Functions of two novel human lectins in glycoprotein clearance and cell migration in the immune system

Abstract

Potential biological roles for two novel human glycan-binding proteins have been investigated. In the first part of the work, analysis of the scavenger receptor C-type lectin (SRCL) leads to the proposal that the primary function of this receptor is to clear glycoproteins released from neutrophils. The expression of SRCL was characterized by screening of cDNA libraries and by immunostaining of tissues. These results confirm and extend earlier reports that SRCL is found in a range of tissues and is expressed by cultured umbilical vein endothelial cells, and demonstrated that SRCL is localized primarily in endothelial cells. SRCL is thus positioned to interact with circulating glycoproteins and cells. Potential ligands were isolated from neutrophils by affinity chromatography and shown by mass spectrometry to comprise lactoferrin and other soluble granule proteins. The interaction between SRCL and lactoferrin was found to be dependent on fucose residues and analysis of lactoferrin N-glycans by mass spectrometry revealed multiple Lewisx epitopes borne on unsialylated triantennary glycans, consistent with selective binding of SRCL to the Lewisx trisaccharide. Transfected cells expressing SRCL were shown to internalize fluorescein-labelled lactoferrin. Taken together, these results provide support for the hypothesis that soluble glycoproteins in neutrophil secondary granules are tagged with the Lewisx epitope to direct their clearance by SRCL after release at sites of inflammation. Prolectin, a novel C-type lectin, was identified by screening of the human genome. Screening of cDNA libraries and immunostaining were used to ascertain that prolectin expression is restricted to dividing B cells in and around germinal centres of lymphoid tissues. This expression pattern, together with the presence of motifs in the cytoplasmic tail of prolectin that may interact with intracellular signalling molecules, leads to the suggestion that prolectin may have a role in the migration of B cells during the adaptive immune response.