The immune microenvironment of colorectal cancer

Colorectal cancer (CRC) progression depends on the close interaction of tumour cells and the tumour microenvironment (TME). Although the TME contributes to poor therapy responses and immune evasion, immune cells within the TME can be therapeutically leveraged, as exemplified by immune checkpoint blockade (ICB). Unfortunately, only a small subset of patients with CRC benefit from ICB therapy; those with immune-activated, microsatellite unstable CRC respond, whereas the predominant group of patients with CRC, those with microsatellite-stable tumours, do not. Although challenging, modulating the TME of CRC to convert these lowly immunogenic and immunosuppressed tumours into immune-activated tumours holds tremendous therapeutic potential. In this Review we provide an overview of the cellular and molecular components of immunity in the TME of CRCs at various stages of disease as well as the mechanisms of immunosuppression and immune evasion. We further describe how systemic and local therapies for CRC impact the tumour and systemic immune microenvironments, and how immunity could serve as a therapeutic and prognostic biomarker. Lastly, we highlight novel immunotherapeutic strategies and approaches that modulate the TME of CRCs to make them amenable to immunotherapy.