The proposed research project aims to understand the relationship between the formulation of mRNA lipid nanoparticles (mRNA-LNPs) and their biodistribution towards myeloid-derived suppressor cells, as well as the functional expression of their mRNA payload. The project has four work packages: WP1 and WP2 focus on the formulation and characterization of LNPs using a design of experiment approach. The biodistribution and functional delivery of the multiple distinct mRNA-LNPs will be simultaneously measured in vivo using a DNA-barcode sequencing approach. WP3 will perform single-cell sequencing to identify the biological mechanisms that steer RNA delivery. Finally, in WP4, the gained knowledge will be used for in situ CRISPR-mediated silencing of SIRPα in myeloid cells in a syngeneic orthotopic triple-negative breast cancer model to evaluate the therapeutic efficacy, with and without concurrent anti-PD-1 treatment. Reprogramming the CD47/SIRPα axis in immunosuppressive myeloid cells in the tumor microenvironment can improve checkpoint inhibition therapy in Triple-Negative Breast Cancer.