The imd gene is required for local Cecropin expression in Drosophila barrier epithelia.

Surfaces of higher eukaryotes are normally covered with microorganisms but are usually not infected by them. Innate immunity and the expression of gene-encoded antimicrobial peptides play important roles in the first line of defence in higher animals. The immune response in Drosophila promotes systemic expression of antimicrobial peptides in response to microbial infection. We now demonstrate that the epidermal cells underlying the cuticle of larvae respond to infected wounds by local expression of the genes for the antimicrobial peptide cecropin A. Thus, the Drosophila epidermis plays an active role in the innate defence against microorganisms. The immune deficiency (imd) gene was found to be a crucial component of the signal-induced epidermal expression in both embryos and larvae. In contrast, melanization, which is part of the wound healing process, is not dependent on the imd gene, indicating that the signalling pathways promoting melanization and antimicrobial peptide gene expression can be uncoupled.