Human papillomavirus has been a cause of infection in humans for thousands of years. The history of papillomaviruses, knowledge of their causative role in benign and malignant disease, and their structural characteristics have led to the development of vaccines to prevent cervical and anogenital cancers. Many questions remain unanswered before HPV vaccines can be optimised; however, the concept of virtual eradication of cervical cancer is not impossible, and remains a realistic aspiration.
Introduction
Papillomaviruses are small, non-enveloped viruses found in several vertebrates, including primates, dogs, rabbits, and cattle, but which are highly host-specific. Human papillomavirus (HPV) is the most common sexually transmitted virus in the world. HPV infects only epithelial cells, either skin or mucous membranes. There are now known to be over 200 types of HPV, with different types causing nothing more than a trivial wart on the finger, or a fatal cervical cancer. HPV virions are icosahedral particles consisting of a protein capsid, or outer coat, surrounding a genome formed of a ring of double-stranded DNA. HPV is a simple virus whose genes code for just eight proteins, two of which (L1 and L2) form the outer protein capsid. Different HPV types are defined when three particular gene sequences (E6, E7 and L1) vary by at least 10%. It is now known that HPVs are necessary precursors for the development of cervical cancer, genital warts and many other types of neoplasia, including penile, anal and laryngeal tumours.
In 2006, the United States Food and Drug Administration approved the first preventative vaccination against HPV, the quadrivalent vaccine ‘Gardasil’, with activity against HPV types 16, 18, 6 and 11— the virus types directly responsible for the development of over 70% of cervical cancers (types 16 and 18) and 90% of genital warts (types 6 and 11). Subsequently, this vaccine, and another bivalent vaccine ‘Cervarix’ have been licensed in at least 80 countries. States implementing national vaccination programmes include the UK, Australia, New Zealand, the Netherlands and parts of Scandinavia.
The background to the development of these vaccines is a long and fascinating story of how a virus came to be recognised as a cause of cancer; how the evolution of specialties such as epidemiology, pathology, virology, immunology, sexual health medicine, gynaecology, molecular biology and pharmaceutics supported the vaccines’ development; and how individual clinicians and scientists from around the globe collaborated in this remarkable achievement. A scientific outcome of this magnitude can only arise from the work of thousands, and arguably it is unrepresentative to single out individuals whose own achievements are attained on the background of work by many others. Therefore, this overview of the history of HPV, warts and cancer, should only be read with the understanding that only the surface of such a vast topic has been scratched, and many scientists who have made great contributions in this field cannot be acknowledged.
Ancient history: the recognition of sexually transmitted lesions
Human papillomavirus is highly stable with little tendency to mutate, and indeed the virus has been around as long as mankind itself. One of the earliest manifestations of HPV infection was identified from the embalmed body of Nakht, an Ancient Egyptian worker on the estate of the Pharoah Setnakht in the 12th century BC, over 3000 years ago. At autopsy in 1974, Nakht was shown to have a wart on the sole of his foot. Warts were well described in classical Greek and Roman medical literature; the term ‘condyloma’ comes from the Greek ‘kondylos’ meaning ‘knuckle’ or ‘knob’; and the term used for the plantar papilloma, ‘verruca’, is Latin. Hippocrates of Kos, the so-called Father of Medicine who lived 400 years BC, referred to genital warts and their treatment with plant extracts:
‘The herb Parthenium parviflorum is used for removing warts from the glans penis….’
Parthenolide, the active component of the parthenium genus of plants, has been shown to be an immune-modulator. This might well account for its apparent efficacy in the treatment of virally mediated skin lesions. References to genital warts and common warts were made by the Roman encyclopaedist Aulus Cornelius Celsus circa AD 25, in his work ‘De Medicina’. Further references exist in the writings of Galen, Greek physician to Roman Emperor Marcus Aurelius, circa AD 160. Galen, it is said, observed a man who sucked or bit off verrucas from the soles of the affected. The Ancient Greeks and Romans recognised that genital warts could be sexually transmitted, but never identified the cause, and medicine in that era was more interested in treatment than prevention. It is highly unlikely that anyone in the Ancient World suspected that warts were related to cervical cancer. At that time, cervical cancer would have been relatively rarely seen because women’s life expectancy was so low; in Ancient Egypt, and even thousands of years later in the Roman Empire, average life expectancy would only be into the mid-30s. With the long latency of HPV infection, women would die of other infection, trauma or obstetric causes long before they would develop cervical cancer. Cervical cancer, however, was surely recognised even in the first century AD. A remarkable set of instruments was excavated from the ‘House of the Surgeon’ in the ruins of Pompeii, destroyed by the eruption of Mount Vesuvius in AD 79. They comprise various vaginal specula, beautifully designed to open and hold back the vaginal walls for inspection of the cervix.
Ancient history: the recognition of sexually transmitted lesions
Human papillomavirus is highly stable with little tendency to mutate, and indeed the virus has been around as long as mankind itself. One of the earliest manifestations of HPV infection was identified from the embalmed body of Nakht, an Ancient Egyptian worker on the estate of the Pharoah Setnakht in the 12th century BC, over 3000 years ago. At autopsy in 1974, Nakht was shown to have a wart on the sole of his foot. Warts were well described in classical Greek and Roman medical literature; the term ‘condyloma’ comes from the Greek ‘kondylos’ meaning ‘knuckle’ or ‘knob’; and the term used for the plantar papilloma, ‘verruca’, is Latin. Hippocrates of Kos, the so-called Father of Medicine who lived 400 years BC, referred to genital warts and their treatment with plant extracts:
‘The herb Parthenium parviflorum is used for removing warts from the glans penis….’
Parthenolide, the active component of the parthenium genus of plants, has been shown to be an immune-modulator. This might well account for its apparent efficacy in the treatment of virally mediated skin lesions. References to genital warts and common warts were made by the Roman encyclopaedist Aulus Cornelius Celsus circa AD 25, in his work ‘De Medicina’. Further references exist in the writings of Galen, Greek physician to Roman Emperor Marcus Aurelius, circa AD 160. Galen, it is said, observed a man who sucked or bit off verrucas from the soles of the affected. The Ancient Greeks and Romans recognised that genital warts could be sexually transmitted, but never identified the cause, and medicine in that era was more interested in treatment than prevention. It is highly unlikely that anyone in the Ancient World suspected that warts were related to cervical cancer. At that time, cervical cancer would have been relatively rarely seen because women’s life expectancy was so low; in Ancient Egypt, and even thousands of years later in the Roman Empire, average life expectancy would only be into the mid-30s. With the long latency of HPV infection, women would die of other infection, trauma or obstetric causes long before they would develop cervical cancer. Cervical cancer, however, was surely recognised even in the first century AD. A remarkable set of instruments was excavated from the ‘House of the Surgeon’ in the ruins of Pompeii, destroyed by the eruption of Mount Vesuvius in AD 79. They comprise various vaginal specula, beautifully designed to open and hold back the vaginal walls for inspection of the cervix.
Post-renaissance: an initial understanding of human papillomavirus epidemiology
The next important step in the history of HPV takes us through the Dark Ages and the Renaissance to 1700 and the work of the Italian Bernardino Ramazzini, who was Professor of Medicine at the Universities of Modena and Padua. His work De Morbis Artificum Diatriba , or Diseases of Workers , justifies the claim that he is the ‘Father of Occupational Medicine.’ In many ways, he also expanded the field of epidemiology whose foundations had been laid by Guillaume de Baillou in the 16th century. Ramazzini outlined the health hazards encountered by workers in 52 occupations, and noted in nuns the virtual absence of cervical cancer but high incidence of breast cancer. He was unable to explain this observation; but it was a major step in identifying sexual contact or celibacy as risk factors in disease. Another Italian physician, Domenico Rigoni-Stern, analysed 80 years of death certificates in Verona and again concluded that there was an excess risk of breast cancer in nuns and single women. It is commonly (if incorrectly) attributed to him that, at the IV Congress of Italian Scientists in 1842, he also made the observation that cervical cancer was relatively common in prostitutes, but unheard of in nuns. Rigoni-Stern did identify that uterine cancers were found more in married rather than unmarried women, and were almost absent in certain orders of nuns. At this time, there was no clear distinction between (squamous) carcinoma of the cervix and (adeno)carcinoma of the uterine corpus; this recognition would not come until the 20th century. The individual to whom the ‘nun factor’ should probably be attributed is Fabien Gagnon, a French Canadian, who served as gynaecologist to several nunneries in Quebec. He noted 12 cases of cancer of the uterine corpus among 13,000 nuns; but never any case of cancer of the uterine cervix, although the incidence in the general population was five to eight times greater. Certainly, by the late 19th century, it was recognised that cervical cancer risk factors were the same as for sexually transmitted infections: early age of onset of sexual activity, multiple partners, association with other sexually transmitted diseases. There was still, however, no explanation for these observations.
Nineteenth and early twentieth centuries: the infectious nature of certain neoplastic lesions
In 1891, Joseph Payne, a physician at St Thomas’s in London, formally reported the infectious nature of warts, describing how warts had developed on his hand after he had curetted a boy’s warts by scraping away the salicylic acid-softened lesions with a scalpel handle and his own thumb nail. Payne concluded that ‘fresh warts are produced by local inoculation of some pathogenic material derived from an existing one.’ The nature of the ‘pathogenic material’ remained unconfirmed for another 15 years.
Confirmation that it was viral in origin came from the experiments of another Italian doctor, Giuseppe Ciuffo, a pathologist at the University of Cagliari. Ciuffo presented at the Fourth Italian Congress of Pathology in Paris, in October 1906:
‘With a series of experiments….I sought to clarify the problem of the etiology, pathogenicity and contagiousness of cutaneous or mucosal papillomatous forms, such as the common wart and condyloma acuminatum, which are assuredly infectious. My principal aim is to be able to find which is the specific microscopic germ or invisible virus that is responsible for these lesions and what are its characteristics, its mode of transmission…and finally to determine whether a comparative clinical, anatomical and bacteriological examination could result in the identification of the responsible agent.’
Ciuffo prepared an extract of warts by morcellating warty tissue, then passing the product through a porcelain filter to remove bacteria and fungi. In the best tradition of scientists of the time, he then proceeded to inoculate himself, and developed warts at the injection site. Cellular matter having been removed by the porcelain filter, Ciuffo concluded that the pathogen causing warts was a virus. It was to be another 43 years before a team at Yale using electron microscopy observed virus-like particles (VLP) in papillomas, and identified this human papillomavirus as the cause of warts.
Meanwhile, there was still no known link between warts, HPV and cervical cancer, and still no idea of the cancer’s cause. The next breakthrough came in 1910–1911, thanks to Peyton Rous, a young Texan virologist working at the Rockefeller Institute in New York. Rous carried out experiments that seemed far-fetched to many. He prepared cell-free filtrates from a malignant sarcoma that had appeared, spontaneously in a hen, and inoculated it into healthy chickens. Surprisingly, the recipients developed tumours of the same type as seen in the original Plymouth hen. This was the first demonstration of a virally induced solid tumour. The disease-causing agent present in the filtrate, now known as the Rous sarcoma virus No. 1, could be propagated by serial passage through chickens or fertilised eggs. Rous’s findings concerning tumour progression were rapidly confirmed in many experimental systems; but his belief about an underlying tumour-inducing virus was received with scepticism. His work was initially met with indifference, but finally recognised as a landmark in modern experimental cancer research. In 1966, aged 87 years, Rous was awarded the Nobel Prize for Medicine. The interval between publication of the work and recognition by the Committee remains the longest for any Nobel award.
Rous continued his interest in transmission of tumours by cell-free extracts. He collaborated with a young colleague at the Rockefeller Institute, Richard Shope, who would one day be honoured for his identification of the influenza virus. It is said that while out hunting, Shope noted a wild cottontail rabbit with ‘horns’ – actually papillomas. In 1933, Shope prepared cell-free extracts from these papillomas and demonstrated that the extracts could be used to transmit the benign lesions to rabbits only; and that more vigorous growths could be produced in domestic breeds than in the natural wild cottontail rabbit host. Rous followed up Shope’s benign papillomas by inducing them to undergo malignant transformation to squamous cell cancers by exposure to tiny quantities of chemical agents, which would not by themselves cause cancer. These experiments were a significant pointer to the underlying viral aetiology of cervical squamous cell cancer in humans.
Post-war twentieth century
Advances in tumour virology and immunology
The Second World War inevitably disrupted many fields of medical research, but the 1950s and 1960s saw progress in oncology that would lead to the development of HPV vaccines.
The concept of tumour immunology had developed rapidly in the 1950s, with the idea that the immune system can recognise ‘self’ and ‘non-self’ antigens. Sir Frank Macfarlane Burnet, an Australian immunologist, and Peter Medawar of University College London, were awarded the Nobel Prize for Medicine in 1960 for their discovery of acquired immunological tolerance. In 1957, Burnet had predicted an immune response against tumours:
‘It is by no means inconceivable that small accumulations of tumour cells may develop and because of their possession of new antigenic potentialities provoke an effective immunological reaction, with regression of the tumour and no clinical hint of its existence.’
This immune surveillance theory paved the way for the concept of generating anti-tumour activity by vaccination, if specific tumour antigenic targets could be identified.
Meanwhile, the role of viruses in human tumours was being pursued as their place in animal cancers had been established by the work of Rous, Shope and others. The first cancer-causing human virus was confirmed by Sir Anthony Epstein and his doctoral student Yvonne Barr, in Bristol in 1964. Epstein-Barr virus was identified in a Burkitt’s lymphoma cell line. The principle of vaccination against viral infection was well-established by the 1960s; surely, it would only be a matter of time before vaccination against a cancer-causing virus could be undertaken?
Viral aetiology of cervical neoplasia
Epidemiological studies had suggested a sexually transmitted aetiology for cervical cancer and, by the early 1970s, there were various infective candidates. The most likely was thought to be herpes simplex virus type 2 (HSV-2); the huge conference facilities at Key Biscayne, Florida, could be filled by the 1972 International Conference on Herpesvirus and Cervical Cancer. Harald zur Hausen, however, a young German Professor of Virology, was not convinced. He believed HSV-2 was unrelated, and dared to present his findings at the International Conference. Zur Hausen could not isolate HSV-2 from his cervical cancer specimens, and felt that HSV identified by other laboratories was either incidental or a contaminant. He, therefore, did not believe that the virus was causative. These findings were in direct contradiction of the perceived belief of the time; zur Hausen later described the ‘stony silence’ that greeted his presentation as ‘the low point’ of his career.
Zur Hausen persisted in his HPV work, however, believing both in the plurality of HPV types and its causal aetiology in cervical neoplasia. In 1977, his group published evidence of different HPV types. Quite independently, similar work was being done in Paris by Gerard Orth et al. It was agreed that HPVs would be numbered in the order of their discovery. Orth’s team sequenced and named HPV 1, whereas zur Hausen’s team found both HPV 6 and 11 in genital warts. Subsequently, it was shown that over 90% of genital warts are caused by these two viruses, which have no oncogenic potential in the cervix.
Thus, by the early 1980s, the viral nature of warts was established, together with the multiplicity of HPV types. But there was still no evidence to support HPV as the cause of cervical cancer — until the breakthrough zur Hausen had been awaiting. In 1982, his research fellow Mathias Durst isolated a new HPV type — the 16th — from a cervical cancer biopsy. HPV 16 was immediately found in half the laboratory’s specimens. The group then went on to find HPV 18; together, these high-risk viruses account for around 70% of the world’s cervical cancers. It was now apparent, if not proven, that cervical cancer probably had an infectious cause with HPV types as the causative agent. As early as 1984, zur Hausen approached pharmaceutical companies to work on an HPV vaccine.
‘The viruses had quite a simple structure and it should have been possible to produce something. But the companies I approached did not believe that this would be profitable and said there were more urgent problems to be solved.’
It would soon be shown that HPV is widespread, and found in many types of benign and malignant lesion ( Table 1 ).
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