Antimicrobial peptides (AMPs) are important defense molecules of the innate immune system. High levels of AMPs are induced in response to infections to fight pathogens, whereas moderate levels induced by metabolic stress are thought to shape commensal microbial communities at barrier tissues. We expressed single AMPs in adult flies either ubiquitously or in the gut by using the inducible GeneSwitch system to tightly regulate AMP expression. We found that activation of single AMPs, including Drosocin, resulted in a significant extension of Drosophila lifespan. These animals showed reduced activity of immune pathways over lifetime, less intestinal regenerative processes, reduced stress response and a delayed loss of gut barrier integrity. Furthermore, intestinal Drosocin induction protected the animals against infections with the natural Drosophila pathogen Pseudomonas entomophila, whereas a germ-reduced environment prevented the lifespan extending effect of Drosocin. Our study provides new insights into the crosstalk of innate immunity, intestinal homeostasis and ageing.

Funding: This work was supported by grants from the DFG to M.H. (SFBs 645, 704 and cluster of excellence ImmunoSensation) ( www.dfg.de ) and the Young Researcher Overseas Visits Program for Vitalizing Brain Circulation by Oversees Training Program Division, International Program Department, Japan Society for the Promotion of Science to T.M. and H.T. ( www.jsps.go.jp/english/ ). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

In Drosophila seven distinct families of inducible AMPs have been identified to date. In response to an infection, they can be expressed systemically in the fat body or locally in epithelial barrier tissues [ 5 ]. AMP expression is regulated by members of the nuclear factor-kappa B family of inducible transactivators, which include the dorsal-related immunity factor (DIF), Relish and Dorsal. These transcription factors are activated upon infection by two major signaling cascades, the Toll and immune deficiency (IMD) pathways [ 5 ]. Additionally, subsets of AMPs can be directly activated by the transcription factors Drosophila Forkhead box O (dFOXO) or Forkhead (FKH), depending on the metabolic status of the fly, demonstrating a cross regulation between metabolism and innate immunity [ 6 , 7 ]. In the midgut AMP expression is not regulated by Toll signaling but by the IMD and the Janus kinase-signal transducers and activators of transcription (JAK-STAT) pathways [ 8 ] and controlled by the negative transcriptional regulator caudal [ 9 ]. It has been described that this local AMP expression at the intestinal barrier is necessary to fight food-borne infections, demonstrated with the Drosophila pathogen Pseudomonas entomophila (Pe) [ 10 , 11 ]. Additionally, the Drosophila intestinal epithelium is not only challenged by pathogenic bacteria, but is also as a host in constant contact to a commensal microbiota [ 12 ], which has to be regulated in composition and density by the immune system [ 13 ]. Both, pathogenic bacteria and the commensal microbiota have a tremendous influence on the intestinal homeostasis, a condition regulated by the immune and stress responses as well as the regenerative activity of the epithelial tissue [ 8 , 9 , 14 ]. The integrity of the intestinal barrier epithelium and the homeostasis of the gut are therefore also tightly linked to organismal health and lifespan [ 15 , 16 ]. In this study we used the fruit fly Drosophila melanogaster to analyze the interplay between intestinal AMP activation, midgut homeostasis and longevity.

In mammals AMPs are expressed in many cell types including immune cells and cells of epithelial barriers like skin and gut [ 3 ]. As just one example, in humans down regulation of alpha-defensins is associated with Crohn’s disease, a chronic inflammatory bowel disease [ 4 ]. In addition to being involved in the pathogen defense, recent studies have shown that AMPs also play a pivotal role in intestinal homeostasis by regulating the composition and abundance of the gut microbiota [ 4 ]. Analyzing functions of AMPs is thus of major interest to biomedical research and to this end we have used Drosophila as a genetic model system.

Results