Fast-spreading diseases are deeply entangled with the history of humankind. A recent example is the SARS-CoV-2 pandemic affecting societies worldwide and the lives of billions of people. Different from earlier pandemics, effective vaccines were found only weeks after the first cases appeared. Even more surprising was that novel mRNA-based vaccines (Elasomeran, Moderna, Comirnaty, BioNTech, and Pfizer) gained attention for their promising therapeutic potential in the prevention of a severe SARS-CoV-2 infection.
The function of these vaccines is based on the principle that mRNA can be transferred to cells and initiate the expression of a foreign protein. The mRNA strands used by Moderna as well as by BioNTech and Pfizer encode the spike protein of SARS-CoV-2, which is essential for the entry of the virus into human cells. The final protein subsequently triggers an immune response leading to the production of antibodies inhibiting a main component of the virus’ life cycle.
How is the mRNA efficiently transferred into the cells? In the 1980s, it was shown that cationic lipids can be used for the transfection of cells with RNA. Over the past 40 years, the composition and structure of the lipids were optimized to minimize their toxicity and increase their efficiency for the delivery of nucleotides. The encapsulation of mRNA in lipid droplets, called lipid nanoparticles (LNPs), was apparently very effective in protecting nucleotides against premature degradation and increasing the transfection rate of the vaccine through an enhanced interaction with the membrane of the target cells. The LNPs deployed in modern vaccines consist of four components, including a titratable aminolipid, a polyethylene glycol (PEG)ylated lipid, phospholipids, and cholesterol. The replacement of cationic lipids with titratable aminolipids was a crucial step in reducing the toxicity of the LNPs. They can be protonated by exposing them to a low pH, leading to an efficient encapsulation of the negatively charged mRNA strands into the LNPs. At a physiological pH (e.g., during administration), the lipids remain neutral, preventing adverse reactions of the body caused by highly positive charged particles. The PEGylated lipids stabilize the LNPs during storage and prevent a rapid clearance of the particles from the body before they reach the target cells. The phospholipids and cholesterol contribute, among other things, to the stability of the vaccine.
Unfortunately, less is known about the organization of LNPs at molecular and atomistic scales. In our work published in the October 18 issue of Biophysical Journal, we characterize the molecular organization and physicochemical properties of the Comirnaty vaccine LNPs. We provide hints for its pH-driven phase transition enabling mRNA release at atomistic resolution. At physiological pH, our simulations suggest an oil-like LNP core that is composed mainly of the aminolipid ALC-0315 and cholesterol. A lipid monolayer formed by cholesterol, phospholipid, ALC-0315, and PEGylated lipids shields the hydrophobic interior from the polar solvent. Protonated aminolipids enveloping mRNA form inverted micellar structures that provide shielding and probably protection from environmental factors. In contrast, at low pH, the Comirnaty lipid composition spontaneously forms lipid bilayers that display a high degree of elasticity. These pH-dependent lipid phases suggest that a change in pH of the environment upon LNP transfer to the endosome probably acts as a trigger for cargo release from the LNP core by turning aminolipids inside out, thereby destabilizing both the LNP shell and the endosomal membrane.
- Marius F.W. Trollman and Rainer A. Bӧckmann