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Apoptotic Thymocyte Clearance in Scavenger Receptor Class A-Deficient Mice Is Apparently Normal

Nick Platt^1,*, Hiroshi Suzuki, Tatsuhiko Kodama and Siamon Gordon^*

^* Sir William Dunn School of Pathology, University of Oxford, Oxford, United Kingdom; and Department of Molecular Biology and Medicine, Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan

Studies of apoptotic cell uptake by phagocytes in vitro have implicated a number of different receptors capable of mediating ingestion. However, there is currently little evidence for involvement of any of these candidate receptors in vivo. Previously, we have shown by the use of a blocking mAb against the class A scavenger receptor (SR-A) and thymic macrophages prepared from SR-A null mice, that this receptor is responsible for 50% of the uptake of apoptotic thymocytes in vitro. In this study we have investigated the frequency of dying cells in the thymus of mice lacking SR-A. Our inability to demonstrate increased frequencies of nonphagocytosed Annexin V⁺, TUNEL⁺, or propidium iodide⁺ apoptotic thymocytes suggests there is no deficiency in apoptotic thymocyte clearance in these mice. Even when the rate of thymocyte apoptosis was increased by exposure of receptor-deficient mice to gamma irradiation, we did not detect a difference in the numbers of dying cells compared with similarly treated wild-type animals. This provides the first direct evidence of redundancy in apoptotic cell clearance mechanisms in vivo.