An attack by a parasitic wasp activates a vigorous cellular immune response in Drosophila larvae. This response is manifested by an increased number of circulating cells, the hemocytes, and by the appearance of a specialized class of hemocyte, the lamellocytes, which participate in the encapsulation and killing of the parasite. To study the molecular mechanisms of this response, we have overexpressed different genes in the hemocytes, by using the GAL4-upstream activating sequence system and a hemocytespecific Hemese-GAL4 driver. Multiple transgenes were tested, representing several important signaling pathways. We found that the proliferation response and the activation of lamellocyte formation are independent phenomena. A drastic increase in the number of circulating hemocytes is caused by receptor tyrosine kinases, such as Egfr, Pvr, and Alk, as well as by the downstream signaling components Ras85D and pointed, supporting the notion that the Ras-mitogen-activated protein kinase pathway regulates hemocyte numbers. In the case of Pvr and Alk, this phenotype also is accompanied by lamellocyte formation. By contrast, constitutively active hopscotch and hemipterous give massive activation of lamellocyte formation with little or no increase in total hemocyte numbers. This finding indicates that both the Jak͞Stat and the Jun kinase pathways affect lamellocyte formation. Still other signals, mediated by aop ACT , Toll 10b , and Rac1 expression, cause a simultaneous increase in lamellocyte and total cell numbers, and the same effect is seen when WNT signaling is suppressed. We conclude that the activation of a cellular response is complex and affected by multiple signaling pathways.

The hemocytes, the blood cells of Drosophila, participate in the humoral and cellular immune defense reactions against microbes and parasites [1-8]. The plasmatocytes, one class of hemocytes, are phagocytically active and play an important role in immunity and development by removing microorganisms as well as apoptotic cells. On the surface of circulating and sessile plasmatocytes, we have now identified a protein, Nimrod C1 (NimC1), which is involved in the phagocytosis of bacteria. Suppression of NimC1 expression in plasmatocytes inhibited the phagocytosis of Staphylococcus aureus. Conversely, overexpression of NimC1 in S2 cells stimulated the phagocytosis of both S. aureus and Escherichia coli. NimC1 is a 90-100 kDa single-pass transmembrane protein with ten characteristic EGF-like repeats (NIM repeats). The nimC1 gene is part of a cluster of ten related nimrod genes at 34E on chromosome 2, and similar clusters of nimrod-like genes are conserved in other insects such as Anopheles and Apis. The Nimrod proteins are related to other putative phagocytosis receptors such as Eater and Draper from D. melanogaster and CED-1 from C. elegans. Together, they form a superfamily that also includes proteins that are encoded in the human genome.

We have identified a previously undescribed transmembrane protein, Hemese, from Drosophila melanogaster blood cells (hemocytes), by using a monoclonal pan-hemocyte antibody. Heavy glycosylation is suggested by the heterogeneous size distribution, ranging between 37 and 70 kDa. Hemese expression is restricted to the cell surfaces of hemocytes of all classes, and to the hematopoietic organs. The sequence of the corresponding gene, Hemese (He), predicts a glycophorin-like protein of 15 kDa, excluding an N-terminal signal peptide, with a single hydrophobic transmembrane region. The extracellular region consists mainly of Ser͞Thr-rich sequence of low complexity, with several potential O-glycosylation sites. Hemese contains phosphotyrosine and the cytoplasmic region has potential phosphorylation sites, suggesting an involvement in signal transduction. Depletion of Hemese by RNA interference has no obvious effect under normal conditions, but the cellular response to parasitic wasps is much enhanced. This finding indicates that Hemese plays a modulatory role in the activation or recruitment of the hemocytes.