Abstract
Human papillomavirus (HPV) is the primary etiological agent of cervical cancer. The high-risk HPV16 is the most prevalent HPV genotype worldwide and its DNA is detectable in more than 50% of all cervical tumours. The consistent over-expression of the HPV E6 and E7 oncogenes is necessary for cervical carcinogenesis. High mortality and morbidity due to HPV-associated disease still remains a major health burden globally. Since the current virus-like particle vaccines exert no therapeutic efficacy, better therapeutic strategies are required for the treatment of pre-existing, potentially cancer-causing, HPV infections. The aims of this study were firstly to understand the immune-modulation by HPV16 E7 on antigen presentation in the epidermal microenvironment and secondly to explore the potential use of HPV16 E6 as a target for immunotherapeutic approach for HPV vaccine candidate.
In HPV infection, Langerhans cells (LC) have been thought to be important in antigen uptake, presentation and consequently initiating the immune response. It was reported previously that the frequency of LC was greatly reduced in HPV16 infections. In this study, the effect of HPV16 E7 in modulating antigen presentation and its downstream effects on the CD8 T cell response was investigated. Replication-defective lentiviral vectors were used to deliver HPV16 E7, a luciferase reporter and the model antigen ovalbumin specifically to the epidermis. It was demonstrated that E7 is capable of reducing the density of LC in the transduced epidermis independent of any viral proteins. When the kinetics of the luciferase gene expression in the presence and absence of HPV16 E7 was monitored, it was observed that E7 delayed the loss of the luciferase expressing lentivirus transduced cells. We also showed that the magnitude of the CD8+ T cell response to skin-expressed ovalbumin was significantly reduced in E7 mice, compared with control mice. However, it was observed that the down-regulation effect occurred independent of LC since LC depletion did not alter the strength of CD8 T cell response in both E7 and control mice, suggesting E7 regulation of other skin-derived dendritic cells in antigen presentation and function.
The therapeutic efficacy of E6-VLP (virus like particles) derived from Rabbit haemorrhagic disease virus against tumour cells expressing HPV16 E6 and E7 was tested in a murine cervical cancer model. It was shown that E6-RHDV-VLP was efficient in inducing an anti-tumour effect against TC-1 tumour cells. Moreover, the incorporation of a universal helper T cell epitope, PADRE into E6-VLP further improved the therapeutic effect and prolonged the median survival, compared to mice immunised with E6-VLP without PADRE. Furthermore, treatment either with anti-CTLA-4 antibody to block T cell suppression or with PC61 (anti-CD25) to deplete regulatory T cells has shown to enhance the vaccine potency afforded by E6-VLP-PADRE.
Data presented here highlight the potential therapeutic use of E6 and/or E7 for the restoration of factors impeding the immune response as well as a target for immunotherapeutic approach against HPV-induced cervical cancer. The information gained from this research contributes to our understanding of HPV regulation of antigen presentation in the skin and may have implications for immunotherapy against HPV pre-cancers.