Thymic nurse cells are sites of thymocyte apoptosis

Thymic stromal cells play important roles in thymocyte differentiation and selection. Thymic nurse cells (TNC) are thymic epithelial stromal cells that envelop two to 200 CD4+CD8+ thymocytes (TNC-T). The mechanism by which TNC complexes form and their role in thymocyte development are unknown. TNC have been implicated as specialized microenvironments for proliferation, positive selection, negative selection, and apoptosis. Using TCR-gamma junctional sequence analysis of thymocytes within individual TNC, eight of ten TNC analyzed were polyclonal, and two showed evidence of oligoclonality. TCR-alpha beta expression was not detectable on most TNC-T and SCID mice, which do not express TCRs because of a defect in TCR gene rearrangement, had normal numbers of thymocyte-bearing TNC. Thus, TCR expression is not necessary for TNC formation. Treatment of mice with Abs to CD3 epsilon, which induces apoptosis in immature thymocytes, resulted in an eightfold increase in TNC per thymus with 95.5% apoptotic TNC-T. These results suggest that a function of TNC is the clearance of nonfunctional, nonselected, apoptotic thymocytes.