An ancient lectin-dependent complement system in an ascidian: novel lectinisolated from the plasma of the solitary ascidian, Halocynthia roretzi

Mannose-binding lectin (MBL) is a C-type lectin involved in the first line of host defense against pathogens and it requires MBL-associated serine pro tease (MASP) for activation of the complement lectin pathway. To elucidate the origin and evolution of MBL, MBL-like lectin was isolated from the plas ma of a urochordate, the solitary ascidian Halocynthia roretzi, using affin ity chromatography on a yeast mannan-Sepharose. SDS-PAGE of the eluted prot eins revealed a major band of similar to 36 kDa (p36). p36 cDNA was cloned from an ascidian hepatopancreas cDNA library. Sequence analysis revealed th at the carboxy-terminal half of the ascidian lectin contains a carbohydrate recognition domain (CRD) that is homologous to C-type lectin, but it lacks a collagen-like domain that is present in mammalian MBLs. Purified p36 bin ds specifically to glucose but not to mannose or N-acetylglucosamine, and i t was designated glucose-binding lectin (GBL). The two ascidian MASPs assoc iated with GBL activate ascidian C3, which had been reported to act as an o psonin. The removal of GBL-MASPs complex from ascidian plasma using Ab agai nst GBL inhibits C3-dependent phagocytosis. These observations strongly sug gest that GBL acts as a recognition molecule and that the primitive complem ent system, consisting of the lectin-proteases complex and C3, played a maj or role in Innate immunity before the evolution of an adaptive immune syste m in vertebrates.