Replication of human papillomaviruses in cell culture

In vitro and Animal Models for Antiviral Therapy in Papillomavirus Infections

The need for antiviral therapies for papillomavirus infections is well recognized but the difficulties of reproducing the infectious cycle of papillomaviruses in vitro has hindered our understanding of virus-cell interactions and the regulation of viral gene expression during permissive growth. Recent advances in understanding the temporal expression and function of papillomavirus proteins has enabled consideration of a targeted approach to papillomavirus chemotherapy and in particular the inhibition of viral replication by targeting the E1 and E2 proteins. There are in vitro culture systems available for the screening of new chemotherapeutic agents, since significant advances have been made with culture systems which promote epithelial differentiation in vitro. However, to date, there are no published data which show that virions generated in vitro can infect keratinocytes and initiate another round of replication in vitro. In vivo animal models are therefore necessary to assess the efficacy of antivirals in preventing and treating viral infection, particularly for the low-risk genital viruses which are on the whole refractory to culture in vitro. Although papillomaviruses affect a wide variety of hosts in a species-specific manner, the animals most useful for modelling papillomavirus infections include the rabbit, ox, mouse, dog, horse, primate and sheep. The ideal animal model should be widely available, easy to house and handle, be large enough to allow for adequate tissue sampling, develop lesions on anatomical sites comparable with those in human diseases and these lesions should be readily accessible for monitoring and ideally should yield large amounts of infectious virus particles for use in both in vivo and in vitro studies. The relative merits of the various papillomavirus animal models available in relation to these criteria are discussed.

Role of innate immunity against human papillomavirus (HPV) infections and effect of adjuvants in promoting specific immune response

During the early stages of human papillomavirus (HPV) infections, the innate immune system creates a pro-inflammatory microenvironment by recruiting innate immune cells to eliminate the infected cells, initiating an effective acquired immune response. However, HPV exhibits a wide range of strategies for evading immune-surveillance, generating an anti-inflammatory microenvironment. The administration of new adjuvants, such as TLR (Toll-like receptors) agonists and alpha-galactosylceramide, has been demonstrated to reverse the anti-inflammatory microenvironment by down-regulating a number of adhesion molecules and chemo-attractants and activating keratinocytes, dendritic (DC), Langerhans (LC), natural killer (NK) or natural killer T (NKT) cells; thus, promoting a strong specific cytotoxic T cell response. Therefore, these adjuvants show promise for the treatment of HPV generated lesions and may be useful to elucidate the unknown roles of immune cells in the natural history of HPV infection. This review focuses on HPV immune evasion mechanisms and on the proposed response of the innate immune system, suggesting a role for the surrounding pro-inflammatory microenvironment and the NK and NKT cells in the clearance of HPV infections.

Viral trans-factor independent replication of human papillomavirus genomes

BACKGROUND

Papillomaviruses (PVs) establish a persistent infection in the proliferating basal cells of the epithelium. The viral genome is replicated and maintained as a low-copy nuclear plasmid in basal keratinocytes. Bovine and human papillomaviruses (BPV and HPV) are known to utilize two viral proteins; E1, a DNA helicase, and E2, a transcription factor, which have been considered essential for viral DNA replication. However, growing evidence suggests that E1 and E2 are not entirely essential for stable replication of HPV.

RESULTS

Here we report that multiple HPV16 mutants, lacking either or both E1 and E2 open reading frame (ORFs) and the long control region (LCR), still support extrachromosomal replication. Our data clearly indicate that HPV16 has a mode of replication, independent of viral trans-factors, E1 and E2, which is achieved by origin activity located outside of the LCR.

HPV: from infection to cancer

Infection with HPV (human papillomavirus) 16 is the cause of 50% or more of cervical cancers in women. HPV16 infection, however, is very common in young sexually active women, but the majority mount an effective immune response and clear infection. Approx. 10% of individuals develop a persistent infection, and it is this cohort who are at risk of cancer progression, with the development of high-grade precursor lesions and eventually invasive carcinoma. Effective evasion of innate immune recognition seems to be the hallmark of HPV infections, since the infectious cycle is one in which viral replication and release is not associated with inflammation. Furthermore, HPV infections disrupt cytokine expression and signalling with the E6 and E7 oncoproteins particularly targeting the type I IFN (interferon) pathway. High doses of IFN can overcome the HPV-mediated abrogation of signalling, and this may be the basis for the therapeutic effects on HPV infections of immune-response modulators such as the imidazoquinolones that induce high levels of type I IFNs by activation of TLR (Toll-like receptor) 7. Using the unique W12 model of cervical carcinogenesis, some of these IFN-related interactions and their relevance in the selection of cells with integrated viral DNA in cancer progression have been investigated. Our data show that episome loss associated with induction of antiviral response genes is a key event in the spontaneous selection of cervical keratinocytes containing integrated HPV16. Exogenous IFN-beta treatment of W12 keratinocytes in which the majority of the population contain episomes results only in the rapid emergence of IFN-resistant cells, loss of episome-containing cells and a selection of cells containing integrated HPV16 in which the expression of the transcriptional repressor E2 is down-regulated, but in which E6 and E7 are up-regulated.

Immune responses to human papillomavirus

The immune system uses innate and adaptive immunity to recognize and combat foreign agents that invade the body, but these methods are sometimes ineffective against human papillomavirus (HPV). HPV has several mechanisms for avoiding the immune system. HPV infects, and multiplies in keratinocytes, which are distant from immune centers and have a naturally short lifespan. The naturally short life cycle of the keratinocyte circumvents the need for the virus to destroy the cell, which would trigger inflammation and immune response. In addition, HPV downregulates the expression of interferon genes. Despite viral immune evasion, the immune system effectively repels most HPV infections, and is associated with strong localized cell mediated immune responses. New prophylactic L1 virus-like protein vaccines for HPV 16 and 18 and HPV 6, 11, 16, and 18 are in phase 3 trials. Available data suggests that these vaccines are safe, produce high levels of antibodies, and are effective at preventing HPV infection.

Oxidative stress in patients with nongenital warts

Comparison of oxidative stress status between subjects with or without warts is absent in the literature. In this study, we evaluated 31 consecutive patients with warts (15 female, 16 male) and 36 control cases with no evidence of disease to determine the effects of oxidative stress in patients with warts. The patients were classified according to the wart type, duration, number, and location of lesions. We measured the indicators of oxidative stress such as catalase (CAT), glucose-6-phosphate dehydrogenase (G6PD), superoxide dismutase (SOD), and malondialdehyde (MDA) in the venous blood by spectrophotometry. There was a statistically significant increase in levels of CAT, G6PD, SOD activities and MDA in the patients with warts compared to the control group (P < .05). However, we could not define a statistically significant correlation between these increased enzyme activities and MDA levels and the type, the duration, the number, and the location of lesions. We determined possible suppression of T cells during oxidative stress that might have a negative effect on the prognosis of the disease. Therefore, we propose an argument for the appropriateness to give priority to immunomodulatory treatment alternatives instead of destructive methods in patients with demonstrated oxidative stress.

Epitope specificity and longevity of a vaccine-induced human T cell response against HPV18

Persistent human papillomavirus (HPV) type 16 and 18 infection can lead to pre-malignant and malignant diseases of the lower genital tract. Several lines of evidence suggest that T cell responses can control HPV infection. However, relative to other human viruses, strong effector memory T cell responses against HPV have been difficult to detect. We used an in vitro stimulation step prior to enzyme-linked immunospot assays to identify IFN-γ-secreting T cells specific for HPV16 and 18 E6/E7 peptides. This allowed the detection of HPV-specific CD4⁺ T cells that were not evident in direct ex vivo assays. T cell responses against HPV16 or 18 peptides were detected in healthy volunteers (7/9) and patients with lower genital tract neoplasia (10/20). Importantly, this assay allowed tracking of vaccine-induced T cell responses in nine patients, following inoculation with a live recombinant vaccinia virus (HPV16 and 18 E6/E7, TA-HPV). Novel vaccine-induced T cell responses were demonstrated in five patients, but no clinical responses (lesion regressions) were seen. For one vaccinated patient, the T cell response was mapped to a single dominant HPV18 E7 epitope and this response was sustained for >3 years. Our data suggest that systemic memory T cells against HPV16 and 18, induced naturally or by TA-HPV vaccination, are relatively rare. Nevertheless, the assay system developed allowed estimation of magnitude, epitope specificity, and longevity of vaccine-induced CD4⁺ T cell responses. This will be useful for vaccine design and measurement of immunological endpoints in clinical trials.

Human papillomavirus molecular biology and pathogenesis

Human papillomavirus (HPV) infection is the most common sexually transmitted disease in the world and accounts for an estimated 11% of the global cancer incidence in women. HPV-16 is the most prevalent type detected in cervical cancer and along with types 18, 31, 33 and 45 has been classified as a class I carcinogen. In addition to cervical cancer, HPVs are also associated with the malignant transformation of other mucosal and skin cancers. Thus, the combination of the malignant potential of HPV and its high prevalence of infection confers to it an importance of generalized clinical and virological significance. The natural history of HPV infection with or without treatment varies from spontaneous regression to persistence. The most important mechanism for wart regression appears to be cell-mediated immunity. Cytokines released by keratinocytes or cells of the immune system may play a part in the induction of an effective immune response against HPV infection and the subsequent regression of lesions. This review discusses the molecular biology, pathogenesis and immunology of HPV infections.

Immunology of human papillomavirus infection in lower genital tract neoplasia

Despite its being a relatively common virus, the study of human papillomavirus infection has lagged behind that of other viruses. Human papillomaviruses do not provoke strong systemic antibody or T-cell responses. Furthermore, the majority of those infected do not display clinical symptoms and are able to clear the virus by unknown mechanisms. In the last decade, however, research into human papillomavirus immunology has blossomed, for two main reasons. First, there is strong circumstantial evidence that the immune system can control papillomavirus infection, since the prevalence of human papillomavirus-associated neoplasia is increased in immunocompromised individuals. Second, the strong association between human papillomavirus infection and cervical cancer has led to attempts to develop prophylactic or therapeutic vaccines. In this chapter, our current knowledge of human papillomavirus immune responses will be reviewed, and how this relates to clinical practice will be discussed.

Regression of canine oral papillomas is associated with infiltration of CD4+ and CD8+ lymphocytes

Canine oral papillomavirus (COPV) infection is used in vaccine development against mucosal papillomaviruses. The predictable, spontaneous regression of the papillomas makes this an attractive system for analysis of cellular immunity. Immunohistochemical analysis of the timing and phenotype of immune cell infiltration revealed a marked influx of leukocytes during wart regression, including abundant CD4+ and CD8+ cells, with CD4+ cells being most numerous. Comparison of these findings, and those of immunohistochemistry using TCRalphabeta-, TCRgammadelta-, CD1a-, CD1c-, CD11a-, CD11b-, CD11c-, CD18-, CD21-, and CD49d-specific monoclonal antibodies, with previously published work in the human, ox, and rabbit models revealed important differences between these systems. Unlike bovine papillomavirus lesions, those of COPV do not have a significant gamma/delta T-cell infiltrate. Furthermore, COPV lesions had numerous CD4+ cells, unlike cottontail rabbit papillomavirus lesions. The lymphocyte infiltrate in the dog resembled that in human papillomavirus lesions, indicating that COPV is an appropriate model for human papillomavirus immunity.

Heterogeneous nuclear ribonucleoprotein C binds exclusively to the functionally important UUUUU-motifs in the human papillomavirus type-1 AU-rich inhibitory element

We have previously identified an inhibitory, 57 nt AU-rich sequence in the HPV-1 late 3' UTR, termed as the HPV-1 AU-rich element (h1ARE). It contains two types of functionally important motifs: two AUUUA sequences and three UUUUU sequences. We have shown that the h1ARE interacts with heterogeneous nuclear ribonucleoprotein (hnRNP) C1/C2 and the ELAV-like HuR protein. While we have shown that HuR binds to both the AUUUA- and the UUUUU-motifs, the interaction between hnRNP C and the h1ARE has not been investigated in detail. Here, we have used recombinant (r)hnRNP C1 to study the interaction between hnRNP C1 and the h1ARE by using the UV cross-linking assay. We demonstrate that (r)hnRNP C1 cross-links specifically to the three functionally important UUUUU-motifs in the h1ARE. In contrast, (r)hnRNP does not UV cross-link to the functionally important AUUUA-motifs in the h1ARE. Conclusively, the binding ability of hnRNP C to the h1ARE correlates with its partially inhibitory function. Additionally, the recombinant AU-rich RNA binding factor 1 (AUF1) was analyzed for binding to the h1ARE by using the UV cross-linking assay, but the results revealed no specificity for the functionally important AUUUA- and UUUUU-motifs.

Overview of vaccinology with special reference to papillomavirus vaccines

Viruses that belong to six different families are a significant cause for neoplasia in man and animals. Among them are the Papillomaviruses that cause uterine cervical cancer in women. Efforts to develop prophylactic vaccines against viruses that cause cancer are now a major research engagement. Vaccinology, the science of vaccines, engages the sciences of immunology and of microbiology, both relying heavily on molecular biology. Successful development of vaccines relies on extensive knowledge of immunology and vaccinology. Present efforts to develop vaccines against cervical cancer caused by Papillomaviruses are focused on use of the structural antigens L1 and L2 of the virus and on the oncoproteins E6 and E7. Work on Papillomavirus vaccines has been brilliantly conceived and executed and some of vaccines are now in clinical trial. Success may follow and Papillomavirus vaccine may join with the hepatitis B virus anti-cancer vaccine in the battle against cancers of man.

Inhibition of human papillomavirus type 16 gene expression by nordihydroguaiaretic acid plant lignan derivatives

Several methylated derivatives of a plant lignan, nordihydroguaiaretic acid (NDGA) were found to be potent anti-viral agents by suppressing Sp1 regulated transcription within the sexually transmitted viruses human immunodeficiency virus (HIV) and herpes simplex virus (HSV). A prominent Sp1 DNA binding site within many human papillomavirus (HPV) promoters has been noted to play an active role in HPV gene expression. In this report it is shown that the three NDGA derivatives, Mal.4, M(4)N, and tetra-acetyl NDGA can also inhibit gene expression from the early promoter P(97) of HPV16. The drug activity on gene expression was measured after DNA transfection of recombinant vector constructs linking the viral promoter and enhancer elements to the luciferase reporter gene. Using the specific luciferase activity as the indicator of gene expression, Mal.4 and M(4)N were found to be active in a dose dependent manner that is in the same range of concentrations reported for the promoters of HIV, HSV, and simian virus 40 (SV40) while tetra-acetyl NDGA was much more active in suppression of the HPV P(97) promoter activity than Mal.4 and M(4)N. The drugs showed limited to no effect on gene expression driven by the adenovirus major late promoter and the cytomegalovirus (CMV) promoter. Hence, such drug derivatives may be significant in the therapy of papillomavirus infections and their associated induced human cancers.

Cytokine profile of draining lymph node lymphocytes in mice grafted with syngeneic keratinocytes expressing human papillomavirus type 16 E7 protein

Studies on the immune response to human papillomaviruses are compromised by the extreme host and tissue specificity of these viruses. To circumvent this, a mouse model system has been used in which antigen is presented via a differentiated, syngeneic keratinocyte graft expressing human papillomavirus type 16 (HPV-16) E7 protein. Using this model, previous studies have shown that animals grafted with a high cell inoculum (1x10(7) NEK 16 cells) exhibit a delayed-type hypersensitivity response that is E7-specific and CD4(+)-mediated, but those receiving a low cell inoculum (5x10(5) NEK 16 cells) are rendered unresponsive to subsequent and repeated antigen challenge. To investigate the mechanisms underlying this phenomenon, we have analysed the early changes in the cytokine profile of the graft-draining lymph node (GDLN) after high- or low-dose grafts. At 4 days post-grafting, there was a peak secretion of IL-2 associated with a decreased secretion of IL-4 by gammadelta-TCR(+) cells in the group receiving 1x10(7) NEK 16 cells. At 5 days post-grafting, there was a peak secretion of IL-10 by CD8(+) cells in both the high- and low-dose graft groups compared with controls. In contrast, low dose-grafted animals showed an increase in IL-4 production by CD8(+) cells at this time-point. Low antigen challenge in this model system is associated with the appearance of a CD8(+) population in the GDLN that secretes both IL-4 and IL-10. This population may represent a Tc2 or Ts subset that could induce further unresponsiveness.

The inhibitory activity of the AU-rich RNA element in the human papillomavirus type 1 late 3' untranslated region correlates with its affinity for the elav-like HuR protein

A 57-nucleotide adenosine- and uridine-rich RNA instability element in the human papillomavirus type 1 late 3' untranslated region termed h1ARE has previously been shown to interact specifically with three nuclear proteins that failed to bind to an inactive mutant RNA. Two of those were identified as the heterogeneous ribonucleoproteins C1 and C2, whereas the third, a 38-kDa, poly(U) binding protein (p38), remained unidentified. Here we show that partially purified p38 reacts with a monoclonal antibody raised against the recently identified elav-like HuR protein, indicating that p38 is the HuR protein. Indeed, recombinant glutathione S-transferase (GST)-HuR protein binds specifically to sites within the h1ARE. Determination of the apparent Kd value of GST-HuR for the h1ARE and the inactive mutant thereof revealed that GST-HuR bound with a more than 50-fold-higher affinity to the wild-type sequence. Therefore, the binding affinity of GST-HuR for the wild-type and mutant h1AREs correlates with their inhibitory activities in transfected cells, strongly suggesting that the HuR protein is involved in the posttranscriptional regulation of human papillomavirus type 1 late-gene expression.

Translational inhibition in vitro of human papillomavirus type 16 L2 mRNA mediated through interaction with heterogenous ribonucleoprotein K and poly(rC)-binding proteins 1 and 2

Human papillomavirus (HPV) type 16 belongs to the group of "high risk" HPV types that are frequently detected in anogenital cancers. The expression of HPV-16 late genes encoding the virus capsid proteins L1 and L2 is restricted to terminally differentiated epithelial cells in the superficial layers of the squamous epithelium. We have previously identified negative elements in the 3' end of L2 RNA that act in cis to reduce mRNA utilization without substantially affecting mRNA levels. The experiments reported here demonstrate the interaction of cellular proteins with an inhibitory sequence present in the coding region of the L2 mRNA. Using RNA gel shift assays and UV cross-linking, we have detected three cellular proteins interacting specifically with the sense strand of the L2 mRNA, two of which were identified as heterogeneous ribonucleoprotein K (hnRNP K) and the poly(rC) binding- protein (PCBP). Recombinant hnRNP K, PCBP-1, and PCBP-2 that were over expressed in bacteria and partially purified bound to the HPV-16 L2 mRNA in a sequence-specific manner. Interestingly, PCBP-1, PCBP-2, and hnRNP K specifically and efficiently inhibited translation of the HPV-16 L2 mRNA in vitro. Therefore, these proteins may play an important role in the regulation of HPV-16 late gene expression and virus production in vivo.

mRNA instability elements in the human papillomavirus type 16 L2 coding region

Human papillomavirus capsid proteins L1 and L2 are detected only in terminally differentiated cells, indicating that expression of the L1 and L2 genes is blocked in dividing cells. The results presented here establish that the human papillomavirus type 16 L2 coding region contains cis-acting inhibitory sequences. When placed downstream of a reporter gene, the human papillomavirus type 16 L2 sequence reduced both mRNA and protein levels in an orientation-dependent manner. Deletion analysis revealed that the L2 sequence contains two cis-acting inhibitory RNA regions. We identified an inhibitory region in the 5'-most 845 nucleotides of L2 that acted by reducing cytoplasmic mRNA stability and a second, weaker inhibitory region in the 3' end of L2. In contrast, human papillomavirus type 1 L1 and L2 genes did not encode strong inhibitory sequences. This result is consistent with observations of high virus production in human papillomavirus type 1-infected tissue, whereas only low levels of human papillomavirus type 16 virions are detectable in infected epithelium. The presence of inhibitory sequences in the L1 and L2 mRNAs may aid the virus in avoiding the host immunosurveillance and in establishing persistent infections.

Human papillomavirus E6 and E7: proteins which deregulate the cell cycle

Numerous clinical, epidemiological and molecular findings link some types of Human Papillomaviruses (HPV) with cancer of the genital tract. They share a common pathway of transformation with a number of DNA tumour viruses, such as Adenovirus and SV40. Although all these viruses are termed 'DNA tumour viruses' and have similar in vitro transforming activities, Human Papillomavirus is the only one so far clearly involved in human cancer. Extensive studies on HPV E6 and E7 proteins have demonstrated their involvement in malignant transformation. E6 and E7 bind the products of tumour suppressor genes, p53 and Rb1, respectively, modifying or inactivating their normal functions. The Rb1 and p53 genes are deleted or mutated in several cancers and both proteins regulate the transcription of genes involved in cell cycle progression control. The E6/p53 and E7/Rb1 interactions result in a deregulation of the cell cycle with loss of control of crucial cellular events, such as DNA replication, DNA repair and apoptosis.