OPEN LETTER TO THE EDITOR

Regulatory T cells (Tregs) are major players in immune home-ostasis, and defects in their number and/or function are associated with a broad range of immunological disorders including autoimmunity, graft-versus-host disease (GvHD), transplant rejection, chronic infections or malignant diseases.1,2 One way of how Tregs actively damp an immune response might be direct induction of apoptosis in activated antigen-presenting cells as well as in conventional effector T cells (Teffs) by secretion of lytic granzyme- and perforin-containing granula. Despite the substantial progress made in our understanding of Treg biology, the cytotoxic potential of Tregs is still under debate. As Tregs are under intense investigation as cellular therapeutics for treatment of GvHD and graft rejection, and might also interfere with novel antitumor strategies,3 this question is not a purely academic one but also implies major consequences for the prospectiv