Invading pathogens provoke robust innate immune responses in Dipteran insects, such as In a systemic bacterial infection, a humoral response is induced in the fat body. Gram-positive bacteria trigger the Toll signaling pathway, whereas gram-negative bacterial infections are signaled the immune deficiency (IMD) pathway. We show here that the RNA interference-mediated silencing of -a member of the proprotein convertase enzyme family-specifically in the fat body, results in a reduction in the expression of antimicrobial peptides. This, in turn, compromises the survival of adult fruit flies in systemic infections that are caused by both gram-positive and -negative bacteria. plays a nonredundant role in the regulation of immune responses, as silencing of , the other member of the enzyme family, had no effect on survival or the expression of antimicrobial peptides upon a systemic infection. does not directly affect the Toll or IMD signaling pathways, but the reduced expression of up-regulates stress response factors in the fat body. We also demonstrate that is a negative regulator of the Janus kinase/signal transducer and activator of transcription signaling pathway, which is implicated in stress responses in the fly. In summary, our data identify as a novel regulator of humoral immunity and cellular stress responses in-Aittomäki, S., Valanne, S., Lehtinen, T., Matikainen, S., Nyman, T. A., Rämet, M., Pesu, M. Proprotein convertase expression in the fat body is essential for a normal antimicrobial peptide response and bacterial host defense.