Research on lepidopteran insects has been instrumental in the discovery and the biochemical understanding of many haemolymph plasma proteins with immune functions. After the identification of inducible bactericidal activity in the haemolymph of the wax moth Galleria mellonella (Stevens, 1962), the first well-characterized insect antimicrobial peptides and proteins (Faye et al., 1975; Hoffmann et al, 1981; Hultmark et al, 1980, 1983; Powning and Davidson, 1973; Steiner et al, 1981) as well as pattern-recognition proteins that function in immune surveillance (Yoshida et al., 1986, 1996; Ochiai and Ashida, 1988; Kang et al., 1998) were initially studied in silkmoths and other caterpillars. Biochemical characterization of the prophenoloxi-dase (proPO) activation system was also pioneered in work with Bombyx mori (Ashida and Brey, 1998).
Insect plasma proteins, including those involved in immune responses, are synthesized primarily by the fat body, with some contribution from haemocytes and other tissues (Dunn, 1990; Kanost et al., 1990). The functions of many plasma immune proteins and the regulation of their gene expression have been characterized in Drosophila melanogaster through extensive genetic analyses (Chapter 2; Ferrandon et al., 2007; Lemaitre and Hoffmann, 2007). Details of immune mechanisms from other arthropod species have also been reviewed (Iwanaga and Lee, 2005; Jiravanichpaisal et al, 2006). In this chapter, we will focus on recent advances in our understanding of plasma proteins in the immune responses of the tobacco hornworm, Manduca sexta, and will point out some related work done with other insect species for background and comparison. Additional reviews on aspects of the M. sexta immune response are available (Kanost et al, 2004; Jiang, 2008; Kanost and Nardi, 2008).
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