Sample Processing

             

Sample considerations

It is important that tumour samples collected for programmed cell death ligand-1 (PD-L1) testing are high quality, in order to more accurately reflect patient PD-L1 expression and ensure confidence in clinical decision-making.

For more information on obtaining tissue for PD-L1 testing in lung cancer, please visit the iD Lung website.

To ensure adequate accuracy of PD-L1 test results, characteristics of tumour samples need to be considered before analysis takes place. Important characteristics associated with PD-L1 testing include sample:

  • Type: Tumour biopsy or cytology samples
  • Age: Fresh or archival
  • Location: Primary or metastatic

Sample type

Routinely processed, formalin-fixed paraffin-embedded (FFPE) tumour biopsy samples containing an adequate number of viable tumour cells (≥100) are suitable for use in non-small cell lung cancer (NSCLC), head and neck squamous cell carcinoma (HNSCC) and urothelial carcinoma (UC).1, 2

The VENTANA SP263 assay is also validated for use with NSCLC FFPE fine-needle aspiration cell blocks for the assessment of tumour cell (TC) PD-L1 expression.3

The use of cytology samples for PD-L1 testing

In this short interview, Professor Keith Kerr (University of Aberdeen, Aberdeen, UK) discusses the potential use of cytology samples for PD-L1 testing.

Sample age

Evidence suggests that PD-L1 staining is concordant between fresh (<3 months old) and archival (<3 years old) tumour biopsies from the same patient.4 However, samples that are older than 3 years may lead to an underestimation of PD-L1 status.5 Therefore, both fresh and archival (<3 years old) tumour samples are suitable for PD-L1 testing.

Sample location

PD-L1 testing on paired primary and metastatic lesions in NSCLC and UC have shown high concordance, suggesting that tumour location does not influence PD-L1 expression.4, 6, 7

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How do I test?

Overview

An overview of the different steps involved in PD-L1 testing can be seen below. Briefly, samples are initially processed and mounted onto microscope slides. Sample sections are then stained for PD-L1 by using a commercial immunohistochemistry (IHC) assay with an automated staining platform. Stained samples are then quality control (QC) checked by a trained pathologist.

Overview of the steps performed during PD-L1 testing is shown below

 

1. Sections cut

Sections cut image
Sections cut image
 

2. Sections mounted

Sections mounted image
Sections mounted image
 

3. Slides dried

Slides dried image
Slides dried image
 

4. Slides labelled

Slides labelled image
Slides labelled image
 

5. Automated IHC stainer

Automated IHC stainer image
Automated IHC stainer image
 

6. Staining quality checked

Staining quality checked
Staining quality checked

IHC, immunohistochemistry

Sample processing

Before PD-L1 expression can be assessed, samples must be appropriately processed in line with the recommended PD-L1 IHC assay recommendations.

Standard tissue sample preparation and processing procedure for PD-L1 testing*3, 8-10

Sample Processing Formalin-Praffin-Sectioning image
Sample Processing Formalin-Praffin-Sectioning image

*The preparation instructions differ for each PD-L1 assay. For further details, refer to specific package insert for individual assays

PD-L1 staining 

Currently, there are five IHC assays, either commercially available or in development, which can be used to determine PD-L1 expression. Each PD-L1 assay is used with an automated platform to ensure consistent sample processing.11

The choice of PD-L1 IHC assay and the staining procedure is used in line with specific immunotherapies according to their respective diagnostic labels. 

Table
Table

*Based on the FDA and CE-IVD diagnostic regulatory approval
BMS, Bristol-Myers Squibb; CE-IVD, Conformité Européenne-in vitro diagnostic; Dx, diagnostic; FDA, Food and Drug Administration; IHC, immunohistochemistry; MSD, Merck Sharp & Dohme; NA, not applicable; PD-L1, programmed cell death ligand-1

To explore the PD-L1 assays in more detail, including the concordance between them, please refer to the Testing Technologies page.

Quality control procedures

During PD-L1 testing, each patient sample is sectioned twice: testing section and negative control section.3, 8-10

  • Testing section: Slide stained with PD-L1 antibody
  • Negative reagent control section: Slide stained with a monoclonal negative control antibody

Apart from the different primary antibodies, the staining procedure used for the two sections is identical. The purpose of the negative control section is to evaluate non-specific staining.3, 8-10

A slide containing a control section is also included within each staining run in order to confirm adequate assay functionality. Examples of control tissues commonly used in PD-L1 testing include benign human tonsil tissue and placental stromal tissue and vasculature, since they contain both PD-L1 positive and negative regions.3, 8-10

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PD-L1 testing interpretation

Stained slides should be scored and interpreted by a qualified pathologist using a light microscope.3,10 Before interpreting patient samples, the control and negative reagent sections should be examined in order to verify the performance of the IHC assay.

Control section

Positive controls should show moderate-to-strong PD-L1 staining; weak or lack of PD-L1 staining in positive controls is not acceptable.3

If control tissues fail to demonstrate anticipated staining, then patient samples within the same staining run are considered invalid.3

Negative reagent section

The negative reagent control section should show limited, non-specific staining within intact cells.

If background PD-L1 staining is excessive, then patient samples within the same staining run are considered invalid.3

Patient Sample

Different compartments of the tumour sections can be
seen following PD-L1 IHC:          

  • TC area: The proportion of TCs with PD-L1 expression
  • Immune cell (IC) area: The proportion of ICs with PD-L1 expression
  • Tumour area: The whole tumour area, regardless of PD-L1 staining

Schematic representation of PD-L1 staining in different tumour compartments

Sample Processing Diagram
Sample Processing Diagram

IC, immune cell; PD-L1, programmed cell death ligand-1; TC, tumour cell

PD-L1 expression is determined by calculating the relative proportion of staining of the different tumour compartments using an algorithm. An algorithm-specific cut-off value is then applied to define PD-L1 expression as high or low.

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PD-L1 algorithms and cut-offs

Different pharmaceutical companies have adopted their own algorithm and cut-offs to determine PD-L1 expression in different cancers, such as NSCLC and bladder cancer.

Non-small cell lung cancer

The majority of algorithms in NSCLC assess the percentage of TCs with PD-L1 expression. However, an algorithm adopted by Roche also considers PD-L1 expression on tumour infiltrating ICs.

Different PD-L1 algorithms to determine PD-L1 expression in NSCLC22-28

sample-processing-diagram-2
sample-processing-diagram-2

A range of cut-offs of PD-L1 expression have been used in NSCLC clinical trials to categorise patients into PD-L1 high and low status.

sample-processing-chart
sample-processing-chart

BMS, Bristol-Myers Squibb; IC, immune cell; MSD, Merck Sharp and Dohme; PD-L1, programmed cell death ligand-1; TC, tumour cell

Bladder cancer

In bladder cancer, most pharmaceutical companies have utilised their own algorithm to determine PD-L1 expression.

Different PD-L1 algorithms to determine PD-L1 expression in bladder cancer30

sample-processing-algorithms-chart-2
sample-processing-algorithms-chart-2

Each algorithm is associated with a specific cut-off value to define patients with high PD-L1 expression.

sample-processing-chart-3
sample-processing-chart-3

*When ICs ≤1% of the tumour area, the tumour is PD-L1 high if all ICs are stained or PD-L1 low if some/no ICs are stained3
BMS, Bristol-Myers Squibb; CPS, combined positive score; IC, immune cell; MSD, Merck Sharp and Dohme; PD-L1, programmed cell death ligand-1; TC, tumour cell

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