PD-L1 Testing for Lung Cancer in 2019: Perspective from the IASLC Pathology Committee

Abstract The recent development of immune checkpoint inhibitors (ICI) has led to promising advances in the treatment of non-small cell and small cell lung cancer patients with advanced or metastatic disease. Most of ICI target the PD-1/PD-L1 axis with the aim of restoring anti-tumor immunity. Multiple clinical trials for ICI have examined a predictive value of PD-L1 protein expression in tumor cells and/or tumor-infiltrating immune cells by immunohistochemistry (IHC), for which different assays with specific IHC platforms were applied. Of those, some PD-L1 IHC assays have been validated for the prescription of the corresponding agent for first- or second-line treatment. However, not all laboratories are equipped with the dedicated platforms and many laboratories have set up in-house or laboratory developed tests, which are more affordable than generally expensive clinical trial-validated assays. Although PD-L1 IHC test is now deployed in the most pathology laboratories, its appropriate implementation and interpretation are critical as a predictive biomarker and can be challenging due to the multiple antibody clones and platforms or assays available and given the typically small size of samples provided. As many articles have been published since the issue of the IASLC Atlas of PD-L1 immunohistochemistry testing in lung cancer, this review by the IASLC pathology committee provides updates on the indications of ICI for lung cancer in 2019, and discusses important considerations on pre-analytical, analytical and post-analytical aspects of PD-L1 IHC testing, including specimen type, validation of assays, external quality assurance and training.