DefectiveNF-kBSignaling inMetastaticHeadandNeckCancer Cells Leads to Enhanced Apoptosis by Double-Stranded RNA

Ligands to several Toll-like receptors (TLR), which mediate innate immune responses and chronic inflammation have been used as adjuvants to immunotherapy to enhance their antitumor activity. In particular, doublestranded RNAs that are cognate ligands of TLR3 have been used to trigger proapoptotic activity in cancer cells. However, a mechanistic understanding of TLR3-mediated apoptosis and its potential involvement in controlling tumor metastasis has been lacking. In this study, we used paired cell lines and fresh tumor specimens, derived from autologous primary andmetastatic head and neck squamous cell carcinoma, to investigate the role of TLR3 signaling in metastatic progression. Compared with primary tumor cells, metastatic tumor cells were highly sensitive to TLR3-mediated apoptosis after double-stranded RNA treatment. Enhanced apoptosis in metastatic cells was dependent on double-stranded RNA and TLR3 and also the TLR3 effector signaling protein TRIF. Downstream responses requiring NF-kBwere critical for apoptosis in metastatic cells, the defects in which could be resuscitated by alternative pathways of NF-kB activation. By elucidating howTLR3 ligands trigger apoptosis in metastatic cells, our findings suggest insights into how to improve their clinical use. Cancer Res; 72(1); 45–55.