Cloning of a novel MHC-encoded serine peptidase highly expressed by cortical epithelial cells of the thymus

Antigen presentation by cortical thymic epithelial cells (cTEC) during the positive selection of T cells has been shown to differ from that of other a ntigen-presenting cells. In the case of MHC class II presentation, cathepsi n L as opposed to cathepsin S is responsible at least in part for the degra dation of invariant chain. Other proteases, however, must be involved. me h ave identified a putative serine protease that is specifically expressed in the thymus. Encoded within the class I major histocompatibility complex (M HC) region, this gene has sequence homology with lysosomal prolylcarboxypep tidase, suggesting that it is a serine protease. We have, therefore, design ated this gene thymus-specific serine protease (TSSP). In situ hybridizatio n and immunofluorescence staining reveal that TSSP is expressed exclusively by cortical thymic epithelial tells, with the strongest staining noted aro und vessels and the thymic capsule. The identification of TSSP further supp orts the theory that MHC class II antigen processing and presentation in th e thymic cortex involves a proteolytic milieu that differs from that of oth er antigen-presenting cells. (C) 1999 Academic Press.