Calcium-dependent lectin activity with novel distribution on thymocyte subsets

In this study we use carbohydrate probes to search for novel cell surface lectins in the immune system. Many carbohydrate binding proteins are members of the C-type lectin superfamily, whose ligand binding is dependent on calcium. To identify potential new members of this superfamily, fluorescein-conjugated carbohydrate polymers were used to probe for calcium-dependent cell surface binding. This approach offers advantages over the use of monoclonal antibody probes since only carbohydrate binding proteins would be identified. We have identified a cell surface lectin, referred to as thy-lec, detected by the probe fucoidan-fuorescein isothiocyanate. This calcium-dependent lectin has a novel distribution on thymocyte subsets. It is present on the surface of immature CD4+8+ and on large, cycling CD4-8- cells and CD8+4- cells, but not on small, mature phenotype CD8+4- or CD4+8- thymocytes. This lectin is not found on mature T cells or other leukocytes in lymph nodes, spleen or bone marrow. It is proposed that this novel cell surface has a function in the maturation of T cells in the thymus.