Many countries have started an immunization program, putting messenger RNA (mRNA) vaccines in the spotlight. Only 66 days separate the sequencing of the SARS-CoV-2 genome from the first clinical trial. This rapid development and high efficiency (95%) have raised questions about the mechanism of operation of these products.
The messenger RNA of the Moderna and BioNTech / Pfizer vaccines carries the information necessary to manufacture the S spike protein of the SARS-CoV-2 coronavirus and thus activate the immune response. The elegance of this method is that no cell cultures are needed to produce attenuated viruses or large factories to synthesize and purify viral proteins. This laborious work is carried out with the machinery of the cell itself. The genetic code is the “Rosetta Stone” that makes it possible to translate a messenger RNA sequence into a protein.
However, messenger RNA is a short-lived fragile molecule. It is degraded in human cells in an average of 10 hours and is destroyed by ribonuclease enzymes. So how is it possible that messenger RNA can be used as a vaccine if as soon as it enters our body it is destroyed by our defenses?
The success of messenger RNA vaccines is the fruit of basic research that has accumulated for more than three decades. Among the many advances that have made this scientific milestone possible, two stand out: the ability to synthesize modified RNA and the transport mechanism to the cell.
Synthetic Messenger RNA: Instructions for Making Proteins
Cellular mechanisms for recognizing and destroying foreign genetic material have been perfected over millions of years of evolution. It has taken time and ingenuity from researchers like Kerikó and his colleague Weissman to come up with a recipe that avoids mRNA degradation. They demonstrated in 2005 that, by modifying one of the components of mRNA, the defenses against synthetic RNA could be suppressed.
The vaccine contains mRNA with a leading cap and a long polyadenine tail on the back so that it is recognized as mRNA within the cell. In addition, a modified form of uracil (U) – one of the four nitrogenous bases that is part of RNA – is used to escape the defenses that help protect us from foreign RNA. For example, in the COVID-19 BioNTech / Pfizer vaccine, each U has been replaced by pseudouridine, denoted by Ψ in the sequence published by WHO.
This ingenious bioengineering ensures that the mRNA is properly processed within the cell, translated into protein (curiously more capable), exposed on the cell membrane, and triggers the immune response against the virus.
When the synthetic RNA is translated and the spike protein is produced in human cells, this protein shows a different conformation than the coronavirus. Thanks to the knowledge acquired about other coronaviruses (MERS, SARS) and the published structure of the spike protein, the mRNA of the vaccine contains two small modifications so that the protein acquires a rigid conformation that resembles the real spike protein of the virus . As mutated variants of the coronavirus are identified, mRNA vaccines could be reprogrammed to rapidly target emerging strains.
Lipid nanoparticles (LNP): protective fat droplets
An effective method of transport is the second ingredient in the mRNA vaccine. Imagine that you buy a very fragile glass vase online. You need a good box, with foam so that it does not break, and the direction so that it reaches its destination. Similarly, mRNA is a very fragile molecule and is transported within a sphere or blob of fat that endocytosis passes through the cell wall and releases its material into the cell.
Delivering nucleic acids to cells has been a challenge for many decades. The first breakthrough came from Robert Langer’s lab at MIT, who showed that certain polymers allow for the gradual release of proteins and other macromolecules.
Studies were published in the 1970s showing that it was possible to encapsulate nucleic acids in small vesicles decorated with the polymer polyethylene glycol (PEG) and thus slow down their degradation. Langer and others developed multi-nanometer fat droplets called lipid nanoparticles (LNPs) that are highly effective at transporting and releasing nucleic acids.
In 2010 they founded Moderna, the company that uses this method in one of the first mRNA vaccines against covid-19 (mRNA-1273). LNPs are used in cancer therapies and it is not the first time that an RNA vaccine using LNP has been licensed. In 2018, the FDA licensed Patisiran, a drug to treat a type of inherited amyloidosis (a rare disease) using interfering RNA.
The COVID-19 vaccine uses lipid nanoparticles made up of three main ingredients: an ionizing cationic lipid (a positively charged lipid), cholesterol, and PEG. Cholesterol ensures fluidity of the particle and is believed to increase the efficiency with which LNP fuses with the cell membrane. Polyethylene glycol increases the potency of the vaccine by making the hydrophobic particle more soluble in water and slowing its degradation. Finally, the ionizing cationic lipid is the key element that facilitates the encapsulation of a negatively charged molecule such as mRNA.
Multiple recipes, different stability
BionTech / Pfizer and Moderna use the same technology to encapsulate mRNA in lipid nanoparticles. However, they differ in the composition of the lipids they use. This, together with possible differences in the structure and thermostability of the mRNA, makes Pfizer recommend storing the vaccine at -70 ° C, while Moderna guarantees the stability of the vaccine at -20 ° C. The details of the recipe to produce LNPs are protected by a patent but can be deduced from the FDA clearance reports mRNA-1273 and BionTech / Pfizer BNT162b2 (Comirnaty).
The Oxford / AstraZeneca ChAdOx1 vaccine uses a modified chimpanzee adenovirus to carry DNA encoding spike protein. Its mechanism is different from the previous ones and it can be stored at 4 ° C.
These vaccines are the great hope in this pandemic and future diseases, but they are also a clear example of the synergies between various disciplines such as molecular biology, nanotechnology and the science of soft matter. Once again, basic science provides the ingredients for a technological revolution, as are mRNA vaccines.
Ibon Santiago, Alexander von Humboldt Research Fellow, Technical University of Munich
This article was originally published on The Conversation. Read the original.
