Soluble Collectin-12 (CL-12) Is a Pattern Recognition Molecule Initiating Complement Activation via the Alternative Pathway.
Soluble defense collagens including the collectins play important roles in innate immunity. Recently, a new member of the collectin family named collectin-12 (CL-12 or CL-P1) has been identified. CL-12 is highly expressed in umbilical cord vascular endothelial cells as a transmembrane receptor and may recognize certain bacteria and fungi, leading to opsonophagocytosis. However, based on its structural and functional similarities with soluble collectins, we hypothesized the existence of a fluid-phase analog of CL-12 released from cells, which may function as a soluble pattern-recognition molecule. Using recombinant CL-12 full length or CL-12 extracellular domain, we determined the occurrence of soluble CL-12 shed from in vitro cultured cells. Western blot showed that soluble recombinant CL-12 migrated with a band corresponding to ∼ 120 kDa under reducing conditions, whereas under nonreducing conditions it presented multimeric assembly forms. Immunoprecipitation and Western blot analysis of human umbilical cord plasma enabled identification of a natural soluble form of CL-12 having an electrophoretic mobility pattern close to that of shed soluble recombinant CL-12. Soluble CL-12 could recognize Aspergillus fumigatus partially through the carbohydrate-recognition domain in a Ca(2+)-independent manner. This led to activation of the alternative pathway of complement exclusively via association with properdin on A. fumigatus as validated by detection of C3b deposition and formation of the terminal complement complex. These results demonstrate the existence of CL-12 in a soluble form and indicate a novel mechanism by which the alternative pathway of complement may be triggered directly by a soluble pattern-recognition molecule.
The Laboratory of Molecular Medicine, Department of Clinical Immunology, Section 7631, Rigshospitalet, Faculty of Health and Medical Sciences, University of Copenhagen, 2100 Copenhagen, Denmark; and.
Ying Jie
YJ
Ma
The Laboratory of Molecular Medicine, Department of Clinical Immunology, Section 7631, Rigshospitalet, Faculty of Health and Medical Sciences, University of Copenhagen, 2100 Copenhagen, Denmark; and.
Estrid
E
Hein
The Laboratory of Molecular Medicine, Department of Clinical Immunology, Section 7631, Rigshospitalet, Faculty of Health and Medical Sciences, University of Copenhagen, 2100 Copenhagen, Denmark; and.
Lea
L
Munthe-Fog
The Laboratory of Molecular Medicine, Department of Clinical Immunology, Section 7631, Rigshospitalet, Faculty of Health and Medical Sciences, University of Copenhagen, 2100 Copenhagen, Denmark; and.
Mikkel-Ole
MO
Skjoedt
The Laboratory of Molecular Medicine, Department of Clinical Immunology, Section 7631, Rigshospitalet, Faculty of Health and Medical Sciences, University of Copenhagen, 2100 Copenhagen, Denmark; and.
Rafael
R
Bayarri-Olmos
Microbiology Section, Department of Experimental Medicine and Biochemical Science, University of Perugia, 06122 Perugia, Italy.
Luigina
L
Romani
The Laboratory of Molecular Medicine, Department of Clinical Immunology, Section 7631, Rigshospitalet, Faculty of Health and Medical Sciences, University of Copenhagen, 2100 Copenhagen, Denmark; and garred@post5.tele.dk.
Peter
P
Garred
D001232:Aspergillus fumigatus / Q000276:immunology*; D037222:Collectins / Q000097:blood / Q000276:immunology*; D003167:Complement Activation / Q000276:immunology*; D003179:Complement C3b / Q000276:immunology; D003170:Complement Pathway, Alternative / Q000276:immunology*; D005260:Female; D005312:Fetal Blood; D061307:Human Umbilical Vein Endothelial Cells / Q000378:metabolism; D006801:Humans; D007113:Immunity, Innate / Q000276:immunology; D011414:Properdin / Q000276:immunology; D051116:Receptors, Scavenger / Q000097:blood / Q000276:immunology*; D013172:Spores, Fungal / Q000276:immunology*
C422442:COLEC12 protein, human; D037222:Collectins; D051116:Receptors, Scavenger; D011414:Properdin; D003179:Complement C3b