One of the most striking immunosuppressive effects associated with PDVs is the suppression of the melanization response. So far only one PDV gene, egf1.0, encoded by MdBV, has been shown to block melanization of haemolymph in Manduca sexta and other insects in vitro. The MdBV Egf family encodes small serine proteinase inhibitor (smapin) homologues (Beck and Strand, 2007) that usually consist of a cysteine-rich trypsin inhibitor-like domain. Egf1.0 was shown to inhibit haemolymph melanization in M. sexta by several mechanisms: (a) inhibition of the catalytic activity of proPO-activating protein PAP-1 and PAP-3; (b) inhibition of pro-PAP1 and pro-PAP3 processing; and (c) prevention of proPO and serine proteinase homologue (SPH) 1 and SPH2 processing (Lu et al., 2008; Beck and Strand, 2007). Egf1.0 could not, however, inhibit PO that had been activated already. Since Egf family homologues have not been identified in other PDVs and serine protease inhibitors are also absent, it will be interesting to determine which genes are involved in suppression of melanization in other systems.
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