Editorial Arjan Kol (1646753)
Big step forward for therapeutic vaccines against HPV and associated diseases
Chronic infection with high-risk-type human papillomaviruses (HPV) may cause up to 10% of malignant diseases in women. These diseases include cancers of the vagina, vulva, cervix, oropharynx and the anus [1,2]. The most common high-risk HPV is HPV-16. It is the main factor associated with development of cervical cancer. HPV-16 is also involved in more than 75% of the cases with vulvar intraepithelial neoplasia (VIN), a chronic premalignant disorder of the vulvar skin.
The genome of HPV consists of early (E) and late (L) genes coding for early and late functions. E6 and E7 may act as oncogenes promoting tumor growth and malignant transformations. The expression of these viral oncogenes, integrated in the host genome, is necessary to maintain the cancerous phenotype [4, 5]. This creates an opportunity to prevent and treat diseases associated with HPV by using anti-viral vaccines.
Currently, the prophylactic vaccines Gardasil and Cervarix are being used for the prevention of infection with HPV-16 and HPV-18. Both vaccines show high efficacy. However, those vaccines don't have a therapeutic effect on an existing HPV infection. Therefore, therapeutic vaccines are being developed for the treatment of an existing HPV infection. These therapeutic vaccines target the HPV antigens present inside the infected cell. Therapeutic vaccines for VIN are needed because the rate of recurrence after surgical treatment is high. Furthermore, the rate of spontaneous regression is very low (1.5%) .
One of the therapeutic vaccines which is currently being developed for the treatment of vulvar intraepithelial neoplasia (VIN) in HPV-16 positive women is a synthetic long-peptide (SLP) vaccine containing overlapping peptides derived from the E6 and E7 proteins of HPV-16. The study described by Kenter et al. focuses on grade 3 vulvar intraepithelial neoplasia (the most severe form of VIN) among women chronically infected with HPV-16  . In this observational phase 2 study twenty HPV-16 positive women with VIN grade 3 were vaccinated with a mix of nine synthetic peptides derived from HPV-16 E6 and four peptides derived from HPV-16 E7 in incomplete Freund's adjuvant. The long peptides represent the entire length of the proteins E6 and E7. The patients received three or four vaccinations.
Clinical efficacy was assessed by looking at the symptoms, lesion size, histological features and the presence or absence of HPV-16 DNA. Furthermore The HPV-16 specific T-cell responses were monitored.
After 12 months of follow-up, there was a clinical response in 15 of 19 patients and there was a complete clinical and histological response in 9 of 19 patients. Vaccine induced T-cell responses were found in all 19 patients.
The authors conclude that the vaccination with a SLP vaccine against HPV-16 E6 and E7 is effective over a period of 12 to 24 months for the treatment of VIN3 and is most likely correlated with the induction of HPV-16 specific immunity.
The study performed by Kenter. et. al. is the first study in which the number of complete responses in women vaccinated with overlapping peptides derived from the E6 and E7 proteins was high. Previous studies using vaccines against E6 and E7 oncoproteins showed much lower complete response rates . The authors of the study explained that the high response rate could be due to a high dose of antigen, efficient dendritic-cell targeting and the absence of antigenic competition from viral vectors .
This is an important study that has shown that vaccination with synthetic long-peptide vaccine seems promising in treating patients with VIN 3 and is definitely a step in the right direction. However, it was an ideal scenario. The weakness of this study is that the investigators only included patients who did not have diseases associated with immunodeficiency, immunosuppressive medication or viral disease. In addition, the spectrum of HPV types found in vulvar squamous cell carcinomas indicate that the efficacy of therapeutic vaccination in reduction of vulvar cancer may be restricted . It is not clear if the patients in the study from Kenter et. al. were tested for other HPV-types, e.g. HPV-18. HPV-18 can also cause VIN. If the patients were infected with another HPV-type it could decrease the effectiveness of the vaccine. This could explain why some patients responded better to the vaccine.
Future studies could compare the non-responders, the partial-responders and the complete-responders with each other in order to find out why the patients react differently. They could look for example at the down- or up-regulation of cytokines and/or the presence of other viruses or diseases. One type of cell that plays an important role is the regulatory T-cell. Regulatory T-cells were found with higher frequency in persistently HPV-infected patients [5,10] . These cells suppress the immune system and could potentially hamper the efficacy of the vaccine. Depletion or inactivation of regulatory T-cells could improve the efficacy of the vaccine .
Another important factor is the immunosuppressive cytokine IL-10, which is produced by a large number of immune cells including the IL-10 producing regulatory T-cells and the naturally occurring suppressor CD4+ T-cells . These are just examples of the many cytokines and cells that play a role in the immune response.
The local environment of the tumor is immunosuppressive or at least not immunostimulatory . Therefore, powerful adjutants, boosters or topical immune stimulatory agents, for example imiquimod , may further enhance the effectiveness of the therapeutic vaccine . Van Seeters et al. have shown that topical treatment with imiquimod induced complete regression of lesion in 9 of 26 VIN 3 patients . The combination of imiquimod with a vaccine could increase the number of complete responses in patients with vulvar intraepithelial neoplasia. New insights in this field can lead to better prevention and treatment strategies in the near future.
After several studies on therapeutic vaccines which generally gave low responses rates, the study by Kenter et al. for the first time shows promising results. However, the efficacy of therapeutic vaccines against HPV and associated disease is still too low. In the study performed by Kenter et al. only 47% of the patients showed a complete response .
Much research has been done on prophylactic vaccines against HPV and considerable progress has been made. These vaccines have been proven highly efficacious. The effectiveness of prophylactic vaccines does not mean that therapeutic vaccines are becoming redundant. In developing countries, there is a continuing need for therapeutic vaccines. These countries don't have financial resources and access to widespread prophylactic vaccination. In addition, screening for pre-malignant disease is minimal in developing countries. Treatment with therapeutic vaccines could considerably improve patients health care.
Several years elapse before women who are infected with HPV develop VIN or other HPV associated diseases. During the period between infection and disease there is a lot of room for therapeutic vaccines. Furthermore, there is a high number of currently infected people. Prophylactic vaccines wont have an effect on those people. This means that therapeutic vaccines wont become obsolete in the next decades.
Hopefully, in the future, we will have vaccines that have both therapeutic and prophylactic effects against a wide range of HPV-types. These vaccines will reduce the rate of recurrence and surgical treatment. In addition, these vaccines will not only be effective against VIN, but also against other HPV associated diseases, such as cervical cancer.
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