C‐Type Lectin S Group <i>TcCTL4</i> Participates in the Immunity of <i>Tribolium castaneum</i>

C-type lectin S (CTL-S) plays a crucial role in pathogen recognition and the activation of immune response. In comparison, the proportion of CTL-S was relatively high in insects, but the study was much smaller than the proportion observed. In this study, we cloned and characterized one CTL-S, TcCTL4, from Tribolium castaneum. Our analysis revealed that TcCTL4 was highly expressed during the early pupal stage, with expression levels exhibiting a tendency to change with developmental stages. Additionally, tissue expression analysis showed a high expression of TcCTL4 in the central nervous system (CNS). Moreover, we observed a significant increase in TcCTL4 transcripts after bacterial challenge. The RNA interference (RNAi) of TcCTL4 before bacterial treatment resulted in a significant reduction in the transcripts of immune factors (IFs) and antimicrobial peptides (AMPs), indicating that TcCTL4 may regulate AMP expression through the activation of the immune signaling pathway. Furthermore, our investigation revealed that the recombinant protein TcCTL4 (rTcCTL4) not only recognized bacteria but also agglutinated bacteria in a Ca²⁺-dependent manner. Enzyme activity analysis suggested that rTcCTL4 could enhance phenoloxidase activity, implying its potential involvement in the prophenoloxidase activation pathway. In conclusion, these results indicate that TcCTL4 is involved in the immunity of T. castaneum, providing valuable insights into insect CTLs.