Cadherins mediate homotypic adhesion between lineage-related cells in epithelia and other tissues. One cadherin, E-cadherin, is also responsible for adhesion of murine epidermal Langerhans cells to keratinocytes in vitro, and may play a role in the localization of Langerhans cells in epidermis. The thymus is another tissue in which important adhesive interactions between bone marrow-derived cells and keratinizing epithelia occur. To determine whether cadherins might be involved in interactions between thymocytes and thymic epithelial cells, we examined thymocytes from C57BL/6 mice of various gestational ages for cadherin expression. Most day 14 (D14) and essentially all D16 isolated fetal thymocytes expressed cell surface E-cadherin. After D16, the proportion of fetal thymocytes expressing E-cadherin and the level of E-cadherin expressed by individual thymocytes decreased with increasing gestational age. A minority of neonatal thymocytes and very few adult thymocytes expressed E-cadherin. E-cadherin was maximally expressed by the least mature (CD4-CD8-, HSA (J11d)high, CD5 (Ly-1)low, CD25 (IL-2R alpha)+) thymocytes. P-cadherin, another epithelial cadherin, was not detected on thymocytes at any stage of development. Immunohistologic studies revealed that thymic epithelial cells also expressed E-cadherin. Similar levels of E-cadherin were expressed by neonatal and adult thymic epithelial cells in situ, and E-cadherin was easily demonstrable on the thymic epithelial cell line, TE-71. In contrast, P-cadherin was transiently expressed by thymic epithelial cells in situ, and only small amounts of P-cadherin were detected on TE-71 cells. These studies demonstrate that thymocytes and thymic epithelial cells each have the capacity to express the homotypic adhesion molecule E-cadherin. E-cadherin may play a role in developmentally regulated interactions between early thymocytes and thymic stromal cells.
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