The identification of thymic nurse cells in vivo and the role of cytoskeletal proteins in thymocyte internalization.

Much debate has been generated about the existence of thymic nurse cells within the thymus. Until now, the authenticity of an epithelial cell capable of internalizing developing thymocytes within the thymic cortex has been in question. Here, we use the thymic nurse cell-specific monoclonal antibody, ph91, to define the in vivo location of thymic nurse cells. For the first time, thymic nurse cells enclosing several thymocytes were detected in the subcapsular region of the thymic cortex in a "honeycomb-like" configuration. In vitro studies show the internalization process using digitalized time-lapse microscopy. Internalized thymocytes have also been reported to interact with macrophages within the TNC complex. The cytoplasmic interaction between thymocytes and macrophages was detected using time-lapse microscopy. Using fluorescence microscopy, we show polymerization of actin within macrophages at the contact point with thymocytes, which is indicative of an immunological synapse. Microfilaments and microtubules within TNCs were shown to be associated with thymocyte binding and internalization, but neither interacted with macrophages. Also, we provide data to show that thymocytes are actively involved in the internalization process. These experiments show for the first time the existence of thymic nurse cells within the thymic microenvironment. They provide a visual documentation of thymocyte uptake by thymic nurse cells, and define an interaction between thymocytes and macrophages within the TNC complex.