Redesigned lipid nanoparticles improve mRNA delivery to lymph nodes

Penn Engineers have redesigned a key component of lipid nanoparticles (LNPs), the delivery vehicles behind mRNA vaccines, to steer the particles toward lymph nodes while reducing off-target delivery to the liver. The advance could make mRNA vaccines more efficient, potentially achieving strong immune protection at lower doses. 

"The more particles that reach the lymph nodes, the fewer particles each dose needs," says Michael J. Mitchell, Associate Professor in Bioengineering (BE) and senior author of a new study in Journal of the American Chemical Society that describes how the researchers modified the ionizable lipid, a key LNP ingredient that helps mRNA enter cells.

In animal models, the new "aroLNPs," whose name refers to the addition of a chemical structure called an "aromatic ring" to the ionizable lipid, delivered at least tenfold less mRNA to the liver compared to the LNP formulation in the Moderna COVID-19 vaccine, while maintaining similar levels of lymph-node delivery. 

AroLNPs could also advance other mRNA therapies, like cancer vaccines and autoimmune-disease treatments. "More precise nanoparticle delivery gives us a new level of control over immune activation," says Mitchell.

Towards the lymph nodes, not the liver

The body contains hundreds of lymph nodes, small organs that serve as immune training grounds. When the immune system detects a foreign presence, specialized immune cells collect pieces of that invader, known as antigens, and carry them to the lymph nodes. There, they present those fragments to other immune cells, teaching them what to attack.

The mRNA vaccines harness this powerful system by delivering mRNA that prompt the body to produce a harmless fragment of a pathogen. Even in the absence of disease, that exposure can prime the immune system to recognize the pathogen in the future.

The lymph nodes are key to this process. That's where the mRNA vaccine teaches the immune system what to guard against."

Hannah Yamagata, a doctoral student in BE and first author of the study

While mRNA vaccines have been tremendously successful, LNP formulations can also result in off-target delivery with less RNA being delivered to the lymph nodes.

"Even with proven mRNA vaccines, not every nanoparticle ends up in the lymph nodes," adds Yamagata. "If we can make the delivery process more precise, we can potentially lower the dose needed to achieve immunity."

The power of aromatic rings

Several years ago, other researchers found that adding a particular aromatic compound with a square shape to the ionizable lipid improved LNP performance. 

In chemistry, "aromatic" refers not to scent, but to a stable, ring-like arrangement of atoms that can influence molecular behavior. 

The Penn team wondered whether other aromatic structures might have similarly beneficial effects. "Small changes in molecular structure can dramatically alter how nanoparticles behave," says Marshall Padilla, a postdoctoral fellow in BE and co-author of the paper. 

The team then created a library of ionizable lipids that included benzene rings. In each variant, the researchers subtly changed where different chemical groups were positioned around the ring, testing how those small structural shifts affected nanoparticle behavior. 

They also incorporated bioreducible disulfide bonds - chemical linkages containing two sulfur atoms that can break apart inside cells - a modification previously shown to enhance performance and reduce toxicity. 

"To our knowledge, this is the first time aromatic rings and bioreducible disulfide bonds have been combined in this way within lipid nanoparticles," says Padilla.

Outperforming FDA-approved formulations

To test the new designs, the team packaged luciferase mRNA, which produces a light-emitting protein when expressed, into the nanoparticles. By measuring the glow of different organs in animal models, the researchers could track where the particles delivered their genetic cargo.

"We were initially just trying to make better-performing lipids," says Yamagata. "When we looked at where the LNPs were going, the shift away from the liver was striking." 

The best-performing aroLNPs delivered tenfold less mRNA to the liver than LNPs formulated with the ionizable lipid used in the Moderna COVID-19 vaccine. At the same time, aroLNPs accumulated just as readily in the lymph nodes, increasing the lymph-node-to-liver delivery ratio by five- to tenfold.

Crucially, reducing off-target nanoparticle delivery did not blunt the particles' ability to train the immune system. When assessed in a vaccine model, the aroLNPs generated antibody responses comparable to ionizable lipids used in clinically approved formulations, as evidenced by similar levels of antibodies, the Y-shaped proteins produced by immune cells that recognize and bind to threats.

AroLNPs also caused minimal increases in systemic proinflammatory cytokines, the immune proteins responsible for vaccine side effects such as fatigue and fever. In other words, the modified nanoparticles could reduce the side effects associated with traditional mRNA vaccines.

"What's exciting is that we were able to redirect where the particles go without losing immune potency, and even reducing side effects," says Yamagata. "That suggests we can design vaccines that are more precise, better tolerated and more efficient."

Future directions

Many next-generation therapies require carefully tuning the immune response, either boosting it, as in cancer vaccines, or quieting it in autoimmune disease. By shifting delivery toward immune tissues and away from off-target organs, aroLNPs may offer a new level of control.

"This is really about precision," says Mitchell. "If we can control where mRNA goes in the body, we can begin to tailor immune responses more deliberately, whether that means turning them up, turning them down or directing them toward a specific target."