Bombyx mori C-Type Lectin 4 Recognizes a Broad Range of Carbohydrates of Microbial Pathogens

C-type lectins (CTLs) are essential pattern-recognition receptors involved in insect innate immunity, mediating defense responses through carbohydrate binding. In this study, we aimed to clone and functionally characterize Bombyx mori CTL4 (BmCTL4), focusing on its immunity-related functions and binding specificity. To this end, the BmCTL4 cDNA was cloned and the recombinant BmCTL4 protein was expressed using a baculovirus expression system. The recombinant protein was then used to assess its binding specificity via carbohydrate-binding and microbial cell-binding assays. BmCTL4 contained a single carbohydrate recognition domain (CRD) with a QPD motif, suggesting specificity for galactose-type sugars. BmCTL4 was upregulated in the fat body following microbial inoculation. Recombinant BmCTL4 exhibited Ca²⁺-dependent binding to microbial cell wall constituents such as LPS, mannan, N-acetyl-D-glucosamine, and peptidoglycan, and directly bound to microbial cell surfaces as confirmed by confocal microscopy. Our findings demonstrate that BmCTL4 acts as a pattern-recognition receptor with broad carbohydrate-binding activity, contributing to the immune defense mechanisms of B. mori.