The cell surface expression and function of the LFA-1 ligand, intercellular adhesion molecule 1 (ICAM-1), on epidermal keratinocytes (EK) was studied. ICAM-1 expression on the surface of cultured EK was either absent or weak, but was induced by treating EK with rIFN-gamma or TNF for 4-48 h. IFN-gamma and TNF were synergistic. IFN-gamma treatment increased T lymphoblast adhesion from less than 2% to 20-40%, with a concentration dependence similar to that seen for ICAM-1 induction. All of the adhesion to EK was inhibited by LFA-1 and ICAM-1 mAbs, but not by HLA-DR, CD2, or LFA-3 mAbs. There was no difference in the level of T lymphoblast adhesion to IFN-gamma-treated allogeneic or autologous EK. ICAM-1 purified from the HeLa epithelioid cell line and reconstituted into planar membranes also supported efficient adhesion of T lymphoblasts that was blocked by LFA-1 mAb bound to the T lymphoblasts or ICAM-1 mAb bound to the planar membranes. T lymphoblasts adherent to EK or ICAM-1 planar membranes were isolated by panning, and surface markers were analyzed by immunofluorescence flow cytometry. The adherent T cells were a phenotypically skewed subpopulation. They were enriched for CD8+ cells and expressed 1.5-2.5-fold higher LFA-1 and CD2 compared with the unseparated population.

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To study the role that epithelial cells of the thymic microenvironment play in promoting activation of immature CD7+, CD2+, CD4-, CD8-(double-negative) human thymocytes, we have isolated thymocyte subsets from normal postnatal thymus and have cocultured autologous double-negative thymocytes with pure populations of thymic epithelial (TE) cells. We report that TE cells directly activate double-negative thymocytes to proliferate and that TE cells enhance the ability of double-negative thymocytes to proliferate in response to stimulation with exogenous interleukin 2. Activated double-negative thymocytes that proliferated in vitro in the presence of TE cells and interleukin 2 remained doublenegative after 23 days in culture. Moreover, TE-cell culture supernatants in the absence of intact TE cells contain interleukin 1, interleukin 3, and granulocyte/macrophage-colonystimulating factor activity for human bone marrow cells and can activate double-negative thymocytes to proliferate. Antibodies against interleukin 1 and against granulocyte/macrophage-colony-stimulating factor inhibited TE-cell-induced thymocyte activation. These data indicate that one role of TE cells in vivo may be to activate double-negative thymocytes to proliferate.The thymus is known to be the primary site of T-cell maturation during fetal and neonatal development (1)(2)(3)(4)(5). However, specific cellular interactions and activation signals necessary for growth and differentiation of T-cell precursors within the human thymus are largely unknown (4-6). Studies of T-cell ontogeny in mice have suggested that the earliest intrathymic T cells have the surface phenotype dLytl + ("d" means that the immunofluorescence is faint, or "dull"), Lyt2-, L3T4-and that a subpopulation of dLytl +, Lyt2 -, L3T4-thymocytes are able to reconstitute the T-cell repertoire (7-13).Lobach and Haynes (14) showed that, during human fetal development, the earliest T-cell precursors are CD7+ prior to entry into the thymus, and these cells acquire the CD2, CD4, CD8, CD3, and CD1 molecules in the thymus between 82 and 14 weeks of fetal gestation. At 8½ weeks of fetal development, intrathymic T cells are CD7+, CD2+, CD4-, CD8-(double-negative) (14)(15)(16).To study the role that thymic epithelial (TE) cells might play in activating subsets of human double-negative thymocytes to proliferate and differentiate into mature T cells, we developed an in vitro coculture system of autologous TE cells and double-negative thymocytes (17). Here we show that TE cells up-regulate interleukin 2 (IL-2) receptor expression and IL-2 responsiveness of double-negative thymocytes and directly activate immature thymocytes to proliferate.Moreover, TE cells produce cytokines capable of providing activation signals to double-negative thymocytes in the absence of direct TE-cell-thymocyte contact.

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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.