Collectins, col-lectins, (collagen-containingC-type lectins) are a part of theinnate immune system. They form a sub-family of collagenous Ca²⁺-dependentlectins of the family ofC-type lectins, which are found in animals. Collectins are solublepattern recognition receptors (PRRs). Their function is to bind to oligosaccharide structure or lipids that are on the surface of microorganisms. Like other PRRs they bindpathogen-associated molecular patterns (PAMPs) anddamage-associated molecular patterns (DAMPs) of oligosaccharide origin. Binding of collectins to microorganisms may trigger elimination of microorganisms byaggregation,complement activation,opsonization, activation ofphagocytosis, or inhibition of microbial growth. Other functions of collectins are modulation ofinflammatory,allergic responses,adaptive immune system and clearance ofapoptotic cells.

Functionally collectins are trimers. Monomeric subunit consists of four parts:

• acysteine-rich domain at the N-terminus
• acollagen-like domain
• acoiled-coil neck domain
• aC-type lectin domain that is also called acarbohydrate recognition domain (CRD)

Recognition of specific parts of microorganism is mediated by CRD in presence of calcium.^[1][2] Affinity of interaction between microbes and collectins depends on the degree of collectin oligomerization and also on the density of ligands on the surface of the microbe.^[3]

Nine types of collectins have been defined:

• MBL =mannan-binding lectin (mannose-binding lectin)
• SP-A =surfactant protein A
• SP-D =surfactant protein D
• CL-L1 =collectin liver 1
• CL-P1 =collectin placenta 1
• CL-43 =Collectin of 43 kDa
• CL-46 =collectin of 46 kDa
• CL-K1 =collectin kidney 1
• Conglutinin^[4]

CL-43, CL-46 and conglutinin are found inbovines.

Collectins can bind to the surface of microorganisms and between carbohydrate ligands. Due to these properties, the interaction can result in aggregation.^[5][6]

Collectins can act as opsonins. There is a specific interaction between collectins and receptors on phagocytic cells which can lead to increased clearance of microorganisms.^[7][8][9] MBL can bind to microorganisms and this interaction can lead to opsonization through complement activation,^[10] or it can opsonize the microorganism directly.^[11] SP-A and SP-D can also interact with microorganisms and phagocytic cells to enhance phagocytosis of the microorganism.^[12]

Collectins have effect on microorganism survival. SP-A and SP-D can bind toLPS (lipopolysaccharide) of bothGram-negative andGram-positive bacteria. SP-A and SP-D can increase permeability of Gram-negative bacterial cell membrane.^[13]

SP-A and SP-D can damp induction of inflammation by LPS orendotoxin. It can be caused by removing the LPS or by binding the LPS toCD14 receptor onmacrophages that can block the inflammatory response.^[14][15][16] SP-A can also bind toTLR2 (toll-like receptor 2). This interaction causes decrease ofTNF-α (tumor necrosis factor-α) production by alveolar macrophages stimulated withpeptidoglycan.^[17] SP-A and SP-D can modulate cytokine production. They modulate the production of oxygen and nitrogen reactive species which are very important for phagocytic cells.^[18][19][20] SP-A and SP-D has s function as chemoattractants for alveolarneutrophils andmonocytes.^[21] MBL can recognize peptidoglycan viaN-acetylglucosamine. This interaction leads to inhibition of ligand-induced inflammatory by macrophage chemokine production.^[22]

SP-A and SP-D can suppress activatedT-lymphocytes andIL-2 (interleukin-2) production.^[23][24] SP-D increases bacterialantigen presentation bydendritic cells^[25] whereas SP-A blocs differentation of the immature dendritic cells.^[26]

Collectins SP-A and SP-D have anti-allergic effects: they inhibitIgE binding toallergens, decreasehistamine release frombasophils, and inhibit T-lymphocyte production in the late phase of the inflammation.^[27][28][29]

Collectins SP-A and SP-D enhance clearance of apoptotic cells by macrophages.^[30][31]

Collectins are linked with activation of lectin pathway of complement activation. At the beginning, there is a binding of collectin to PAMPs or DAMPs. Collectin MBL is involved in activation of the lectin complement pathway.^[32][33] There are three serine proteases, MASP-1, 2 and 3 (MBL-associated serine proteases), which participate in activation of the lectin pathway. MASP-2 has a cleavage activity and it is essential for forming lectin C3 and C5 convertases and for activation of the complement.^[34][35][36]

For more information and details see reviews:^[37][38][39]

• Lectins

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