Abstract
The serum-mannan binding protein (S-MBP) is a calcium-dependent C-type lectin specific for mannose and N-acetylglucosamine. S-MBP is known as a host defense factor involved in innate immunity, where the target ligands for S-MBP should be on the surface of exogenous microorganisms. In this study, we tried to find endogenous ligands for this endogenous lectin. Among the cells tested, only the lymphocytes from thymus of BALB/c mice expressed ligands for S-MBP on their surface, those from bone marrow, spleen, mesenteric lymph nodes and peripheral blood all being negative. Interestingly, among the thymocytes, only the immature thymocytes with the CD4+CD8+CD3low phenotype expressed ligands for S-MBP, and ligands for S-MBP decreased on their maturation. A major cell surface glycoprotein bearing S-MBP ligands was isolated and identified as CD45RO, which is a transmembrane protein with tyrosine phosphatase activity. Deglycosylation experiments with N-glycanase and endoglycosidase H indicated that the S-MBP ligands on thymic CD45 are high mannose type or hybrid type N-linked oligosaccharides. This unique presentation of S-MBP ligands on this special CD45 isoform suggested the possibility that the oligosaccharide portion of CD45 on immature thymocytes is associated with the maturation, development or selection events of thymocytes.
Highlights
The serum mannan-binding protein (S-MBP)1 has been isolated from various mammalian sera and characterized as a calcium-dependent C-type lectin, which recognizes mannose and N-acetylglucosamine [1,2,3,4]
Lymphocytes obtained from bone marrow, thymus, spleen, mesenteric lymph nodes, and peripheral blood of 5-week-old male BALB/c mice were incubated with fluorescein isothiocyanate (FITC)-labeled rabbit serum-mannan binding protein (S-MBP) in the presence of Ca2ϩ and analyzed with a flow cytometer
Thymocytes were stained heavily with FITC-labeled rabbit S-MBP, the level of which was about 100-fold higher than the control value, whereas other lymphocytes did not show significant staining. These results indicated that thymocytes can be differentiated into two groups with regard to reactivity to S-MBP
Summary
S-MBP; serum-mannan binding protein; MBP, mannan-binding protein; mAb, monoclonal antibody; CD, cluster of differentiation; PNA, peanut agglutinin; PAGE, polyacrylamide gel electrophoresis; FITC, fluorescein isothiocyanate; PE, phycoerythrin; CHAPS, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid. A variety of microorganisms have manno-oligosaccharide structures on their surface, while mammalian cells generally do not. For this reason, S-MBP recognizes exogenous microorganisms. We demonstrated that S-MBP exhibits complement-dependent cytotoxic activity toward mammalian cells that express high mannose type oligosaccharides [11], suggesting the possibility that S-MBP eliminates abnormal mammalian cells that express high mannose type oligosaccharides on their surface. The major glycoprotein which carried S-MBP ligands was identified as CD45 (T-200, leukocyte common antigen). This thymic CD45 carried characteristic oligosaccharides that were not carried by the CD45 molecules from other tissues. The biological significance of this finding is discussed with regard to the function of this transmembrane protein with tyrosine phosphatase activity
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