Immune checkpoint blockade (ICB) has significant clinical activity in diverse cancer classes and can induce durable remissions in even refractory advanced disease. However, only a minority of cancer patients treated with ICB have long-term benefits, and ICB treatment is associated with significant, potentially life-threatening, autoimmune side effects. There is a great need to develop biomarkers of response to guide patient selection to maximize the chance of benefit and prevent unnecessary toxicity, and current biomarkers do not have optimal positive or negative predictive value. A variety of potential biomarkers are currently being developed, including those based on assessment of checkpoint protein expression, evaluation of tumor-intrinsic features including mutation burden and viral infection, evaluation of features of the tumor immune microenvironment including nature of immune cell infiltration, and features of the host such as composition of the gut microbiome. Better understanding of the underlying fundamental mechanisms of immune response and resistance to ICB, along with the use of complementary assays that interrogate distinct features of the tumor, the tumor microenvironment, and host immune system, will allow more precise use of these therapies to optimize patient outcomes.

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