A novel cell surface molecule on cortical epithelial cells of the human thymus

A novel cell surface molecule, DN4, defined by an mAb raised against human thymic epithelial cells, showed a specificity for epithelial cells of the thymic cortex. This antigen was not expressed at detectable levels on any other types of tissues in the human body except for the thymus and bone marrow. Immunohistochemical analysis revealed that the reactivity of anti-DN4 mAb was restricted to the thymic cortex, and the antigen-expressing cells were arranged in a reticular network with long processes between thymocytes. The cellular nature of DN4-positive cells was identified as cortical epithelial cells, as DN4 was expressed in a subpopulation of freshly prepared thymic stromal cells which contain a large amount of keratin and expression of DN4 was strictly confined to the cortical area within the thymus on immunohistochemical analysis of frozen tissues. Immunofluorescence and flow cytometric analysis revealed that a subpopulation of bone marrow cells was also positive for DN4 (20%). The large blasts of normal bone marrow cells were clearly labeled with anti-DN4 mAb, in contrast to small-sized bone marrow cells. This finding suggests that DN4 seems to be transiently expressed in certain blastic stages during the differentiation of bone marrow cells.