Role of mannan-binding protein, MBP, in innate immunity.

Mannan-binding protein (MBP) is a C-type lectin, which binds to carbohydrates on the surface of some microorganisms and kills them through the activation of complement. This complement activation pathway is called the lectin pathway. MBP also kills mammalian cells that express MBP ligands on their surfaces via the lectin pathway. Recently, we found anti-tumor activity of MBP in vivo using tumor cells transplanted into nude mice. We propose to call this anti-tumor effect mannan-binding protein-dependent cell-mediated cytotoxicity (MDCC), because it does not require complement activation, and the involvement of some immune cells was assumed. Very recently, MBP was demonstrated to be selectively expressed in epithelial cells of the small intestine. This finding may suggest that MBP plays an important role in the small intestine as a host defense factor.