For the development of therapeutic cancer vaccines, tumor antigens need to be identified that are either specific or aberrantly expressed in tumor cells compared to normal cells. The cyclin-dependent kinase inhibitor p16INK4a is strongly overexpressed in human papilloma virus (HPV)-induced tumors, whereas it is barely detectable in normal tissue. Therefore, it is an established surrogate marker for high risk HPV infections and considered to be an interesting target for therapeutic vaccination in cancers associated with HPV. p16INK4a expression has also been detected in non-HPV-related tumor entities as colorectal and small cell lung cancer, suggesting p16INK4a as a broad tumor associated antigen that is not only specific for HPV-induced cancers.
In a phase I/IIa trial to monitor toxicity and immunogenicity of a p16INK4a peptide vaccine in patients with advanced HPV-associated, p16INK4a-overexpressing cancers, we could show the induction of a humoral and cellular immune response against p16INK4a without any severe vaccine-related side effects.
Presently we are establishing a p16INK4a-positive tumor mouse model in order to explore the effect of a p16INK4a-based vaccine on tumor growth and its potential to be combined with current immunotherapies.
Pools of long, synthetic peptides covering the whole p16INK4a sequences were injected into female C57BL/6 mice. Establishing the p16INK4a-positive tumor mouse model, C57BL/6 mice were challenged with p16INK4a-expressing TC-1 cells before and respectively after the peptide immunization to analyse the tumor response of a therapeutic and respectively prophylactic vaccine approach.
The p16INK4a-based vaccination with long, synthetic peptides induced an antibody response against p16INK4a detected by ELISA as well as p16INK4a-specific IFNɣ-producing T cells measured by ELISpot. We are currently performing the tumor regression and protection experiments with p16INK4a-positive TC-1 tumor cells in C57BL/6 mice.
The established murine system allows to address the question whether a p16INK4a-based vaccine is able to induce the regression of an established p16INK4a-positive tumor and/or to prevent the further outgrowth of a tumor expressing p16INK4a. The generation an effective tumor response against p16INK4a could lead to a new therapeutic approach for HPV-induced cancers as well as for tumors overexpressing p16INK4a independent of an HPV infection.