November 8, 2023

PD-L1 (Programmed Death-Ligand 1) is a protein found on the surface of normal, healthy cells and some cancer cells. It is called an immune checkpoint protein because it acts to turn down the activity of immune cells called T cells which normally detect abnormal cells such as cancer cells and remove them from the body. Cancer cells that express this protein escape attack by T cells by activating a protein on the T cell called PD-1.

Testing for PD-L1

PD-L1 expression is frequently seen in cancers of the stomach, liver, kidney, esophagus, pancreas, ovary, bladder, lung, and head and neck. Doctors test for this protein to help determine which patients may benefit from treatments that target the PD-1/PD-L1 pathway, such as immune checkpoint inhibitors.

To test for PD-L1 expression, pathologists typically perform a test called immunohistochemistry (IHC) on a tissue sample from the tumour. In this test, a specific antibody against PD-L1 is applied to the tissue section and then detected using a secondary antibody attached to a dye.

The level of protein expression is then counted and scored based on the intensity and percentage of positive cells. For lung cancers, pathologists use a score called the total proportion score (TPS). For most other types of cancer, pathologists use a score called the combined positive score (CPS).

This picture shows PD-L1 expressing cells (brown) demonstrated by immunohistochemistry.
This picture shows PD-L1 expressing cells (brown) demonstrated by immunohistochemistry.

About this article

This article was written by doctors to help you read and understand your pathology report. Contact us if you have questions about this article or your pathology report. For a complete introduction to your pathology report, read this article.

Other helpful resources

Atlas of pathology
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