Understanding PD-L1 Testing in Breast Cancer: A Practical Approach

Abstract

<b><i>Background:</i></b> Immune checkpoint inhibitors (ICI) have changed therapy strategies for cancer patients tremendously. Some approved ICI acquire testing of PD-L1 expression on tumor and/or immune cells. However, since PD-L1 testing is a comprehensive issue with various assays, antibody clones, scoring methods, and cut-offs, we aimed to summarize the recommendations and technical and histopathological issues of diagnostic PD-L1 assessment with an emphasis on invasive breast cancer (IBC). <b><i>Summary:</i></b> Besides other (pre)analytical considerations, selecting the most adequate PD-L1 immunohistochemical assay/antibody clone is important. In-house assay validation, prediagnostic training, and internal and external quality assurance should be implemented. The current most relevant PD-L1 assays and scores will be explained in this review. Moreover, recommendations for PD-L1 testing in IBC are outlined. <b><i>Key Messages:</i></b> Atezolizumab plus nab-paclitaxel therapy is approved for adult patients with locally advanced or metastatic triple negative breast cancer (mTNBC), if the tumor-associated immune cells express PD-L1. – This PD-L1 immune cell positivity is defined as an immune cell (IC) score, which refers to the area occupied by PD-L1 positive immune cells (lymphocytes, dendritic cells, macrophages, and granulocytes) as a percentage of the whole tumor area. The cut-off is an IC score ≥1%. In the approval study for atezolizumab in mTNBC, IC score was assessed using the Ventana PD-L1 SP142 assay. Other assays or laboratory developed tests may be used depending on country-specific drug approvals. However, harmonization studies have to show whether other PD-L1 tests are reliable and of clinical value to predict the response of breast cancer patients to ICI.

Currently Used Laboratory Methodologies for Assays Detecting PD-1, PD-L1, PD-L2 and Soluble PD-L1 in Patients with Metastatic Breast Cancer

“…Therefore, a clinical trial included only biopsied or surgical specimens collected within 90 days [36]. Biopsies or surgically excised specimens should be fixed in neutral-buffered formalin for 6-72 h, depending on the sample size [37]. It is recommended that whole-slide sections with 2.5-4 µm thickness are cut from representative areas of the FFPE tumor tissue.…”

“…The binding of PD-L1 to PD-1 leads to the inhibition of the T cell response, enabling self-tolerance and immune tolerance to prevent excessive immune reactions [65]. In malignant condition, cancer cells also present PD-L1 to inactivate and exhaust the T cell activation [37,66]. This mechanism involves immune escape or local suppression of the immune system (Figure 2).…”

Association of Galectin 9 Expression with Immune Cell Infiltration, Programmed Cell Death Ligand-1 Expression, and Patient’s Clinical Outcome in Triple-Negative Breast Cancer

“…The percentage of PD-L1 expression in tumor-infiltrating immune cells was calculated as the proportion of the intratumoral or peritumoral stromal rim occupied by PD-L1-positive immune cells of any staining intensity. Positivity was defined as ≥1% in both tumor cells and immune cells [33]. All slides that were immunohistochemically stained were read by two experienced pathologists blinded to other clinical information.…”

In Vitro Examinations of Cell Death Induction and the Immune Phenotype of Cancer Cells Following Radiative-Based Hyperthermia with 915 MHz in Combination with Radiotherapy

“…Another important issue is the heterogeneity of the biomaterial itself. For example, the biopsies were taken from the primary tumor or metastatic tumor and previously collected as part of a treatment option [57]. There may be differences in immune cell population composition and PD-L1 status, especially for the tumor itself [4,58].…”