An Orf Virus-based therapeutic vaccine for the treatment of papillomavirus-induced tumors.

: The Orf virus (ORFV) represents a suitable vector for the generation of efficient, prophylactic antiviral vaccines against different pathogens. The present study investigates for the first time the therapeutic application of ORFV vector-based vaccines against tumors induced by Cottontail Rabbit Papillomavirus (CRPV). ORFV-CRPV recombinants were constructed expressing the early CRPV gene E1, E2, E7, or LE6, respectively. In two independent experiments we used in total 23 rabbits which immunized with a mixture of the four ORFV-CRPV recombinants or empty ORFV vector as control five weeks after the appearance of skin tumors. For the determination of the therapeutic efficacy the subsequent growth of the tumors was recorded. In the first experiment, we could demonstrate that three immunizations with the combined four ORFV-CRPV recombinants of rabbits with high tumor burden resulted in significant growth retardation of the tumors as compared to the control. A second experiment was performed to test the therapeutic effect of 5 doses of the combined vaccine in rabbits with a lower tumor burden compared to non-immunized rabbits. Tumor growth was significantly reduced after immunization and one vaccinated rabbit even displayed complete tumor regression until the end of the observation period at 26 weeks. Results of delayed-type hypersensitivity (DTH) skin tests suggest the induction of a cellular immune response mediated by the ORFV-CRPV vaccine. The data presented show for the first time a therapeutic potential of the ORFV vector platform and encourage further studies for the development of a therapeutic vaccine against virus-induced tumors.IMPORTANCE Viral vectors are widely used for the development of therapeutic vaccines for the treatment of tumors. In our study we have used the Orf Virus (ORFV) strain D1701-V for the generation of recombinant vaccines expressing the Cottontail Rabbit Papillomavirus (CRPV) early proteins E1, E2, LE6, and E7, respectively. The therapeutic efficacy of the ORFV-CRPV vaccines was evaluated in two independent experiments using the outbred CRPV rabbit model. In both experiments the immunization achieves significant suppression of tumor growth. In total, 84.6 % of all outbred animals benefit from the ORFV-CRPV vaccination showing reduction in tumor size and significant tumor growth inhibition including one animal with complete tumor regression without recurrence.