What Are PD-1 and PD-L1?

Learn how these proteins help people with cancer and their healthcare providers make treatment decisions.

A close-up of a microscope. Testing for cancer biomarkers like PD-1 and PD-L1 involves biopsies and lab tests.

Biomarker testing looks for the presence of certain biological molecules on cancer cells. When certain biomarkers are present in certain levels, it gives healthcare providers an idea of the processes the cancer is using to grow and spread—and predict what therapies might be most effective in treating that cancer. Certain anti-cancer treatments work by disrupting these processes.

PD-1 and PD-L1 proteins are one example of biomarkers that are found in some cancers. Here we look at the role of these proteins in the body, how cancer cells use these proteins to fuel their growth, and how certain anti-cancer drugs target these proteins.

Immune checkpoint inhibitors

The immune system attacks and destroys cells, pathogens, and foreign substances that enter the body. To keep the immune system from damaging healthy cells, the body also contains proteins called immune checkpoints.

PD-1 and PD-L1 are immune checkpoint inhibitors. Together, they form what is called the PD-1/PD-L1 pathway.

  1. PD-1 is a protein found on immune cells called T cells.
  2. PD-L1 is a protein found on normal healthy cells.
  3. When PD-1 binds to PD-L1, it stops the immune system from attacking the normal healthy cells.

Keep in mind that this is a simplified explanation of a complex biological process.

PD-L1 on cancer cells

Some cancer cells produce high amounts of PD-L1. These high levels of PD-L1 essentially act as a disguise, letting the cancerous cells appear normal and avoid attack when they encounter T cells. This allows the cancerous cells to continue to grow and proliferate.

PD-1/PD-L1 inhibitors

Some anti-cancer drugs work by disrupting the PD-1/PD-L1 pathway. These drugs are called PD-1 inhibitors or PD-L1 inhibitors, depending on which protein they act on. When this immune checkpoint process is disrupted, T cells will attack the cancer cells.

PD-1 and PD-L1 inhibitors are categorized as immunotherapies—cancer treatments that work by helping the body’s immune system fight cancer. These drugs may be prescribed alone, or in combination with other cancer therapies, such as chemotherapy.

Your healthcare providers may order tests to measure the levels of PD-L1. This test involves taking a biopsy of the cancer cells, which are then examined in a lab. PD-L1 levels are one piece of information that your healthcare providers will use to help predict what cancer treatments may be effective.

Remember that every case of cancer is different, and the best source of information about your diagnosis is your healthcare team.

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