| Drosophila immunity has become of great interest during the past few years due to its evolutionary conservation in mammals. For example, the fly immune system activates NF-kappaB signaling in response to pathogens as does the vertebrate innate immune system. Our lab has been interested in how pathogens are recognized in Drosophila, and how these recognition processes induce downstream effector molecules, such as the antimicrobial peptide Diptericin. An ethylmethane sulfonate-induced mutagenesis screen was performed in the lab and identified more than 40 mutants that did not induce a Diptericin-lacZ reporter gene upon E. coli injection. These were named immune response deficient ( ird) mutants. My thesis work was to characterize the mutant phenotypes, clone genes responsible for the phenotypes and reveal how the signaling pathways are affected by these mutations.; First, I categorized the mutants into different groups based on their induction of various antimicrobial peptides and proteolytic processing of Relish, a NF-kappaB-like transcription factor essential for antibacterial responses. The mutants fell into two groups: mutants defective in both antibacterial and antifungal responses and mutants specifically defective in antibacterial responses. Among the mutants in the first group are ird15 and ird6, which I molecularly characterized. Both genes fail to induce antibacterial and antifungal peptide genes at normal level. ird15 is allelic to scribble and ird6 is allelic to kurtz/modulo. Both genes affect the cellular organization of the fat body, the tissue responsible for antimicrobial peptide production. Therefore, it appears that the immune phenotype in these mutants may be an indirect consequence of abnormalities in cell architecture.; ird7 belongs to the second group, that is, those specifically defective in the antibacterial response. ird7 mutants completely fail to activate Relish upon E. coli injection. I found mutations in a gene encoding peptidoglycan recognition protein-LC (PGRP-LC) in both alleles of ird7 and showed that PGRP-LC transgenes rescued the mutant phenotype in ird7, demonstrating that PGRP-LC is required for the antibacterial response. Peptidoglycan is a component of bacterial cell walls and a potent activator of innate immune responses in both flies and mammals. The results suggest that Ird7/PRGP protein acts as a receptor molecule that directly recognizes bacterial pathogens, and preliminary data suggest that Ird7/PGRP protein binds certain types of Gram-positive and Gram-negative bacteria. Four PGRP genes have been identified in the human genome. Given the evolutionary conservation of many proteins required for innate immune responses, it will be important to evaluate whether PGRPs function as a family of pathogen recognition proteins in human innate immune responses. |
