An immune system protects organisms against infections. During my PhD,I have focused on the local immune responses of Drosophila and on their links with the systemic immune response. The systemic response is induced in the fat body by the presence of bacteria in the body cavity while the local response is initiated when bacteria accumulate within an epithelial tissue. Some local gut infections however have been shown to also induce systemic responses in the absence of bacteria in the body cavity. Thus, a signal from the gut is detected by the fat body. It has been proposed that bacterial peptidoglycan, passing across the gut epithelium, could be this signal. In the first project of my PhD, I characterized a new, genital, mode of infection that induces both a local response of the genital tract and a systemic fat body response. Experiments based on genital deposition of peptidoglycan reveal that this molecule is the signal that passes from the genital tract to the fat body. Preliminary results suggest that its passage requires transcytosis. In my second project, I investigated the function of PGRP-LA. PGRPs are regulators and effectors of the immune response in animals. A transcriptome analysis in larval tracheae suggests that PGRP-LA is involved in the maintenance of a basal level of immune response in healthy conditions. Preliminary results suggest that PGRP-LA is also required for a localised response of the fat body surrounding the salivary glands. These studies provide new insights into the immune responses of the tracheae and genital tract and into the communication between barrier epithelia, specifically the salivary glands and the genital tract, and the fat body
