Cellular Immune Response Involving Multinucleated Giant Hemocytes with Two-Step Genome Amplification in the Drosophilid Zaprionus indianus

Cellular Immunity of Drosophila willistoni Reveals Novel Complexity in Insect Anti-Parasitoid Defense

Coevolution of hosts and their parasites has shaped heterogeneity of effector hemocyte types, providing immune defense reactions with variable effectiveness. In this work, we characterize hemocytes of Drosophila willistoni, a species that has evolved a cellular immune system with extensive variation and a high degree of plasticity. Monoclonal antibodies were raised and used in indirect immunofluorescence experiments to characterize hemocyte subpopulations, follow their functional features and differentiation. Pagocytosis and parasitization assays were used to determine the functional characteristics of hemocyte types. Samples were visualized using confocal and epifluorescence microscopy. We identified a new multinucleated giant hemocyte (MGH) type, which differentiates in the course of the cellular immune response to parasitoids. These cells differentiate in the circulation through nuclear division and cell fusion, and can also be derived from the central hematopoietic organ, the lymph gland. They have a binary function as they take up bacteria by phagocytosis and are involved in the encapsulation and elimination of the parasitoid. Here, we show that, in response to large foreign particles, such as parasitoids, MGHs differentiate, have a binary function and contribute to a highly effective cellular immune response, similar to the foreign body giant cells of vertebrates.

Germline and Somatic Cell Syncytia in Insects

Syncytia are common in the animal and plant kingdoms both under normal and pathological conditions. They form through cell fusion or division of a founder cell without cytokinesis. A particular type of syncytia occurs in invertebrate and vertebrate gametogenesis when the founder cell divides several times with partial cytokinesis producing a cyst (nest) of germ line cells connected by cytoplasmic bridges. The ultimate destiny of the cyst's cells differs between animal groups. Either all cells of the cyst become the gametes or some cells endoreplicate or polyploidize to become the nurse cells (trophocytes). Although many types of syncytia are permanent, the germ cell syncytium is temporary, and eventually, it separates into individual gametes. In this chapter, we give an overview of syncytium types and focus on the germline and somatic cell syncytia in various groups of insects. We also describe the multinuclear giant cells, which form through repetitive nuclear divisions and cytoplasm hypertrophy, but without cell fusion, and the accessory nuclei, which bud off the oocyte nucleus, migrate to its cortex and become included in the early embryonic syncytium.

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.

Differential immune responses in new and old fruit fly-parasitoid associations: Implications for their management

The oriental fruit fly, Bactrocera dorsalis (Hendel), and marula fruit fly, Ceratitis cosyra (Walker), are major fruit-infesting tephritids across sub-Saharan Africa. Biological control of these pests using parasitic wasps has been widely adopted but with varying levels of success. Most studies investigating host-parasitoid models have focused on functional and evolutionary aspects leaving a knowledge gap about the physiological mechanisms underpinning the efficacy of parasitoids as biocontrol agents of tephritids. To better understand these physiological mechanisms, we investigated changes in the cellular immune responses of C. cosyra and B. dorsalis when exposed to the parasitic wasps, Diachasmimorpha longicaudata (Ashmaed) and Psyttalia cosyrae (Wilkinson). We found that B. dorsalis was more resistant to parasitisation, had a higher hemocyte count, and encapsulated more parasitoid eggs compared to C. cosyra, achieving up to 100% encapsulation when exposed to P. cosyrae. Exposing B. dorsalis to either parasitoid species induced the formation of a rare cell type, the giant multinucleated hemocyte, which was not observed in C. cosyra. Furthermore, compared to P. cosyrae-parasitized larvae, those of both host species parasitized by D. longicaudata had lower encapsulation rates, hemocyte counts and spreading abilities and yielded a higher number of parasitoid progeny with the highest parasitoid emergence (72.13%) recorded in C. cosyra. These results demonstrate that cellular immune responses are central to host-parasitoid interaction in tephritid fruit flies and further suggest that D. longicaudata presents greater potential as a biocontrol agent of B. dorsalis and C. cosyra in horticultural cropping systems.

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.

Adaptations and counter-adaptations in Drosophila host-parasitoid interactions: advances in the molecular mechanisms

Both hosts and parasitoids evolved a diverse array of traits and strategies for their antagonistic interactions, affecting their chances of encounter, attack and survival after parasitoid attack. This review summarizes the recent progress that has been made in elucidating the molecular mechanisms of these adaptations and counter-adaptations in various Drosophila host-parasitoid interactions. For the hosts, it focuses on the neurobiological and genetic control of strategies in Drosophila adults and larvae of avoidance or escape behaviours upon sensing the parasitoids, and the immunological defences involving diverse classes of haemocytes. For the parasitoids, it highlights their behavioural strategies in host finding, as well as the rich variety of venom components that evolved and were partially acquired through horizontal gene transfer. Recent studies revealed the mechanisms by which these venom components manipulate their parasitized hosts in exhibiting escape behaviour to avoid superparasitism, and their counter-strategies to evade or obstruct the hosts' immunological defences.

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.