Peptidoglycan recognition protein L1 regulates intestinal immunity and microbial homeostasis  via the IMD pathway in the silkworm, <i>Bombyx mori</i>

Peptidoglycan recognition proteins (PGRPs), evolutionarily conserved pattern recognition receptors, act as receptors and regulators in insect Toll and IMD (immune deficiency) signaling pathways. Despite prior identification of silkworm long PGRP (PGRP-L) genes, their physiological roles remain poorly characterized. Here, we investigated the sequence features and expression patterns of silkworm PGRP-Ls, focusing on the role of PGRP-L1 in intestinal immunity and gut microbiota regulation. We identified and cloned 6 silkworm PGRP-L genes, and the proteins encoded by PGRP-L1, L4, L5, and L6 may function as nonamidolytic immune receptors predominantly expressed in the midgut. PGRP-L1 protein resides on the cell membrane of the midgut epithelium near the intestinal lumen and can directly bind to pathogens and peptidoglycans. Genetic and pathogen stimulation analyses revealed that PGRP-L1 knockout suppresses, while overexpression enhances, IMD pathway activation, specifically regulating antimicrobial peptides (AMPs) and Relish1 expression in the midgut. Under mulberry feeding, both PGRP-L1 knockout and overexpression significantly enhanced early larval growth; however, only individuals overexpressing PGRP-L1 maintained survival rates comparable to wild type individuals and improved economic traits. Additionally, PGRP-L1 knockout in artificial diet-fed larvae resulted in reduced microbial diversity, increased dominance of Enterococcus, and developmental arrest, whereas overexpression enhanced microbial richness and larval survival rates. These results established PGRP-L1 as a key regulator of gut immunity and microbiota homeostasis in silkworms and may operate through IMD-mediated AMP production and selective microbial control. Our findings provide insights into insect immune mechanisms and potential strategies for optimizing the gut health of economically important insects.