New insights into Drosophila larval haemocyte functions through genome-wide analysis.

Drosophila blood cells or haemocytes comprise three cell lineages, plasmatocytes, crystal cells and lamellocytes, involved in immune functions such as phagocytosis, melanisation and encapsulation. Transcriptional profiling of activities of distinct haemocyte populations and from naive or infected larvae, was performed to find genes contributing to haemocyte functions. Of the 13 000 genes represented on the microarray, over 2500 exhibited significantly enriched transcription in haemocytes. Among these were genes encoding integrins, peptidoglycan recognition proteins (PGRPs), scavenger receptors, lectins, cell adhesion molecules and serine proteases. One relevant outcome of this analysis was the gain of new insights into the lamellocyte encapsulation process. We showed that lamellocytes require betaPS integrin for encapsulation and that they transcribe one prophenoloxidase gene enabling them to produce the enzyme necessary for melanisation of the capsule. A second compelling observation was that following infection, the gene encoding the cytokine Spatzle was uniquely upregulated in haemocytes and not the fat body. This shows that Drosophila haemocytes produce a signal molecule ready to be activated through cleavage after pathogen recognition, informing distant tissues of infection.

graal: a Drosophila gene coding for several mosaic serine proteases.

Serine proteases play vital roles in several biological processes such as development and immunity. We have characterized Graal, a large multi-domain serine protease from Drosophila. Graal is spliced in at least three transcripts that are present throughout development. The domains found in Graal proteins are: chitin-binding domains (CBD), scavenger receptor cysteine-rich (SRCR) domains, low density lipoprotein receptor cysteine-rich (LDLR-CR) domains, histidine and proline-rich domains, a NGGYQPP-repeat domain and a serine protease domain. The last 2370 nucleotides of these RNAs are identical and encode a His-rich domain, two SRCR domains, two LDLR-CR domains and a protease domain. The transcription of graal is upregulated after fungal or bacterial infection. Analysis of the Iso1 (y;cn,sp,bw) strain shows that graal transcription is impaired in this fly line due to the insertion of a retrotransposon in the sixth exon. However, no phenotype could be observed consecutive to the absence of graal full length transcripts, particularly in the context of an immune challenge.

Drosophila blood cells.

Drosophila blood cells or haemocytes belong to three lineages: plasmatocytes, crystal cells and lamellocytes. There is no equivalent of a lymphoid lineage in insects which have no adaptive immunity. Haematopoiesis is under the control of a number of transcription factors and signalling pathways (such as GATA factors, JAK/STAT or Notch pathways) most of which have homologues which participate in the control of mammalian haematopoiesis. Drosophila plasmatocytes are professional phagocytes reminiscent of the cells from the mammalian monocyte/macrophage lineage. Several receptors responsible for recognition of microorganisms or apoptotic corpses have been identified, which include a Scavenger Receptor, a CD36 homologue and a peptidoglycan recognition protein. Crystal cells contain the enzymes necessary for humoral melanization that accompanies a number of immune reactions. The production of melanin generates, as by-products, cytotoxic free radicals that are believed to participate in the killing of pathogens. Finally, lamellocytes represent a cell type that specifically differentiates after parasitism of Drosophila larvae and forms a capsule around the invader. Encapsulation together with melanization eventually kill the parasite within the capsule.

PVF2, a PDGF/VEGF-like growth factor, induces hemocyte proliferation in Drosophila larvae.

Blood cells play a crucial role in both morphogenetic and immunological processes in Drosophila, yet the factors regulating their proliferation remain largely unknown. In order to address this question, we raised antibodies against a tumorous blood cell line and identified an antigenic determinant that marks the surface of prohemocytes and also circulating plasmatocytes in larvae. This antigen was identified as a Drosophila homolog of the mammalian receptor for platelet-derived growth factor (PDGF)/vascular endothelial growth factor (VEGF). The Drosophila receptor controls cell proliferation in vitro. By overexpressing in vivo one of its putative ligands, PVF2, we induced a dramatic increase in circulating hemocytes. These results identify the PDGF/VEGF receptor homolog and one of its ligands as important players in Drosophila hematopoiesis.