INDUCIBLE ANTIBACTERIAL PEPTIDES OF INSECTS

Insects belonging to the phylogenetically recent orders of Lepidoptera, Diptera, Hymenoptera and Coleoptera possess a remarkably efficient antibacterial defense mechanism involving the synthesis of several potent antibacterial peptide families with a large spectrum of activity. The best known of these molecules are the cecropins (4 kDa) which form two amphipathic helices and the insect defensins (4 kDa) characterized by an amphipathic-alpha-helix linked via two disulfide bridges to an antiparallel-beta-sheet (cysteine stabilized alpha-helix motif). The peptide sequences of seven other inducible antibacterial peptide families have been established over the last years. These are predominantly small cationic proline rich peptides (2-3 kDa) or larger glycine rich polypeptides (8-27 kDa). The synthesis of the antibacterial peptides takes place in the fat body, a functional equivalent of the vertebrate liver. The antibacterial response is activated by non-clonally distributed receptors recognizing structural motifs primarily present on procaryotic cell envelopes (lipopolysaccharides, peptidoglycans). The response is rapid (several hours) and short-lived (several days). There is no indication at present for the existence of an immune memory in these insect orders. The antibacterial response of insects shares many of the characteristics of the acute phase response in mammals.