Human papillomavirus type 16 (HPV-16) actively recruits DNA methyltransferases (DNMTs) and/or histone deacetylases (HDACs) via the viral oncoprotein, E7, to strategically regulate viral gene expression during the viral life cycle in order to evade immune recognition and clearance by the host. We evaluated the role of epigenetic alterations in HPV-associated head and neck squamous cell carcinomas (HPV-HNSCCs), and mapped all potential DNA methylation sites within the HPV-16 epigenome in 22 patients with HPV-HNSCC. We identified 110 CpG sites within the HPV-16 viral methylome and reported that 45% (10 of the 22) of advanced stage HNSCCs were methylated in greater than 50% of the viral DNA CpG sites, with methylation clustered in the E1, E5, L1, and L2 genes (1). This finding suggests that the majority of the HPV viral proteins are not being expressed in the cancer cell. Furthermore, similar to HPV-associated cervical lesions, we found that as the number of viral DNA copies increased within a cancer cell the extent of viral DNA methylation also increased, which supports the hypothesis that there is active transcriptional suppression of the majority of the copies of the viral genome and, thus, expression of the immunogenic HPV viral antigens. The silencing of the redundant copies may be advantageous and even critical for clonal selection during carcinogenesis and may also facilitate immune evasion by the virus through decreased expression of immunogenic viral antigens. Another mechanism of immune evasion utilized by HPV and reported by our group was the activation of the Programmed Death-1 (PD-1): Programmed Death Ligand 1 (PD-L1) immune checkpoint pathway. Our findings supported a model in which the PD-1:PD-L1 immune checkpoint pathway becomes induced as an adaptive immune resistance mechanism of tumor against host. Preliminary results from a multi-institutional phase I study (www.clinicaltrials.gov; NCT01848834) demonstrated a 20% objective response rate in HPV-HNSCC patients with locally recurrent and/or metastatic disease treated with anti-PD-1 monotherapy. We hypothesize that head and neck cancer patients who failed anti-PD-1 monotherapy most likely failed due to a paucity of CD8+ T cells present within the tumor microenvironment. Thus, we are exploring whether administration of demethylating agents can re-activate the transcriptionally silent viral genomes and increase the expression of immunogenic HPV antigens to increase the frequency of infiltrating cytotoxic CD8 + T cells and, thus, convert immune checkpoint blockade failures to responders.
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1.Park IS et al, Cancer Prev Res 2011; 4(2):207-17