Mitochondrial perturbations in immune cells enhance cell-mediated innate immunity in Drosophila

Mitochondria participate in various cellular processes including energy metabolism, apoptosis, autophagy, ROS production, stress responses, inflammation and immunity. We investigated the effects of immune tissue specific mitochondrial perturbations on immune responses at the organismal level. Drosophila melanogaster model was utilized to knockdown genes from oxidative phosphorylation (OXPHOS) complexes cI-V, by targeting the two main immune tissues of Drosophila, the fat body and the immune cells (hemocytes). While fat body targeted OXPHOS perturbations caused detrimental effects on the viability and immunity, hemocyte specific perturbations led to enhanced immunocompetence of the host. This was accompanied by the formation of melanized hemocyte aggregates (melanotic nodules), a sign of activated cell-mediated innate immunity. Furthermore, hemocyte specific OXPHOS perturbations caused proliferation and immune activation of the hemocytes, resulting to a similar hemocyte profile as seen upon infection, and the animals ultimately had an enhanced immune response when infected with a parasitoid. Our data shows that hemocyte targeted OXPHOS perturbations cause mitochondrial membrane depolarization and upregulation of genes associated with mitochondrial unfolded protein response, including aerobic glycolysis and reactive oxygen species. Overall, we show that while the effects of mitochondrial perturbations on immune responses are highly tissue specific, mild mitochondrial dysfunction can be beneficial in immune-challenged individuals and is causing variation in infection outcomes between individuals. The GAL4-UAS system was utilized to silence OXPHOS complex I gene UQCR-C1 specifically in hemocytes. We used the combination of two hemocyte GAL4 lines (He-GAL4 and Hml[delta]-GAL4, HH-GAL4 for short) to drive the expression of shRNA for UQCR-C1 (UAS-UQCR-C1) in hemocytes. As a control, the w1118 line was crossed to the HH-GAL4 line. We collected the hemocyte samples at two timepoints: early (90 hours post egg lay) and late (120 hours post egg lay). In addition, we included wasp-infected (Leptopilina boulardi) late timepoint samples. For the infections, larvae were kept with parasitoid wasps for two hours. The hemocyte samples were collected 48 h after infection start. Hemocytes were also collected from uninfected, age-matched larvae.