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PD-L1 Expression: Biomarker for Immunotherapy

PD-L1 expression refers to the presence of the PD-L1 protein on tumor and immune cells. It is a key biomarker used to guide immunotherapy decisions in cancer treatment.

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Things worth knowing about "PD-L1 Expression"

PD-L1 expression refers to the presence of the PD-L1 protein on tumor and immune cells. It is a key biomarker used to guide immunotherapy decisions in cancer treatment.

What Is PD-L1 Expression?

PD-L1 expression (Programmed Death-Ligand 1) refers to the presence of the surface protein PD-L1 on tumor cells and immune cells. PD-L1 is an immune checkpoint protein that plays a central role in regulating the body's immune response. When PD-L1 binds to its receptor PD-1 on T cells (a type of white blood cell), it suppresses the activity of those immune cells. Cancer cells exploit this mechanism to shield themselves from immune destruction.

Biological Background

Under normal physiological conditions, the PD-1/PD-L1 axis prevents excessive immune reactions and protects healthy tissue from autoimmune damage. However, many tumor types overexpress PD-L1, meaning they produce this protein at abnormally high levels. This allows cancer cells to actively suppress the immune response and evade destruction by T cells.

Clinical Significance as a Biomarker

PD-L1 expression is now routinely used as a predictive biomarker to determine whether a patient is likely to respond to immune checkpoint therapy. In many cancer types, higher PD-L1 expression on tumor cells is associated with a greater likelihood of responding to PD-1 or PD-L1 inhibitors.

Relevant Cancer Types

Non-small cell lung cancer (NSCLC): PD-L1 expression is particularly well established as a biomarker here.
Urothelial carcinoma (bladder cancer)
Triple-negative breast cancer
Gastric and gastroesophageal junction cancer
Cervical cancer
Melanoma

Measuring PD-L1 Expression

PD-L1 expression is typically measured using immunohistochemical (IHC) staining on tumor tissue samples (biopsies). The result is reported as a Tumor Proportion Score (TPS) or Combined Positive Score (CPS), reflecting the percentage of PD-L1-positive cells.

TPS: The proportion of PD-L1-positive tumor cells relative to the total number of viable tumor cells.
CPS: Also accounts for PD-L1-positive immune cells within the tumor microenvironment.

Cutoff values (e.g., TPS ≥ 1%, TPS ≥ 50%) often determine which therapies are approved or recommended for a given patient.

Therapeutic Implications

PD-L1 expression guides treatment decisions regarding the use of immune checkpoint inhibitors. Approved agents include:

Pembrolizumab (anti-PD-1) – e.g., for NSCLC with TPS ≥ 50%
Atezolizumab (anti-PD-L1) – e.g., for urothelial carcinoma
Durvalumab (anti-PD-L1) – e.g., for lung cancer after chemoradiotherapy
Nivolumab (anti-PD-1) – e.g., for melanoma and NSCLC

Importantly, low or absent PD-L1 expression does not completely rule out a treatment response, as other factors such as tumor mutational burden (TMB) and microsatellite instability (MSI) also play a role.

Limitations and Challenges

Interpreting PD-L1 expression is complex and comes with several challenges:

Different diagnostic assays (e.g., 22C3, 28-8, SP142) do not produce fully comparable results.
PD-L1 expression can vary spatially and over time within a tumor (tumor heterogeneity).
Not all PD-L1-positive tumors respond to checkpoint inhibitors.
Full standardization of measurement methods has not yet been achieved.

References

Herbst RS, Soria JC, Kowanetz M et al. - Predictive correlates of response to the anti-PD-L1 antibody MPDL3280A in cancer patients. Nature. 2014;515(7528):563-567.
Doroshow DB, Bhalla S, Beasley MB et al. - PD-L1 as a biomarker of response to immune-checkpoint inhibitors. Nature Reviews Clinical Oncology. 2021;18(6):345-362.
National Comprehensive Cancer Network (NCCN) - Clinical Practice Guidelines in Oncology: Non-Small Cell Lung Cancer. Version 2024. Available at: www.nccn.org.

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