Specific zinc finger-induced methylation of PD-L1 promoter inhibits its expression

DNA methylation of promoter regions is often associated with epigenetic silencing of gene expression, and DNA methyltransferase (DNMTs) has been used to suppress gene expression. In order to explore the synergistic roles of two methyltransferase members Dnmt3a and Dnmt1, we constructed expression plasmid that could express a recombinant DNMTs consisting of the C-terminal domains of both Dnmt3a and Dnmt1 fused to a zinc finger domain which binds to the PD-L1 promoter of human prostate cancer cells (DU145). Programmed death ligand 1 (PD-L1, B7-H1, CD-274) is a transmembrane protein widely expressed on antigen-presenting and other immune cells. The interaction of PD-L1 with its receptor PD-1 is considered an 'immune checkpoint' for possible cancer therapy. DU145 cells treated with the Dnmt3aC-1C plasmid showed significantly reduced expression of PD-L1 as compared to Dnmt3aC or Dnmt1C alone. Our results show that the fusion of Dnmt1 improves the methylation activity of Dnmt3a and enhances its biological functions. This combinatorial strategy can be used to better control PD-L1 expression to support cytotoxic T lymphocytes (CTL) response against tumors.