Human Papillomavirus (HPV) Upregulates the Cellular Deubiquitinase UCHL1 to Suppress the Keratinocyte's Innate Immune Response

Abstract

Persistent infection of basal keratinocytes with high-risk human papillomavirus (hrHPV) may cause cancer. Keratinocytes are equipped with different pattern recognition receptors (PRRs) but hrHPV has developed ways to dampen their signals resulting in minimal inflammation and evasion of host immunity for sustained periods of time. To understand the mechanisms underlying hrHPV's capacity to evade immunity, we studied PRR signaling in non, newly, and persistently hrHPV-infected keratinocytes. We found that active infection with hrHPV hampered the relay of signals downstream of the PRRs to the nucleus, thereby affecting the production of type-I interferon and pro-inflammatory cytokines and chemokines. This suppression was shown to depend on hrHPV-induced expression of the cellular protein ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) in keratinocytes. UCHL1 accomplished this by inhibiting tumor necrosis factor receptor-associated factor 3 (TRAF3) K63 poly-ubiquitination which lead to lower levels of TRAF3 bound to TANK-binding kinase 1 and a reduced phosphorylation of interferon regulatory factor 3. Furthermore, UCHL1 mediated the degradation of the NF-kappa-B essential modulator with as result the suppression of p65 phosphorylation and canonical NF-κB signaling. We conclude that hrHPV exploits the cellular protein UCHL1 to evade host innate immunity by suppressing PRR-induced keratinocyte-mediated production of interferons, cytokines and chemokines, which normally results in the attraction and activation of an adaptive immune response. This identifies UCHL1 as a negative regulator of PRR-induced immune responses and consequently its virus-increased expression as a strategy for hrHPV to persist.

Activation of NF-κB by Human Papillomavirus 16 E1 Limits E1-Dependent Viral Replication through Degradation of E1

“…Although the expression level of exogenous E1 in our experiments could be much higher than that during initial amplification, a lower level of E1 might be sufficient to induce NF-B activation. Karim et al reported that an increase in NF-B activity was detected within 24 h after infection with HPV16 virions isolated from raft cultures (50). Although their interpretation was that NF-B is activated as part of an innate immune response, their results did not exclude the possibility that E1-dependent initial amplification contributes to activation of NF-B.…”

“…ATM has been shown to activate the IB kinase (IKK) complex by directly phosphorylating its regulatory subunit, the NF-B essential modulator (NEMO), and this activation of IKK leads to phosphorylation-dependent degradation of IB␣ (49). Interestingly, it was recently shown that degradation of NEMO is enhanced in human keratinocytes containing episomal HPV16 genomes, compared to primary human keratinocytes (50). Therefore, activation of NF-B by the ATM-mediated pathway through NEMO could be inherently limited in HCK1T-HPV16 cells.…”

Therapeutic cancer vaccines

“…Moreover, many of these vaccines are directed against viral structures involved in malignant transformation (Table 2). This advantage can be exploited, because the vaccine can deliver antigen and stimulate robust effector T cell responses, whereas the natural infection is subject to the myriad intricate immune evasion and immunosuppression mechanisms these viruses have developed in the course of evolution (92)(93)(94)(95)(96)(97)(98). Nevertheless, these vaccines have to work against the same peripheral cell tolerance-, anergy-, and suppression-driving mechanisms operating in nononcogenic, persistent viral infections and nonviral cancers.…”

Early Defensive Mechanisms against Human Papillomavirus Infection

“…UCHL1 proved to be a potent negative regulator of PRR-induced immune responses in KC and was exploited by high-risk HPV to prevent the production of IFNs, cytokines, and chemokines and the attraction of professional immune cells. Mechanistic studies showed that UCHL1-mediated suppression of nuclear factor B (NF-B) and IFN response factor (IRF) signaling pathways by destabilizing NF-B essential modulator and preventing polyubiquitination of TNF receptor-associated factor 3, respectively (43). HPV-transformed KC tend to lose expression of UCHL1, suggesting that different mechanisms of immune escape operate in these cells.…”

“…KC constitutively secrete low levels of soluble immune mediators, including cytokines, chemokines, and growth factors (46)(47)(48), the production of which can be elevated in response to proinflammatory stimuli, including PRR activation and autocrine or paracrine stimulation with inflammatory cytokines (31,43,46,47,49). Several cytokines, including type I IFNs, TNF-␣, IL-1␣, IL-4, IL-13, and transforming growth factor ␤ (TGF-␤), share the ability to inhibit HPV early gene expression (50)(51)(52)(53)(54).…”