FAS LIGAND EXPRESSION IS RESTRICTED TO NONLYMPHOID THYMIC COMPONENTS IN-SITU

The cell surface receptor Fas (Apo-1/CD95) and its ligand (FasL) are m ediators of apoptosis that have been shown to be implicated in activat ion-induced death of mature T cells and in killing mediated by cytolyt ic T cells, The role of the Fas pathway in apoptosis associated with t hymic selection events is, however, controversial, Although Fas and Fa sL are known to be expressed in the thymus, the nature and in vivo loc alization of FasL-expressing cells have not been determined, Using rec ently developed anti-FasL Abs in combination with in situ hybridizatio n on tissue sections, we show in this work that FasL-expressing cells are present in the thymus, particularly within the medulla, FasL mRNA was detected readily in thymic stromal cell extracts, but not in isola ted thymocytes, Moreover, immunohistochemical analysis of serial tissu e sections stained with Abs against FasL in conjunction with epithelia l and dendritic cell markers indicated that both thymic epithelial and dendritic cells express FasL in situ, The coexistence of FasL-express ing stromal cells and Fas-expressing thymocytes may have important imp lications for the role of the Fas pathway in apoptosis associated with thymic selection events.