Sequential plasmid vaccinations targeting normal melanocytes protects against challenge with melanoma in a syngenic murine model

Abstract

2570 Background: Malignant melanoma is one of a handful of tumors responsive to immunotherapy. Tumor regression correlates with an expanded epitope response and induction of vitiligo. In fact, expansion of the immune response to include multiple melanoma differentiation antigen (MDA) targets correlates with clinical response in newer vaccine approaches. We hypothesized that a vaccine designed to induce epitope spreading against melanocytic antigens may be useful in melanoma. By directly targeting normal melanocytes in situ, we hoped to induce both autoimmunity and a broad anti-melanocytic response. To accomplish this, we utilized sequential plasmid vaccinations in an immune competent murine model for melanoma. Methods: C57/b6 mice were vaccinated with plasmid DNA encoding either the gene for chicken ovalbumin (cOVA) or empty vector intramuscularly (IM) weekly for two weeks. Mice (n=9) were then vaccinated intradermally (ID) with either cOVA or a control gene under the control of the tyrosinase promoter weekly for three weeks. Mice were challenged with subcutaneous injections of 105 B16 melanoma tumor cells or B16 containing the transgene cOVA (B16cOVA). Tumor size and survival were followed. Results: Vaccination with the cOVA gene IM resulted in 90% to 100% protection against B16cOVA tumor cells compared to 0% to 10% of control plasmid injected mice. Challenge with B16 resulted in equally poor survival between the two groups with 100% of mice being sacrificed by day 26. Surprisingly, if the mice were vaccinated with the cOVA gene ID under the control of the tyrosinase promoter after the initial IM vaccination with cOVA and subsequent tumor challenge, B16 growth was delayed compared to controls. Delay was not observed without the initial IM cOVA vaccination. Conclusions: Ectopic expression of antigens for which a strong pre-existing cellular immune response exists may allow targeting of an immune response to normally tolerant antigens. This system provides a relatively simple approach in understanding the requirements of both autoimmunity and cancer immunotherapy. No significant financial relationships to disclose.