Functional Downregulation of PD-L1 and PD-L2 by CpG and non-CpG Oligonucleotides in Melanoma Cells

SIMPLE SUMMARY: Although metastatic melanoma is still not a curable disease, targeting of immunologically relevant checkpoints represents a turning point in the treatment. Particularly, targeting the interaction between PD-L1 and its referring receptor PD-1 with antibodies has been shown to activate T-cell function abrogating the evasion of tumor cells from immune recognition. Here, we present another approach that interferes with this system by showing that treatment of melanoma cells with oligonucleotides reduces the expression of PD-L1 (and PD-L2) on tumor cells. Specifically, non-CpG-6-PTO, an ODN that forms superstructures known as G-quartets, has been found to inhibit the interferon-γ-induced signaling cascade which fosters PD-L1 expression. These findings suggest a new therapeutic strategy to interfere with one of the most important immune checkpoints. ABSTRACT: The clinical application of immune checkpoint inhibitors represents a breakthrough progress in the treatment of metastasized melanoma and other tumor entities. In the present study, it was hypothesized that oligonucleotides (ODNs), known as modulators of the immune response, have an impact on the endogenous expression of checkpoint molecules, namely PD-L1 and PD-L2 (PD-L1/2). IFNγ-stimulated melanoma cells (A375, SK-Mel-28) were treated with different synthetically manufactured oligonucleotides which differed in sequence, length and backbone composition. It was found that a variety of different ODN sequences significantly suppressed PD-L1/2 expression. This effect was dependent on length and phosphorothioate (PTO) backbone. In particular, a sequence containing solely guanines (nCpG-6-PTO) was highly effective in downregulating PD-L1/2 at the protein, mRNA and promoter levels. Mechanistically, we gave evidence that ODNs with G-quartet-forming motifs suppress the interferon signaling axis (JAK/STAT/IRF1). Our findings identify a subset of ODNs as interesting pharmacological compounds that could expand the arsenal of targeted therapies to combat the immunological escape of tumor cells.