Degradation of invariant chain (li) is a critical step in major histocompatibility complex class II-restricted antigen presentation. Cathepsin L was found to be necessary for li degradation in cortical thymic epithelial cells (cTECs), but not in bone marrow (BM)derived antigen-presenting cells (APCs). Consequently, positive selection of CD4(+) T cells, was reduced. Because different cysteine proteinases are responsible for specific li degradation steps in cTECs and BM-derived APCs, the proteolytic environment in cells mediating positive and negative selection may be distinct. The identification of a protease involved in class II presentation in a tissue-specific manner suggests a potential means of manipulating CD4(+) T cell responsiveness in vivo.