A molecule expressed on accessory cells, activated T cells, and thymic epithelium is a marker and promoter of T cell activation.

T cell maturation results in part from direct cell-cell interactions between developing thymocytes and thymic stromal cells. Identification of the cell surface molecules involved in these interactions has been approached by production of mAbs reactive to thymic stromal cell surface Ags. A mAb against one such Ag, mouse thymic stroma (MTS) mAb MTS 23, stains a subset of thymic medullary epithelium by immunohistology. In addition, it was found to detect, by flow cytometry, an Ag constitutively expressed on peripheral B cells and macrophages as well as thymic and splenic dendritic cells. This Ag was also up-regulated on T cells and thymocytes within 24 to 48 h after activation. We then investigated whether the Ag identified by MTS 23 may represent a functional accessory molecule. MTS 23 was able to block up to 75% of T cell proliferation in soluble anti-CD3 and Ag-induced responses in a dose-dependent manner, but not under conditions in which no APCs were required. The molecule detected by this mAb has an apparent molecular mass of 120 kDa under reducing and nonreducing conditions. On the basis of these molecular properties and expression pattern, it is therefore postulated that MTS 23 may detect a novel accessory molecule important for T cell activation. Its expression on thymic epithelium is consistent with the notion that T cell development is not solely a consequence of unique molecular interactions, but also of signals arising from combinations of interactions involving molecules also expressed extrathymically.