The emerging role of immunosurveillance in dictating metastatic spread in breast cancer.

It is now well known that the immune system can recognize transformed cells and control the initiation and growth of some cancers, a process termed tumor immunosurveillance. Key regulators of this process have been described in the primary tumor setting, where the balance of protumor and antitumor responses dictates tumor initiation and progression. Accumulating evidence suggests that immunosurveillance may also be critical for regulating metastatic spread, the most fatal aspect of cancer, and that mechanisms of overcoming immune control may be quite different from those at the primary site. Our recent findings support loss of type I interferon (IFN) signaling as a tumor-cell intrinsic mechanism of evading metastasis-specific immune responses in breast cancer. We revealed that type I IFN-induced innate (natural killer) and adaptive (CD8(+) T cell) responses suppressed bone metastatic growth and this was associated with decreased accumulation of immune suppressor cells (myeloid-derived suppressor cells). This review summarizes recent findings that are in support of tumor-induced immunosurveillance in regulating metastatic spread, including evidence that immune regulation of primary tumors may be distinct from those dictating metastasis.