Theabrownin from Pu-erh Tea Restores Intestinal Immunity via Gut Microbiota–Metabolite Network in Immunosuppressed Mice

Theabrownin (TB), a polymeric pigment derived from ripened Pu-erh tea, demonstrates beneficial effects on lipid reduction, weight management, and mitigation of metabolic syndrome. However, the role of TB in restoring immune function in vivo has not been studied. This study evaluated the therapeutic effects and underlying mechanisms of TB in cyclophosphamide (CTX)-induced immunosuppressed mice. TB was administered at doses of 1.215, 0.608, and 0.304 mg/kg body weight, with levamisole hydrochloride (40 mg/kg) serving as a positive drug control. Each experimental group comprised eight mice. Evaluations primarily consisted of histopathological examination, enzyme-linked immunosorbent assay, Western blotting, flow cytometry, 16S rRNA sequencing, and ultra-performance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS)-based untargeted metabolomics. The results demonstrate that high- and medium-dose TB treatment significantly alleviated immunosuppression, as evidenced by restored body and organ weights, colon length, splenic and colonic histoarchitecture, blood cell counts, and immunoglobulin levels. These improvements correlated with upregulated colonic cytokines, claudin-1, occludin, toll-like receptor 2/4, and reduced regulatory T cell populations. High- and medium-dose TB treatment also enriched beneficial gut microbiota, including Lactobacillus and Candidatus saccharimonas, accompanied by strong microbiota–metabolite correlations (M2 = 0.725, p = 0.003). Metabolic KEGG pathway enrichment analysis revealed that moderate-dose TB significantly activated caffeine metabolism and histidine metabolism, accompanied by the enrichment of multiple immune signaling pathways driven by diacylglycerol series lipid metabolites, whereas high-dose TB induced a greater number of immune signaling pathways. Overall, TB may reverse CTX-induced immunosuppression by integrating gut microbiota–metabolite reprogramming, highlighting TB’s potential as an immunomodulatory functional component of medicine-food homologous products.