T cell-mediated tumor suppression by a self-amplifying mRNA-based melanoma vaccine in a syngeneic mouse model

Melanoma is a highly aggressive form of cancer that occurs due to the malignant transformation of melanocytes. Despite the availability of a variety of therapeutic options, melanoma treatment remains a significant challenge and new approaches are constantly being explored. In this study, we evaluated self-amplifying mRNA (saRNA)-based melanoma vaccine candidates targeting Melan-A, a melanoma-associated antigen (MAA), and assessed the immunogenicity and efficacy. The candidates were formulated with lipid inorganic nanoparticles (LIONs) and administered intramuscularly into mice. The immunogenicity was evaluated by enzyme-linked immunosorbent spot (ELISpot) assay and emzyme-linked immunoassay (ELISA). The results showed that one of the candidates, LS2-Melan-A (TM+), namely saMelan-A/LION, triggered the humoral immune response by producing immunoglobulin G (IgG) antibodies specific to Melan-A, rather than T cell-mediated immunity. Further studies were conducted to assess the therapeutic and preventive efficacy against melanoma using the B16F10 mouse melanoma model. In a therapeutic efficacy study, significant suppression of tumor growth was observed in the saMelan-A/LION administered group. Additionally, Fluorescence-activated cell sorting (FACS) and immunohistochemistry (IHC) indicated the increases in the CD8⁺/CD4⁺ T cell ratio and tumor-infiltrating cytotoxic T cells. In the preventive efficacy study, the saMelan-A/LION substantially delayed tumor progression by inducing an antigen-specific T cell immune response. The results of the ELISpot assay demonstrated that the T cell response specific to Melan-A was significantly increased in the saMelan-A/LION group, different from the result of the immunogenicity study. This indicates that additional exposure to antigens through tumor challenge after vaccination can enhance antigen-specific immune response. Furthermore, the FACS analysis showed an enhanced ratio of effector memory T cells to central memory T cells within the CD8⁺ and CD4⁺ cells. These findings suggest that saMelan-A/LION has the potential to be utilized as a therapeutic and preventive vaccine for melanoma.