Structure and function of collectin liver 1 (CL-L1) and collectin 11 (CL-11, CL-K1).

The collectins are a group of innate immune proteins structurally characterized by their content of a carbohydrate recognition domain and a collagen-like region. Collectin liver 1 (CL-L1) and collectin 11 (CL-11, alias collectin kidney 1, CL-K1) are the more recently described members of this group. Their genomic organization and protein structure reveal many similarities. However, CL-11 is a serum protein, whereas CL-L1 appears to be restricted to the cytosol of cells such as hepatocytes. Specificity analyses of the CRDs reveal some differences in their preferences for saccharides: CL-11 binds most avidly to l-fucose and d-mannose, whereas CL-L1 shows preference for d-mannose, d-fucose, N-acetylglucosamine, and surprisingly also d-galactose. CL-11 binds to various microorganisms including Escherichia coli, Candida albicans and Influenza A virus. Polymorphisms in the CL-11 gene (COLEC11) leading to deficiencies have recently been identified as causative for 3MC syndrome. The 3MC syndrome is associated with a wide spectrum of developmental features including facial dysmorphism, cognitive impairment, hearing loss and vesicorenal anomalies. Similar polymorphic associations were reported for the mannan-binding lectin-associated serine protease 3 (MASP-3), and falls into line with the observation that CL-11 is found in circulating complexes with MASP-1/3. These findings suggest dual or overlapping functions of CL-11 in innate immunity and in fetal development.

Collectin 11 (CL-11, CL-K1) is a MASP-1/3-associated plasma collectin with microbial-binding activity.

Collectins play important roles in the innate immune defense against microorganisms. Recently, a new collectin, collectin 11 (CL-11 or CL-K1), was identified via database searches. In present work, we characterize the structural and functional properties of CL-11. Under nonreducing conditions, in gel permeation chromatography recombinant CL-11 forms disulfide-linked oligomers of 100 and 200 kDa. A mAb-based ELISA estimates the concentration of CL-11 in plasma to be 2.1 µg/ml, and the presence of CL-11 in plasma was further verified by Western blotting and mass spectrometry. Mannan-binding lectin-associated serine protease 1 (MASP-1) copurified with CL-11 and the interaction in plasma with MASP-1 and/or MASP-3 was further demonstrated using ELISA. We identified the adrenal glands, the kidneys, and the liver as primary sites of expression. CL-11 lectin activity was demonstrated by ELISA and showed that CL-11 has preference for l-fucose and d-mannose. We finally show that CL-11 binds to intact bacteria, fungi, and viruses and that CL-11 decreases influenza A virus infectivity and forms complexes with DNA. On the basis of the significant concentration of CL-11 in circulation and CL-11's interaction with various microorganisms and MASP-1 and/or MASP-3, it is conceivable that CL-11 plays a role in activation of the complement system and in the defense against invading microorganisms.

Expression and tissue localization of collectin placenta 1 (CL-P1, SRCL) in human tissues.

Collectin placenta-1 (CL-P1), also known as scavenger receptor with C-type lectin (SRCL), is a type II membrane glycoprotein that shares structural features with both collectins and type A scavenger receptors. CL-P1 was originally cloned from the placenta and found to be associated with endothelial cells. It binds via its lectin domain to desialyated Lewis X containing glycoproteins and it is able to facilitate internalization of bound ligands. Via positively charged residues in the collagen-like region it binds to negatively charged components of microbial membranes. It has previously been proposed that CL-P1 plays a role in the host defense system and in the clearance of glycoproteins from the blood. With the aims of determining the detailed tissue expression of human CL-P1 we expressed CL-P1 recombinantly in both E. coli and CHO cells, and raised monoclonal antibodies against human CL-P1. Three monoclonal antibodies were characterized and used in immunohistochemical analyses of a panel of cryo- and formalin-fixed sections. We find that CL-P1 mainly associates with cytotrophoblasts and syncytiotrophoblasts of the placenta, alveolar macrophages and to a less degree with macrophage-like and stromal cells of the tonsils. By real-time RT-PCR we verified that the placenta is also the main organ of CL-P1 synthesis. The only source of endothelial cells whereto CL-P1 associates are umbilical cord vein endothelial cells (human umbilical vein endothelial cells, HUVEC). In vitro cultured HUVECs express both the CL-P1 mRNA and show anti-CL-P1 immunoreactivity but CL-P1 locates mainly to the cytosol and not to the membrane of these cells. We conclude that CL-P1 is not a common membrane protein on endothelial cells found in normal tissues under steady state conditions.