Nonspecific esterase released from thymic macrophages accumulates in the apoptotic thymocytes: an indication for this enzyme participating in the clearance of apoptotic thymocytes

In the mouse thymus, a large number of developing thymocytes die through apoptosis each day. It has been proposed that thymic macrophages are responsible for clearance of the massive number of thymocytes that die through apoptosis. The detailed clearance mechanism by which macrophages remove the apoptotic cells is not clear. Our in vitro studies in this report show that nonspecific esterase (NSE), a cytochemical marker enzyme of macrophages, was secreted from thymic macrophages as a consequence of stimulation by interaction with thymocytes, and the esterase accumulated in these macrophage-binding thymocytes (MBT). TUNEL staining demonstrated that these MBT were undergoing apoptosis. The inability to exclude eosin Y and the presence of pores on the plasma membrane were further evidence for the disintegration of these MBT. In vivo, the release of NSE was evident by the presence of NSE activity in the extracellular space between the macrophages and apoptotic thymocytes under the transmission electron microscope after dexamethasone injection, which causes massive apoptosis of thymocytes. Inhibition study showed that the inhibition of NSE delayed the MBT progressing to the late apoptotic phase. These results suggest that the NSE released from macrophages is involved in the clearance of apoptotic thymocytes.