PD-L1 expression in non-small cell lung cancer: heterogeneity by pathologic types, tissue sampling and metastasis

BACKGROUND: Programmed cell death ligand-1 (PD-L1) is a predictive marker of anti-PD-1/PD-L1 therapy response. Intra-tumour heterogeneity of PD-L1 expression has been reported in non-small cell lung cancer (NSCLC), but comprehensive studies regarding the determination of PD-L1 expression in different materials are still lacking. Therefore, we aimed to compare PD-L1 expression in paired tumour samples and in different specimen types. METHODS: A total of 1,002 resected NSCLC specimens, 35 biopsy specimens and 54 endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) samples were performed PD-L1 immunohistochemistry (IHC) testing using the 22C3 assay. PD-L1 expression was evaluated using the tumour proportion score (TPS) and categorized into three levels: negative (TPS <1%), low expression (TPS 1–49%) and high expression (TPS ≥50%). RESULTS: A total of 1,002 resected NSCLC specimens, including 852 adenocarcinomas (ADCs) and 150 squamous cell carcinomas (SCCs); 35 paired biopsy and resected samples; 54 paired cell block and biopsy samples; 53 paired blocks from the same resected tissue and 49 paired primary and metastatic lesion samples were included in this study. Interestingly, high PD-L1 expression was significantly more frequent in poorly differentiated subtypes than in well-differentiated subtypes in the ADC subgroup (P<0.001). In the SCC subgroup, high PD-L1 expression was significantly more associated with the nonkeratinizing type than the keratinizing type (P=0.001). PD-L1 expression differed between cell blocks and matched biopsy specimens (discordance rate =11.1%, 6/54) and between biopsy and matched resected specimens (discordance rate =31.4%, 11/35). PD-L1 expression differed between different paraffin blocks from the same resected specimen (discordance rate =35.8%, 19/53), and the discordance rate of PD-L1 expression between primary tumours and matched lymph node metastases was 28.6% (14/49). CONCLUSIONS: Discordant PD-L1 expression is not uncommon in NSCLC and warrants additional studies and serious consideration when interpreting PD-L1 test results. Initial negative test results may lead to repeat PD-L1 testing in additional samples or the use of a different clone if necessary.