UV Radiation Activates Toll-Like Receptor 9 Expression in Primary Human Keratinocytes, an Event Inhibited by Human Papillomavirus 38 E6 and E7 Oncoproteins.

Several lines of evidence indicate that cutaneous human papillomavirus (HPV) types belonging to the beta genus of the HPV phylogenetic tree synergize with UV radiation in the development of skin cancer. Accordingly, the E6 and E7 oncoproteins from some beta HPV types are able to deregulate pathways related to immune response and cellular transformation. Toll-like receptor 9 (TLR9), in addition to playing a role in innate immunity, has been shown to be involved in the cellular stress response. Using primary human keratinocytes as experimental models, we have shown that UV irradiation (and other cellular stresses) activates TLR9 expression. This event is closely linked to p53 activation. Silencing the expression of p53 or deleting its encoding gene affected the activation of TLR9 expression after UV irradiation. Using various strategies, we have also shown that the transcription factors p53 and c-Jun are recruited onto a specific region of the TLR9 promoter after UV irradiation. Importantly, the E6 and E7 oncoproteins from beta HPV38, by inducing the accumulation of the p53 antagonist ΔNp73α, prevent the UV-mediated recruitment of these transcription factors onto the TLR9 promoter, with subsequent impairment of TLR9 gene expression. This study provides new insight into the mechanism that mediates TLR9 upregulation in response to cellular stresses. In addition, we show that HPV38 E6 and E7 are able to interfere with this mechanism, providing another explanation for the possible cooperation of beta HPV types with UV radiation in skin carcinogenesis.IMPORTANCE Beta HPV types have been suggested to act as cofactors in UV-induced skin carcinogenesis by altering several cellular mechanisms activated by UV radiation. We show that the expression of TLR9, a sensor of damage-associated molecular patterns produced during cellular stress, is activated by UV radiation in primary human keratinocytes (PHKs). Two transcription factors known to be activated by UV radiation, p53 and c-Jun, play key roles in UV-activated TLR9 expression. The E6 and E7 oncoproteins from beta HPV38 strongly inhibit UV-activated TLR9 expression by preventing the recruitment of p53 and c-Jun to the TLR9 promoter. Our findings provide additional support for the role that beta HPV types play in skin carcinogenesis by preventing activation of specific pathways upon exposure of PHKs to UV radiation.

Downregulation of Toll-Like Receptor 9 Expression by Beta Human Papillomavirus 38 and Implications for Cell Cycle Control.

UNLABELLED: Innate immunity is the first line of host defense against infections. Many oncogenic viruses can deregulate several immune-related pathways to guarantee the persistence of the infection. Here, we show that the cutaneous human papillomavirus 38 (HPV38) E6 and E7 oncoproteins suppress the expression of the double-stranded DNA sensor Toll-like receptor 9 (TLR9) in human foreskin keratinocytes (HFK), a key mediator of the antiviral innate immune host response. In particular, HPV38 E7 induces TLR9 mRNA downregulation by promoting accumulation of ΔNp73α, an antagonist of p53 and p73. Inhibition of ΔNp73α expression by antisense oligonucleotide in HPV38 E6/E7 HFK strongly rescues mRNA levels of TLR9, highlighting a key role of ΔNp73α in this event. Chromatin immunoprecipitation experiments showed that ΔNp73α is part of a negative transcriptional regulatory complex with IκB kinase beta (IKKß) that binds to a NF-κB responsive element within the TLR9 promoter. In addition, the Polycomb protein enhancer of zeste homolog 2 (EZH2), responsible for gene expression silencing, is also recruited into the complex, leading to histone 3 trimethylation at lysine 27 (H3K27me3) in the same region of the TLR9 promoter. Ectopic expression of TLR9 in HPV38 E6/E7 cells resulted in an accumulation of the cell cycle inhibitors p21(WAF1) and p27(Kip1), decreased CDK2-associated kinase activity, and inhibition of cellular proliferation. In summary, our data show that HPV38, similarly to other viruses with well-known oncogenic activity, can downregulate TLR9 expression. In addition, they highlight a new role for TLR9 in cell cycle regulation. IMPORTANCE: The mucosal high-risk HPV types have been clearly associated with human carcinogenesis. Emerging lines of evidence suggest the involvement of certain cutaneous HPV types in development of skin squamous cell carcinoma, although this association is still under debate. Oncogenic viruses have evolved different strategies to hijack the host immune system in order to guarantee the persistence of the infection. Their capability to evade the immune system is as important as their ability to promote cellular transformation. Therefore, understanding the viral mechanisms involved in viral persistence is a valid tool to evaluate their potential role in human carcinogenesis. Here, we show that E6 and E7 oncoproteins from the cutaneous HPV38 downregulate the expression of the double-stranded DNA sensor TLR9 of innate immunity. We also present evidence that the HPV38-mediated downregulation of TLR9 expression, in addition to its potential impact on the innate immune response, is linked to cell cycle deregulation.

EBV latent membrane protein 1 is a negative regulator of TLR9.

EBV infects most of the human population and is associated with a number of human diseases including cancers. Moreover, evasion of the immune system and chronic infection is an essential step for EBV-associated diseases. In this paper, we show that EBV can alter the regulation and expression of TLRs, the key effector molecules of the innate immune response. EBV infection of human primary B cells resulted in the inhibition of TLR9 functionality. Stimulation of TLR9 on primary B cells led to the production of IL-6, TNF-α, and IgG, which was inhibited in cells infected with EBV. The virus exerts its inhibitory function by decreasing TLR9 mRNA and protein levels. This event was observed at early time points after EBV infection of primary cells, as well as in an immortalized lymphoblastoid cell line. We determined that the EBV oncoprotein latent membrane protein 1 (LMP1) is a strong inhibitor of TLR9 transcription. Overexpression of LMP1 in B cells reduced TLR9 promoter activity, mRNA, and protein levels. LMP1 mutants altered in activating the NF-κB pathway prevented TLR9 promoter deregulation. Blocking the NF-κB pathway recovered TLR9 promoter activity. Mutating the NF-κB cis element on the TLR9 promoter restored luciferase transcription in the presence of LMP1. Finally, deletion of the LMP1 gene in the EBV genome abolished the ability of the virus to induce TLR9 downregulation. Our study describes a mechanism used by EBV to suppress the host immune response by deregulating the TLR9 transcript through LMP1-mediated NF-κB activation.

C/EBP{delta} and STAT-1 are required for TLR8 transcriptional activity.

Toll-like receptor 8 (TLR8), which is expressed primarily in myeloid cells, plays a central role in initiating immune responses to viral single-stranded RNA. Despite the great interest in the field of TLR8 research, very little is known in terms of TLR8 biology and its transcriptional regulation. Here, we describe the isolation of the hTLR8 promoter and the characterization of the molecular mechanisms involved in its regulation. Reporter gene analysis and ChIP assays demonstrated that the hTLR8 regulation of the basal transcription is regulated via three C/EBP cis-acting elements that required C/EBPδ and C/EBPß activity. In addition, we observed that R848 stimulation increases TLR8 transcriptional activity via an enhanced binding of C/EBPδ, and not C/EBPß, to its responsive sites within the TLR8 promoter. Moreover, we showed that IFN-γ also increased TLR8 transcription activity via the binding of STAT1 transcription factor to IFN-γ activated sequence elements on the TLR8 promoter and enhanced TLR8 functionality. These results shed new light on the mechanisms involved during TLR8-mediated innate immune response.

T-bet and Eomes instruct the development of two distinct natural killer cell lineages in the liver and in the bone marrow.

Trail(+)DX5(-)Eomes(-) natural killer (NK) cells arise in the mouse fetal liver and persist in the adult liver. Their relationships with Trail(-)DX5(+) NK cells remain controversial. We generated a novel Eomes-GFP reporter murine model to address this question. We found that Eomes(-) NK cells are not precursors of classical Eomes(+) NK cells but rather constitute a distinct lineage of innate lymphoid cells. Eomes(-) NK cells are strictly dependent on both T-bet and IL-15, similarly to NKT cells. We observed that, in the liver, expression of T-bet in progenitors represses Eomes expression and the development of Eomes(+) NK cells. Reciprocally, the bone marrow (BM) microenvironment restricts T-bet expression in developing NK cells. Ectopic expression of T-bet forces the development of Eomes(-) NK cells, demonstrating that repression of T-bet is essential for the development of Eomes(+) NK cells. Gene profile analyses show that Eomes(-) NK cells share part of their transcriptional program with NKT cells, including genes involved in liver homing and NK cell receptors. Moreover, Eomes(-) NK cells produce a broad range of cytokines, including IL-2 and TNF in vitro and in vivo, during immune responses against vaccinia virus. Thus, mutually exclusive expression of T-bet and Eomes drives the development of different NK cell lineages with complementary functions.

The human papillomavirus type 16 E7 oncoprotein induces a transcriptional repressor complex on the Toll-like receptor 9 promoter.

Human papillomavirus type 16 (HPV16) and other oncogenic viruses have been reported to deregulate immunity by suppressing the function of the double-stranded DNA innate sensor TLR9. However, the mechanisms leading to these events remain to be elucidated. We show that infection of human epithelial cells with HPV16 promotes the formation of an inhibitory transcriptional complex containing NF-κBp50-p65 and ERα induced by the E7 oncoprotein. The E7-mediated transcriptional complex also recruited the histone demethylase JARID1B and histone deacetylase HDAC1. The entire complex bound to a specific region on the TLR9 promoter, which resulted in decreased methylation and acetylation of histones upstream of the TLR9 transcriptional start site. The involvement of NF-κB and ERα in the TLR9 down-regulation by HPV16 E7 was fully confirmed in cervical tissues from human patients. Importantly, we present evidence that the HPV16-induced TLR9 down-regulation affects the interferon response which negatively regulates viral infection. Our studies highlight a novel HPV16-mediated mechanism that combines epigenetic and transcriptional events to suppress a key innate immune sensor.

Cell proliferation and survival induced by Toll-like receptors is antagonized by type I IFNs.

TRIF is an adaptor protein associated with the signaling by Toll-like receptor (TLR)3 and TLR4 for the induction of type I IFNs. Here, we demonstrate a mechanism by which TLR signaling controls cell proliferation and survival. We show that TLR3 and TLR4 can induce cell cycle entry via TRIF, which targets the cell cycle inhibitor p27(kip1) for relocalization, phosphorylation by cyclin/cdk complexes, and proteasome degradation. These events are antagonized by type I IFN induced by the TRIF pathway. Furthermore, in human dendritic cells treated with TLR3, TLR4, or TLR5 ligands, we demonstrate that IFN signaling modulates p27(kip1) degradation and apoptosis, identifying an immunoregulatory "switching" function of type I IFNs. These findings reveal a previously uncharacterized function of TLR signaling in cell proliferation and survival.

TLR9 expression and function is abolished by the cervical cancer-associated human papillomavirus type 16.

Cervical cancer development is linked to the persistent infection by high-risk mucosal human papillomaviruses (HPVs) types. The E6 and E7 major oncoproteins from this dsDNA virus play a key role in the deregulation of the cell cycle, apoptosis, and adaptive immune surveillance. In this study, we show for the first time that HPV type 16 (HPV16), the most carcinogenic type among the high-risk subgroup, interferes with innate immunity by affecting the expression of TLRs. Infection of human primary keratinocytes with HPV16 E6 and E7 recombinant retroviruses inhibits TLR9 transcription and hence functional loss of TLR9-regulated pathways. Similar findings were achieved in HPV16-positive cancer-derived cell lines and primary cervical cancers, demonstrating that this event occurs also in an in vivo context. Interestingly, E6 and E7 from the low-risk HPV type 6 are unable to down-regulate the TLR9 promoter. In addition, E6 and E7 from the high-risk HPV type 18, which are known to persist less competently in the host than HPV16, have reduced efficiency compared with HPV16 in inhibiting TLR9 transcription. Furthermore, a CpG motif derived from the HPV16 E6 DNA sequence activated TLR9, indicating this virus is able to initiate innate responses via the receptor it later down-regulates. This study reveals a novel mechanism used by HPV16 to suppress the host immune response by deregulating the TLR9 transcript, providing evidence that abolishing innate responses may be a crucial step involved in the carcinogenic events mediated by HPVs.

Toll-like receptor signaling stimulates cell cycle entry and progression in fibroblasts.

Toll-like receptors (TLRs) are proteins involved in recognition of foreign pathogen-associated molecular patterns and activation of processes leading to innate immune recognition. We show that stimulation of fibroblasts with a TLR5 ligand, flagellin, can induce proliferation of serum-starved cells or prevent cell cycle exit upon serum withdrawal independently of autologous growth factor secretion. Other TLR ligands, such as poly(I:C) and lipopolysaccharide, can have a similar effect only if the action of type I interferons is blocked. Flagellin stimulation can prevent cell cycle arrest induced by overexpression of exogenous cyclin-dependent kinase inhibitor p27. Stimulation of TLR5 and overexpression of MyD88, but not TRIF, TIRAP, or TRAM, result in p27 degradation, which can be suppressed by dominant negative Akt and mutation of the p27 C-terminal Thr(187) site. These data provide evidence for a nonimmune and cell autonomous role of TLR signaling, whereby TLR stimulation provides a positive signal for cell division.

Recognition of double-stranded RNA by human toll-like receptor 3 and downstream receptor signaling requires multimerization and an acidic pH.

Studies involving Toll-like receptor 3 (TLR3)-deficient mice suggest that this receptor binds double-stranded RNA. In the present study, we analyzed ligand/receptor interactions and receptor-proximal events leading to TLR3 activation. The mutagenesis approach showed that certain cysteine residues and glycosylation in TLR3 amino-terminal leucine-rich repeats were necessary for ligand-induced signaling. Furthermore, inactive mutants had a dominant negative effect, suggesting that the signaling module is a multimer. We constructed a chimeric molecule fusing the amino-terminal ectodomain of TLR3 to the transmembrane and carboxyl terminal domains of CD32a containing an immunoreceptor tyrosine-based motif. Expression of TLR3-CD32 in HEK293T cells and the myeloid cell line U937 resulted in surface localization of the receptor, whereas the nonrecombinant molecule was intracellularly localized. The synthetic double-stranded RNAs poly(I-C) and poly(A-U) induced calcium mobilization in a TLR3-CD32 stably transfected U937 clone but not in control cells transfected with other constructs. An anti-TLR3 antibody also induced Ca(2+) flux but only when cross-linked by a secondary anti-immunoglobulin antibody, confirming that multimerization by the ligand is a requirement for signaling. The inhibitors of lysosome maturation, bafilomycin and chloroquine, inhibited the poly(I-C)-induced biological response in immune cells, showing that TLR3 interacted with its ligand in acidic subcellular compartments. Furthermore, TLR3-CD32 activation with poly(I-C) was only observed within a narrow pH window (pH 5.7-6.7), whereas anti-TLR3-mediated Ca(2+) flux was pH-insensitive. The importance of an acidic pH for TLR3-ligand interaction becomes critical when using oligomeric poly(I-C) (15-40-mers). These observations demonstrate that engagement of TLR3 by poly(I-C) at an acidic pH, probably in early phagolysosomes or endosomes, induces receptor aggregation leading to signaling.

Differential induction of gene promoter constructs by constitutively active human TLRs.

Antigen presenting cells can sense microorganisms through activation of members of the Toll like receptor family (TLRs), which initiate signals leading to transcription of many inflammation-associated genes. TLRs and IL-1R, through their TIR domains, activate NFkappaB and mitogen-activated protein kinase pathways and upregulate a set of specific target genes. Recent evidence points to several differences in signaling pathways activated by individual TLRs. To evaluate the basic signaling potential of individual TIR signaling domains, we generated constitutively active versions of all known human TLRs by fusing mouse CD4 extracellular portion with the TLR transmembrane and TIR domains. A panel of promoters from genes known to be activated by TLRs as well as artificial promoter constructs with transcription factor binding sites were selected to measure their response in the presence of constitutively active CD4TLR fusion molecules. These studies show for the first time that a unique panel of promoters appears to be highly induced by CD4TLR1, 6 (TLRs that usually function through heterodimerisation with TLR2), and CD4TLR10. We also observed that CD4TLR4 is the most potent gene activator compared to all other ten human TLRs. Preliminary analyses of several promoter deletions showed that TLRs use different sequence elements to activate these reporters. In addition, since different ligands for a single TLR (e.g., TLR9) can induce different pathways, the CD4TLR fusions seem to activate all the pathways and therefore can be used to assess the overall signaling capacity of a given TLR. Finally, analysis of promoter constructs induced by the only orphan TLR, TLR10, allowed the identification of the ENA78 promoter as a tool for screening its ligands.