Compositions and methods for modified dendrimer nanoparticle (“MDNP”) delivery of therapeutic, prophylactic and/or diagnostic agent such as large repRNA molecules to the cells of a subject have been developed. MDNPs efficiently drive proliferation of antigen-specific T cells against intracellular antigen, and potentiate antigen-specific antibody responses. MDNPs can be multiplexed to deliver two or more different repRNAs to modify expression kinetics of encoded antigens and to simultaneous deliver repRNAs and mRNAs including the same UTR elements that promote expression of encoded antigens.