PD-L1 expression and immune escape in melanoma resistance to MAPK inhibitors

Purpose: To examine the relationship between immune activity, PD-L1 expression and tumor cell signaling, in metastatic melanomas prior to and during treatment with targeted MAPK inhibitors. <p>Experimental design: Thirty-eight tumors from 17 patients treated with BRAF inhibitor (n=12) or combination BRAF/MEK inhibitors (n=5) with known PD-L1 expression were analyzed. RNA expression arrays were performed on all pre-treatment (PRE, n=17), early during treatment (EDT, n=8) and progression (PROG, n=13) biopsies. HLA-A/HLA-DPB1 expression was assessed by immunohistochemistry (IHC).</p> <p>Results: Gene set enrichment analysis (GSEA) of PRE, EDT and PROG melanomas revealed that transcriptome signatures indicative of immune cell activation were strongly positively correlated with PD-L1 staining. In contrast, MAPK signaling and canonical Wnt/-ß-catenin activity were negatively associated with PD-L1 melanoma expression. The expression of PD-L1 and immune activation signatures did not simply reflect the degree or type of immune cell infiltration, and was not sufficient for tumor response to MAPK inhibition.</p> Conclusions: PD-L1 expression correlates with immune cells and immune activity signatures in melanoma, but is not sufficient for tumor response to MAPK inhibition, as many PRE and PROG melanomas displayed both PD-L1 positivity and immune activation signatures. This confirms that immune escape is common in MAPK inhibitor-treated tumors. This has important implications for the selection of second-line immunotherapy because analysis of mechanisms of immune escape will likely be required to identify patients likely to respond to such therapies.

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