Regulatory T cells (Tregs) play an indispensable role in the immune system as they are involved in the prevention of autoimmune diseases, allergies, infection-induced organ pathology, transplant rejection as well as graft versus host disease (GvHD) by suppression of effector T cells and other immune cells (Sakaguchi, 2004). However, Tregs can also dampen immune responses against tumors, as described for various types of cancer (Zou, 2006; Curiel, 2008). Conversely, in certain situations, Tregs can also protect against cancer by controlling cancer-associated inflammation (Gounaris et al., 2009). Therefore, Tregs are subject to intense investigations.