High-risk HPVs infect keratinocytes (KCs) and successfully suppress host immunity for up to two years despite the fact that KCs are well equipped to detect and initiate immune responses to invading pathogens.Viral persistence is achieved by active interference with KCs innate and adaptive immune mechanisms. We studied how hrHPV exploits cellular proteins to interfere with innate and adaptive immune pathway signaling.
An unique model for hrHPV infection, resembling the natural infection with hrHPV as closely as possible, was used for several types of analyses including micro-array, immunohistochemistry, antibody blocking, small inhibitors, siRNA and several biochemical methods to study the expression of immune receptors by KCs, to study the response of infected and non-infected KCs to innate and adaptive stimuli and to unravel how hrHPV interferes with these pathways.
We assessed signaling via the pattern recognition receptors (PRR), interferon gamma and tumor necrosis factor alpha in non, newly, and persistently hrHPV-infected keratinocytes. We found that active infection with hrHPV hampered the relay of signals downstream of these innate and adaptive immune stimuli, thereby affecting the production of type-I interferon and pro-inflammatory cytokines. In depth studies revealed that hrHPV exploits the cellular proteins UCHL1 and IFRD1 to interfere with NFκB signaling. UCHL1 hampered the interferon pathway by interacting with and deubiquitinating K63-linked polyubiquitin chains from TRAF3, resulting in reduced TBK1 – TRAF3 interaction, IRF3 phosphorylation and IFNβ expression. PRR-induced NFκB signaling was also attenuated through binding of UCHL1 to TRAF6, thereby influencing the Ub status of TRAF6. Furthermore, UCHL1 exacerbated NEMO degradation (1). In addition, hrHPV upregulates epidermal growth factor receptor (EGFR) gene and surface expression via the E5, E6 and E7 proteins. EGFR signaling through mTOR, RAF and/or MEK1, resulted in the increased expression of IFRD1, which mediates RelA K310 deacetylation by HDAC1/3 and as a result in the attenuated transcriptional activity of NFκB1. As a result, hrHPV also hampered subsequent immune cell attraction and the response of KCs to incoming signals from the immune system (2).
Thus hrHPV exploits the cellular proteins UCHL1 and IRFD1 to evade host innate immunity by suppressing immune signaling induced keratinocyte-mediated production of interferons, cytokines and chemokines, which normally results in the attraction and activation of an adaptive immune response, enabling the virus to persist. It is highly likely that these mechanism also play a role in other viral infections too and even extents to tumors.
1. R. Karim, B. Tummers, C. Meyers, C. Backendorf, R. Offringa, G.J.B. van Ommen, C.J.M. Melief, D. Guardavaccaro, J.M. Boer, S.H. van der Burg (2013) Human papillomavirus (HPV) upregulates the cellular deubiquitinase UCHL1 to suppress the keratinocyte's innate immune response. PLoS Pathogens 9(5):e1003384
2. B. Tummers, R. Goedemans, L.P.L. Pelascini, E.S. Jordanova, E.M.G. van Esch, C. Meyers, C.J.M. Melief, J.M. Boer, S.H. van der Burg (2015) The interferon-related developmental regulator 1 is used by Human papillomavirus to suppress NF-kappaB activation. Nature Communications, 6:6537