Collectin

Collectins are soluble pattern recognition receptors (PRRs) belonging to the superfamily of collagen containing C-type lectins.

Eight collectins have been identified including mannan-binding lectin (MBL), surfactant protein A (SP-A), surfactant protein D (SP-D), collectin liver 1 (CL-L1), collectin placenta 1 (CL-P1), conglutinin, collectin of 43 kDa (CL-43) and collectin of 46 kDa (CL-46).

These molecules have been implicated as major modulators of the innate immune system where they have a key role in the first line of defense against invading microorganisms by binding to them so macrophages know to dispose of them.^[1]

Functionally, collectins are usually trimers with the number of trimeric units differing among the collectin family.

Structural domains

The collectin monomers comprise four structural domains;

• a cysteine-rich domain at the N-terminus.
• a collagen domain.
• a coiled-coil neck domain .
• a C-type lectin domain that is also called a carbohydrate recognition domain (CRD). Collectins selectively bind to specific complex carbohydrates of microbes using their CRDs.^[2]

Responses

The binding of collectins to microbes causes several immune related responses.

• They cause the formation of aggregates of the microorganisms, opsonize the microorganisms to increase their phagocytosis and some upregulate of the activity of other PRRs such as the mannose receptor.

• Collectins can also induce the production of pro-inflammatory molecules like cytokines, and reactive oxygen species in phagocytes by interacting with other cell surface receptors or by scavenging of bacterial molecules like lipopolysaccharide (LPS).

• Some collectins (e.g. MBL) activate the lectin pathway of the complement system to increase membrane permeability of microorganisms and cause the destruction of that microbe.^[2]

References

1. ^ http://www.jimmunol.org/cgi/content/full/169/10/5726
2. ^ ^{a b} van de Wetering et al. Collectins: Players of the innate immune system. Eur. J. Biochem. 271, 1229-1249 (2004)