Why is immunotherapy effective (or not) in patients with MSI/MMRD tumors?

In the last few years, immunotherapy has revolutionized the oncology landscape by targeting the host immune system. Blocking immune checkpoints such as cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell death-1 (PD-1) and its ligand (PD-L1 or B7-H1), has proven its efficacy in several solid cancers. Recently, several clinical studies have demonstrated a significant improvement in clinical response to the anti-PD-1-based immunotherapy in a subset of patients with microsatellite instability-high (MSI-H)/mismatch repair (MMR)-deficient tumors that accumulate short insertion/deletion mutations notably in coding microsatellites regions of the genome. Thus, the responsiveness of MSI cancers to immune checkpoint inhibitors can be explained by the increased rate of putative frameshift peptide neoantigens and the immunogenic tumor microenvironment. However, not all MSI tumors respond to immunotherapy. The current review will summarize how and why MMR deficiency has emerged as an important predictor of sensitivity for immunotherapy-based strategies. We will also discuss tumor-cell intrinsic genetic and immune-related features of MSI tumors that can modulate immune checkpoint blockade response and explain primary and/or acquired resistance to anti-PD-1 therapy. Finally, we will also discuss about emerging scores which can define more precisely the immune context of the tumor microenvironment and thus better evaluate prognosis and predict response to Immune Checkpoint Blockade.