Human cells infected with HPVs are able to escape from host immunosuppression leading to the development of cancers. HPV oncogene E7 has been reported to have specific biological features to affect host immune systems such as MHC class I antigen processing and presentation. Our aims are to investigate whether and how HPV16 E7 down-regulates MHC class I to induce carcinogenesis in virus-infected keratinocytes.
Four established systems: (HPV16 E7-transgenic mouse keratinocytes (E7-KCs), human primary keratinocyte cultures, two-cell co-cultures and mice model were used to achieve our aims.
HPV16 E7 protein inhibits interferon-γ (IFN-γ)-mediated enhancement of keratinocyte antigen processing and T-cell lysis, leading to the blockade of interferon regulatory factor-1 (IRF-1) and TAP-1 expression in E7-KCs. HPV16 E7 prevents IFN-γ-induced phosphorylation of STAT1(Tyr701) to decrease MHC class I antigen presentation through Cytotoxic T lymphocytes (CTLs), leading to escape of IFN-γ-treated E7-KCs from immune surveillance (CTLs-mediated killing) through the JAK/STAT1/IRF-1 signaling pathway. In human primary keratinocyte cultures, expression of HPV16 E7 significantly increased the activities of PI3K/Akt and MAPK as indicated by increased protein levels of PI3K, p-PI3K, p-AKT and p-MAPK accompanied by a dramatically decreased level of pRb protein. The expression of HPV 16 E7 substantially inhibited expression of IRF-1, STAT1 and PKCδ, in contrast to the marked upregulation of STAT3 and PKCα. HPV16 E7 expression also markedly increased the frequencies of G0/1 cells, leading to an increase in keratinocyte survival/proliferation. Two-cell co-cultures revealed that HPV16 E7-transduced lymphoma cells (E7-EL4) were shown to fuse with mouse primary keratinocytes, leading to distinct changes of the KC morphology within seven weeks. The fusion of E7-EL4 cells with keratinocytes that induced formation of spheroid-like colonies and multiple-cell tumor colonies was associated with upregulated expression of several oncogenes including c-Jun, c-Fos and k-Ras and abnormal expression of several MHC class I molecules. Furthermore, mice subcutaneously challenged with E7-EL4 cells could also develop skin tumors. These skin tumors showed substantial disruption of spectrin/adducin-based cytoskeletons and significantly increased protein levels of p-PI3K/p-AKT.
All data reveal that E7 down-regulated MHC class I to mediate immune escape, which could play a crucial role in human cancers caused by infections with HPVs. E7 oncogene is a potential target for developing immunotherapy of HPV-induced cancers.