• Esmeralda Celia Marginean and Barbara Melosky.
• From the Department of Pathology, University of Ottawa, Ottawa, Ontario, Canada (Dr Marginean); the Gastrointestinal Pathology Section, The Ottawa Hospital, Ottawa (Dr Marginean); the Department of Medical Oncology, University of British Columbia, Vancouver, Canada (Dr Melosky); and the Department of Oncology, British Columbia Cancer Agency, Vancouver (Dr Melosky).
• Arch. Pathol. Lab. Med. 2018 Jan 1; 142 (1): 26-34.

Context- The world of oncology has changed dramatically in the past few years with the introduction of checkpoint inhibitors and immunotherapy. The promising findings of a small, phase 2 clinical trial that led to the US Food and Drug Administration breakthrough designation and approval of the anti-programmed death receptor-1 (PD-1) drug pembrolizumab (Keytruda, Merck, Kenilworth, New Jersey) to treat metastatic/refractory microsatellite instability-high colorectal cancer (CRC) has significantly boosted interest in immunomodulatory therapies in microsatellite instability-high CRC.Objectives- To review the immune response to cancer and the role of immune checkpoints, focusing on the technical and interpretation challenges of PD-1/programmed death ligand-1 (PD-L1) testing by pathologists and the clinical implications of the test and the therapeutic potential of treating CRC with checkpoint inhibitors.Data Sources- A PubMed review was performed of articles pertaining to CRC, microsatellite instability and mismatch repair systems, molecular classification, immune response, PD-1/PD-L1, and immunotherapy.Conclusions- Exciting success with anti-PD-1/PD-L1 and anticytotoxic T-lymphocyte-associated protein 4 (CTLA4) checkpoint inhibitors has already been reported in melanoma and in lung and renal carcinomas. Recently, microsatellite instability-high CRCs, expressing PD-L1 by immunohistochemistry, regardless of the level of that PD-L1 expression, appeared to respond to checkpoint blockades with anti-PD-1 or anti-PD-L1 agents, whereas microsatellite-stable tumors were much less responsive. With microsatellite instability routinely tested by most centers, studies that include larger cohorts are required to study the predictive role of PD-1/PD-L1 expression in microsatellite instability-high CRC, to assess which immunohistochemistry antibodies to use, to refine the scoring criteria, and to critically analyze the interpretation pitfalls.

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