A C1qDC (CgC1qDC-6) with a collagen-like domain mediates hemocyte phagocytosis and migration in oysters

Abstract Most of the bivalve C1q domain containing proteins (C1qDCs) are either only composed of the globular head domain, or contain an N-terminal coiled-coil domain, presumed to cover a role in oligomerization. On the other hand, collagen regions, widespread in vertebrate C1qDCs, are very uncommon in bivalves. In the present study, a C1qDC with a collagen-like domain (designated Cg C1qDC-6) was identified from the Pacific oyster Crassostrea gigas and its possible involvement in immune responses was also characterized. The coding sequence of Cg C1qDC-6 was of 756 bp, encoding a peptide of 251 amino acids with an N-terminal signal peptide, a central collagen-like domain, and a C-terminal ghC1q domain. Cg C1qDC-6 was clustered with the C1qDCs from several mollusks in the phylogenetic tree. Cg C1qDC-6 was detected at both mRNA and protein levels in all tested tissues including hepatopancreas, gonad, gill, mantle, adductor muscle, and hemocytes. The recombinant Cg C1qDC-6 protein (r Cg C1qDC-6) exhibited binding activity to various pathogen-associated molecular patterns (PAMPs) including LPS, PGN, mannose and Poly I:C, and microorganisms including Gram-negative bacteria ( Escherichia coli and Vibrio splendidus ), Gram-positive bacteria ( Micrococcus luteus and Staphylococcus aureus ), and fungus ( Pichia pastoris ). The phagocytic rates of oyster hemocytes towards V . splendidus pre-incubation with r Cg C1qDC-6 were significantly enhanced ( p Cg C1qDC-6 could mediate the migration of oyster hemocytes in a dose-dependent manner, which exhibited a positive chemotactic effect at low concentration ( Cg C1qDC-6 could serve as a pattern recognition receptor and mediate the hemocyte phagocytosis and migration to eliminate the invading pathogens.