Human papilloma viruses (HPV) are members of the papillomaviridae family that infect epithelial cells exclusively. After gaining entry into the cells of the epithelial basal layer, replication of the virus occurs in the nuclei of the infected cells, and the production of mature virions occurs in the suprabasal epithelial cell layers 1. HPV infection is highly transmissible, has a variable incubation period that can culminate in latent infection with low HPV DNA copy-number in basal cells insufficient to support transmissibility; in subclinical infection that is active but without clinical signs; or in clinical infection leading to benign, potentially malignant or malignant epithelial lesions. Many of these manifestations of HPV infection can undergo spontaneous resolution 2.

HPV infection is the most common of all sexually transmitted diseases. It is estimated that two thirds of those who have had sexual contact with HPV-infected persons, will become infected 3. Oral HPV infection can be acquired by oral-genital contact, by mouth-to-mouth contact, or possibly by autoinoculation 45; and in infants by mother-to-child transmission 67. How HPV infection of the upper respiratory tract occurs is not clear, but it may be by mucous carriage of virally infected squames from the mouth, or from the mouth of another person to the oropharynx and larynx 8.

The clinical manifestations and the microscopical features of HPV-associated lesions vary with the anatomical site affected and with the genotype of the HPV 7. The variability of the clinical and microscopical appearances and the course of HPV infection are governed by the complex interactions between the specific HPV genotype, viral genetic variables, host immune response and the phenotype of the infected epithelial cell against a background of differing environments and life-styles 9.

Immunosuppressed people are at significantly greater risk of developing HPV infection and of experiencing a more aggressive course of infection than immunocompetent people. The immune system plays an important rôle in controlling HPV infection 210, but since the intracellular HPV is shielded from the host immune surveillance until the virus has been sufficiently amplified or the infected keratinocytes exfoliate and disintegrate, the immune response remains relatively low-level compared to the immune response to viruses that are not confined to epithelial cells 1011.

More than 100 types of HPV have been identified, differing in the regulatory sequences and coding potential of their genomes 91012. Because of the unequivocal implication of HPV in the aetiopathogenesis of uterine cervical squamous cell carcinoma (SCC), HPV types in this context have been well studied and categorized into low-risk and high-risk types according to their potential for causing SCC 13. Examples of low-risk HPV genotypes are HPV-6, 11, 42, 43, 44 and of high-risk HPV genotypes are HPV-16, 18, 31, 33, 35, 45, 51, 52, 56, 58 and 59 6. Distinct clinical manifestations are associated with specific HPV genotypes 26131415161718 (Table 1).

In a small minority of women with persistent vulvo-vaginal infection with high-risk HPV genotypes, cervical SCC may develop as long as 12-15 years after the initial infection 12. High-risk HPV genotypes, in particular HPV-16, have also been implicated in the development of oral and oropharyngeal SCC 1415192021222324.

Low-risk HPV genotypes only rarely have been implicated in malignant transformation 6. In the mouth, low-risk HPV infection is associated with a variety of benign lesions including squamous cell papilloma, verruca vulgaris (common wart), condyloma acuminatum and focal epithelial hyperplasia (Heck disease), but can also be detected in potentially malignant epithelial lesions such as leukoplakia and erythroplakia 2526.

High-risk HPV genotypes, in particular HPV-16 and -18 are prevalent in potentially malignant oral epithelial lesions 272829 and in oral SCC 3031; but the cells of these lesions typically show low viral load, infrequent viral integration into the cellular genome, and the transformed cells seldom contain active transcriptional E6/E7 mRNA 313233.

All types of HPV have the same general organization of the genome: double-stranded circular DNA that carries early (E) open reading frames (ORFs) encoding for non-structural regulatory proteins, and late (L) ORFs encoding for capsid proteins. Noncoding upstream regulatory region (URR) encompassing the origin of replication, the E6/E7 gene promoter, and enhancers and silencers, is located between the early and the late regions (Figure 1) 23.

Early ORFs encode for E1, E2, E4, E5, E6 and E7 proteins. E1 and E2 proteins bind to viral origin of replication, recruit cellular proteins that mediate viral DNA replication, and regulate viral gene expression 37. E1 and E2 are also responsible for the maintenance of viral DNA as an episome during initial HPV infection, and also during latent infection in the basal cell layer 31034. E2 facilitates the segregation of the HPV genome during cell division resulting in the distribution of HPV DNA into daughter cells 1034; and is involved in the packaging of HPV DNA and in the promotion of virion assembly. Furthermore, E2 has the capacity to repress the activity of E6/E7 promoter 34.

The integration of HPV DNA within the cellular genome that is evident in some HPV-associated malignancies disrupts the E2 ORF leading to loss of E2 repressing function 35, thus permitting free transactivation of E6/E7 promoter by cellular transcription factors 211 and possibly by other factors such as nutritional agents, resulting in the increased expression of E6 and E7 oncoproteins 34.

E3 ORF does not encode for proteins 2. The function of E4 and E5 HPV proteins is not completely known 3. E4 protein is assumed to modulate HPV DNA amplification and structural gene expression 1034. E5 protein participates in HPV DNA amplification. It is also associated with down-regulation of gap junction intercellular communications, with expression of major histocompatibility complex (MHC) class 1 molecules, and with activation of growth factor receptors. Thus E5 protein has the capacity to promote immune evasion 3436.

Little is known about the action of E6 and E7 proteins of low-risk HPV genotypes during latent and subclinical HPV infection 23. During productive HPV infection high- and low-risk HPV E6 and E7 facilitate the maintenance of the viral episome in suprabasal matured epithelial cells 3. E6 and E7 also play an important rôle in the increased cell proliferation and the extended cell survival of HPV-associated malignancies, by altering the cell cycle regulatory factors 3637. E6 of high-risk HPV can induce ubiquitin-mediated degradation of p53 tumour suppressor protein 3839; and E7 of high-risk HPV genotypes can bind to and inactivate the Rb suppressor gene and other cellular proteins associated with cell cycle regulation 3940. Consequently E6 and E7 of high-risk HPV genotypes are instrumental in the pathogenesis of HPV-associated malignancies.

E6 and E7 of low-risk HPV probably play a rôle in the pathogenesis of HPV-associated benign tumours and only rarely are implicated in malignant transformation. It is unclear what properties E6 and E7 of high-risk HPV genotypes possess that the E6 and E7 of low-risk HPV genotypes do not possess, that enable them to induce malignant transformation of some epithelial cells infected with high-risk HPV. One possibility is that E6 and E7 proteins of high-risk HPV genotypes have a greater affinity for the regulatory factors of the cell cycle, thus causing an exaggerated dysregulation of mechanisms of cell growth and of cell survival 2.

E6 and E7 are not expressed in cells of the basal layer with low-risk HPV infection 2, and it is uncertain whether E6 and E7 oncoproteins of high-risk HPV genotypes are expressed in basal cells during productive HPV infection that does not give rise to malignant changes 10.

In high-risk HPV infection associated with intraepithelial neoplasia or with invasive malignancy, E6 and E7 oncoproteins are expressed in progenitor basal layer keratinocytes mediating basal cell proliferation and genomic instability, making the infected cells susceptible to additional genetic alterations leading to cell transformation and possibly to the subsequent development of SCC 35. The biological effects induced by E6 and E7 of high-risk HPV have been investigated in great detail. Little is known about the biological properties and mode of action of E6 and E7 proteins of low-risk HPV genotypes 2.

E8 ORF has been identified only in bovine papillomavirus 2. L1 ORF encodes for major capsid protein and L2 ORF for minor capsid protein 2. L1 protein is expressed after L2 protein in the viral replication cycle and mediates packaging of HPV particles and virion assembly. L2 protein interacts with E2 protein, facilitates the transport of L1 protein to the nucleus, and plays a role in the encapsulation of viral DNA 10.

The cells of the epithelium comprise two populations of cells, the progenitor and the maturing. The former cells are based in the basal and parabasal layers, have the capacity to proliferate by mitotic division following which some of the newly divided (amplifying) cells enter a maturation process and gradually rise through the several morphologically distinct cell layers to the surface, while some remain to maintain the integrity of the basal layer. In the mouth this process of maturation follows two main patterns: keratinization, and non-keratinization 41.

Some authors refer to this process of maturation as differentiation 2310. The authors of this paper prefer the term maturation, as we think that the term differentiation should be restricted to the development of specific populations of stem cells arising from the zygote that form particular cell compartments and possess distinct characteristics such as function, shape and rate of turnover 42.

Whether the cells that give rise to new progenitor cells and those that give rise to cells destined to go through the process of maturation are in fact distinct sub-populations, or whether the fates of all cells resulting from mitoses in the basal cell layer are determined by changing local microenvironment, is uncertain.

The HPV cycle is influenced by the stage of maturation of the infected keratinocyte in the squamous epithelium 2310. The production of virions is restricted to mature suprabasal epithelial cells where all the virus genes are expressed 43. Why HPV virions can be produced only by suprabasal postmitotic infected epithelial cells is not known 3.

HPV infection starts when the viruses enter epithelial basal cells which are referred to as the target cells of the virus 1044. Once inside the target cell, the viral DNA undergoes uncoating and is transported to the nucleus where the HPV genome will persist as multiple episomal copies 10. In HPV-infected basal cells, E1 and E2 proteins are expressed and they regulate early viral DNA transcription from the early promoter prior to production of virions. When expression of E2 is more pronounced, E2 represses viral DNA replication by blocking cellular transcription factors, thus controlling the number of HPV DNA copies in the basal cell by a process analogous to negative feedback 3. A stable number of about twenty to one hundred HPV DNA copies per cell is regulated by E1 and E2 viral proteins in the basal cells for as long as the infection persists 3.

In cases of high-risk HPV infection, E6 and E7 may also be expressed in HPV-infected basal cells, and the epithelium may then enter a proliferative phase characterized by an increasing number of HPV-infected basal cells 10, culminating in intraepithelial or invasive neoplasm.

Why HPV can infect basal cells in the first instance but not the cells of other epithelial layers is not known; but this is probably related to specific surface receptors confined to basal cells 2. The attachment of the virus to the basal cell membrane receptor may be mediated by heparin sulphate, and the bound virus enters the cell by a slow process of endocytosis 10. As the basal cells divide, E2 mediates distribution of some HPV DNA copies to daughter cells, while some copies remain in the progenitor cells, in both cases, as episomes 35.

In latent HPV infection the affected epithelium looks both clinically and histologically normal because the gene expression of the latent genes is restricted to E1 and E2 10. The virus can remain latent for long periods, can be cleared by the immune system, or can be activated and initiate productive infection 35.

As the epithelial cells mature, the HPV cycle progresses to productive replication 1043. In HPV infected epithelium where virions are being produced the matured epithelial cells express HPV E6 and E7 proteins in the suprabasal layers. E6 prevents apoptosis that normally occurs in response to viral infection; and E7 has the capacity to overcome the physiological blockade of DNA synthesis at the G1/S phase of the cell cycle in these post-mitotic cells. Thus E7 activates the cellular DNA replication mechanism allowing matured epithelial cells to re-enter the S phase of the cell cycle, and although this does not usually result in full replication of the cellular genome it makes the cellular replication machinery available for viral DNA replication 3353745.

Multiplication of the viral genome, synthesis of early gene proteins (E2, E4, E6, E7) and late gene proteins (L1, L2), assembly of virions, and release of virions from exfoliated epithelial cells occur at progressive stages of maturation of postmitotic HPV-infected epithelial cells. It is suggested that specific cellular factors associated with epithelial cell maturation activate late viral promoter located within E7 ORF resulting in expression of high levels of E1 and E2 viral replication proteins which activate late viral gene expression. This increase in E1 and E2 expression leads to HPV amplification resulting in an increase in viral genome numbers parallel with the stages of epithelial cell maturation. Eventually, mediated by L1 and L2 proteins, the virus escapes from the shedding epithelial cells 1039.

Unlike the early promoter, the late promoter is not suppressed by high levels of E2 3. The amplification of HPV genome in the maturing epithelium is modulated by E4 and E5 proteins, and E6 and E7 proteins ensure the maintenance of the HPV genome as a stable replicating episome. As outlined above, under certain circumstances E6 and E7 have the capacity to stimulate suprabasal postmitotic cells to re-enter the S-phase of the cell cycle resulting in the development of proliferative epithelial lesions 3. However, during subclinical HPV infection, although viral production continues, for as yet unknown reasons, the ability of E6/E7 to mediate the re-entry of suprabasal post-mitotic epithelial cells at the S phase of the cell cycle is limited, suprabasal proliferation of epithelial cells does not occur, and proliferative epithelial lesions do not develop 10. In some instances, normal appearing epithelial cells with subclinical infection may be adjacent to the HPV-induced lesion 7.

HPV-induced benign lesions are characterized by proliferation of cells of all epithelial layers 46. This proliferation is most probably mediated by E6/E7 proteins of mainly low-risk HPV. The proliferation manifests microscopically as acanthosis, hyperkeratosis and parakeratosis 746. Koilocytes in the upper layers of the epithelium are evident 7.

Microscopic examination of high-risk HPV-induced high-grade intraepithelial neoplasia shows greatly increased numbers of mitotic figures and features of cellular atypia, and extension of these multiplying atypical cells into the suprabasal layer of the epithelium as the outcome of expression of high-risk HPV E6/E7 oncoproteins in the basal cells 35.

In persons who have been exposed to HPV, several HPV types have been detected in their apparently normal oral mucosa, and in certain specific benign, potentially malignant and malignant oral lesions, in the aetiology of which they appear to be implicated. In infected persons, in clinically and histologically normal looking oral mucosa, the HPV exists in a latent form as a low-copy number episome in basal epithelial cells 17. In HPV-associated lesions, there is a substantial increase in intraepithelial HPV DNA copy-numbers 39.

Oral HPV infection is acquired primarily by sexual transmission, or less frequently by non-sexual direct transmission, by mother to child transmission or by autoinoculation. Persons who practise oral-genital sex, those who have had a number of sexual partners, and those who are immunocompromised are at greater risk of acquiring oral HPV infection 4514474849.

HPV is epitheliotropic, and it may be present in a latent form, a subclinical form or in a form which in association with other ill-defined factors can induce benign or malignant epithelial neoplasms. Figure 2 summarizes the different manifestations of HPV infection and the relationships between latent, subclinical and clinical HPV infection.

While some viral proteins are expressed in HPV-infected basal cells independently of the cell maturation process regulating early viral DNA transcription from early promotor prior to production of virions, the productive phase of the viral cycle is dependent on specific cellular factors associated with the progressive stages of maturation of post-mitotic cells as the cells gradually rise trough the layers of epithelium to the surface, and exfoliate.

Persistent high-risk HPV infection in a subset of subjects with HPV-cytopositive oropharyngeal epithelium induces genomic instability in these cells, making them susceptible to additional genetic alterations leading to cell transformation and to subsequent development of squamous cell carcinoma. In these oropharyngeal squamous cell carcinomas there is a causal association between HPV infection and the malignancy. In contrast, the nature of the link between high-risk HPV genotypes and potentially malignant and malignant oral epithelial lesions is not known.

Low-risk HPV infection of oral epithelium is associated with squamous cell papilloma, verruca vulgaris (common wart), condyloma acuminatum and focal epithelial hyperplasia (Heck disease); and possibly with the potentially malignant oral epithelial lesions, leukoplakia and erythroplakia.

The authors declare that they have no competing interests.

LF and RAGK contributed to the literature review. LF, JL and NHW contributed to the conception of the article. LF, JL, NHW and RAG contributed to the manuscript preparation. Each author reviewed the paper for content and contributed to the manuscript. All authors read and approved the final manuscript.