Validated PD-L1 immunohistochemistry assays (E1L3N & SP142) reveal similar immune cell staining patterns in melanoma when using the same sensitive detection system

AIMS: The tumor cell and/or immune cell PD-L1 expression is considered as a potential biomarker for anti-PD1 and anti-PD-L1 immunotherapy. Currently, different PD-L1 assays are used. This study aims to compare the staining patterns of two PD-L1 antibody clones in melanoma metastases and correlate them with PD-L1 mRNA expression.

METHODS: The immunohistochemistry assays were independently optimized and validated on a Ventana Benchmark Ultra (E1L3N) and XT (SP142), using the same detection system. In total, 46 melanoma metastases were stained with both validated immunohistochemistry assays. Stained slides were digitized for qualitative and semi-quantitative evaluation; scoring by pathologist and semi-automated software analysis. A subset of 21 melanoma metastases was selected for quantification of the PD-L1 mRNA expression (Nanostring).

RESULTS: Accuracy and precision criteria were met for both assays. PD-L1 protein and mRNA expression showed remarkably good Spearman coefficients of 0.90 (E1L3N) and 0.87 (SP142). Despite the remarkable correlation between both PD-L1 assays in expression patterns and quantification values (ρ > 0.9), E1L3N showed significantly more tumor cell staining than SP142.

CONCLUSIONS: E1L3N and SP142 IHC assays were successfully and independently optimized and validated for sensitive and accurate PD-L1 detection. Concordance was best for immune cell scoring, while E1L3N tended to detect more tumor cells. Determination of the clinically relevant cut-off values for immune cell versus tumor cell detection requires further research. This article is protected by copyright. All rights reserved.