Super-enhancer receives signals from the extracellular matrix to induce PD-L1-mediated immune evasion via integrin/ BRAF/TAK1/ERK/ETV4 signaling

Objective:PD-L1 and PD-L2 expression levels determine immune evasion and the therapeutic efficacy of immune checkpoint blockade.The factors that drive inducible PD-L1 expression have been extensively studied,but mechanisms that result in constitutive PD-L1 expression in cancer cells are largely unknown.Methods:DNA elements were deleted in cells by CRISPR/Cas9-mediated knockout.Protein function was inhibited by chemical inhibitors.Protein levels were examined by Western blot,mRNA levels were examined by real-time RT-PCR,and surface protein expression was determined by cellular immunofluorescence and flow cytometry.Immune evasion was examined by in vitro T cell-mediated killing.Results:We determined the core regions(chr9:5,496,378–5,499,663)of a previously identified PD-L1L2-super-enhancer(SE).Through systematic analysis,we found that the E26 transformation-specific(ETS)variant transcription factor(ETV4)bound to this core DNA region but not to DNA surrounding PD-L1L2SE.Genetic knockout of ETV4 dramatically reduced the expressions of both PD-L1 and PD-L2.ETV4 transcription was dependent on ERK activation,and BRAF/TAK1-induced ERK activation was dependent on extracellular signaling fromαvβ3 integrin,which profoundly affected ETV4 transcription and PD-L1/L2 expression.Genetic silencing or pharmacological inhibition of components of the PD-L1L2-SE-associated pathway rendered cancer cells susceptible to T cell-mediated killing.Conclusions:We identified a pathway originating from the extracellular matrix that signaled via integrin/BRAF/TAK1/ERK/ETV4 to PD-L1L2-SE to induce PD-L1-mediated immune evasion.These results provided new insights into PD-L1L2-SE activation and pathways associated with immune checkpoint regulation in cancer.