Human Papillomavirus (HPV) - Virology

Human Papillomavirus (HPV)

More than 90% of cervical cancer cases are found to have the human papillomavirus, which is also the most frequent infection linked to female malignancies.

The HPV virus has more than 100 subtypes. Multiple epithelial lesions and malignancies, primarily on cutaneous and mucosal surfaces, are its major associations.

It is the most widespread STD in the world and has a major negative influence on a person's social life.

Anogenital and cutaneous warts are caused by low-risk HPVs (LR-HPVs), whereas oropharyngeal malignancies and anogenital cancers such cervical, anal, vulvar, vaginal, and penile cancers are caused by high-risk HPVs (HR-HPVs).

Although HPV does not directly cause cancer, it can be brought on by a number of other factors, including smoking, exposure to UV rays and other carcinogens, a lack of folate, immunosuppression, and pregnancy.

Table of Contents

• Structure of Human Papillomavirus (HPV)
• Genome Structure of Human Papillomavirus (HPV)
• Epidemiology of Human Papillomavirus (HPV) 
• Transmission of Human Papillomavirus (HPV)
• Replication of Human Papillomavirus (HPV)
• Early-phase of viral replication
• Late-stage of viral replication
• Pathogenesis of Human Papillomavirus (HPV)
• Clinical Manifestations of Human Papillomavirus (HPV)
• Mucosal human papillomavirus infections:
• Cutaneous human papillomavirus infections:
• Diagnosis of Human Papillomavirus (HPV)
• Treatment of HPV
• Vaccination of HPV
• Prevention and Control of HPV

Structure of Human Papillomavirus (HPV)

• The papillomaviruses have an icosahedral structure, are tiny, non-enveloped, and measure 52–55 nm in diameter.
• A single double-stranded DNA molecule is contained within a protein capsid made up of 72 pentameric capsomers.
• Two structural proteins, including the viral proteins L1 (Late 1) and L2 (Late 2), make up the capsid.
• At 55 kDa, the L1 protein accounts for 80% of viral proteins. The size of the L2 is 70 kDa. The viral encoding of the L1 and L2 proteins is the same.
• A variable number of L2 molecules, which are partially exposed on the surface of the virion, are present in the viral coat together with 360 molecules of L1 protein, which are organised into 72 capsomers.
• In mammals and non-mammals, the expression of L1 alone or in combination results in Virus-like particles (VLPs).
• Virion density in cesium chloride is 1.34 g/ml, while the sedimentation coefficient (S20, W) is 300.

Genome Structure of Human Papillomavirus (HPV)

• Approximately eight ORFs are transcribed into a single double-stranded DNA molecule with a genome size of 800 bp in the genomes of all HPV strains.
• The early region (E), the late region (L), and a sizable non-coding portion known as the long control region can all be classified as functional areas of the ORF (LCR).
• The L region codes for structural proteins (L2-L2) vital for viral assembly, the E region codes for proteins (E1-E7) required for replication, and the LCR includes the cis-elements required for viral DNA replication and transcription.
• The papillomaviruses varies in size and number of ORFs, but they all include well-conserved core genes for replication (E1 and E2) and packing (L1 and L2), as well as very diverse genes for cell entrance, immunological infiltration, and viral release (E6, E7, E5, and E4).

Epidemiology of Human Papillomavirus (HPV)

• The HPV virus has more than 180 known subtypes.
• Verruca vulgaris and verruca plantaris, which are cutaneous warts on the hands and feet, are brought on by HPV subtypes 1, 2, 4, 27, and 57. These are transmitted by coming into touch with skin that has epidermal damage or by infected insects.
• Condyloma acuminatum and HPV subtypes 6 and 11 are frequently linked to anogenital warts. They are known as low-risk HPV, and both juvenile and adult recurrent respiratory papillomatosis have been linked to them.
• Precancerous and cancerous lesions of the cervix, male and female anogenital regions, and oropharyngeal region are most frequently caused by subtypes 16 and 18, respectively.
• The development of cervical cancer is linked to the high-risk HPV subtypes 31, 33, 35, 45, 52, and 58.
• Despite being regarded as sexually transmitted diseases, both low-risk and high-risk HPV can spread through other types of intimate contact.
• According to recent research, 39.9% of women and 45.2% of men in people aged 18 to 59 have HPV, according to the Centers for Disease Control and Prevention (CDC).
• The most prevalent sexually transmitted infection in both the United States and the rest of the globe is the genital HPV infection.

Transmission of Human Papillomavirus (HPV)

Various HPV subtypes have an impact on the vaginal region, throat, and mouth. Despite being a prevalent STD, it has the potential to remain asymptomatic and go away on its own. The virus does not need to be penetrated during intercourse to propagate.

It may transmit via:

• Skin-to-skin contact and the genital region
• oral, vaginal, or anal sex
• Sex toys being shared

Among the risk factors for becoming infected with HPV are:

• Sexual activity, the first sexual experience's age, and the total number of partners
• Smoking
• oral contraceptives are used (for more than 5 years)
• consume betel nuts
• radiation and UV light exposure

Replication of Human Papillomavirus (HPV)

• Through microscopic tears, the HPV penetrates the cells. The single-layered squamous cellular interface between the endo and ectocervix is where HR-HPV, a form of HPV that causes cervical and other cancers, may enter the cell. The basal layer cells' surface or the basement membrane include cellular receptors, such as heparin sulphate proteoglycans (HSPGs), which the HPV L1 capsid protein interacts to.
• Endocytosis, which shares the greatest similarities with micropinocytosis, is how HPV enters the cell.
• The virus passes via the trans-Golgi apparatus and membrane-bound cytoplasmic elements.
• The viral episomal genome is subsequently transferred into the nucleus through nuclear pores or by the nuclear membrane rupturing during mitosis via a tubulin-mediated route.

The following steps of viral replication:

Early-phase of viral replication

• Early transcription starts as soon as the nuclear entrance occurs.
• The E2 engages the cellular DNA replication machinery by binding to the E1, which then binds as a dimer of hexamers to the origin of the replication site.
• In the initial replication, the HPV genome produces 50–100 episomal copies per nucleus.

Late-stage of viral replication

• Virion development and viral DNA replication are both involved.
• Viral E1 and E2 proteins must express themselves more strongly throughout this phase.
• It is distinguished by the viral main late promoter's activation, which is located in the E7 gene area and leads to enhanced expression of E1 and E2, as well as E4 and E5.
• Thousands of offspring viral genomes are produced at the late stage of viral replication, which most likely utilises the rolling circle process for DNA replication.
• After viral genome amplification and capsid protein synthesis, the virion is formed in the nucleus.
• The superficial keratinocytes are the site of viral maturation, which ultimately results in the release of the infectious virion.

Pathogenesis of Human Papillomavirus (HPV)

• Despite their high incidence rates, HPV infections typically go away on their own within a year or two. The continued infection has a significant role in the emergence of cervical and other malignancies.
• Depending on the HPV type, HPVs cause benign or malignant lesions in cutaneous or mucosal epithelial cells.
• Because the host immune response mechanisms are so important in regulating HPV infection, immunocompromised people have higher prevalence rates.

• The virus does not lyse the target cells after infection or generate viremia, which limits the exposure of viral antigen and helps the virus elude the host immune response.
• The MHC-1 gene can be downregulated by the HPV, and it can also interfere with the oncoproteins E5, E6, and E7-mediated interferon (IFN) pathway.
• The Golgi apparatus transports HLA molecules and intracellular viral antigenic peptide complexes to the cell surface, where cytotoxic CD8+ T lymphocytes detect them.
• The infected cells then undergo apoptosis as a result of the activated cytotoxic T lymphocytes (CTLs).
• The cytokines secreted by T helper cells promote proliferation, maintain cytotoxic lymphocytes, and activate B cells to produce antibodies.

Clinical Manifestations of Human Papillomavirus (HPV)

The clinical symptoms of the HPV include:

Mucosal human papillomavirus infections:

• Condyloma acuminatum (Present as papules, nodules, or soft, filiform, pinkish, sessile, or pedunculated growths in the genital area)
• Focal epithelial hyperplasia (disease of the oral mucosa more common in children and women)
• Cervical neoplasia and cervical cancer
• Other anogenital cancers (including those of the vulva, vagina, penis, and anus)
• Head and neck cancer

Cutaneous human papillomavirus infections:

• Common warts
• Plantar warts
• Flat warts
• Filiform warts
• Pigmented warts
• Epidermal cysts
• Skin cancer (Bowen’s disease)

Diagnosis of Human Papillomavirus (HPV)

HPV cannot be detected using conventional techniques like electron microscopy, cell culture, or certain immunological tests. Among the crucial techniques for its detection are:

• Warts/Condyloma detection

By analysing the pigmentation, induration, ulceration, and other characteristics of warts, one may visually check for genital or oral warts, and a biopsy can confirm this.

• Colposcopy and acetic acid test

Following the administration of an acetic acid solution and the collection of samples from lesions suspected of being neoplastic, a colposcopy is performed to examine the cervix, vagina, and occasionally the vulva. The degree of acetowhite lesions, surface contour, mosaic pattern, and punctuation are used to score the colposcopic results. Based on the anomalies of these parameters, the severity of lesions is deduced.

• Biopsy 

Colposcopic results that demonstrate high abnormalities, low-grade alterations with severe dyskaryosis, or a lesion that extends to the canal need excisional biopsy.

Genital warts are characterised by the presence of koilocytes, mature squamous cells with a distinct perinuclear zone.

• DNA techniques

HPV detection was first carried out using direct probe hybridization techniques including dot blotting and Southern blotting. However, they needed a lot of DNA in clinical samples, were time-consuming, labor-intensive, and had limited sensitivity. The two primary methods for routine viral detection are as follows:

• Hybrid capture HPV DNA Test 2 (hc2)

The FDA has given the HC2 with Pap test its OK. It can identify HPV as little as 1 pg/ml. Although it is unable to identify the precise HPV type, it can distinguish between the "low-risk" genotype group (6, 11, 42, 43, 44) and the "high-risk" genotype group (16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 68).

• Polymerase chain reaction

The HPV sequences found in the biological sample are amplified specifically.

• Pap smear or Pap test

In the beginning, it was explained by Papanicolaou and Traut. It makes use of scrapings that are mounted on a glass slide from the squamocolumnar junction. This technique may be used to identify malignant and premalignant alterations as well as viral diseases like HPV and herpes virus. For cytological analysis, scrapings from the upper lateral region of the vaginal wall can be collected.

Treatment of HPV

• There are several ways to treat cutaneous warts, including surgery, cryotherapy, irritating or immunomodulating drugs, and laser excision.
• Podophyllin resin, podophyllotoxin, organic acids like salicylic acid, trichloroacetic acid, and bichloroacetic acid, and cytostatic drugs like bleomycin, cidofovir, and 5-fluorouracil are a few examples of pharmaceuticals used to treat mucosal and cutaneous warts.
• In immunocompetent people, oropharyngeal and anogenital warts can both be treated.
• For the growth of HPV-related cancer, resection, chemotherapy, and/or radiation may be necessary.

Vaccination of HPV

• Currently licenced and available in the US is a 9-valent recombinant protein subunit HPV vaccination.
• The L1 major capsid protein is the main antigen utilised for HPV vaccination.
• Recombinant technology is used to create the L1 protein, which self-assembles into particles that resemble viruses (VLP).
• It is intramuscular to deliver the vaccination.
• Routine immunisation is not permitted for persons over the age of 45 but is advised for both genders at age 11 or 12.

Prevention and Control of HPV

Although most sexually active persons eventually get the HPV, there are steps that may be taken to lessen the risk.

The following are a few of the preventative measures:

• At 11 to 12 years old, children should have their first vaccine. Vaccination is advised up to the age of 26; however, if given after the commencement of sexual activity, the vaccine's effectiveness is reduced.
• using dental dams and/or condoms during oral, anal, or vaginal intercourse.
• Women with a history of invasive cervical cancer, high-grade CIN, or adenocarcinoma in-suite should get an annual vaginal cuff Pap test (BIII).
• The incidence of anal cancer in PWH can be decreased in HIV seropositive individuals by screening for lesions and treating precancerous lesions.