Spontaneous and Vaccine-Induced Clearance of Mus Musculus Papillomavirus 1 Infection

Mus musculus papillomavirus 1 (MmuPV1/MusPV1) induces persistent papillomas in immunodeficient mice but not in common laboratory strains. To facilitate the study of immune control, we sought an outbred and immunocompetent laboratory mouse strain in which persistent papillomas could be established. We found that challenge of SKH1 mice (Crl:SKH1-Hrhr) with MmuPV1 by scarification on their tail resulted in three clinical outcomes: (i) persistent (>2-month) papillomas (∼20%); (ii) transient papillomas that spontaneously regress, typically within 2 months (∼15%); and (iii) no visible papillomas and viral clearance (∼65%). SKH1 mice with persistent papillomas were treated by using a candidate preventive/therapeutic naked-DNA vaccine that expresses human calreticulin (hCRT) fused in frame to MmuPV1 E6 (mE6) and mE7 early proteins and residues 11 to 200 of the late protein L2 (hCRTmE6/mE7/mL2). Three intramuscular DNA vaccinations were delivered biweekly via in vivo electroporation, and both humoral and CD8 T cell responses were mapped and measured. Previously persistent papillomas disappeared within 2 months after the final vaccination. Coincident virologic clearance was confirmed by in situ hybridization and a failure of disease to recur after CD3 T cell depletion. Vaccination induced strong mE6 and mE7 CD8(+) T cell responses in all mice, although they were significantly weaker in mice that initially presented with persistent warts than in those that spontaneously cleared their infection. A human papillomavirus 16 (HPV16)-targeted version of the DNA vaccine also induced L2 antibodies and protected mice from vaginal challenge with an HPV16 pseudovirus. Thus, MmuPV1 challenge of SKH1 mice is a promising model of spontaneous and immunotherapy-directed clearances of HPV-related disease. IMPORTANCE High-risk-type human papillomaviruses (hrHPVs) cause 5% of all cancer cases worldwide, notably cervical, anogenital, and oropharyngeal cancers. Since preventative HPV vaccines have not been widely used in many countries and do not impact existing infections, there is considerable interest in the development of therapeutic vaccines to address existing disease and infections. The strict tropism of HPV requires the use of animal papillomavirus models for therapeutic vaccine development. However, MmuPV1 failed to grow in common laboratory strains of mice with an intact immune system. We show that MmuPV1 challenge of the outbred immunocompetent SKH1 strain produces both transient and persistent papillomas and that vaccination of the mice with a DNA expressing an MmuPV1 E6E7L2 fusion with calreticulin can rapidly clear persistent papillomas. Furthermore, an HPV16-targeted version of the DNA can protect against vaginal challenge with HPV16, suggesting the promise of this approach to both prevent and treat papillomavirus-related disease.