The immune system has to be ignorant against the self-cells to avoid autoimmunity, but it should also be able to eliminate the self-cells that are noxious for the organism such as tumor cells. Transforming Growth Factor beta (TGF-β) is a master regulatory cytokine of the immune system. Interestingly, this cytokine is highly produced by the tumor micro-environment and it is known to contribute also to tumor growth. We revealed that within the immune system the target-cells of the regulatory effects of TGF-β are T lymphocytes and that TGF-β signaling represses their activation against self-cells. Our lab now studies the molecular and cellular mechanisms responsible for the control of peripheral T cell tolerance to self-cells by TGF-β and analyses their effects on autoimmune diseases and tumor growth.