Human scavenger receptor B1 is involved in recognition of apoptotic thymocytes by thymic nurse cells

Recognition and uptake of apoptotic cells by neighboring phagocytes is essential for the clearance of dying cells without accompanying inflammation or tissue damage. In the thymus, many apoptotic cells are generated in the process of negative selection, and both thymic macrophages (professional phagocytes) and nursing thymic epithelial cells (nursing TEC; nonprofessional phagocytes) recognize and ingest them. However the receptors responsible for this recognition and uptake have not been identified. In the present study, we have established a human nursing TEC line and examined the expression of several genes of the scavenger receptor family considered to be potential receptors for apoptotic cells. Human scavenger receptor-Ri (hSR-B1)/CLA-1, previously shown to recognize apoptotic cells, was strongly expressed in nursing TEC, whereas there was little or no expression of the other scavenger receptors tested: scavenger receptor class A, CD36, or CD68. Suppression of hSR-B1/CLA-1 expression using antisense oligonucleotides decreased the binding of apoptotic thymocytes to nursing TEC by more than 40%. These results indicate that hSR-B1/CLA-1 may play a major role in the clearance of apoptotic cells in the thymus, mediating the recognition and ingestion of apoptotic thymocytes by nursing TEC.