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Human papillomaviruses (HPV) form a family of more than 100 members (HPV types). Many of them are harmless passengers in the epithelium, others cause benign warts located at the skin or on mucosal surfaces (oral cavity, genital tract). Infection by certain (so called high-risk) HPV types may, after many years of persistence in the epithelium and in a small proportion of the infected people, lead to malignant transformation of cells resulting in growth of malignant tumors.
The prototype of high risk human papillomaviruses is HPV 16. The transforming potential of these viruses is best studied in the case of cervical cancer. This disease is the second most frequent malignant female cancer worldwide, claiming more than 200,000 lives every year. Most of the cases occur in countries of the Third World where screening programs for early detection of cervical cancer do not exist. Since a few months two vaccines became commercially available which prevent infection by the high-risk types 16 and 18 that are causing about 70% of cervical cancer cases. They consist of so called virus-like particles (VLP) that form by recombinant expression of the major structural protein L1. Together with several colleagues we have contributed to the development of VLPs.
HPV infections related to cervical cancer are mostly transmitted through sexual intercourse; hence effective immunization must take place in teenagers at an age before they become sexually active. Due to the long time lapse between infection and the onset of the tumor, it will take many years before the reduction of cancer incidence becomes visible. We are focusing on the development of vaccines which are expected to be useful in already infected individuals and thus would yield an earlier benefit (“therapeutic vaccine”). The most promising target antigens for an attack of immune cells are the viral oncoproteins proteins E6 and E7 which are expressed in every tumor cell. In fact it was shown that the induction of cytotoxic T cells directed against these viral proteins show an inhibitory effect on the growth of HPV positive tumor cells in laboratory animals.
It is our major goal to develop optimized methods for the induction of immune responses against the early proteins of HPV 16, the prototype of cancer-related papillomaviruses. Our studies are primarily on a preclinical level (experiments in cells in culture and in laboratory animals) but it is planned that the most promising approaches shall enter first clinical trials as soon as possible.
1) We have generated chimeric virus-like particles (CVLP) of HPV 16 which assemble from 360 copies of a fusion protein between the major structural protein L1 and the 55 N-terminal amino acids of E7. CVLPs proved to be highly immunogenic in mice (induction of L1-specific neutralizing antibodies and L1- and E7-specific cytotoxic T cells) as well as in a first trials in humans. We are now extending the concept of chimeric particles to substructures of CVLPs ("chimeric capsomeres") that consist only of five molecules of the L1-E7 fusion protein. In comparison to CVLPs, chimeric capsomeres proved (i) to be of similar immunogenicity in animal experiments, (ii) to tolerate the incoprporation of longer sequences onto the L1 molecule (e.g. full-length E7 or other putative tumor antigens), (iii) to be easier to manufacture (expression in E. coli) and (iv) to be of superior stability under non-refrigerated conditions (of particular relevance for use in developing countries). We plan a phase I clinical trial in Sao Paulo together with colleagues from the US and Brazil.
2) Vaccination with eukaryotic expression plasmids (“DNA vaccination”) was shown in recent years to be highly efficient both in animal experiments and first clinical trials in humans. The use of an oncogene such as HPV 16 E7 as DNA vaccine would be unethical because of the risk to induce tumors by the vaccine itself. We therefore have rearranged (“shuffled”) sequences of the E7 gene in a way that its transforming activity was destroyed yet the ability to induce a T cell response against the authentic E7 protein is retained. We expect that a second generation of shuffled E6 and E7 genes (improved immunogenicity and with additional safety features) can enter into clinical trials in the near future.
3) Whereas skin warts occur mostly in children and regress spontaneously they appear frequently in immunosuppressed individuals such as organ transplant recipients where they occur as multiple and therapy-resistant lesions. We are developing a VLP-based vaccine for prevention of these extremely bothering skin warts in patients with organ failure that shall be immunized while they are prepared for the transplantation and have not yet received immunosuppressive therapy.