Certain cutaneous human papillomaviruses (HPVs), which are ubiquitous and acquired early during childhood, can cause a variety of skin tumors and are likely involved in the development of non-melanoma skin cancer, especially in immunosuppressed patients. Hence, the burden of these clinical manifestations demands for a prophylactic approach. To evaluate whether protective efficacy of a vaccine is potentially translatable to patients, we used the rodent Mastomys coucha that is naturally infected with Mastomys natalensis papillomavirus (MnPV). This skin type papillomavirus induces not only benign skin tumours, such as papillomas and keratoacanthomas, but also squamous cell carcinomas, thereby allowing a straightforward read-out for successful vaccination in a small immunocompetent laboratory animal. Here, we examined the efficacy of a virus-like particle (VLP)-based vaccine on either previously or newly established infections. VLPs raise a strong and long-lasting neutralizing antibody response that confers protection even under systemic long-term cyclosporine A treatment. Remarkably, the vaccine completely prevents the appearance of benign as well as malignant skin tumors. Protection involves the maintenance of a low viral load in the skin by an antibody-dependent prevention of virus spread. Our results provide first evidence that VLPs elicit an effective immune response in the skin under immunocompetent and immunosuppressed conditions in an outbred animal model, irrespective of the infection status at the time of vaccination. These findings provide the basis for the clinical development of potent vaccination strategies against cutaneous HPV infections and HPV-induced tumors, especially in patients awaiting organ transplantation. Organ transplant recipients (OTR) frequently suffer from fulminant warts that are induced by cutaneous human papillomaviruses (HPV). Moreover, some skin HPV types may also be involved in the development of non-melanoma skin cancer. Mimicking the situation of immunosuppressed OTR who acquire cutaneous HPV infections already in childhood, we explored the efficacy of a vaccine in infected animals that additionally underwent immunosuppression. We demonstrate for the first time the success of a vaccine against a skin papillomavirus in a natural outbred animal system, which completely prevents both benign and malignant skin tumor formation even under immunosuppressed conditions. Hence, our study provides the basis for clinical development of a vaccine against cutaneous HPV infections, which may be particularly useful in transplant recipients.

There is still controversy in the scientific field about whether certain types of cutaneous human papillomaviruses (HPVs) are causally involved in the development of non-melanoma skin cancer (NMSC). Deciphering the etiological role of cutaneous HPVs requires – besides tissue culture systems – appropriate preclinical models to match the obtained results with clinical data from affected patients. Clear scientific evidence about the etiology and underlying mechanisms involved in NMSC development is fundamental to provide reasonable arguments for public health institutions to classify at least certain cutaneous HPVs as group 1 carcinogens. This in turn would have implications on fundraising institutions and health care decision makers to force – similarly as for anogenital cancer – the implementation of a broad vaccination program against “high-risk” cutaneous HPVs to prevent NMSC as the most frequent cancer worldwide. Precise knowledge of the multi-step progression from normal cells to cancer is a prerequisite to understand the functional and clinical impact of cofactors that affect the individual outcome and the personalized treatment of a disease. This overview summarizes not only recent arguments that favor the acceptance of a viral etiology in NMSC development but also reflects aspects of causality in medicine, the use of empirically meaningful model systems and strategies for prevention.

Cutaneous human papillomaviruses (HPVs) are considered as cofactors for non-melanoma skin cancer (NMSC) development, especially in association with UVB. Extensively studied transgenic mouse models failed to mimic all aspects of virus-host interactions starting from primary infection to the appearance of a tumor. Using the natural model Mastomys coucha, which reflects the human situation in many aspects, we provide the first evidence that only UVB and Mastomys natalensis papillomavirus (MnPV) infection strongly promote NMSC formation. Using UVB exposures that correspond to UV indices of different geographical regions, irradiated animals developed either well-differentiated keratinizing squamous cell carcinomas (SCCs), still supporting productive infections with high viral loads and transcriptional activity, or poorly differentiated non-keratinizing SCCs almost lacking MnPV DNA and in turn, early and late viral transcription. Intriguingly, animals with the latter phenotype, however, still showed strong seropositivity, clearly verifying a preceding MnPV infection. Of note, the mere presence of MnPV could induce γH2AX foci, indicating that viral infection without prior UVB exposure can already perturb genome stability of the host cell. Moreover, as shown both under in vitro and in vivo conditions, MnPV E6/E7 expression also attenuates the excision repair of cyclobutane pyrimidine dimers upon UVB irradiation, suggesting a viral impact on the DNA damage response. While mutations of Ras family members (e.g. Hras, Kras, and Nras) were absent, the majority of SCCs harbored—like in humans—Trp53 mutations especially at two hot-spots in the DNA-binding domain, resulting in a loss of function that favored tumor dedifferentiation, counter-selective for viral maintenance. Such a constellation provides a reasonable explanation for making continuous viral presence dispensable during skin carcinogenesis as observed in patients with NMSC. Epidemiological data already strongly suggest an involvement of cutaneous papillomaviruses in the development of NMSC. However, since the viral DNA is frequently lost during progression from precursor lesions to NMSC—which is in contrast to high-risk mucosal HPVs in the context of anogenital cancer—their etiological role is still controversially discussed. Although extensively studied in experimentally infected or transgenic models, studies in this context are still hampered by the lack of suitable preclinical models mimicking all stages as found in humans, starting from primary infection to the appearance of a tumor. Here, we provide the first evidence that only the cooperation between UVB and cutaneous papillomavirus infection strongly favors the development of skin tumors in the natural model Mastomys coucha. This study strongly supports the importance of cutaneous papillomaviruses in NMSC development and explains the loss of viral DNA during malignization of UV-induced tumors, a feature commonly observed in human SCCs.

Cutaneous papillomaviruses are associated with specific skin diseases, such as extensive wart formation and the development of non-melanoma skin cancer (NMSC), especially in immunosuppressed patients. Hence, clinical approaches are required that prevent such lesions. Licensed human papillomavirus (HPV) vaccines confer type-restricted protection against HPV types 6, 11, 16 and 18, responsible of 90% of genital warts and 70% of cervical cancers, respectively. However, they do not protect against less prevalent high-risk types or cutaneous HPVs. Over the past few years, several studies explored the potential of developing vaccines targeting cutaneous papillomaviruses. These vaccines showed to be immunogenic and prevent skin tumor formation in certain animal models. Furthermore, under conditions mimicking the ones found in the intended target population (i.e., immunosuppression and in the presence of an already established infection before vaccination), recent preclinical data shows that immunization can still be effective. Strategies are currently focused on finding vaccine formulations that can confer protection against a broad range of papillomavirus-associated diseases. The state-of-the-art of these approaches and the future directions in the field will be presented.

Mastomys coucha, an African rodent, is a useful animal model of papillomavirus infection, as it develops both premalignant and malignant skin tumors as a consequence of a persistent infection with Mastomys natalensis papillomavirus (MnPV). In this study, we mapped the MnPV transcriptome in productive lesions by both classical molecular techniques and high-throughput RNA sequencing. Combination of these methods revealed a complex and comprehensive transcription map, with novel splicing events not described in other papillomaviruses. Furthermore, these splicing occurrences could potentially lead to the expression of novel E2, E1^E4, E7 and L2 isoforms. Expression level estimation of each transcript showed that late-region mRNAs amply outnumber early transcripts, with species coding for L1 and E1^E4 being the most abundant. In summary, the full transcription map assembled in this study will allow us to further understand MnPV gene expression and the mechanisms which lead to natural tumor development.