Inducible response

Evidence for the existence of an inducible antiviral response has also been obtained in several other invertebrate species. For example, injection of dsRNA in shrimps or sandflies can induce an antiviral response (Robalino et al., 2005; Pitaluga et al., 2008). In the crayfish Pacifastacus leniusculus, the large DNA virus white spot syndrome virus (WSSV family Nimaviridae) induces expression of the antilipopolysaccharide factor (ALF), which interferes with WSSV replication in vitro and in vivo (Liu et al., 2006). The mode of action and the mechanism of regulation of ALF remain to be characterized. Finally and of great potential interest, infection of the vector insect A. aegypti by arboviruses also leads to altered gene expression. For example SINV infection leads to changes in expression in 135 genes, some of which are strongly upregulated in the midgut (Sanders et al., 2005). Interestingly, one of these strongly induced genes is the ortho-logue of Unc93b, which plays a critical role in TLR-mediated antiviral defences in mammals (Beutler et al., 2007). Infection with dengue virus also leads to induction of many genes, including antimicrobial peptides, and a bias towards the Toll and JAK/ STAT pathways. Furthermore, silencing of MyD88, a key component of the Toll pathway, resulted in a small but significant increase in dengue viral load in the midgut of infected mosquitoes, supporting the concept that upon infection the Toll pathway regulates expression of antiviral molecules (Xi et al., 2008). An evolutionarily conserved antiviral role of the JAK/STAT pathway is also supported by independent studies in other invertebrate models. The induction of STAT DNA-binding activity in the mosquito cell line C6/36 has been reported following infection by the flavivirus Japanese encephalitis virus (Lin et al, 2004). In addition, WSSV, which infects shrimps, also induces STAT-binding activity in infected animals, and subverts it to enhance the expression of its immediate-early genes (Liu et al, 2006).

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