Host immune suppression by toxoneuron nigriceps polidnavirus: use of Drtosophila melanogaster as an experimental model system.

Host regulation strategies adopted by parasitic Hymenoptera are increasingly attracting the scientific interest, since several data support the idea that these strategies include targeting of evolutionarily conserved pathways. This is in particular emerging from molecular and functional analyses of several parasitoid-associated polydnaviruses (PDVs), whose gene products, expressed in the tissues of parasitized hosts, interact with key signaling molecules (Pennacchio and Strand. 2006; Espagne et al. 2004; Falabella et al. 2003; Falabella et al. 2007; Provost et al. 2004; Thoetkiattikul et al. 2005). T. nigriceps bracovirus (TnBV) is the PDV associated with Toxoneuron nigriceps (Hymenoptera, Braconidae), the endophagous larval parasitoid of the tobacco budworm Heliotis virescens (Lepidoptera, Noctuide). We recently identified a host gene, named 102, which is targeted by a small TnBV transcript. Several data indicate that this gene is involved in the immune response, but its mode of action is still unclear. Since the 102 gene is highly conserved throughout evolution, to investigate the function of the protein encoded by the 102 gene, we decided to use Drosophila melanogaster as a model system, since it offers a wide range of molecular genetic tools not available in other systems: its genome is fully sequenced; detailed knowledge about the molecular pathways regulating cellular immune responses is available; several sophisticated genetic and molecular genetic techniques have been developed and can be used as powerful tools to dissect the functional mechanisms underlying a wide range of biological processes. In Drosophila melanogaster two putative protein sequences, showing 38% and 34% identity, respectively, with the translation product of the 102 gene, were found by Blast analyses. One of these is highly expressed in larval hemocytes, like its Heliotis virescence counterpart. Moreover, as reported in the Flyatlas database (http://www.flyatlas.org/), the gene is strongly expressed also in the fat body. I took advantage of the well-established RNAi strategies based on the GAL4/UAS binary system to interfere with the expression of this gene in immuno-competent tissues (i.e. hemocytes and fat body) of Drosophila melanogaster. First of all, I evaluated the potential impact of this tissue-specific RNA interference on viability and found a dramatic lethal effect approaching 100%. Lethal phase measurements indicated that mortality was prominent in larval and pupal stages. Morphological analyses of the larvae in which the 102 gene had been targeted by RNA interference showed the presence of huge melanotic masses freely floating in the hemocoel. This tumorous-like phenotype is usually associated with altered and/or excessive performance of the immune system. Accordingly, hemocyte counts indicated that hemocyte overproliferation occurred in larvae, and increased numbers of both plasmatocytes and lamellocytes were found in their hemolymph. Furthermore, in these larvae, the sessile hemocyte population, was also affected. Finally, morphological analyses of melanotic masses clearly identified their association with hemocytes, forming a cellular capsule around them.