We have investigated the blood cell types present in Drosophila at postembryonic stages and have analysed their modifications during development and under immune conditions. The anterior lobes of the larval hematopoietic organ or lymph gland contain numerous active secretory cells, plasmatocytes, few crystal cells, and a number of undifferentiated prohemocytes. The posterior lobes contain essentially prohemocytes. The blood cell population in larval hemolymph differs and consists mainly of plasmatocytes which are phagocytes, and of a low percentage of crystal cells which reportedly play a role in humoral melanisation. We show that the cells in the lymph gland can differentiate into a given blood cell lineage when solicited. Under normal nonimmune conditions, we observe a massive differentiation into active macrophages at the onset of metamorphosis in all lobes. Simultaneously, circulating plasmatocytes modify their adhesion and phagocytic properties to become pupal macrophages. All phagocytic cells participate in metamorphosis by ingesting doomed larval tissues. The most dramatic effect on larval hematopoiesis was observed following infestation by a parasitoid wasp. Cells within all lymph gland lobes, including prohemocytes from posterior lobes, massively differentiate into a new cell type specifically devoted to encapsulation, the lamellocyte.

Drosophila haemocytes (blood cells) originate from a specialized haematopoietic organ-the lymph gland. Larval haematopoietic progenitors (prohaemocytes) give rise to three types of circulating haemocytes: plasmatocytes, crystal cells and lamellocytes. Lamellocytes, which are devoted to encapsulation of large foreign bodies, only differentiate in response to specific immune threats, such as parasitization by wasps. Here we show that a small cluster of signalling cells, termed the PSC (posterior signalling centre), controls the balance between multipotent prohaemocytes and differentiating haemocytes, and is necessary for the massive differentiation of lamellocytes that follows parasitization. Communication between the PSC and haematopoietic progenitors strictly depends on the PSC-restricted expression of Collier, the Drosophila orthologue of mammalian early B-cell factor. PSC cells act, in a non-cell-autonomous manner, to maintain JAK/STAT signalling activity in prohaemocytes, preventing their premature differentiation. Serrate-mediated Notch signalling from the PSC is required to maintain normal levels of col transcription. The key role of the PSC in controlling blood cell homeostasis is reminiscent of interactions between haematopoietic progenitors and their micro-environment in vertebrates, thus further highlighting the interest of Drosophila as a model system for studying the evolution of haematopoiesis and cellular innate immunity.

In this paper we report a recessive mutation, immune deficiency (imd), that impairs the inducibility of all genes encoding antibacterial peptides during the immune response of Drosophila. When challenged with bacteria, flies carrying this mutation show a lower survival rate than wild-type flies. We also report that, in contrast to the antibacterial peptides, the antifungal peptide drosomycin remains inducible in a homozygous imd mutant background. These results point to the existence of two different pathways leading to the expression of two types of target genes, encoding either the antibacterial peptides or the antifungal peptide drosomycin.The powerful innate defense of higher insects involves proteolytic cascades (coagulation and phenoloxidase cascades), phagocytosis and encapsulation of invading microorganisms, and the synthesis by the fat body of a battery of potent antimicrobial peptides (reviewed in refs. 1 and 2). In Drosophila, several genes encoding inducible antibacterial peptides [cecropins (3, 4); diptericin (5); defensin (6); drosocin (7); M. Charlet, personal communication] and one inducible antifungal peptide [drosomycin (8); L.M. and J.-M.R., unpublished results] have been cloned. Understanding the mechanism of the coordinate control of their expression after immune challenge (e.g., septic injury) is a major goal in the field. Significant similarities exist between the control of antimicrobial peptide gene expression in insects and that of acute phase response genes in mammals (reviewed in refs. 1 and 2). This is illustrated by the involvement of common cis-regulatory motifs in the promoters of most of the insect and mammalian immune genes [e.g., NF-KB and NF-IL6 response elements, interferon consensus regulatory sequences (9-11)]. Furthermore, members of the Rel/NF-KB family play a crucial role in the transactivation of mammalian acute phase response genes; similarly, Rel proteins have been recently implicated in the control of the immune genes in Drosophila (12, 13) as the genes encoding the two Rel proteins dorsal (dl) and Dif (dorsal-related immune factor) are up-regulated in the fat body upon immune challenge and both proteins are translocated in the nucleus (refs. 12 and 13; B.L. and E.N., unpublished results). The precise roles of these proteins in the immune response of Drosophila are not yet established (discussed in refs. 14 and 15).While analyzing the expression of antibacterial genes in a mutant of the phenoloxidase cascade, we have found, by serendipity, a recessive mutation, immune deficiency (imd), that impairs the inducibility of the antibacterial peptides described so far in Drosophila. When challenged with bacteria, flies carrying this mutation show a lower survival rate than wild-type flies. We also report that, in contrast to the antibacterial peptides, the antifungal peptide drosomycin remains inducible in a homozygous imd mutant background. These results point to the existence of two different pathways leading either to the expression of the genes encoding antibact...

Phagocytosis is a complex, evolutionarily conserved process that plays a central role in host defense against infection. We have identified a predicted transmembrane protein, Eater, which is involved in phagocytosis in Drosophila. Transcriptional silencing of the eater gene in a macrophage cell line led to a significant reduction in the binding and internalization of bacteria. Moreover, the N terminus of the Eater protein mediated direct microbial binding which could be inhibited with scavenger receptor ligands, acetylated, and oxidized low-density lipoprotein. In vivo, eater expression was restricted to blood cells. Flies lacking the eater gene displayed normal responses in NF-kappaB-like Toll and IMD signaling pathways but showed impaired phagocytosis and decreased survival after bacterial infection. Our results suggest that Eater is a major phagocytic receptor for a broad range of bacterial pathogens in Drosophila and provide a powerful model to address the role of phagocytosis in vivo.