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Diffuse large B cell lymphoma (DLBCL) presents a critical clinical challenge due to declining chemosensitivity and difficult-to-manage dose-limiting toxicities. Although gut microbiota modulation shows potential for "toxicity reduction and efficacy enhancement", its mechanism in DLBCL remains unclear. Comparative analysis revealed a marked reduction of beneficial bacteria in patients with DLBCL versus healthy volunteers, with a marked decrease in the abundance of core probiotics, particularly Limosilactobacillus reuteri. Fecal microbiota transplantation from healthy donors into DLBCL mouse models reduced tumor burden, improved chemosensitivity, and alleviated intestinal toxicity. A core probiotic strain, L. reuteri HG001, was isolated and shown to replicate these effects alone, with the tryptophan metabolite indole-3-carbaldehyde (ICAld) identified as the key component responsible for its adjunctive antitumor activity. Mechanistic studies demonstrated that ICAld exerts significant adjunctive antitumor effects both in vitro and in vivo in a dose-dependent manner in mouse models; it acts by activating the aryl hydrocarbon receptor (AHR)/cytochrome P450 family 1 subfamily A member 1 (CYP1A1)/reactive oxygen species (ROS) axis, inhibiting the phosphatidylinositol 3-kinase (PI3K)/AKT/mechanistic target of rapamycin (mTOR) signaling pathway, promoting apoptosis, and synergizing with cyclophosphamide. An aryl hydrocarbon receptor antagonist reversed both the chemosensitizing and intestinal protective effects of L. reuteri HG001 and ICAld. This study elucidates a microbiota-mediated mechanism in DLBCL and supports L. reuteri HG001 as a probiotic adjuvant to enhance therapy while reducing toxicity.