Immunohistochemistry for prediction of response to immunotherapy

For almost a decade predicting the response of malignant tumours to treatment with immunotherapies has been dependent on PD-L1 expression by immunohistochemistry. Much has been learned about the uniquely challenging application of PD-L1 immunohistochemistry which is utilized in a constantly expanding array of tumour types and clinical contexts. More recently, other predictive biomarkers have acquired clinical validation including tumour mismatch repair deficiency. In this short review, I will assess the strengths and weaknesses of PD-L1 immunohistochemistry, including pre-analytics, analytics and post-analytics in the context of the complex underlying biology. In doing so, I hope to provide some practical guidance that will help optimize its application in routine practice. I will also highlight the various contexts in which immunohistochemistry for PD-L1 and mis-match repair is used to predict response to immunotherapies and how ongoing developments of novel immunohistochemistry based biomarkers might assist in strengthening the ability to predict response to immunotherapies.