| Summary: | High tumour programmed cell death-ligand 1 (PD-L1) expression is associated with poor progression-free survival (PFS) after tyrosine kinase inhibitor (TKI) therapy in <i>ALK</i>-rearranged non-small cell lung cancer (NSCLC). However, the characteristics of the tumour microenvironment (TME) and their prognostic values in <i>ALK</i>-rearranged NSCLC are unknown. Here, we collected tumour tissues from pretreated <i>ALK</i>-rearranged NSCLC patients, immunohistochemical staining was used to assess PD-L1 expression, and tumour-infiltrating immune cells were determined via multiplex immunofluorescence staining (mIF). Our data showed that the median values of PFS for the high PD-L1 group and low PD-L1 group who received ALK-TKI treatment were 4.4 and 16.4 months, respectively (<i>p</i> = 0.008). The median overall survival (OS) of the two groups was 24.0 months and not reached, respectively (<i>p</i> = 0.021). Via univariate and multivariate analyses, a high PD-L1 expression and a worse ECOG PS were determined to be independent prognostic factors of OS (HR = 3.35, 95% CI: 1.23–9.11, <i>p</i> = 0.018; HR = 6.42, 95% CI: 1.45–28.44, <i>p</i> = 0.014, respectively). In addition, the high PD-L1 group had increased Tregs and exhausted CD8+ T cells in both the tumour and stroma (all <i>p</i> < 0.05). High PD-L1 expression was an adverse predictive and prognostic biomarker for <i>ALK</i>-rearranged NSCLC. The characteristics of the TME in patients with high PD-L1 expression were shown to have an immunosuppressive status.
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