Can lipid nanoparticles mask endotoxin contamination in vaccine testing?

The short version: yes, lipid nanoparticles can interfere with endotoxin testing, and the standard assays are not reliable for them without extensive validation.

The evidence directly shows that nanoparticles interfere with the Limulus amebocyte lysate (LAL) assay, the standard test for endotoxin. Silica nanoparticles, including PEGylated versions, either enhanced or inhibited the gel-clot LAL reaction depending on particle concentration and surface properties [3]. This is not a theoretical risk; the study measured it. A 2019 methods paper on testing nano-formulations acknowledges this problem explicitly, recommending serial dilutions and spike-recovery controls to mitigate interference [4]. The paper notes that PEGylated liposomal doxorubicin interfered with the chromogenic LAL format at low dilutions [4]. These are the same class of lipid-based particles used in mRNA vaccines.

The mechanism is straightforward. Nanoparticles have large surface areas and charged surfaces that can bind to the LAL reagent proteins, activate the clotting cascade non-specifically, or physically block the reaction [3, 4]. The result is a false negative (masked endotoxin) or a false positive (apparent endotoxin where there is none). The 2019 guide states that the key control, the inhibition-enhancement control (IEC), must show 50–200% spike recovery for a result to be valid [4]. If the nanoparticle formulation fails that control at the test dilution, the assay result is meaningless.

The retrieved studies do not test this question directly for the specific LNPs in mRNA vaccines. The silica nanoparticle study [3] and the LAL methods paper [4] are the closest evidence. The 2024 monocyte activation test (MAT) study [1] and the transgenic cell-line assay [2] are alternative pyrogen tests that may be less prone to nanoparticle interference, but neither study tested them against LNPs. The MAT uses human immune cells and detects pyrogens by IL-6 release [1]; the cell-line assay uses an NF-κB reporter [2]. Both could theoretically be affected by LNPs, but that was not studied.

The practical implication is that a manufacturer can report an endotoxin level below the limit when the actual level is higher, because the LNP masked it. The burden is on the manufacturer to prove the assay works in their specific formulation. The 2019 guide provides the protocol for that validation [4], but whether every manufacturer runs it properly and publishes the results is a separate question.

My call: lipid nanoparticles can mask endotoxin in the standard LAL test, and the evidence does not confirm that the alternative tests (MAT, cell-line) are immune to this interference. Confidence: moderate.