In the 1980s, Professor Harald zur Hausen, the long-standing chairman of the German Cancer Research Center (DKFZ), came up with convincing evidence that viruses can cause cancer in humans. He showed that certain human papillomavirus (HPV) types, in particular the sexually transmitted types HPV 16 and HPV 17, caused cervical cancer. Zur Hausen’s life work, for which he was awarded the Nobel Prize in Physiology or Medicine in 2008, was crowned with the development of vaccines that protect women against cervical cancer. This was the first successful anti-cancer vaccine in the world (see article entitled “Papillomaviruses as cancer-causing agents and how they can be fought off”; see link on right-hand side).

Since then, the awareness that virus-induced carcinogenesis is a relatively common principle rather than a one-off case has gradually been taken on board by cancer researchers. Harald zur Hausen continues indefatigably collecting further evidence for virus-induced carcinogenesis. The difficulty is that many factors need to coincide and it usually takes many years for the tumour to be diagnosed in humans.

The contributions made by individual factors are difficult to predict, let alone control. Model systems that come as close as possible to the situation in humans therefore need to be developed.

Professor Frank Rösl, head of the Department of Viral Transformation Mechanisms at the DKFZ, and his colleagues used a unique animal model for investigating the role of papillomaviruses in the development of skin tumours: the multimammate mouse Mastomys coucha. Like the majority of people, the mouse colony in the DKFZ is naturally infected with papillomaviruses. The mice are infected with papillomavirus type MnPV (Mastomys natalensis papillomavirus), which is very similar to human HPV types. Mastomys mice are infected with MnPV very soon after birth.

In the majority of mice, the viruses do not cause malignant tumours, but symptoms such as warts (papillomas) and benign tumours (keratoacanthomas) in the skin. However, in both humans and mice, these tumours can develop into non-melanoma skin cancer (NMSC). The transformation of benign tumours into malignant NMSC can be efficiently induced in an experimental situation by a single application of a carcinogen followed by a repeated challenge with a tumour promoter (which enhances tumour growth, but is not carcinogenic).

NMSC is not only one of the most frequent cancers in the world, but there is also an alarming increase in skin cancer incidence worldwide. In Germany, the number of people diagnosed with NMSC has doubled in recent years. Fortunately, this type of cancer can be identified relatively easily due to its exposed location on the skin surface. It does not spread easily and can also be cured when diagnosed at an early stage. However, as the cancer cells grow through the whole tissue and large tissue areas need to be surgically removed, the consequences might still be devastating due to disfiguring scars, explained Prof. Petra Boukamp, head of the Department of the Genetics of Skin Carcinogenesis at the DKFZ. UV radiation is regarded as the main risk factor for the development of non-melanoma skin cancer; extensive sunbathing and excessive use of solaria have a significant influence on the increase of NMSC in Germany.

People with suppressed immune systems have a high incidence of non-melanoma skin cancer and a large number of noticeable warts. A Swedish study showed that this was the case in 56 percent of all organ transplant recipients who had to take immunosuppressive drugs for a prolonged period. The incidence of NMSC in these patients is up to 250-fold higher than in the general population.

The link between infection with papillomaviruses and simultaneous exposure to UV radiation and the risk of developing a tumour seemed obvious to Professor Rösl and his team. HPV infection and simultaneous exposure to UV radiation increases tumour risk, but the immune system is normally able to fight the infection. “Since sunlight contains UV radiation and we cannot completely avoid contact with it, our approach was to develop a vaccination against types of HPV found in the skin,” said Rösl. The researchers used the mouse Mastomys coucha as an experimental system for the development of the vaccine.

The researchers used so-called virus-like particles (VLP) to immunize the mice against the virus. These particles consist of papillomavirus shells but lack infectious DNA. Following vaccination, the animals produced antibodies against the virus shells, which the researchers were able to detect in the animals’ blood. “If the antibodies come into contact with real, infectious viruses, the immune system can fight these invaders,” said Dr. Sabrina E. Vinzón, first author of the preclinical study which was carried out in cooperation with the research group of Prof. Dr. Nindl at the Charité Hospital in Berlin. 

None of the vaccinated mice developed a tumour, neither benign nor malignant. Using numerous experimental approaches, the researchers were able to show that this protection was due to the VLP antibodies of the vaccine which neutralized the viruses that lead to the development of tumours. Rösl and his team have established an MnPV-free Mastomys colony at the DKFZ. When the mice, which were kept under sterile conditions, were infected with MnPV, they developed benign and malignant tumours, just like naturally infected animals. However, none of the virus-free mice that were vaccinated prior to infection with MnPV developed a tumour.

The specific effect of the vaccine was shown to impressive effect in experiments with mice that were given drugs to suppress their immune system. Chronic immunosuppression was achieved with cyclosporine A. These animals developed benign and malignant skin tumours, as is common for immunospressed organ recipients. However, when the immunosuppressed mice were given the vaccine against MnPV virus shells, the mice did not develop virus-induced skin tumours. They were protected against MnPV infection. Mice with intact immune systems that had been infected with the virus prior to the vaccination did not develop tumours either.

These findings are both exciting and encouraging. They do not provide convincing evidence for the role of papillomaviruses in the development of NMSC, but they do provide a solid basis for the clinical development of vaccination strategies that provide an effective protection against HPV-induced skin lesions and skin cancer, in particular in immunosuppressed organ recipients. It is not yet foreseeable whether and when a vaccination will be available for humans.

Original publication:
Vinzón SE, Braspenning-Wesch B, Müller M, Geissler EK, Nindl I, Gröne H-J, Schäfer K, Rösl F: Protective Vaccination against Papillomavirus-Induced Skin Tumors under Immunocompetent and Immunosuppressive Conditions: A Preclinical Study Using a Natural Outbred Animal Model
PLoS Pathog 10(2): e1003924.doi:10.1371/journal.ppat.10003924