PD-L1 Immunohistochemical Detection in Tumor Cells and Tumor Microenvironment: Main Considerations on the Use of Tissue Micro Arrays.

PD-1/PD-L1 (programmed death 1/programmed death ligand 1) pathway plays a critical role in immune escape of tumor cells. Recent studies have described that PD-L1 is heterogeneously expressed in various types of cancer, although its prognostic/predictive value is still uncertain. These problems are partly due to a not well defined operating protocol for its detection by immunohistochemistry, but also because most of the studies conducted on large case series were made by Tissue Micro Array (TMA). We are going to discuss this latter point, to highlight that TMA must be set up in an appropriate manner, especially for some markers, such as PD-L1, which, besides being poorly expressed in tumor cells, can be expressed by cells of the tumor microenvironment.