Ficolin

Ficolins (Fi+Col+Lin) are a group of oligomeric lectins with N-terminal collagen-like domain and a C-terminal fibrinogen-like domain. The primary ficolin structure contains 288 amino acids. The combination of collagen-like and fibrinogen-like domain allows the protein to form a basic subunit containing a triple helical tail and a trio of globular heads.^[2]

Ficolins are produced in the liver by hepatocytes and in the lung by alveolar cells type II, neutrophils and monocytes.^[3]

We now know that innate immune recognition mechanisms are sophisticated. Exocrine secretions provide a variety of soluble factors that are able to protect the body from potential pathogens.^[4]

Together with pentraxins, collectins and C1q molecules, ficolins constitute the soluble pattern-recognition molecules (PRMs) which play an important role in humoral innate immunity.^[4] Ficolins recognise carbohydrate structures on pathogens' surfaces as their pathogen-associated molecular pattern (PAMP) and activate the lectin pathway of the complement cascade.^[3][5] Specifically, ficolins bind to acetyl groups present in certain bacterial molecules, such as N-acetylglucosamine, a component of peptidoglycan in the bacterial cell wall.^[1][6] When ficolins bind to their PAMP ligands by their C-terminal fibrinogen-like domain,^[1] they initiate the proteolytic complement cascade, facilitated by the mannose-binding protein-associated serine proteases (MASPs) that ficolins are associated to and co-circulate with.^[1][6] Serine proteases then cleave a number of soluble complement proteins leading to complement activation, opsonisation, generation of proinflammatory mediators, and cell lysis.^[7]

Collectins and ficolins are also called collagenous lectins. The collectin family constitutes calcium-dependent proteins. In contrast, the ficolin family does not bind to PAMPs in a calcium-dependent way.^[3]