(436g) Enhancing mRNA Vaccine Immunogenicity By Engineering the Lipid Nanoparticle and mRNA-Encoded Antigen

mRNA vaccines have demonstrated great success against SARS-CoV-2. However, further clinical translation of mRNA vaccines will require eliciting optimal immune responses with desirable safety profiles. To further improve the safety and efficacy of mRNA vaccines, we here engineered a multiply self-adjuvanted mRNA vaccine system whereby both the ionizable lipid as well as the mRNA-encoded antigen are designed to potentiate the elicited immune response. We hypothesized that incorporating multiple self-adjuvanting properties directly into the mRNA-LNP formulation would spatially and temporally restrict immunostimulation to sites of antigen presence, improving beneficial, antigen-specific immune responses while limiting detrimental systemic inflammation.

We first screened a combinatorial library of 480 biodegradable ionizable lipids with adjuvanting cyclic amine headgroups, identifying a top ionizable lipid with enhanced innate immunostimulatory properties leading to a potentiated immune response when compared to the FDA-approved ionizable lipid MC3. We then designed an adjuvanted mRNA transcript encoding SARS-CoV-2 model antigens genetically fused to C3d, a terminal degradation product of complement, as a molecular adjuvant. Genetic fusion of C3d to the encoded antigen targets the fusion protein to complement receptor 2 on B-cells and follicular dendritic cells, lowering the barrier to activation of these cells which play central roles in coordinating the innate and adaptive immune responses. Compared to the mRNA encoding viral antigens alone, the inclusion of C3d in the mRNA transcript increased the magnitude of antigen-specific antibody titers by at least ten-fold in mouse sera. Notably, the enhanced immune response elicited by C3d did not elevate pro-inflammatory cytokines systemically, suggesting a favorable safety profile for this adjuvanting strategy. In mice, intramuscular and intranasal vaccination with LNPs combining the adjuvanted ionizable lipid and mRNA transcript resulted in synergistically improved immune responses to a SARS-CoV-2 Delta variant antigen when compared to unadjuvanted vaccines. Our results demonstrate that a multiply adjuvanted system holds great potential in improving the efficacy, safety, and ease of administration of mRNA vaccines.