Brightfield Multiplex Immunohistochemistry Assay for PD-L1 Evaluation in Challenging Melanoma Samples

Targeting the PD1/PD-L1 immune checkpoint pathway has rapidly become a therapeutic strategy for melanoma patients. Indeed, the quantification of PD-L1 expression by immunohistochemistry (IHC) in melanoma samples is already required, in some contexts, to allow access to anti-PD-1/PD-L1 immunotherapy. Despite a rising demand for PD-L1 testing, paralleling increasing cumulative experience in its assessment and quantification, it is fair to recognize that PD-L1 evaluation in melanoma samples still presents some critical issues. The aim of this technical report is to develop and validate a multiplex double staining protocol for PD-L1/SOX10 in Ventana Benchmark Ultra for routine practice. Our results show that double labeling provides the necessary tools to identify PD-L1+ melanoma cells clearly. The simultaneous visualization of 2 different proteins targets allows the topographical relationship between the 2 labeling to be evaluated within the context of the tissue morphology. Future studies are needed to test this technique's real-world applicability and effectiveness in implementing interpathologist agreement.