No abstract

The thymus plays a critical role in the generation of immunocompetent T lymphocytes. In the thymus, lymphocytes are in close contact with epithelial cells, and this contact is necessary for T-cell maturation. Using cultured human thymic epithelial (TE) cells, we have found that human thymocytes bind to human TE cells in vitro. Thymocytes bound to both allogeneic and autologous TE cells and to the epidermoid carcinoma cell line A431 but did not bind to epidermal keratinocytes or to thymic fibroblasts. Thymocyte binding to TE cells was trypsin-and cytochalasin B-sensitive. Indirect immunofluorescence assays showed that both mature (T6-, T3+) and immature (T6+, T3-) thymocytes bound TE cells. In our system, TE-thymocyte binding was not inhibited by antibodies to class I or class II major histocompatibility antigens. In vitro binding of thymocytes to TE cells may represent a correlate of in vivo TE-thymocyte interactions and provides a model system for the study of human intrathymic T-lymphocyte maturation and activation.Microscopists have long appreciated the extensive physical contact between the lymphoid and nonlymphoid elements of the thymus (1, 2). Direct contact of lymphoid and nonlymphoid elements within the thymus is necessary for generation of functionally mature, antigen-specific, major histocompatibility complex (MHC)-restricted T lymphocytes (3-6). Although the precise role played by the nonlymphoid component of the thymus is poorly understood, a number of interactions of developing thymocytes with nonlymphoid elements have been identified, including the formation of lymphoepithelial cell complexes in vivo, called thymic nurse cells (7), and the binding of thymocytes to macrophages and dendritic cells (8, 9). Farr et al. (10) recently observed a subpopulation of thymocytes within the thymic cortex expressing low levels of surface T-lymphocyte antigen receptors (Ti). Where these thymocytes were found in contact with epithelial cell processes, Ti molecules were localized in the region of thymocyte-epithelial contact. Recent development of methods for the long-term culture of human thymic-stromal elements (11), as well as development of monoclonal reagents specific for nonlymphoid components of human thymus (12-15), have made it possible to investigate thymocyte-stromal interactions in vitro. In this report we present evidence that human thymocytes bind to both autologous and allogeneic thymic epithelial (TE) tTo whom reprint requests should be addressed. 6588The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

We have established long term in vitro cultures of human thymic epithelium and human epidermis free of contaminating fibroblasts. The cultured cells were examined using a panel of monoclonal antibodies which were raised against human thymic stroma and recognize tissue specific differentiation antigens of human epidermis and thymic epithelium. A subset of cultured epidermal cells (50%) and thymic epithelial cells (18%) expressed the TE-4 antigen characteristic of basal keratinocytes in skin and endocrine epithelium found in the subcapsular cortex and medulla of the thymus. Subpopulations of the cultured cells expressed the antigens detected by antibodies TE-8 and TE-15. In tissue sections antibodies TE-8 and TE-15 bound to the stratum granulosum and stratum corneum of skin and to the Hassall's bodies of thymus, and therefore recognize antigens characteristic of late stages of keratinized epithelial differentiation. In addition, a subset of thymic epithelial cells expressed the antigen detected by antibody TE-3 which is expressed by nonendocrine thymic epithelium found in the thymic cortex. Thus, in vitro cultures of both epidermal and thymic epithelial cells expressed the entire array of differentiation antigens detected by our panel of monoclonal antibodies. This approach can be used to evaluate the role of components of the thymic microenvironment at various stages of differentiation on developing T lymphocytes. In addition, the cultured epidermal cells can be used to evaluate epidermis as a site of extrathymic T cell maturation.