Single-particle multi-parametric microscopy reveals structural, size, and payload heterogeneity in mRNA-loaded lipid nanoparticles

Deciphering the heterogeneity of mRNA-containing lipid nanoparticles (LNPs) is essential for understanding the relationship between their microscopic properties and therapeutic function. Here, by combining alternating laser excitation (ALEX) with Convex Lens-induced Confinement (CLiC) microscopy, we simultaneously measure size, multi-color fluorescence, mRNA payload, and Forster resonance energy transfer (FRET) of individual suspended LNPs containing labeled lipid and mRNA molecules. By varying formulation parameters, including ionizable lipids, formulation buffers, and molecular ratios, we investigated and correlated key microscopic properties for relevant vaccine formulations. While the per-particle lipid fluorescence was lower for empty versus mRNA-loaded particles for all formulations, the relative size of empty versus mRNA-loaded particles depended upon the formulation and intraparticle structure. When comparing CLiC-ALEX to cryogenic transmission electron microscopy measurements (Cryo-TEM), for the LNP formulations that display blebs, the fraction of bleb LNPs is in close agreement with the fraction of mRNA-containing LNPs. CLiC-ALEX also enabled quantification of the per-particle mRNA fluorescence and FRET signals, and thus the heterogeneity in the mRNA copy number and mRNA-LNP structural arrangements, where the results were compared with biophysical estimates based on the LNP formulations. These rigorous biophysical insights are critical to inform our understanding of structure-activity relationships and inform rational design of nanomedicines.