Background: Merkel Cell Carcinoma (MCC) is a neuroendocrine skin cancer associated with integration of a truncated form of Merkel Cell Polyomavirus Large T Antigen (LTA) in most U.S. cases. LTA is a robust antigen and its expression is required for proliferation of MCC, rendering it an ideal target for an mRNA therapeutic vaccine approach. Methods: B16-F10 murine melanoma cells that express the truncated LTA oncoprotein were generated for use as an in vivoMCC model. In vitro transcription was used to create an optimized mRNA coding for the LTA and was encapsulated in lipid nanoparticles for intramuscular injection. LTA expressing tumor cells were injected subcutaneously in C57BL/6 mice and treated with LTA mRNA or placebo +/- anti-PD1 antibodies. Tumor growth and survival were tracked, and flow cytometry of ex-vivo tumor samples was used to characterize immune infiltration. Separate mice were treated with a three-dose series of LTA mRNA or placebo 60 days prior to tumor cell injection, and tumor growth and survival were measured. MCC patient PBMCs were collected and used as a model for human in vitro vaccination. Monocyte derived dendritic cells were transfected with LTA mRNA or placebo and used to stimulate the remainder of the PBMC pool every seven days. LTA specific T cell proportion, activation markers, cytokine release, and MCC specific tumor cell killing were measured. Results: Treatment with LTA mRNA suppressed tumor growth compared to placebo (day 21 mean volume 47.28 mm³ vs. 575.12mm³ P<.001) and prolonged median survival (46 vs. 24 days P<.001). Combination treatment with anti-PD1 resulted in 100% tumor regression compared to 50% with anti-PD1 only (median survival 117 days) and 0% with placebo (median survival 28.5 days). Flow cytometry of ex vivotumors from LTA mRNA treated mice revealed increased immune infiltration (mean CD45+ 11.56% vs. 7.81% P=.037) with an increased proportion of CD3+ cells (mean 69.74% vs. 57.36% P=.0013) and CD8+ cells with increased cytotoxic markers (MFI GZMB 26785 vs. 13607 P=.0026). Prophylactic vaccination with LTA mRNA resulted in 80% tumor rejection vs. 0% with placebo (median survival undefined vs. 31.5 days P<.0001). Furthermore, in the prophylactic vaccination group there were no detectable tumors until day 60 following tumor challenge. In vitro vaccination of patient PBMCs resulted in expansion of LTA tetramer specific CD8+ T cells by day 10 (0.3% vs. 0.077%). Exposure of treated cells to antigen loaded DCs resulted in increased IFNg release by day 10 (21.78ng/mL vs. 5.35ng/mL P<.0001) and increased specific MCC tumor cell killing on day 14 (mean tumor death 9.14% vs. 4.18% P=.0001). Conclusions: Therapeutic vaccination with LTA targeting mRNA suppresses tumor growth and prolongs survival in vivo by increasing infiltration of cytotoxic immune populations. In vitrohuman vaccination increases the proportion LTA specific CD8+ T cells, IFNg release, and specific killing of MCC cells.