A novel C-type lectin 4E from Cromileptes altivelis (CaCTL4E) participates in antibacterial innate immunity

Abstract

C-type lectins (CTLs) play a crucial role as pattern recognition receptors (PRRs) in the innate immune system. Nevertheless, the underlying mechanisms of CTL-mediated immune recognition and response in bony fish remain poorly understood. In the current study, a CTL member, namely CaCTL4E, was characterized in Cromileptes altivelis, and its biological functions and roles in response to bacterial infection were analyzed. The CaCTL4E gene encodes 254 amino acids with a 765 bp coding sequence and exhibits typical C-type lectin structural features, including a carbohydrate recognition domain (CRD), six conserved cysteine residues, a WIGL motif, and a predicted mannose-binding EPN motif. In healthy C. altivelis, CaCTL4E was differentially expressed in all tested tissues, with a predominant expression in the liver. Notably, upon Vibrio harveyi stimulation, the expression of CaCTL4E in the spleen and head kidney was significantly upregulated, indicating its active involvement in the immune response. The recombinant CaCTL4E (rCaCTL4E) exhibited the ability to agglutinate red blood cells and four bacteria (V. harveyi, Edwardsiella tarda, Streptococcus agalactiae, and Staphylococcus aureus), as well as to bind these bacteria and four pathogen-associated molecular patterns (PAMPs: lipopolysaccharide, peptidoglycan, mannose, and galactose). Interestingly, the binding of rCaCTL4E to PAMPs competitively inhibited its bacterial binding and agglutination activities. Moreover, rCaCTL4E directly inhibited the growth of the four bacteria in vitro. In vivo, the overexpression or knockdown of CaCTL4E led to a decrease or increase in the bacterial burden, respectively. Importantly, rCaCTL4E also exhibited immunomodulatory properties by enhancing the phagocytic activity of head kidney macrophages. These results provide evidence that CaCTL4E acts as the important PRR, and lay a foundation for further understanding the process of CaCTL4E participating in the immune response to pathogenic infections.