The blood cells, or hemocytes, in Drosophila participate in the immune response through the production of antimicrobial peptides, the phagocytosis of bacteria, and the encapsulation of larger foreign particles such as parasitic eggs; these immune reactions are mediated by phylogenetically conserved mechanisms. The encapsulation reaction is analogous to the formation of granuloma in vertebrates, and is mediated by large specialized cells, the lamellocytes. The origin of the lamellocytes has not been formally established, although it has been suggested that they are derived from the lymph gland, which is generally considered to be the main hematopoietic organ in the Drosophila larva. However, it was recently observed that a subepidermal population of sessile blood cells is released into the circulation in response to a parasitoid wasp infection. We set out to analyze this phenomenon systematically. As a result, we define the sessile hemocytes as a novel hematopoietic compartment, and the main source of lamellocytes.

Eater is an EGF-like repeat transmembrane receptor of the Nimrod family and is expressed in Drosophila hemocytes. Eater was initially identified for its role in phagocytosis of both Gram-positive and Gram-negative bacteria. We have deleted eater and show that it appears to be required for efficient phagocytosis of Gram-positive but not Gram-negative bacteria. However, the most striking phenotype of eater deficient larvae is the near absence of sessile hemocytes, both plasmatocyte and crystal cell types. The eater deletion is the first loss of function mutation identified that causes absence of the sessile hemocyte state. Our study shows that Eater is required cell-autonomously in plasmatocytes for sessility. However, the presence of crystal cells in the sessile compartment requires Eater in plasmatocytes. We also show that eater deficient hemocytes exhibit a cell adhesion defect. Collectively, our data uncovers a new requirement of Eater in enabling hemocyte attachment at the sessile compartment and points to a possible role of Nimrod family members in hemocyte adhesion.

The NimC1 molecule has been described as a phagocytosis receptor, and is being used as a marker for professional phagocytes, the plasmatocytes, in Drosophila melanogaster. In studies including tumor-biology, developmental biology, and cell mediated immunity, monoclonal antibodies (P1a and P1b) to the NimC1 antigen are used. As we observed that these antibodies did not react with plasmatocytes of several strains and genetic combinations, a molecular analysis was performed on the structure of the nimC1 gene. In these strains we found 2 deletions and an insertion within the nimC1 gene, which may result in the production of a truncated NimC1 protein. The NimC1 positivity was regained by recombining the mutation with a wild-type allele or by using nimC1 mutant lines under heterozygous conditions. By means of these procedures or using the recombined stock, NimC1 can be used as a marker for phagocytic cells in the majority of the possible genetic backgrounds.

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 withDrosophilagenetics, may now change this picture. This review aims to make sense of recently published data, focusing onDrosophila melanogasterand 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 inDrosophilaand granular cells in other insects. (2) A conserved class of cells that control melanin deposition around parasites and wounds, called crystal cells inD. melanogaster, and oenocytoids in other insects. (3) A new class of cells, the primocytes, so far only identified inD. 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 inD. 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.

We identified and characterized a so far unrecognized cell type, dubbed the multinucleated giant hemocyte (MGH), in the ananassae subgroup of Drosophilidae. Here, we describe the functional and ultrastructural characteristics of this novel blood cell type as well as its characterization with a set of discriminative immunological markers. MGHs are encapsulating cells that isolate and kill the parasite without melanization. They share some properties with but differ considerably from lamellocytes, the encapsulating cells of Drosophila melanogaster, the broadly used model organism in studies of innate immunity. MGHs are nonproliferative effector cells that are derived from phagocytic cells of the sessile tissue and the circulation, but do not exhibit phagocytic activity. In contrast to lamellocytes, MGHs are gigantic cells with filamentous projections and contain many nuclei, which are the result of the fusion of several cells. Although the structure of lamellocytes and MGHs differ remarkably, their function in the elimination of parasites is similar, which is potentially the result of the convergent evolution of interactions between hosts and parasites in different geographic regions. MGHs are highly motile and share several features with mammalian multinucleated giant cells, a syncytium of macrophages formed during granulomatous inflammation.