N-acetylchitohexaose induces immune priming against Pseudomonas aeruginosa infection in the silkworm, Bombyx mori

Lysine Motif (LysM) domain-containing receptors are evolutionarily conserved pattern recognition receptors (PRRs) that serve as key mediators of glycan sensing and innate immune activation in plants and mammals. In invertebrates, however, their role in activating innate immunity remains poorly understood, although some evidence for immunosuppressive functions exists. In this study, we performed in silico structural analyses and identified a putative B. mori LYSMD3 homolog (XP_004933441.1). This protein exhibits high structural similarity in the LysM domain to human LYSMD3, with a root-mean-square deviation (RMSD) of 0.559 [A], indicating close structural alignment. RNA-seq analysis of hemocytes isolated from silkworm larvae injected with N-acetylchitohexaose (GN6), a chitin-derived oligosaccharide and known ligand of human LYSMD3, revealed transcriptional activation of innate immune effectors, including antimicrobial peptide (AMP) genes such as cecropins. GN6 also induced cecropin transcription in isolated hemocytes in vitro, and western blotting of hemolymph confirmed elevated Cecropin B protein levels. Furthermore, GN6 and chitin significantly improved survival against P. aeruginosa infection, with median effective doses (ED) values of 0.62 and 0.48 {micro}g/larva, respectively. In contrast, N-acetylglucosamine (GlcNAc) and shorter oligosaccharides (GN2-GN5) were ineffective. These findings provide the first molecular-level evidence of a putative glycan receptor in silkworms based on structural similarity to known LysM domains. Moreover, GN6-induced antimicrobial peptide expression and enhanced infection resistance demonstrate immune priming in this model, supporting an evolutionarily conserved glycan-sensing pathway in invertebrates.