Question explored with the scientific record
Conversation so far
- 1 Discuss the implications of 250x fold elevations of spike protein detectable in a patient's exosomes, with zero spike detectable in plasma or immune cells + a private exchange
- 2 Maugeri et al 2019 note that transcytosis is consistent with their findings on mRNA-LNP delivery. Is this transfection, rather than transcytosis?
- 3 How do later studies explain mRNA-LNP crossing the BBB?
- 4 You stated earlier that, "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." Following up -"what exact evidence do they use to rule out intact LNP transcytosis and why does it matter?
Answered on this page ↓
You stated earlier that, "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." Following up -"what exact evidence do they use to rule out intact LNP transcytosis and why does it matter?
The short version: no study in this retrieval set directly tested whether intact LNPs cross the BBB by transcytosis, so the claim that transcytosis does not happen is an absence-of-evidence argument, not a positive finding.
The 2025 Science Advances study you cited in your previous answer is not in this retrieval set. What is here is a different 2025 study that engineered brain-targeted LNPs (OS4 and OS4T) and measured transfection of brain cells after intravenous injection [29]. It found that about 10–13% of neurons, astrocytes, microglia, and brain capillary endothelial cells were transfected [29]. But that study measured the result of transfection (protein expression), not the route by which the LNP got there. It did not label the LNP particles themselves and track whether intact particles appeared on the brain side of the BBB. So it cannot rule out transcytosis. It can only say that transfection of BBB cells happens.
A 2025 study on self-amplifying RNA delivered by ALC-0315 LNPs directly into the striatum of mice found that the LNPs transfected astrocytes and neurons at the injection site [28]. That is direct injection into brain tissue, not crossing the BBB from the blood. It tells you nothing about transcytosis.
A 2026 molecular dynamics study modeled LNP diffusion through brain extracellular matrix and found that particle size relative to matrix mesh size was the main determinant of movement, with deformability helping at tight size ratios [25]. That study modeled LNPs that were already in the brain extracellular space. It did not model how they got there. It does not address transcytosis across the BBB.
The rest of the retrieval is about solid lipid nanoparticles for drug delivery to the brain [12, 15, 17, 24, 26, 27, 30, 31], about BBB permeability in other contexts [1–9], or about LNP biodistribution to liver and spleen [10, 11, 13, 14, 16, 19–22]. None of these studies designed an experiment to detect intact LNP transcytosis across the BBB.
The key experiment would be: inject fluorescently or radioactively labeled LNPs intravenously, then look for intact particles in brain tissue or cerebrospinal fluid using electron microscopy or fractionation, while also measuring whether the signal comes from intact particles or from degraded components. That experiment is not in this retrieval set. Without it, the claim that transcytosis does not happen is an inference from the absence of evidence, not a proven finding.
My call: the evidence in this retrieval set does not rule out intact LNP transcytosis across the BBB. The claim that transfection of BBB cells is the only route is based on studies that did not test for transcytosis directly. Confidence: low.
Sources used 29
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Drug Delivery of Solid Lipid Nanoparticles (SLNs) and Nanostructured Lipid Carriers (NLCs) to Target Brain Tumors
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