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 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 alpha beta(+) thymocytes by day 18 of gestation. In short-term cultures. stimulation of unfractionated adult thymocytes with concanavalin A increased gp40 expression, particularly among CD3(ht) and CD3(int) thymocyte populations. This demonstration that Ep-CAM. initially considered to be expressed primarily epithelial cells. is also expressed by thymocytes. T cells and antigen-presenting cells. mises 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.