The costimulatory molecule B7 is expressed in the medullary region of the murine thymus.

Several laboratories, including our own, have previously shown that the B7 antigen, a glycoprotein expressed on activated B cells, macrophages and dendritic cells, can provide a potent costimulatory signal for peripheral murine T lymphocytes. In the present report we have analysed the expression and function of B7 in the murine thymus. The expression of B7 was demonstrated histochemically. B7-expressing cells were present in the thymic medulla but virtually absent from the cortex. Further analysis by immunofluorescence and flow fluorocytometry revealed that the B7-positive cells also expressed major histocompatibility complex (MHC) class II molecules. Both epithelial and dendritic cells expressed the B7 antigen. Finally, although we have demonstrated expression of mB7 in the murine thymus, we have been unable to detect a function for this antigen in this organ. The implications of these findings are discussed.