Inducible expression of non-classical innate immune genes by a novel DAP-type peptidoglycan sensing and Toll signaling pathway 2266405

Introduction The Imd and Toll pathways regulate the Drosophila innate response, with each pathway activating distinct NF-κB factors and target genes. These innate immunity signaling cascades are well-studied individually, but how they intersect or cross-regulate is poorly characterized. Methods To identify Toll and Imd interactions, we subjected either wild type, Toll, or Imd pathway mutant flies to a classic Imd challenge — systemic infection with the Gram-negative bacteria Ecc15 and RNA was isolated from the entire animal and analyzed by RNAseq or qRT-PCR. Results These data identified a suit of Ecc15-induced genes that require both Imd and Toll. These Imd/Toll target genes are neither classic Toll genes nor typical Imd targets. These studies revealed that either dead bacteria or DAP-type peptidoglycan alone is sufficient to trigger the induction of two of these Imd/Toll genes, Diedel and TotM, while classic Toll activators, like M. luteus, failed to induce these genes. Nonetheless, Diedel and TotM induction required nearly the entire Toll pathway, except for PGRP-SA - the Lys-type peptidoglycan sensor. Instead, the DAP-type sensor PGRP-SD, along with the orphan co-receptor GNBP2, were critical for this Imd/Toll response. Conclusion These data identify a novel Drosophila innate sensing pathway, where DAP-peptidoglycan triggers a non-canonical Toll-mediated response through the sensor PGRP-SD and co-receptor GNBP2, and contributes to immune gene regulation in conjunction with classic Imd signaling.