TLR9 and head and neck squamous cell carcinoma: In addition, co-culturing of HPV HNSCC cells with fibroblasts can enhance PD-L2 expression in fibroblasts through Toll-like receptor 9 (TLR9), which can be blocked by the oligodeoxynucleotide TTAGGG in the absence of IFN-γ, TNF-α, and CD81 stimulation [+81, 82].