What Is PD-L1?

  • A PD-L1 test helps doctors determine whether a patient is likely to benefit from immune checkpoint inhibitors.
  • Immune checkpoint inhibitors unleash an immune system attack on cancer cells.

Medically reviewed by Patrick Ott, MD, PhD

A PD-L1 test helps doctors determine whether a patient is likely to benefit from cancer drugs known as immune checkpoint inhibitors. It involves sending a piece of tumor tissue to a lab for analysis.

What is PD-L1?

PD-L1 is a protein that allows some cells to escape an attack by the immune system. Extending from the cancer cell surface, PD-L1 interacts with a protein called PD-1 on important immune system cells called T cells. This coupling — known as an immune checkpoint — instructs the T cell to leave the tumor cell alone. Checkpoint inhibitor drugs prevent the PD-1/PD-L1 meeting from taking place. Without receiving the “stop” signal from the PD-L1 protein, the T cells can go ahead an attack the tumor cells.

Susceptible cancers

Drugs inhibiting the PD-1/PD-L1 checkpoint have been approved by the Food and Drug Administration for several cancers, including:

They are in clinical testing for a variety of other cancer types. A PD-L1 test measures how much PD-L1 a tumor “expresses,” or produces.

checkpoint inhibitors, immunotherapy

Tumors that express high amounts of PD-L1 may be more susceptible to checkpoint inhibitors than those that express less. Patients should ask their cancer physicians whether a PD-L1 test is appropriate for them.

Although helpful in determining which patients may respond to certain drugs, the test is not infallible. Some tumors that test for high levels of PD-L1 may not respond to checkpoint inhibitors, and those that test for low levels may have a strong response. Cancer cells are complex, and a variety of factors can influence how susceptible they are to such drugs.