Only dull CD3+ thymocytes bind to thymic epithelial cells. the binding is elicited by both CD2/LFA-3 and LFA-1/ICAM-1 interactions

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Abstract
In view of the necessity for thymocytes to interact with thymic epithelial cells to differentiate into mature T cells, this study analyzed the binding between human thymocytes, cultured thymic epithelial cells (CTEC) and the required adhesion molecules. Immediately after separation, thymic epithelial cells (TEC) readily expressed ICAM‐1, which is one of the ligands of LFA‐1 cell adhesion molecules. However, the ICAM‐1 expression was gradually lost upon culture of TEC. IFN‐γ re‐induced ICAM‐1 on the CTEC, and the ability of CTEC to bind to thymocytes was also increased by IFN‐γ treatment. The increase in binding seemed to be caused by the LFA‐1/ICAM‐1 interaction, since it was inhibited by anti‐ICAM‐1 monoclonal antibody (mAb) and anti‐LFA‐1 mAb. This suggests that the LFA‐1/ICAM‐1 interaction is also involved in vivo with the binding of thymocytes to TEC, which have been shown to express ICAM‐1. To better understand the nature of the cells involved in binding, thymocytes were sorted into CD3‐, CD3dull+, and CD3bright+ subsets (which are supposed to represent the immature, intermediate and mature stages of differentiation, respectively), and were examined for their binding to IFN‐γ‐treated CTEC. The result showed that only the CD3dull+ subset bound to CTEC. CD3‐, CD3bright+ cells and peripheral blood T lymphocytes did not bind, but they were induced to bind by neuramidase treatment. All these bindings were inhibited by anti‐LFA‐1 mAb and anti‐CD2 mAb. These findings indicate that CD3dull+ cells can bind to TEC via CD2/LFA‐3 and LFA‐1/ICAM‐1 interactions. Other cells seemed not to bind to TEC because of sialylation.