Molecular characterization of <scp><i>Bombyx mori</i></scp> C‐type lectin 21 in innate immune responses

Abstract Insect C‐type lectins (CTLs) play a vital role in recognizing and binding pathogen‐associated molecules, such as carbohydrate moieties, to initiate immune responses. Although the characteristics of Bombyx mori C‐type lectin 21 ( Bm CTL21), named Bm LBP, have been reported, its molecular mechanisms, including expression patterns during immune responses and recognition and binding behaviors, remain poorly understood. Therefore, this study aims to explore the molecular characterization of Bm CTL21 in innate immune responses. The findings reveal that Bm CTL21 is upregulated in the fat body and subsequently transported to hemocytes, where it binds to the surface of Gram‐negative bacteria during microbial infection. Additionally, Bm CTL21 RNAi treatment leads to a decline in Bm CTL21 expression, which results in the suppression of phenoloxidase activity. Moreover, recombinant Bm CTL21 was found to recognize carbohydrates, such as lipopolysaccharide, mannan and N‐ acetyl‐ d ‐glucosamine, in a Ca 2+ ‐independent manner, and binds to the cell walls of Gram‐negative bacteria, Gram‐positive bacteria and fungi. In summary, this study offers new insights into the molecular mechanisms and functional behaviors of Bm CTL21 in the innate immune response of silkworms.