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Detection of an autologous ligand for mannan-binding lectin on human B lymphocytes
I Downing1, S L MacDonald, M L Turner
1Cell Therapy Group, Scottish National Blood Transfusion Service National Science Laboratory, Edinburgh, UK. ian.downing@snbts.csa.scot.nhs.uk
Insights
Mannan-binding lectin (MBL) binds to B lymphocytes and natural killer cells, in addition to monocytes and dendritic cells. This interaction via MBL's carbohydrate-recognition domain may have functional significance.
Area of Science:
- Immunology
- Molecular Biology
- Glycobiology
Background:
- Mannan-binding lectin (MBL) is a key soluble pattern-recognition protein involved in innate immunity.
- MBL recognizes carbohydrate structures on pathogens and self-cells via its C-type carbohydrate recognition domain.
- Previous studies indicated MBL binding to monocytes and dendritic cells.
Purpose of the Study:
- To investigate the interaction of MBL with fresh human peripheral blood cells.
- To characterize MBL binding to B lymphocytes and compare it with binding to monocytes and dendritic cells.
Main Methods:
- Flow cytometry was used to assess MBL binding to various human peripheral blood cells.
- Inhibition assays were performed using specific sugars (mannose, GlcNAc, galactose), mannan, autologous serum, and C1q.
Main Results:
- MBL was found to bind specifically to B lymphocytes and natural killer cells.
- MBL binding to B cells was observed at physiological and optimal at supraphysiological concentrations.
- Binding to B cells and dendritic cells was inhibited by mannose, mannan, and autologous serum, while monocytes showed different sensitivity.
Conclusions:
- MBL binds to differently glycosylated ligands on B lymphocytes, natural killer cells, dendritic cells, and monocytes via its carbohydrate-recognition domain.
- The observed binding suggests potential functional roles for MBL at extravascular sites.
- Functional significance may be restricted to individuals with MBL-sufficient genotypes.
Abstract:
Mannan-binding lectin (MBL) is a collectin and a major soluble pattern-recognition protein. MBL can distinguish self from nonself and altered self using its C-type carbohydrate recognition domain and may also interact via its collagen-like region with autologous cells. Recently, it was found that MBL could bind to adherent cells (monocytes) and dendritic cells in a specific and sugar-sensitive manner. We have now investigated the interaction of MBL with fresh human peripheral blood cells and report binding to B lymphocytes and natural killer cells. The binding to B lymphocytes was studied in detail and was compared with the binding of MBL to monocytes and dendritic cells. Binding of MBL to B cells was evident at physiological MBL and calcium concentrations but was optimal at supraphysiological MBL concentrations. It was readily inhibited by autologous serum, mannan, mannose, GlcNAc and (to a lesser extent) galactose but not by C1q. A similar, but not identical, inhibition profile was observed with dendritic cells, but monocytes were not sensitive to mannose or mannan. We conclude that MBL is capable of binding to differently glycosylated ligands on several autologous cell types via its carbohydrate-recognition domain. We speculate that this could have functional significance at extravascular sites, but perhaps only in individuals possessing MBL genotypes conferring MBL sufficiency.
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