Phosphorothioate-Modified TLR9 Ligands Protect Cancer Cells against TRAIL-Induced Apoptosis

Hypomethylated CpG oligodeoxynucleotides (CpG ODNs) target TLR9 expressed by immune cells and are currently being evaluated as adjuvants in clinical trials. However, TLR signaling can promote some tumor growth and immune evasion, such as in multiple myeloma (MM). Therefore, deciphering the effects of CpG ODNs on cancer cells will help in preventing these adverse effects and in designing future clinical trials. TLR activation induces multiple signaling pathways, notably NF-kappaB that has been involved in the resistance to TRAIL. Thus, we wondered if CpG ODNs could modulate TRAIL-induced apoptosis in different models of tumors. Here, we show that TLR9+ (NCI-H929, NAN6, KMM1) and TLR9- MM cells (MM1S) were protected by CpG ODNs against recombinant TRAIL-induced apoptosis. By using two fully human, agonist mAbs directed against TRAIL receptors DR4 and DR5 (mapatumumab and lexatumumab, respectively), we show that the protection was restricted to DR5-induced apoptosis. Similar results were observed for two colon cancer (C45 and Colo205) and two breast cancer cell lines (HCC1569 and Cal51). The protection of CpG ODNs was mediated by its nuclease-resistant phosphorothioate backbone independent of TLR9. We next demonstrated by surface plasmon resonance that phosphorothioate-modified CpG ODNs directly bound to either TRAIL or lexatumumab and then decreased their binding to DR5. Finally, NK cell lysis of a DR5-sensitive MM cell line (NCI-H929) through TRAIL was partially inhibited by phosphorothioate-modified CpG ODNs. In conclusion, our results suggest that the phosphorothioate modification of CpG ODNs could dampen the clinical efficacy of CpG ODN-based adjuvants by altering TRAIL/TRAIL receptor interaction.