Procyanidin Suppresses Tumor Growth by Activating the B-Cell MAPK Pathway through Remodulation of the Gut Microbiota and Metabolites in Hepatocellular Carcinoma

The mortality of hepatocellular carcinoma (HCC) is high. Plant-derived bioactive compounds have emerged as potential therapies for HCC. Procyanidin (PAC) has been shown to possess immune-modulating and anti-tumor properties. However, the role and mechanism of total PAC in treating HCC remain unclear. We established subcutaneous and orthotopic HCC mouse models to assess the effect of PAC on tumor growth. Multi-omics analyses and <i>in vitro</i> experiments were conducted to investigate the changes in the gut microbiota, related-metabolites, and the tumor microenvironment (TME). 16S rDNA sequencing revealed that PAC could reshape the gut microbiota, notably increasing <i>Lactobacillus murinus</i> abundance. Furthermore, transplantation of <i>Lactobacillus murinus</i> reduced tumor volumes in mice. Single-cell RNA sequencing showed upregulation of the MAPK pathway in B cells within the TME. Metabolomic analysis suggests that 5-Hydroxytryptophan (5-HTP) derived from <i>Lactobacillus murinus</i> was significantly increased in B cells from mesenteric lymph nodes (MLNs) in the PAC-treated group. <i>In vitro</i> experiments revealed that 5-HTP could significantly upregulate the MAPK pathway in B cells. Additionally, 5-HTP-educated B cells could activate IFN-γ⁺CD8⁺T cells through B cell-T cell interactions, indicating that 5-HTP is a key metabolite in the therapeutic effect of PAC. Finally, feeding 5-HTP to HCC mice reduced tumor volume, upregulated the MAPK pathway in B cells from MLNs, and activated IFN-γ⁺CD8⁺T cells in the TME. PAC reshapes the gut microbiota and metabolites, upregulates the MAPK pathway in B cells from MLNs, and activates CD8⁺T cells in the TME through the gut-liver axis, thereby inhibiting HCC progression.