Epitope mapping of monoclonal antibodies specific for a macrophage lectin: a calcium-dependent epitope is in the carbohydrate recognition domain

Identification of Sialoadhesin as a Dominant Lymph Node Counter-receptor for Mouse Macrophage Galactose-type C-type Lectin 1

In the sensitization phase of contact hypersensitivity in mice, dermal macrophages (MOs) expressing MO galactose-type C-type lectin1 (MGL1) are known to migrate from the dermis to lymph nodes (LNs) where they accumulate in the subcapsular sinus, interfollicular regions, and areas surrounding high endothelial venules. We hypothesize that the interactions between MGL1 and its ligands determine the localizations of MGL1-positive cells within the LNs. In the present study, our major aim was to isolate MGL1 counter-receptor(s) from lysates of LNs using affinity chromatography with immobilized recombinant MGL1. Fractions bound and eluted with EDTA were analyzed by SDS-PAGE and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. One of the predominant components was sialoadhesin (Sn, Siglec-1). Sn from lysates of LNs was immobilized on microtiter plates precoated with anti-Sn monoclonal antibody, and binding of recombinant MGL1 and adhesion of cells expressing MGL1 were tested. The binding of recombinant MGL1 to Sn was shown to be dependent on Ca2+ and N-glycans on Sn. MGL1-transfected Chinese hamster ovary cells adhered to the Sn-coated plates, whereas mock transfectants did not. Immunohistochemical localization of anti-Sn monoclonal antibody in LN coincided with the subcapsular sinus area to which recombinant MGL1 was bound. Furthermore, the distribution of MGL1+ cells after sensitization with FITC was demonstrated to overlap with that of Sn within the subcapsular sinus of draining LNs. These results suggest that Sn acts as an endogenous counter-receptor for MGL1.

Interactions of surfactant protein D with fatty acids

Surfactant Protein D (SP-D) plays important roles in antimicrobial host defense, inflammatory and immune regulation, and pulmonary surfactant homeostasis. The best-characterized endogenous ligand is phosphatidylinositol; however, this lipid interaction at least in part involves the carbohydrate moiety. In this study we observed that SP-D binds specifically to saturated, unsaturated, and hydroxylated fatty acids (FA). Binding of biotinylated-SP-D to FAs or biotinylated FA to SP-D was dose-dependent, saturable, and specifically competed by the corresponding unlabeled probe. Specific binding to FA chains was also demonstrated by solution phase competition for FA binding to acrylodan-labeled FA binding protein (ADIFAB), and by overlay of thin layer chromatograms with SP-D. Maximal binding to FA was dependent on calcium, and binding was localized to the neck and carbohydrate recognition domains (CRD) using recombinant trimeric neck+CRDs. Saccharide ligands showed complex, dose-dependent effects on FA binding, and FAs showed dose- and physical state-dependent effects on the binding of SP-D to mannan. In addition, CD spectroscopy suggested alterations in SP-D structure associated with binding to monomeric FA. Together, the findings indicate specific binding of FA to one or more sites in the CRD. We speculate that the binding of SP-D to the fatty acyl chains of surfactant lipids, microbial ligands, or other complex lipids contributes to the diverse biological functions of SP-D in vivo.

Molecular cloning and characterization of a novel mouse macrophage C-type lectin, mMGL2, which has a distinct carbohydrate specificity from mMGL1

A novel mouse macrophage galactose-type C-type lectin 2 (mMGL2) was identified by BLAST analysis of expressed sequence tags. The sequence of mMGL2 is highly homologous to the mMGL, which should now be called mMGL1. The open reading frame of mMGL2 contains a sequence corresponding to a type II transmembrane protein with 332 amino acids having a single extracellular C-type lectin domain. The 3'-untranslated region included long terminal repeats of mouse early transposon. The Mgl2 gene was cloned from a 129/SvJ mouse genomic library and sequenced. The gene spans 7,136 base pairs and consists of 10 exons, which is similar to the genomic organization of mMGL1. The reverse transcriptase-PCR analysis indicates that mMGL2 is expressed in cell lines and normal mouse tissues in a macrophage-restricted manner, also very similar to that of mMGL1. The mMGL2 mRNA was also detected in mMGL1-positive cells, which were sorted from thioglycollate-induced peritoneal cells with a mMGL1-specific monoclonal antibody, LOM-8.7. The soluble recombinant proteins of mMGL2 exhibited carbohydrate specificity for alpha- and beta-GalNAc-conjugated soluble polyacrylamides, whereas mMGL1 preferentially bound Lewis X-conjugated soluble polyacrylamides in solid phase assays. These two lectins may function cooperatively as recognition and endocytic molecules on macrophages and related cells.