Inducible Humoral Antibacterial Immunity in Insects

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Insects have effective immune systems composed of both cellular (Boman et al., 1978; Boman and Hultmark, 1981; Salt, 1970) and humoral components (Boman, 1982; Boman et al., 1986; Boman and Hultmark, 1987; Faye et al., 1975; Gotz and Boman, 1985). Cellular immune reactions include phagocytosis, nodule formation and encapsulation (Gotz and Boman, 1985; Ratcliffe and Rowley, 1979; Ratcliffe et al., 1985), whereas humoral immune reactions involve synthesis and release of several antibacterial immune proteins (Boman and Hultmark, 1987; Gotz and Boman, 1985), some capable of killing both gram-positive and gram-negative bacteria. The expression of this multicomponent humoral immune system requires de novo synthesis of RNA and proteins with broad antibacterial activity. There are at least three families of antibacterial proteins: lysozyme, cecropins and attacins (Boman et al., 1986; Boman and Hultmark, 1987; Boman and Steiner, 1981). Humoral immunity studies in insects have been conducted mostly in lepidopterans (Briggs, 1958; Chadwick, 1967; Hoffmann et al., 1981; Stephens and Marshall, 1962) and in diapausing pupae of silkmoths, e.g. Anthaeraea pernyi, Samia cynthia and Hyalophora cecropia (Boman and Hultmark, 1987; Boman et al., 1974; Faye et al., 1975; Hultmark et al., 1980). Pupae of H. cecropia constitute a particularly good model for studying humoral immunity as they are large (5–10 g) and contain 1–2 ml of haemolymph. They also undergo a long diapause (6–9 months) that allows immune proteins to be synthesized without much interference from other biosynthetic processes (Boman et al., 1986; Boman and Hultmark, 1981).