Carrimycin Ameliorates Dextran Sulfate Sodium-Induced Ulcerative Colitis in Mice by Modulating Gut Microbiota and Restoring Th17/Treg Balance

Technical Summary: Carrimycin Ameliorates Dextran Sulfate Sodium-Induced Ulcerative Colitis in Mice

Problem Statement
Ulcerative colitis (UC) is a chronic inflammatory bowel disease characterized by mucosal inflammation, epithelial barrier breakdown, and a dysregulated immune response. Current therapeutic strategies, including 5-aminosalicylates, corticosteroids, and biologics, often face limitations regarding long-term safety, economic burden, and the risk of opportunistic infections. A critical pathological feature of UC is the disruption of the Th17/Treg immune balance and gut dysbiosis, specifically the expansion of pathogenic bacteria like Escherichia-Shigella and the depletion of beneficial commensals. There is an unmet need for novel, safe, and cost-effective oral small-molecule therapeutics that can simultaneously address immune dysregulation and microbiota homeostasis.

Methodology
The study utilized a murine model of dextran sulfate sodium (DSS)-induced colitis in male C57BL/6 mice. The experimental design included three groups: a control group, a DSS-induced model group, and a Carrimycin (CA)-treated group (10 mg/kg, oral gavage).

• Clinical and Histological Assessment: Disease progression was monitored via body weight, Disease Activity Index (DAI), colon length, and spleen index. Histopathological changes were evaluated using H&E staining and a standardized pathological scoring system.
• Immune Profiling: Systemic and local immune responses were analyzed using flow cytometry. This included quantifying peripheral blood and colonic lamina propria mononuclear cells (LPMCs) for CD3+ T cells, B220+ B cells, Gr-1+ neutrophils, and the specific differentiation of Th17 (CD4+IL-17A+) and Treg (CD4+CD25+Foxp3+) cells.
• Cytokine Analysis: Serum levels of pro-inflammatory and regulatory cytokines (IL-6, TNF-α, IFN-γ, IL-17A, IL-10, etc.) were measured via Cytometric Bead Array (CBA).
• In Vitro Differentiation Assays: Naïve CD4+ T cells were isolated and induced to differentiate into Th17 or Treg lineages in the presence of varying concentrations of CA (0, 0.5, and 1 μM) to assess direct immunomodulatory effects.
• Microbiota Analysis: Fecal samples underwent 16S rRNA gene sequencing (V3–V4 regions) to evaluate alpha and beta diversity, community structure, and specific taxonomic shifts.

Key Contributions and Results
The study demonstrates that Carrimycin effectively ameliorates DSS-induced colitis through a multi-target mechanism involving immune regulation and microbiota reconstruction.

1. Clinical and Histological Improvement: CA treatment significantly attenuated DSS-induced symptoms, including weight loss and high DAI scores. Histopathological analysis confirmed that CA preserved colonic mucosal integrity, reduced inflammatory cell infiltration, and decreased the pathological score from 6.9 ± 0.3 (model) to 4.5 ± 0.4 (treated). While CA showed a trend toward improving colon length and reducing splenomegaly, these macroscopic changes did not reach statistical significance at the 10 mg/kg dose.
2. Systemic and Local Immune Modulation: CA intervention significantly reduced serum levels of key pro-inflammatory cytokines, most notably IL-6 (decreased by ~52%), as well as TNF-α, IFN-γ, and IL-17A. Flow cytometry revealed that CA restored the balance of immune cell populations, specifically by downregulating the frequency of Th17 cells and upregulating Treg cells in the colonic lamina propria.
3. Direct Immunomodulatory Mechanism: In vitro differentiation assays confirmed that CA directly inhibits Th17 differentiation and promotes Treg differentiation in a dose-dependent manner, independent of antimicrobial activity. This suggests a direct cell-autonomous effect on T-cell lineage commitment, likely mediated by the modulation of transcription factors (RORγt and Foxp3) or upstream signaling pathways.
4. Microbiota Reconstruction: 16S rRNA sequencing indicated that DSS induction caused severe gut dysbiosis, characterized by reduced alpha diversity and a massive expansion of Escherichia-Shigella. CA treatment partially restored microbial diversity and significantly reduced the relative abundance of pathogenic Escherichia-Shigella. Concurrently, CA enriched beneficial commensals, particularly Akkermansia, and restored Alistipes and Alloprevotella, suggesting a role in enhancing short-chain fatty acid (SCFA) metabolism.

Significance and Claims
The authors conclude that Carrimycin represents a promising therapeutic candidate for ulcerative colitis management. The study posits that CA's efficacy stems from a dual mechanism:

1. Immunomodulation: It restores the Th17/Treg homeostatic balance by directly suppressing pro-inflammatory Th17 responses and enhancing regulatory Treg populations, thereby reducing mucosal inflammation.
2. Microbiota Remodeling: It selectively eradicates pathogenic bacteria (e.g., Escherichia-Shigella) while enriching protective commensals (e.g., Akkermansia), thereby breaking the cycle of dysbiosis and inflammation.

The paper highlights that these effects occur at a low oral dose (10 mg/kg), suggesting a favorable therapeutic window where immunomodulatory benefits are achieved without the severe microecological disruption often associated with high-dose antibiotic therapy. The findings provide a scientific rationale for repurposing Carrimycin as a multi-target immunomodulatory agent for UC, offering a potential alternative to current therapies with a distinct mechanism of action involving the "Microbiota-Immune" axis.