Abstract: BACKGROUND: Mesenchymal stem cell-derived exosomes, as intercellular communication mediators, have been shown to inhibit neuroinflammation and promote angiogenesis. However, there are few studies on the mechanism of action of mesenchymal stem cell-derived exosomes in alleviating the damage of the blood-brain barrier in septic encephalopathy.
OBJECTIVE: To explore the protective effect and mechanism of exosomes derived from human umbilical cord mesenchymal stem cells on the blood-brain barrier of mice with septic encephalopathy
METHODS: Exosomes were isolated from human umbilical cord mesenchymal stem cell culture medium by ultracentrifugation. Fifty-one C57/BL6 mice were randomly divided into sham operation group (n=17), model group (n=17), and treatment group (n=17). The latter two groups were intraperitoneally injected with lipopolysaccharide to establish the sepsis encephalopathy model. The treatment group was injected with 50 μg exosomes 0.5 hours after modeling. After 24 hours, the water content of brain tissue and the permeability of blood-brain barrier in mice were detected by mouse dry wet gravity method and Evans blue method. The neuronal damage in the mouse hippocampus was detected by Nissl staining. The fluorescence intensity of zonula occludens protein 1 and occludin protein in cerebral cortex was observed by immunofluorescence staining. The expression levels of tumor necrosis factor-α, interleukin-1β, zonula occludens protein 1, occludin protein, high mobility group protein B1, toll like receptor 4, nuclear factor κB, and matrix metalloproteinase 9 in cerebral cortex were detected by western blot assay. 
RESULTS AND CONCLUSION: (1) Compared with the sham operation group, the brain water content and Evans blue content in the model group were significantly increased (P < 0.05), while compared with the model group, the brain water content and Evans blue content in the treatment group were significantly decreased (P < 0.05). (2) Nissl staining results showed that compared with the sham operation group, the morphology and arrangement of neurons in hippocampal CA1 area of the model group were irregular, and the number of neurons was significantly decreased (P < 0.05). Compared with the model group, the morphology and arrangement of neurons in hippocampal CA1 area of the treatment group were regular, and the number of neurons was significantly increased (P < 0.05). (3) Immunofluorescence staining results showed that compared with the sham operation group, the fluorescence intensity of zonula occludens protein 1 and occludin protein in the cerebral cortex of the model group was significantly decreased, and compared with the model group, the fluorescence intensity of zonula occludens protein 1 and occludin protein in the cerebral cortex of the treatment group was significantly increased. (4) Western blot assay results showed that compared with the sham operation group, the expression levels of tumor necrosis factor-α, interleukin-1β, high mobility group protein B1, toll like receptor 4, nuclear factor κB, and matrix metalloproteinase-9 were significantly increased in the cerebral cortex of the model group (P < 0.05), while the expression levels of tumor necrosis factor-α, interleukin-1β, high mobility group protein B1, toll like receptor 4, nuclear factor kappa B, and matrix metalloproteinase-9 were significantly decreased in the cerebral cortex of the treatment group (P < 0.05). (5) The results showed that human umbilical cord mesenchymal stem cell-derived exosomes could reduce the damage of blood-brain barrier and the permeability of blood-brain barrier in mice with sepsis encephalopathy, and its mechanism was related to the inhibition of high mobility group protein B1/toll like receptor 4/matrix metalloproteinase 9 related inflammatory pathway by exosomes. 

Key words: human umbilical cord mesenchymal stem cell, exosome, lipopolysaccharide, septic encephalopathy, blood-brain barrier, high mobility group protein B1, matrix metalloproteinase 9, neuroinflammation