Critical roles of sea cucumber C-type lectin in non-self recognition and bacterial clearance.

C-type lectin is one important pattern recognition receptor (PRR) that plays crucial roles in multiple immune responses. A C-type lectin from sea cucumber Apostichopus japonicus (AjCTL-1) was characterized in the present study. The amino acid sequence of AjCTL-1 shared high similarities with other C-type lectins from invertebrates and vertebrates. The C-type lectin domain (CTLD) of AjCTL-1 contained a Ca(2+)-binding site 2 and four conserved cysteine residues. AjCTL-1 mRNA expression patterns in tissues and after bacterial challenge were then analysed. Quantitative PCR revealed that AjCTL-1 mRNA was widely expressed in the tested tissues of healthy sea cucumber. The highest expression level occurred in gonad followed by body wall, coelomocytes, tentacle, intestinum and longitudinal muscle, and the lowest expression level was in respiratory tree. AjCTL-1 mRNA expression in coelomocytes was significantly induced by gram-negative Listonella anguillarum and gram-positive Micrococcus luteus, with different up-regulation patterns post-challenge. Recombinant AjCTL-1 exhibited the ability to bind peptidoglycan directly, agglutinate M. luteus, Staphylococcus aureus and Escherichia coli, in a Ca(2+)-dependant manner, and enhance the phagocytosis of coelomocytes against E. coli in vitro. The results indicated that AjCTL-1 could act as a PRR in Apostichopus japonicus and had critical roles in non-self recognition and bacterial clearance against invading microbes.