The C-Type lectin AcCTL5 from apis cerana mediates anti- nosema ceranae defense via agglutination and hemolymph melanization

Unlike vertebrates, which use adaptive immunity, insects depend on humoral and cellular innate immunity to resist pathogens, with pattern recognition receptors (PRRs) like C-type lectins (CTLs) critical for recognizing pathogen-associated molecular patterns (PAMPs). CTLs are a diverse, mostly Ca2 + -dependent glycoprotein superfamily defined by conserved Carbohydrate Recognition Domains (CRDs), enabling ligand binding and mediating immune defenses in invertebrates. Apis cerana, a vital pollinator, face severe threats from pathogens such as Nosema ceranae, a major cause of nosemosis that damages midguts, reduces vitality, and triggers colony collapse. This highlights the urgency of studying bee immune mechanisms. Here, we identified AcCTL5 (a CTL from A. cerana), prepared, purified its recombinant protein, and characterized its functions. Recombinant AcCTL5 strongly agglutinated N. ceranae, bound significantly to PAMPs (galactose, peptidoglycan, lipoteichoic acid, lipopolysaccharide), reduced honeybee infection rates, and participated in prophenoloxidase activation and hemolymph melanization. These results suggested that AcCTL5 may play an important role in the immune defense response of A. cerana against N. ceranae, and lay the foundation for further research on the molecular mechanism of AcCTL5 involved in the innate immunity of A. cerana.