable to vertebrate respiration.

A specific innate immune response silences the virulence of Pseudomonas aeruginosa in a latent infection model in the Drosophila melanogaster host

Microbial pathogenicity often depends on the route of infection. For instance, P. aeruginosa or S. marcescens cause acute systemic infections when low numbers of bacteria are injected into D. melanogaster flies whereas flies succumb much slower to the continuous ingestion of these pathogens, even though both manage to escape from the gut compartment and reach the hemocoel. Here, we have developed a latent P. aeruginosa infection model by feeding flies on the bacteria for a short period. The bacteria stably colonize internal tissues yet hardly cause any damage since latently-infected flies live almost as long as noninfected control flies. The apparently dormant bacteria display particular characteristics in terms of bacterial colony morphology, composition of the outer cell wall, and motility. The virulence of these bacteria can however be reactivated upon wounding the host. We show that melanization but not the cellular or the systemic humoral response is the predominant host defense that establishes latency and may coerce the bacteria to a dormant state. In addition, the lasting activation of the melanization responses in latently-infected flies provides a degree of protection to the host against a secondary fungal infection. Latent infection by an ingested pathogen protects against a variety of homologous or heterologous systemic secondary infectious challenges, a situation previously described for the endosymbiotic Wolbachia bacteria, a guard against viral infections.

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.

Mosquito pericardial cells upregulate Cecropin expression after an immune challenge

Most mosquito-transmitted pathogens grow or replicate in the midgut before invading the salivary glands. Pathogens are exposed to several immunological factors along the way. Recently, it was shown that hemocytes gather near the periostial region of the heart to efficiently phagocytose pathogens circulating in the hemolymph. Nerveless, not all pathogens can be phagocyted by hemocytes and eliminated by lysis. Interestingly, some studies have shown that pericardial cells (PCs) surrounding periostial regions, may produce humoral factors, such as lysozymes. Our current work provides evidence that Anopheles albimanus PCs are a major producer of Cecropin 1 (Cec1). Furthermore, our findings reveal that after an immunological challenge, PCs upregulate Cec1 expression. We conclude that PCs are positioned in a strategic location that could allow releasing humoral components, such as cecropin, to lyse pathogens on the heart or circulating in the hemolymph, implying that PCs could play a significant role in the systemic immune response.

A clash on the Toll pathway: competitive action between pesticides and zymosan A on components of innate immunity in Apis mellifera

Background

The immune system of honeybees includes multiple pathways that may be affected by pesticide exposure decreasing the immune competencies of bees and increasing their susceptibility to diseases like the fungal Nosema spp. infection, which is detected in collapsed colonies.

Methods

To better understand the effect of the co-presence of multiple pesticides that interact with bees like imidacloprid and amitraz, we evaluated the expression of immune-related genes in honeybee hemocytes.

Results

Imidacloprid, amitraz, and the immune activator, zymosan A, mainly affect the gene expression in the Toll pathway.

Discussion

Imidacloprid, amitraz, and zymosan A have a synergistic or an antagonistic relationship on gene expression depending on the level of immune signaling. The presence of multiple risk factors like pesticides and pathogens requires the assessment of their complex interaction, which has differential effects on the innate immunity of honeybees as seen in this study.

Drosophila immune priming to Enterococcus faecalis relies on immune tolerance rather than resistance

Innate immune priming increases an organism's survival of a second infection after an initial, non-lethal infection. We used Drosophila melanogaster and an insect-derived strain of Enterococcus faecalis to study transcriptional control of priming. In contrast to other pathogens, the enhanced survival in primed animals does not correlate with decreased E. faecalis load. Further analysis shows that primed organisms tolerate, rather than resist infection. Using RNA-seq of immune tissues, we found many genes were upregulated in only primed flies, suggesting a distinct transcriptional program in response to initial and secondary infections. In contrast, few genes continuously express throughout the experiment or more efficiently re-activate upon reinfection. Priming experiments in immune deficient mutants revealed Imd is largely dispensable for responding to a single infection but needed to fully prime. Together, this indicates the fly's innate immune response is plastic-differing in immune strategy, transcriptional program, and pathway use depending on infection history.

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.

The immune deficiency and c-Jun N-terminal kinase pathways drive the functional integration of the immune and circulatory systems of mosquitoes

The immune and circulatory systems of animals are functionally integrated. In mammals, the spleen and lymph nodes filter and destroy microbes circulating in the blood and lymph, respectively. In insects, immune cells that surround the heart valves (ostia), called periostial haemocytes, destroy pathogens in the areas of the body that experience the swiftest haemolymph (blood) flow. An infection recruits additional periostial haemocytes, amplifying heart-associated immune responses. Although the structural mechanics of periostial haemocyte aggregation have been defined, the genetic factors that regulate this process remain less understood. Here, we conducted RNA sequencing in the African malaria mosquito, Anopheles gambiae, and discovered that an infection upregulates multiple components of the immune deficiency (IMD) and c-Jun N-terminal kinase (JNK) pathways in the heart with periostial haemocytes. This upregulation is greater in the heart with periostial haemocytes than in the circulating haemocytes or the entire abdomen. RNA interference-based knockdown then showed that the IMD and JNK pathways drive periostial haemocyte aggregation and alter phagocytosis and melanization on the heart, thereby demonstrating that these pathways regulate the functional integration between the immune and circulatory systems. Understanding how insects fight infection lays the foundation for novel strategies that could protect beneficial insects and harm detrimental ones.

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.

Carbohydrate metabolism is a determinant for the host specificity of baculovirus infections

Baculoviruses Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and Bombyx mori nucleopolyhedrovirus (BmNPV) have highly similar genome sequences but exhibit no overlap in their host range. After baculovirus infects nonpermissive larvae (e.g., AcMNPV infecting B. mori or BmNPV infecting Spodoptera litura), we found that stored carbohydrates, including hemolymph trehalose and fat body glycogen, are rapidly transformed into glucose; enzymes involved in glycolysis and the TCA cycle are upregulated and produce more ATP; adenosine signaling that regulates glycolytic activity is also increased. Subsequently, phagocytosis in cellular immunity and the expression of genes involved in humoral immunity increase significantly. Moreover, inhibiting glycolysis and the expression of gloverins in nonpermissive hosts increased baculovirus infectivity, indicating that the stimulated energy production is designed to support the immune response against infection. Our study highlights that alteration of the host's carbohydrate metabolism is an important factor determining the host specificity of baculoviruses, in addition to viral factors.

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Nonpermissive infections by AcMNPV and BmNPV alter host carbohydrate metabolism

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Increased carbohydrate metabolism produces energy to launch immune responses

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Immune responses including antimicrobial peptide production inhibit virus infection

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Host metabolic alterations affect the determination of virus's host specificity

Identification and the immunological role of two Nimrod family genes in the silkworm, Bombyx mori

In animals, immune signaling pathways and effector molecules participate in attenuating microbial infection. Recent work has shown that the Nimrod family proteins can directly bind to bacteria, and this binding leads to bacterial phagocytosis. Although the Nimrod gene family has been reported in many non-drosophilids, their functions remain unexplored in most insect species. Here, we report two members (Nimrod-B and Draper) of the Nimrod gene family from Bombyx mori and analyzed their role in immunity. The two genes were ubiquitously expressed in the tested tissues; but, they transcribed preferentially in immune tissues. The developmental profiles showed that BmNimrod-B and BmDraper transcription levels were highest in the pupal stages. Challenge with microbial pathogens induced the transcription levels of all two genes at different time points. Knockdown of BmDraper decreased the bacterial clearance and increased their replication relative to the control group, whereas, BmNimrod-B suppression had a non-significant effect on them. Furthermore, the mortality rate was increased after BmDraper silencing. The knockdown of these genes did not significantly affect the production of antimicrobial peptides following E. coli infection. Taken together, the Nimrod family genes play a crucial role in host defense by positively regulating the antibacterial immune response in silkworm B. mori.

Pvr and distinct downstream signaling factors are required for hemocyte spreading and epidermal wound closure at Drosophila larval wound sites

Tissue injury is typically accompanied by inflammation. In Drosophila melanogaster larvae, wound-induced inflammation involves adhesive capture of hemocytes at the wound surface followed by hemocyte spreading to assume a flat, lamellar morphology. The factors that mediate this cell spreading at the wound site are not known. Here, we discover a role for the platelet-derived growth factor/vascular endothelial growth factor-related receptor (Pvr) and its ligand, Pvf1, in blood cell spreading at the wound site. Pvr and Pvf1 are required for spreading in vivo and in an in vitro spreading assay where spreading can be directly induced by Pvf1 application or by constitutive Pvr activation. In an effort to identify factors that act downstream of Pvr, we performed a genetic screen in which select candidates were tested to determine if they could suppress the lethality of Pvr overexpression in the larval epidermis. Some of the suppressors identified are required for epidermal wound closure (WC), another Pvr-mediated wound response, some are required for hemocyte spreading in vitro, and some are required for both. One of the downstream factors, Mask, is also required for efficient wound-induced hemocyte spreading in vivo. Our data reveal that Pvr signaling is required for wound responses in hemocytes (cell spreading) and defines distinct downstream signaling factors that are required for either epidermal WC or hemocyte spreading.

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.

Yorkie-Cactus (IκBα)-JNK axis promotes tumor growth and progression in Drosophila

Presence of inflammatory factors in the tumor microenvironment is well-documented yet their specific role in tumorigenesis is elusive. The core inflammatory pathways like the Toll-Like Receptor (TLR) and the Tumor Necrosis Factor (TNF) pathway are conserved in Drosophila. We induced GFP-marked epithelial tumors by expressing activated oncogenic forms of RasV12 or Yorkie (Yki3SA, mammalian YAP) in scribble deficient cells (scribRNAi, mammalian SCRIB) to study the role of inflammatory factors in tumorigenesis. Similar to RasV12scribRNAi, we found that Yki3SAscribRNAi form invasive neoplastic lethal tumors that induce a systemic inflammatory response. We identified Cactus (Cact, mammalian IκBα), the negative regulator of TLR, as a key player in tumor growth. Cact accumulates in the cytoplasm in Drosophila tumor models, similar to squamous cell carcinoma in mice models and human patients where cytoplasmic IκBα favors oncogenic transformation. Further, cact is transcriptionally upregulated in tumors, and downregulation of Cact affects tumor growth. We investigated if TLR or TNF pathway affect tumor growth through activation of Jun N-terminal Kinase (JNK) pathway and its target Matrix Metalloprotease1 (MMP1). Genetically manipulating levels of TLR components or TNF receptors showed that Cact acts upstream of JNK signaling and regulates JNK via a non-canonical mechanism during tumorigenesis. Further, Hippo coactivator Yki transcriptionally regulates cact expression, and downregulation of Yki or Cact is sufficient to cause downregulation of JNK-mediated signaling that promotes tumorigenesis. Here, we report a link between Hippo, IκBα and JNK signaling that may induce inflammation and innate immune response in tumorigenesis.

Regulators and signalling in insect antimicrobial innate immunity: Functional molecules and cellular pathways

Insects possess an immune system that protects them from attacks by various pathogenic microorganisms that would otherwise threaten their survival. Immune mechanisms may deal directly with the pathogens by eliminating them from the host organism or disarm them by suppressing the synthesis of toxins and virulence factors that promote the invasion and destructive action of the intruder within the host. Insects have been established as outstanding models for studying immune system regulation because innate immunity can be explored as an integrated system at the level of the whole organism. Innate immunity in insects consists of basal immunity that controls the constitutive synthesis of effector molecules such as antimicrobial peptides, and inducible immunity that is activated after detection of a microbe or its product(s). Activation and coordination of innate immune defenses in insects involve evolutionary conserved immune factors. Previous research in insects has led to the identification and characterization of distinct immune signalling pathways that modulate the response to microbial infections. This work has not only advanced the field of insect immunology, but it has also rekindled interest in the innate immune system of mammals. Here we review the current knowledge on key molecular components of insect immunity and discuss the opportunities they present for confronting infectious diseases in humans.

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.

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.

The Leukemic Fly: Promises and Challenges

Leukemia involves different types of blood cancers, which lead to significant mortality and morbidity. Murine models of leukemia have been instrumental in understanding the biology of the disease and identifying therapeutics. However, such models are time consuming and expensive in high throughput genetic and drug screening. Drosophilamelanogaster has emerged as an invaluable in vivo model for studying different diseases, including cancer. Fruit flies possess several hematopoietic processes and compartments that are in close resemblance to their mammalian counterparts. A number of studies succeeded in characterizing the fly's response upon the expression of human leukemogenic proteins in hematopoietic and non-hematopoietic tissues. Moreover, some of these studies showed that these models are amenable to genetic screening. However, none were reported to be tested for drug screening. In this review, we describe the Drosophila hematopoietic system, briefly focusing on leukemic diseases in which fruit flies have been used. We discuss myeloid and lymphoid leukemia fruit fly models and we further highlight their roles for future therapeutic screening. In conclusion, fruit fly leukemia models constitute an interesting area which could speed up the process of integrating new therapeutics when complemented with mammalian models.

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.

Drosophila as a Model Organism in Host-Pathogen Interaction Studies

Owing to the genetic similarities and conserved pathways between a fruit fly and mammals, the use of the Drosophila model as a platform to unveil novel mechanisms of infection and disease progression has been justified and widely instigated. Gaining proper insight into host-pathogen interactions and identifying chief factors involved in host defense and pathogen virulence in Drosophila serves as a foundation to establish novel strategies for infectious disease prevention and control in higher organisms, including humans.

Snellenius manilae bracovirus suppresses the host immune system by regulating extracellular adenosine levels in Spodoptera litura

Sufficient energy supply to the host immune system is important for resisting pathogens. Therefore, during pathogen infection, the host metabolism is reassigned from storage, growth, and development to the immune system. Previous studies in Drosophila melanogaster have demonstrated that systemic metabolic switching upon an immune challenge is activated by extracellular adenosine signaling, modulating carbohydrate mobilization and redistributing energy to the hemocytes. In the present study, we discovered that symbiotic virus (SmBV) of the parasitoid wasp Snellenius manilae is able to down-regulate the extracellular adenosine of its host, Spodoptera litura, to inhibit metabolism switching. The decreased carbohydrate mobilization, glycogenolysis, and ATP synthesis upon infection results in the host being unable to supply energy to its immune system, thus benefitting the development of wasp larvae. When we added adenosine to the infected S. litura larvae, we observed enhanced host immune responses that decreased the pupation rate of S. manilae. Previous studies showed that after pathogen infection, the host activates its adenosine pathway to trigger immune responses. However, our results suggest a different model: we found that in S. manilae, SmBV modulates the host adenosine pathway such that wasp eggs and larvae can evade the host immune response.

The Insect Circulatory System: Structure, Function, and Evolution

Although the insect circulatory system is involved in a multitude of vital physiological processes, it has gone grossly understudied. This review highlights this critical physiological system by detailing the structure and function of the circulatory organs, including the dorsal heart and the accessory pulsatile organs that supply hemolymph to the appendages. It also emphasizes how the circulatory system develops and ages and how, by means of reflex bleeding and functional integration with the immune system, it supports mechanisms for defense against predators and microbial invaders, respectively. Beyond that, this review details evolutionary trends and novelties associated with this system, as well as the ways in which this system also plays critical roles in thermoregulation and tracheal ventilation in high-performance fliers. Finally, this review highlights how novel discoveries could be harnessed for the control of vector-borne diseases and for translational medicine, and it details principal knowledge gaps that necessitate further investigation.

Pattern recognition receptors in Drosophila immune responses

Insects, which lack the adaptive immune system, have developed sophisticated innate immune system consisting of humoral and cellular immune responses to defend against invading microorganisms. Non-self recognition of microbes is the front line of the innate immune system. Repertoires of pattern recognition receptors (PRRs) recognize the conserved pathogen-associated molecular patterns (PAMPs) present in microbes, such as lipopolysaccharide (LPS), peptidoglycan (PGN), lipoteichoic acid (LTA) and β-1, 3-glucans, and induce innate immune responses. In this review, we summarize current knowledge of the structure, classification and roles of PRRs in innate immunity of the model organism Drosophila melanogaster, focusing mainly on the peptidoglycan recognition proteins (PGRPs), Gram-negative bacteria-binding proteins (GNBPs), scavenger receptors (SRs), thioester-containing proteins (TEPs), and lectins.

Adult Drosophila Lack Hematopoiesis but Rely on a Blood Cell Reservoir at the Respiratory Epithelia to Relay Infection Signals to Surrounding Tissues

The use of adult Drosophila melanogaster as a model for hematopoiesis or organismal immunity has been debated. Addressing this question, we identify an extensive reservoir of blood cells (hemocytes) at the respiratory epithelia (tracheal air sacs) of the thorax and head. Lineage tracing and functional analyses demonstrate that the majority of adult hemocytes are phagocytic macrophages (plasmatocytes) from the embryonic lineage that parallels vertebrate tissue macrophages. Surprisingly, we find no sign of adult hemocyte expansion. Instead, hemocytes play a role in relaying an innate immune response to the blood cell reservoir: through Imd signaling and the Jak/Stat pathway ligand Upd3, hemocytes act as sentinels of bacterial infection, inducing expression of the antimicrobial peptide Drosocin in respiratory epithelia and colocalizing fat body domains. Drosocin expression in turn promotes animal survival after infection. Our work identifies a multi-signal relay of organismal humoral immunity, establishing adult Drosophila as model for inter-organ immunity.

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

Eater and NimC1 are transmembrane receptors of the Drosophila Nimrod family, specifically expressed in haemocytes, the insect blood cells. Previous ex vivo and in vivoRNAi studies have pointed to their role in the phagocytosis of bacteria. Here, we have created a novel NimC1 null mutant to re-evaluate the role of NimC1, alone or in combination with Eater, in the cellular immune response. We show that NimC1 functions as an adhesion molecule ex vivo, but in contrast to Eater it is not required for haemocyte sessility in vivo. Ex vivo phagocytosis assays and electron microscopy experiments confirmed that Eater is the main phagocytic receptor for Gram-positive, but not Gram-negative bacteria, and contributes to microbe tethering to haemocytes. Surprisingly, NimC1 deletion did not impair phagocytosis of bacteria, nor their adhesion to the haemocytes. However, phagocytosis of both types of bacteria was almost abolished in NimC11 ;eater1 haemocytes. This indicates that both receptors contribute synergistically to the phagocytosis of bacteria, but that Eater can bypass the requirement for NimC1. Finally, we uncovered that NimC1, but not Eater, is essential for uptake of latex beads and zymosan particles. We conclude that Eater and NimC1 are the two main receptors for phagocytosis of bacteria in Drosophila, and that each receptor likely plays distinct roles in microbial uptake.

Phagocytosis in Drosophila: From molecules and cellular machinery to physiology

Phagocytosis is an evolutionarily conserved mechanism that plays a key role in both host defence and tissue homeostasis in multicellular organisms. A range of surface receptors expressed on different cell types allow discriminating between self and non-self (or altered) material, thus enabling phagocytosis of pathogens and apoptotic cells. The phagocytosis process can be divided into four main steps: 1) binding of the phagocyte to the target particle, 2) particle internalization and phagosome formation, through remodelling of the plasma membrane, 3) phagosome maturation, and 4) particle destruction in the phagolysosome. In this review, we describe our present knowledge on phagocytosis in the fruit fly Drosophila melanogaster, assessing each of the key steps involved in engulfment of both apoptotic cells and bacteria. We also assess the physiological role of phagocytosis in host defence, development and tissue homeostasis.

The Axial Organ and the Pharynx Are Sites of Hematopoiesis in the Sea Urchin

Background: The location of coelomocyte proliferation in adult sea urchins is unknown and speculations since the early 1800s have been based on microanatomy and tracer uptake studies. In adult sea urchins (Strongylocentrotus purpuratus) with down-regulated immune systems, coelomocyte numbers increase in response to immune challenge, and whether some or all of these cells are newly proliferated is not known. The gene regulatory network that encodes transcription factors that control hematopoiesis in embryonic and larval sea urchins has not been investigated in adults. Hence, to identify the hematopoietic tissue in adult sea urchins, cell proliferation, expression of phagocyte specific genes, and expression of genes encoding transcription factors that function in the conserved regulatory network that controls hematopoiesis in embryonic and larval sea urchins were investigated for several tissues. Results: Cell proliferation was induced in adult sea urchins either by immune challenge through injection of heat-killed Vibrio diazotrophicus or by cell depletion through aspiration of coelomic fluid. In response to either of these stimuli, newly proliferated coelomocytes constitute only about 10% of the cells in the coelomic fluid. In tissues, newly proliferated cells and cells that express SpTransformer proteins (formerly Sp185/333) that are markers for phagocytes are present in the axial organ, gonad, pharynx, esophagus, and gut with no differences among tissues. The expression level of genes encoding transcription factors that regulate hematopoiesis show that both the axial organ and the pharynx have elevated expression compared to coelomocytes, esophagus, gut, and gonad. Similarly, an RNAseq dataset shows similar results for the axial organ and pharynx, but also suggests that the axial organ may be a site for removal and recycling of cells in the coelomic cavity. Conclusions: Results presented here are consistent with previous speculations that the axial organ may be a site of coelomocyte proliferation and that it may also be a center for cellular removal and recycling. A second site, the pharynx, may also have hematopoietic activity, a tissue that has been assumed to function only as part of the intestinal tract.

Headcase is a Repressor of Lamellocyte Fate in Drosophila melanogaster

Due to the evolutionary conservation of the regulation of hematopoiesis, Drosophila provides an excellent model organism to study blood cell differentiation and hematopoietic stem cell (HSC) maintenance. The larvae of Drosophila melanogaster respond to immune induction with the production of special effector blood cells, the lamellocytes, which encapsulate and subsequently kill the invader. Lamellocytes differentiate as a result of a concerted action of all three hematopoietic compartments of the larva: the lymph gland, the circulating hemocytes, and the sessile tissue. Within the lymph gland, the communication of the functional zones, the maintenance of HSC fate, and the differentiation of effector blood cells are regulated by a complex network of signaling pathways. Applying gene conversion, mutational analysis, and a candidate based genetic interaction screen, we investigated the role of Headcase (Hdc), the homolog of the tumor suppressor HECA in the hematopoiesis of Drosophila. We found that naive loss-of-function hdc mutant larvae produce lamellocytes, showing that Hdc has a repressive role in effector blood cell differentiation. We demonstrate that hdc genetically interacts with the Hedgehog and the Decapentaplegic pathways in the hematopoietic niche of the lymph gland. By adding further details to the model of blood cell fate regulation in the lymph gland of the larva, our findings contribute to the better understanding of HSC maintenance.

Functional analysis of RIP toxins from the Drosophila endosymbiont Spiroplasma poulsonii

Background

Insects frequently live in close relationship with symbiotic bacteria that carry out beneficial functions for their host, like protection against parasites and viruses. However, in some cases, the mutualistic nature of such associations is put into question because of detrimental phenotypes caused by the symbiont. One example is the association between the vertically transmitted facultative endosymbiont Spiroplasma poulsonii and its natural host Drosophila melanogaster. Whereas S. poulsonii protects its host against parasitoid wasps and nematodes by the action of toxins from the family of Ribosome Inactivating Proteins (RIPs), the presence of S. poulsonii has been reported to reduce host’s life span and to kill male embryos by a toxin called Spaid. In this work, we investigate the harmful effects of Spiroplasma RIPs on Drosophila in the absence of parasite infection.

Results

We show that only two Spiroplasma RIPs (SpRIP1 and SpRIP2) among the five RIP genes encoded in the S. poulsonii genome are significantly expressed during the whole Drosophila life cycle. Heterologous expression of SpRIP1 and 2 in uninfected flies confirms their toxicity, as indicated by a reduction of Drosophila lifespan and hemocyte number. We also show that RIPs can cause the death of some embryos, including females.

Conclusion

Our results indicate that RIPs released by S. poulsonii contribute to the reduction of host lifespan and embryo mortality. This suggests that SpRIPs may impact the insect-symbiont homeostasis beyond their protective function against parasites.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1410-1) contains supplementary material, which is available to authorized users.

Drosophila as a Genetic Model for Hematopoiesis

In this FlyBook chapter, we present a survey of the current literature on the development of the hematopoietic system in Drosophila The Drosophila blood system consists entirely of cells that function in innate immunity, tissue integrity, wound healing, and various forms of stress response, and are therefore functionally similar to myeloid cells in mammals. The primary cell types are specialized for phagocytic, melanization, and encapsulation functions. As in mammalian systems, multiple sites of hematopoiesis are evident in Drosophila and the mechanisms involved in this process employ many of the same molecular strategies that exemplify blood development in humans. Drosophila blood progenitors respond to internal and external stress by coopting developmental pathways that involve both local and systemic signals. An important goal of these Drosophila studies is to develop the tools and mechanisms critical to further our understanding of human hematopoiesis during homeostasis and dysfunction.

Embryonic hematopoiesis modulates the inflammatory response and larval hematopoiesis in Drosophila

Recent lineage tracing analyses have significantly improved our understanding of immune system development and highlighted the importance of the different hematopoietic waves. The current challenge is to understand whether these waves interact and whether this affects the function of the immune system. Here we report a molecular pathway regulating the immune response and involving the communication between embryonic and larval hematopoietic waves in Drosophila. Down-regulating the transcription factor Gcm specific to embryonic hematopoiesis enhances the larval phenotypes induced by over-expressing the pro-inflammatory Jak/Stat pathway or by wasp infestation. Gcm works by modulating the transduction of the Upd cytokines to the site of larval hematopoiesis and hence the response to chronic (Jak/Stat over-expression) and acute (wasp infestation) immune challenges. Thus, homeostatic interactions control the function of the immune system in physiology and pathology. Our data also indicate that a transiently expressed developmental pathway has a long-lasting effect on the immune response.

Extraction of Hemocytes from Drosophila melanogaster Larvae for Microbial Infection and Analysis

During the pathogenic infection of Drosophila melanogaster, hemocytes play an important role in the immune response throughout the infection. Thus, the goal of this protocol is to develop a method to visualize the pathogen invasion in a specific immune compartment of flies, namely hemocytes. Using the method presented here, up to 3 × 10⁶ live hemocytes can be obtained from 200 Drosophila 3^rd instar larvae in 30 min for ex vivo infection. Alternatively, hemocytes can be infected in vivo through injection of 3^rd instar larvae followed by hemocyte extraction up to 24 h post-infection. These infected primary cells were fixed, stained, and imaged using confocal microscopy. Then, 3D representations were generated from the images to definitively show pathogen invasion. Additionally, high-quality RNA for qRT-PCR can be obtained for the detection of pathogen mRNA following infection, and sufficient protein can be extracted from these cells for Western blot analysis. Taken together, we present a method for definite reconciliation of pathogen invasion and confirmation of infection using bacterial and viral pathogen types and an efficient method for hemocyte extraction to obtain enough live hemocytes from Drosophila larvae for ex vivo and in vivo infection experiments.

The human Smoothened inhibitor PF-04449913 induces exit from quiescence and loss of multipotent Drosophila hematopoietic progenitor cells

The efficient treatment of hematological malignancies as Acute Myeloid Leukemia, myelofibrosis and Chronic Myeloid Leukemia, requires the elimination of cancer-initiating cells and the prevention of disease relapse through targeting pathways that stimulate generation and maintenance of these cells. In mammals, inhibition of Smoothened, the key mediator of the Hedgehog signaling pathway, reduces Chronic Myeloid Leukemia progression and propagation. These findings make Smo a candidate target to inhibit maintenance of leukemia-initiating cells. In Drosophila melanogaster the same pathway maintains the hematopoietic precursor cells of the lymph gland, the hematopoietic organ that develops in the larva. Using Drosophila as an in vivo model, we investigated the mode of action of PF-04449913, a small-molecule inhibitor of the human Smo protein. Drosophila larvae fed with PF-04449913 showed traits of altered hematopoietic homeostasis. These include the development of melanotic nodules, increase of circulating hemocytes, the size increase of the lymph gland and accelerated differentiation of blood cells likely due to the exit of multi-potent precursors from quiescence. Importantly, the Smo inhibition can lead to the complete loss of hematopoietic precursors. We conclude that PF-04449913 inhibits Drosophila Smo blocking the Hh signaling pathway and causing the loss of hematopoietic precursor cells. Interestingly, this is the effect expected in patients treated with PF-04449913: number decrease of cancer initiating cells in the bone marrow to reduce the risk of leukemia relapse. Altogether our results indicate that Drosophila comprises a model system for the in vivo study of molecules that target evolutionary conserved pathways implicated in human hematological malignancies.

Thioester-containing proteins regulate the Toll pathway and play a role in Drosophila defence against microbial pathogens and parasitoid wasps

BACKGROUND

Members of the thioester-containing protein (TEP) family contribute to host defence in both insects and mammals. However, their role in the immune response of Drosophila is elusive. In this study, we address the role of TEPs in Drosophila immunity by generating a mutant fly line, referred to as TEPq Δ , lacking the four immune-inducible TEPs, TEP1, 2, 3 and 4.

RESULTS

Survival analyses with TEPq Δ flies reveal the importance of these proteins in defence against entomopathogenic fungi, Gram-positive bacteria and parasitoid wasps. Our results confirm that TEPs are required for efficient phagocytosis of bacteria, notably for the two Gram-positive species tested, Staphylococcus aureus and Enterococcus faecalis. Furthermore, we show that TEPq Δ flies have reduced Toll pathway activation upon microbial infection, resulting in lower expression of antimicrobial peptide genes. Epistatic analyses suggest that TEPs function upstream or independently of the serine protease ModSP at an initial stage of Toll pathway activation.

CONCLUSIONS

Collectively, our study brings new insights into the role of TEPs in insect immunity. It reveals that TEPs participate in both humoral and cellular arms of immune response in Drosophila. In particular, it shows the importance of TEPs in defence against Gram-positive bacteria and entomopathogenic fungi, notably by promoting Toll pathway activation.

A Genetic Screen Reveals an Unexpected Role for Yorkie Signaling in JAK/STAT-Dependent Hematopoietic Malignancies in Drosophila melanogaster

A gain-of-function mutation in the tyrosine kinase JAK2 (JAK2^V617F) causes human myeloproliferative neoplasms (MPNs). These patients present with high numbers of myeloid lineage cells and have numerous complications. Since current MPN therapies are not curative, there is a need to find new regulators and targets of Janus kinase/Signal transducer and activator of transcription (JAK/STAT) signaling that may represent additional clinical interventions . Drosophila melanogaster offers a low complexity model to study MPNs as JAK/STAT signaling is simplified with only one JAK [Hopscotch (Hop)] and one STAT (Stat92E). hop^{Tumorous-lethal(Tum-l)} is a gain-of-function mutation that causes dramatic expansion of myeloid cells, which then form lethal melanotic tumors. Through an F1 deficiency (Df) screen, we identified 11 suppressors and 35 enhancers of melanotic tumors in hop^Tum-l animals. Dfs that uncover the Hippo (Hpo) pathway genes expanded (ex) and warts (wts) strongly enhanced the hop^Tum-l tumor burden, as did mutations in ex, wts, and other Hpo pathway genes. Target genes of the Hpo pathway effector Yorkie (Yki) were significantly upregulated in hop^Tum-l blood cells, indicating that Yki signaling was increased. Ectopic hematopoietic activation of Yki in otherwise wild-type animals increased hemocyte proliferation but did not induce melanotic tumors. However, hematopoietic depletion of Yki significantly reduced the hop^Tum-l tumor burden, demonstrating that Yki is required for melanotic tumors in this background. These results support a model in which elevated Yki signaling increases the number of hemocytes, which become melanotic tumors as a result of elevated JAK/STAT signaling.

Regulation of Drosophila hematopoietic sites by Activin-β from active sensory neurons

An outstanding question in animal development, tissue homeostasis and disease is how cell populations adapt to sensory inputs. During Drosophila larval development, hematopoietic sites are in direct contact with sensory neuron clusters of the peripheral nervous system (PNS), and blood cells (hemocytes) require the PNS for their survival and recruitment to these microenvironments, known as Hematopoietic Pockets. Here we report that Activin-β, a TGF-β family ligand, is expressed by sensory neurons of the PNS and regulates the proliferation and adhesion of hemocytes. These hemocyte responses depend on PNS activity, as shown by agonist treatment and transient silencing of sensory neurons. Activin-β has a key role in this regulation, which is apparent from reporter expression and mutant analyses. This mechanism of local sensory neurons controlling blood cell adaptation invites evolutionary parallels with vertebrate hematopoietic progenitors and the independent myeloid system of tissue macrophages, whose regulation by local microenvironments remain undefined.

Macrophage Functions in Tissue Patterning and Disease: New Insights from the Fly

Macrophages are multifunctional innate immune cells that seed all tissues within the body and play disparate roles throughout development and in adult tissues, both in health and disease. Their complex developmental origins and many of their functions are being deciphered in mammalian tissues, but opportunities for live imaging and the genetic tractability of Drosophila are offering complementary insights into how these fascinating cells integrate a multitude of guidance cues to fulfill their many tasks and migrate to distant sites to either direct developmental patterning or raise an inflammatory response.

Advances in Myeloid-Like Cell Origins and Functions in the Model Organism Drosophila melanogaster

Drosophila has long served as a valuable model for deciphering many biological processes, including immune responses. Indeed, the genetic tractability of this organism is particularly suited for large-scale analyses. Studies performed during the last 3 decades have proven that the signaling pathways that regulate the innate immune response are conserved between Drosophila and mammals. This review summarizes the recent advances on Drosophila hematopoiesis and immune cellular responses, with a particular emphasis on phagocytosis.

The Drosophila CD36 Homologue croquemort Is Required to Maintain Immune and Gut Homeostasis during Development and Aging

Phagocytosis is an ancient mechanism central to both tissue homeostasis and immune defense. Both the identity of the receptors that mediate bacterial phagocytosis and the nature of the interactions between phagocytosis and other defense mechanisms remain elusive. Here, we report that Croquemort (Crq), a Drosophila member of the CD36 family of scavenger receptors, is required for microbial phagocytosis and efficient bacterial clearance. Flies mutant for crq are susceptible to environmental microbes during development and succumb to a variety of microbial infections as adults. Crq acts parallel to the Toll and Imd pathways to eliminate bacteria via phagocytosis. crq mutant flies exhibit enhanced and prolonged immune and cytokine induction accompanied by premature gut dysplasia and decreased lifespan. The chronic state of immune activation in crq mutant flies is further regulated by negative regulators of the Imd pathway. Altogether, our data demonstrate that Crq plays a key role in maintaining immune and organismal homeostasis.

Drosophila hematopoiesis under normal conditions and in response to immune stress

The emergence of hematopoietic progenitors and their differentiation into various highly specialized blood cell types constitute a finely tuned process. Unveiling the genetic cascades that control blood cell progenitor fate and understanding how they are modulated in response to environmental changes are two major challenges in the field of hematopoiesis. In the last 20 years, many studies have established important functional analogies between blood cell development in vertebrates and in the fruit fly, Drosophila melanogaster. Thereby, Drosophila has emerged as a powerful genetic model for studying mechanisms that control hematopoiesis during normal development or in pathological situations. Moreover, recent advances in Drosophila have highlighted how intricate cell communication networks and microenvironmental cues regulate blood cell homeostasis. They have also revealed the striking plasticity of Drosophila mature blood cells and the presence of different sites of hematopoiesis in the larva. This review provides an overview of Drosophila hematopoiesis during development and summarizes our current knowledge on the molecular processes controlling larval hematopoiesis, both under normal conditions and in response to an immune challenge, such as wasp parasitism.

Transdifferentiation and Proliferation in Two Distinct Hemocyte Lineages in Drosophila melanogaster Larvae after Wasp Infection

Cellular immune responses require the generation and recruitment of diverse blood cell types that recognize and kill pathogens. In Drosophila melanogaster larvae, immune-inducible lamellocytes participate in recognizing and killing parasitoid wasp eggs. However, the sequence of events required for lamellocyte generation remains controversial. To study the cellular immune system, we developed a flow cytometry approach using in vivo reporters for lamellocytes as well as for plasmatocytes, the main hemocyte type in healthy larvae. We found that two different blood cell lineages, the plasmatocyte and lamellocyte lineages, contribute to the generation of lamellocytes in a demand-adapted hematopoietic process. Plasmatocytes transdifferentiate into lamellocyte-like cells in situ directly on the wasp egg. In parallel, a novel population of infection-induced cells, which we named lamelloblasts, appears in the circulation. Lamelloblasts proliferate vigorously and develop into the major class of circulating lamellocytes. Our data indicate that lamellocyte differentiation upon wasp parasitism is a plastic and dynamic process. Flow cytometry with in vivo hemocyte reporters can be used to study this phenomenon in detail.

Macrophages and cellular immunity in Drosophila melanogaster

The invertebrate Drosophila melanogaster has been a powerful model for understanding blood cell development and immunity. Drosophila is a holometabolous insect, which transitions through a series of life stages from embryo, larva and pupa to adulthood. In spite of this, remarkable parallels exist between Drosophila and vertebrate macrophages, both in terms of development and function. More than 90% of Drosophila blood cells (hemocytes) are macrophages (plasmatocytes), making this highly tractable genetic system attractive for studying a variety of questions in macrophage biology. In vertebrates, recent findings revealed that macrophages have two independent origins: self-renewing macrophages, which reside and proliferate in local microenvironments in a variety of tissues, and macrophages of the monocyte lineage, which derive from hematopoietic stem or progenitor cells. Like vertebrates, Drosophila possesses two macrophage lineages with a conserved dual ontogeny. These parallels allow us to take advantage of the Drosophila model when investigating macrophage lineage specification, maintenance and amplification, and the induction of macrophages and their progenitors by local microenvironments and systemic cues. Beyond macrophage development, Drosophila further serves as a paradigm for understanding the mechanisms underlying macrophage function and cellular immunity in infection, tissue homeostasis and cancer, throughout development and adult life.