Treg(s) in Cancer: Friends or Foe?

Immune escape is a hallmark of cancer. Regulatory T cells (Treg) have been described to maintain peripheral tolerance. The role of Treg in cancer is ambiguous, as they are central inhibitory regulators in solid tumors, whereas during inflammation-driven tumorigenesis they prevent cancer initiation by restraining inflammation. As a consequence, under conditions with chronic inflammation that may initiate malignant transformation, application rather than depletion of Treg may be helpful. In solid tumors, however, the success story of immune-activating antibodies targeting checkpoint molecules of T cell activation fuels the hope that Treg inactivation or depletion may additionally boost anti-tumor immune response. In this review we summarize important aspects on the dual role of Treg in cancer to provide a rationale for future Treg targeting attempts.