Abstract 4110: Identification of tumor peptides for therapeutic mRNA vaccination in a mouse multiple myeloma model

Abstract Background: Multiple myeloma (MM) is a plasma cell cancer for which immunotherapy holds the promise of durable control. To date, cancer vaccines mainly focused on tumor-associated antigens with survivin eliciting protective T-cell responses in both preclinical and clinical studies. Given the success of mRNA vaccines in medicine, we developed Galsomes, lipid nanoparticles containing mRNA and α-galactosyl ceramide to activate both CD8+ T cells and invariant natural killer (iNK) T cells, both potential anti-MM effectors. Materials and Methods: Mouse 5T33MM cells were studied by PCR and flow cytometry (FCM) for expression of survivin and CD1d, respectively. Intravenous (IV) prime-boost Galsome vaccination was performed with a 7-day interval in C57BL6/KalwRij mice that received an IV injection of 5x106 5T33MM cells 3 days earlier. As a control, mice were injected with saline. After 2 weeks, MM burden was studied by serum electrophoresis and FCM, incl. expression of CD1d and PD-L1 on MM cells. Activation of iNKT cells and survivin-specific T cells was studied by measuring IFN-γ secretion in ELISA and dextramer staining. Immunopeptidomics was used to identify new vaccine candidates for which Galsome vaccination is ongoing. Result: The 5T33MM cells showed expression of survivin and CD1d, making it a suitable model for Galsome vaccination. Two weeks following prime-boost vaccination, the serum M-protein was reduced, though not significantly. In contrast, MM cells were significantly reduced in bone marrow and spleen. Expression of CD1d and PD-L1 on MM cells was high but unchanged by Galsome vaccination. Activation of iNKT cells and survivin-specific T cells following prime vaccination was shown. However, following boost vaccination these cell populations reduced significantly compared to control. We repeated prime-boost vaccination in healthy mice, observing a similar outcome. This suggests that the negative effect of the boost Galsome vaccination was not related to immune dysfunctionality due to MM progression. So, we studied the impact of survivin mRNA in the vaccine, as survivin is also expressed in activated T cells, making them a potential target. We observed that iNKT cells and ovalbumin-specific T cells were more reduced when ovalbumin mRNA was paired with survivin mRNA in the Galsomes. To identify other vaccine candidates, we studied peptides eluted from MHC-I/peptide complexes on 5T33MM cells. We selected 4 vaccine candidates including the MM idiotype based on immunogenicity, pathway classification and abundance. mRNA and Galsomes were prepared, and the efficacy of these Galsomes in 5T33MM mice is currently under evaluation. Conclusion: These data show that Galsome vaccination has the potential to reduce MM cell levels. To reach higher efficacy, careful selection of the vaccine antigen is required. Moreover, combination with other immunotherapies might be needed to reach durable disease control. Citation Format: Arne Van der Vreken, Fabien Thery, Sofie Meulewaeter, Karin Vanderkerken, Elke De Bruyne, Kim De Veirman, Lorenzo Franceschini, Rein Verbeke, Francis Impens, Ine Lentacker, Karine Breckpot, Eline Menu. Identification of tumor peptides for therapeutic mRNA vaccination in a mouse multiple myeloma model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4110.