The Pathology Dictionary Team
February 23, 2023

What is PD-L1?

PD-L1 (Programmed Death-Ligand 1) is a protein found on the surface of normal, healthy cells and some cancer cells. PD-L1 is called an immune checkpoint protein because it acts to turn down the activity of immune cells called T cells.

Why do cancer cells express PD-L1?

T cells are able to detect abnormal cells such as cancer cells and remove them from the body. Cancer cells that express PD-L1 escape attack by T cells by activating a protein on the T cell called PD-1.

What types of cancer express PD-L1?

PD-L1 is frequently seen in cancers of the stomach, liver, kidney, esophagus, pancreas, ovary, bladder, lung, and head and neck.

Why do pathologists test for PD-L1?

PD-L1 expression on cancer cells can be used as a biomarker to predict which patients may benefit from treatments that target the PD-1/PD-L1 pathway, such as immune checkpoint inhibitors.

How do pathologists test for PD-L1?

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 PD-L1 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).

Immunohistochemistry for PD-L1. The PD-L1-expressing tumour cells appear brown.

What does positive for PD-L1 mean?

Positive for PD-L1 means that the cells in the tumour sample express the PD-L1 protein. Depending on the level of expression, this may be used as an indication for treatment with an immune checkpoint inhibitor.

What does negative for PD-L1 mean?

Negative for PD-L1 means that the cells in the tumour sample do not express the PD-L1 protein. Immune checkpoint inhibitors are less effective in patients with tumours that are negative for PD-L1.

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