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The murine homolog of human Ep‐CAM, a homotypic adhesion molecule, is expressed by thymocytes and thymic epithelial cells

In this report, we demonstrate that gp40, a molecule previously shown to be expressed by thymic epithelial cell lines in vitro and by thymic epithelial cells in vivo, is the murine homolog of human Ep‐CAM, a calcium‐independent homotypic adhesion molecule. gp40 is also expressed at low levels by thymocytes and peripheral T cells. In the adult thymus, gp40 expression was inversely related to the state of thymocyte maturation, with the highest levels associated with CD4− CD8− and CD4+CD8+ thymocyte populations. Ultrastructural immunohistochemistry revealed gp40 localization to areas of thymocyte/epithelial contact and demonstrated that gp40 is also expressed by thymic dendritic cells. During fetal development, thymocytes at days 14–16 of gestation expressed high levels of gp40. At later stages, the observed decline in the frequency of gp40+ cells and levels of expression correlated with the emergence of αβ+ thymocytes by day 18 of gestation. In short‐term cultures, stimulation of unfractionated adult thymocytes with concanavalin A increased gp40 expression, particularly among CD3hi and CD3int thymocyte populations. This demonstration that Ep‐CAM, initially considered to be expressed primarily by epithelial cells, is also expressed by thymocytes, T cells and antigen‐presenting cells, raises the possibility that Ep‐CAM may contribute to adhesive interactions between thymocytes and epithelial cells or dendritic cells, either in the context of thymocyte development or peripheral T cell trafficking and function.