CTSL-2 upon specifically recognizing Vibrio splendidus directly cleaves complement C3 to promote the bacterial phagocytosis and degradation in oyster.

Cathepsin L (CTSL) as a cysteine cathepsin protease mediates complement C3 cleavage and pathogen degradation. In the present study, a CTSL homolog was identified from Crassostrea gigas (designated as CgCTSL-2). Its mRNA expression increased significantly in hemocytes after Vibrio splendidus stimulation. The activity of rCgCTSL-2 was induced after incubation with LPS or V. splendidus in Ca²⁺-dependent manner. rCgCTSL-2 could specifically bound V. splendidus in Ca²⁺-dependent manner. The co-localization of rCgCTSL-2 and V. splendidus was observed in cell-free hemolymph. Upon binding V. splendidus, CgCTSL-2 interacted with CgC3 in cell-free hemolymph and hemocytes. CgC3 fragments in CgCTSL-2-RNAi oysters and full length CgC3 in rCgCTSL-2-treated oysters were both reduced in cell-free hemolymph, respectively. CgC3 fragments were accumulated in CgCTSL-2-RNAi or rCgCTSL-2-treated oysters. The co-localizations of V. splendidus, CgC3, CgCD18, CgCTSL-2 and lysosomes were observed in hemocytes. These results suggested that CgCTSL-2 upon binding V. splendidus directly interacted with CgC3 to lead to CgC3 cleavage and then CgC3 fragments coated on V. splendidus were mediated by CgCD18 into CTSL-2-lysosome pathway.