Enterococcus faecium L-3 Ameliorates the Severity of Experimental Allergic Encephalomyelitis in Rats by Influencing Gut Microbiota, Enterocytes, and Immune Functions

This study explores the mechanisms of the modulating effects of the probiotic strain Enterococcus faecium L-3 on the course of experimental allergic/autoimmune encephalomyelitis (EAE) in rats, a widely used model of multiple sclerosis. EAE was induced in female Wistar rats by a single subcutaneous injection of spinal cord homogenate in complete Freund’s adjuvant. After EAE induction, the rats were divided into two groups, one of which received E. faecium L-3 at a dose of 5 × 108 lg CFU/mL from day 2 to day 16 post induction. The effects of E. faecium L-3 were assessed by the severity of EAE, population composition of immune cells (flow cytometry), cytokine levels (ELISA and RT-PCR), taxonomic composition of the gut microbiome (16S rRNA gene sequencing), and ultrastructural alterations in the small intestinal epithelium (electron microscopy). Rats treated with E. faecium L-3 exhibited a delay in the onset of neurological symptoms and a reduction in their severity compared to the untreated group. E. faecium L-3 administration led to significant changes in the composition of the gut microbiome during EAE. At the peak of the disease, probiotic-treated rats showed a higher content of Bacteroidota and a lower content of Bacillota (Firmicutes) compared to the control group. Shifts in the gut microbiome also persisted throughout the recovery phase and were associated with a more complete recovery. Probiotic administration reduced the blood level of the circulating pro-inflammatory cytokine MCP-1, while simultaneously increasing the level of the regulatory cytokine TGF-β. Ultrastructural analysis of the intestinal epithelium showed that a course of E. faecium L-3 administration reduced enterocyte damage at the peak of the disease and promoted intestinal mucosa restoration in the recovery phase. The obtained results suggest that the protective effect of E. faecium L-3 in EAE is implemented locally through modulation of the gut microbiome composition, stimulation of IL-10 expression in mesenteric lymph nodes, maintenance and restoration of enterocyte integrity in the small intestine, and systemically through immunoregulatory mechanisms mediated by TGF-β-producing T cells. Overall, the results indicate that E. faecium L-3 could be a useful supplement to existing multiple sclerosis treatments after confirmation of its protective effect in randomized clinical trials.