Papillomaviruses (PVs) induce benign squamous cell lesions that can progress to carcinomas. Two of the most extensive studied PVs are human papillomavirus type 16 (HPV-16) and bovine papillomavirus type 4 (BPV-4), which are the causative agents of genital cancer in humans and alimentary cancer in cattle respectively. There is a growing body of evidence that HPVs that infect cutaneous epithelia are contributory factors to non melanoma skin cancer (NMSC) in both immunosuppressed and immunocompetent individuals. The viral proteins E5, E6 and E7 induce cell transformation and in concert help the virus escape the host immune response and establish a persistent productive infection. The aim of the group is the study of the different aspects of the biology of the virus in the context of tumorigenesis and immune evasion.
Virus-cofactor interactions and cell transformation
The natural co-carcinogen of BPV-4 is bracken fern. One of the mutagens present in the fern is the tetra-hydroxyflavone quercetin. Exposure to quercetin of cells partially transformed by E7 leads to full oncogenic transformation. This effect is due to the increased activity of the viral transcriptional promoter in the presence of quercetin (thus increasing the expression of the viral transforming proteins), and to the ability of E7 to abrogate quercetin-induced G1 arrest (thus allowing the expansion of quercetin-damaged cells into fully transformed clones).
We have observed similar effects in human keratinocytes expressing the HPV-18 oncoproteins. In control keratinocytes quercetin induces G1 arrest by up-regulating p27kip; in HPV-18 cells quercetin induces an increase in the level of E7 and a concomitant increase in the levels of Skp2, the negative regulator of p27kip. This leads to inhibition of p27kip activity and to abrogation of quercetin-induced G1 arrest, leading to proliferation and expansion of quercetin-damaged cells. These observations strengthen the association, observed particularly in developing countries, between human oesophageal carcinoma, HPV infection and bracken fern in the diet.
Evasion of the host immune response
Expression of E5 results not only in alterations of the cytoskeleton and Golgi apparatus, but also in the loss of major histocompatibility complex class I (MHC I). MHC I is required for the presentation of antigenic peptides to cytotoxic T-lymphocytes (CTL) and therefore its loss is a possible direct way in which the virus, via E5, avoids the immune system. E5 retains MHC I in the Golgi apparatus (GA). Retention in the GA and inhibition of traffic from the GA to the cell surface is due to at least two events: the impeded acidification of the GA (stemming from the interaction between E5 and 16k subunit c) and the interaction between E5 and the heavy chain (HC) component of MHC I. While E5 interacts with, and down-regulates, many different classical MHC I, E5 does not down-regulate non-classical MHC I. One of the function of non-classical MHC I is the inhibition of natural killer (NK) cells, and thus E5 has the potential to inhibit both CTL and NK killing of PV infected cells. We have mapped the domain of E5 that interacts with HC to the first hydrophobic domain and we are currently mapping the domain of HC that interacts with E5. We are also investigating the effect of the loss of MHC I on the immune response to the virus.
Not all HPVs have an E5 ORF. Most of the numerous cutaneous HPVs lack E5 and therefore other viral proteins must by-pass the host immune response. We are currently analysing how the E6 and E7 proteins of HPV-38 (a virus found in up to 50% of NMSC) impact on the interferon (IFN) signal transduction pathway. A number of steps along the IFN pathway are inhibited by HPV-38 E6 and/or E7, including TAP-1, IRF-1 and STAT-1. Similar events have also been observed for HPV-16 E6 and E7. Thus, the three oncoproteins together potentially contribute to the evasion of the host immune response by modulating or inhibiting different parts of both adaptive and innate immune response.
