Proliferative Signals Regulate Hemocyte Development and Intestinal Immunity in Drosophila melanogaster.
The innate immune system is an ancient line of resistance against intrusive microbial threats. This system integrates cellular, humoral, and barrier defenses to generate a protective immune response. These divisions are indispensable and conserved between Drosophila and mammals. Hemocytes are the immune cells of Drosophila, and they resemble vertebrate myeloid lineages in their ontogeny and function. Similar to tissue macrophages, the Drosophila hemocytes have roles in development, tissue homeostasis and defense against pathogens. Conserved constituents of the platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) receptor tyrosine kinase (RTK) families have defined roles in these processes. Signaling through the PDGF/VEGF-related receptor (Pvr) mediates the establishment and dispersal of hemocytes during embryogenesis. The Pvr ligands, Pvf2 and Pvf3 are implicated in hemocyte cellular proliferation, size and chemotactic guidance of hemocyte. However, the precise role that Pvf2 and Pvf3 play in hemocytes is unclear due to the lack of available mutants. To determine Pvf functions in vivo, I generated a genomic deletion that simultaneously disrupts Pvf2 and Pvf3. From my studies, I identified contributions of Pvf2 and Pvf3 to the trophic maintenance of hemocytes. I uncovered a novel role for Pvfs in invasive migrations and found that Pvf2 and Pvf3 are not required as guidance cues during hemocyte migration, but act locally in epithelial cells to coordinate trans-epithelial movements. Additionally, I observed that Pvr activation in hemocytes attenuates immune signaling through the immune deficiency pathway (IMD). These observations redefined the role of growth factor signaling in Drosophila hemocyte migration and uncover novel roles for hemocyte invasive migration. I then assessed the role that proliferative and immune responses play in the Drosophila gut epithelium. Unlike the sterile hemolymph of the body cavity, the epithelial barriers of the Drosophila gut interface with a dense heterogeneous population of microbes. Cells of the epithelium must relay intrinsic and extrinsic signals to coordinate epithelial renewal programs and immune response. Pathogenic challenge of the epithelium drives a local humoral response through the IMD pathway and a burst of intestinal stem cell (ISC) proliferation to maintain barrier integrity. While Pvf-Pvr signaling and several additional intrinsic regulators are essential for ISC homeostatic proliferation, it is unclear if the microbiota impact these systems during infection. To this end, I assessed gut epithelial IMD and regenerative responses in Drosophila devoid of a microbiota. I found that in the absence of infection, the microbiota stimulate basal IMD signaling are essential to maintain the ISCs. I also found that during infection the gut microbiota dramatically enhance survival, support epithelial compensatory proliferation and exhibit a regulatory impact on the IMD response. Together my findings define growth factor signaling roles through Pvfs in hemocyte survival and invasive migration and identify roles for the microbiota in gut epithelial regeneration and immune responses during infection.