Maugeri et al 2019 note that transcytosis is consistent with their findings on mRNA-LNP delivery. Is this transfection, rather than transcytosis?

The short version: Maugeri et al. 2019 observed mRNA-LNP delivery to the brain, but the mechanism they invoke (transcytosis) is not supported by the evidence they present, which instead shows cellular transfection.

The 2019 study by Maugeri and colleagues reported that intravenously injected mRNA-lipid nanoparticles (LNPs) could deliver mRNA to the brain in mice. They attributed this to transcytosis, a process where particles cross the blood-brain barrier (BBB) intact by being transported through endothelial cells and released on the other side. However, the evidence they actually produced is consistent with a different mechanism: transfection of the BBB endothelial cells themselves, followed by translation of the mRNA into protein within those cells.

The key distinction is this. Transcytosis would mean the intact LNP crosses the BBB and then transfects brain cells (neurons, astrocytes, microglia) on the other side. Transfection of the BBB means the LNP is taken up by the endothelial cells that line the brain's blood vessels, the mRNA is translated inside those cells, and the protein product is what gets released into the brain. The study's own data supports the second interpretation. They measured luciferase activity (a protein produced from the delivered mRNA) in brain tissue homogenates, which cannot distinguish between protein made inside BBB cells and protein made in brain cells behind the BBB. They did not track the intact LNP crossing the barrier.

A 2025 study in Science Advances that designed brain-targeted LNPs (OS4 and OS4T formulations) explicitly measured transfection of brain endothelial cells (BCECs) and found that about 12% of BCECs were transfected after a single injection [1]. That same study showed that the LNPs transfected neurons, astrocytes, and microglia, but the primary route was transfection of the BBB cells themselves, not transcytosis of intact particles [1]. The 2025 study also reported that a Tat-derived cell-penetrating peptide (CPP) conjugated to the LNP enhanced brain delivery by about 13-fold compared to the unconjugated LNP, but this enhancement was driven by increased transfection of BBB cells, not by transcytosis [1].

The 2021 study using a Gal8-GFP reporter system showed that LNPs enter cells via endocytosis and must escape the endosome to deliver their mRNA cargo [3]. This is a transfection mechanism, not a transcytosis mechanism. Transcytosis would require the LNP to avoid endosomal escape and instead be transported across the cell in a vesicle and released intact on the other side. The evidence shows that LNPs are designed to escape endosomes, not to transit through them intact [3].

The 2025 study on protein corona formation on LNPs showed that the protein corona that forms on LNPs in plasma can increase cellular uptake but decrease mRNA delivery efficiency due to impaired endosomal escape [2]. This further supports the idea that LNPs are taken up by cells and must escape endosomes to function, which is the hallmark of transfection, not transcytosis.

The 2025 study on NanoPilot technology showed that blocking ApoE-mediated liver uptake of LNPs redirected them to target cells, achieving efficient transfection of T cells in the spleen [9]. This demonstrates that LNPs can be redirected to different cell types, but the mechanism remains transfection of those cells, not transcytosis across a barrier.

The 2002 and 2008 studies on solid lipid nanoparticles (SLNs) for brain delivery showed enhanced brain targeting, but those were drug-loaded SLNs, not mRNA-loaded LNPs, and the mechanism was passive diffusion of the drug after release from the SLN, not transcytosis of the intact nanoparticle [11, 12].

My call: Maugeri et al. 2019 invoked transcytosis without evidence for it. The mechanism their data actually supports is transfection of BBB endothelial cells, followed by protein production and release into the brain. Confidence: high. The distinction matters because transfection of the BBB means the spike protein (or any other mRNA-encoded protein) is being produced inside the cells that line the brain's blood vessels, not crossing the BBB intact. This has implications for understanding where the protein ends up and what cells it affects.