Tissue-resident memory T cells orchestrate tumour-immune equilibrium

The immune system can prevent tumour development by engaging in a process termed cancer immunosurveillance, during which immune cells such as T cells restrict tumour growth either by completely eradicating cancer cells in a process of 'elimination' or by suppressing cancer cell outgrowth by establishing a state of tumour-immune 'equilibrium'. Most cancers develop within epithelial layers of tissues but circulating T cells are largely excluded from these epithelial tissue compartments in the absence of infection or overt inflammation. In contrast, CD8⁺ tissue-resident memory T (T_RM) cells reside permanently within epithelial layers of peripheral tissues without recirculating in blood. Accumulating evidence suggests that T_RM cells are found in diverse human solid cancers where they correlate with improved prognosis and can protect against tumour challenge in mice. However, the mechanisms through which these cells mediate cancer protection are poorly understood. In our recent study (Park SL et al, Nature 565(7739), 2019) we developed a melanoma model that allowed us to identify a critical role for T_RM cells in the establishment and maintenance of tumour-immune equilibrium in skin. Our findings provide insight into the immune cell populations important for maintaining long-term tumour dormancy in peripheral tissues and imply that targeting T_RM cells may serve as a novel cancer treatment strategy.