Journal Information
- Article link: https://doi.org/10.1016/j.neurot.2026.e00892
- Journal: Neurotherapeutics (2026, Vol. 23, e00892)
- Impact Factor: about 5 (approximate)
- About the journal: Neurotherapeutics is the official journal of the American Society for Experimental NeuroTherapeutics. It covers novel therapies and translational research for neurological diseases and is a highly regarded academic journal in the field of neurotherapeutics. This article is published as open access (CC BY).
Summary
When small extracellular vesicles (small extracellular vesicle, sEV = a fraction rich in exosomes) derived from human bone marrow mesenchymal stem cells (MSC) are administered intravenously (IV) to spinal cord injury (spinal cord injury, SCI) model rats, motor function recovery is obtained. This study focused on “how you deliver them” rather than “how much you give”, comparing daily IV injections over 3 days with continuous infusion by osmotic pump (3 or 6 days), matched for total administered dose.
The findings are as follows.
- Even at the same total dose, administering it spread out over time produces a greater therapeutic effect than giving it all at once.
- Continuous infusion led to a faster onset of motor recovery than daily injection, and extending administration to 6 days further enhanced recovery (the total dose being the same).
- Mechanistically, hMSC-sEV are rich in microRNA (miRNA) that target fibrosis pathways; they are taken up by M2 macrophages at the injury site and suppress the expression of extracellular matrix (extracellular matrix, ECM) production genes, reducing fibronectin and collagen 1 and 5 at the injury site.
In other words, continuous infusion of hMSC-sEV is thought to bring about greater motor recovery than the same dose of daily injection by modulating macrophage-mediated ECM deposition.
Background
In spinal cord injury, the cellular composition, gene expression, and ECM deposition change greatly over time and space at and around the injury site. After injury, activation of resident microglia and infiltration of macrophages occur, and along with the inflammatory response the blood-spinal cord barrier breaks down. The inflammatory M1 macrophages that dominate early on eventually transition to anti-inflammatory M2 macrophages, which secrete ECM components and create an environment favorable for axonal regeneration. However, when the production of fibronectin and collagen becomes excessive, a mature fibrotic scar once again becomes a barrier to regeneration. Therefore, “balanced remodeling” of the ECM, rather than its “excessive accumulation,” is important for successful axonal regeneration.
Even when MSC are administered IV, they do not reach the target site; most are trapped in the lungs and remain there for a few days. This suggests that MSC may act indirectly via soluble factors or vesicles. Indeed, it has been reported that the MSC-derived sEV/exosome fraction can reproduce much of the therapeutic effect of the parent cells in a variety of injury and disease models. In the authors’ previous work, a single large-dose administration had no effect, whereas dividing the same dose over 3 days did produce an effect. From this observation, the present study tests the hypothesis that “the sustained delivery itself may be the key.”
Methods and Key Results
- Characterization of sEV: hMSC-sEV were 70-150 nm in diameter (mean about 130 nm), rich in the exosome markers CD63, CD9, and Alix, and almost negative for the endoplasmic reticulum marker Calnexin (little contamination by intracellular components).
- 3-day protocol: Both the daily IV injection group and the continuous infusion group recovered motor function (BBB score) significantly better than the PBS group. Continuous infusion showed a tendency toward an earlier onset of recovery.
- 6-day protocol: Here the difference became clear: the continuous infusion group was significantly superior to the daily injection group, and achieved the highest final BBB score (6-day continuous infusion group, 12.9 ± 0.9). Even at the same total dose, extending the administration time boosted the effect.
- Mechanism (macrophages and ECM): Compared with control human serum-derived sEV, hMSC-sEV were rich in miRNA that target fibrosis pathways. Uptake of hMSC-sEV by M2 macrophages significantly suppressed the expression of genes involved in ECM production. Furthermore, in sEV-treated rats, fibronectin and collagen 1 and 5 at the injury site were reduced compared with the PBS group.
- Body weight and growth: sEV administration also promoted recovery of body weight and growth, but this did not depend on the administration method (daily injection or continuous infusion). In contrast, only motor recovery depended on “how it was delivered.”
Significance and Reflections
This study clearly showed that in MSC-sEV therapy for spinal cord injury, “how continuously it is delivered” rather than “the total administered dose” governs the therapeutic effect. With clinical application in mind, it is important to design protocols based on continuous administration rather than a single bolus dose, and the study also suggests a concrete therapeutic target: modulation of the fibrosis (ECM) pathway. When using exosomes as a “drug,” not only the dose but also the timing and duration of administration determine the effect—this is a paper rich in implications from the perspective of formulation and administration design in regenerative medicine.