Abstract
Cecropins are small helical secreted peptides with antimicrobial activity that are widely distributed among insects. Genes encoding cecropins are strongly induced upon infection, pointing to their role in host-defense. In Drosophila, four cecropin genes clustered in the genome (CecA1, CecA2, CecB and CecC) are expressed upon infection downstream of the Toll and Imd pathways. In this study, we generated a short deletion ΔCecA-C removing the whole cecropin locus. Using the ΔCecA-C deficiency alone or in combination with other antimicrobial peptide (AMP) mutations, we addressed the function of cecropins in the systemic immune response. ΔCecA-C flies were viable and resisted challenge with various microbes as wild-type. However, removing ΔCecA-C in flies already lacking ten other AMP genes revealed a role for cecropins in defense against Gram-negative bacteria and fungi. Measurements of pathogen loads confirm that cecropins contribute to the control of certain Gram-negative bacteria, notably Enterobacter cloacae and Providencia heimbachae. Collectively, our work provides the first genetic demonstration of a role for cecropins in insect host defense, and confirms their in vivo activity primarily against Gram-negative bacteria and fungi. Generation of a fly line (ΔAMP14) that lacks fourteen immune inducible AMPs provides a powerful tool to address the function of these immune effectors in host-pathogen interactions and beyond.
