Mannose receptor RpMR1 of Manila clam (Ruditapes philippinarum) defense against Vibrio anguillarum infection.

The mannose receptor (MR) is a member of the C-type lectin superfamily and a type I transmembrane protein that functions as a pattern recognition receptor (PRR) in immune responses. In this study, we identified 13 MR genes (RpMR1-13) in the genome of Ruditapes philippinarum and investigated their expression profiles following Vibrio anguillarum challenge. Notably, RpMR1, RpMR2, RpMR3, and RpMR4 exhibited peak expression at 72 h post-infection. We successfully purified the recombinant RpMR1 protein and demonstrated its antibacterial activity against three Gram-negative bacteria (V. splendidus, V. alginolyticus, and V. anguillarum), though it had no effect on Gram-positive bacteria. Furthermore, in vivo injection of RpMR1 significantly reduced mortality in R. philippinarum following V. anguillarum infection. To explore role of RpMR1 in immune signaling, we performed RNA interference (dsRNA-RpMR1) and observed successful gene silencing. Subsequent qRT-PCR analysis revealed that RpMR1 knockdown significantly suppressed TLR4 expression (P< 0.05) under V. anguillarum stress, confirming an interaction between RpMR1 and TLR4 in the immune response. This study provides the first functional evidence of mannose receptor-mediated immunity in mollusks, offering new insights into the molecular defense mechanisms of R. philippinarum against bacterial infection.