Identification and functional characterization of an Ig domain-containing C-type lectin involved in antibacterial innate immunity in Eriocheir sinensis.

C-type lectins (CTLs) in invertebrates exhibit molecular and structural diversity, which underpins their versatile functions. In crustaceans, although several immunoglobulin (Ig) domain-containing CTLs have been reported to participate in immune defense, our understanding of this molecule class remains limited. In this study, we identified a novel Ig domain-containing CTL from the Chinese mitten crab Eriocheir sinensis, designated as EsIgLec3. This lectin is widely expressed in multiple tissues, and its expression is significantly upregulated upon challenge with Staphylococcus aureus, Aeromonas hydrophila, and Vibrio parahaemolyticus. Knockdown and recombinant protein injection experiments demonstrated that EsIgLec3 positively regulates key components of the immune deficiency (IMD) pathway, nuclear transport-related proteins, and antimicrobial peptide (AMP) expression, while negatively regulating the negative regulator of the IMD pathway during bacterial infection. Notably, these regulatory effects were more pronounced against the two Gram-negative bacteria tested. Additionally, EsIgLec3 promotes active phenoloxidase (PO) activity following bacterial challenge. It's worth noting that the full-length EsIgLec3 protein exhibited stronger bacterial and carbohydrate binding capacities than the carbohydrate-recognition domain (CRD)-only fragment, suggesting that the Ig domain may assist the CRD in pattern recognition. Collectively, these findings indicate that EsIgLec3 contributes to antibacterial defense via pattern recognition, IMD pathway modulation, and PO activation, though the role of its Ig domain remains to be elucidated. This work advances our understanding of Ig-containing CTLs and identifies new targets for combating bacterial diseases in crustaceans.