Cathepsin W expressed exclusively in CD8+ T cells and NK cells, is secreted during target cell killing but is not essential for cytotoxicity in human CTLs

Objective Cathepsin W (CatW, lymphopain) is a putative cysteine protease with restricted expression to natural killer (NK) cells and CD8+ T cells and so far unknown function and properties. Here, we characterize in detail, the regulation of human CatW during T-cell development in response to different stimuli and its functional involvement in cytotoxic lymphocyte effector function.Materials and Methods Western blots and real time polymerase chain reaction of sorted, unstimulated, and stimulated cell subsets (thymocytes, T cells, NK cells) and their culture supernatants were used to study regulation and expression of CatW. Primary CD8+ T cells and short-term T-cell lines were transfected with small interfering RNA to study the involvement of CatW in effector function such as target cell killing and interferon-γ production.Results Levels of CatW expression correlate closely with cytotoxic capacity both during development and in response to factors influencing cytotoxicity. Furthermore, CatW is secreted during specific target cell killing. However, knockdown of CatW expression by small interfering RNA neither influences target cell killing nor interferon-γ production.Conclusion Despite being expressed in the effector subset of CD8+ and NK cells and of being released during target cell killing, our functional inhibition studies exclude an essential role of CatW in the process of cytotoxicity.