A prominent response during the Drosophila host defence is the induction of proteolytic cascades, some of which lead to localized melanization of pathogen surfaces, while others activate one of the major players in the systemic antimicrobial response, the Toll pathway. Despite the fact that gain‐of‐function mutations in the Toll receptor gene result in melanization, a clear link between Toll activation and the melanization reaction has not been firmly established. Here, we present evidence for the coordination of hemolymph‐borne melanization with activation of the Toll pathway in the Drosophila host defence. The melanization reaction requires Toll pathway activation and depends on the removal of the Drosophila serine protease inhibitor Serpin27A. Flies deficient for this serpin exhibit spontaneous melanization in larvae and adults. Microbial challenge induces its removal from the hemolymph through Toll‐dependent transcription of an acute phase immune reaction component.
In drosophila, fungal and Gram-positive bacterial molecular determinants are detected by circulating pattern recognition receptors (PRRs). Previous findings suggest that these PRRs activate yet unidentified serine protease cascades culminating in the cleavage of Spaetzle, the endogenous Toll receptor ligand, and triggering the immune response. We demonstrate here that the Grass protease defines a common activation cascade for PRR-mediated fungal and Gram-positive bacterial detection. The serine protease Persephone, previously shown to be specific for fungal detection in a cascade activated by secreted fungal proteases, was also required for sensing of proteases elicited by bacteria in the hemolymph. Hence, Persephone defines a parallel proteolytic cascade activated by danger signals such as abnormal proteolytic activities.
The response of the fruit fly Drosophila melanogaster to various microorganism infections relies on a multilayered defense. The epithelia constitute a first and efficient barrier. Innate immunity is activated when microorganisms succeed in entering the body cavity of the fly. Invading microorganisms are killed by the combined action of cellular and humoral processes. They are phagocytosed by specialized blood cells, surrounded by toxic melanin, or lysed by antibacterial peptides secreted into the hemolymph by fat body cells. During the last few years, research has focused on the mechanisms of microbial recognition by various pattern recognition receptors and of the subsequent induction of antimicrobial peptide expression. The cellular arm of the Drosophila innate immune system, which was somehow neglected, now constitutes the new frontier.
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